Article

Asperities, barriers, characteristic earthquakes and strong motion prediction ( Japan).

July 1984
Journal of Geophysical Research Atmospheres 89(NB7):5867-5872

July 1984
89(NB7):5867-5872

Authors:

Recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip. Stable asperities and barriers, which survive many earthquakes, can explain these results. The activity of Volcano Usu, Japan, since 1977 which produced a dacite dome with a total upheaval of 180 m, offered a great opportunity to study the recurrence behaior of a fault. Seismologists at Hokkaido University revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall, along which the dome moved upward. This family can be explained by the existence of a stable asperity on the crater wall and is called the ``asperity-type'' family. They found, however, another type of family of earthquakes, which share the same fault plane but show a great range of variation in the amount of slip. The behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers, and this family is referred to as the ``barrier-type'' family. This type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from California, and the results reveal the stability of local stress drop among different earthquakes. The agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault.

TIME DISTANCE ANALYSIS OF A MINE SCALE EVENT

Thesis

Full-text available

Aug 2022

Benjamin Ollila

Mining induced seismic events greater then Nuttli Magnitude 3.0 are difficult to understand, have high potential consequences and are becoming increasingly common in Canada. The term mine scale event (MSE) is used to describe a seismic event in which the mechanisms and processes involved take place on a scale similar to that of the mine. A MSE from Nickel Rim South Mine was investigated using seismic data to explain its time, location and large magnitude. A novel tool, Time Distance Analysis was developed to identify spatial-temporal trends in seismicity around the MSE. Guidelines were developed to account for the unknown spatial and temporal extent of the processes that led to and were affected by the MSE. The results showed that preceding seismicity tended to coalesce around the eventual hypocenter of the MSE while subsequent seismicity migrated away. The coalescence was interpreted to represent the deterioration of a fault asperity, leading to an eventual rupture. After the MSE occurred, the dispersion of seismicity was interpreted to represent an unloading of the source region.

Tuzla earthquake swarm in Turkey

Article

May 2022
ACTA GEOPHYS

Long-lasting earthquake clusters are common in western Anatolia. One of them has been active in the southwestern part of the Biga peninsula. We had studied this cluster in terms of strain energy produced over time and revealed its interesting characteristics. The seismicity in this clustering region is not normal in terms of both the number of earthquakes, duration of the activity, magnitude/frequency and mainshock/aftershock relations, and the strain energy produced. An abnormal seismic activity with a set of medium-sized earthquakes without a mainshock, which are indistinguishable in size occurred in this clustering region in 2017. Interesting features that are characteristic of earthquake swarms are probably related to the abnormal physical properties of the crust. The 2017 Tuzla activity, where there is also no notable aftershock activity, could be associated with a phase of the swarming phenomenon itself. For this, we approached the 2017 activity in terms of the fact that it may not be part of the active tectonic system. Apart from the presence of high geotherm and hot springs in the region, the crust has been weakened by pre-existing intense fault zones that have developed in previous deformation regimes since the paleo-tectonic periods. Historical and instrumental period large earthquakes have caused loss of life and property in Biga peninsula due to an existing active fault zone. The Tuzla region probably is in this zone extending NE-SW in the peninsula. However, it is very difficult or impossible to distinguish possible foreshocks or precursory phenomenon of a future large earthquake from the background activity of the earthquake swarms.

The impact of erosion on fault segmentation in thrust belts: Insights from thermochronology and fluvial shear stress analysis (southern Longmen Shan, eastern Tibet)

Article

Full-text available

Oct 2021
GEOMORPHOLOGY

Along the southern Longmen Shan thrust belt in eastern Tibet, the Dayi subsegment stands out as it remained unruptured in both the 2008 Wenchuan and the 2013 Lushan earthquakes. Whether localized weak materials or differential erosion caused this fault segmentation is unclear. According to the critical-taper wedge theory, these two mechanisms would generate distinct morphotectonic features and erosional distributions, such as a recess vs. a salient structure at the range front, and higher vs. lower erosion rate than that of the adjacent areas along strike in the hinterland, respectively. We integrated two independent methods to evaluate these mechanisms. We obtained 10 apatite fission track and two apatite (U-Th)/He dates from the Dayi subsegment, which yield ages of ~3–44 Ma, and deduced an average erosion rate of 0.5–0.6 mm/yr and 0.2–0.4 mm/yr since ~6–8 Ma in the hanging-wall and footwall of the Shuangshi-Dachuan Fault, respectively. We also calculated the fluvial shear stress and erosion rate at 708 sites along five rivers that flow across the faults in the southern Longmen Shan. These two methods yield consistent mean erosion rates, implying an exhumational steady state since at least 5 Ma. Intriguingly, a systematic, heterogeneous pattern of erosion is found on a 3-by-3 grid along and across the Dayi subsegment. In the hinterland, the erosion rate in the Dayi subsegment is lower than that of the adjacent areas along strike, whereas at the range front, the erosion rate in the Dayi subsegment is greater. The spatial patterns of erosion and topography suggest that differential erosion plays a major, but not exclusive, role in regulating the fault segmentation and earthquakes in the Longmen Shan. Moreover, the Dayi subsegment could have acted as a barrier during the recent great earthquakes, posing substantial seismic potential to this region.

RELACIÓN ENTRE LA DISTRIBUCIÓN DE LAS AMPLITUDES DE LOS TSUNAMIS Y LA EVOLUCIÓN TEMPORAL DEL PROCESO DE RUPTURA DE FUENTES SÍSMICAS.

Thesis

Full-text available

Jun 2019

Hermann Schwarze

En el presente trabajo se analiza la respuesta del agua al proceso temporal de ruptura de terremotos submarinos. Dicho comportamiento es estudiado por medio de la modelación de tsunamis generados por distintos tipos de fuentes sísmicas. Esto construyendo modelos de fallas finitas que permiten aislar los parámetros cinemáticos de las rupturas, para luego ser usados como entrada de la modelación de tsunamis. Los resultados muestran que desde el punto de vista cinemático las grandes amplitudes de tsunamis son una “respuesta” del agua a las bajas velocidades de ruptura, al tamaño de la columna de agua sobre la fuente y la ubicación del hipocentro en el plano de falla. Este estudio fue hecho considerando dos tipos de escenarios; se utilizó una batimetría simple, compuesta por un fondo plano y un talud que se eleva desde el fondo del mar hasta la costa y una batimetría realista del Norte de Chile, de iguar resolución espacial. Estas dos batimetrías contaron con dos superficies de falla asociadas; para el caso simple, se tuvo una superficie plana rectangular y para el caso del Norte de Chile, se construyeron muchos planos superpuestos sobre el slab en esa zona. Cada superficie de falla utilizó una distribución de slip uniforme y estocástica y para la simulación del terremoto se construyó un modelo cinemático de fuente que permitió controlar la velocidad de ruptura y el Rise Time. Entre los resultados obtenidos se destaca el hecho de que las bajas velocidades de ruptura pueden amplificar hasta 8 veces la altura del Run-Up, con respecto al caso de una fuente instantánea. También se observa que para velocidades lentas, el efecto de la directividad de la fuente controla la distribución espacial de las amplitudes del tsunami, haciendo que las mayores amplitudes del Run-Up se concentren en las costas más cercanas a la proyección horizontal de la dirección de propagación de la ruptura. También se observa que las amplitudes del tsunami dependen de la relación entre la velocidad de ruptura y la profundidad de la batimetría, por lo que se concluye que los terremotos tsunamigénicos de distintas partes del mundo logran excitar tsunamis que se propagan a diferentes velocidades de ruptura, dependiendo de la profundidad de la batimetría sobre la fuente. Finalmente, se observa que los terremotos tsunamigénicos que propagan sus rupturas con cambios abruptos de velocidad, de rápido a lento, logran amplificar la altura de los tsunamis más que las rupturas que se propagan a una velocidad constante

Reconstruction of the earthquake rupture process through coherent teleseismic imaging and statistical modeling

Thesis

Nov 2019

Marina Corradini

Many studies have attempted to illuminate rupture complexities of large earthquakes through the use of coherent imaging techniques such as back-projection (BP). Recently, Fukahata et al. (2013) suggested that, from a theoretical point of view, the BP image of the rupture is related to the slip motion on the fault. However, the quantitative relationship between the BP images and the physical properties of the earthquake rupture process still remains unclear.Our work aims at clarifying how BP images of the radiated wavefield can be used to infer spatial heterogeneities in slip and rupture velocity along the fault. We simulate different rupture processes using a line source model. For each rupture model, we calculate synthetic seismograms at three teleseismic arrays and we apply the BP technique to identify the sources of high-frequency (HF) radiation. This procedure allows for the comparison of the BP images with the originating rupture model, and thus the interpretation of HF emissions in terms of along-fault variation of the three kinematic parameters: rise time, final slip, rupture velocity. Our results show that the HF peaks retrieved from BP analysis are most closely associated with space-time heterogeneities of slip acceleration. We verify our findings on two major earthquakes that occurred 9 years apart on the strike-slip Swan Islands fault: the Mw 7.3 2009 and the Mw 7.5 2018 North of Hondurasearthquakes. Both events followed a simple linear geometry, making them suitable for comparison with our synthetic approach. Despite the simple geometry, both slip-rate functions are complex, with several subevents. Our preliminary results show that the BP image of HF emissions allows to estimate a rupture length and velocity which are compatible with other studies and that strong HF radiation corresponds to the areas of large variability of the moment-rate function. An outstanding question is whether one can use the BP image of the earthquake to retrieve the kinematic parameters along the fault. We build on the findings obtained in the synthetic examples by training a neural network model to directly predict the kinematic parameters along the fault, given an input BP image. We train the network on a large number of different synthetic rupture processes and their BP images, with the goal of identifying the statistical link between HF radiation and rupture kinematic parameters. Our results show that the neural network applied to the BP image of the earthquake is able to predict the values of rise time and rupture velocity along the fault, as well as thecentral position of the heterogeneity, but not the absolute slip values, to which the HF BP approach is relatively insensitive. Our work sheds some light on the gap currently existing between the theoretical description of the generation of HF radiation and the observations of HF emissions obtained by coherent imaging techniques, tackling possible courses of action and suggesting new perspectives.

Laboratory Earthquake Rupture Interactions With a High Normal Stress Bump

Article

Full-text available

Nov 2023

To better understand how normal stress heterogeneity affects earthquake rupture, we conducted laboratory experiments on a 760 mm poly (methyl‐mathacrylate) PMMA sample with a 25 mm “bump” of locally higher normal stress (∆σbt). We systematically varied the sample‐average normal stress (σn‾ $\overline{{\sigma }_{n}}$) and bump prominence (∆σbt/σn‾ ${{\increment}\sigma }_{\text{bt}}/\overline{{\sigma }_{n}}$). For bumps with lower prominence (∆σbt/σn‾<6 ${{\increment}\sigma }_{\text{bt}}/\overline{{\sigma }_{n}}< 6$) the rupture simply propagated through the bump and produced regular sequences of periodic stick‐slip events. Bumps with higher prominence (∆σbt/σn‾>6 ${{\increment}\sigma }_{\text{bt}}/\overline{{\sigma }_{n}} > 6$) produced complex rupture sequences with variable timing and ruptures sizes, and this complexity persisted for multiple stick‐slip supercycles. During some events, the bump remained locked and acted as a barrier that completely stopped rupture. In other events, a dynamic rupture front terminated at the locked bump, but rupture reinitiated on the other side of the bump after a brief pause of 0.3–1 ms. Only when stress on the bump was near critical did the bump slip and unload built up strain energy in one large event. Thus, a sufficiently prominent bump acted as a barrier (energy sink) when it was far from critically stressed and as an asperity (energy source) when it was near critically stressed. Similar to an earthquake gate, the bump never acted as a permanent barrier. In the experiments, we resolve the above rupture interactions with a bump as separate rupture phases; however, when observed through the lens of seismology, it may appear as one continuous rupture that speeds up and slows down. The complicated rupture‐bump interactions also produced enhanced high frequency seismic waves recorded with piezoelectric sensors.

A Stochastic Model for Earthquake Rupture

Preprint

Full-text available

Aug 2023

Analytical solutions for the dynamics of asymmetric many-body systems are impractical to obtain, and numerical solutions usually exhibit chaotic behavior if interactions between bodies are considered.To address these challenges, stochastic approaches have been widely employed in modelling many-body systems.Following Langevin's approach, we propose a stochastic dynamic model for the earthquake rupture process, in which complexity in degrees of freedom is reduced by introducing a random force that accounts for uncertainties in faulting plane heterogeneity and structural collisions.By treating the tectonic process as a Coulomb friction process, the proposed Langevin equation can be viewed as a stochastic variant of Newton's second law, thereby attributing physical significance to the equation through the realization of stochastic processes as sample paths.This study analyzes synthetic events generated numerically with the Langevin equation to determine the energy--duration relationship and solves the corresponding Fokker--Planck equation to obtain the theoretical rupture slip distribution.The results for the energy--duration relationship suggest the existence of a universal scaling law, with the scaling exponent varying under different slip velocity thresholds used to define synthetic events.Regarding the slip distribution, solutions obtained assuming relatively large external driving forces align closely with the truncated exponential model, characterizing rupture models of large earthquake events worldwide.The proposed Langevin equation establishes a physical foundation that elucidates the physics governing the scaling parameter of such laws and reveals the connection between the scaling parameter and the dissipative and environmental noise effects of the faulting system.

Variability of Spectral Estimates of Stress Drop Reconciled by Radiated Energy

Article

Full-text available

Jun 2022

A review of a collection of theoretical source spectral models revealed: (1) Despite the well-known variation in predicting static stress drop Δσs from the seismic moment and corner frequency, all models, especially the three conventional models, suggest that earthquakes radiate about half of the available strain energy into the surrounding medium. This similarity justifies a less model-dependent approach to estimate Δσs, though estimates for natural earthquakes rely on apparent seismic radiation efficiency (=2σa/Δσs; σa is apparent stress of an earthquake). (2) When one attempts to use Δσs and spectral models to make predictions, such as apparent stress σa, there is a model-dependent discrepancy between the σa inferred from theoretical energy partitioning and the σa predicted using spherical mean corner frequency. Their ratio cp varies significantly from 1.0 for the Brune (1970, 1971) model to 6.38 for the Madariaga (1976) model. If one uses spectral models to predict the ground motion, cp must be considered. (3) We infer that the constancy of the “stress parameter” (Δσ˜) found in engineering seismology (e.g., Boore, 1983; Atkinson and Beresnev, 1998) is similar to having constant apparent stress, σa (e.g., Ide and Beroza, 2001). The observation that Δσ˜ is generally larger than the average static stress drop Δσs for global M >5.5 shallow crustal earthquakes in active tectonic regions implies that these earthquakes radiate, on average, more seismic energy than predicted from the conventional dynamic crack models.

Spatial-Temporal Characterization of the San Andreas Fault by Fault-Zone Trapped Waves at Seismic Experiment Site, Parkfield, California

Article

Full-text available

Aug 2021

Yong-Gang Li

In this article, we review our previous research for spatial and temporal characterizations of the San Andreas Fault (SAF) at Parkfield, using the fault-zone trapped wave (FZTW) since the middle 1980s. Parkfield, California has been taken as a scientific seismic experimental site in the USA since the 1970s, and the SAF is the target fault to investigate earthquake physics and forecasting. More than ten types of field experiments (including seismic, geophysical, geochemical, geodetic and so on) have been carried out at this experimental site since then. In the fall of 2003, a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth (SAFOD) site; the main-hole (MH) passed a ~200-m-wide low-velocity zone (LVZ) with highly fractured rocks of the SAF at a depth of ~3.2 km below the wellhead on the ground level (Hickman et al., 2005; Zoback, 2007; Lockner et al., 2011). Borehole seismographs were installed in the SAFOD MH in 2004, which were located within the LVZ of the fault at ~3-km depth to probe the internal structure and physical properties of the SAF. On September 28 2004, a M6 earthquake occurred ~15 km southeast of the town of Parkfield. The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake. This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF, California, will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models, the fault models and the earthquake forecasting models in global seismogenic regions.

