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Introduction to the Special Issue on the 2011 Tohoku Earthquake and Tsunami
Abstract
The 11 March 2011 Tohoku earthquake (05:46:24 UTC) involved a massive rupture of the plate‐boundary fault along which the Pacific plate thrusts under northeastern Honshu, Japan. It was the fourth‐largest recorded earthquake, with seismic‐moment estimates of 3–5×10^(22) N•m (M_w 9.0). The event produced widespread strong ground shaking in northern Honshu; in some locations ground accelerations exceeded 2g. Rupture extended ∼200 km along dip, spanning the entire width of the seismogenic zone from the Japan trench to below the Honshu coastline, and the aftershock‐zone length extended ∼500 km along strike of the subduction zone. The average fault slip over the entire rupture area was ∼10 m, but some estimates indicate ∼25 m of slip located around the hypocentral region and extraordinary slip of up to 60–80 m in the shallow megathrust extending to the trench. The faulting‐generated seafloor deformation produced a devastating tsunami that resulted in 5–10‐km inundation of the coastal plains, runup of up to 40 m along the Sanriku coastline, and catastrophic failure of the backup power systems at the Fukushima Daiichi nuclear power station, which precipitated a reactor meltdown and radiation release. About 18,131 lives appear to have been lost, 2829 people are still missing, and 6194 people were injured (as reported 28 September 2012 by the Fire and Disaster Management Agency of Japan) and over a half million were displaced, mainly due to the tsunami impact on coastal towns, where tsunami heights significantly exceeded harbor tsunami walls and coastal berms.
... The main rupture was on the megathrust where the Pacific plate subducts beneath the Okhostk plate at an average rate of 8.5 cm/yr ( Ozawa et al., 2011) ( Fig. 1). The zone of greatest megathrust displacement on the upper plate was modeled and documented based on seismic, geodetic, and geological data (e.g., Yagi and Fukahata, 2011;Lay et al., 2013) ( Fig. 1). The huge rupture extended for ~200 km along dip from the Japan Trench to below the Honshu coastline, and the length of the aftershock zone extended for ~500 km along the subduction zone ( Lay et al., 2013). ...
... The zone of greatest megathrust displacement on the upper plate was modeled and documented based on seismic, geodetic, and geological data (e.g., Yagi and Fukahata, 2011;Lay et al., 2013) ( Fig. 1). The huge rupture extended for ~200 km along dip from the Japan Trench to below the Honshu coastline, and the length of the aftershock zone extended for ~500 km along the subduction zone ( Lay et al., 2013). This rupture reached the trench at the ...
The A.D. 2011 Tohoku-Oki Mw 9 earthquake ruptured the megathrust up to the Japan Trench with a large displacement and caused a catastrophic tsunami. This study is the first to use short-lived radioisotopes, including those emitted by the damaged nuclear reactors at the Fukushima Daiichi nuclear power plant (Japan), to document the remobilization of the upper few centimeters of sediment as a highly significant process triggered by the earthquake and its aftershocks. Targeting the post-earthquake environment allowed characterization of the sedimentary signature of this event for a better understanding of paleoearthquakes in Japan and other tectonically active boundary areas. The results stem from 23 piston cores recovered by the 2013 expedition NT13-19 of the Japan Agency for Marine-Earth Science and Technology. We document submarine homogeneous muddy flow deposits that were triggered by ground motion in 2011. They are highly enriched with excess (xs) xs210Pb, requiring only centimeters-deep sediment remobilization over large areas of the seafloor. Some contain 134Cs and 137Cs radioisotopes derived from the Fukushima nuclear reactors, indicating that sedimentation persisted for at least 30 days after the main shock. We found these deposits at all sampling sites in an ~5000 km2 area of the seafloor in 4000-6000 m of water depth. The study area extends for ~260 km parallel to the strike of the trench. The thickness of this "Tohoku layer" (3-200 cm) increases toward the zone of maximum megathrust slip, where deposits are thickest. These results demonstrate that the shaking of the seafloor above large megathrust ruptures near the trench remobilized surficial unconsolidated sediment for hundreds of kilometers. The characteristics of these deposits may typify deposits resulting from large fault slips like that of the Tohoku-Oki earthquake, but also other earthquake deposits, contributing to their identification in the sedimentary record globally.
