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On 19 October 2020, an Mw 7.6 earthquake occurred within the Shumagin Islands, Alaska, which is the largest strike-slip earthquake occurred in the shallow subducted plates with abundant seismic observations. Here, we relocated the earthquake sequence, implemented back-projection analyses, and finite-fault inversion to investigate the source processes of the mainshock, and calculated mainshock focal mechanisms using the polarities of P waves and W-phase inversion, respectively. Our results show that the faulting of the mainshock can be divided into two segments with the initial rupture along a steep plane (strike = 15°, dip = 81°) and propagation southeastward along a more shallowly dipping plane (strike = 344°, dip = 48°). The inferred strikes of the mainshock faults are similar to the orientations of preexisting structures in the source region, likely indicating that the 2020 Mw 7.6 earthquake ruptured along the preexisting plate fabric in the downgoing plate. The fabrics are located at the boundary with significant variations of the plate coupling, indicating that these structures within the subducting plate may affect the interplate coupling or as a result of the varying interplate coupling in subduction zones.

SUMMARYDetecting P-wave onsets for online processing is an important component for real-time seismology. As earthquake early warning systems around the world come into operation, the importance of reliable P-wave detection has increased, since the accuracy of the earthquake information depends primarily on the quality of the detection. In addition to the accuracy of arrival time determination, the robustness in the presence of noise and the speed of detection are important factors in the methods used for the earthquake early warning. In this paper, we tried to improve the P-wave detection method designed for real-time processing of continuous waveforms. We used the new Tpd method, and proposed a refinement algorithm to determine the P-wave arrival time. Applying the refinement process substantially decreases the errors of the P-wave arrival time. Using 606 strong motion records of the 2011 Tohoku earthquake sequence to test the refinement methods, the median of the error was decreased from 0.15 to 0.04 s. Only three P-wave arrivals were missed by the best threshold. Our results show that the Tpd method provides better accuracy for estimating the P-wave arrival time compared to the STA/LTA method. The Tpd method also shows better performance in detecting the P-wave arrivals of the target earthquakes in the presence of noise and coda of previous earthquakes. The Tpd method can be computed quickly, so it would be suitable for the implementation in earthquake early warning systems.

The 2022 volcanic eruption in Tonga caused an unusually large tsunami around the Pacific. It travels with a faster apparent velocity and has larger amplitudes at long distances than what would be expected from a conventional tsunami from the volcanic source. This tsunami was generated by the moving atmospheric Lamb wave and traveled at the speed of the Lamb wave (0.31 km/s). Japanese data showed the amplitude of this first tsunami becomes small when approaching the coast, due to the weaker air-sea coupling at the shallow depth. This wave split when passing the continental slope, and traveled at the speed of the ocean gravity wave. Therefore, the tsunami observed at the coast is delayed by thousands of seconds from the passage of the Lamb wave. Tsunamis generated by this atmospheric mechanism have not been previously observed by modern digital recording systems and should be considered in the tsunami warning systems.

The El Niño-Southern Oscillation (ENSO) produces large shifts in ocean and atmospheric conditions in the Pacific and Indian Ocean basins, including major changes in sea level in equatorial regions across the Pacific basin, including the Pacific Rim1-6, 18. The Pacific Ocean rim is also well known as a region prone to a high frequency of earthquakes16,17, which has a profound influence on lives, economies, and ecosystems in the region. An ability to better estimate earthquake risk frequency would be of enormous value in preparedness and response planning16. Here we show that ENSO-driven sea-level variability, measured in the equatorial, northern or overall Pacific Ocean is coherent with subsequent variability in earthquake frequency in the western Pacific Ocean Basin. While the statistical link is evident for M4.7 to M5.5 and ≥M6.5 earthquakes, it is strongest for M4.7 to M5.5 earthquakes. We hypothesize that the relationship is due to a systematic influence of the climate-ocean system on the lithospheric stress field and plate coupling along subduction zones. We suggest these results would underpin an ability to better assess earthquake frequency risk in and around the western Pacific several months in advance.

