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To examine the change in stress between before and after the Tohoku-oki Mw9.0 earthquake, we performed stress measurements after the earthquake in the Kamaishi mine in Iwate prefecture in northern Japan, located near the northern termination of the mainshock rupture, following previous measurements before the earthquake in the same mine. The results showed that the magnitudes of the three-dimensional principal stresses and the vertical stress drastically increased after the mainshock and, at 1 year after the earthquake, were more than double those before the earthquake. The principal stress magnitudes then decreased with time and returned to almost pre-earthquake levels at about 3 years after the earthquake. These changes can be interpreted in terms of coseismic rupture of the mainshock and the occurrence of aftershocks in the Sanriku-oki low-seismicity region (SLSR), where the Kamaishi mine is located. The drastic increase in stress suggests that the SLSR may act as a barrier to further rupture propagation., 東北地方太平洋沖地震前後で地殻に加わった力の増減と余震との相関を実測で解明 --地下数百mでも地震すべり破壊挙動の理解は可能--. 京都大学プレスリリース. 2017-09-07.

On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

To understand the generation mechanism of the 2011 Tohoku-Oki earthquake, we analyzed the spatial and temporal distribution of the cumulative seismic moment (ΣM0) for all earthquakes along the Japan Trench listed in the global centroid moment tensor catalog between January 1976 and November 2011. Three areas with distinct characteristics of ΣM0 are identified: (1) in the 2011 Tohoku-Oki source area, the ΣM0 released in the down-dip portion (≥30 km) was greater than that in the up-dip portion (

Temporal change in the stress field after the 2011 Tohoku earthquake was observed by stress tensor inversions of focal mechanisms of earthquakes near the source region. The maximum compressive stress (σ1) axis before the earthquake has a direction toward the plate convergence, dipping oceanward at an angle of 25–30 degrees. Its dip angle significantly increased by 30–35 degrees after the earthquake, and σ1 axis came to intersect with the plate interface at a high angle of about 80 degrees. By using the observed rotation of σ1 axis, we estimated the ratio of mainshock stress drop to the background deviatoric stress Δτ/τ to be 0.9–0.95. This shows that the deviatoric stress causing the Mw 9.0 earthquake was mostly released by the earthquake, or the stress drop during the earthquake was nearly complete. Adopting the average stress drop obtained by GPS observation data, the deviatoric stress magnitude is estimated to be 21–22 MPa. This suggests the plate interface is weak. The nearly complete stress drop caused a high dip angle of σ1 axis, which is the reason why not a small number of normal fault type aftershocks have occurred.

We conduct a systematic survey of remote triggering of earthquakes in north China following the 2008 Mw 7.9 Wenchuan earthquake. We identify triggered earthquakes as impulsive seismic energies with clear P and S arrivals on 5 Hz high-pass-filtered three-component velocity seismograms during and immediately after the passage of teleseismic waves. We find clearly triggered seismic activity near the Babaoshan and Huangzhuang-Gaoliying faults southwest of Beijing, and near the aftershock zone of the 1976 MW 7.6 Tangshan earthquake. While several earthquakes occur during and immediately after the teleseismic waves in the aftershock zone of the 1975 Mw 7.0 Haicheng earthquake, the change of seismicity is not significant enough to establish the direct triggering relationship. Our results suggest that intraplate regions with active faults associated with major earthquakes during historic or recent times are susceptible to remote triggering. We note that this does not always guarantee the triggering to occur, indicating that other conditions are needed. Since none of these regions is associated with any active geothermal or volcanic activity, we infer that dynamic triggering could be ubiquitous and occur in a wide range of tectonic environments.

