Article

Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone

April 2006
Tectonophysics 417(1):33-51

April 2006
417(1):33-51

DOI:10.1016/j.tecto.2005.04.013

Authors:

Non-volcanic deep low-frequency tremors in southwest Japan exhibit a strong temporal and spatial correlation with slow slip detected by the dense seismic network. The tremor signal is characterized by a low-frequency vibration with a predominant frequency of 0.5–5 Hz without distinct P- or S-wave onset. The tremors are located using the coherent pattern of envelopes over many stations, and are estimated to occur near the transition zone on the plate boundary on the forearc side along the strike of the descending Philippine Sea plate. The belt-like distribution of tremors consists of many clusters. In western Shikoku, the major tremor activity has a recurrence interval of approximately six months, with each episode lasting over a week. The tremor source area migrates during each episode along the strike of the subducting plate with a migration velocity of about 10 km/day. Slow slip events occur contemporaneously with this tremor activity, with a coincident estimated source area that also migrates during each episode. The coupling of tremor and slow slip in western Shikoku is very similar to the episodic tremor and slip phenomenon reported for the Cascadia margin in northwest North America. The duration and recurrence interval of these episodes varies between tremor clusters even on the same subduction zone, attributable to regional difference in the frictional properties of the plate interface.

Cellular Automaton Equivalence of the Process of 2011 Mw 9 East Japan Earthquake Recorded at Kesennuma City

Preprint

Full-text available

Mar 2022

Hiroyuki Kikuchi

This paper links the seismic process of a megathrust earthquake with a cellular automaton (CA) and paves the way for understanding the seismic process by investigating CA. During the seismic process of the Great East Japan Earthquake (GEJE) of magnitude 9, anomalous noise with large amplitude in the frequency range of 3 Hz to 10Hz is observed in the Fourier spectrum. The anomalous noise and the normal ground vibrations in the GEJE process are generalized by introducing $\alpha$ -tremor, which is defined as the curvature of the Fourier amplitude spectrum of the velocity. It is found that the binarization of the vibration velocity conserves the $\alpha$-tremor, and the binarized velocity and velocity are equivalent. The binarization allows the recorded data to be directly compared with CA since CA is inherently binary, and since both the binarized velocity deviation and the CA are considered to follow the same master equation which represents the thermodynamics of a stochastically fluctuating non-equilibrium system. It is found that the recorded data in the GEJE process is a subset of the CA and the former can be reproduced by the latter. The scope of this research is limited to the specific earthquake GEJE and the data recorded at a specific seismic station. Further investigation is required for generalization.

Étude pétrophysique et hydrogéologique des sédiments entrant dans les zones de subduction

Thesis

Jul 2021

Jade Dutilleul

Cette étude explore l’influence sur le type de sismicité exprimé par le méga-chevauchement des propriétés lithologiques, pétrophysiques et hydrogéologiques des sédiments entrant dans les systèmes de subduction, et de l’évolution de ces propriétés, à partir d’échantillons et de diagraphies issus d’expéditions du Programme International de Découverte des Océans (IODP) dans 1) la marge nord d’Hikurangi caractérisée par une sismicité modérée, des séismes lents et des tsunami earthquakes, 2) la marge de Nankai où ont été documentés des méga-séismes tsunamigènes, des séismes lents et des tsunami earthquakes et 3) la marge nord de Sumatra surépaissie, où des méga-séismes tsunamigènes ont principalement été enregistrés. Dans la marge nord de Sumatra, un intervalle riche en smectites est interprété comme la zone de formation du futur décollement. Celui-ci coïncide au voisinage de la fosse avec un réflecteur sismique de forte amplitude et de polarité négative induites par les fluides produits par la déshydratation pré-subduction de la partie inférieure de l’épaisse séquence sédimentaire entrante. La mégaséisme tsunamigène de 2004 aurait été favorisé par la lithification avancée des sédiments entrants ainsi que par le drainage des fluides du décollement par des failles du prisme externe. A contrario, dans la marge nord d’Hikurangi, le décollement se formerait dans des carbonates pélagiques ou des faciès volcanoclastiques altérés riches en fluides. Les contrastes de composition et de propriétés hydrogéologiques et pétrophysiques de la séquence sédimentaire entrante impliquent une distribution hétérogène des fluides le long de l’interface de subduction, avec des pressions de fluide potentiellement exacerbées au voisinage d’un mont sous-marin subduit sous la partie externe de la marge, zone source de séismes lents. Combinées, ces hétérogénéités seraient à l’origine de propriétés de friction hétérogènes favorables à la formation de séismes lents et de tsunami earthquakes. Les séismes lents sont supposés 1) se propager jusqu’à la fosse le long de l’interface de subduction ou de failles satellitaires et 2) libérer régulièrement une partie de la déformation accumulée par le méga-chevauchement, ce qui expliquerait la sismicité modérée de la région. L’intégration de ces résultats dans des modèles de production de fluide ou de couplage mécanique et sismique est une étape essentielle à l’amélioration de la gestion du risque sismique et de tsunami en contexte de subduction.

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.

Influence of structure and pore pressure of plate interface on tectonic tremor in the Nankai subduction zone, Japan

Article

Full-text available

Mar 2021
EARTH PLANET SC LETT

Episodes of tectonic tremor are observed in the Nankai accretionary prism close to the trough axis; however, their non-impulsive signals and the distortion of seismic signals in the accretionary prism make it challenging to accurately estimate their locations. Reliable tremor locations would help to characterize geological conditions favourable for tremor generation and could offer insights into the up-dip limit of coseismic ruptures. Here we report a new approach to pinpoint tremor locations based on seismic phase polarization. The estimated epicentres of tremor distribution mostly coincide with low-velocity shear zone or thick underthrust sediments where high pore pressures are inferred, and the distribution of tremor appears to be controlled by characteristics of the shear zone. A tremor episode occurred a couple days after the off-Mie earthquake clearly shows migration at a velocity similar to that of very low frequency earthquakes. Migration of tremor initiated around the outer ridge where the estimated pore pressure is greatest. The spatio-temporal tremor distribution indicates that pore pressure relaxation could trigger the tremor migration. The occurrence of tremor thus may indicate a weak shallow plate interface that is prone to slip following large earthquakes. Based on the information of tremor migration, the permeability along the fault can be estimated as 3.7×10−12 m².

The Slow Slip of Viscous Faults

Article

Full-text available

May 2019

We examine a simple mechanism for the spatiotemporal evolution of transient, slow slip. We consider the problem of slip on a fault that lies within an elastic continuum and whose strength is proportional to sliding rate. This rate dependence may correspond to a viscously deforming shear zone or the linearization of a nonlinear, rate‐dependent fault strength. We examine the response of such a fault to external forcing, such as local increases in shear stress or pore fluid pressure. We show that the slip and slip rate are governed by a type of diffusion equation, the solution of which is found using a Green's function approach. We derive the long‐time, self‐similar asymptotic expansion for slip or slip rate, which depend on both time t and a similarity coordinate η = x/t, where x denotes fault position. The similarity coordinate shows a departure from classical diffusion and is owed to the nonlocal nature of elastic interaction among points on an interface between elastic half‐spaces. We demonstrate the solution and asymptotic analysis of several example problems. Following sudden impositions of loading, we show that slip rate ultimately decays as 1/t while spreading proportionally to t, implying both a logarithmic accumulation of displacement and a constant moment rate. We discuss the implication for models of postseismic slip as well as spontaneously emerging slow slip events.

The slow slip of viscous faults

Preprint

Full-text available

Jun 2018

We examine a simple mechanism for the spatio-temporal evolution of transient, slow slip. We consider the problem of slip on a fault that lies within an elastic continuum and whose strength is proportional to sliding rate. This rate dependence may correspond to a viscously deforming shear zone or the linearization of a non-linear, rate-dependent fault strength. We examine the response of such a fault to external forcing, such as local increases in shear stress or pore fluid pressure. We show that the slip and slip rate are governed by a type of diffusion equation, the solution of which is found using a Green’s function approach. We derive the long-time, self-similar asymptotic expansion for slip or slip rate, which depend on both time t and a similarity coordinate η = x/t, where x denotes fault position. The similarity coordinate shows a departure from classical diffusion and is owed to the non-local nature of elastic interaction among points on an interface between elastic half-spaces. We demonstrate the solution and asymptotic analysis of several example problems. Following sudden impositions of loading, we show that slip rate ultimately decays as 1/t while spreading proportionally to t, implying both a logarithmic accumulation of displacement as well as a constant moment rate. We discuss the implication for models of post-seismic slip as well as spontaneously emerging slow slip events.

Comparison of Continuously Recorded Seismic Wavefields in Tectonic and Volcanic Environments Based on the Network Covariance Matrix

Article

Full-text available

Dec 2023

Extended and sufficiently dense seismic networks capture spatiotemporal properties of the continuously recorded wavefields and can be used to compute the level of their coherence at different frequencies via the analysis of the network covariance matrix, which has been successfully used to study volcanic seismicity. Here, we present an application of the covariance matrix method in a subduction zone environment. We show that most coherent signals identified through the covariance matrix analysis are related to regional earthquakes with the wavefield properties affected by the scattering, which depends on the source location. Tectonic tremors, on the other hand, are not characterized by a high level of coherence. We compare real data results with a set of synthetic tests aimed at mimicking the properties of seismic sources and the main features of wave propagation. We conclude that highly coherent volcanic tremor wavefields could be produced in two ways: by a spatially localized group of monochromatic seismic sources or by a single source located in a highly heterogeneous medium. In both cases, the stability of the source position is a necessary condition to reproduce the observations in volcanoes. On the other hand, the low coherence of tectonic tremor wavefields can be explained by a spatially extended distribution of sources, in agreement with large portions of the subduction interface being nearly simultaneously involved in the episodes of slow deformation.

Slow slip events in the Kanto and Tokai regions of central Japan detected using GNSS data during 1994‐2020

Article

Full-text available

Feb 2021
GEOCHEM GEOPHY GEOSY

Takuya Nishimura

Slow slip events (SSEs) along subduction zones play an important role in accommodating relative plate motion. SSEs interplay with large megathrust earthquakes and other slow earthquakes, including low frequency and very low frequency earthquakes. The Kanto and Tokai regions of central Japan host frequent slow and large earthquakes, with significant differences in slip behavior along the subduction zones in the Suruga Trough, Sagami Trough, and Japan Trench. In this study, we conducted a systematic search to estimate the fault models and durations of short‐term SSEs using continuous Global Navigation Satellite System data collected from 1994 to 2020. We detected 176 potential SSEs with moment magnitudes of 5.3–7.0 and durations of 0–80 days from the time series. Along the Sagami Trough, two shallow regions at a depth of 10–20 km host Mw ≥ 6.5 SSEs off of the Boso Peninsula and accommodate most of the relative plate motion aseismically. Some SSEs also occur on the deep plate interface down to ∼50 km without tectonic tremors. Along the Japan Trench, the cumulative slip of the SSEs exhibits a bi‐modal depth distribution to avoid the large slip areas of past megathrust earthquakes at 30–40 km depth. The shallow SSEs are in the same depth range (10–30 km) as tectonic tremors, but are spatially separate from tremors along the trench. The detected SSEs have limited temporal correlations with other slow earthquakes and earthquake swarms, which suggests that many factors control the genesis of slow and regular earthquakes.

Co‐Occurrence of Low and Very Low Frequency Earthquakes Explained From Dynamic Modeling

Article

Full-text available

May 2024
GEOPHYS RES LETT

Very low‐frequency earthquakes (VLFs) are characterized by longer source duration and smaller stress drop than regular earthquakes of similar magnitude. Recent studies have shown their frequent correlation with low‐frequency earthquakes (LFEs) on shared faults. The underlying source processes governing the occurrence of VLFs and their interaction with LFEs remain elusive. Here, we employ a slip‐weakening model for slow earthquakes. By comparing the source parameters of simulations and observations, it is suggested that VLFs are slow self‐arresting earthquakes that self‐terminate within the nucleation patch. Additionally, we adopt a composite model to reproduce the records of the simultaneous occurrences of a VLF and an LFE in the Nankai area. Our results present the possibility that VLFs, LFEs, and regular earthquakes can be distinguished using a unified dynamic framework.