Universal precursor seismicity pattern before locked-segment rupture and evolutionary rule for landmark earthquakes

Article

Full-text available

Oct 2021

Despite extensive investigations, no precursor patterns for reliably predicting major earthquakes have been identified as yet. Seismogenic locked segments that can accumulate adequate strain energy to cause major earthquakes are highly heterogeneous and low brittle. The progressive cracking of the locked segments with these properties can produce an interesting seismic phenomenon: a landmark earthquake and a sequence of smaller subsequent earthquakes (pre-shocks) always arise prior to another landmark earthquake within a well-defined seismic zone and its current seismic period. Applying a mechanical model, magnitude constraint conditions, and case study data of 62 worldwide seismic zones, we show that two adjacent landmark earthquakes reliably occur at the volume-expansion point and peak-stress point (rupture) of a locked segment; thus, the former is an identified precursor for the latter. Such a precursor seismicity pattern before the locked-segment rupture has definite physical meanings, and it is universal regardless of the focal depth. Because the evolution of landmark earthquakes follows a deterministic rule described by the model, they are predictable. The results of this study lay a firm physical foundation for reliably predicting the occurrence of future landmark earthquakes in a seismic zone and can greatly improve our understanding of earthquake generation mechanism.

Evolution of Shear Rupture Along a Prescribed Interface Using the Discontinuous Deformation Analysis Method

Article

Full-text available

May 2024
ROCK MECH ROCK ENG

Laboratory experiments suggest that the evolution of in-plane shear rupture along an interface separating two elastic blocks typically shows a transition from slow to fast slip. In contrast to the commonly used continuum mechanics-based approaches, here we study the shear rupture process along a weak interface using the discontinuous deformation analysis (DDA) method. We incorporate a slip-weakening constitutive friction law to simulate the initiation and propagation of shear rupture under external conditions of a constant normal load and a steadily increased shear load. As the shear load increases, our modeling results reveal a sharp transition from episodic expansion and arrest to unstable runaway rupture, consistent with previous experimental results. In the stage of dynamic runaway, rupture velocity is limited by the Rayleigh wave velocity. We further investigate the effects of external loading conditions including load point velocity and normal stress on rupture behavior. We find that the dynamic rupture velocity increases with load point velocity and normal stress, also consistent with previous studies. Our results indicate that the DDA method can well capture some of the general characteristics of shear rupture process and, hence, can be applied to study other aspects of dynamic shear ruptures.

Holocene Earthquake Cycles of an Active Tectonic Block Boundary Fault Zone: A Case Study in the Qilian–Haiyuan Fault Zone, Northeastern Tibet Plateau

Article

Full-text available

Aug 2023

Fault zones along active tectonic block boundaries are a significant source of devastating continental earthquakes. Strong earthquakes produce disruptions of sediment and induce characteristic sediments near the fault, which serve as valuable sedimentary evidence for identifying and dating of paleoearthquakes. In this study, we aimed to reconstruct the earthquake history of the Qilian–Haiyuan fault zone in the northeastern Tibetan Plateau during the Holocene. We reanalyzed forty-four trenches and used the sedimentary sequences, event indicators, and age constraints to determine the earthquake history. Our analysis revealed the paleoearthquakes of 6 subsidiary faults of the Qilian–Haiyuan fault zone with accurate event ages and rupture extents. Based on the spatial and temporal distributions of strong earthquakes since 10 ka, we identified five earthquake clusters around the central-eastern Qilian–Haiyuan fault zone including seven rupture cascades where the earthquakes migrated gradually from east to west. The existing seismic gap reveals that the latest migration may not yet be complete and suggests a high probability of M ≥ 7 earthquakes occurring on the Jinqianghe fault, Maomaoshan fault, and the central part of the Lenglongling faults. We concluded that, in order to better understand earthquake cycles and seismic hazards, it is important to consider a fault zone as a whole, including multiple faults and their interaction on the earthquake triggering between nearby faults.

Analysis of spatiotemporal variations in b-values before the 6.8-magnitude earthquake in Luding, Sichuan, China, on September 5, 2022

Preprint

Full-text available

Jun 2023

Using the earthquake catalog provided by the Sichuan Earthquake Network Center, spatial and temporal b -value scans were calculated for large- and small-scale regions based on assessing the completeness of the earthquake catalog and aftershock removal. The results show that (1) b -values in the large-scale region ranged from 0.689 to 1.169, with a mean value of 0.928, while the b -values in the small-scale region ranged from 0.694 to 1.223, with a mean value of 0.925. The b -values in the study area were below the mean value before the medium and strong earthquake occurrence, and all exhibited the anomalous feature of a sudden drop-low peak rise. (2) The sliding rate of the northwest section of the Xianshui River Fracture Zone was higher than that of the southeast section; therefore, a large amount of stress was accumulated in the mill-west section of the southeast section, leading to a 6.8-magnitude earthquake in Luding. Before the earthquake, the study area was a low b -value area. The b -value decreased within a short period after the earthquake, dividing the area into concave and convex bodies. This area still has a future risk of moderate to strong earthquakes. (3) The error in the b -values for most of the earthquakes in the large- and small-scale regions is between 0.05 and 0.15, and only individual grid points have larger b -value errors (>0.2), indicating high confidence in the information. In addition, when conducting a b -value study, choosing a suitable study area is important to avoid missing the b-value anomaly area.

Depth-dependent seismic anomalies and potential asperity linked to fluid-driven crustal structure in Garhwal region, NW Himalaya

Article

Jun 2023
TECTONOPHYSICS

Characterizing the stress heterogeneity and crustal complexities are essential to understand the important aspects of rupture development and its propagation in a tectonically active region. We analyze a continuous seismic data (1999-2020) to estimate stress drops, b-value, fractaldimension (Dc) and focal mechanisms using a local network of 14 broadband stations.

Deprem ve Jeoloji

Book

Full-text available

Apr 2023

Ramazan Demirtaş

Integrated mechanical environment of pre- and post-rupture fault and asperity origin of the 2011 giant Tohoku-Oki earthquake

Article

Full-text available

Dec 2022

It is a key to know mechanical environment (ME) of pre- and post-rupture fault of giant earthquakes at subduction zones for predicting earthquake and tsunami disaster. However, we know little about its details till now. In this paper, using the inverted stress change three hours before and three hours after the mainshock in the rupture zone of the 2011 Tohoku-Oki M w 9.0 earthquake, we show a quantitative integrated ME in the rupture zone, including principal stress, pore-fluid pressure and friction strength. We discover from this environment a large asperity composed of two asperities induced by relatively high friction coefficients and relatively lower pore-fluid pressures. The integrate ME quantitatively explained the reasons of the overshoot and relatively lower shear strength of the trench, which caused huge displacement and tsunami at the trench. We suggest that the asperities favor the horst and graben structure system which provides a geology environment for interseismic stress accumulation and thus for breeding the megathrust tsunami earthquake.

The Luangwa Rift Active Fault Database and fault reactivation along the southwestern branch of the East African Rift

Article

Full-text available

Nov 2022
SE

Seismic hazard assessment in slow straining regions is challenging because earthquake catalogues only record events from approximately the last 100 years, whereas earthquake recurrence times on individual faults can exceed 1000 years. Systematic mapping of active faults allows fault sources to be used within probabilistic seismic hazard assessment, which overcomes the problems of short-term earthquake records. We use Shuttle Radar Topography Mission (SRTM) data to analyse surface deformation in the Luangwa Rift in Zambia and develop the Luangwa Rift Active Fault Database (LRAFD). The LRAFD is an open-source geospatial database containing active fault traces and their attributes and is freely available at https://doi.org/10.5281/zenodo.6513691. We identified 18 faults that display evidence for Quaternary activity, and empirical relationships suggest that these faults could cause earthquakes up to Mw 8.1, which would exceed the magnitude of historically recorded events in southern Africa. On the four most prominent faults, the median height of Quaternary fault scarps varies between 12.9 ± 0.4 and 19.2 ± 0.9 m, which suggests they were formed by multiple earthquakes. Deformation is focused on the edges of the Luangwa Rift: the most prominent Quaternary fault scarps occur along the 207 km long Chipola and 142 km long Molaza faults, which are the rift border faults and the longest faults in the region. We associate the scarp on the Molaza Fault with possible surface ruptures from two 20th century earthquakes. Thus, the LRAFD reveals new insights into active faulting in southern Africa and presents a framework for evaluating future seismic hazard.

Earthquake cycles and physical modeling of the process leading up to a large earthquake

Article

Jun 2014
EARTH PLANETS SPACE

Mitiyasu Ohnaka

Seismicity, Fault Architecture, and Slip Mode of the Westernmost Gofar Transform Fault

Article

Full-text available

Nov 2022

Oceanic transform faults accommodate plate motions through both seismic and aseismic slips. However, deformation partition and slip mode interaction at these faults remain elusive mainly limited by rare observations. We use 1‐year ocean bottom seismometer data collected in 2008 to detect and locate earthquakes at the westernmost Gofar transform fault. The ultra‐fast slipping rate of Gofar results in ∼30,000 earthquakes during the observational period, providing an excellent opportunity to investigate interrelations between the slip mode, seismicity, and fault architecture at an unprecedented resolution. Earthquake distribution indicates that the ∼100‐km‐long Gofar transform fault is distinctly segmented into five zones, including one zone contouring a M6 earthquake that was captured by the experiment. Further, a barrier zone east of the M6 earthquake hosted abundant foreshocks preceding the M6 event and halted its active seismicity afterward. The barrier zone has two layers of earthquakes at depth, and they responded to the M6 earthquake differently. Additionally, a zone connecting to the East Pacific Rise had quasi‐periodic earthquake swarms. The seismicity segmentation suggests that the Gofar fault has multiple slip modes occurring in adjacent fault patches. Spatiotemporal characteristics of the earthquakes suggest that complex fault architecture and fluid–rock interaction play primary roles in modulating the slip modes at Gofar, possibly involving multiple concurrent physical processes.

Volcanic tremor associated with the Surtsey eruption of 1963–1967

Article

Full-text available

Oct 2022
JOKULL

The formation of the island of Surtsey over 3.5 years, remains one of the best-documented volcanic, island-forming eruptions to date. The basaltic submarine volcanic activity was detected on November 14, 1963, where ocean depth was 130 m prior to the eruption at the southern end of the Vestmannaeyjar archipelago. The eruptions occurred in several phases involving explosive and effusive activity, including the initial submarine phase on November 12–13, 1963. Separate phases of subaerial volcanic activity occurred during November 14, 1963–January 1964, January–April 1964, April 1964–May 1965, May–October 1965, December 1965– August 1966, and August 1966–June 1967. Seismic data quality from this period is inferior compared to that of modern monitoring systems. Four permanent seismic stations were operated in Iceland at the time, whereof only two, located at 115 and 140 km distance, had the sensitivity to record tremor from Surtsey. Nevertheless, the scanned analog seismograms (http://seismis.hi.is/) show that the eruptive activity was accompanied by considerable seismic activity, both earthquakes, and volcanic tremor. Earthquakes were primarily associated with changes in vent location. Both spasmodic and harmonic tremor was identified, both with low (<3 Hz) and higher (3–5 Hz) characteristic frequencies. The results indicate a complicated relationship between tremor and magma flow rate or style of activity. During the explosive eruption, the highest magma flow rates occurred in the first 10–20 days, a period with little observed tremor. The highest tremor is observed in December 1963– March 1964, after the discharge rates had dropped substantially, and on a timescale of hours-to-days, no clear relationship between tremor and eruption style is observed. The same applies to the effusive activity, where no seismic tremor was observed during most of the effusive eruption of Surtungur, despite the fact that magma flow rates were ∼3 times higher than during later phases where some tremor was observed

Earth’s gradients as the engine of plate tectonics and earthquakes

Article

Full-text available

Sep 2022
RIV NUOVO CIMENTO

The processes occurring on the Earth are controlled by several gradients. The surface of the Planet is featured by complex geological patterns produced by both endogenous and exogenous phenomena. The lack of direct investigations still makes Earth interior poorly understood and prevents complete clarification of the mechanisms ruling geodynamics and tectonics. Nowadays, slab-pull is considered the force with the greatest impact on plate motions, but also ridge-push, trench suction and physico-chemical heterogeneities are thought to play an important role. However, several counterarguments suggest that these mechanisms are insufficient to explain plate tectonics. While large part of the scientific community agreed that either bottom-up or top-down driven mantle convection is the cause of lithospheric displacements, geodetic observations and geodynamic models also support an astronomical contribution to plate motions. Moreover, several evidences indicate that tectonic plates follow a mainstream and how the lithosphere has a roughly westerly drift with respect to the asthenospheric mantle. An even more wide-open debate rises for the occurrence of earthquakes, which should be framed within the different tectonic setting, which affects the spatial and temporal properties of seismicity. In extensional regions, the dominant source of energy is given by gravitational potential, whereas in strike-slip faults and thrusts, earthquakes mainly dissipate elastic potential energy indeed. In the present article, a review is given of the most significant results of the last years in the field of geodynamics and earthquake geology following the common thread of gradients, which ultimately shape our planet.

A New Method of Seismic Source Parameter Estimation of a Locked-Segment Cracking Event

Article

Full-text available

Jun 2022
GEOFLUIDS

Locked segments are widely present in the slip surface of large rock slopes and seismogenic faults of rock underground engineering. Each cracking occasion of the locked segment results in a seismic event. Accurate determination of the seismic source parameters of a locked-segment cracking event is crucial for the reliable evaluation of rock-mass stability associated with slopes and underground openings. The theoretical framework for calculating seismic source parameters in previous studies is mostly based on the stick-slip model, which is not applicable to describing the locked segment’s damage process, and research on seismic source parameter estimation of a locked-segment cracking event is insufficient. Hence, based on the principle of energy conversion and distribution during the locked segment’s damage process, we proposed an equation for the radiated seismic energy of a locked-segment cracking event. Using this equation, we established a mechanical relationship between the earthquake magnitude and the stress drop or shear strain increment (or maximum coseismic displacement) of a locked-segment cracking event. Typical case studies of rock slope and rock underground engineering showed that the proposed calculation method of seismic source parameters was reliable. In addition, this paper discusses the controversy surrounding the relationship between earthquake magnitude and stress drop. Relevant results lay a firm physical foundation to accurately calculate the seismic source parameters of a locked-segment cracking event and obtain detailed insights into the generation mechanism of the locked-segment cracking event.