We present some singular characteristics of the 2011 off the Pacific coast of Tohoku Earthquake in comparison with other megathrust earthquakes, such as the 1960 Chilean and the 2004 Sumatra-Andaman earthquakes. In addition to the conventional along-strike segmentation, along-dip segmentation of the fault area or subduction zone is an important feature for the Tohoku subduction zone, as indicated by the difference in background seismicity: virtually no seismicity in shallow segments but active with large events repeating in deep segments. The interaction between along-dip segments (deep and shallow segments) led to the great 2011 Tohoku earthquake. The along-dip segmentation results in plane or two-dimensional rupture propagation on a coseismic fault. Significant along-strike variability is also important for the 2011 Tohoku earthquake, with segments of both weak (e.g., slow or tsunami earthquakes) and strong plate couplings located adjacent to each other. In contrast, every segment appears to be with strong plate coupling for other megathrust earthquakes. One exception is the 1964 Alaska earthquake that shares a certain degree of common characteristics with the 2011 Tohoku earthquake: two distinct seismogenic zones along the dip direction of the trench, that is, the along-dip segmentation is noticeable. Significant along-strike variability also characterizes the activities in and around the subduction zone of the 1964 Alaska earthquake, including a creeping segment and a tsunami-earthquake segment.

Strong ground motions from the 2011 off the Pacific coast of Tohoku Earthquake, the most powerful earthquake to have occurred in and around Japan after the installation of a modern seismic network, were recorded for more than 300 seconds by a dense and wide-span seismic network, the Metropolitan Seismic Observation Network (MeSO-net), installed around the Tokyo metropolitan area about 200 km away from the epicenter. We investigate the rupture process of the earthquake in space and time by performing semblance-enhanced stacking analysis of the waveforms in a frequency range of 0.05 to 0.5 Hz. By projecting the power of the stacked waveforms to an assumed fault plane, the rupture propagation image of the large and complex earthquake has been successfully obtained. The seismic energy was mainly generated from the off-shore areas of about 100 km away from the coast in Miyagi and Fukushima Prefectures. The shallow and eastern part of the fault along the Japan trench off Miyagi Prefecture released strong seismic energy which might have been related to the excitation of gigantic tsunami. In contrast, the southern shallow part of the fault plane, off Ibaraki Prefecture, released only minor seismic energy. Our analysis suggests that the focal areas combining both the officially-forecasted Miyagi-oki earthquake and those of historical earthquakes that occurred off the coast of Fukushima Prefecture in 1938 were broken, resulting in the 2011 great M 9 earthquake.

An eastern half of the focal region of the 1854 Ansei-Tokai earthquake was not ruptured at the 1944 Tonankai earthquake and the area around Suruga Bay remained as a seismic gap (Ishibashi, 1981). As to the cause that the fault motion in 1944 did not extend to the eastern part, Mogi (1981) and Pollitz and Sacks (1995) considered effects of the 1891 Nobi earthquake of M8.0. In this paper, we evaluate quantitatively effects of nearby large earthquakes, the 1891 Nobi, the 1923 Kanto and the 1944 Tonankai earthquakes, on the occurrence time of the Tokai earthquake that has been anticipated to occur soon to fill the seismic gap, using a three-dimensional simulation model with a rate- and state-dependent friction law (Ruina, 1983). It is suggested that the occurrence of the Tokai earthquake was delayed by the 1891 Nobi earthquake, while it was advanced by the 1923 Kanto and the 1944 Tonankai earthquakes. We show that the sense and degree of the effect of nearby large earthquakes on the occurrence time of the anticipated Tokai earthquake depend on whether and how much the stress perturbation produced by the nearby earthquakes strengthens or weakens the stress accumulated in the coupling region, especially in the highly shear-stressed ring zone (Kuroki et al., 2002). However it is difficult to say definitely how much the Tokai earthquake was delayed or advanced, because the magnitude of the effect is very sensitive to the location of the fault planes.

Hypocenters of main shocks and aftershocks of the 1933 M=7.1, 1936 M=7.4, 1937 M=7.1 and 1978 M=7.4 Miyagi-oki earthquakes are relocated using S-P times reported in the Seismological Bulletin of the Japan Meteorological Agency (JMA) and those re-read from original smoked-paper seismograms observed at the Mizusawa station of the National Astronomical Observatory of Japan (NAOJ) and the Mukaiyama station of Tohoku University. In order to reduce the error caused by inaccuracies of the arrival times and the small number of seismic observation stations, we determined the hypocenters by using a grid search method that assumed that the events occurred at the boundary between the subducting Pacific plate and the overriding plate. The main shock epicenters of these four earthquakes were determined to be close to each other, while the distributions of their aftershocks seem to disperse on the upper boundary of the Pacific plate. These distributions show that aftershock areas of the 1933, 1936 and 1937 events partly overlap with that of the 1978 event and occupy its easternmost, central and westernmost portions, respectively. This result suggests that the 1933, 1936 and 1937 events possibly ruptured a part of the source area of the 1978 event, i.e., its eastern, central and western portions, respectively.