Imaging the changes in the clustering properties of deep low-frequency earthquakes caused by the occurrence of the 2010 long-term slow slip event in Western Shikoku, Japan

Conference Paper

Full-text available

Sep 2022

By using the space-time cross-correlation integral as a statistical method to perform a self-consistent analysis, we aim to reveal the changes in space-time patterns of deep low frequency activity across the western part of the Nankai subduction region. We present an imaging method derived from the gradient of the cross-correlation integral in respect to space-time domain which captures the characteristic clustering pattern of ETS episodes. This allows us to evaluate the influence of the 2010 long-term slow slip episode on the space-time patters in the segment directly affected by the long-term slip, as well as the neighboring segment. Both segments show significant change in the spatio-temporal clustering during the L-SSE event, as well as small changes in the before and after clustering patterns.

Segmentation characteristics of deep, low-frequency tremors in Shikoku, Japan using machine learning approaches

Article

Full-text available

Mar 2023
EARTH PLANETS SPACE

Shikoku island, southwestern Japan lies in the western Nankai Trough and showcases along-strike segmentation of slow earthquake behavior. Whether the spatial variation of tremor behavior reflects the regional differences in structure/source properties and how much such differences can be recognized by the seismic signals themselves are two questions addressed in this paper. Taking advantage of advanced methods in recognizing and classifying signals using machine learning approaches, we attempt to answer them by conducting signal classification experiments in Shikoku. Based on the tremor catalog from 1 June 2014 to 31 March 2015, the tremors recorded in four different areas were treated as different classes and segmented into 60-s-long signals. The number of tremors in four different areas (A to D, from west to east) reached 15,000, 31,000, 10,000, and 16,000, respectively. To efficiently distinguish between tremors from different areas, we applied a k-nearest neighbor (k-NN) classifier with Fisher’s class separability criteria to select the optimal feature subset. The resulting classification performance reached more than 90% at all 12 stations. We further designed a triangle test to select the features that can better represent the differences in source properties between areas. We found that the most efficient features were associated with (1) the number of peaks in the temporal evolution of discrete Fourier transforms and (2) the energy distribution in the autocorrelation function (ACF). To match the difference in behavior revealed by the ACF, the size of the tremor zone, which mainly controls how long the seismic energy lasts in a tremor episode, was determined to be largest in Area B and smallest in Area C. The heterogeneity of the asperities in a tremor zone, which may control how spiky the tremor signals developed over time, was determined to be strong in Areas B and C. Together with previously documented variations in slow earthquake behavior in the same area, we finally propose a conceptual model that provides a better understanding of the regional differences in the tremor source properties in Shikoku, Japan.

Complex Interaction, Striations, and Periodicity of Deep Tremor Surrounding a Tremor Gap in the Central Nankai Subduction Zone, Japan

Article

Full-text available

Feb 2023

Plain Language Summary Slow earthquakes are imperceptible slip events occurring close to the generation areas of megathrust earthquakes in many subduction zones. The smallest member of a family of slow earthquakes is tectonic tremor, and it is thought that the occurrence of background slow slip events and the physical properties of the plate interface can be inferred from tremor activity. The Nankai subduction zone in southwest Japan, which is one of the most active slow earthquake regions in the world, is characterized by a gap in tremor distribution associated with a complicated plate geometry. To understand tremor surrounding this gap, we analyzed the tremor activity in the central Nankai subduction zone observed by a temporary dense seismic observation network. We discovered an interaction of tremor activities across the tremor gap, suggesting that undetectable slow slip events occur in the gap. These tremor activities also interacted with other geodetically observed slow slip events. We also revealed that the spatial distribution and periodicity of the tremor events were associated with the plate subduction history and the physical properties of the plate interface. Our findings contribute to fundamental understanding of subduction processes and will improve future modeling of subduction earthquakes.

The 2019 East Coast Slow Slip Event, New Zealand: Spatiotemporal Evolution and Associated Seismicity

Article

Full-text available

Oct 2022
MAR GEOD

Slow slip events (SSEs) are interpreted as the transient quasi-static fault deformation in the deep transition zone from locked to freely slipping in many subduction zones. Using continuous Global Positioning System (cGPS) data collected in New Zealand, we estimate the spatiotemporal evolution model during the 2019 SSE and analyze the influence of subduction interface heterogeneity on seismicity during SSEs at the Hikurangi margin. The results reveal that the 2019 SSE extends from the northern (Gisborne) to the central (Hawke's Bay) Hikurangi subduction interface and decays rapidly within approximately 3-4 weeks. It releases a total seismic moment of about 4.83ⅹ1019 N·m (Mw 6.8), with a significant slip in Gisborne and a secondary slip in Hawke's Bay. The slip depths are similar, but peaks, durations, and rates differ slightly. By combining previous SSEs (2011-2019), diverse characteristics are concluded, i.e., shorter duration and more frequency in Gisborne and relatively longer duration and less frequency in Hawke's Bay. The seismicity offshore and onshore indicates along-strike variations, which appear to be spatially correlated with the variations in topography, such as subduction seamounts. The heterogeneities on the subduction interface are related to the spatiotemporal distribution of SSEs and seismicity along the Hikurangi margin.

Slow slip events and flank instability at Mt. Etna volcano (Italy)

Article

May 2022
TECTONOPHYSICS

We analyzed a set of 11 slow slip events occurred during the 2006–2016 period and affecting the GNSS (Global Navigation Satellite System) stations of the unstable flank of Mt. Etna volcano. Observed surface deformation for most of the detected slow slip events, concentrates on the south-eastern edge of the unstable flank while the slow slip events involving the north-eastern edge are less frequent. Such a pattern highlights the existence of two distinct families of events, involving two contiguous sectors of the unstable flank, which occasionally slip together in large slow slip events. The modelled slips also highlight that both contiguous sectors extend ~10–12 km off-shore, on areas where active tectonic lineaments such as the ESE (northward of Catania Canyon) and the N102° (along the southern slope of the Riposto Ridge) ones have been recently discovered. Equivalent seismic moments of slow slip events occurred in the last ten years (corresponding to magnitudes in the range 5.4–5.9) are larger than those associated to seismic events observed in the last 200 years, suggesting that most of the deformation affecting the eastern flank occurs aseismically.

Cellular Automaton Equivalence of Megathrust Earthquake Process

Preprint

Full-text available

Jan 2022

Hiroyuki Kikuchi

This paper links the seismic process of a megathrust earthquake with a cellular automaton (CA) and paves the way for understanding the seismic process by investigating CA. During the seismic process of the Great East Japan Earthquake (GEJE) of magnitude 9, anomalous noise with large amplitude in the frequency range of 3 Hz to 10Hz is observed in the Fourier spectrum. The anomalous noise and the normal ground vibrations in the GEJE process are generalized by introducing ɑ-tremor, which is defined as the curvature of the Fourier amplitude spectrum of the velocity. It is found that the binarization of the vibration velocity conserves the ɑ-tremor, and the binarized velocity and velocity are equivalent. The binarization allows the recorded data to be directly compared with CA since CA is inherently binary, and since both the binarized velocity deviation and the CA are considered to follow the same master equation which represents the thermodynamics of a stochastically fluctuating non-equilibrium system. It is found that the recorded data in the GEJE process is a subset of the CA and the former can be reproduced by the latter.

Complexity of Deep Low‐Frequency Earthquake Activity in Shikoku (Japan) Imaged From the Analysis of Continuous Seismic Data

Article

Full-text available

Nov 2021

We derive an extensive catalog of low‐frequency earthquakes (LFEs) in western and central Shikoku applying an automatic, coherency‐based detection and location method to 4‐year long continuous data from selected seismic stations. Our catalog is the first LFE catalog in this area that does not rely on a priory templates and results from a homogeneous data processing scheme. It allows investigating in detail main characteristics of LFE activity in space and time. We observe clear heterogeneity of the LFE space distribution in the along‐strike direction of the subducting Philippine Sea plate corresponding to segments with different sizes, some of which are highly productive in LFE generation. More detailed statistical and correlation‐based analysis of LFE occurrence patterns allows quantification of the along‐strike segmentation and examination of the migration and event interaction during the LFE sequences and inter‐sequence periods. The analysis indicates that a strong interaction exists among LFE sources during slow slip events, but it otherwise varies significantly among the along‐strike segments. We suggest that the observed segmentation of LFE activity is related to static heterogeneity, such as structural property variations along the subduction interface, or dynamic heterogeneity, corresponding to memory‐dependent stress variations or possible fluid transients. We also confirm that regions with the highest LFE productivity correspond to spots of tectonic tremor triggered by teleseismic earthquakes' surface waves supporting a localized fluid‐rich environment with possible fluid transients. Our results illustrate how high‐resolution LFE catalog can contribute to the characterization and quantification of slow earthquake processes through detailed statistical analysis of their activity.

Constraints From Exhumed Rocks on the Seismic Signature of the Deep Subduction Interface

Article

Full-text available

Sep 2021
GEOPHYS RES LETT

Low Velocity Zones (LVZs) with anomalously high Vp-Vs ratios occur along the downdip extents of subduction megathrusts in most modern subduction zones and are collocated with complex seismic and transient deformation patterns. LVZs are attributed to high pore fluid pressures, but the spatial correlation between the LVZ and the subduction interface, as well as the rock types that define them, remain unclear. We characterize the seismic signature of a fossil subduction interface shear zone in northern California that is sourced from the same depth range as modern LVZs. Deformation was distributed across 3 km of dominantly metasedimentary rocks, with periodic strain localization to km-scale ultramafic lenses. We estimate seismic velocities accounting for mineral and fracture anisotropy, constrained by microstructural observations and field measurements, resulting in a Vp/Vs of 2.0. Comparable thicknesses and velocities suggest that LVZs represent, at least in part, the subduction interface shear zone.

Post‐Earthquake Fold Growth Imaged in the Qaidam Basin, China, With Interferometric Synthetic Aperture Radar

Article

Full-text available

Mar 2021

Questions regarding the development of folds and their interactions with the seismic faults within thrust systems remain unanswered. However, estimating fault slip and earthquake hazards using surface observations and kinematic models of folding requires an understanding of how the shortening is accommodated during the different phases of the earthquake cycle. Here, we construct 16‐years of interferometric synthetic aperture radar time series across the North Qaidam thrust system (NE Tibet), where three Mw 6.3 earthquakes occurred along basement faults underlying shortened folded sediments. The analysis reveals spatio‐temporal changes of post‐earthquake surface displacement rates and patterns, which continue more than 10 years after the seismic events. The decomposition of the Sentinel‐1 ascending and descending line of sight velocities into vertical and shortening post‐earthquake components indicates that long‐term transient uplift and shortening is in agreement with the deformation that might be expected from kinematic models of folding. Long‐term uplift coincides spatially with young anticlines observed in the geomorphology, with steep gradients in the forelimbs, gentle gradients in the back‐limbs, an absence of subsidence in the footwalls, and higher gradients along interpreted bedding planes. Long‐term shortening is also different from the surface displacements expected for typical time‐varying creep on a narrow dislocation interface and shows rates higher than the average convergence across the whole region. These findings provide evidence for anelastic fold buckling during the post‐earthquake phase and highlight the contribution of distributed aseismic deformation to the growth of topography.

Tracking Deep Sediment Underplating in a Fossil Subduction Margin: Implications for Interface Rheology and Mass and Volatile Recycling

Article

Full-text available

Mar 2021
GEOCHEM GEOPHY GEOSY

The architecture and mechanical properties of the subduction interface impact large-scale subduction processes, including mass and volatile recycling, upper-plate orogenesis, and seismic behavior. The nature of the deep subduction interface, where a dominantly frictional megathrust likely transitions to a distributed ductile shear zone, is poorly understood, due to a lack of constraints on rock types, strain distribution, and interface thickness in this depth range. We characterized these factors in the Condrey Mountain Schist, a Late Jurassic to Early Cretaceous subduction complex in northern California that consists of an upper and lower unit. The Lower Condrey unit is predominantly pelagic and hemipelagic metasediment with m- to km-scale metamafic and metaserpentinitic ultramafic lenses all deformed at epidote blueschist facies (0.8-1.0 GPa, 450ºC). Major and trace element geochemistry suggest tectonic erosion of the overriding plate sourced all ultramafic and some mafic lenses. We identified two major ductile thrust zones responsible for Lower Condrey unit assembly, with earlier strain distributed across the structural thickness between the ductile thrusts. The Lower Condrey unit records distributed deformation across a sediment-dominated, 2+ km thick shear zone, possibly consistent with Low Velocity Zones (LVZs) observed in modern subduction zones, despite subducting along a sediment poor, tectonically erosive margin. Periodic strain localization occurred when rheological heterogeneities (i.e. km-scale ultramafic lenses) entered the interface, facilitating underplating that preserved 10-60% of the incoming sediment. Modern mass and volatile budgets do not account for erosive margin underplating, so improved quantification is crucial for predicting mass and volatile net flux to Earth's interior.