Estimation of the maximum earthquakes magnitude based on potential brittle volume and strain rate: The Italy test case

Article

Full-text available

May 2022
TECTONOPHYSICS

One major critical issue in seismic hazard analysis deals with the computation of the maximum earthquake magnitude expected for a given region. Its estimation is usually based on the analysis of past seismicity that is incomplete by definition, or derived from the dimension of faults through empirical relationships with the intrinsic uncertainty in source characterization. Here, we propose a workflow aimed at providing a time-independent estimate for the maximum possible magnitude based on geological and geophysical evidence. Our estimate is also source unrelated as it is constrained by the seismic brittle volume of the crust that scales with the effective seismic energy. The seismic brittle volume is calculated considering fault kinematics and rock rheology (i.e., the brittle-ductile transition depth) over a grid that covers the entire study area. The maximum earthquake magnitude is calculated at each point of the grid based on a volume/magnitude empirical relationship. We apply this model to Italy for which we propose a map of the maximum possible magnitudes. Maximum predicted magnitudes are 7.3 ± 0.25 for thrust faulting, 7.6 ± 0.77 for normal faulting and 7.6 ± 0.37 for strike-slip faulting. These magnitudes are locally higher than the historical record. This could be due to an overestimation of the involved volumes; smaller volumes and lower magnitudes may occur where faults are detached at decollements shallower than the brittle ductile transition or where they behave aseismically. Alternatively, strong or major earthquakes could be possible, but they have longer recurrence time and they have never been recorded yet in Italy. Regardless these values are fully reliable or not, the recurrence of earthquakes with the predicted magnitude is related to current strain rates. We conclude that a large part of the Italian territory is prone to trigger M > 5 earthquakes.

Geological and geotechnical investigation of the seismic ground response characteristics in some urban and suburban sites in Chile exposed to large seismic threats

Article

Full-text available

Aug 2022
B EARTHQ ENG

The central area of Chile’s Valparaiso Region has been classified as a seismic gap for a major earthquake, which makes it very important to understand the seismic hazard of the zone. Generally, seismic codes consider a qualitative classification of sites to estimate the possible damage in the case of an earthquake scenario. Estimating the values of acceleration could be very important to prevent damages and increase preparedness for these rare events. In this research, a qualitative and quantitative estimation of seismic hazard is performed in the study area (Valparaiso region between Papudo and San Antonio 32°–34° S). This is achieved through an integrated and relatively economical approach which considers the information from Geology, Geophysical experiments (Gravity and seismic methods), and Geotechnical analyses. The results of the geophysical survey and geology information allow dividing the zone into five site types through a new proposal of site classification that depends not only on the Vs30, but also on the sites predominant period (T0), which is an innovation of this work for the Chilean code. The Peak Ground Acceleration (PGA) values in the study zone were estimated using a Ground Motion Predictive Equation developed for the Chilean subduction zone. Additionally, we consider three different seismic scenarios according to the history of events in Central Chile. The results of this quantitative analysis show PGA values up to 0.52 g for the median and 1.2 g for the 84th percentile of the scenarios. Overall, the highest accelerations (PGA) are in zones with low shear wave velocities (< 500 m/s), a long predominant period (> 0.4 s) and where geology establishes the presence of low stiffness soils. The comparison of response spectra from the model against records from 2010 Maule and 1985 Valparaiso earthquakes shows available models tend to overpredict the intensities.

Recurrence and Clustering of Large Earthquakes along the Northern Boundary of Ordos Block: Constraining Paleoearthquakes by an Improved Multiple Trench Constraining Method

Article

Full-text available

Apr 2022

Tectonic belts along active tectonic block boundaries comprise one or more active faults; along which, large earthquakes recur. Therefore, it is important to establish the recurrence behavior of large earthquakes along such boundary zones for studying their characteristics and developments. Many paleoearthquake studies make it possible to investigate the recurrence behavior of large earthquakes along the northern boundary of the Ordos block (NBOB). Based on the previous studies, data from 52 trenches were collected to reconstruct prehistoric earthquakes using an improved multiple trench constraining method. This method is based on paleoearthquake indicators and trench location distribution to constrain the rupture time and length, thereby reducing the selection bias of fixed rupture length to construct additional rupture scenarios. The results suggest that the NBOB comprises four normal faults (from west to east): the Langshan Piedmont Fault (LPF), Sertengshan Piedmont Fault (SPF), Wulashan Piedmont Fault (WPF), and Daqingshan Piedmont Fault (DPF); along which, six, seven, eight, and six paleoearthquakes have occurred within approximately 15,000 yr, respectively. In addition, recurrence behaviors of the individual faults exhibit remarkable periodicity. The regional fault network along the NBOB reveals clustered characteristics with six clusters propagating either westward or eastward and a recurrence time of approximately 1,300 yr. Large earthquake events have occurred along the LPF, WPF, and DPF according to the most recent cluster; however, earthquakes were absent along the SPF, and no evidence of large earthquakes was observed along the NBOB after the 849 CE earthquake. Thus, we discuss the possibility of occurrence of large earthquakes along the SPF after the 849 CE earthquake based on earthquake recurrence and cluster migration behavior. Additional research is required to assess the potential risk of the occurrence of a large earthquake along the SPF in the future.

Numerical precursory study on strong earthquakes in southern and Baja California

Article

Apr 2022

We examined the spatio-temporal variation of correlation fractal dimension (DC) for M≥2.5 earthquakes in southern and Baja California to ascertain the incidence of seismic precursors before strong earthquakes. The unforeseen 2019 Ridgecrest events have restructured the crustal stress patterns in the adjacent regions signifying the prerequisite for a caveat. The current research attempts to discern the existence of numerical precursors of strong earthquakes (Mw > 7) based on the correlation integral method. A time window of three decades (1990-2020) was analyzed to study the relative changes in the distribution of earthquake clusters and the subsequent variations reflected in DC values. We have successfully demonstrated the numerical precursors for four stronger main shocks (1992 Landers Mw 7.3, 1999 Hectormine Mw 7.1, 2010 El-Mayor Cucapah Mw 7.2, and 2019 Ridgecrest Mw 7.1) through the relative changes in DC. In addition, our study has confirmed the remarkable correlation between low values of DC and spatio-temporal clustering of earthquakes for the study region. Though the present study confers the application of DC on the recognition of seismicity patterns before major events, the constant monitoring of the same can give insights into earthquake forecasting and seismic risk analysis of unstable tectonic terrains.

Temporal and spatial changes in the b-value prior to the 2021 Luxian MS 6.0 earthquake in Sichuan, China

Article

Full-text available

Apr 2022

Using the seismic data collected since 2010 by the China Earthquake Networks Center and based on the maximum likelihood method, we analyzed the temporal and spatial anomalies of the b-value prior to the MS 6.0 Luxian earthquake on September 16, 2021. The results are as follows. (1) The Luxian earthquake broke the historical record of no M ≥ 6 earthquake occurring in the Huayingshan fault zone, revealing that the Huayingshan fault zone still has the seismogenic ability of medium and strong earthquakes. (2) Before the MS 6.0 Luxian earthquake, the b-values around the focal areas exhibited increasing–peaking–decreasing anomalous characteristics, indicating that this is an effective method of predicting the occurrence of moderate to strong earthquakes in the region. (3) In the past 10 years, the b-values of the middle segment of the Huayingshan fault zone have been relatively low, but the b-value of this section may be in an abnormal stage of slow increase, indicating that this segment may be in a stage of stress accumulation and concentration and is preparing for moderate to strong earthquakes. The middle segment of the Huayingshan fault zone may become the site of moderate to strong earthquakes in the future. We should continue to pay attention to it.

Fluids Triggered the 2021 Mw 6.1 Yangbi Earthquake at an Unmapped Fault: Implications for the Tectonics at the Northern End of the Red River Fault

Article

Full-text available

Jan 2022

On 21 May 2021 a magnitude Mw 6.1 earthquake occurred in Yangbi region, Yunan, China, which was widely felt and caused heavy casualties. Imaging of the source region was conducted using our improved double-difference tomography method on the huge data set recorded by 107 temporary stations of ChinArray-I and 62 permanent stations. Pronounced structural heterogeneities across the rupture source region are discovered and locations of the hypocenters of the Yangbi earthquake sequence are significantly improved as the output of the inversion. The relocated Yangbi earthquake sequence is distributed at an unmapped fault that is almost parallel and adjacent (∼15 km distance) to the Tongdian–Weishan fault (TWF) at the northern end of the Red River fault zone. Our high-resolution 3D velocity models show significant high-velocity and low-VP/VS ratios in the upper crust of the rupture zone, suggesting the existence of an asperity for the event. More importantly, low-VS and high-VP/VS anomalies below 10 km depth are imaged underlying the source region, indicating the existence of fluids and potential melts at those depths. Upward migration of the fluids and potential melts into the rupture zone could have weakened the locked asperity and triggered the occurrence of the Yangbi earthquake. The triggering effect by upflow fluids could explain why the Yangbi earthquake did not occur at the adjacent TWF where a high-stress accumulation was expected. We speculate that the fluids and potential melts in the mid-to-lower crust might have originated either from crustal channel flow from the southeast Tibet or from local upwelling related to subduction of the Indian slab to the west.

Segmentation and Holocene Behavior of the Middle Strand of the North Anatolian Fault (NW Turkey)

Article

Full-text available

Nov 2021
TECTONICS

The North Anatolian Fault (NAF) in the Marmara region is composed of three parallel strands all separated by ∼50 km. The activity of the middle strand, which borders the southern edge of the Marmara Sea, is much debated because of its present‐day very low seismicity. This contrasts with historical, archeological and paleoseismological evidence, which suggest several destructive earthquakes have occurred during the last 2000 years. Our study aims to better constrain seismic hazard on the middle strand by exploring its Holocene paleoseismicity. For this, we mapped 148 km of the eastern part of the middle strand, using high‐resolution satellite imagery. A series of landforms offset by the middle strand activity have been systematically measured to recover the past ruptures. Three Late Pleistocene‐Holocene terraces have been dated with the terrestrial cosmogenic nuclide method, constraining a horizontal slip rate of 4.4−2.8+10.6 mm/yr. The statistical analysis of the offsets evidences several major ruptures preserved in the landscape, with coseismic lateral displacements ranging between 3 and 6.5 m. This corresponds to Mw ∼7.3 earthquakes able to propagate along several fault segments. Historical seismicity and paleoseismology data suggest that the last large earthquakes along the middle strand of the NAF (MNAF) happened in 1065 CE and between the 14th and 18th centuries CE. Since then, the MNAF may have accumulated enough stress to generate a destructive rupture.

The Effect of Depth‐Dependent Stress in Controlling Free‐Surface‐Induced Supershear Rupture on Strike‐Slip Faults

Article

Full-text available

May 2021

Supershear rupture is crucial in determining ground motion characteristics and its various transition mechanisms is in key connection with rupture dynamics, geometrical complexities and stress heterogeneities. Using 3D dynamic rupture models with numerous depth‐dependent stress regimes, we show that depth‐dependent stress plays a major role in controlling the occurrence of free‐surface‐induced (FSI) supershear ruptures. The effect of stress gradient, initial normal stress at the free surface, fault width and critical depth of depth‐dependent stress regime are examined. The 2D tangent rupture velocity distributions show that generation of a sustained FSI supershear rupture prefers a larger stress gradient and initial normal stress at the free surface. The truncation of fault width could transit the rupture to a sub‐Rayleigh rupture. We propose a dimensionless stress parameter representing a measure of the overall level of depth‐dependence in the initial normal stress, and a larger moment magnitude is more likely to trigger a sustained FSI supershear rupture. By comparing the near‐field ground motions, three distinct ground motion characteristics are presented using the sustainability of Mach fronts and displacement polarization. This work helps to understand the occurrence of FSI supershear ruptures and its relationship with underground depth‐dependent stresses.

Development of Laser-Based System to Determine Fault Surface Morphology at the Micron Scale

Article

May 2021
MEASUREMENT

Fault surface morphology plays an important role in the earthquake rupture process. However, determining the frictional surface morphology at the micron scale is challenging owing to the lack of reliable measurement instruments. To overcome the existing instruments' limitations, we developed a novel system. The system effectively measures the steep slope generated in the adjacent fault striations and fault steps. The single measurement area is large, with a value of 30 cm × 30 cm. Furthermore, the developed instrument has a horizontal (X and Y orientations) and vertical (Z orientation) positioning uncertainty of 3.5 μm and 4.5 μm (Z orientation), and a horizontal and vertical resolution of 0.5 μm and 0.076 μm, respectively, which are better than those of existing instruments. The proposed device can be used to practically evaluate the wear surface's shape to obtain high-accurate point cloud data, satisfying the engineering requirements for morphological analyses.

Segmentation of the Wassuk Range normal fault system, Nevada (USA): Implications for earthquake rupture and Walker Lane dynamics

Article

Mar 2021

Normal faults are commonly segmented along strike, with segments that localize strain and influence propagation of slip during earthquakes. Although the geometry of segments can be constrained by fault mapping, it is challenging to determine seismically relevant segments along a fault zone. Because slip histories, geometries, and strengths of linkages between normal fault segments fundamentally control the propagation of rupture during earthquakes, and differences in segment slip rates result in differential uplift of adjacent footwalls, we used along-strike changes in footwall morphology to detect fault segments and the relative strength of the mechanical links between them. We applied a new geomorphic analysis protocol to the Wassuk Range fault, Nevada, within the actively deforming Walker Lane. The protocol examines characteristics of footwall morphology, including range-crest continuity, bedrock-channel long profiles, catchment area variability, and footwall relief, to detect changes in strike-parallel footwall characteristics. Results revealed six domains with significant differences in morphology that we used to identify seismically relevant fault segments and segment boundaries. We integrated our results with previous studies to determine relative strength of links between the six segments, informing seismic hazard assessment. When combined with recent geodetic studies, our results have implications for the future evolution of the Walker Lane, suggesting changes in the accommodation of strain across the region. Our analysis demonstrates the power of this method to efficiently detect along-strike changes in footwall morphology related to fault behavior, permitting future researchers to perform reconnaissance assessment of normal fault segmentation worldwide.

Structure-controlled asperities of the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes, NE Tibet, China, revealed by high-resolution seismic tomography

Article

Full-text available

Mar 2021

Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

Séismes lents et variations du couplage intersismique : le cas de la subduction Mexicaine vue par géodésie spatiale.

Thesis

Nov 2020

Louise Maubant

La multiplication depuis 20 ans d'observations de déformations transitoires au cours du cycle sismique a mis en avant la question de l'impact de ces déformations dans l'estimation de l'aléa sismique. Ainsi, dans les zones de subduction, le rôle des séismes lents sur la variabilité spatiale et temporelle du couplage intersismique et sur l'existence de lacunes sismiques est une question de première importance. Nous avons abordé cette problématique par l'analyse d'observations géodésiques et par la modélisation numérique, en se focalisant sur la subduction mexicaine. Cette zone est une cible pertinente pour étudier ces questions en raison de la présence de séismes lents qui font partie des plus grands observés au monde. La géométrie de cette subduction est également favorable aux observations géodésiques. Un premier volet de cette thèse a été consacré à l’étude du séisme lent de 2017-2018 qui a duré plusieurs mois dans la région de Guerrero. Pour cela, de nouvelles observations ont été faites par interférométrie radar satellitaires (InSAR) utilisant les données Sentinel-1, combinées aux données du réseau GPS permanent. Les données InSAR ont permis d’améliorer significativement la couverture et la résolution spatiale des mesures de déformation du sol par rapport aux études précédentes. La fréquence d’acquisition de ces données est de 6 à 12 jours. Un travail méthodologique sur l’extraction du signal tectonique inclus dans les séries temporelles InSAR a été nécessaire en raison de la grande superficie de la zone d’étude où sont présents de fort gradients topographiques. Deux méthodes de séparation de sources ont été employées. La première approche est une décomposition paramétrique, dans laquelle la forme fonctionnelle des signaux de déformation est imposée, et les signaux atmosphériques sont décrits en utilisant comme contrainte des séries temporelles de délais troposphériques zénithaux issus du GPS. La seconde approche utilise l’analyse en composantes indépendantes (ICA) des séries temporelles InSAR, ne nécessitant pas d’à priori sur le signal recherché. Les deux méthodes fournissent des résultats cohérents et permettent de séparer le signal atmosphérique, sans corrections préalables, du signal tectonique. A partir des cartes de séries temporelles de déplacements validées par les mesures GPS, le glissement du séisme lent sur l’interface de subduction est inversé. La distribution spatiale du glissement est cohérente avec celle des évènements précédents et confirme une localisation à la limite de la zone sismique. L’influence de séismes distants sur la cinématique de ce type d’évènement est également confirmée par ces observations.Dans une seconde partie, les déformations intersismiques sur une zone couvrant environ 1000 km de la subduction mexicaine de Jalisco à Oaxaca sont également analysées à partir de mesures InSAR et GPS. Les variations latérales de couplage le long de la subduction entre 2016 et 2019 sont établies pour la première fois de façon homogène sur l’ensemble de cette zone. Entre Michaocan et Jalisco, où de grands séismes ont eu lieu, on retrouve une zone à fort couplage. L’analyse montre bien l’importance que peuvent avoir à l’échelle de temps de quelques années les signaux transitoires comme les séismes lents sur la variabilité du couplage mesurée par géodésie spatiale. La dernière partie de cette thèse, aborde ce problème par une modélisation numérique du cycle sismique sur un plan de faille en 3D, basée sur des lois de friction de type « rate and state ». Cette modélisation permet de reproduire certaines caractéristiques de subduction mexicaine et de replacer les 20 ans d’observations géodésiques à l’échelle de plusieurs cycles sismiques. Les résultats préliminaires donnent des pistes de réflexions intéressantes sur la question de la possibilité qu’un séisme puisse se produire dans la lacune sismique de Guerrero et sur le rôle des séismes lents sur le faible couplage observé dans cette région.