We have estimated the spatio-temporal slip distribution of the 12 May, 2008, Wenchuan earthquake from teleseismic P-wave data. Teleseismic body wave observations are useful geophysical observations for estimating the overall seismic source process immediately after a great earthquake. However, it is difficult to constrain the slip vectors of later sub-events during a large earthquake such as the 2008 Wenchuan earthquake because the modeling error due to the Green’s function uncertainty generally increases with time. In this study, we have applied a new waveform inversion method that incorporates the uncertainty of the Green’s function, and we have assumed a simple fault plane model to evaluate our inversion scheme for a rapid analysis of the seismic source process. The results show that the slip history of the Wenchuan earthquake include six distinct sub-events. The derived source parameters were a seismic moment M0 = 1.4 × 1021 Nm (MW 8.0) and a source duration = 125 s. The total slip vector distribution and segmentations obtained in our study are consistent with results obtained from InSAR data. The manner of the rupture propagation suggests that the two major source faults, the Beichuan fault and the Pengguan fault, intersected in the deep part of the seismogenic zone.

Based on the model of oblique convergence, we predicted the slip rate on the megathrust plate boundary in the western Sunda arc and compared it with the slip distribution of the 2004 M w 9.0 Sumatra-Andaman earthquake. The slip directions on the megathrust plate boundary, which are necessary for predicting the slip rate, were determined using the fault plane solutions of medium-sized earthquakes occurring for the last 30 years in the 2004 rupture zone. At long wavelengths the predicted slip rate correlates well with the latitudinal variation of the coseismic slip, suggesting that kinematics described by the model of oblique convergence operated over a long interseismic period and that the seismic coupling coefficient has been generally uniform along the strike of the rupture zone. Using the data for the same period of time, we estimated the total seismic moment released by the earthquakes on the Sumatra fault and its submarine continuation located on the backarc side of the Nicobar and Andaman islands. The ratio of the observed seismic moment to the seismic moment predicted from the model of oblique convergence is small. The residual may either be stored for generation of future earthquakes or was taken up by the fault creep at shallow depths.

Earthquake predictability is a fundamental problem of seismology. Using a sophisticated model, a Bayesian approach with lognormal distribution on the renewal process, we theoretically formulated a method to calculate the conditional probability of a forthcoming recurrent event and forecast the probabilities of small interplate repeating earthquakes along the Japan Trench. The numbers of forecast sequences for 12 months were 93 for July 2006 to June 2007, 127 for 2008, 145 for 2009, and 163 for 2010. Forecasts except for 2006–07 were posted on a web site for impartial testing. Consistencies of the probabilities with catalog data of two early experiments were so good that they were statistically accepted. However, the 2009 forecasts were rejected by the statistical tests, mainly due to a large slow slip event on the plate boundary triggered by two events with M 7.0 and M 6.9. All 365 forecasts of the three experiments were statistically accepted by consistency tests. Comparison tests and the relative/receiver operating characteristic confirm that our model has significantly higher performance in probabilistic forecast than the exponential distribution model on the Poisson process. Therefore, we conclude that the occurrence of microrepeaters is statistically dependent on elapsed time since the last event and is not random in time.