Post-Earthquake Fold Growth Imaged in the Qaidam basin, China, With InSAR

Preprint

Nov 2020

Geodetic Measurements of Slow Slip Events Southeast of Parkfield, CA

Research

Full-text available

May 2020

Tremor and low-frequency earthquakes are presumed to be indicative of surrounding slow, aseismic slip that is often below geodetic detection thresholds. This study uses data from borehole seismometers and long-baseline laser strainmeters to observe both the seismic and geodetic signatures of episodic tremor and slip on the Parkfield region of the San Andreas Fault near Cholame, CA. The observed occurrence rates of both the tremors and co-located families of low-frequency earthquakes are not steady, but instead exhibit quasi-periodic bursts of increased activity. We show that these periods of elevated seismic activity correlate with statistically significant stacked strain signals consisting of 44 slow slip events. Modeled individual slow slip events and their total summed moment, which are constrained by seismic signals and stacked strain respectively, indicate that the individual moment magnitudes of these events range from Mw 4.6−5.2. We find that the measured geodetic signal likely precedes the seismic signal by several hours, consistent with the aseismic slip preceding, and driving the observed seismic tremor activity. We confirm that strike-slip faults, in addition to subduction zones, are capable of producing episodic tremor and slip.

Rapid fluid infiltration and permeability enhancement during middle–lower crustal fracturing: Evidence from amphibolite–granulite-facies fluid–rock reaction zones, Sør Rondane Mountains, East Antarctica

Article

Apr 2020
LITHOS

Permeability is a key control on fluid infiltration in the crust. However, quantitative geological constraints on crustal permeability are limited, particularly with regards to its temporal evolution. Here we constrain the permeability evolution in the middle–lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing. We investigated mafic granulite and orthopyroxene–hornblende schist (opx–hbl schist) samples from Mefjell, Sør Rondane Mountains, East Antarctica. Millimetre-scale amphibolite-facies reaction zones occur along fractures in these rocks. In the mafic granulite, two zones were identified: (i) reaction zones (1–2 mm thick); and (ii) mafic granulite host rock. The opx–hbl schist sample can be divided into the following three zones: (i) actinolite–cummingtonite zones (1.4 mm thick); (ii) actinolite–orthopyroxene zones (1.6 mm thick); and (iii) host rock. These zones are evident from the modal mineralogy, reaction textures, and trace element profiles. The P–T conditions of fluid infiltration are estimated to be 0.55 GPa and 620°C for the mafic granulite, and 0.3 GPa and 450°C for the opx–hbl schist, respectively. Chlorine concentrations in apatite grains show a gradual decrease from the fractures towards the host rocks. Chlorine concentration profiles suggest that the dominant processes of chlorine transport were advection with minor diffusion for both samples. Based on these results, the timescales of fluid infiltration are constrained to be ~8 h for the mafic granulite and ~10 h for the opx–hbl schist. The pressure gradient across the reaction zones was estimated from the H2O activity in the reaction zones to be 0.4–1.4 MPa/mm. The permeability of the host rock and fractures were estimated to be 10−20–10−22 and 10−8–10−9 m2, respectively. Our results show that rapid infiltration of Cl-bearing fluids (~10 h) occurred due to a limited fluid flux from low-permeability (10−20–10−22 m2) host rocks. The low-permeability media led to fluid accumulation and further fracturing. The spatio-averaged permeability then increased by more than several orders of magnitude (10−10–10−16 m2) and the fluid pressure decreased. The contrasting permeability between the host rocks and fractures reveals permeability enhancements associated with crustal fracturing on timescales comparable to geophysical observations. Compared with average long-term (Myr) permeability estimates from previous studies (e.g., 10–18 m2), the permeability obtained from the fracture–reaction zone systems shows large fluctuations in the middle–lower crust (10−20–10−22 to 10−10–10−16 m2).

Article

Full-text available

Apr 2020

Low-frequency earthquakes are a particular class of slow earthquakes that provide a unique source of information on the physical processes along a subduction zone during the preparation of large earthquakes. Despite increasing detection of these events in recent years, their source mechanisms are still poorly characterised, and the relation between their magnitude and size remains controversial. Here, we present the source characterisation of more than 10,000 low-frequency earthquakes that occurred during tremor sequences in 2012-2016 along the Nankai subduction zone in western Shikoku, Japan. We show that the scaling of seismic moment versus corner frequency for these events is compatible with an inverse of the cube law, as widely observed for regular earthquakes. Their radiation, however, appears depleted in high-frequency content when compared to regular earthquakes. The displacement spectrum decays beyond the corner frequency with an omega-cube power law. Our result is consistent with shear rupture as the source mechanism for low-frequency earthquakes, and suggests a self-similar rupture process and constant stress drop. When investigating the dependence of the stress drop value on the rupture speed, we found that low-frequency earthquakes might propagate at lower rupture velocity than regular earthquakes, releasing smaller stress drop.

Etude des propriétés mécaniques et de la déformation transitoire dans les zones de subduction à partir de l'analyse de l'activité sismique, le cas du Chili

Thesis

Full-text available

Jan 2019

Florent Aden-Antoniow

Les zones de subduction sont les endroits les plus sismiquement actifs du monde et sont également le siège de méga-tremblement de Terre comme celui qui frappa le Chili en 1960 d’une magnitude estimée à 9.5. Les dernières grandes ruptures ayant frappées le Chili ont permis de mettre en évidence des intéractions sismiques-asismiques complexes. Afin d’étudier ces interactions nous nous sommes penchés sur deux crises sismiques : l’essaim sismique d’Avril 2017 ; la phase préparatoire du séisme d’Iquique (M w 8.1) du 1er Avril 2014. L’essaim sismique qui pris place près de la ville de Valparaiso dans une région connue pour avoir eu l’expérience de très grands séismes par le passé en 1730 et en 1906 notamment. Dans le but d’étudier la dynamique de cet essaim nous avons construit un catalogue riche de plus de 2000 séisme composant la séquence. Une activité sismique intense commença deux jours avec le séisme principal de la séquence M w 6.9 et fut accompagné par un glissement progressif le long de l’interface que nous avons observé à la fois dans les données GPS mais également à l’aide de repeaters. Enfin, un séisme de magnitude 8,2, survenu le 1er avril 2014 près de la ville de Iquique, a rompu un tiers de la lacune sismique au nord du Chili. Cet séisme a été précédé d’une séquence d’essaims sismiques précurseurs qui semblent avoir été également accompagnés par des glissements stables de l’interface de subduction. A l’aide d’un catalogue de séisme plus complet, nous avons pu analyser finement la préparation du séisme d’Iquique. Nous mettons en évidence, à travers une approche statistique, l’apparition d’une quiescence sismique dans la région du choc principale. Nous lions cette quiescence à un glissement asismique profond potentiellement relié à la circulation de fluide sous pression le long de l’interface de subduction.

Independent Component Analysis and Parametric Approach for Source Separation in InSAR Time Series at Regional Scale: Application to the 2017–2018 Slow Slip Event in Guerrero (Mexico)

Article

Full-text available

Mar 2020

Separating different sources of signal in Interferometric Synthetic Aperture Radar (InSAR) studies over large areas is challenging, especially between the long‐wavelength changes of atmospheric conditions and tectonic deformations, both correlated to elevation. In this study, we focus on the 2017–2018 slow slip event (SSE) in the Guerrero state (Mexico) where (1) the permanent GPS network has a low spatial density (less than 30 stations in an area of 300 × 300 km) with uneven distribution; (2) the tropospheric phase delays can be as high as 20 cm of apparent ground displacements, with a complex temporal evolution; (3) the tested global weather models fail to correct interferograms with enough accuracy (with residual tropospheric signal higher than the tectonic signal); and (4) the surface displacement caused by the seismic cycle shows complex interactions between seismic sequences and aseismic events. To extract the SSE signal from Sentinel‐1 InSAR time series, we test two different approaches. The first (parametric method) consists of a least squares linear inversion, imposing a functional form for each deformation or atmospheric component. The second uses independent component analysis of the InSAR time series. We obtain time series maps of surface displacements along the radar line of sight associated with the SSE and validate these results with a comparison to GPS. Combining those two approaches, we propose a method to separate atmospheric delays and tectonic deformation on time series data not corrected from atmospheric delays. From the extracted ground deformation maps, we propose a first‐order slip inversion model at the subduction interface during this SSE.

IOP Conference Series: Earth and Environmental Science Slow Strain Waves in Rocks as Potential Precursors to Seismic Hazard

Article

Full-text available

Dec 2019

Spatiotemporal distribution of low-frequency earthquakes in Southwest Japan: Evidence for fluid migration and magmatic activity

Article

Jan 2018
J ASIAN EARTH SCI

Low-frequency earthquakes (LFEs) in the lower crust and uppermost mantle are widely observed in Southwest Japan, and they occur not only along the subducting Philippine Sea (PHS) slab interface but also beneath active arc volcanoes. The volcanic LFEs are still not well understood because of their limited quantities and less reliable hypocenter locations. In this work, seismic tomography is used to determine detailed three-dimensional (3-D) P- and S-wave velocity (Vp and Vs) models of the crust and upper mantle beneath Southwest Japan, and then the obtained 3-D Vp and Vs models are used to relocate the volcanic LFEs precisely. The results show that the volcanic LFEs can be classified into two types: pipe-like and swarm-like LFEs, and both of them are located in or around zones of low-velocity and high-Poisson’s ratio anomalies in the crust and uppermost mantle beneath the active volcanoes. The pipe-like LFEs may be related to the fluid migration from the lower crust or the uppermost mantle, whereas the swarm-like LFEs may be related to local magmatic activities or small magma chambers. The number of LFEs sometimes increases sharply before or after a nearby large crustal earthquake which may cause cracks and fluid migration. The spatiotemporal distribution of the LFEs may indicate the track of migrating fluids. As compared with the tectonic LFEs along the PHS slab interface, the volcanic LFEs are more sensitive to fluid migration and local magmatic activities. High pore pressures play an important role in triggering both types of LFEs in Southwest Japan.

Ordinary and Slow Earthquakes Reproduced in a Simple Continuum System With Stochastic Temporal Stress Fluctuations

Article

Full-text available

Dec 2019
GEOPHYS RES LETT

Earthquakes are complex and diverse, ranging from energetic and destructive megathrust earthquakes to slow earthquakes. Complex earthquake rupture processes are generally modeled by prescribing a deterministic system and solving deterministic differential equations. However, such approaches cannot adequately capture the temporal evolution of a complex fault system because of the inherent unpredictability that results from various unprescribed processes. Such unpredictability may be better represented by time‐dependent stochastic fluctuations in addition to the deterministic estimates. Here, we demonstrate that the consideration of time‐dependent stochastic stress fluctuations in ordinary crack simulations can reproduce a variety of ruptures, including both crack‐like and pulse‐like ordinary (fast) earthquakes and slow earthquakes, by simply changing the strength drop and the initial stress level. The results indicate that stochasticity is effective for reproducing and better understanding the diversity of earthquakes, including slow earthquakes.