The 2018 Mw 6.4 Hualien earthquake generated a large peak-to-peak velocity of over 2 m/s, with a period of 3 s at the south end of the Milun fault, which resulted in the collapse of five buildings. To investigate the shallow subsurface soil structure and evaluate possible effects on the ground motion and building damage, we performed microtremor measurements in the Hualien basin. Based on the velocity structure jointly inverted from both Rayleigh-wave dispersion curves and microtremor horizontal-to-vertical spectral ratio data, we found that the shallow subsurface structure generally deepens from west to east. Close to the Milun fault, the structure becomes shallower, which is consistent with faulting during the 2018 earthquake and the long-term tectonic displacement. There is no significant variation for the site conditions in the north–south direction that can explain the large peak ground velocity in the south. As a result of the dense measurements in the heavily damaged area, where three high-rise buildings totally collapsed, these locations have the average S-wave velocity of the upper 30 m (AVS30) values and are relatively high compared to the more distant area from the Meilun River. This is somewhat unusual, because lower AVS30 values indicating softer ground conditions are expected close to the river. We did not find any characteristic subsurface soil structure that may contribute to the building collapses. The large 3 s pulse was probably generated by source effects, rather than subsurface soil amplification.

The rapid and reliable estimation of the magnitudes of large earthquakes is critical for determining the potential shaking damage and tsunami hazards. The primary challenge to rapidly and accurately estimating the magnitude of large earthquakes is the need to wait for the full waveform in order to calculate the source parameters. We used data of the M≥7.0 shallow earthquakes in Japan from 2008 to 2016, recorded at 10°–60° (regional to teleseismic distances), to establish an operational method to quickly determine their magnitudes. Our results suggest that earthquake magnitudes can be estimated accurately 6–12 min after their origin times. The only time‐limiting factor on our method is the epicentral distances to the seismic stations. For the case of the 2011 great Tohoku earthquake, the magnitude was estimated as M 8.9–9.1 at 6–12 min after the origin time. Resolutions of the results were further investigated by bootstrap and jackknife tests and subarray analysis. Therefore, we propose building a system for determining the magnitude of large earthquakes in and around Japan using real‐time seismic data in China and worldwide. This will assist in disaster mitigation immediately after a damaging earthquake, especially for the purpose of tsunami evacuation and emergency rescue.

The 17 June 2018 MJMA 6.1 (Mw 5.5) Osaka earthquake exhibits a large non–double‐couple component (∼26%), and its aftershock sequence shows a complicated spatial pattern. To better understand the ruptured faults, we relocate the earthquake sequence using P and S arrival times and waveform cross correlations and calculate the focal mechanisms of all MJMA≥2.5 (Mw≥2.3) earthquakes within three months after the mainshock using P‐wave first‐motion polarities and S/P amplitude ratios. Relocated aftershocks image several faults, the northeast‐striking strike‐slip fault, the north‐northwest‐striking reverse fault, and at least two small northwest‐striking features. P‐wave first motions of the mainshock indicate nearly a pure thrust mechanism. We deduce that the earthquake sequence started from a north‐northwest‐striking reverse fault and propagated to a northeast‐striking strike‐slip fault. The aligned strike‐slip aftershocks occurring in the vicinity of the northeast‐striking strike‐slip fault delineates the growth of several newly formed or reactivated northwest‐striking Riedel shears that are conjugated to the northeast‐striking strike‐slip fault.