... The recent destructive earthquakes that occurred in Japan (2011; e.g., Lay et al. 2013) and New Zealand (2010-2011 earthquake sequence; e.g., Quigley et al. 2012) have clearly pointed out that traditional seismic hazard assessment based only on vibratory ground motion data needs to be integrated with information about the local vulnerability of the territory to earthquake occurrence. Nowadays, a huge amount of information about the characteristics of Earthquake Environmental Effects (EEEs; i.e., any phenomena generated by a seismic event in the natural environment; Michetti et al. 2004Guerrieri et al. 2007) is available for a very large number of earthquakes that occurred not only in the instrumental period and in historical time but also in the prehistorical period (paleoearthquakes, e.g., Mc Calpin 2009;Reicherter et al. 2009). ...
The recent destructive earthquakes that occurred in Japan (2011; e.g., Lay et al. 2013) and New Zealand
(2010–2011 earthquake sequence; e.g., Quigley et al. 2012) have clearly pointed out that traditional
seismic hazard assessment based only on vibratory ground motion data needs to be integrated with
information about the local vulnerability of the territory to earthquake occurrence. Nowadays, a huge
amount of information about the characteristics of Earthquake Environmental Effects (EEEs; i.e., any
phenomena generated by a seismic event in the natural environment; Michetti et al. 2004, 2007; Guerrieri
et al. 2007) is available for a very large number of earthquakes that occurred not only in the instrumental
period and in historical time but also in the prehistorical period (paleoearthquakes, e.g., Mc Calpin 2009;
Reicherter et al. 2009). However, available information located in different sources (scientific papers,
historical documents, professional reports) can often be difficult to access.
... A major nuclear disaster occurred in 2011 in the coastal region of the Fukushima prefecture of Japan following the March 11th earthquake and large tsunami. This accident triggered: a vivid scientific (e.g., Lay et al. 2013), technical (e.g., Mori and Eisner 2013) and public discussion on the safety of nuclear power plants (NPPs) worldwide; the key role of geosciences in properly assessing maximum magnitudes (e.g., Zoller et al. 2014) and earthquake hazard levels over different time scales (e.g., Satake et al. 2013;Hoshiba and Aoki 2015); and the need to account correctly for the likelihood of extreme events in the engineering design of critical facilities. Over 4 years later, the dramatic social, environmental and overall estimated economic impact (*150-250 billion USD according to different sources) of the meltdown of three of the plant's six reactors still dominates the public and political debate on nuclear safety. ...
Based on our experience in the project REAKT, we present a methodological framework to evaluate the potential benefits and costs of using earthquake early warning (EEW) and operational earthquake forecasting (OEF) for real-time mitigation of seismic risk at nuclear facilities. We focus on evaluating the reliability, significance and usefulness of the aforementioned real-time risk-mitigation tools and on the communication of real-time earthquake information to end-users. We find that EEW and OEF have significant potential for the reduction of seismic risk at nuclear plants, although much scientific research and testing is still necessary to optimise their operation for these sensitive and highly-regulated facilities. While our test bed was Switzerland, the methodology presented here is of general interest to the community of EEW researchers and end-users and its scope is significantly beyond its specific application within REAKT.
... In the example of great megathrust earthquakes, such as the 2004 Sumatra-Andaman earthquake (e.g., Shearer and Bürgmann, 2010) or the 2011 Tohoku-Oki earthquake (e.g., Lay et al., 2013), using different methodologies to analyze the coseismic slip distribution leads to different resulting slip distributions. The case of the 2006 M7.8 Java tsunami earthquake (JTE), which occurred in a shallow portion of the megathrust along the Java trench, is similar. ...