Migrations and Clusters of Shallow Very Low Frequency Earthquakes in the Regions Surrounding Shear Stress Accumulation Peaks Along the Nankai Trough

Article

Full-text available

Nov 2019
GEOPHYS RES LETT

Plain Language Summary Shallow slow earthquakes with rupture speeds between those of regular earthquakes and stable sliding often occur in the shallower extents of expected megathrust rupture zones. Such shallow slow earthquakes are useful for monitoring the slip behavior of shallow plate boundaries. The spatiotemporal variations of shallow slow earthquakes are related to tectonic characteristics including stress accumulation, friction properties, and the presence of pore fluids. To investigate the spatiotemporal variations of shallow slow earthquakes occurring along the Nankai Trough, the activities of shallow very low frequency earthquakes (SVLFEs) were examined using template matching and cross‐correlation relocation analysis. It was found that activity patterns of SVLFEs differed in each analyzed segment due to the different structural characteristics of the shallow Philippine Sea Plate boundary. A detailed spatial distribution revealed that SVLFEs tended to cluster or migrate in the regions surrounding the boundary stress accumulation peaks on the Philippine Sea Plate boundary. The obtained results suggest that shallow slow earthquakes occur and migrate to release accumulated stress in transitional regions between high‐strength (strongly locked) and low‐strength (stable sliding) zones on the shallow plate boundary.

IOP Conference Series: Earth and Environmental Science Slow Strain Waves in Rocks as Potential Precursors to Seismic Hazard

Conference Paper

Full-text available

Sep 2019

Slow Strain Waves in Rocks as Potential Precursors to Seismic Hazard

Article

Full-text available

Sep 2019

The article reports on instrumental monitoring of rock deformation in the South Baikal geodynamic test site. Based on the analysis of the time series of data sets, two groups of strain components are distinguished by differences in their origin and wave properties. Strain components in the first group are caused by external factors, such as lunar-solar tides and atmospheric pressure variations. The second group includes non-periodic, random and periodic (constant) components generated by internal tectonics. Non-periodic components are manifested as single deformation impulses that differ in intensity and form, and are related to slow displacements along large active faults, which occur outside the monitoring sites. Besides, non-periodic components are related to fast displacements at block interfaces within the monitoring sites. The periodic (constant) component is related to the India-Eurasian collision. It is represented by slow strain waves, which lengths amount to 400–500 m, with amplitudes of few microns, and periods of (1-3)·10 ⁻⁴ Hz. The strain wave directions and rates of their migration in space vary with time. The identified trends of changes of the wave parameters in the South Baikal geodynamic polygon can be related to the preparation of a strong earthquake.

Length scales and types of heterogeneities along the deep subduction interface: Insights from exhumed rocks on Syros Island, Greece

Article

Full-text available

Jun 2019
GEOSPHERE

We use structural and microstructural observations from exhumed subduction-related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that represent different oceanic protoliths and deformation conditions within a subduction interface shear zone, including: (1) prograde subduction of oceanic crust to eclogite facies; (2) exhumation of oceanic crust from eclogite through blueschist-greenschist facies; and (3) exhumation of mixed mafic crust and sediments from eclogite through blueschist-greenschist facies. All three localities preserve rheological heterogeneities that reflect metamorphism of primary lithological, geochemical, and/or textural variations in the subducted protoliths and that take the form of brittle pods and lenses within a viscous matrix. Microstructural observations indicate that the matrix lithologies (blueschists and quartz-rich metasediments) deformed by distributed power-law viscous flow accommodated by dislocation creep in multiple mineral phases. We estimate bulk shear zone viscosities ranging from ~10^18 to 10^20 Pa-s, depending on the relative proportion of sediments to (partially eclogitized) oceanic crust. Eclogite and coarse-grained blueschist heterogeneities within the matrix preserve multiple generations of dilational shear fractures and veins formed under high-pressure conditions. The veins commonly show coeval or overprinting viscous shear, suggesting repeated cycles of frictional and viscous strain. These geologic observations are consistent with a mechanical model of episodic tremor and slow slip (ETS), in which the deep subduction interface is a rheologically heterogeneous distributed shear zone comprising transiently brittle (potentially tremor-genic) sub-patches within a larger, viscously creeping interface patch. Based on our observations of outcrop and map areas of heterogeneous patches and the sizes, distributions, and amounts of brittle offset recorded by heterogeneities, we estimate that simultaneous brittle failure of heterogeneities could produce tremor bursts with equivalent seismic moments of 4.5 × 10^9–4.7 × 10^14 N m, consistent with seismic moments estimated from geophysical data at active subduction zones.

Scaled Energy Estimation for Shallow Slow Earthquakes

Article

Full-text available

Feb 2019

Deep and shallow slow earthquakes occur at the edge of the seismogenic zone in the Nankai subduction zone. Deep slow earthquakes occur under high‐temperature and high‐pressure conditions, whereas shallow slow earthquakes occur at lower temperatures and pressures. Although these two types of slow earthquake show qualitatively similar behaviors, such as slow deformation and depleted high‐frequency seismic signals, their similarities have not yet been evaluated in quantitative ways. In this work, we analyze shallow tectonic tremors accompanied by very low frequency earthquakes off Kii Peninsula, Japan. We estimate the seismic energy of the shallow tectonic tremors by correcting for the site amplifications of ocean bottom seismic stations and seismic attenuation in the shallow accretionary prism. The estimated seismic energy rates of these tectonic tremors are compared with seismic moment rates of accompanying very low frequency earthquakes, revealing that the scaled energy of shallow slow earthquakes is roughly constant, ~10⁻⁹ to 10⁻⁸, which is 0 to 1 order of magnitude larger than the typical scaled energy of deep slow earthquakes.

Low-Frequency Earthquakes and Pore Pressure Transients in Subduction Zones

Article

Oct 2018

Low-frequency earthquakes (LFEs) have been observed in subduction zones and some major tectonic faults and may well be the most important constituents of tectonic tremors. In subduction zones, they were initially attributed to fluids released by dehydration reactions in downgoing slabs. Their seismic radiation pattern, however, is consistent with shear slip on the subduction interface, and this rapidly became the favored model. Recent studies indicate that the source duration of LFEs does not scale with magnitude, which can hardly be explained by shear rupture. We revisit the characteristics of LFE events in subduction zones as retrieved from local seismic arrays. We demonstrate that they can be explained equally well by forces acting in the direction of fluid motion. Such forces may be generated by a fast local pressure variations associated with unsteady fluid motion. The amount of fluid required for LFE activity is consistent with dehydration reaction rates.

Frictional Behavior of Input Sediments to the Hikurangi Trench, New Zealand

Article

Full-text available

Aug 2018
GEOCHEM GEOPHY GEOSY

The Hikurangi subduction zone hosts shallow slow-slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow-slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity-weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow-slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10-km depth without requiring elevated pore fluid pressures. The transition to velocity-strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow-sliding velocities, rather than accommodating deformation through ordinary earthquakes.

Migrations of released seismic energy in various geodynamic conditions

Article

Full-text available

Mar 2018

Anna Novopashina

The properties of slow seismic activity migration have been revealed by the space-time analysis of the total earthquake energy (LgEsum). Our study of seismic activity covers the fragments of the Central Asian, Pacific and Alpine seismic belts: the Baikal rift system (BRS, Russia), the San Andreas fault zone (California, USA), the Christchurch fault (New Zealand), the North and East Anatolian faults (Turkey), the Philippine subduction zone, and the central fragment of the Mid-Atlantic oceanic ridge. The chains of LgEsum clusters mark the propagation of the maximum stresses front in the weaker crust areas, the zones of fault dynamic influence, and the regions of conjugated tectonic structures. The migration process is characterized by a periodicity, changes in direction, and similar modular values of the migration rates within a single fault segment (or a fault zone), which is probably related to the mechanical and rheological crust and upper mantle properties. The data analysis shows that a strong earthquake source may occur at a location wherein the front of seismic activity propagates with periodical changes in direction, and such a source can develop within a period that is multiple of the migration fluctuations, probably associated with the influence of external periodic factors. The main periods of migration fluctuations (2–4 years, and 9–13 years, in different ratios) are present in the seismic regimes of different seismic belts. The migration rate, as well as the propagation velocity of the maximum stresses front, directly depends on the velocity of movements between the plates in the region.

Laboratory Hydrofractures as Analogs to Tectonic Tremors

Article

Full-text available

Jan 2024

The fracture of Earth materials occurs over a wide range of time and length scales. Physical conditions, particularly the stress field and Earth material properties, may condition rupture in a specific fracture regime. In nature, fast and slow fractures occur concurrently: tectonic tremor events are fast enough to emit seismic waves and frequently accompany slow earthquakes, which are too slow to emit seismic waves and are referred to as aseismic slip events. In this study, we generate simultaneous seismic and aseismic processes in a laboratory setting by driving a penny-shaped crack in a transparent sample with pressurized fluid. We leverage synchronized high-speed imaging and high-frequency acoustic emission (AE) sensing to visualize and listen to the various sequences of propagation (breaks) and arrest (sticks) of a fracture undergoing stick-break instabilities. Slow radial crack propagation is facilitated by fast tangential fractures. Fluid viscosity and pressure regulate the fracture dynamics of slow and fast events, and control the inter-event time and the energy released during individual fast events. These AE signals share behaviors with observations of episodic tremors in Cascadia, United States; these include: (a) bursty or intermittent slow propagation, and (b) nearly linear scaling of radiated energy with area. Our laboratory experiments provide a plausible model of tectonic tremor as an indicative of hydraulic fracturing facilitating shear slip during slow earthquakes.

Friction of hornblende and tremolite at hydrothermal conditions with low effective normal stress and high pore pressure

Article

Aug 2023
TECTONOPHYSICS

Short‐Lived and Voluminous Fluid‐Flow in a Single Fracture Related to Seismic Events in the Middle Crust

Article

Full-text available

Mar 2023
GEOPHYS RES LETT

Plain Language Summary Short‐lived fluid flow in the crust modifies the hydrological properties of rocks and controls the earthquakes triggering. However, there are limited numerical constraints on the fluid volumes that can be rapidly transported. This study focuses on fluid flow through a single fracture in metamorphic rocks. We discuss the relationship between estimated fluid volumes and a series of low‐magnitude fracturing events, such as tremors and other types of slow slip events in the lower‐middle crust. Specifically, we analyze unique geological and geochemical evidence preserved in fluid‐rock reaction zones to approximate the duration of fluid infiltration and the volume of fluids transported. We use two independent methods for constraining generated seismic moment and magnitude based on fluid volumes and single fracture geometry. The transportation of fluid volumes through a fracture (10¹–10⁴ m³) may be related to short seismic events, as suggested by duration (∼10 hr) and cumulative magnitude, representing the maximum values as 2.0–3.8. We observed a dramatic change in hydrological properties: from low permeable rocks to high‐permeable fractures, which are not dead‐end and can effectively transport a large volume of fluids in a short time. Such fluid infiltration can possibly trigger seismic activity above the earthquake source regions.

Segmentation characteristics of deep, low-frequency tremors in Shikoku, Japan using machine learning approaches

Preprint

Full-text available

Sep 2022

Shikoku island, Japan lies in the western Nankai Trough and showcases along-strike segmentation of slow earthquake behavior. Whether the spatial variation of tremor behavior reflects the regional differences in structure and source properties and how much such differences can be recognized by the seismic signals themselves are two questions addressed in this paper. Taking advantage of advanced methods in recognizing and classifying signals using machine learning approaches, we attempt to answer them by conducting signal classification experiments in Shikoku. Based on the tremor catalog from 1 June 2014 to 31 March 2015, the tremors recorded in four different areas were treated as different classes and segmented into 60-s-long signals. The number of tremors in four different areas (A to D, from east to west) reached 15000, 31000, 10000, and 16000, respectively. To efficiently distinguish between tremors from different areas, we applied a k-nearest neighbor ( k -NN) classifier with Fisher’s class separability criteria to select the optimal feature subset. The resulting classification performance reached more than 90% at all 12 stations. We further designed a triangle test to select the features that can better represent the differences in source properties between areas. We found that the most efficient features were associated with (1) the number of peaks in the temporal evolution of discrete Fourier transforms and (2) the energy distribution in the autocorrelation function (ACF). In order to match the difference in behavior revealed by the ACF, the size of the tremor zone, which mainly controls how long the seismic energy lasts in a tremor episode, was determined to be largest in Area B and smallest in Area C. The heterogeneity of the asperities in a tremor zone, which may control how spiky the tremor signals become over time, was determined to be strong in Areas B and C. Together with previously documented variations in slow earthquake behavior in the same area, we finally propose a conceptual model that provides a better understanding of the regional differences in the tremor sources of Shikoku.

Synchronous Tremor Modulation During the Passage of 2012 Super-typhoon Jelawat in Nankai Trough: By Chance or Real Consequence?