We investigate the spatiotemporal pattern of crustal anisotropy in the source area of the 2004 Niigata earthquake (M 6.8) that occurred in the northern segment of the Niigata–Kobe tectonic zone, central Japan, by measuring shear‐wave splitting parameters from waveform data of local earthquakes. Our results show that the fast polarization directions in the upper crust have spatial variations across the region of the earthquake that are likely caused by both structural and stress field effects. The northwest–southeast direction near the northeastern end of the source zone (beneath station N.NGOH) and the east–west direction to the southwest (beneath station N.KWNH) are consistent with the spatial variation of the orientation of the maximum compression of the local stress field. Fast polarization directions at other stations tend to align in the directions of active faults and folds and thus are considered to be structure induced. These spatial patterns were unaffected by the earthquake. However, at two stations (N.NGOH and N.KWNH) we observe an increase in both the average and scatter of the normalized delay times (δt) during the aftershock period. In addition, two stations (HIROKA and N.YNTH) that are located in the strike‐normal direction east of the source area show an increase in the average of the normalized δt and a rotation of up to 90° of the fast direction immediately after the mainshock. We also notice that stations located very close to the source fault (DP.YMK and DP.OJK) show larger average delay times compared with stations farther away (HIROKA and N.YNTH) during the postseismic stage. To explain the temporal changes in the strength of the anisotropy, we speculate that spatiotemporal variations in microcrack development in and around the source area could be caused by static stress changes due to tectonic deformation and the earthquake rupture.

This chapter focuses on group discussion sessions targeting the Priority Areas of the Sendai Framework for Disaster Risk Reduction 2015–2030. Day one group discussion session efforts were on Priority Area One—Understanding Disaster Risks; and Day two emphasis was on Priority Areas 2, 3 and 4.

Relocation of earthquakes recorded by the agency for meteorology, climatology and geophysics (BMKG) in Indonesia and inversions of global positioning system (GPS) data reveal clear seismic gaps to the south of the island of Java. These gaps may be related to potential sources of future megathrust earthquakes in the region. To assess the expected inundation hazard, tsunami modeling was conducted based on several scenarios involving large tsunamigenic earthquakes generated by ruptures along segments of the megathrust south of Java. The worst-case scenario, in which the two megathrust segments spanning Java rupture simultaneously, shows that tsunami heights can reach ~ 20 m and ~ 12 m on the south coast of West and East Java, respectively, with an average maximum height of 4.5 m along the entire south coast of Java. These results support recent calls for a strengthening of the existing Indonesian Tsunami Early Warning System (InaTEWS), especially in Java, the most densely populated island in Indonesia.

Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project (JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪ Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪ Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪ Dynamic friction during the earthquake was lower than static friction. ▪ Complete stress drop is possible, and stress reversal is plausible.

The Central and East Java region is part of the Sunda Arc which has an important role in producing destructive earthquakes and volcanic complexes as a result of the subduction of the Indo-Australian plate under the Eurasian plate. Seismic tomography is one geophysical tool that is adaptable to understanding the mechanism process related to tectonic activity, seismicity, and volcanism. We collected a series of waveforms from 1,519 events in the period January 2009 to September 2017 and re-picked 11,192 phases for P- and S-waves at 34 stations of the BMKG network. We determined the 3-D P- and S-wave velocity structure beneath this high-risk region down to a depth of 200 km. In this study, we compare the tomographic images and relocated seismicity in order to represent the subducted slab geometry and the features in the seismic zones, i.e. the 2006 Yogyakarta earthquake zone (Opak fault), south of the mainland, and the 1994 Banyuwangi earthquake zone. Low-velocity anomalies beneath the volcanoes, i.e. Merapi, Merbabu, Kelud, Semeru, Bromo, and Ijen also imply the existence of fluid material and possible partial melting of the upper mantle which migrated from the subducted slab.

We apply a back-projection analysis to determine the locations and timing of the sources of short-period (0.5 to 2 s) energy generated by the 13 November 2016 Mw 7.8 Kaikoura, New Zealand earthquake using data from Australian and Southeast Asia. The sources of strong short-period energy are distributed northeast of the epicenter at distances of 70 to 80 km during the time period of 70 to 80 s after the initiation. The locations of sources of long-period energy derived from global seismic and local GPS data are close to the northeastern edge of the source area, and complementary to the areas of short-period energy which occur in the converging region of the Upper Kowhal, Papatea, and Jordan Thrust faults. The obvious frequency dependence might be attributed to complexities in fault geometry, possible rupture in the subduction interface, or varying focal mechanisms during the earthquake.