After M W 7.8 Nepal earthquake occurred, the rearrangement of stresses in the crust commonly leads to subsequent damaging earthquakes. We present the calculations of the coseismic stress changes that resulted from the 25th April event using models of regional faults designed according to south Tibet-Nepal structure, and show that some indicative significant stress increases. We calculate static stress changes caused by the displacement of a fault on which dislocations happen and an earthquake occurs. A M W 7.3 earthquake broke on 12 May at a distance of ~ 130 km SEE of the M W 7.8 earthquake, whose focus roughly located on high Coulomb stress change (CSC) site. Aftershocks (first 15 days after the mainshock) are associated with stress increase zone caused by the main rupture. We set receiver faults with specified strikes, dips, and rakes, on which the stresses imparted by the source fault are resolved. Four group normal faults to the north of the Nepal earthquake seismogenic fault were set as receiver faults and variant results followed. We provide a discussion on Coulomb stress transfer for the seismogenic fault, which is useful to identify potential future rupture zones.

The Northern Miyagi earthquake (Mj 6.4) on 26 July, 2003, was a shallow crustal earthquake produced by high-angle reverse faulting. To construct a realistic geologic model for this fault system from depth to the surface, seismic reflection profiling was carried out across the northern part of the source fault of this earthquake. The common mid-point seismic reflection data were acquired using a vibrator truck along a 12 km-long seismic line. The obtained seismic profile portrays a Miocene half-graben bounded by a west-dipping fault. Consistent with gravity anomaly data, the maximum thickness of the basin fill probably reaches 3 km. From the regional geology, this basin-bounding normal fault forms the eastern edge of the northern Honshu rift system and was produced by rapid extension during 17–15 Ma. The deeper extension of the fault revealed by seismic profiling coincides with the planar distribution of aftershocks. The hypocentral distribution of the aftershocks shows a concentration on a plane dipping 55 degrees to the west with listric geometry. Thus, the basin inversion has been performed using the same fault; the 2003 Northern Miyagi earthquake was generated by fault reactivation of a Miocene normal fault.

The high strain rates of the Niigata-Kobe Tectonic Zone (NKTZ) of central Japan have been simulated using a viscoelastic finite element model of great interplate earthquake cycles that includes the subducting Pacific plate and lateral changes in crustal structure beneath NKTZ. As a result of long-term loading at trenches, the elastic crust over a viscoelastic mantle can transfer stress or strain rate not only as an average of geological time scales, but also during an interplate earthquake cycle. Thus, if the earthquake cycle is several times longer than the relaxation time of the viscoelastic system, deformation of an inland area far from trenches responds to the cycles of interplate earthquakes, and the crust deforms like an elastic sheet during the later stage of an earthquake cycle, allowing efficient transmission of stress during this period. Therefore, in the case of long interplate earthquake cycle, the NKTZ crustal structure with small thickness and rigidity enhances local interseismic strain concentration there. The “plate coupling ratio” (0.4∼0.5) between Pacific and Amurian plates and local crustal heterogeneity beneath the high strain rate zone explain the large strain rate belt if the earthquake cycle offshore Kanto is enough long.

The 11 March 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) produced megathrust displacements of at least 40 m. The resulting tsunami devastated the Honshu coast southwest of regions struck by earthquake-generated tsunami in 1611, 1896 and 1933. The 1896 Meiji-Sanriku earthquake was also an underthrusting earthquake, but the 1933 Sanriku-oki earthquake was a trench-slope normal faulting event; both generated inundation heights of 10 to 25 m along the coast of Iwate prefecture. Possible occurrence of a great outer trench-slope earthquake seaward of the 2011 Tohoku Earthquake along a southwestward extension of the 1933 fault zone is a concern. The second largest 2011 aftershock, an outer rise Mw 7.7 normal faulting earthquake occurred near the southern end of the 1933 rupture. Additional aftershock activity has been distributed along a trend below the trench and diffusely spread in the outer rise, seaward of the megathrust region where the largest slip occurred. Coulomb stress perturbations of at least 5–10 bars are calculated for outer rise normal fault geometries for mainshock slip models. Whether a future great trench slope event will occur is uncertain, but the potential tsunamigenic hazard can be gauged by the huge inundations accompanying the 1933 rupture.