Contribution to post-pronostic decision : a new framework based on prongnostic/decision interaction

Thesis

Full-text available

Oct 2020

Omar Bougacha

With the emergence of prognostics and health management (PHM) methodology, companies are trying to fully exploit the data sources they have to build models for their assets and predict their future behavior. Therefore, the decision-making process is no longer based on only historical and actual data, but it integrates future information. On one hand, decisions in the PHM context are based on future information. On the other hand, the estimation of future conditions of the systems requires a knowledge on its future loads and usage (i.e. future decisions).Several works have studied the decision-making process in the PHM context. However, none of these works have addressed the prognostic-decision interdependency. Moreover, most of the post-prognostic decision-making works present several omissions: 1) How to emphasize the relationship between the prognostic and decision-making modules? 2) How often should the PHM process be launched? 3) How long ahead should the decision-making module plan? 4) How to better/fully use prognostic information in the decision process? 5) How to clarify the applicability of the PHM framework? 6) How to make the PHM framework robust to the system's conditions changes? These questions are addressed along this thesis. They resulted in modifying the existing PHM framework to enhance the decision-making process. The main contributions of this thesis are:• A summary of the PHM challenges followed by a thorough survey on post-prognostics decision-making and its growing interest as a strategy to exploit prognostic information to maintain and manage life cycle of critical machinery.• Some adaptation to the existing PHM framework are proposed to enhance the decision-making process. Three loops are proposed to improve the prognostic accuracy by eliminating uncertainty sources that are caused by unknown future loads and conditions, improve the performance of the decision-making process by dynamically incorporating prognostic information, and improve the reactivity of the overall PHM framework to new operating conditions.• The instantiation and implementation of the proposed post-prognostic decision framework on two different case studies. A first application studies the joint problem of maintenance and production scheduling on a single multi-purpose machine. A second application addresses the joint problem of maintenance planning and mission assignment for a fleet of rolling stock units.• The utility of applying the proposed framework, the duration of the decision horizon, and the frequency of execution are discussed.

BİGA YARIMADASINDA DEPREM KÜMELENMELERİ

Thesis

Aug 2020

Biga yarımadasında tarih boyunca yıkıcı depremlere sebep olmuş olan KD-GB yönlü uzanan diri faylar bulunmaktadır. Büyük depremlerin bazen öncesinde (öncü şoklar) ve sonrasında ise neredeyse mutlaka zamanla azalan küçük ve orta büyüklükte artçı şoklar görülür. Bu şoklar, bölgelerin sismisitesinde geçici kümelenmeler teşkil ederler. Biga yarımadasında tektonik hareketler genel olarak KD-GB doğrultusunda uzanan doğrultu atımlı faylarda gerçekleşmekte olup bunlardaki toplam yıllık sağ yanal kayma hızları GPS ölçümleri ile 7 mm olarak belirlenmiştir. Bu hareket çalışmamızda Kapıdağı yarımadası-Manyas hattından güneybatıya Edremit körfezine doğru ortalama ~30 km genişliğe ve ~150 km uzunluğa sahip Biga yarımadası fay zonu olarak anılmış bir zon ile alınmaktadır. Hareket bu zonu temsil eden paralel faylara paylaşacak şekilde dağılmaktadır. Biga yarımadası sismisitesinde genel olarak birisi Biga ve diğeri Bababurnu civarında olmak üzere iki alan göze çarpar. Araştırmamızda bu iki alandaki, arka plan depremselliği, ana depremler ve bunlarla ilgili şoklardan kaynaklanan enerji çıkışları ele alınarak alanlardaki kümelenmelerin birbirleri ile benzerlik ve farklılıkları anlaşılmaya çalışılmıştır. Biga civarındaki sismisite Bababurnu alanındakine nazaran hem deprem sayısı hem de hasıl olan enerji bakımından çok daha sakin olup 1983 yılında orta büyüklükte (Ms=5.8) bir depremle başlayıp artçı depremlerle sönümlenen bir etkinlikle temsil edilir. Bababurnu alanında ise üç ayrı etkinlik mevcut olup bunlardan en batıdaki 2013 yılında orta büyüklükte (Ms=5.1) bir depremle başlayıp azalan artçı şoklarla sönümlenen bir deprem aktivitesi olan 2013 etkinliğidir. Bu bakımdan 1983 Biga etkinliği ile benzerlik gösterir. En doğudaki Bababurnu alanını temsil eden çerçeve içerisinde kalan Ayvacık civarındaki 2018 etkinliği ise bu tezin kapsamı dışında tutulmuştur. Bababurnu alanındaki diğer etkinlik Tuzla bölgesindeki 2017 yılında vii başlamıştır. Etkinliğin artçıları ile ilgili enerji değişimi deprem fırtınası özelliğindeki arka plan depremselliğinin baskın enerjisi içerisinde ayırt edilememekte maskelenmektedir. Ayrıca, 2017 Tuzla etkinliği bir hafta içerisinde gerçekleşmiş büyüklükleri 5.3 ile 5.6 (Ms) arasında değişen dört adet depremle başlar. Yani etkinlik anormal bir şekilde artçılarından büyüklük bakımından ayrık duran bir ana şoka sahip değildir. Tuzla bölgesinin hem bu bakımdan hem de arka planda bir deprem fırtınasına sahip olması bakımından özel bir durumu olduğu düşünülmüştür. Bunun sebebi kabuğun, birçok veriyle birlikte yüksek sıcaklıktaki (173 °C) Tuzla jeotermal sahasının da işaret ettiği üzere, bölgede yüksek sıcaklığa maruz kalması gibi olaylardan kaynaklanan, anormal elastik/rheolojik özellikleri ve geçmişteki tektonik dönemlerin bıraktığı fay yapılarının kabukta yoğun zayıflık zonu ağlarını teşkil ediyor olması olabilir. Deprem fırtınalı bir yerde oluşan bir orta büyüklükte depremin büyük bir depremin öncüsü olabileceği değerlendirmesinin son derece sakıncalı olabileceği bu çalışmadaki Tuzla etkinliğinin deprem fırtınasından dolayı ayrt edilememesinden anlaşılmaktadır. https://tez.yok.gov.tr/UlusalTezMerkezi/TezGoster?key=fl0Kw4p1rmMDotyKRdYv1KNHPvW3yAtM45SO0o3boEIQWwmNUZ78o94YsbxVv7qP ABSTRACT EARTHQUAKE CLUSTERS IN BİGA PENINSULA Rıza BAYSAL Çanakkale Onsekiz Mart University School of Graduate Studies Master of Science Thesis in Geophysical Engineering Advisor: Doç. Dr. Tolga KOMUT 28/08/2020, 41 On the Biga peninsula, there are NE-SW trending active faults that have caused devastating earthquakes throughout the history. After major earthquakes, aftershocks almost always occur, and they decrease over time. On the other hand, sometimes fore-shocks are seen before major earthquakes. In the seismicity of the regions, these shocks constitute temporary clusters. In the Biga peninsula, tectonic movements are generally in NE-SW direction on strike-slip faults. Their total right lateral shear rate is approximately 7 mm/yr according to GPS measurements. In our study, this annual slip is taken by a zone known as the Biga peninsula fault zone, ~30 km (average) wide and ~150 km long, from the Kapıdağı peninsula-Manyas line to the southwest towards Edremit gulf. The movement is distributed to the parallel faults representing this zone. In the seismicity of the Biga peninsula, in general two areas are remarkable, one of them near Biga and the other around Bababurnu. In our study, we tried to understand similarities and differences of the clusters by dealing with background seismicity, main earthquakes and energy release related to these earthquake activities. Seismicity around Biga is much quieter than in the Bababurnu area, in terms of both the number of earthquakes and the energy generated, and is represented by an activity that started with a medium-sized (Ms=5.8) earthquake in 1983 and was followed by aftershocks. There are three different activities in the Bababurnu area, the western one in 2013, which is a 2013 activity that started with a medium-sized (Ms=5.1) earthquake and was followed by decreasing aftershocks. In this respect, this is like the 1983 Biga event that have aftershocks. In the easternmost of the area, the activity around Ayvacık village that ix took place in 2018, was out of the scope of the thesis. The other activity in the area is Tuzla activity that starts in 2017. The energy release change related to the aftershocks of the event is masked by the dominant energy of background seismicity that has earthquake storm character. In addition, 2017 Tuzla event starts with four earthquakes having magnitudes ranging between 5.3-5.6 (Ms) that occurred within a week. In other words, none of the shocks in terms of their magnitude is major. It was suggested that Tuzla region has a special situation in terms of both lack of major earthquake and having an earthquake storm. The reason for this may be the abnormal elastic/rheological situation of the crust, caused by abnormal cases such as its high temperature that appeared for example as very hot (173 ° C) geothermal spring in the Tuzla region. The other cause may be the fault structures left by paleo-tectonic regimes constitute many nets of weakness zones in the crust. It appears that from the fact that Tuzla activity in this study was not detectable due to the earthquake storm, an interpretation that a medium-sized earthquake occurring in an earthquake storm may be extremely objectionable. https://tez.yok.gov.tr/UlusalTezMerkezi/TezGoster?key=fl0Kw4p1rmMDotyKRdYv1KNHPvW3yAtM45SO0o3boEIQWwmNUZ78o94YsbxVv7qP

Unraveling the roles of fault asperities over earthquake cycles

Article

Jun 2024
EARTH PLANET SC LETT

On the role of trans-lithospheric faults in the long-term seismotectonic segmentation of active margins: a case study in the Andes

Preprint

Full-text available

May 2024

Plate coupling play a fundamental role in the way in which seismic energy is released during the seismic cycle. This process includes quasi-instantaneous release during megathrust earthquakes and long-term creep. Both mechanisms can coexist in a given subducting margin, defining a seismotectonic segmentation in which seismically active segments are separated by zones in which ruptures stop, classified for simplicity as asperities and barrier, respectively. The spatiotemporal stability of this segmentation has been a matter of debate in the seismological community for decades. At this regard, we explore in this paper the potential role of the interaction between geological heterogeneities in the overriding plate and fluids released from the subducting slab towards the subduction channel. As a case study, we take the convergence between the Nazca and South American plates between 18°–40° S, given its relatively simple convergence style and the availability of a high-quality instrumental and historical record. We postulate that trans-lithospheric faults striking at a high angle with respect to the trench behave as large fluid sinks that create the appropriate conditions for the development of barriers and promote the growth of highly coupled asperity domains in their periphery. We tested this hypothesis against key short- and long-term observations in the study area, obtaining consistent results. If the spatial distribution of asperities is controlled by the geology of the overriding plate, seismic risk assessment could be established with better confidence.

Analysis of spatiotemporal variations in b-values before the 6.8-magnitude earthquake in Luding, Sichuan, China, on September 5, 2022

Article

May 2024
ACTA GEOPHYS

Using the earthquake catalog provided by the Sichuan Earthquake Network Center, spatial and temporal b-value variations were calculated for in regional and local scales based on assessing the completeness of the earthquake catalog and declustering. The results show that (1) b-value temporal variations in regional scale ranged from 0.689 to 1.169, with a mean value of 0.928; while, the local-scale temporal variations ranged from 0.694 to 1.223, with a mean value of 0.925. The b-values in the study area were below the mean value before the moderate and large earthquakes occurrence, and all b-values exhibited the anomalous feature of a sudden decrease before the earthquake low peak rise after the earthquake. (2) The seismotectonic characteristic of the area is the higher value of slip rate of the NW section of Xianshui River Fault Zone; therefore, a large amount of stress was accumulated in the Moxi section of the SE section, leading to a M = 6.8 earthquake in Luding. Before the earthquake, the study area has a low b-value area. The b-value decreased within a short period after the earthquake, dividing the area into asperity. This area still has a future risk of moderate to strong earthquakes. (3) The error in the b-values for most of the earthquakes in the regional and local scales regions is between 0.05 and 0.15, and only individual grid points have larger b-value errors (> 0.2), indicating high confidence in the information. In addition, when conducting a b-value study, choosing a suitable study area is important to avoid missing the b-value anomaly area.

Evaluation of seismicity and seismotectonics in the Alborz Mountains: insights from seismic parameters, Northern Iran

Article

Full-text available

May 2024
J SEISMOL

The Alborz Mountains are among the areas exhibiting high tectonic and seismic activity in northern Iran. Studying key parameters of tectonic structures, including quantitative analysis and observational studies, in such active regions is essential to identify potential active faults and assess the consequent seismic hazards. This study focuses on seismicity and seismotectonics by analyzing seismic parameters, including b-value, mean seismic activity rate, earthquake recurrence time, seismic moment, and fractal dimension derived from micro and teleseismic data. The b-values vary between 0.6 and 1.1 in the tectonically active parts of the study area, corresponding with the reverse/thrust and strike-slip active faults. Large earthquakes might be prone to occur at 10–25 km depth because both catalogues show low b-values (b < 1.0) concentrations at this depth range. The high fractal dimension (> 1.5), high seismic activity rate, large seismic moment parameters, and its continuously increasing trend. Short recurrence periods (20–50 years) of M 6.5 events also emphasize the high seismic activity and high seismic hazard. On the other hand, the prevalence of low b-values is notably observed in areas encompassing densely populated cities such as Rasht, Lahijan, Amol, Babol, Sari, Behshahr, Gorgan, and the megacity of Tehran. Furthermore, we have identified asperities where the Gorgan Plain, the Khazar, and the Alamutrud Fault Zones are located. These findings emphasize the seismic hazard potential in the identified areas and urban centers within the study area. Therefore, particular attention should be directed towards areas exhibiting low b-values when assessing and mitigating seismic hazards. It underscores the necessity for additional focus on seismic hazard assessment and implementation of mitigation strategies in the Alborz region.