Fast estimates of magnitude and source extent of large earthquakes are fundamental for disaster mitigation. However, resolving these estimates within 10-20 min after origin time remains challenging. Here we propose a robust algorithm to resolve magnitude and source length of large earthquakes using seismic data recorded by regional arrays and global stations. We estimate source length and source duration by back-projecting seismic array data. Then the source duration and the maximum amplitude of the teleseismic P-wave displacement waveforms are used jointly to estimate magnitude. We apply this method to 74 shallow earthquakes that occurred within epicentral distances of 30-85 degrees to Hi-net (2004-2014). The estimated magnitudes are similar to moment magnitudes estimated from W-phase inversions (USGS), with standard deviations of 0.14-0.19 depending on the global station distributions. Application of this method to multiple regional seismic arrays, could benefit tsunami warning systems and emergency response to large global earthquakes.

The Global Alliance of Disaster Research Institutes held its 3rd Global Summit of Research Institutes for Disaster Risk Reduction at the Disaster Prevention Research Institute, Kyoto University, Japan, 19–21 March, 2017. The Global Alliance seeks to contribute to enhancing disaster risk reduction (DRR) and disaster resilience through the collaboration of research organizations around the world. The summit aim was to expand the platform for bridging science and policy making by evaluating the evidence base needed to meet the expected outcomes and actions of the Sendai Framework for Disaster Risk Reduction 2015–2030 and its Science and Technology Roadmap. The summit reflected the international nature of collaborative research and action. A pre-conference questionnaire filled out by Global Alliance members identified 323 research projects that are indicative of current research. These were categorized to support seven parallel discussion sessions related to the Sendai Framework priorities for action. Four discussion sessions focused on research that aims to deepen the understanding of disaster risks. Three cross-cutting sessions focused on research that is aimed at the priorities for action on governance, resilience, and recovery. Discussion summaries were presented in plenary sessions in support of outcomes for widely enhancing the science and policy of DRR.

Gross morphological structures of the Main Himalayan Thrust control the rupture length of the 2015 Mw 7.8 Nepal earthquake., The Himalaya orogenic belt produces frequent large earthquakes that affect population centers along a length of over 2500 km. The 2015 Gorkha, Nepal earthquake (Mw 7.8) ruptured the Main Himalayan Thrust (MHT) and allows direct measurements of the behavior of the continental collision zone. We study the MHT using seismic waveforms recorded by local stations that completely cover the aftershock zone. The MHT exhibits clear lateral variation along geologic strike, with the Lesser Himalayan ramp having moderate dip on the MHT beneath the mainshock area and a flatter and deeper MHT beneath the eastern end of the aftershock zone. East of the aftershock zone, seismic wave speed increases at MHT depths, perhaps due to subduction of an Indian basement ridge. A similar magnitude wave speed change occurs at the western end of the aftershock zone. These gross morphological structures of the MHT controlled the rupture length of the Gorkha earthquake.

A precursory sequence of repeating earthquakes was recorded before the Rausu landslide in Hokkaido, Japan on April 24, 2015. There were two seismic sequences with each consisting of very similar waveforms and leading up to significant landslide movements. The nearly-identical waveform shapes indicate similar source locations and mechanisms, so repeated events originated on a particular small area. This sequence is interpreted as stick-slip movement on a small patch leading up to the larger landslide failure. Our observations show that heterogeneous structure, such as asperities on the slip surface, can play an important role in the initiation of landslides, adding a new aspect to the conventional understanding of mechanisms controlling large mass movements.

High frequency energy radiation of large earthquakes is a key to evaluating shaking damage and is an important source characteristic for understanding rupture dynamics. We developed a new inversion method, Image Deconvolution Back-Projection (IDBP) to retrieve high frequency energy radiation of seismic sources by linear inversion of observed images from a back-projection approach. The observed back-projection image for multiple sources is considered as a convolution of the image of the true radiated energy and the array response for a point source. The array response that spreads energy both in space and time is evaluated by using data of a smaller reference earthquake that can be assumed to be a point source. The synthetic test of the method shows that the spatial and temporal resolution of the source is much better than that for the conventional back-projection method. We applied this new method to the 2001 M w 7.8 Kunlun earthquake using data recorded by Hi-net in Japan. The new method resolves a sharp image of the high frequency energy radiation with a significant portion of supershear rupture.