We investigate three available coseismic fault models of the 2006 M7.8 Java tsunami earthquake, as reported by Fujii and Satake (2006), Bilek and Engdahl (2007), and Yagi and Fukahata (2011), in order to find the best coseismic model based on mechanisms of postseismic deformation associated with viscoelastic relaxation and afterslip. We construct a preliminary rheological model using vertical data, obtaining a final rheological model after we include horizontal and vertical components of afterslip in the further process. Our analysis indicates that the coseismic fault model of Fujii and Satake (2006) provides a better and more realistic result for a rheological model than the others. The best-fit rheological model calculated using the coseismic fault model of Fujii and Satake (2006) comprises a 60±5km elastic layer thickness with a viscosity of 2.0±1.0×1017Pas in the asthenosphere. Also, we find that afterslip is dominant over the horizontal displacements, while viscoelastic relaxation is dominant over the vertical displacement. Additionally, in comparison to the coseismic displacement found through GPS data taken at BAKO station, our calculation indicates that Fujii and Satake (2006) modeled coseismic displacements with less GPS data misfit than the other examined models. Finally, we emphasize that our methodology for evaluating the best coseismic fault model can satisfactorily explain the postseismic deformation of the 2006 Java tsunami earthquake.
The tsunami source models of the 2011 Tohoku Earthquake, which reproduce the wave gauge record offshore the Fukushima Daiichi Nuclear Power Plant, were obtained by the joint inversion of tsunami waveform data, 1-second sampling GPS data, and sea bottom crustal deformation data. For improving the reproducibility of the wave gauge record, we used an approach that includes a “hypothetical tsunami waveform” data in the joint inversion. The hypothetical tsunami waveforms were defined at the depth of 50 m offshore the Fukushima Daiichi Nuclear Power Plant,. The tsunami waves at the position of the wave gauge were simulated based on nonlinear long-wave theory using the tsunami source models obtained by the joint inversion including the hypothetical tsunami waveforms. The final modified hypothetical tsunami waveform, which minimized the sum of squared residuals between observed and calculated tsunami waves at the position of wave gauge, was searched among many hypothetical tsunami waveforms. By including the modified hypothetical tsunami waveform in the joint inversion, we obtained the tsunami source models reproducing the wave gauge record well. A similar approach using the hypothetical tsunami waveform data was also applied for improving the fit between the observed and calculated tsunami heights from Hokkaido to Chiba prefectures. It also became clear that the combined use of the 1-second sampling GPS data in the inversion is effective for constraining the characteristics of the fault rupture process.
Faults are ubiquitous throughout the Earth's crust. The majority are silent for decades to centuries, until they suddenly rupture and produce earthquakes. With a focus on shallow continental active-tectonic regions, this paper reviews a subset of faults that have a different behavior. These unusual faults slowly creep for long periods of time and produce many small earthquakes. The presence of fault creep and the related microseismicity help illuminate faults that might not otherwise be located in fine detail, but there is also the question of how creeping faults contribute to seismic hazard. It appears that well-recorded creeping fault earthquakes of up to magnitude 6.6 that have occurred in shallow continental regions produce similar fault-surface rupture areas and similar peak ground shaking as their locked fault counterparts of the same earthquake magnitude. The behavior of much larger earthquakes on shallow creeping continental faults is less well known, because there is a dearth of comprehensive observations. Computational simulations provide an opportunity to fill the gaps in our understanding, particularly of the dynamic processes that occur during large earthquake rupture and arrest.
The Great East Japan Earthquake Tsunami on March 11, 2011, caused unprecedented damage mainly in northeast Japan. This paper introduces the characteristics of the tsunami and resultant damage. The mechanism of the breaching of coastal structures and the effect of surviving structures on damage reduction in land are discussed. Then, a two-level tsunami mitigation concept, proposed and adopted by a committee hosted by the Government for recovery and reconstruction, is introduced. Within this framework, coastal structures are required to be resilient to external forces exceeding the design level. Various technologies that have been developed based on experience and research after the tsunami are introduced.
The takeoff point in this paper is a random variable X for which large positive values are desired. When its probability distribution is highly skewed, the possibility of a long fat-tailed distribution can lead to an expected value, μ = E[X], which is unduly optimistic. For the reverse case when small values of X are desired, the ideas in this paper are applied to -X. This issue of over-optimism in the expected value is particularly important when a mission-critical random variable is involved. For example, when considering earthquake intensity or flood levels, reporting an understated expected value to a technically unsophisticated general public would be considered by many as highly undesirable. The Conservative Expected Value (CEV) of X, denoted by CEV,(X) is a new definition provided in this paper. It is a metric which we argue is particularly useful when risk aversion must be highly emphasized. At the same time, while being conservative, the CEV is also aimed at avoiding reflection of excessive pessimism. When, then CEV(X), while being conservative, is defined in such a way so as not to be unduly pessimistic. In classical analysis, enhancement of a calculation often includes the variance σ² = var(X). However, when large X-values are desired, this may further distort one's overview of the risk at hand; e.g., if X is profit, values above the mean should not be penalized. With these one-sidedness considerations in mind, we work with a new reward-risk pair, the CEV and the so-called Conservative Semi-Variance (CSV) of X, denoted by CSV(X). Whereas the CEV definition is entirely new, the CSV definition is motivated by ideas in the area of finance. This paper also illustrates calculation of the CEV and CSV pair for a number of classical probability distributions and includes description of a number of properties of these metrics which suggest that this new theory is mathematically rich. Finally, we demonstrate the potential for application of the theory via two numerical examples.