Article

Feb 2022

Bhaskar Kundu

Episodic tremor and slow-slip events are sensitive to the exogenous stress perturbations process. Although tidal and remote triggering phenomena of tremors are well-established facts; however, the triggering mechanism induced by low-barometric pressure of typhoons or larger storms remains poorly addressed. In this paper, a time-synchronous tremor modulation is presented from the northern Kii Peninsula in western Japan, associated with a large Super Typhoon Jelawat, which occurred on September 30, 2012. It has been argued that such tremor excitation may not be correlated with other types of signal, such as the short-term or long-term slow-slip events, tidal effect, or remote triggering. The atmospheric low-pressure condition during the passage of super typhoon Jelawat causes vertical crustal uplift by a negative load and hence possibly enhances the thrust motion due to unclamping effects on the fault. Therefore, it is suggested that the synchronous tremor modulation process in the northern Kii Peninsula is a real consequence induced by super typhoon Jelawat.

Tidal Calibration of Multicomponent Borehole Strainmeters and Validation of the Method Using Surface Waves

Preprint

Full-text available

Jun 2021

We conducted in-situ calibration of fifteen multicomponent borehole strainmeters deployed in and around the expected focal zones of the Nankai megathrust earthquake. The in-situ calibration method compares tidal strain observed by the borehole strainmeters with predicted tidal strains from the solid Earth’s tide and oceanic tidal loading. Then we obtained a calibration matrix to transfer observed strain data to the regional strain field. We estimated the oceanic tidal loading accurately using a Green’s function, which takes the depth of deployment into consideration. We calculated four sets of calibration matrices using combinations of any three of a group of four gauges as well as a calibration matrix using all four gauges. The estimated calibration matrix was validated by comparing observed seismic strain waves after applying the calibration matrix with theoretical seismic strain waves excited by the 2010 Chile earthquake (Mw 8.8). The in-situ calibration was found to be appropriate for all eleven Ishii-type borehole strainmeters and for one of the four Gladwin Tensor Strainmeters (GTSMs). It was also effective with respect to two shear strains for two of the other three GTSMs.

Changes of Event Size Distribution During Episodes of Shallow Tectonic Tremor, Nankai Trough

Article

Full-text available

Feb 2021
GEOPHYS RES LETT

Plain Language Summary The size distribution of slow earthquakes mostly follows a power law like that of ordinary earthquakes, in which the logarithm of event numbers is negatively proportional to the logarithm of event sizes, with an exponential taper for events larger than a cut‐off magnitude. We investigated the changes in this size distribution during two prolonged episodes of shallow tectonic tremor that occurred on the plate interface along the Nankai trough, southwestern Japan. We found that the ratio of smaller events increased and the cut‐off magnitude decreased as tremor activity decreased late in each episode. We interpreted this observation by using a model of slow earthquakes that divides a source fault into small cells and updates slip on each cell probabilistically. The model can explain the changes in the tremor size distribution by a decrease in the probability of event ignition or an increase in the energy dissipation during fault slip. This result implies that the accumulated stress or the pore‐fluid pressure on the source fault decreased when the tremor was less active. Because the tremor source migrates during the course of each episode, these changes indicate that the source characteristics of tremor vary at different times or locations.

Friction properties of hornblende and implications for slow-slip events in subduction zones

Article

Dec 2020
TECTONOPHYSICS

In order to understand the friction properties of amphibole as a primary constituent mineral in subduction zones, we used hornblende as the simulated gouge material to conduct two series of shearing experiments at temperatures from 101 to 607 °C under confining pressures of 136 MPa and 83 MPa, a pore pressure of 30 MPa and loading rates of 0.0488–1.22 μm/s. Steady-state friction coefficients in the high confining pressure series exhibited values between 0.70 and 0.72 with an average of ~0.71, without showing a systematic temperature dependence. The counterpart in the low confining pressure series ranged from 0.67 to 0.73 and exhibited a temperature-dependent decline for temperatures above 400 °C. Velocity-weakening behaviors were observed at 303, 505 and 607 °C in the high confining pressure series, and at 303 and 403 °C in the low confining pressure series. The |a-b| parameter associated with velocity weakening of hornblende in our high and low confining pressure experiments are 0.0005–0.0014 and 0.0014–0.0015, respectively. The b values, which represent the evolution effect in friction, were found to increase with increasing temperature, suggesting a thermally-activated process that produced creep at asperity. Ubiquitously precipitated particles in the deformed gouge suggest that dissolution-precipitation creep may be the most likely mechanism associated with the evolution effect and therefore velocity weakening. The very low velocity weakening observed for hornblende (|a-b| ≤ 0.0015) provides a favorable condition for nucleation of slow-slip events to occur in subduction zones over a wider range of effective normal stress.

Underground Structural Anomalies and Slow Earthquake Activities Around Seismogenic Megathrust Earthquake Zone as Revealed by Inland Seismic Observations

Article

Apr 2009

Seismogenic zones of interplate megathrust earthquakes along the Nankai Trough can be subdivided into several segments. At each segment, seismic rupture has occurred at a recurrence interval of about one century. In many cases, some neighboring segments ruptured simultaneously or sequentially after a short interval. One of the factors that controls the properties of such seismic ruptures is the underground structure, including the plate configuration and heterogeneity around the subducting plate (slab) interface. To clarify the mechanism of megathrust earthquakes, detailed surveys and analyses of the underground structures are required. Moreover, the detection of seismic phenomena on the plate interface is important in discussing interplate coupling and interactions between such events and megathrust earthquakes. More and better knowledge of the underground structure around the plate interface has been accumulated by analyses of high-quality data from high-density seismograph networks in inland areas and from joint seismic explorations of the sea and land. Moreover, knowledge regarding a wide variety of newly detected slow earthquakes has contributed toward our understanding of the subducting plate interface. Additional information about underground structures, such as slab segmentations, is expected to provide a better understanding of the occurrence of megathrust earthquakes.

Geodetic Measurements of Slow‐Slip Events Southeast of Parkfield, CA

Article

Full-text available

May 2020

Tremor and low‐frequency earthquakes are presumed to be indicative of surrounding slow, aseismic slip that is often below geodetic detection thresholds. This study uses data from borehole seismometers and long‐baseline laser strainmeters to observe both the seismic and geodetic signatures of episodic tremor and slip on the Parkfield region of the San Andreas Fault near Cholame, CA. The observed occurrence rates of both the tremors and co‐located families of low‐frequency earthquakes are not steady but instead exhibit quasiperiodic bursts of increased activity. We show that these periods of elevated seismic activity correlate with statistically significant stacked strain signals consisting of 44 slow‐slip events. Modeled individual slow‐slip events and their total summed moment, which are constrained by seismic signals and stacked strain, respectively, indicate that the individual moment magnitudes of these events range from Mw 4.6–5.2. We find that the measured geodetic signal likely precedes the seismic signal by several hours, consistent with the aseismic slip preceding and driving the observed seismic tremor activity. We confirm that strike‐slip faults, in addition to subduction zones, are capable of producing episodic tremor and slip.

Slow Slip Events in New Zealand

Article

May 2020

Laura Wallace

Continuously operating global positioning system sites in the North Island of New Zealand have revealed a diverse range of slow motion earthquakes on the Hikurangi subduction zone. These slow slip events (SSEs) exhibit diverse characteristics, from shallow (<15 km), short (<1 month), frequent (every 1–2 years) events in the northern part of the subduction zone to deep (>30 km), long (>1 year), less frequent (approximately every 5 years) SSEs in the southern part of the subduction zone. Hikurangi SSEs show intriguing relationships to interseismic coupling, seismicity, and tectonic tremor, and they exhibit a diversity of interactions with large, regional earthquakes. Due to the marked along-strike variations in Hikurangi SSE characteristics, which coincide with changes in physical characteristics of the subduction margin, the Hikurangi subduction zone presents a globally unique natural laboratory to resolve outstanding questions regarding the origin of episodic, slow fault slip behavior. ▪ New Zealand's Hikurangi subduction zone hosts slow slip events with a diverse range of depth, size, duration, and recurrence characteristics. ▪ Hikurangi slow slip events show intriguing relationships with seismicity from small earthquakes and tremor to larger earthquakes. ▪ Slow slip events play a major role in the accommodation of plate motion at the Hikurangi subduction zone. ▪ Many aspects of the Hikurangi subduction zone make it an ideal natural laboratory to resolve the physical processes controlling slow slip. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The slow deformation waves as a possible precursor of seismic hazard

Article

Jan 2019

Simulating Short-Term Evolution of Slow Slip Influenced by Fault Heterogeneities And Tides: Simulating Short-Term Evolution of Slow Slip

Article

Sep 2018

In this study, we analyze high-resolution tremor catalogs from northern Cascadia, Guerrero, and northern Kii Peninsula. We find that tremor often occurs in short bursts that repeatedly occupy the same source area within a slow slip event. We hypothesize that these bursts are driven by loading from slow slip in areas surrounding the tremor zone. Adopting a rate-and-state friction law with a velocity-weakening to velocity-strengthening transition, we develop a finite fault model in which the size of the slow slip zone is larger than that of the tremor zone. Tidal forcing is added. Many asperities are randomly distributed within the tremor zone in order to generate burst-like slip evolution while maintaining reasonable propagation speeds of the main slow slip front. We successfully reproduce the increasing recurrence intervals of the bursts as the main front moves across the tremor zone, as well as tidally modulated secondary fronts well behind the main front.

Episodic Aseismic Slip at Plate Boundaries

Chapter

Jan 2015

S.Y. Schwartz

Slip in earthquakes accounts for only a fraction of plate tectonic displacements. Until recently, the aseismic component of plate motions had been assumed to take place primarily as steady-state creep, although no direct observations of creep at depth exist. This chapter summarizes the accumulating evidence that a large fraction of aseismic slip occurs episodically rather than continuously. Several types of episodic aseismic slip processes have been documented, distinguished by when they occur in the earthquake cycle (precursory, interseismic, or afterslip) and what they represent: deviations in slip rate from long-term rates or discrete slow slip events (SSEs). Interseismic SSEs at convergent margins are the most commonly observed mode of episodic aseismic motion. SSEs at several subduction zones correlate with seismic tremor, a signal that is commonly associated with active volcanoes. Given their correspondence, it is likely that slow slip and tremor are manifestations of the same process. Aseismic slip events concentrate near Earth's surface, at the up- or down-dip edge of the seismogenic zone, or at complementary locations to strongly locked patches or co-seismic asperities within the seismogenic zone. The nucleation of aseismic slip events at frictional transitions is supported by rate- and state-dependent frictional modeling. The fairly common occurrence of aseismic slip following moderate to large earthquakes (afterslip) suggests a relationship between higher velocity ruptures (earthquakes) and slow slip. Although isolated episodes of interseismic slow slip can be related to nearby earthquake activity, a consistent triggering pattern has yet to emerge and further study is clearly warranted.

New, improved version of the Generic Mapping Tool released

Article

Full-text available

Nov 1998

New, improved version of Generic Mapping Tools released

Article

Full-text available

Nov 1998
Eos Trans. AGU

Version 3.1 of the Generic Mapping Tools (GMT) has been released. More than 6000 scientists worldwide are currently using this free, public domain collection of UNIX tools that contains programs serving a variety of research functions. GMT allows users to manipulate (x,y) and (x,y,z) data, and generate PostScript illustrations, including simple x-y diagrams, contour maps, color images, and artificially illuminated, perspective, and/or shaded-relief plots using a variety of map projections (see Wessel and Smith [1991] and Wessel and Smith [1995], for details.). GMT has been installed under UNIX on most types of workstations and both IBM-compatible and Macintosh personal computers.

Katsumata, A. & Kamaya, N. Low-frequency continuous tremor around the Moho discontinuity away from volcanoes in the southwest Japan. Geophys. Res. Lett. 30 doi:10.1029/2002GL015981

Article

Full-text available

Jan 2003
GEOPHYS RES LETT

Recent enhancement of seismic networks in the Japan Islands revealed occurrence of low-frequency continuous tremors of a beltlike distribution in the southwest Japan, where the subducting Philippine Sea plate reaches depths of 30-40 km. Source depth of the tremor is estimated by selecting tremor segments with relatively clear P-wave onsets. The source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust, mantle wedge, and the subducting slab. The long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor. The most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments. The northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water.