Studying rupture speeds of shallow earthquakes is of broad interest because it has a large effect on the strong near-field shaking that causes damage during earthquakes, and it is an important parameter that reflects stress levels and energy on a slipping fault. However, resolving rupture speed is difficult in standard waveform inversion methods due to limited near-field observations and the tradeoff between rupture speed and fault size for teleseismic observations. Here we applied back-projection methods to estimate the rupture speeds of 15 M w ≥ 7.8 dip-slip and 8 M w ≥ 7.5 strike-slip earthquakes for which direct P waves are well recorded in Japan on Hi-net, or in North America on USArray. We found that all strike-slip events had very fast average rupture speeds of 3.0–5.0 km/s, which are near or greater than the local shear wave velocity (supershear). These values are faster than for thrust and normal faulting earthquakes that generally rupture with speeds of 1.0–3.0 km/s.

The 25 April 2015, M(w)7.8 Gorkha, Nepal, earthquake ruptured a shallow section of the Indian-Eurasian plate boundary by reverse faulting with NNE-SSW compression, consistent with the direction of current Indian-Eurasian continental collision. The Gorkha main shock and aftershocks were recorded by permanent global and regional arrays and by a temporary local broadband array near the China-Nepal border deployed prior to the Gorkha main shock. We relocate 272 earthquakes with M-w > 3.5 by applying a multiscale double-difference earthquake relocation technique to arrival times of direct and depth phases recorded globally and locally. We determine a well-constrained depth of 18.5 km for the main shock hypocenter which places it on the Main Himalayan Thrust (MHT). Many of the aftershocks at shallower depths illuminate faulting structure in the hanging wall with dip angles that are steeper than the MHT. This system of thrust faults of the Lesser Himalaya may accommodate most of the elastic strain of the Himalayan orogeny.

We apply a backprojection analysis to determine the locations and timing of the sources of short‐period (0.5–5 s) energy generated by the 2015 Nepal earthquake. We use data from four different arrays at varying azimuths in Europe, China, Japan, and Australia, which show generally consistent features for the rupture propagation. The sources of strong short‐period energy are generally distributed east of the epicenter at distances of 10–100 km during the time period of 25–55 s after the initiation. The rupture speed was ∼1.0 km/s for the first 20 s then accelerated to ∼3.0 km/s for the remaining 30–40 s. The locations of sources of short‐period energy are close to the down‐dip edge of the fault and are complementary to the areas of large fault slip that occur further up‐dip. The Nepal earthquake might be another example in which regions of large fault slip do not coincide with the source areas of short‐period energy, which are likely associated with the damaging strong ground motions. Online Material: Animations of backprojections for the 2015 Nepal earthquake using four different arrays.

In order to investigate the seismic velocity structure in the region of concentrated severe damage during the 2016 Kumamoto earthquake sequence, we conducted continuous seismic observations in the central area of Mashiki Town, Kumamoto Prefecture. During 4 days of observations at eight temporary sites, 2 months after the mainshock, recordings from 30 aftershocks (1.7 ≤ M j ≤ 4.3, 1.9 km ≤ depth ≤ 13.5 km) were obtained. The aftershock data showed that site amplifications at approximately 1 Hz are dominant in a zone where almost no buildings were damaged along the Akitsu riverside, whereas site amplifications at higher than 3 Hz are observed in the heavily damaged zones. Our data also showed that the peak acceleration and velocity amplitudes, as well as seismic intensities for the small events in the less damaged zone, are clearly larger than those in the damaged zones, implying that the damage distribution is inconsistent with site response based on linear site amplifications. The estimated phase velocities of Rayleigh waves using the aftershock and microtremor data show dispersive characteristics in the lower frequency range (0.26 ≤ f ≤ 1.27 Hz), but the values are substantially smaller than those derived from the P–S logging model at the nearest KiK-net strong-motion observation station KMMH16. The derived microtremor horizontal-to-vertical spectral ratios and earthquake radial-to-vertical (R/V) spectral ratios show common distinct peaks at around 0.4 Hz, which are possibly related to the response of deep sedimentary layers beneath the area. The refined velocity structure model that better accounts for both the phase velocity and common dominant peak indicates that the values of S wave velocity (Vs) above the bedrock layer (Vs = 2700 m/s) are smaller than those inferred from the logging model and the depth to the bedrock layer could be much deeper (about 600 m) in comparison with the logging model (234 m). The derived R/V spectral ratio at station KMMH16 also shows a distinct peak at 0.4 Hz, suggesting that there is no large difference of deep sedimentary structure between the observation area and station KMMH16.