We analyzed the far-field co-seismic response of the M W 9.0 Tohoku-Oki earthquake, which occurred on March 11th 2011 at the Japan Trench plate boundary. Our analysis indicates that the far-field co-seismic displacement was very sensitive to the magnitude of this event, and that a significant co-seismic surface displacement from earthquakes in the Japan Trench region can be observed in Eurasia only for events of M W ≥ 8.0. We also analyzed the temporal characteristics of the near-field post-seismic deformation caused by the afterslip and the viscoelastic relaxation following the Japan earthquake. Next, we performed a simulation to analyze the influence of the two post-seismic effects previously mentioned on the far-field post-seismic crustal deformation. The simulation results help explain the post-seismic crustal deformation observed on the Chinese mainland 1.5 years after the event. Fitting results revealed that after the M W 9.0 Tohoku-Oki earthquake, the afterslip decayed exponentially, and may eventually disappear after 4 years. The far-field post-seismic displacement in Eurasia caused by the viscoelastic relaxation following this earthquake will reach the same magnitude as the co-seismic displacement in approximately 10 years. In addition, the co- and post-seismic Coulomb stress on several NE-trending faults in the northeastern and northern regions of the Chinese mainland were significantly enhanced because of the M W 9.0 earthquake, especially on the Yilan-Yitong and the Dunhua-Mishan faults (the northern section of the Tan-Lu fault zone) as well as the Yalujiang and the Fuyu-Zhaodong faults.

A large Mw = 8.1 earthquake occurred off southeast coast of Antarctica near the Balleny Island region on March 25, 1998. We inverted teleseismic body-wave records to determine the rupture pattern using an iterative deconvolution method. The source parameters obtained are: the centroid depth=20km, (strike, dip, rake)= (282, 83, −1), the seismic moment M0 = 1.6 × 1021 Nm (Mw = 8.1), the length L = 200 km, and the average slip D = 4.4 m. This earthquake occurred in the mid-plate but there has been no reports of such large earthquakes in this region. Furthermore, the source mechanism cannot be related to the plate motion inferred from the nearby transform faults. Therefore this earthquake is not a usual tectonic event. Here we show that the compressional axis of our source mechanism coincides with the horizontal crustal motion predicted by the Earth’s response to present-day and past ice mass changes in Antarctica. Our result suggests that the 1998 Antarctica earthquake is caused by the postglacial rebound in the Antarctica.

We estimate the source model of the 2005 Miyagi-Oki earthquake, which consists of two source patches called “strong motion generation areas (SMGAs)”, to explain the broadband strong motions. The location of the first SMGA is determined from the arrival time differences between the first motion and the first pulse in theP-wave portion. Based on the broadband waveform modeling obtained using the empirical Green’s function method, we estimate the parameters of the two SMGAs. The estimated location of the two SMGAs coincides with the two major large slip areas inferred from the inversion of strong-motion and teleseismic waveforms, which implies that the broadband strong motion of the 2005 Miyagi-Oki earthquake mainly radiates from the asperities. The deeper SMGA has a larger stress drop (34.1 MPa) than the shallower one (17.6 MPa). Comparison of the locations of SMGAs between the 2005 and 1978 Miyagi-Oki earthquakes suggests that they do not overlap with each other. The interplate earthquakes, including this event, were found to have smaller SMGAs than those predicted from the relationship of crustal earthquakes for a given seismic moment. This implies that the stress drop of the SMGAs for interplate earthquakes is larger than that for crustal earthquakes.

The strength of seismogenic faults is fundamental to earthquake mechanics and plate tectonics, affecting many subsidiary processes in the solid earth. The key to understanding fault strength is determining the pore pressures and hydraulic properties within the faults and surrounding crust. The debate has lasted over decades, with evidence provided for both strong fault and weak fault scenarios. A recently proposed hypothesis for quantifying the strength at which faults fail uses earthquake scaling arguments to show that earthquakes fail over a range of strengths in the upper 15–20 km, and at near-lithostatic pore pressure below this depth. This observation, if correct, has important implications for crustal hydraulics, plate tectonics, and earthquake hazard assessment. This paper summarizes the arguments for high pore pressure faulting, and explores its implications for earthquake stress drops, strength of the lithosphere, and earthquake scaling. The hope is to establish a general framework for quantifying the role of fluids in the earthquake process, and to demonstrate that high fluid pressure may dominate failure of the brittle crust.