Surface deformation to shear and tensile faults in a halfspace

Article

Full-text available

Aug 1985

Yoshimitsu Okada

A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent power- ful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.

A decade of GEONET: 1994-2003-The continuous GPS observation in Japan and its impact on earthquake studies

Article

Full-text available

Aug 2004
EARTH PLANETS SPACE

Takeshi Sagiya

The dense continuous GPS network of Japan, now called GEONET, has been operated since 1994 by the Geo-graphical Survey Institute. GEONET provides precise daily coordinates of all the stations, with which displacement rates and strain rates are calculated nationwide. Various characteristics of tectonic deformation in the Japanese Is-lands have been revealed. GEONET is also quite useful in earthquake studies, precisely detecting co-seismic, post-seismic, and inter-seismic deformation signals. These observations are utilized to infer physical processes at earthquake sources. Slow slip events on plate boundaries have been found from GPS data. Such slow events provide an important constraint on the mechanism of faulting. On the other hand, there has been no success in detecting pre-seismic deformation. Lack of a precursory signal before the 2003 Tokachi-Oki (M8.0) earthquake has posed a serious question to short-term earthquake prediction. GEONET enables a good linkage between monitoring and modeling studies, opening a possibility of practical data assimilation. For further contribution to earthquake studies, it is necessary to continue GEONET with high traceability on the details in observation and analysis.

A slow thrust slip event following the two 1996 Hyuganada Earthquakes beneath the Bungo Channel, southwest Japan

Article

Full-text available

Nov 1999

We report a "slow thrust slip event" that occurred beneath the Bungo Channel region, southwest-ern Japan. On Oct. and Dec., 1996, two Hyuganada earthquakes (both M w = 6.7), followed by afterslips, occurred. In addition, a crustal movement character-ized by an extremely slow rise was observed around the Bungo Channel, about 200 km north from the epicen-ters, and continued for about 300 days long. Assuming a slow slip on the plate boundary, we estimate its du-ration and surface displacements from GPS time series data by curve-fitting, and then, determine the fault slip distribution. We found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the Hyuga-nada earthquakes. Occurrence of the slow thrust slip event suggests that this kind of event may be a charac-teristic mode of stress release at a transition region of interplate coupling.

Low-frequency tremors, intraslab and interplate earthquakes in Southwest Japan - From a viewpoint of slab dehydration

Article

Full-text available

Nov 2003

1] Low-frequency tremors (LFT) recently found in the Southwest Japan fore-arc likely occur due to hydro-fracturing from fluids leaving the dehydrating Philippine Sea slab. However, there are some places without such LFT; they are N. Izu-Kanto, E. Shikoku, and S. Kyushu. These are the places where island-arc type crust is subducting. We propose that dehydration of the subducted crust does not occur beneath these regions, because it is composed mainly of tonalite, lacking the quantity of hydrous minerals seen in normal subducting oceanic crust. We show that almost no earthquake occurs within the subducted crust in such regions, consistent with dehydration embrittlement hypothesis for intraslab seismicity. The lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes, because pore fluid pressure is more difficult to rise at the thrust zone. This might be reflected in the irregular occurrence of great earthquakes in these regions. INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7220 Seismology: Oceanic crust; 7230 Seismology: Seismicity and seismotectonics; 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 8123 Tectonophysics: Dynamics, seismotectonics. Citation: Seno, T., and T. Yamasaki, Low-frequency tremors, intraslab and interplate earthquakes in Southwest Japan—from a viewpoint of slab dehydration, Geophys. Res. Lett., 30(22), 2171, doi:10.1029/2003GL018349, 2003.

Repeating short- and long-term slow slip events with deep tremor activity around the Bungo Channel region, southwest Japan

Article

Full-text available

Oct 2005

We report the repeating occurrence of short-and long-term slow slip events (SSE) which are accompanied by deep tremor activity around the Bungo channel region, southwest Japan. Both of these activities are detected by NIED Hi-net, which is composed of densely distributed observatories equipped with a set of tiltmeter and a high-sensitivity seismograph. Since the short-term SSE is small in magnitude, GPS can detect only the long-term SSE. Some of these episodes have nearly the same surface deformation pattern. This shows the existence of 'slow slip patches' on a plate interface, where the episodic slow slip is the characteristic slip behavior. We observe a change in periodicity and size of the short-term episode after the onset of the long-term SSE. Moreover, the long-term slow slip accelerates when the short-term activity takes place. This suggests that there is an interaction between these two types of SSEs.

Recent progress of seismic observation networks in Japan—Hi-net, F-net, K-NET and KiK-net

Article

Full-text available

Aug 2004
EARTH PLANETS SPACE

After the disastrous 1995 Kobe earthquake, a new national project has started to drastically improve seismic observation system in Japan. A large number of strong-motion, high-sensitivity, and broadband seismographs were installed to construct dense and uniform networks covering the whole of Japan. The new high-sensitivity seismograph network consisting of 696 stations is called Hi-net, while the broadband seismograph network consisting of 71 stations is called F-net. At most of Hi-net stations strong-motion seismographs are also equipped both at depth and the ground surface. The network of these 659 stations with an uphole/downhole pair of strong-motion seismographs is called KiK-net, while another network consisting of 1034 strong-motion seismographs installed at the ground surface is called K-NET. Here, all the station numbers are as of April 2003. High-sensitivity data from Hi-net and pre-existing seismic networks operated by various institutions have been transmitted to and processed by the Japan

A Silent Slip Event on the Deeper Cascadia Subduction Interface

Article

Full-text available

Jun 2001

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore. In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion. No seismicity was associated with this event. The sudden displacements are best explained by approximately 2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses.

Periodic Slow Earthquakes from the Cascadia Subduction Zone

Article

Full-text available

Apr 2002
SCIENCE

Hi-net: High sensitivity seismograph network, Japan

Article

Jan 2002

K. Obara

Geometry of the seismic Philippine Sea slab beneath the region from Ise Bay to western Shikoku southwest Japan [in Japanese with English abstract]

Article

Dec 2004

We inferred minutely the geometry of the upper surface of the seismic Philippine Sea (PHS) slab beneath the region from Ise Bay to western Shikoku, southwest Japan, based on hypocentral distribution. Despite the importance of the slab geometry for the subduction zone seismotectonics as well as for the assessment and forecasting of destructive interplate and slab earthquakes, the shape of the PHS slab has been obscure and controversial. We selected well-determined hypocenters during the period from Oct. 1997 through Dec. 2002 from the integrated hypocenter database prepared by the Japan Meteorological Agency. At first, we composed depth data set of the slab upper surface from vertical cross sections of hypocentral distribution along longitude and latitude of every 0.1 degree, and drew first-trial isodepth contours of the upper surface of the PHS slab assuming that the upper boundary of slab earthquake distribution roughly coincides with the slab upper surface. Then, in many areas we made vertical cross sections of hypocenter distribution in many directions including inferred dip-direction and strike of the slab, and revised isodepth contours. The main results are as follows: (1) In the region to the east of Lake Biwa, northwestward gently-dipping PHS slab reaches around the depth of 35km, in which disastrous slab earthquakes presumably took place. (2) To the northeast it continues to the slab beneath the central Tokai district, but to the southwest it is obscure whether it continues to the slab beneath the eastern Kii Peninsula or it is separated from the latter. (3) The slab beneath the middle part of the Kii Peninsula may be torn into two parts, with the southwestern part underlying the northeastern part. (4) There are double seismic zones beneath the southern Kii Peninsula. (5) In the west part of the Kii Peninsula no slab earthquake occurs in deeper part, whereas just in this area a remarkable shallow crustal swarm earthquake activity exists. (6) Beneath Shikoku the slab is being subducted with gentle dip angle and forms a broad ridge. The slab looks continuous from Shikoku to Kyusyu. Recently the PHS slab geometry has been investigated by seismic refraction/reflection profilings and receiver function analyses. Those results are shallower than our result in eastern Shikoku suggesting that slab earthquakes in this region occur in the oceanic mantle. We relocated slab earthquakes in this region using a velocity structure model consistent with the result of seismic profiling and found that the shallower main activity lays in the slab mantle of the assumed model with deeper remarkable events beneath it. In order to clarify the true slab geometry it is essentially important to know in which part of the slab, oceanic crust or mantle, slab earthquakes are actually taking place in each region.

Seismicity and a focal mechanism of low-frequency earthquakes occurring in the western part of Tochigi prefecture

Article

Aug 1996

Low-frequency earthquakes lying near the Moho discontinuity have been, recently, observed just beneath the volcanic front. In this paper, we analyzed seismograms recorded in the western part of Tochigi prefecture from September 1992 to January 1993 and from July 1994 to January 1995, in order to reveal the characteristic and seismicity of low-frequency earthquakes in this region. The seismograms of the low-frequency earthquake on November 17, 1992, are also analyzed by using a waveform inversion method to get a focal mechanism of low-frequency earthquake. Major results are as follows. 1) 269 low-frequency earthquakes were recorded for 15 months in the western part of Tochigi prefecture, and their activity is similar to an earthquake swarm. 2) The depths of low-frequency earthquakes during September, 1992-January, 1993 range from 39 km to 30 km, but most of the events occurred about 30 km deep. On the other hand, all low-frequency earthquakes which occurred during July, 1994-January, 1995 lay at 27 km deep. 3) There are two groups of low-frequency earthquake family composed by earthquakes having waveforms similar to each other. The hypocenters of earthquake family migrated about 1 km from west to east during August, 1994 to January, 1995. 4) The low-frequency earthquake of November 17, 1992 has a focal mechanism of strike-slip type with a maximum compression axis directed west-northwest to east-southeast which is consistent with a regional tectonic stress. The source models composed by a single force, an open crack, or CLDV can not explain the observed seismic waveforms of this low-frequency earthquake.

Internal deformation due to shear and tensile faults in a half-space[J]

Article

Apr 1992

Yoshimitsu Okada

A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. These expressions are particularly compact and systematically composed of terms representing deformations in an infinite medium, a term related to surface deformation and that is multiplied by the depth of observation point. Several practical suggestions to avoid mathematical singularities and computational instabilities are also presented. The expressions derived here represent powerful tools both for the observational and theoretical analyses of static field changes associated with earthquake and volcanic phenomena.

Low Frequency Earthquakes around Moho beneath the Volcanic Front in the Kanto District, Central Japan

Article

Dec 1993

Activity of low frequency (LF) earthquakes in the depth range from 25 to 50 km was investigated in central Japan, by using data obtained by the seismic network of National Research Institute for Earth Science and Disaster Prevention. A total of 15 LF events were found in five regions, southwest Fukushima Pref., north Tochigi Pref., west Tochigi Pref,. central Gunma Pref., and Izu-Oshima, from 1980 to 1992. All of the LF earthquakes are located along the volcanic front in the Kanto district. Predominant frequencies of the LF earthquakes, magnitudes of which range from 1.4 to 2.7, are 1 to 5 Hz, significantly lower than those of typical microearthquakes of the same magnitude range. The seismograms are dominated by clear P and S phases, S amplitudes being larger than P amplitudes at most of the stations. P waves of the LF earthquakes are often contaminated by high frequency waves at close stations. In most cases, more than two earthquakes successively occurred in a short time. Spectra of the LF earthquakes in southwest Fukushima, west Tochigi and Izu-Oshima exhibit a single strong peak at almost same frequency for P and S waves, while those in the other regions show more complicated structure, presumably reflecting complicated spectra at the sources. Although source process of the LF earthquakes are not modeled in this study, large amplitude ratios of P and S phases, which are estimated to be 5.6 in average, suggest a contribution of shear mode rather than tensile mode at the sources.

Low-frequency continuous tremor around the Moho discontinuity away from volcanoes in the southwest Japan

Article

Jan 2003

Activity of Deep Low-frequency Microearthquakes and Their Moment Tensors in Northeastern Japan

Article

May 2000

Most of shallow inland earthquakes in northeastern Japan are distributed at depths shallower than 15 km, but some occur at anomalously large depths (20-45 km). They have low predominant frequencies for both P and S waves. We confirmed these deep low-frequency microearthquakes (deep LF events) and relocated swarms of deep LF events in the period from 1976 to May 1999 by station corrections obtained for events located accurately using dense seismic network. They are isolated from normal shallow earthquakes and occur beneath active volcanoes and/or in or around P wave low-velocity zones. The seismic activity of deep LF events varies in time. Most seismically active areas for deep LF events are two areas near Iwate volcano and Kurikoma/Narugo volcanoes, where large earthquakes with magnitudes 6.1 and 5.9 occurred recently. We determined moment tensors of three deep LF events near Narugo volcano by waveform inversion. Some of these include non-double couple components.