J. D. Kirkpatrick, M. Strasser, S. Kodaira, J. Sample, J. Mori, and S. Saito Department of Geosciences, Colorado State University, 1482 Campus Delivery, Fort Collins, CO 80523, USA Geological Institute, ETH Zurich, NO G 46, Sonneggstrasse 5, 8092 Zurich, Switzerland Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan School of Earth Sciences and Environmental Sustainability, Northern Arizona University, 602 S Humphreys, Flagstaff, AZ 86011, USA Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Research and Development Center for Ocean Drilling Science, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan

This is the publisher’s final pdf. The published article is copyrighted by the Geological Society of America and can be found at: http://geology.gsapubs.org/.

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (M w 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Our mineralogical analyses using X-ray diffraction reveal that the shallow portion of the fault zone that caused the earthquake is significantly enriched in smectite compared to the surrounding sediments, which may be intimately linked to the tsunamigenic shallow faulting. For comparison, we also analyzed mineralogical features of incoming sediments just prior to subduction, recovered on the outer rise of the Japan Trench (Site 436, Deep Sea Drilling Project Leg 56), and found a characteristic smectite-rich horizon in the uppermost ∼5 m of the pelagic clay layer. This horizon should be mechanically weak and will become the future plate boundary fault, as observed in the JFAST cores. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes that occur over a broad area of the Japan Trench plate boundary, which would result in large tsunamis for this region.

Several major fault-drilling projects have captured the interseismic and postseismic periods of earthquakes. However, near-field observations of faults immediately before and during an earthquake remain elusive due to the unpredictable nature of seismicity. The Scientific Exploration of Induced SeisMicity and Stress (SEISMS) workshop met in March 2017 to discuss the value of a drilling experiment where a fault is instrumented in advance of an earthquake induced through controlled fluid injection. The workshop participants articulated three key issues that could most effectively be addressed by such an experiment: (1) predictive understanding of the propensity for seismicity in reaction to human forcing, (2) identification of earthquake nucleation processes, and (3) constraints on the factors controlling earthquake size. A systematic review of previous injection experiments exposed important observational gaps in all of these areas. The participants discussed the instrumentation and technological needs as well as faults and tectonic areas that are feasible from both a societal and scientific standpoint.

A major International Ocean Discovery Program (IODP) workshop covering scientific ocean drilling in the southwest Pacific Ocean was held in Sydney, Australia, in late 2012. The workshop covered all fields of geoscience, and drilling targets in the area from the Equator to Antarctica. High-quality contributions and a positive and cooperative atmosphere ensured its success. The four science themes of the new IODP science plan were addressed. An additional resource-oriented theme considered possible co-investment opportunities involving IODP vessels. As a result of the workshop, existing proposals were revised and new ones written for the April 2013 deadline. Many of the proposals are broad and multidisciplinary in nature, hence broadening the scientific knowledge that can be produced by using the IODP infrastructure. This report briefly outlines the workshop and the related drilling plans.