The recent Tohoku Japan earthquake clearly demonstrated that aftershocks following large magnitude earthquakes can be a source of significant hazard. In this study, we have followed the Reasenberg and Jones (1989) approach to describe the aftershock activity based on: 1) Gutenberg-Richter model (GRM) specifying the relationship between the magnitude and total number of earthquakes in any region and time period, and 2) modified Omori model (MOM) defining the decay of the aftershock activity with time. The aftershock occurrence is represented by a non-stationary Poisson process whose rate varies with time after the main shock. Since the Tohoku earthquake occurred in a seismically active region, it is essentially impossible to differentiate aftershocks from background earthquakes in the catalog. Hence this study fitted the MOM and GRM to all the events in the catalog in order to estimate the post-Tohoku seismicity rates. We have used the JMA catalog with magnitude larger than or equal to M3.5 in order to estimate the seismicity that may cause damage to the built environment. The Method of Maximum Likelihood is used to estimate the parameters in MOM and GRM. The pre-Tohoku seismicity is calculated based on the observed seismicity since 1983 and that for post-Tohoku is based on the seismicity for one year following the Tohoku event. In this study, we consider only the change in the seismicity in the background sources. The change in seismicity rates in the shallow crustal faults, deep background sources, or the subduction sources are not considered in this study. In order to find out the change in the seismic risk in Japan after the Tohoku earthquake, we have estimated the average annual loss (AAL) for a well distributed building portfolio in Japan. We have calculated the AAL for each year from 2013 to 2017 in order to illustrate the evolution of risk with time following the Tohoku event. There is significant uncertainty in the estimation of the GRM and MOM parameters resulting in uncertainty in the estimation of risk and this is also addressed in this study. In addition, we investigate the suitability of MOM in forward prediction of post-event risks in a regional scale. The observed seismicity in the second year following the Tohoku earthquake is compared with those estimated based only on the first year post-event data.
The canonical model of fault coupling assumes that slip is partitioned into fixed asperities that display stick-slip behavior and regions that creep stably. We show that this simple asperity model is inconsistent with GPS-derived deformation in northern Japan associated with interseismic coupling on the subduction interface and the transient response to Mw 6.3-7.2 earthquakes during 2003-2011. Comparisons of GPS data with simulations of earthquakes on asperities and associated velocity-strengthening afterslip require that afterslip overlaps areas of the fault that ruptured in previous earthquakes, including the 2011 Mw 9 Tohoku-oki earthquake. Whereas about 55% of the plate interface ruptured in earthquakes during 2003-2011, we infer that only 9% of the plate interface was fully locked between earthquakes. Inferred locked asperities are roughly 25% the size of rupture areas determined by seismic source inversions. These smaller asperities are consistent with interseismic strain accumulation in 2009, although more extensive locking is required a decade earlier in 1998.