Structural Heterogeneity Controlled Rupture Process of the 2021 Mw 7.1 Fukushima, Japan, Earthquake Revealed by Joint Inversion of Seismic and Geodetic Data

Article

Nov 2023

We determined the rupture model of the 2021 Mw 7.1 Fukushima earthquake near northeastern Japan in this study and adopted this model to investigate the cause of this earthquake and its aftershocks. The rupture model was obtained through joint inversion of teleseismic, strong-motion and geodetic data. It is shown that the slips were predominantly distributed on the southwest side of the earthquake epicenter, indicating a unilateral rupture event. We observed that the seismic moment was released in three time periods, producing four slip patches on the fault plane. Through comparison, we demonstrated that our joint inversion model was more reliable in describing the rupture process of the Fukushima earthquake than the automatic inversion models determined using only strong-motion data. By jointly analyzing the slip distribution and seismic velocity structure, we found a good correlation between the slip patches and VP/VS anomalies, suggesting that structural heterogeneities along the fault zone played a critical role in controlling the rupture process of the Fukushima earthquake. In addition, most aftershocks were located in the region characterized by small slips and high VP/VS, and we demonstrated that they were caused by stress changes due to the presence of fluids and the rupture of the mainshock.

Seismic response analysis of pile-supported wharf under three types of near-fault ground motion

Article

Nov 2023

The 28 October 2022 Mw 3.8 Goesan Earthquake Sequence in Central Korea: Stress Drop, Aftershock Triggering, and Fault Interaction

Article

Aug 2023

We identified the causative fault of the 2022 Goesan, Korea, earthquake sequence based on the precise relocation of the sequence that revealed a 0.8 km-long fault plane striking east-southeast–west-northwest. The fault plane encompasses the largest foreshock, the mainshock, and the majority of the aftershocks. The orientation of the fault plane is consistent with the left-lateral strike-slip motion along the east-southeast (106°) striking nodal plane of the focal mechanism. The Jogok fault system recently mapped in the source area runs through the mainshock epicenter with a consistent strike and left-lateral strike-slip motion, which suggests that it is the likely causative fault of the 2022 Mw 3.8 Goesan earthquake sequence. It is a rare case of assigning a causative fault for a small-sized (Mw 3.8) earthquake with some confidence in a typical stable continental region setting, albeit no surface break observed due to deep focal depth (~13 km) and the small size of the event. Aftershocks on the main fault plane, and on the adjacent subparallel fault patches seemed to be triggered by the increase in Coulomb stress caused by the mainshock. Two large aftershocks on the subparallel fault patches show slightly higher stress drops than the large foreshock and mainshock on the main fault plane, likely due to high frictional strength on those fault patches. Events of the 2022 Goesan earthquake sequence progressed rapidly in time and appear to be high stress-drop events compared with other earthquakes that occurred in other regions in Korea, probably due to the long quiescent period in the Goesan earthquake epicentral region.

Seismic Performance of Vertical Irregular Steel Frame Structure under Mainshock-Aftershock

Article

May 2023

The majority of the structures are involved with architectural importance and it is highly impossible to achieve uniform structural properties in all directions. Hence earthquake resistant design codes considered it as irregular frames based on relative difference in the story properties. In many cases these irregularities are responsible for structural collapse of buildings under earthquake ground motions. The seismic response of buildings with irregular distribution of stiffness along the height may be differentfrom that ofregular building. Also, past earthquakes showed that structure may be subjected to sequence of ground motions but current codes do not have guidelines for such cases. It is considered that aftershock do not cause any more damage to damaged structure by the preceding mainshock ground motion. In this study steel moment resisting frame buildings are evaluated to understand the seismic response of vertical stiffness irregular frames subjected to mainshock-aftershock ground motions. The 9-story steel moment resisting frame building situated in Los Angeles is used in this study was originally developed as part of the SAC steel project. In this study soft and stiff storey case was considered at three different locations along the height, i.e., at bottom storey, mid-height storey and top storey. The single modification factor is used for irregularity. For comparison purpose dynamic properties of regular and irregular frames are kept same. Two sets of 6 records were selected representing a seismic hazard level of 2% and 10% probability of exceedance in 50 years respectively as mainshock. Two sets of 30 mainshockaftershock ground motion are considered for the study. These ground motions were developed using randomized approach. A non-linear time history analysis of regular and irregular building is carried out separately under mainshock and mainshockaftershock. The effect of building irregularity was studied for single storey modification at bottom storey, mid-storey and top storey with comparison to regular building. The comparison of a regular and irregular building is carried out in terms of maximum roof displacement and interstorey drift ratio. Also, comparison of frames under mainshock and mainshock-aftershock is done.

Influence of regional background stress fields on the spontaneous rupture of the major faults around Xiluodu dam, China

Article

Oct 2022

Simulations of the spontaneous rupture of potential earthquakes in the vicinity of reservoir dams can provide accurate parameters for seismic resilience assessment, which is essential for improving the seismic performance of reservoir dams. In simulations of potential spontaneous ruptures, fault geometry, regional stress fields, constitutive parameters of the fault friction law, and many other factors control the slip rate, morphology, and dislocation of the rupture, thereby affecting the simulated ground motion parameters. The focus of this study was to elucidate the effects of the background stress field on the nucleation and propagation of spontaneous ruptures based on the factors influencing potential M > 7 earthquake events on the Leibo Middle Fault (LBMF) and the Mabian-Yanjing Fault (MB-YJF) in the Xiluodu dam (XLD) region. Our simulation results show that the magnitude of the regional background stress field plays a decisive role in whether a destructive earthquake exceeding the critical magnitude will occur. We found that the direction and magnitude of the regional stress significantly affect the range of rupture propagation on the fault plane, and fault geometry affects the spatial distribution of the rupture range. Under the same regional stress field magnitude and orientation, a more destructive, high-magnitude earthquake is more likely to occur on the LBMF than on the MB-YJF.

Investigation on variations of apparent stress in the region in and around the rupture volume preceding the occurrence of the 2021 Alaska MW8.2 earthquake

Article

Jun 2022

On July 29, 2021, a large earthquake of MW8.2 occurred south of the Alaska Peninsula. To investigate the spatial-temporal changes of crustal stress in the earthquake-stricken area before this event, we selected 159 earthquakes of 4.7 ≤ MW ≤ 6.9 that occurred in the epicentral region and its surroundings between January 1980 and June 2021 to study the temporal variation and spatial distribution of their apparent stress. In addition, we analyzed the correlation between seismic activities and Earth’s rotation and explored the seismogenic process of this earthquake. The crustal stress rose from January 2008 to December 2016. This period was followed by a sub-instability stage from January 2017 until the occurrence of the MW8.2 earthquake. The average rate of apparent stress change in the first five years of the stress increase period was roughly 2.3 times that in the last four years. The lateral distribution of the apparent stress shows that the areas with apparent stress greater than 1.0 MPa exhibited an expanding trend during the seismogenic process. The maximum apparent stress was located at the earthquake epicenter during the last four years. The distribution of the apparent stress in the E-W vertical cross section revealed that an apparent stress gap formed around the hypocenter during the first five years of the stress increase period, surrounded by areas of relatively high apparent stress. After the Alaska earthquake, most parts of this gap were filled in by aftershocks. The seismic activities during the sub-instability stage exhibited a significant correlation with Earth’s rotation.

Characterization of the San Andreas Fault by Fault-Zone Trapped Waves at Seismic Experiment Site, Parkfield, California: A Review

Chapter

Apr 2022

Yong-Gang Li

The San Andreas Fault (SAF) at Parkfield, California has been taken as a seismic experimental site since the 1970s. The San Andreas Fault Observatory at Depth (SAFOD) measured a ~200-m-wide damage zone with a seismic velocity reduction of ~25–30% within the mature SAF at ~3 km depth. Observations and 3-D finite-difference simulations of fault-zone trapped waves (FZTWs) recorded in a sequence of seismic experiments conducted at the Parkfield SAF characterized a low-velocity waveguide in the SAF, likely extending from the surface to ~7-km depth with a width taper from ~200 m to ~100 m, within which seismic wave velocities are reduced by 20–40% and Q is 15–50. The FZTW data recorded before and after the 2004 M6 Parkfield earthquake show that the SAF co-seismically weakened with seismic wave velocities reduced by ~2.5% and consequently healed with seismic velocity recovery by ~1.25% within approximate three months after the mainshock. This chapter is a retrospective review of the results from our previous experiments at the Parkfield SAF, California, we expect that it will be valuable for researchers who are carrying out seismic experiments at the active faults to develop the community models of seismic wave velocity, fault structure and earthquake forecasting in the China Seismic Experimental Site (CSES) and global earthquake regions.

Review and Update on Some Connections between a Spring-Block SOC Model and Actual Seismicity in the Case of Subduction Zones

Article

Full-text available

Mar 2022
Entropy

The self-organized critical (SOC) spring-block models are accessible and powerful computational tools for the study of seismic subduction. This work aims to highlight some important findings through an integrative approach of several actual seismic properties, reproduced by using the Olami, Feder, and Christensen (OFC) SOC model and some variations of it. A few interesting updates are also included. These results encompass some properties of the power laws present in the model, such as the Gutenberg-Richter (GR) law, the correlation between the parameters a and b of the linear frequency-magnitude relationship, the stepped plots for cumulative seismicity, and the distribution of the recurrence times of large earthquakes. The spring-block model has been related to other relevant properties of seismic phenomena, such as the fractal distribution of fault sizes, and can be combined with the work of Aki, who established an interesting relationship between the fractal dimension and the b-value of the Gutenberg-Richter relationship. Also included is the work incorporating the idea of asperities, which allowed us to incorporate several inhomogeneous models in the spring-block automaton. Finally, the incorporation of a Ruff-Kanamori-type diagram for synthetic seismicity, which is in reasonable accordance with the original Ruff and Kanamori diagram for real seismicity, is discussed.

Seismic hazard calculation using the equivalent magnitude obtained from numerical modelling

Article

May 2022
INT J ROCK MECH MIN

Numerical modelling is widely used to estimate seismic potential in mines. Several numerical methods, along with different interpretation techniques, allow estimating the seismic potential at different stages of a mine or tunnel construction. However, none of the existing methods considers the magnitude of seismic events as a distribution of values, rather all methods focus on obtaining a single value representative of the seismic conditions for each excavation stage. The objective of this study was to develop a method based on the implicit generation of a synthetic distribution of seismic data and on the results of numerical modelling, providing a means to decompose the modelling results into a statistical distribution of maximum magnitudes. The results of this method provide a statistical distribution of the maximum magnitudes, based on existing plastic modelling results and assumptions about the failure mechanism. This work provides an engineering tool to evaluate the probability of the maximum seismic event.

Dual threshold search method for asperity boundary determination based on geodetic and seismic catalog data

Article

Mar 2022

As an important model for explaining the seismic rupture mode, the asperity model plays an important role in studying the stress accumulation of faults and the location of earthquake initiation. Taking Qilian-Haiyuan fault as an example, this paper combines geodetic method and b-value method to propose a multi-source observation data fusion detection method that accurately determines the asperity boundary named dual threshold search method. The method is based on the criterion that the b-value asperity boundary should be most consistent with the slip deficit rate asperity boundary. Then the optimal threshold combination of slip deficit rate and b-value is obtained through threshold search, which can be used to determine the boundary of the asperity. Based on this method, the study finds that there are four potential asperities on the Qilian-Haiyuan fault: two asperities (A1 and A2) are on the Tuolaishan segment and the other two asperities (B and C) are on Lenglongling segment and Jinqianghe segment, respectively. Among them, the lengths of asperities A1 and A2 on Tuolaishan segment are 17.0 km and 64.8 km, respectively. And the lower boundaries are 5.5 km and 15.5 km, respectively; The length of asperity B on Lenglongling segment is 70.7 km, and the lower boundary is 10.2 km. The length of asperity C on Jinqianghe segment is 42.3 km, and the lower boundary is 8.3 km.

High-frequency radiation from dynamic earthquake fault models

Article

Full-text available

Jan 1983
ANN GEOPHYS-GERMANY

Raúl Madariaga

Dasht-e Bayaz Fault, Iran: Earthquake and Earlier Related Structures in Bed Rock I I

Article

Full-text available

John Tchalenko

An 18-kin-long segment of bed rock of the Dasht-e Ba~az earthquake fault was studied in detail to define the 1968 earthquake-related and earlier tectonic de-formations. Ground displacements that ac-companied the earthquake coincided pre-cisely with the pre-existing east-trending fault trace. Maximum components of offset were 4m left-lateral and 1 m south side rela-tively down. The bedrock displacement oc-curred along new tension fractures that strike on average at 30 °, as well as along reactivated pre-existing structures. Earlier tectonic deformation also produced tension fractures (post-Pliocene), conjugate shears (Pliocene), and tension joints (pre-Pliocene), and all are consistent with 47 ° to 55 ° tectonic compression. The study cov-ered three points: (1) the ° to 45 ° angle measured between the major principal stress direction indicated by the earthquake fractures and the fault; (2) the apparent constancy of the stress field direction during the three early phases and the 1968 defor-mation; and (3) the "gap" and "anti-Riedel" structure shown by the overall fauk trace, which, we suggest, are characteristic of situations of kinematic restraint and are associated with a nonuniformly propagat-ing rupture.

A specific barrier model for the quantitative description of inhomogeneous faulting and the prediction of strong ground motion. Part II. Applications of the model

Article

Full-text available

Jun 1983

The specific barrier model, which was described in detail by Papageorgiou and Aki (1983), is applied to a set of five moderate to strong Californian earthquakes: Kern County (1952); San Fernando (1971); Borrego Mountain (1968); Long Beach (1933); and Parkfield (1966). Source parameters such as barrier intervals, local stress drops, cohesive zone size, and cohesive stress are inferred. The analysis of the San Fernando earthquake of 1971 revealed a strong frequency dependence of Qp, suggesting that the high frequencies may not be as strongly attenuated as initially thought. This suggests that the fall-off at high frequencies of observed spectra at a site may not be a propagation path effect but primarily a source effect. The cut-off frequency observed on the source acceleration power spectra of all the events analyzed in this paper, is interpreted in terms of the cohesive zone size and cohesive stress which arrest the localized fractures that occur on the fault plane. It was found that the barrier interval, as inferred by the specific barrier model, increases with the increase in maximum slip. The ratio of the latter to the former represents the local strain drop and was found to increase slightly (factor of 2) with earthquake size. This verifies the relation between barrier interval and maximum slip which has been observed by Aki et al. (1977) who inferred the barrier interval by different methods (i.e., surface measurement of fault slip, seismic measurement of rise time, scaling law of seismic spectrum). Striking similarities with respect to source parameters were found between the Fort Tejon (1857) and Kern County (1952) earthquakes as well as between the Long Beach (1933) and Parkfield (1966) earthquakes. The former are characterized by long barrier intervals and large slips while the latter are characterized by short barrier intervals and small slips. San Fernando (1971) and Borrego Mountain (1968) earthquakes lie in between these two extremes.

Regional variation of the scaling law of earthquake source spectra

Article

Full-text available

Jan 1978

Time-Predictable recurrence model for large earthquakes

Article

Full-text available

Apr 1980

We present historical and geomorphological evidence of a regularity in earthquake recurrence at three different sites of plate convergence around the Japan arcs. The regularity shows that the larger an earthquake is, the longer is the following quiet period. In other words, the time interval between two successive large earthquakes is approximately proportional to the amount of coseismic displacement of the preceding earthquake and not of the following earthquake. The regularity eanbles us, in principle, to predict the approximate occurrence time of earthquakes. The data set includes 1) a historical document describing repeated measurements of water depth at Murotsu near the focal region of Nankaido earthquakes, 2) precise levelling and 14C dating of Holocene uplifted terraces in the southern boso peninsula facing the Sagami trough, and 3) similar geomorphological data on exposed Holocene coral reefs in Kikai Island along the Ryukyu arc.