The Integrated Ocean Drilling Program (IODP) Expedition 343, Japan Trench Fast Drilling Project (JFAST) drilled and cored across the plate-boundary decollement at Site C0019 near the Japan Trench, where large slip occurred during the 2011 Tohoku-Oki earthquake ( M w 9.0). Anisotropy of magnetic susceptibility (AMS) data obtained from core samples show a striking change in magnetic fabric across the decollement. In the frontal prism above the decollement, the maximum AMS axes display a strong preferred orientation in the northeast–southwest direction, with the intermediate and minimum AMS axes distributed in the vertical plane. In the underthrust sediments below the decollement, the maximum and intermediate AMS axes are subhorizontal but variable in direction, and the minimum AMS axes display an approximately vertical preferred orientation. The AMS ellipsoids for all samples have an oblate component, but the AMS ellipsoids of the underthrust sediments are generally less oblate than those of the frontal prism. The magnetic fabric of the sediments in the prism are consistent with horizontal tectonic shortening nearly parallel to the plate convergence direction in the Japan Trench, while that in the underthrust sediments represents a vertical, uniaxial strain. The magnetic fabrics of the frontal prism and underthrust sediments indicate an abrupt change in strain across the decollement at shallow depths, and imply that large co-seismic slip occurred along a weak plate-boundary decollement that is mainly decoupled over the long-term.

Using the Japan Meteorological Agency earthquake catalog since 1923, we investigated the seismic activity prior to the 2011 M w 9.0 Tohoku‐Oki earthquake with reference to the slip distribution, which is characterized by two high‐slip (≥20 m) patches separated by a zone of relatively low slip. The peak of the northern high‐slip patch is located near the trench while the peak of the southern high‐slip patch is situated about 40 km southeast of the mainshock epicenter, about 70 km away from the trench. It is estimated that the mainshock ruptured the southern high‐slip patch first and then extended to a larger adjacent region, including the northern high‐slip patch. The epicenters of foreshock activity that started two days before the mainshock are distributed in the western edge of the northern high‐slip patch where other prominent activity, such as the 1981 event, occurred during the past 90 years. Based on the spatiotemporal seismicity pattern around the two high‐slip patches, we infer that the foreshock activity triggered the 2011 Tohoku‐Oki earthquake because the increased stress from the foreshock activity was able to overcome the strength of the southern high‐slip patch, which had been sufficiently weakened by a series of large surrounding earthquakes since 2003. Other prominent activity, such as the 1981 event, failed to trigger such a great earthquake because similar stress condition had not been established at those times. The doughnut‐shaped seismicity pattern that formed around the peak of the southern high‐slip patch suggests the existence of an extremely strong patch that had not been ruptured by the surrounding large earthquakes for a long period of time.

The Yamasaki fault zone in southwestern Japan currently has a high potential for producing a large damaging earthquake. We carried out a seismic tomographic study to determine detailed crustal structures for the region. The velocity model clearly images a low-velocity and high V p /V s (high Poisson’s ratio) anomaly in the lower crust beneath the Yamasaki fault zone at a depth of ~15–20 km. This anomaly may be associated with the existence of partially-melted minerals. The existence of this anomaly below the fault zone may contribute to changing the long-term stress concentration in the seismogenic zone.

Water at the Bottom of a Well Earthquakes generate numerous fractures as they propagate through an underground fault zone. These fractures strongly influence the way in which fluids flow in the subsurface, and the permeability of fault zones is often used as a proxy for the extent of fracturing. Following the 2008 M w 7.9 Wenchuan earthquake in central China, several wells were drilled in and around the fault zone to understand the mechanics of the earthquake. Because the bottoms of these deep boreholes were open, the water levels in the wells were sensitive to tidal forces acting on the surrounding rock. Through continuous measurements of water levels over 1.5 years, Xue et al. (p. 1555 ) found that the rate at which water was pumped in and out of the borehole was proportional to the permeability of the fault zone, providing a direct way to measure the evolution of the hydrologic properties of a fault zone following a major earthquake. Permeability decreased ∼25% during that time, suggesting that fractures generated in fault zones heal relatively rapidly.