The 2011 M-w 9.0 Tohoku, Japan, earthquake triggered deep tectonic tremor and shallow microearthquakes in numerous places worldwide. Here, we conduct a systematic survey of triggered tremor in regions where ambient or triggered tremor has been previously identified. Tremor was triggered in the following regions: south-central Alaska, the Aleutian Arc, Shikoku in southwest Japan, the North Island of New Zealand, southern Oregon, the Parkfield-Cholame section of the San Andreas fault in central California, the San Jacinto fault in southern California, Taiwan, and Vancouver Island. We find no evidence of triggered tremor in the Calaveras fault in northern California. One of the most important factors in controlling the triggering potential is the amplitude of the surface waves. Data examined in this study suggest that the threshold amplitude for triggering tremor is similar to 0.1 cm/s, which is equivalent to a dynamic stress threshold of similar to 10 kilopascals. The incidence angles of the teleseismic surface waves also affect the triggering potentials of Love and Rayleigh waves. The results of this study confirm that both Love and Rayleigh waves contribute to triggering tremor in many regions. In regions where both ambient and triggered tremor are known to occur, tremor triggered by the Tohoku event generally occurred at similar locations with previously identified ambient and/or triggered tremor, further supporting the notion that although the driving forces of triggered and ambient tremor differ, they share similar mechanisms. We find a positive relationship between the amplitudes of the triggering waves and those of the triggered tremor, which is consistent with the prediction of the clock-advance model.
Recently, there have been numerous great (M-w >= 8), devastating earthquakes, with a rate in the last seven years that is 260% of the average rate during the 111-year seismological history. Each great earthquake presents an opportunity to study a major fault at the very beginning and end of the inferred seismic cycle. In this work, we use these events as both targets and sources to probe susceptibility to dynamic triggering in the epicentral region before and after a large earthquake. This study also carefully addresses the possibility that large earthquakes interact in a cascade of remotely triggered sequences that culminates in further large earthquakes. We seek evidence of triggering associated with the 16 great M-w >= 8 events that occurred between 1998 and 2011, using regional and global earthquake catalogs to measure changes in interevent time statistics. Statistical significance is calculated with respect to a nonstationary reference model that includes mainshock-aftershock clustering. We find limited evidence that a few great earthquakes triggered an increase in seismicity at the site of the next great earthquake in the sequence. However, this evidence is not corroborated by all statistical tests nor all earthquake catalogs. Systematic triggered rate changes in the years to decades before each great earthquake are less than 19% at the 95% confidence level, too small to explain the observed rate increase. The catalogs are insufficient for the purpose of resolving more moderate triggering expected from previous studies. We calculate that an improvement in completeness magnitude from 3.7 to 3.5 could resolve the expected triggering signal in the International Seismic Center (ISC) catalog taken as a whole, but an improvement to M 2.0 would be needed to consistently resolve triggering on a regional basis.
In this article we discuss the character and spatial pattern of coseismic landslides from the eastern Honshu region of Japan, which was strongly shaken in the 2011 Tohoku earthquake. We developed a detailed geospatial database of 3477 landslides based on postearthquake field surveys and examination of high-resolution satellite imagery across a 28; 380 km(2) landslide study area. Analysis of the database shows that a substantial majority (80%) of landslides occurred in Quaternary soil and Neogene rock units. Despite their abundance in the study area, relatively few landslides occurred in pre-Neogene rocks (i.e., older than 23 Ma). Further examination of the data showed that the most common types of landslides were (1) disrupted landslides in Neogene sedimentary rocks and (2) lateral spreading in Quaternary sediments. However, we found that coseismic landslide erosion (i.e., debris mobilization) was almost fully dominated by lateral spreading within Quaternary sediments. When comparing the landslide inventory with ground motions recorded by dense regional seismic arrays, we found no statistically significant correlation between landslide intensity and ground motion within the study area.
The implementation of a new Tsunami Intensity Scale is proposed because a vast amount of data has been collected from the two megatsunamis that took place in the Indian Ocean in 2004 and northeast Japan in 2011. The newly proposed scale is 12-grade and is based on the assessment of a large number of objective criteria that are easily accessible and incorporated into six groups. As a result, it does not saturate as six-grade scales do. More specifically, the estimation of intensity values takes into account: (1) the quantities of the phenomena, (2) the direct impact on humans, (3) the impact on mobile objects, (4) the impact on coastal infrastructure, (5) the impact on the environment, and (6) the impact on structures. The new scale is compatible with the widely used EMS1998 and ESI2007 scales and has a reliable horizontal correspondence throughout the groups of criteria. It is easily and directly applicable to all environments, and particularly useful for the outlining of microzones of different intensities in any tsunami-affected area. It can be implemented for any land use/cover type, such as urban, rural, industrial, touristic, and so on, and at the same time, morphologic diversity of affected areas is not an obstacle to application.