Geometry of the Subducting Philippine Sea Plate and the Crustal and Upper Mantle Structure beneath Eastern Shikoku Island Revealed by Seismic Refraction/Wide-angle Reflection Profiling

Article

Apr 2002

Large earthquakes occur along the Nankai trough, SW Japan, where the Philippine Sea plate is subducting beneath southwestern Japan arc. To understand tectonics related to the occurrence of these large earthquakes, we need to elucidate crustal and upper mantle structures of the subducting Philippine Sea plate and the overlying southwestern Japan arc. In the summer of 1999, we conducted a highly dense onshore-offshore integrated seismic experiment in the eastern part of Shikoku Island and the adjacent Nankai trough, SW Japan. Controlled seismic signals both from land explosives and air-gun sources were recorded by the land seismic stations. We obtained high signal-to-noise ratio data along the entire length of the land profile. Analyzing these explosive and air-gun data recorded by the land seismic stations, we could obtain a detailed geometry of the subducting Philippine Sea plate and the crustal and upper mantle structure beneath eastern Shikoku Island. The signals from the air-gun source are especially useful to reveal a structure of an ocean-continental transition zone, which is not obtained using only land explosive data. The uppermost crust beneath eastern Shikoku Island is covered with a surface layer with velocities of 4.0-5.0km/s. The surface velocity shows a remarkable lateral change at the geological boundary between granitic rocks and an accretionary belt. The velocity of the uppermost crust is obtained as 5.7km/s. The island arc Moho is about 33km deep beneath northeastern Shikoku Island and the crustal thickness seems to thin toward the north. Beneath the southern edge of the land profile, the top of the subducting Philippine Sea plate is located at a depth of about 18km with a dip angle of approximately 12 degrees. The subduction angle steepens beneath this point. The top of the subducting plate can be traced to a depth of about 30km, almost parallel to the Wadati-Benioff seismic plane. This subduction angle is shallower than that beneath the Kii peninsula. Hypocenter distribution associated with the underthrusting of the Philippine Sea plate beneath eastern Shikoku Island is located beneath the subducting oceanic Moho. The seismicity underlies a contact zone between the nreanic and island-arc crusts.

Surface Deformation due to Shear and Tensile Faults in a Half Space

Article

Jan 1992

Yoshimitsu Okada

A complete set of closed analytical expressions is presented in a unified manner for the internal displacements and strains due to shear and tensile faults in a half-space for both point and finite rectangular sources. Several practical suggestions to avoid mathematical singularities and computational instabilities are presented. -from Author

Materials Science and Seismological Approaches to Understanding Seismogenic Processes Time Sequence of Deep Low-frequency Tremors in the Southwest Japan Subduction Zone: Triggering Phenomena and Periodic Activity

Article

Jan 2003

Kazushige Obara

Non-volcanic deep low-frequency tremors detected in southwest Japan are distributed in the forearc side along the strike of the descending Philippine Sea plate. The source depth of the tremor corresponds to the slab interface or the Moho discontinuity. The time sequence of the tremor activity is characterized by long durations from hours to weeks. The mobility and the successive occurrence of the tremor are thought to be related to the existence of fluid liberated from the slab by a dehydration process. The spatial distribution of the tremors is not homogeneous in a narrow belt but is spatially clustered. The major activity of the tremors with relatively long time durations is also clustered periodically, with a period of 2-3 months in the east and middle of Shikoku area and about 6 months in the west of Shikoku. On the other hand, tremors are sometimes triggered by local earthquakes or teleseismic waves. The periodicity of the tremor activity may represent a stable accumulation of fluid with a stable subduction process and the triggering phenomenon implies the unstable condition of the occurrence of tremors.

Deep Low-Frequency Earthquakes Associated With Active Faults in Central - Southwest Japan

Article

Dec 2001

S. Ohmi

In previous studies, deep low-frequency (DLF) earthquakes have been discussed in association with fluid (magma) activity around volcanoes. There are several examples of DLF activity beneath active faults in central - southwest Japan. In some cases, there are no volcanoes in the vicinity of the faults. On October 6, 2000, the Western Tottori Earthquake (Mj7.3) occurred in the Chugoku district, southwest Japan. In the focal region, several DLF earthquakes were observed prior to the earthquake and the DLF activity increased after the mainshock (Ohmi & Obara, 2001). The events are located at 25 - 35 km depth in the focal region with a magnitude range up to 1.8. There is a Quaternary volcano (Mt. Daisen) 25 km away. In central Kyoto prefecture, southwest Japan, DLF earthquakes have been observed since 1978. From 1998 through August 2001 there have been 15 DLF events with magnitudes up to 1.6. They are located below the Mitoke Fault system at depths of 30 - 35 km. Continuous shallow swarm activity is also observed in the region and an Mj5.1 earthquake occurred in August 2001. There are no volcanoes in the area. In central Japan, DLF earthquakes were mainly concentrated in the Hida Mountain range (Japan Alps). One prominent area of activity is near the northern-east part of the Atotsugawa fault, where the Atotsugawa fault enters the region of volcanoes in the Hida Mountain range. In this area the DLF events have magnitudes up to 2.3 with focal depths around 25 - 35 km. Mt. Tateyama, which is an active volcano, is located 10 km from the focal region of the DLF's. There is also DLF activity beneath an active fault in Toyama prefecture in central Japan, an area without volcanoes in the near vicinity. Seismic tomography analysis (e.g. Zhao et al., 2000) indicates the existence of a low-velocity body at depths from the lower-crust to the upper-mantle in the focal region of these DLF's. The distribution of DLF activities, described above, corresponds well to the low-velocity region rather than to the distribution of volcanoes. The DLF occurences are probably direct evidence of fluid activity in the seismogenic zone, which might be injected from the lower-crust beneath active faults.

Configuration of the Subducting Philippine Sea Plate and the Crustal and Upper Mantle Structure Beneath the southwestern Japan arc, Revealed by Seismic Refraction/Wide-angle Reflection Profiling

Article

Dec 2003

The Nankai trough region, where the Philippine Sea Plate is subducted beneath the SW Japan arc, is a well-known seismogenic zone of interplate earthquakes. A detailed crustal and upper mantle structures of the subducted Philippine Sea Plate and the overlying SW Japan arc is inevitably important to constrain the physical process of earthquake occurrence as well as the evolution process of this margin. In the summer of 2002, we conducted an onshore-offshore seismic experiment across the SW Japan arc. In this experiment, about 2,300 seismometers were deployed on a 235-km-long onshore line with NS direction, on which 10 explosives shots were fired. Data collected from on the onshore line have high signal-to-noise ratios, from which we can easily recognize prominent wide-angle reflections from the deep crust and the subducted plate boundary. Analyzing these explosive data, we could obtain a detailed configuration of the subducting Philippine Sea plate and the crustal and upper mantle structure beneath the SW Japan arc. Beneath the southern edge of the land profile, the top of the subducting Philippine Sea plate is located at a depth of about 18 km with a dip angle of approximately 12 degrees. The top of the subducting plate can be traced to a depth of about 35 km. The island arc Moho is about 33 km deep beneath northeastern Shikoku Island and the crustal thickness is thinning toward the north.

Characteristic silent earthquakes in the eastern part of the Boso peninsula, Central Japan

Article

Mar 2003
GEOPHYS RES LETT

Analysis of Global Positioning System (GPS) data shows transient crustal deformation in the Boso peninsula, central Japan, for about fifty days from October 2002. The southeastward horizontal displacements detected suggest the occurrence of interplate aseismic slip between the Philippine Sea plate and the North American plate, six years after a similar event in 1996. We estimate an area of aseismic slip with moment magnitude (Mw) 6.6 centered off the Boso peninsula, adjacent to associated seismic activity. Slip evolution of the 2002 aeismic slip started on October 4 and migrated from north to south for about ten days, followed by gradual subsidence and restarting of slip which lasted until ∼December 2, propagating more southward. Similarities in focal area and slip process between the 1996 and the 2002 events suggest the Boso aseismic slips as characteristic silent earthquakes, together with the repetitive occurrence of seismic swarm in a similar region every six to seven years. A more tightly coupled southern area that did not release enough energy in the 1996 event may have caused the differences in magnitude, central slip area, and duration between the 1996 and the 2002 events, if the hypothesized recurrence interval for strain accumulation is true, based on swarm seismicity.

Kinetics of Hydrofracturing and Metamorphic Veining

Article

Jan 1989
GEOLOGY

Tadao Nishiyama

Veins in metamorphic rocks result from the interplay of two processes: deformation and mineral reactions. Two kinds of veins have been recognized: segregation and hydrofracturing. This paper examines quantitatively the conditions for hydrofracturing and associated vein formation due to devolatilization reactions. The inflection point of the reaction curve defines a boundary between the reaction-enhanced ductility regime (high-pressure side) and the hydrofracturing regime in a pressure-temperature diagram. Low strain rates and low permeability of rocks, together with the irreversible reaction, are important factors for hydrofracturing. Combination of reaction kinetics and percolation theory enables us to estimate the time required for macroscopic hydrofracturing. Model calculations are carried out at 1 kbar for the reaction serpentine + brucite = forsterite + water. It takes several tens of years for macroscopic hydrofracturing to develop. The result depends on rock texture (the lattice type in percolation theory) and superheating degrees; however, hydrofracturing will generally not exceed 200 yr.

Aseismic slip and low-frequency earthquakes in the Bungo Channel, southwestern Japan

Article

Apr 2004

Analysis of Global Positioning System (GPS) data has shown transient crustal deformation in the Bungo channel, in southwestern Japan, since late August 2003, six years after a similar event in 1996-1997. Considering the southeastward motion at many of the GPS sites and the similarity between the 1996-1997 and 2003 events, it is highly likely that the transients were caused by aseismic interplate slip between the Philippine Sea plate and the Amurian plate. The 2003 aseismic slip is estimated beneath the Bungo channel and the relative slip increased with time on the west side, with a moment magnitude (Mw) amounting to 7.0. In contrast to the gradual subsidence of low-frequency earthquakes which began and ended in coincidence with the 2003 event, the 2003 transient crustal deformation proceeded at a roughly constant rate until the end of November 2003. The Bungo channel area may release energy accumulated from the subduction of the Philippine Sea plate by aseismic slip.

Configuration of subducting Philippine Sea plate beneath southwest Japan revealed from receiver function analysis based on the multivariate autoregressive model

Article

Apr 2004

We construct a detailed image of velocity discontinuity beneath southwest Japan by using receiver functions. We first propose an improved receiver function estimation method based on the statistical multivariate autoregressive model. Then we apply the new method to the teleseismic waveform data recorded by the high-density seismograph network in southwest Japan. The results show a clear velocity discontinuity at 30 km depth beneath the southern Shikoku region. This discontinuity, corresponding to the boundary between the oceanic crust and the high-velocity layer (the oceanic Moho) of the Philippine Sea plate (PHS), continues down to the north indicating that the aseismic PHS extends to the central Chugoku region. The continental Moho is also clearly imaged beneath the Chugoku region. The depth contour of the PHS shows a rather complicated feature with ridges and valleys. The most significant ridge is located around longitude 133°E, west of which the contours are basically directed north-south, changing to west-east to the east. Beneath the western part of this ridge, hypocenters of microearthquakes are located above this velocity discontinuity, while earthquakes mainly occur below the discontinuity, within the oceanic mantle, beneath the eastern part of the ridge.