The 2011 Tohoku-Oki earthquake ruptured a large source region, which included areas with a range of past M 7 to M 8 earthquakes, varying stress accumulation, and different structural features. This megathrust event was substantially larger than anticipated in this region, though there were indications that the sequences of events in recent times were insufficient to relieve the full accumulated strain in the relatively rapid subduction of the Pacific plate. The source process time of about 150 s included rupture of an area of very large slip (30 to 60 m) on the shallow portion of the megathrust, updip of the hypocenter. The area of large slip produced large amplitude low-frequency radiation, while the dominant high-frequency radiation was generated from deeper sources down-dip of the hypocenter. Real-time information systems in Japan were able to issue timely warnings of the strong shaking and tsunami, but the complicated pattern of rupture growth led to an underestimate in the initial estimates of magnitude and tsunami threat. Variations in the plate coupling and heterogeneities of physical properties in the megathrust zone characterize the different source areas that ruptured together during the earthquake. Tomographic images of shear-wave and bulk-sound speeds show subtle changes of physical properties that may be associated with coupling condition and present clues for understanding the rupture process of this Mw 9 earthquake, which combined the source areas of many past earthquakes.

Coulomb stress triggering is examined using well-determined aftershock focal mechanisms and source models of the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake. We tested several slip distributions obtained by inverting onshore GPS-derived coseismic displacements under different a priori constraints on the initial fault parameters. The aftershock focal mechanisms are most consistent with the Coulomb stress change calculated for a slip distribution having a center of slip close to the trench. This demonstrates the capability of the Coulomb stress change to help constrain the slip distribution that is otherwise difficult to determine. Coulomb stress changes for normal-fault aftershocks near the Japan Trench are found to be strongly dependent on the slip on the shallow portion of the fault. This fact suggests the possibility that the slip on the shallow portion of the fault can be better constrained by combining information of the Coulomb stress change with other available data. The case of normal-fault aftershocks near some trench segment which are calculated to be negatively stressed shows such an example, suggesting that the actual slip on the shallow portion of the fault is larger than that inverted from GPS-derived coseismic displacements.

We performed a damage survey of buildings and carried out microtremor observations in the source region of the 2015 Gorkha earthquake. Our survey area spans the Kathmandu valley and areas to the east and north of the valley. Damage of buildings in the Kathmandu valley was localized, and the percentage of the totally collapsed buildings was less than 5 %. East of the Kathmandu valley, especially in Sindhupalchok district, damage of buildings was more severe. In the center of Chautara and Bahrabise, towns in Sindhupalchok district, the percentage of the totally collapsed houses exceeded 40 %. North of the Kathmandu valley, the damage was moderate, and 20–30 % of the buildings were totally collapsed in Dhunche. Based on the past studies and our microtremor observations near the strong motion station, the H/V spectrum in Kathmandu has a peak at around 0.3 Hz, which reflects the velocity contrast of the deep sedimentary basin. The H/V spectra in Bahrabise, Chautara, and Dhunche do not show clear peaks, which suggests that the sites have stiff soil conditions. Therefore, the more severe damage outside the Kathmandu valley compared with the relatively light damage levels in the valley is probably due to the source characteristics of the earthquake and/or the seismic performance of buildings, rather than the local site conditions.

Near-trench slip during large megathrust earthquakes (megaquakes) is an important factor in the generation of destructive tsunamis. We proposed a new approach to assessing the near-trench slip potential quantitatively by integrating laboratory-derived properties of fault materials and simulations of fault weakening and rupture propagation. Although the permeability of the sandy Nankai Trough materials are higher than that of the clayey materials from the Japan Trench, dynamic weakening by thermally pressurized fluid is greater at the Nankai Trough owing to higher friction, although initially overpressured fluid at the Nankai Trough restrains the fault weakening. Dynamic rupture simulations reproduced the large slip near the trench observed in the 2011 Tohoku-oki earthquake and predicted the possibility of a large slip of over 30 m for the impending megaquake at the Nankai Trough. Our integrative approach is applicable globally to subduction zones as a novel tool for the prediction of extreme tsunami-producing near-trench slip., 「ちきゅう」の断層掘削試料の分析と動力学解析による南海トラフ地震での断層すべり量の定量的評価. 京都大学プレスリリース. 2016-06-27.