A multiple time window inversion of 53 high-sampling tsunami waveforms on ocean-bottom pressure, Global Positioning System, coastal wave, and tide gauges shows a temporal and spatial slip distribution during the 2011 Tohoku earthquake. The fault rupture started near the hypocenter and propagated into both deep and shallow parts of the plate interface. A very large slip (approximately 25 m) in the deep part off Miyagi at a location similar to the previous 869 Jogan earthquake model was responsible for the initial rise of tsunami waveforms and the recorded tsunami inundation in the Sendai and Ishinomaki plains. A huge slip, up to 69 m, occurred in the shallow part near the trench axis 3 min after the rupture initiation. This delayed shallow rupture extended for 400 km with more than a 10-m slip, at a location similar to the 1896 Sanriku tsunami earthquake, and was responsible for the peak amplitudes of the tsunami waveforms and the maximum tsunami heights measured on the northern Sanriku coast, 100 km north of the largest slip. The average slip on the entire fault was 9.5 m, and the total seismic moment was 4.2 x 10(22) N . m (M-w 9.0). The large horizontal displacement of seafloor slope was responsible for 20%-40% of tsunami amplitudes. The 2011 deep slip alone could reproduce the distribution of the 869 tsunami deposits, indicating that the 869 Jogan earthquake source could be similar to the 2011 earthquake, at least in the deep-plate interface. The large tsunami at the Fukushima nuclear power station is due to either the combination of a deep and shallow slip or a triggering of a shallow slip by a deep slip, which was not accounted for in the previous tsunami-hazard assessments.
Changes in groundwater levels on Jeju Island, which is similar to 1500 km from the epicenter of the 2011 M-w 9.0 Off the Pacific Coast of Tohoku earthquake in Japan, were analyzed. The results show a series of water level fluctuations related to the foreshock (M-w 7.3 and M-w 6.1) and aftershock (M-w 7.9) in addition to the mainshock (M-w 9.0). The groundwater-level changes in response to the earthquake were oscillatory, and the groundwater levels at some wells had an irregular pattern after the M-w 9.0 earthquake, although they recovered in five to seven days. This phenomenon may reflect the unstable elastic aquifer properties that are present for a period of time after a large earthquake. In addition, the successive groundwater-level change was different for each magnitude and well location. The magnitude increases the groundwater-level change, but the response amplitude is also dependent on the hydrogeological characteristics at the well. On Jeju Island, the groundwater-level changes due to the earthquake generally increased where there was more volcanic hard rock and more permeable layers, but these changes were inversely correlated with the presence of sedimentary deposits with less permeability and less restrictive characteristics.
The 2011 M-w 6.6 Fukushima earthquake occurred in Iwaki City, similar to 250 km southwest of the epicenter of the 2011 M-w 9.0 Tohoku (Japan) earthquake, and produced two subparallel similar to 15-23-km-long surface rupture zones with distinct displacements along two pre-existing normal faults: the Shionohira and Yunodake faults, which strike north-northwest-south-southeast and northwest-southeast (both dipping southwest), respectively. Field investigations and structural analysis of the coseismic shear zones developed within the Shionohira and Yunodake fault zones, reveal that (1) the coseismic shear zones consist of a fault core that includes a narrow fault gouge zone of < 10 cm in width (generally 1-5 cm), a fault breccia zone of < 50 cm in width, and a damage zone of 5-50 m in width that is composed of cataclastic rocks, fractures, and subsidiary faults; (2) the foliations characterized by S-C fabrics developed in the shear zones indicate a dominantly normal fault-slip sense, consistent with that observed along the coseismic surface ruptures; and (3) veinlet cataclastic rocks composed of unconsolidated fault gouges and fine-grained materials occur as simple discrete veinlets and multiple, cross-cutting, interconnected networks of veinlets. These newly documented structural characteristics for two coseismic fault/shear zones and associated cataclastic rocks indicate that the locations of coseismic slip zones associated with the 2011 Fukushima earthquake were controlled by pre-existing fault/shear zones central to the Shionohira and Yunodake faults, which have repeatedly moved as seismogenic normal faults since the timing of formation of associated cataclastic rocks. Our results demonstrate that the structural analyses of coseismic shear/fault zones provide a powerful tool to study the nature and active faulting history of seismological faults.