Fault Behavior and Characteristic Earthquakes: Examples From the Wasatch and San Andreas Fault Zones

Article

Full-text available

Jul 1984

Palaeoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp-derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.-from Authors

Dasht-e Bayaz Fault, Iran: Earthquake and Earlier Related Structures in Bed Rock

Article

Full-text available

May 1975
GEOL SOC AM BULL, GSA Bulletin

An 18-km-long segment of bed rock of the Dasht-e Baȳaz earthquake fault was studied in detail to define the 1968 earthquake-related and earlier tectonic deformations. Ground displacements that accompanied the earthquake coincided precisely with the pre-existing east-trending fault trace. Maximum components of offset were 4 m left-lateral and 1 m south side relatively down. The bedrock displacement occurred along new tension fractures that strike on average at 50°, as well as along reactivated pre-existing structures. Earlier tectonic deformation also produced tension fractures (post-Pliocene), conjugate shears (Pliocene), and tension joints (pre-Pliocene), and all are consistent with 47° to 55° tectonic compression. The study covered three points: (1) the 40° to 45° angle measured between the major principal stress direction indicated by the earthquake fractures and the fault; (2) the apparent constancy of the stress field direction during the three early phases and the 1968 deformation; and (3) the "gap" and "anti-Riedel" structure shown by the overall fault trace, which, we suggest, are characteristic of situations of kinematic restraint and are associated with a nonuniformly propagating rupture.

Dynamics of an Expanding Circular Fault

Article

Full-text available

Jun 1976

Raúl Madariaga

We study a plane circular model of a frictional fault using numerical methods. The model is dynamic since we specify the effective stress at the fault. In one model we assume that the fault appears instantaneously in the medium; in another, that the rupture nucleates at the center and that rupture proceeds at constant subsonic velocity until it suddenly stops. The total source slip is larger at the center and the rise time is also longer at the center of the fault. The dynamic slip overshoots the static slip by 15 to 35 per cent. As a consequence, the stress drop is larger than the effective stress and the apparent stress is less than one half the effective stress. The far-field radiation is discussed in detail. We distinguish three spectral regions. First, the usual constant low-frequency level. Second, an intermediate region controlled by the fault size and, finally, the high-frequency asymptote. The central region includes the corner frequency and is quite complicated. The corner frequency is shown to be inversely proportional to the width of the far-field displacement pulse which, in turn, is related to the time lag between the stopping phases. The average corner frequency of S waves v0s is related to the final source radius, a, by v0s = 0.21 β/α. The corner frequency of P waves is larger than v0s by an average factor of 1.5.

Theory of time-dependent rupture in the Earth

Article

Full-text available

Oct 1980

Using the concepts of fracture mechanics, we develop a theory of the earthquake mechanism which includes the phenomenon of subcritical crack growth. The theory specifically predicts the following phenomena: slow earthquakes, multiple events, delayed multiple events (doublets), postseismic rupture growth and afterslip, foreshocks, and aftershocks. The theory also predicts that there must be a nucleation stage prior to an earthquake and suggests a physical mechanism by which one earthquake may 'trigger' another. -from Authors

SEISMOLOGICAL ASPECTS OF STRONG MOTION SEISMOLOGY.

Article

Jan 1979

Thomas C. Hanks

The paper summarizes observations and model techniques appropriate to the estimation of long-period strong ground motion; discusses recent developments in the estimation of high-frequency strong ground motion via the root-mean-square acceleration; and addresses the magnitude-dependence of peak accelerations by developing a connection between magnitude saturation and the larger peak accelerations.

A Simulation of Earthquake Occurrence: Part 5 An Interpretation of Aftershock Phenomena

Article

May 1976

Michio OTSUKA

According to the chain-reaction type source model which the author has proposed throughout this series of paper, areas of variety of size in which seismic energy is still left unliberated are known to be dispersed within the source domain of the main shock. The assumption that these regions are cause of aftershock is shown to be successfully applied in explaining various aspects of the phenomena.

Recurrent Occurrence of the Earthquakes with Similar Wave Forms and Its Related Problems

Article

Aug 1975

In the course of examining the wide dynamic range records of the aftershocks of the Tokachi-oki Earthquake of 1968, the seventy two shocks having similar wave forms out of about 50, 000 aftershocks were found by means of a simple but systematic data sorting. High correlations in P and later phases are clear not only among the events of the same order of magnitude but also among the events with considerably different magnitudes (M=1-4.6). The recurrent occurrence of such events can be traced for the length of about 50 days, which is much longer duration compared with so-called twin or triple shocks reported so far. The activity is, as a whole, a swarm type; however, it is composed of several foreshock-main shock-aftershock sequences. The determination of hypocenters for the several large shocks shows that the events are closely distributed in space within the range of 5km, which suggests that they have common spatial and dynamical characters. Hereby, the group of earthquakes having a similar wave character can be called an Earthquake Family, which may be the most basic unit of sequence of shocks. The nature of this family is characterized by a small b-value of 0.43 (total number=68) in the Gutenberg-Richter's formula: this result is in qualitative harmony with Mogi's explanation on b-value. The variations of P wave spectral amplitude ratio with magnitude are compared with the ω²- and ω³-theoretical source models and the ω²-model agrees well with the observed data in the frequency range between 4.6 and 16.2Hz. The average Qα in the crust is estimated to be 400 from the frequency dependence of the difference between the observed and theoretical amplitude ratio.

Notes on stream channels offset by the San Andreas fault, southern Coast Ranges, California

Article

Jan 1968

R.E. Wallace

Active faults and earthquakes-the geological aspect

Article

Jan 1976

T. Matsuda

Characteristic earthquakes on intraplate normal faults

Article

Jan 1981

The surface faults associated with the Rikuu earthquake of 1896

Article

Jan 1980

A dynamic model for far-field acceleration

Article

Aug 1982

John Boatwright

A model for the far-field acceleration radiated by an incoherent rupture is constructed by combining Madariaga's (1977) theory for the high-frequency radiation from crack models of faulting with a simple statistical source model. By extending Madariaga's results to acceleration pulses with finite durations, the peak acceleration of a pulse radiated by a single stop or start of a crack tip is shown to depend on the dynamic stress drop of the subevent, the total change in rupture velocity, and the ratio of the subevent radius to the acceleration pulse width. An incoherent rupture is approximated by a sample from a self-similar distribution of coherent subevents. Assuming the subevents fit together without overlapping, the high-frequency level of the acceleration spectra depends linearly on the rms dynamic stress drop, the average change in rupture velocity, and the square root of the overall rupture area. The high-frequency level is independent, to first order, of the rupture complexity. Following Hanks (1979), simple approximations are derived for the relation between the rms dynamic stress drop and the rms acceleration, averaged over the pulse duration. This relation necessarily depends on the shape of the body-wave spectra. The body waves radiated by 10 small earthquakes near Monticello Dam, South Carolina, are analyzed to test these results. The average change of rupture velocity of Δv = 0.8β associated with the radiation of the acceleration pulses is estimated by comparing the rms acceleration contained in the P waves to that in the S waves. The rms dynamic stress drops of the 10 events, estimated from the rms accelerations, range from 0.4 to 1.9 bars and are strongly correlated with estimates of the apparent stress.

A dynamic source model for the San Fernando earthquake

Article

Dec 1978

Michel Bouchon

We model the San Fernando earthquake as a propagating rupture in a half-space, using for the slip-time-history on the fault plane analytical expressions which approximate the slip functions of dynamic crack models obtained by Das and Aki (1977a, b). We synthesize the strong ground motions and accelerations at the Pacoima Dam site and compute the teleseismic signals for different models of cracks. Three major featuras of the data–the strong pulse associated with the beginning of the rupture, the high acceleration phase on the Pacoima Dam records, and the presence of ripples on the teleseismic seismograms–which are not compatible with a smooth rupture process, are well explained by a crack with barriers model where the rupture encounters, along the fault plane, barriers or obstacles of high strength materials which may remain unbroken after the passage of the rupture front. A high-stress drop (400 to 500 bars) is required in the hypocentral area to explain the high-amplitude short-duration first pulse of the teleseismic records. This indicates a high level of tectonic stress in the area. A study of the earthquake series following the main shock shows that the aftershocks which took place in the region where major slipping occurred during the earthquake may represent the release of some of the barriers.

Notes on stream channels offset by the San Andreas fault, southern Coast Ranges, California, in Dickinson, W.R., and Grantz, Arthur, editors, Proceedings of conference on geologic problems of San Andreas fault system

Article

Jan 1968

R.E. Wallace

Active Faults, Paleoseismology, and Earthquake Hazards in the Western United States

Article

Jan 1981

Robert E. Wallace

Active faults are those that may have displacement within a future period of concern to humans. Studies of prehistoric earthquakes --paleoseismology-- show that average recurrence intervals for large displacements and related earthquakes on most active faults in the western US are generally longer than 1000 years; for many the average recurrence is greater than 10 000 years. Only on the San Andreas fault and its major branches are average recurrence intervals as short as 10-200 years. In the Great Basin province, a central part may have had no major displacement events (surface faulting events that could accompany a M?7 or larger earthquake) in late Quaternary time whereas in the western and eastern parts the generalized rate of faulting is in the range of 5 x 10-5 to 5 x 10-6 events per year per 1000 km2. In some localized areas of western Nevada rates are higher. Probabilistic expressions of the likelihood of future behavior of active faults are needed. -Author

Body wave spectra from propagating shear waves

Article

Jan 1973

An earthquake source model studied in this paper predicts a higher value of P wave corner frequency than S wave corner frequency; the ratio of P to S wave corner frequency is about 1.3 on the average. This result owes mainly to the slip characteristics on the fault, that is, the center of the fault slips for a longer time than the edges and consequently a greater relative displacement takes place near the center. Relationships of source dimension with the corner frequencies for P and S waves are derived to estimate the source dimension of earthquakes from teleseismic body wave spectra. The far-field spectra from the present model of equidimensional rupture propagation demonstrate the spectral decay of ω-2 at high frequencies. The seismic efficiency is found to be independent of the size of source dimension and expressed as a function of initial stress, stress drop, and rupture velocity. Being interpreted in terms of this model, the ratio of frictional stress to final stress can be estimated from observations of rupture velocity.

RMS accelerations and spectral amplitudes of strong ground motion during the San Fernando, California earthquake

Article

Oct 1980

An estimate of the Fourier amplitude spectrum of horizontal shear-wave motion is obtained using the Brune seismic source model in the presence of anelasUc attenuation, from which the root-mean-square acceleration (arms) can be calculated. Predicted spectral amplitudes and values of a,ms are compared to observations for 160 free-field or basement-level components of horizontal acceleration of the San Fernando earthquake. For a stress drop of 50 bars and a faulting duration of 10 sec, observed and predicted spectral amplitudes and values of arms are in reasonable agreement: the observations conform to the predicted attenuation with distance (which is frequency-dependent for spectral amplitudes and proportional to R -3/2 for arms), but exceed predicted values by an average factor of about 3. Some of this difference is attributable to azimuthal variation in source excitation and the highly nonuniform distribution of stations with azimuth; the remainder results from the high-stress drop associated with the initial rupture for this earthquake. Predictions made by the model are nonetheless in agreement, both in terms of attenuation and amplitude, with spectral amplitudes estimated by empirical models calibrated with records from a wide variety of earthquakes.

Metal vapour ion lasers with excitation by transverse radio-frequency discharge

Article

Jan 1981

Experiments with CW and pulsed laser operation using transverse RF discharge excitation of inert gases and Cd, Zn, Hg, Se, Tl, and Cu vapors in ion transitions in the range 325-806 nm are reported. The TRFD process was applied from 3-40 MHz in quartz discharge tubes specifically to optimize Cd ion laser lines oscillations. The CW mode actually allowed 1 ms pulse length and the optimum buffer gas pressure was found to vary reciprocally with the tube bore diameter. Electron deexcitation processes were found to cause the saturation of charge-transfer excited transitions while the saturation of the Penning reaction excited transitions created Cd ion resonant radiation trapping. Higher power was obtained in the pulsed mode, although output power declined with an increasing repetition rate. A model for the TRFD method is presented, including consideration of the ionization and excitation of He, optimum Cd pressure, and optimum excitation power.

A stochastic fault model: 1. Static case

Article

Jul 1980

D. J. Andrews

The number-size distribution of earthquakes requires that irregularities exist on a fault at all length scales. The assumption of self-similar irregularity is used to formulate a stochastic description of the faulting process. A random irregularity is termed self similar if it remains statistically similar upon a change of length scale. Self-similar geometric irregularity of a fault surface is represented in this model by stress and friction functions that fluctuate self similarly on a plane. If the set of rupture areas of all earthquakes on the brittle portion of a fault plane is assumed to be self similar, then the number of ruptures with area greater than A is proportional to 1/A. If stress drop is independent of earthquake size, then the number of earthquakes with moment greater than M0 is proportional to M0-2/3. The size of an earthquake is determined by spatial fluctuation of the initial stress and sliding friction functions. The spectrum of the stress function is related to both the average stress drop as a function of earthquake size and the number-moment distribution. A model of the slip and stress change functions of an earthquake is constructed in the Fourier transform domain. While the stress function becomes smoother in an earthquake at the length scale of the rupture, it becomes rougher at shorter length scales to prepare the fault for future smaller earthquakes. Seismicity is a cascade of stored elastic energy from longer to shorter wavelengths.

Coupling of energy between tectonic processes and earthquakes

Article

Jan 1978

D. J. Andrews

The elastic field energy change associated with a faulting process is indeterminate, because work may be done on distant bounding surfaces. Savage (1969) has shown that plastic strain, the source of the earth's self-stress, must be considered in the resolution of this question (Steketee's paradox). In a self-stressed earth, faulting will decrease the elastic field energy. Over a period of time following the faulting, distant viscoelastic regions can be expected to respond in such a way as to return stress to a value appropriate to steady flow. In the course of this response, work will be performed on the brittle seismogenic region to increase elastic field energy smoothly throughout the region. Steketee's result for a region bounded by a surface of constant stress is appropriate to the question of tectonic reloading, if account is taken of irregular slip due to past earthquakes. The energy increase due to tectonic reloading is not necessarily equal to energy released in the earthquake and has a different spectral decomposition. Although there is no energy balance in the seismogenic region for a single earthquake, there must be such a balance for a cycle of earthquakes of all sizes.

Recurrence models and Parkfield, California, earthquakes ( USA).

Article

May 1984

Main shocks of the earthquake sequences that occurred on the Parkfield section of the San Andreas fault in central California in 1922, 1934, and 1966 are characterized by southeast rupture expansion over the same 20- to 30-km-long section of the fault. Whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10%, the seismic moment for 1966 is 20% greater than for the earlier events to within a precision of 20%. The Parkfield ara seismicity, in general, seems well described by a recurring moderate size characteristic earthquake, repeating the same epicenter, magnitude, seismic moment, rupture area, and southeast direction of rupture expansion. An unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the Parkfield earthquakes in 1857, 1818, 1901, 1922, 1934, and 1966 represent a strictly periodic process. Assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 and 1934 period, the next characteristic Parkfield earthquake should occur between 1983 and 1993.