Repeating earthquake activities associated with the Philippine Sea Plate subduction in the Kanto district, central Japan: A new plate configuration revealed by interplate aseismic slips

Article

Apr 2006
TECTONOPHYSICS

We detect repeating earthquakes associated with the Philippine Sea plate subduction to reveal the plate configuration. In the Kanto district, we find 140 repeating earthquake groups with 428 events by waveform similarity analysis. Most repeating earthquakes in the eastern part of the Kanto district occur with a regular time interval. They have thrust-type focal mechanisms and are distributed near the upper surface of the Philippine Sea plate. These observations indicate that the repeating earthquakes there occur as a repetition of ruptures on the isolated patches distributed on the plate boundary owing to the concentration of stress caused by aseismic slips in the surrounding areas. This shows that the distributions of repeating earthquakes suggest the aseismic slips in the surrounding areas of small patches. We determine spatial distributions of repeating earthquakes in the eastern part of the Kanto district and find that they correspond to the upper boundary of the Philippine Sea plate, that is, the upper boundary of the oceanic crust layer of the Philippine Sea plate. The plate geometry around Choshi is newly constrained by repeating earthquake data and a rather flat geometry in the eastern part of the Kanto district is revealed. The obtained geometry suggests uplift of the Philippine Sea plate due to the collision with the Pacific plate beneath Choshi.Repeating earthquakes in the western part of the Kanto district have extremely shorter recurrence times, and their focal mechanisms are not of the thrust types. These repeating earthquakes are classified as “burst type” activity and likely to occur on the preexistent fault planes which are distributed around the “collision zone” between the Philippine Sea plate and the inland plate. The variation among the repeating earthquake activities in the Kanto district indicates that regular repetition of repeating earthquakes is possible only on the plate boundary with a smooth and simple geometry.

Structure of the upper part of the Philippine Sea plate estimated by later phases of upper mantle earthquakes in and around Shikoku, Japan

Article

May 2000
TECTONOPHYSICS

Takahiro Ohkura

In the Chugoku, Kinki and Shikoku districts, southwest Japan, a distinct pair of later P and S phases is frequently observed after P and S phases for earthquakes at depths less than 60km occurring in the vicinity of the west Seto Inland Sea, caused by the subduction of the Philippine Sea (PHS) plate. The observed phases are dominant in all three components with an apparent velocity almost equal to the P and S velocity of the lower crust. If the upper mantle earthquakes take place within a low-velocity oceanic crust overlying the high-velocity PHS plate, and if this oceanic crust is in contact with the continental lower crust between the sources and the receivers, waves traveling through the oceanic and the lower crust can be observed as later phases. Travel times and synthetic waveforms were calculated using a three-dimensional ray tracing method and a three-dimensional Gaussian-beam method. The calculated travel times and amplitudes of the later phases are in good accordance with the observed values. These indicate that in the subducting oceanic crust, a gabbroic phase remains without transformation to eclogitic rocks to depths of about 60km. Similar later phases are observed in the Chugoku district from the upper mantle earthquakes beneath the Shikoku district. These indicate that the continental lower crust is in contact with the subducting oceanic crust beneath Shikoku and that no mantle wedge exists.

Characteristic activity of subcrustal earthquakes along the outer zone of southwestern Japan

Article

Jan 1997

Deep low-frequency earthquakes beneath the focal area of the Mw 6.7 2000 Western Tottori earthquake

Article

Aug 2002

On October 6, 2000, an Mw = 6.7 crustal earthquake occurred in western Tottori prefecture, southwest Japan. Beneath the focal region of the earthquake, deep low-frequency (DLF) earthquakes were observed at depths of around 30 km. Five DLF earthquakes were detected within 3 years before the mainshock and more than 60 DLF earthquakes were observed during the 13 months after the mainshock. We investigated the focal mechanism of the DLF earthquake that occurred 9 hours before the mainshock, using amplitude ratios of the S-waves to the P-waves and polarization patterns of the S-waves. Both analyses indicated that a single-force source mechanism is more preferable than a double-couple source mechanism, which suggests the transport of fluid such as water or magma. This event is probably another example of DLF earthquakes that occur beneath active fault zones.

Time Sequence of Deep Low-frequency Tremors in the Southwest Japan Subduction Zone : Triggering Phenomena and Periodic Activity

Article

Jan 2003

Kazushige OBARA

Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone

Article

Dec 2004

1] Episodic slow slip events have been recognized by means of tilt changes in the western Shikoku area, southwest Japan. The crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non-volcanic deep tremor activities in this area. Observed tilt changes can be explained by slow slip events occurring around the source area of tremors. In each episode, the source of the slow slip event and tremor migrate simultaneously. The spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 7230 Seismology: Seismicity and seismotectonics; 8045 Structural Geology: Role of fluids; 8150 Tectonophysics: Plate boundary—general (3040); 8159 Te ct o n op h y s i c s : R he o l o gy — c r u s t a n d l i t h o s ph e r e . Citation: Obara, K., H. Hirose, F. Yamamizu, and K. Kasahara (2004), Episodic slow slip events accompanied by non-volcanic tremors in southwest Japan subduction zone, Geophys. Res. Lett., 31, L23602, doi:10.1029/2004GL020848.

The seismogenic zone of subduction thrust faults

Article

Sep 1997
ISL ARC

Abstract Subduction thrust faults generate earthquakes over a limited depth range. They are aseismic in their seaward updip portions and landward downdip of a critical point. The seaward shallow aseismic zone, commonly beneath accreted sediments, may be a consequence of unconsolidated sediments, especially stable-sliding smectite clays. Such clays are dehydrated and the fault may become seismogenic where the temperature reaches 100--150°C, that is, at a 5--15 km depth. Two factors may determine the downdip seismogenic limit. For subduction of young hot oceanic lithosphere beneath large accretionary sedimentary prisms and beneath continental crust, the transition to aseismic stable sliding is temperature controlled. The maximum temperature for seismic behavior in crustal rocks is ∼ 350°C, regardless of the presence of water. In addition, great earthquake ruptures initiated at less than this temperature may propagate with decreasing slip to where the temperature is ∼ 450°C. For subduction beneath thin island arc crust and beneath continental crust in some areas, the forearc mantle is reached by the thrust shallower than the 350°C temperature. The forearc upper mantle probably is aseismic because of stable-sliding serpentinite hydrated by water from the underthrusting oceanic crust and sediments. For many subduction zones the downdip seismogenic width defined by these limits is much less than previously assumed. Within the narrowly defined seismic zone, most of the convergence may occur in earthquakes. Numerical thermal models have been employed to estimate temperatures on the subduction thrust planes of four continental subduction zones. For Cascadia and Southwest Japan where very young and hot plates are subducting, the downdip seismogenic limit on the subduction thrust is thermally controlled and is shallow. For Alaska and most of Chile, the forearc mantle is reached before the critical temperature, and mantle serpentinite provides the limit. In all four regions, the seismogenic zones so defined agree with estimates of the extent of great earthquake rupture, and with the downdip extent of the interseismic locked zone.

Deep, low-frequency microearthquakes in or around seismic low-velocity zones beneath active volcanos in northeastern Japan

Article

May 1994
TECTONOPHYSICS

Precise hypocenter relocations have been made for shallow intraplate microearthquakes beneath northeastern Japan by applying source-region station corrections. The relocated hypocenter distribution shows that most of the events are confined to the upper 15 km of the crust which forms the brittle seismogenic zone in this volcanic arc. It also shows that exceptionally deep microearthquakes, well below the base of the seismic-aseismic boundary, occur at twelve locations of northeastern Japan. They occur in the lowermost crust or in the uppermost mantle, where rheological properties of rocks are presumed to be in the ductile regime. All the 153 events presently detected in the lowermost crust or in the uppermost mantle have anomalously low predominant frequencies (1–5.5 Hz) both for P and S waves, in contrast to those (8–20 Hz) of normal shallow events in the brittle seismogenic zone. The magnitudes of these events are small (M ⩽ 2.2). Moreover, all of these events are located beneath active volcanoes and/or near P wave low-velocity zones in the uppermost mantle. The proximity of these events to the active volcanoes and other anomalous features suggest that these deep low-frequency events are generated by magmatic activity of mantle diapirs in the uppermost mantle.

Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan

Article

Jun 2002
SCIENCE

Kazushige Obara

Deep long-period tremors were recognized and located in a nonvolcanic region in southwest Japan. Epicenters of the tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 kilometers. The depth of the tremors averaged about 30 kilometers, near the Mohorovic discontinuity. Each tremor lasted for at most a few weeks. The location of the tremors within the subduction zone indicates that the tremors may have been caused by fluid generated by dehydration processes from the slab.

Detection and Monitoring of Ongoing Aseismic Slip in the Tokai Region Central Japan

Article

Dec 2002
SCIENCE

Analysis of global positioning system data shows that the rate of crustal deformations in the Tokai region of Japan, a seismic gap area, changed over the past 18 months. Kalman filtering analysis shows aseismic slip on the plate boundary in the western Tokai region centered on Lake Hamana, adjacent to the anticipated Tokai earthquake source area. The cumulative moment magnitude reaches 6.7 in June 2002 with a relative slip increase northeast of Lake Haman from January 2002. An existence of aseismic slip in the western Tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 Tonankai earthquake.

Episodic Tremor and Slip on the Cascadia Subduction Zone: The Chatter of Silent Slip

Article

Jul 2003
SCIENCE

We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.

Periodic slow earthquakes from the Cascadia subduction zone Geometry of the seismic Philippine Sea slab beneath the region from Ise Bay to western Shikoku, southwest Japan

Jan 2002
139-152

M M Miller
T Melbourne
D J Johnson
W Q Sumner
T Miyoshi
K Ishibashi

Miller, M.M., Melbourne, T., Johnson, D.J., Sumner, W.Q., 2002. Periodic slow earthquakes from the Cascadia subduction zone. Science 295, 2423. Miyoshi, T., Ishibashi, K., 2004. Geometry of the seismic Philippine Sea slab beneath the region from Ise Bay to western Shikoku, southwest Japan. J. Seismol. Soc. Japan (Zisin) 57, 139 – 152 (in Japanese with English abstract).

Tectonics of Convergent Plate Boundaries. Univ

Jan 2002
271

G Kimura

Kimura, G., 2002. Tectonics of Convergent Plate Boundaries. Univ. Tokyo Press. 271pp. (in Japanese).

No low-frequency tremor implying no dehydration and infrequent great interplate earthquakes Configuration of subducting Philippine Sea plate beneath southwest Japan revealed from receiver function analysis based on the multivar-iate autoregressive model

Jan 2003
10-1029

T Seno
T Yamasaki
K Shiomi
H Sato
K Obara
M Ohtake

Seno, T., Yamasaki, T., 2003. No low-frequency tremor implying no dehydration and infrequent great interplate earthquakes. Geophys. Res. Lett. 30 (22), 2171, doi:10.1029/2003GRL018349. Shiomi, K., Sato, H., Obara, K., Ohtake, M., 2004. Configuration of subducting Philippine Sea plate beneath southwest Japan revealed from receiver function analysis based on the multivar-iate autoregressive model. J. Geophys. Res. 109, doi:10.1029/ 2003JB002774. K. Obara, H. Hirose / Tectonophysics 417 (2006) 33–51

On the Long-Term Evaluation of Earthquakes in the Nankai Trough

Jan 2001

Earthquake Headquarters
Promotion

Headquarters for Earthquake Research Promotion, 2001. On the Long-Term Evaluation of Earthquakes in the Nankai Trough (in Japanese).

Nationwide activity of low-frequency earth-quakes in the lower crust in Japan

Jan 2000
002

N Nishide
T Hashimoto
J Funasaki
H Nakazawa
M Oka
H Ueno
N Yamada
I Sasakawa
K Maeda
K Sugimoto
T Takashima

Nishide, N., Hashimoto, T., Funasaki, J., Nakazawa, H., Oka, M., Ueno, H., Yamada, N., Sasakawa, I., Maeda, K., Sugimoto, K., Takashima, T., 2000. Nationwide activity of low-frequency earth-quakes in the lower crust in Japan. Abstr. Japan. Earth Planet. Sci. Joint Meeting, sk-p002 (in Japanese).

No low-frequency tremor implying no dehydration and infrequent great interplate earthquakes

Jan 2003
2171

Seno

Seno, T., Yamasaki, T., 2003. No low-frequency tremor implying no dehydration and infrequent great interplate earthquakes. Geophys. Res. Lett. 30 (22), 2171, doi:10.1029/2003GRL018349.

Nationwide activity of low-frequency earthquakes in the lower crust in Japan

Jan 2000

Nishide

Non-volcanic deep low-frequency tremors accompanying slow slips in the southwest Japan subduction zone

Abstract

No full-text available

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