The gigantic M-w 9.0 11 March 2011 Tohoku-oki earthquake suddenly changed the overriding inland area to an extensional stress regime and triggered massive seismic swarms in the coastal region. The largest earthquake of M-w 6.6 struck southern Fukushima on 11 April 2011 and ruptured two previously mapped faults, the northwest (NW)-trending Yunodake fault and the north-northwest (NNW)-trending Itozawa fault. Clear 15-km-long and 14-km-long surface ruptures appeared along both faults, respectively, exhibiting a predominantly normal sense of slip, down to the west. The maximum vertical offset on the Yunodake fault is similar to 0.8 m, whereas that on the Itozawa fault is similar to 2.1 m. The Itozawa fault rupture is in part marked by uphill-facing scarps, which is discordant with the large-scale topography but consistent with saddled ridges and ponded alluvium. The Yunodake fault, which bounds a Neogene half-graben structure juxtaposing Mesozoic metamorphic rocks against Neogene sedimentary rocks, shows left-lateral deflections of streams. Seismological data and interviews of local residents revealed that the two subparallel faults ruptured simultaneously. Based on the location of its hypocenter, we, however, interpret that the Itozawa rupture was primary and then triggered normal faulting on the Yunodake fault under the heightened Coulomb stress caused by the preceding Tohoku-oki earthquake. Our paleoseismic trench across the Itozawa fault exposed evidence for a penultimate earthquake that occurred sometime between 12,620 and 17,410 cal yr B.P. There is no evidence that the Itozawa fault ruptured during or immediately after the A.D. 869 Jogan earthquake, which is believed to be the penultimate giant megathrust earthquake along the Japan trench.
After the 11 March 2011 M-w 9.0 Tohoku earthquake, Japan, a swarm of shallow normal-faulting earthquakes was triggered in a localized region beneath the border between Fukushima and Ibaraki prefectures in eastern Honshu. Here we examine the coseismic displacement field and a fault model of the largest event of the swarm, the 11 April 2011 M-w 6.6 Iwaki earthquake. Radar interferometry applied to data of the Japanese ALOS satellite reveals complex ruptures associated with the Iwaki earthquake. In particular, the interferogram clearly shows multiple surface ruptures along each of the subparallel Yunodake and Itozawa faults. The slip distributions on these faults, as estimated by inverting the interferogram, suggest that the faults dip 60 degrees-70 degrees westward and that slip on the Itozawa fault extended to a splay fault at depths shallower than about 5 km. Our results indicate that the highland area west of the Itozawa fault was downthrown by up to 2.4 m during the Iwaki earthquake.
The dynamic stresses that are associated with the energetic seismic waves generated by the M-w 9.0 Tohoku earthquake off the northeast coast of Japan triggered bursts of tectonic tremor beneath the Parkfield section of the San Andreas fault (SAF) at an epicentral distance of similar to 8200 km. The onset of tremor begins midway through the similar to 100-s-period S-wave arrival, with a minor burst coinciding with the SHSH arrival, as recorded on the nearby broadband seismic station PKD. A more pronounced burst coincides with the Love arrival, followed by a series of impulsive tremor bursts apparently modulated by the 20- to 30-s-period Rayleigh wave. The triggered tremor was located at depths between 20 and 30 km beneath the surface trace of the fault, with the burst coincident with the S wave centered beneath the fault 30 km northwest of Parkfield. Most of the subsequent activity, including the tremor coincident with the SHSH arrival, was concentrated beneath a stretch of the fault extending from 10 to 40 km southeast of Parkfield. The seismic waves from the Tohoku epicenter form a horizontal incidence angle of similar to 14 degrees, with respect to the local strike of the SAF. Computed peak dynamic Coulomb stresses on the fault at tremor depths are in the 0.7-10 kPa range. The apparent modulation of tremor bursts by the small, strike-parallel Rayleigh-wave stresses (similar to 0.7 kPa) is likely enabled by pore pressure variations driven by the Rayleigh-wave dilatational stress. These results are consistent with the strike-parallel dynamic stresses (delta tau(s)) associated with the S, SHSH, and surface-wave phases triggering small increments of dextral slip on the fault with a low friction (mu similar to 0.2). The vertical dynamic stresses delta tau(d) do not trigger tremor with vertical or oblique slip under this simple Coulomb failure model.