A Stochastic Fault Model 2. Time-Dependent Case

Article

Nov 1981

D. J. Andrews

A random model of fault motion in an earthquake is formulated by assuming that the slip velocity is a random function of position and time truncated at zero, so that it does not have negative values. This random function is chosen to be self-affine; that is, on change of length scale, the function is multiplied by a scale factor but is otherwise unchanged statistically. A snapshot of slip velocity at a given time resembles a cluster of islands with rough topography; the final slip function is a smoother island or cluster of islands. In the Fourier transform domain, shear traction on the fault equals the slip velocity times an impedance function. The fact that this impedance function has a pole at zero frequency implies that traction and slip velocity cannot have the same spectral dependence in space and time. To describe stress fluctuations of the order of 100 bars when smoothed over a length of kilometers and of the order of kilobars at the grain size, shear traction must have a one-dimensional power spectrum is space proportional to the reciprocal wave number. Then the one-dimensional power spectrum for the slip velocity is proportional to the reciprocal wave number squared and for slip to its cube. If slip velocity has the same power law spectrum in time as in space, then the spectrum of ground acceleration will be flat (white noise) both on the fault and in the far field.

b Values and ω −γ Seismic Source Models: Implications for Tectonic Stress Variations Along Active Crustal Fault Zones and the Estimation of High-Frequency Strong Ground Motion

Article

May 1979

Thomas C. Hanks

In this study the tectonic stress along active crustal fault zones is taken to be of the form (y) + Aa,(x, y), where (y) is the average tectonic stress at depth y and Aa,(x, y) is a seismologically observable, essentially random function of both fault plane coordinates; the stress differences arising in the course of crustal faulting are derived from Aa,(x, y). Empirically known frequency of occurrence statistics, moment-magnitude relationships, and the constancy of earthquake stress drops may be used to infer that the number of earthquakes N of dimension >r is of the form N 1/? and that the spectral composition of Aao(x, y) is of the form I'œ.(k)l /k , where A%(k) is the two-dimensional Fourier transform of Aa,(x, y) expressed in radial wave number k. The 3/= 2 model of the far-field shear wave displacement spectrum is consistent with the spectral composition I,(k)ll/k s, provided that the number of contributions to the spectral representation of the radiated field at frequency f goes as (k/ko), consistent with the quasi-static frequency of occurrence relation N 1/?; k0 is a reference wave number associated with the reciprocal source dimension. Separately, a variety of seismologic observations suggests that the 3/ = 2 model is the one generally, although certainly not always, applicable to the high-frequency spectral decay of the far-field radiation of earthquakes. In this framework, then, b values near 1, the general validity of the 3/ = 2 model, and the constancy of earthquake stress drops independent of size are all related to the average spectral composition of Aa,(x, y), Il/kL Should one of these change as a result of premonitory effects leading to failure, as has been specifically proposed for b values, it seems likely that one or all of the other characteristics will change as well from their normative values. Irrespective of these associations, the far-field, high-frequency shear radiation for the 3/= 2 model in the presence of anelastic attenuation may be interpreted as band-limited, finite duration white noise in acceleration. Its rms value, a .... is given by the expression arms -' 0.8512x/(2*r)/106) (Aa/pR)(fmax/fo)/a, where Aa is the earthquake stress drop, p is density, R is hypocentral distance, f0 is the spectral corner frequency, and fmax is determined by R and specific attenuation I/Q. For several reasons, one of which is that it may be estimated in the absence of empirically defined ground motion correlations, arms holds considerable promise as a measure of high-frequency strong ground motion for engineering purposes.

Characterization of barriers on a seismic fault

Article

Jan 1979

Keiiti Aki

Recently, Bouchon (1979a) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1066 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 109 erg cm-2. Using the formulas of Ida (1973b), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.

Seismological Aspects of the Guatemala Earthquake of February 4, 1976

Article

Jul 1978

Detailed analyses of teleseismic surface waves and body waves from the Guatemala earthquake of February 4, 1976, show the following: (1) Left lateral displacement along a vertical fault with a strike varying from N66°E to N98°E is consistent with the teleseismic data. (2) The seismic moment was 2.6×1027 dyn cm. The directivity of the surface wave radiation indicates an asymmetric (1:2.3) bilateral faulting with a total length of 250 km. In modeling the displacement a rupture velocity of 3 km/s was used, and the fault curvature was included. (3) If a fault width of 15 km is assumed, the average offset is estimated to be about 2 m. This value is about twice as large as the average surface offset. (4) Although the observed directivity suggests a uniform overall displacement along the fault, the body wave analysis suggests that the earthquake consists of as many as 10 independent events, each having a seismic moment of 1.3-5.3×1026 dyn cm and a fault length of about 10 km. The spatial separation of these events varies from 14 to 40 km. This multiple-shock sequence suggests that the rupture propagation is jagged and partially incoherent with an average velocity of 2 km/s. (5) The average stress drop estimated from surface waves is about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of a heterogeneous distribution of the mechanical properties along the fault, which may be caused by either asperities, differences in strength, differences in pore pressure, differences in slip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important in assessing the effect of large earthquakes along other transform faults like the San Andreas.

Discrete wave number representation of elastic wave field in three-space dimensions.

Article

Jul 1979

Michel Bouchon

We present the generalization to three dimensions of the discrete wave number representation method of Bouchon and Aki (1977). The method is developed to study the near field of a three-dimensional seismic source embedded in a layered medium. The elastic wave fields are represented by a superposition of plane waves propagating in discrete directions. The discretization is exact and results from a periodic two-dimensional arrangement of sources. The accuracy of the method is checked, in the case of a rectangular dislocation source radiating in an infinite medium, by comparing the results obtained with Madariaga's (1978) exact solution. Examples of the calculation of strong ground motion produced by a thrust fault and a strike slip fault are presented.

Fault plane with barriers: A versatile earthquake model

Article

Dec 1977

Shear cracks with finite cohesive forces can propagate by skipping past barriers. The barriers left behind may remain unbroken or may eventually break because of subsequent increase in dynamic stress depending on the ratio of barrier strength to tectonic stress. This model can explain a variety of observations on rupture in the earth, including (1) segmentation of the fault or ruptured zone in earthquakes and rock bursts, (2) ripples in seismograms which cannot be explained by path effect, and (3) departure of the scaling law of the seismic spectrum from that based upon the similarity assumption. The model also explains why the simple uniform dislocation model sometimes works better than the crack model without barriers. It also predicts, contrary to common belief, that an earthquake with low average stress drop may generate relatively greater amounts of high-frequency waves than an earthquake with high average stress drop. One important consequence of of our barrier model is the possibility of predicting the occurence of aftershocks by analyzing the source spectrum of the main shock.

Geophysical characteristics of dacite volcanism --- the 1977 1978 eruption of Usu volcano

Article

Mar 1981
J VOLCANOL GEOTH RES

The 1977–1978 eruption of Usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism. The activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas; the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes.The hypocenters of the earthquakes occurring beneath Usu volcano have been located precisely. Their distribution defines an earthquake-free zone which underlies the area of doming within the summit crater. This zone is regarded as occupied by viscous magma. The domings within the summit crater forming the cryptodomes have amounted to about 160 m. In addition to uplift they showed thrusting towards the northeast. As a result, the northeastern foot of the volcano has contracted by about 150 m. The relation between crustal deformation and earthquake occurrence is examined, and it is found that the abrupt domings are accompanied by the larger earthquakes (M = 3–4.3). Both the seismic activity and the ground deformation are shown to have a unique and common energy source.The energy of activities of Usu volcano consists of the explosive type, the deformation type and the seismic type; the second and the third are in parallel with each other in discharges, and both energies are complementary to the explosive energy. The explosive energy and the seismic energy have been calculated for an explosion sequence, and it is concluded that the deformation energy is about 10 times greater than the seismic energy. The discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption, except for a small increase at the end of January 1978. Eruptions are governed not only by the supply of the energies but also by the depth of the magma, which has gradually approached the surface. The last eruption occurred in October 1978; however, the crustal deformations and the earthquake swarms are still proceeding as of January 1980, albeit at a lower rate of activity.

Source mechanisms of Usu volcano, Japan, earthquakes and their tectonic implications

Article

Jun 1983
PHYS EARTH PLANET IN

Minoru TAKEO

An intensive earthquake swarm and remarkable surface deformation due to dacitic doming have taken place at the Usu volcano in southwestern Hokkaido, Japan, since August, 1977. A major U-shaped fault has formed in the summit crater, and the amount of upheaval reached ~ 180 m for ~ 5 years. In order to reveal the quantitative relation between this doming deformation and the associated earthquakes, the mechanisms of the latter have been studied. For this purpose, broadband and wide dynamic range seismic observations were performed in a near-field region of the Usu volcano for the period from June, 1979 to July, 1980. Long-period seismometers and a digitally recording accelerometer system were used. For relatively large earthquakes, the data from the digital recording system permit very accurate calculations of ground displacements by employing the recursive-filter method. Source parameters such as seismic moment, average dislocation, stress drop and fault dimensions for five earthquake families have been determined from a comparison of the observed ground displacements with synthetic ones. Synthetic displacements were computed using Haskell's formulation for relatively deep earthquakes. The discrete-wavenumber (DW) method was used for relatively shallow earthquakes. The stress drop varies widely among earthquakes in the same earthquake family. The cumulative seismic deformation caused by the large earthquakes is almost consistent with the observed surface deformation. It is suggested that the large earthquakes are caused by stick-slip motions at the two sides of the U-shaped fault.

Effects of fault interaction on moment, stress drop, and strain energy release

Article

Mar 1981

Solutions for colinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment, stress drop, and static energy release. Two models, the barrier model and the asperity model, are considered. In the asperity model, the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases; areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or 'asperities' across which slip occurs only at the time of a seismic event. In the barrier model, slip zones separated by unbroken ligaments (barriers) are introduced into a uniformly stressed medium to approximate the non-uniform fault propagation proposed by Das and Aki.-Authors

A Review of Geological Evidence for Recurrence Times of Large Earthquakes

Article

Jan 1981

Kerry E. Sieh

The geological record of the past several thousand years contains valuable information for evaluating the earthquake potential of the earth's major fault systems. Geologists have begun to characterize past and, presumably, future behavior of active faults and recurrence intervals for large earthquakes by studying 1) uplifted marine terraces, 2) fault-scarp morphology, 3) physiographic features offset along faults, and 4) faulted or otherwise deformed young sediments. -Author

The Faulting Mechanism of the San Fernando Earthquake

Article

Mar 1974

Thomas C. Hanks

'The large displacement pulse occurring at Pacoima dam approximately 2.5 s after the triggering of the accelerogram is identified as the shear radiation emanating from the point of initial rupture, this interpretation suggesting that the San Fernando, California, earthquake was initiated with massive but localized rupture in the hypocentral region. On the basis of the resulting S-P time at Pacoima dam and teleseismic observations of the reflected phase pP, together with the estimated uncertainties in the local hypocentral determination of Allen et al. (1973), the point of initial rupture is located at 34ø27.0'N, 118ø24.(rW and h -- 13 kin. The breakout phases resulting from the rupture of the earth's surface are tentatively identified on the Pacoima dam accelerograms and teleseismic World-Wide Standard Seismograph Network seismograms. For the San Fernando earthquake the breakout phases do not appear to be particularly energetic, but this may only reflect the especially energetic nature of the initial rupture phases. The inferred length of faulting and the estimated origin times of the initial rupture and the breakout phases yield an average rupture velocity. of at least 2.8 km/s. Although they are subject to the uncertainties of the suggested effects of source propagation and of single-station observations, the source parameters of the initial rupture are estimated as follows: source dimension, 6-3 km; stress drop, 350-1400 bars; average slip, 4.6-9.2 m; and seismic moment, 1.7-0.85 X 10 dynes cm. A 0.8-s time interval after the arrival of Sx contributes 40% of the radiated energy flux at Pacoima dam; the initial rupture event may have generated as much as 80% of the total energy radiated by the San Fernando earthquake. The stress difference accompanying the initial rupture is comparable to an estimate of the north-south compresslye stress that is assumed to maintain the continued uplift of the San Gabriel Mountains; the stress drop obtained for the entire faulting process, however, is smaller than either of these by 1-2 orders of magnitude. That the San Fernando earthquake was apparently initiated with massive but localized failure not representative statically or dynamically of the full faulting process most likely reflects a highly nonuniform distribution of strain energy density in the incipient source region. This possibility adds a new dimension to laboratory studies, numerical models, and conventional seismic investigations of the earthquake mechanism.

Theoretical Basis of Some Empirical Relations in Seismology

Article

Oct 1975
B SEISMOL SOC AM

Empirical relations involving seismic moment M_o, magnitude M_S, energy E_S and fault dimension L (or area S) are discussed on the basis of an extensive set of earthquake data (M_S ≧ 6) and simple crack and dynamic dislocation models. The relation between log S and log M_o is remarkably linear (slope ∼ 2/3) indicating a constant stress drop Δσ; Δσ = 30, 100 and 60 bars are obtained for inter-plate, intra-plate and “average” earthquakes, respectively. Except for very large earthquakes, the relation M_S ∼ (2/3) log M_o ∼ 2 log L is established by the data. This is consistent with the dynamic dislocation model for point dislocation rise times and rupture times of most earthquakes. For very large earthquakes M_S ∼ (1/3) log M_o ∼ log L ∼ (1/3) log E_S. For very small earthquakes M_S ∼ log M_o ∼ 3 log L ∼ log E_S. Scaling rules are assumed and justified. This model predicts log E_S ∼ 1.5 M_S ∼ 3 log L which is consistent with the Gutenberg-Richter relation. Since the static energy is proportional to σ̅L^3, where σ̅ is the average stress, this relation suggests a constant apparent stress ησ̅ where η is the efficiency. The earthquake data suggest ησ̅ ~ 1/2 Δσ. These relations lead to log S ∼ M_S consistent with the empirical relation. This relation together with a simple geometrical argument explains the magnitude-frequency relation log N ∼ − M_S.

The character of high-frequency strong ground motion

Article

Dec 1981
B SEISMOL SOC AM

Analysis of more than 300 horizontal components of ground acceleration written by the San Fernando earthquake, eight other moderate-to-large California earthquakes, and seven Oroville aftershocks reveal that these acceleration time histories are, to a very good approximation, band-limited white Gaussian noise within the S-wave arrival window; the band limitation is defined by the spectral corner frequency f0 and fmax, the highest frequency passed by the accelerograph or the Earth's attenuation, and the S-wave arrival window is (0 ≦ t − R/β ≦ Td), where R is distance, β is shear-wave velocity, and Td is the faulting duration. An examination of the root-mean-square acceleration (arms) characteristics of these records for 0 ≦ t − R/β ≦ Td in terms of the relation a rms = 0.85 ( 2 π ) 106 2 Δ σ ϕ R f max f o where Δσ is the earthquake stress drop, yields the surprising result that all 16 earthquakes have stress drops, as determined by record values of arms, very nearly equal to 100 bars (within a factor of 2). The source dependence of arms thus depends solely on the parameter 1/fo, which increases only as the one-sixth power of seismic moment for constant stress drop earthquakes. Put another way, model and record arms are in agreement within a factor of 2 approximately 85 per cent of the time for Δσ = 100 bars and knowledge of 1/fo. On the basis that acceleration time histories are finite-duration, band-limited, white Gaussian noise, for any of which arms is fixed by Δσ = 100 bars and 1/fo, we can estimate the peak accelerations (amax) for all of these records with considerable accuracy (50 per cent or less). The relation is a max = a rms 2 In ( 2 f max f o ) , where arms is defined above. With less accuracy, this relation fits the peak acceleration set of Hanks and Johnson (1976) as well, again with Δσ = 100 bars. At a fixed, close distance, we determine the magnitude dependence of amax to be log amax ∝ 0.30 M for 4≲M=ML≲612, close to that recently determined empirically by Joyner and Boore (1981) for 5.0 ≦ M ≦ 7.7, their coefficient on M (moment magnitude) being 0.25 ± 0.04. In the model presented here, the magnitude dependence of peak acceleration is a function of faulting duration alone; larger earthquakes have larger peak accelerations because they last longer, not because they are intrinsically more powerful at the high frequencies controlling peak acceleration. These well-behaved characteristics of high-frequency strong ground motion also suggest that the stress differences which develop in the course of crustal faulting are comparably well behaved, both in the average stress release across the characteristic source dimension and in the spectral composition and distribution of stress differences that develop across smaller dimensions.

Scaling Law of Seismic Spectrum

Article

Feb 1967

Keiiti Aki

Dynamic rupture process on a three dimensional fault with non-uniform frictions and near-field seismic waves

Article

Aug 1978
GEOPHYS J INT

Dynamical rupture process on the fault is investigated in a quasi-three-dimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress. The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method. If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it. The rise time of the source function and the final displacements are larger around the centre of the fault. In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena. In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage. These regions could be the source of aftershocks at a next stage. The stick-slip faulting and irregular rupture propagation radiate high-frequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults.

Earthquake Stress Drops, Ambient Tectonic Stresses and Stresses That Drive Plate Motions

Article

Jan 1977

Thomas C. Hanks

A variety of geophysical observations suggests that the upper portion of the lithosphere, herein referred to as the elastic plate, has long-term material properties and frictional strength significantly greater than the lower lithosphere. If the average frictional stress along the non-ridge margin of the elastic plate is of the order of a kilobar, as suggested by the many observations of the frictional strength of rocks at mid-crustal conditions of pressure and temperature, the only viable mechanism for driving the motion of the elastic plate is a basal shear stress of several tens of bars. Kilobars of tectonic stress are then an ambient, steady condition of the earth's crust and uppermost mantle. The approximate equality of the basal shear stress and the average crustal earthquake stress drop, the localization of strain release for major plate margin earthquakes, and the rough equivalence of plate margin slip rates and gross plate motion rates suggest that the stress drops of major plate margin earthquakes are controlled by the elastic release of the basal shear stress in the vicinity of the plate margin, despite the existence of kilobars of tectonic stress existing across vertical planes parallel to the plate margin. If the stress differences available to be released at the time of faulting are distributed in a random, white fasbion with a mean-square value determined by the average earthquake stress drop, the frequency of occurrence of constant stress drop earthquakes will be proportional to reciprocal faulting area, in accordance with empirically known frequency of occurrence statistics.

Seismological significance of the 1977–1978 eruptions and the magma intrusion process of Usu volcano, Hokkaido

Article

Mar 1981
J VOLCANOL GEOTH RES

An earthquake swarm, and the major pumice eruptions in August 1977 which followed, marked the start of the dacitic doming activity of Usu volcano in southwestern Hokkaido, Japan. The sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data. The distribution of the abundant hypocenters shows clearly an earthquake-free zone beneath the summit crater. The hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations, considered due to magma intrusion into this earthquake-free zone. The earthquake mechanism solutions are mostly of dip-slip type and are interpreted in terms of the doming deformations. The existence of earthquake families (earthquakes with similar waveforms) is the main cause of the peculiar occurrence of earthquakes in space, time and magnitude. The concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm, and the observed episodic deformations.

Tectonic stress and spectra of shear waves from earthquakes. J Geophys Res

Article

Sep 1970

James N. Brune

An earthquake model is derived by considering the effective stress available to accelerate the sides of the fault. The model describes near- and far-field displacement-time functions and spectra and includes the effect of fractional stress drop. It successfully explains the near- and far-field spectra observed for earthquakes and indicates that effective stresses are of the order of 100 bars. For this stress, the estimated upper limit of near-fault particle velocity is 100 cm/sec, and the estimated upper limit for accelerations is approximately 2g at 10 Hz and proportionally lower for lower frequencies. The near field displacement u is approximately given by u(t) = (sigma/mu) beta r (1 - exp(-t/r)) where sigma is the effective stress, mu is the rigidity, beta is the shear wave velocity, and tau is of the order of the dimension of the fault divided by the shear-wave velocity. The corresponding spectrum is Omega(omega) = (sigma beta)/mu 1/(omega(omega^2 + tau^-2)^(1/2)). The rms average far-field spectru

High frequency radiation from crack (stress drop) model of earthquake faulting

Article

Dec 1977

Raúl Madariaga

We present a theory for the radiation of high-frequency waves by earthquake faults. We model the fault as a planar region in which the stress drops to the kinematic friction during slip. This model is entirely equivalent to a shear crack. For two-dimensional fault models we show that the high frequencies originate from the stress and slip velocity concentrations in the vicinity of the fault's edges. These stress concentrations radiate when the crack expands with accelerated motion. The most efficient generation of high-frequency waves occurs when the rupture velocity changes abruptly. In this case, the displacement spectrum has an ω⁻² behaviour at high frequencies. The excitation is proportional to the intensity of the stress concentration near the crack tips and to the change in the focusing factor due to rupture velocity. We extend these two-dimensional results to more general three-dimensional fault models in the case when the rupture velocity changes simultaneously on the rupture front. Results are similar to those described for two-dimensional faults. We apply the theory to the case of a circular fault that grows at constant velocity and stops suddenly. The present theory is in excellent agreement with a numerical solution of the same problem. Our results provide upper bounds to the high-frequency radiation from more realistic models in which rupture velocity does not change suddenly. The ω⁻² is the minimum possible decay at high frequencies for any crack model of the source.

Study of Elastic Shocks Caused by the Fracture of Heterogeneous Materials and its Relations to Earthquake Phenomena

Article

Jan 1962

Kiyoo Mogi

地殻に作用する歪力に因る地殻の急激な破壊乃至はそれに類似した現象に伴って地震が発生するという考えは,(比較的浅い)地震の原因として多くの研究者に支持されているように思われる.このような観点から地震現象を見るならば,地殻の破壊特性を明らかにすることが,地震発生の諸問題を解く重要な手掛りとなると考えられる.本論は,このような立場から,地殻の破壊に関する性質に類似すると思われる不均質媒質の破壊特性を実験的に研究して,地震現象を明らかにする手掛りを得ようと試みたものである.地殼の破壊に関する性質を特徴づける重要な要素として,まず脆性と構造的不均一性があげられる.従って,上述の目的のためには,これらの性質に着目して,破壊の諸性質を調べることが有益な結果を与えると考えられる.そこで今回は,種々の不均質度をもつ脆性媒質として,軽石,結晶質岩石,およびガラス等をとり,一様等速増加応力および一定持続荷重を加えた場合に生ずる破壊群,とくにそれに伴ったElastic Shocks(衝撃性弾性波)の発生過程およびその大きさ分布を,主として統計的に研究したが,次にその結果を要約する.(1)花園岩や軽石のような不均質脆性媒質に比較的一応な等速増加応力を加えた場合,応力の増加と共にElastic Shocksが著しく発生する.これに対して,安山岩のうちのA(K1)やガラスおよび松脂等の均質な媒質では,試験体の全面的破断に至るまで,Elastic Shocksの発生は極めて少ないかまたは全く認められない.上述のようなElastic Shocksの頻発は不均質脆性媒質の特性である.(2)不均質脆性媒質に一定持約応力を作用させた場合に,直ちにElastic Shocksが頻発し,以後次第に減少する.ただし,試料の巨視的破砕(または破断)の発生の直前に再び著しい増加を示す.一定応力下のこのような現象もまた不均質脆性体に特有である.(3)岩石試料の一定荷重のもとにおけるElastic Shocksの発生経過は,岩石のcreepの機構を明らかにする一つの手掛りを与える.即ち,transient creepの機構が高圧実験の結果から岩石内の微小破壊によると推定されたが(E.C.Robertson), Elastic Shocksの測定の結果,このような微小破壊群の発生が直接確められた.なお,一般にElastic Shocksの測定は,媒質内の破壊発生を探知する有力な手段となり得る.(4)Elastic Shocksの発生は媒質の脆性破壊に因るものであることから,次のような量をもつて不均質媒質の脆性度の尺度とすることができる.B=2Wef/S・εnそれによると,花崗岩は一般に安山岩よりも著しく脆性的である.(5)軽石,花崗岩および安山岩等の破壊に伴ったElastic Shocksの大きさ分布はほぼ一定の統計法則に従っている.即ち,その最大振巾に関しては,地震で知られている石本・飯田の統計式が適用され,結晶質岩石試料では,その指数mが1.5～2.0で一般の地震の場合に類似し,また不均質度の著しく高い軽石では,指数mが明らかに大きい値(2～3)を示す.均一応力のもとでの均質媒質では,微小破壊は発生しがたく,このような分布とは異っている(或いはmが著しく小さい傾向を示す).このように均一な応力のもとで石本・飯田の統計式が成立し,指数mが如何なる値をとるかは,媒質の不均質度に関係するもので,不均質度が著しいほど指数mの値が大きい傾向が認められる.(6)均質媒質に不均一応力が作用した場合にも石本・飯田の統計式が成り立つ場合があることから,この統計式の成立は媒質内の応力分布の不均一性に関係すると考えられる.この結果にもとづいて,二,三の仮定のもとに石本・飯田の統計式を導びいた.それによれば,指数,mは媒質内の応力分布の不均一度と共に増加する.(7)Elastic Shocksの時間間隔の頻度分布は,定常状態では指数分布を示す.任意の過程もそれをいくつかの定常的過程に分ければ,それぞれについて指数分布を示す.(8)一定持続応力のもとでのElastic Shocksの頻度は,初期の段階をのぞけば,凡そ指数函数的に減少する.従つて,一定応力状態のもとでは,一定の遷移確率をもつて発生すると考えられる.(9)Elastic Shocksのこのような時間特性は,一種の確率過程としてよく説明される.このような発生特性を定量的に調べるために,応力分布の一層明確な,巨視的破断について,破壊時間を測定して,その発生確率と応力との関係を求めた.このような試料の巨視的破断の発生も,またその局部的破壊群(即ちElastic Shocks)の発生も確率過程論的破壊論によつて説明される.この結果にもとづいて,Elastic Shocksの頻度曲線と応力状態との関係が求められる.次に,地殻が不均質脆性媒質であり,地震はその局部的破壊に伴う弾性波であると考えれば,こわらのElastic Shocksに関して得られた結果の多くが,地震の場合に適用され,次のような結果が導かれる.(10)Elastic Shocksの大きさ分布に関する上述の結果を地震に適用すれば,地震の場合に石本・飯田の統計式が成立し,指数mが1.5～2,0であることは,地殼ではある程度応力が不均一に分布していること,そしてそれが結晶質岩石に応力が作用した場合の内部の応力分布の不均一さと類似の程度(但し長さの尺度が非常にちがう)であることを示している.(11)火山地震のうちやや深い所に発生する地震では指数mが1.5～2.0程度で一般の地震と同程度の値を示すが,活動的な火山の火口直下の極く浅い所に発生する地震ではmが著しく大きい値(2～4)を示す.これは,火口直下の浅い部分では,媒質が極度に不均質状態にあり,また作用する応力も不均一に集中的に作用するためであると考えられ,一方やや深い部分では,他の地殻の部分と大差のないことを示すものと考えられる.(12)地殼に外応力が作用した場合に発生する地震の時間特性は,上述のElastic Shocksのそれに対応するものと考えられる.例えば,定常状態における時間間隔の分布はやはり指数分布で表わされる.(13)若干の仮定のもとに,地震の頻度曲線から地殼の応力の変化状態が推定される可能性がある.一例として,余震の場合を取扱つたが,余震域の応力は本震後急激に減少し,次第に一定値に近づくことが推定された.

3. Waveform and Spectral Features of Earthquake Swarms and Foreshocks : in Special Reference to Earthquake Prediction

Article

Jan 1983

Masaru Tsujiura

Through the analyses of waveforms and spectra for the earthquake swarm, foreshock and ordinary seismic activities, some differences in the activity mode are found among those activities. The most striking difference is the "similarity of waveform". The earthquake swarm activity which occurred in a certain short time interval mainly consists of events with similar waveforms, belonging to the event group called "similar earthquakes" or an "earthquake family". On the other hand, the foreshock activity consists of events with an individual waveform character. Similarly, the rate of the occurrence of earthquake families is very low in ordinary seismic activity. The epicenters of earthquakes in a family are distributed within a small area, about 400meters for a family with Mmax≅3.0, in contrast with a wide area for foreshocks. The source spectra of earthquake swarms also show some features differing from those of other activities. The corner frequencies of events in the same family are almost constant, and their values depend on the size of the largest earthquake within the family. On the other hand, the corner frequencies of foreshocks differ from event to event, and there is no simple linear relation between the corner frequency and the earthquake size. Such behavior suggests that the earthquakes in a family occur on the same fault plane as a repeated slipping or a repeated incomplete rupture, and foreshocks occur independently in a heterogeneous zone near the main shock. If such differences are applicable for the other sequences, continuous monitoring of waveforms and spectra gives us some useful information for distinguishing a foreshock sequence from an earthquake swarm. ある地域における地震活動が活発化した場合,それが群発地震であるか,前震であるかを識別することは地震学的にも,又地震予知の見地からも極めて重要な問題である.ここでは,群発地震についての波形,並びにスペクトル解析から,群発地震のもつ特徴を明らかにすると共に,同様な解析を前震活動にも適用し,両者の性質の違いを見出そうとするものである.対象とした地域は関東地方である.堂平観測所並びにその衛星観測所の資料から,過去14年間で15個の群発地震,及び3個の前震活動を選び,それぞれの波形解析及びスペクトル解析を行なった. I.群発地震の波形解析早送り記録の重ね合わせにより波形について調べた結果:1.ある時間帯(活動度により異なる)に発生する地震は非常に似かよった波形を示し,所謂"相似地震"によって構成される. 2.一連の相似地震の活動が終ると,別の相似地震群が隣接地域において発生する(活動が高い場合).3.相似地震の震源域は数百米である(Mmax≈3). 4.相似地震に含まれる最大地震は,その系列の後半に発生する. II.群発地震のスペクトル解析アナログ型バソドパスフイルターの記録を用い,それぞれの相似地震群の震源スペクトルを求めた.主な結果は: 1.相似地震のコーナー周波数は,ある範囲の地震(ΔM≈2)に対し略一定である. 2.ある相似地震のコーナー周波数は,その地震群に含まれる最大地震の大きさに依存する. 3.従って,相似地震に含まれる最大地震の大きさはストレスドロップ量(スベリ量)によって制限される.III.前震の波形及びスペクトル解析群発地震で用いた同じ方法により,前震活動及び一部定常活動の地震についても解析を行なった.主な結果は: 1.前震はそれぞれ独立した波形を持ち,群発地震で見られたような相似地震は現われない. 2.前震活動におけるそれぞれの地震のコーナー周波数は複雑に分布し,群発地震でみられたような地震の大きさ(モーメント)と震源の大きさとの間で単純な直線関係は見られない. 3.定常活動における相似地震の発生率は極めて低い. 以上,群発地震,前震及び定常活動における地震について,それぞれの活動様式の特徴を明らかにした.この様な特徴はそれぞれの地震群の発生様式の違いに起因するものであろうと推定される.すなわち,群発地震は局所的な応力集中の場で発生し,そのため極めてせまい範囲おそらく同じ断層面上で,繰り返し発生するのに対し,前震は,広域応力場での発生のため,広範囲の領域で発生したことを反映したものであろう.従って,若しこの楼な違いが,他の前震及び群発地震についても適用されるならば,地震波形並びにスペクトルの継続的な監視が地震予知のための1資料として役立つことが期待される.

Matsuda The occurrence rate of earthquakes on Quaternary

S G C H Wesnousky
K Scholz
T Shimazaki

Yamashina Surface faults associated with the Izu-Hanto-Oki earthquake of 1974

T K Matsuda

The characteristic earthquake model: Implications to recurrence on the San Andreas fault (abstract)

Jan 1983
61

Coppersmith K. J.

Asperities, barriers, characteristic earthquakes and strong motion prediction ( Japan).

Abstract

No full-text available

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