We investigated the crustal deformation associated with the 2011 off the Pacific coast of Tohoku Earthquake (M 9.0) that occurred on March 11, 2011, along the plate boundary off Tohoku district, northeastern Japan, based on dense GPS observation. Coseismic displacements due to this event were applied to estimate the causal interplate slip by means of a geodetic inversion analysis. The major slip area is located around the asperities of the 1981 Miyagi-oki (M 7.2) and 2003 Fukushima-oki (M 6.8) earthquakes and the maximum slip is estimated as being up to 35 m. The estimated slip distribution suggests that the asperities of the Miyagi-oki earthquake in 1978 (M 7.4) that had not been ruptured during the Miyagi-oki earthquake in 2005 were ruptured as a part of the main shock fault of the 2011 off the Pacific coast of Tohoku Earthquake.

The occurrence rate of the characteristic earthquake (CE) was compared with seismicity in nine source regions of interplate earthquake (i.e., the regions off Shikotan Island, off Nemuro, off Tokachi, off Northern Sanriku, off Miyagi, far off Miyagi, and the Tonankai, Nankai, and Kanto regions) by combining instrumental data and the recurrence interval of CEs evaluated by the Headquarters for Earthquake Research Promotion (HERP) in Japan. The recurrence interval of the Nankai earthquake was estimated on the basis of long historical records and found to be one of the least uncertain ones. We used the unified catalog of earthquakes obtained by the Japan Meteorological Agency (JMA) on the basis of a recently improved seismic network, together with the old JMA catalog. Seismicity of all the interplate source regions indicated that the number of observed events was much less than the number of events predicted from the Gutenberg-Richter (G-R) relation and the occurrence rate of the CE. In all regions except for far off Miyagi, the CEs occurred during the interval of the earthquake catalog. Thus, our data set included the highest seismicity period during an earthquake cycle. In the region off Tokachi where the 1952 and 2003 Tokachi-Oki earthquakes occurred, the magnitude frequency distribution (MFD) during one seismic cycle exhibited a magnitude gap of 1.1 between the CE and the other events. Therefore, our results favored the CE model. Moreover, we conversely estimated an average recurrence interval of the CEs in each region, based on the assumption that the G-R relation holds. Most estimated recurrence intervals were far longer than the evaluation given by HERP.

We have estimated the spatial distribution of interplate coupling in and around the source area for the 2011 Tohoku earthquake from small repeating earthquake data. The source area of the 2011 Tohoku earthquake exhibited a relatively high coupling coefficient (>0.5) for the period from 1993 to 2007 and was surrounded by areas of low coupling (

On March 11th, 2011, a moment magnitude 9.0 earthquake struck off the coast of northeast Honshu, Japan, generating what may well turn out to be the most costly natural disaster ever. In the hours following the event, the U.S. Geological Survey National Earthquake Information Center led a rapid response to characterize the earthquake in terms of its location, size, faulting source, shaking and slip distributions, and population exposure, in order to place the disaster in a framework necessary for timely humanitarian response. As part of this effort, fast finite-fault inversions using globally distributed body- and surface-wave data were used to estimate the slip distribution of the earthquake rupture. Models generated within 7 hours of the earthquake origin time indicated that the event ruptured a fault up to 300 km long, roughly centered on the earthquake hypocenter, and involved peak slips of 20 m or more. Updates since this preliminary solution improve the details of this inversion solution and thus our understanding of the rupture process. However, significant observations such as the up-dip nature of rupture propagation and the along-strike length of faulting did not significantly change, demonstrating the usefulness of rapid source characterization for understanding the first order characteristics of major earthquakes.

The 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) occurred on June 14, 2008 in a zone of concentrated deformation along the mountain backbone of Tohoku encompassing SW Iwate and NW Miyagi Prefectures, NE Japan. This earthquake was a shallow intraplate earthquake with the reverse-type focal mechanism. Regular seismic activity is high in this concentrated deformation zone. We have performed regional-scale seismic tomography in and around the focal area and found that the distinct low-velocity regions are continuously distributed from the mantle wedge to the lower crust just below the focal area and active seismic zones. This low-velocity zone can be interpreted to be a region of partial melting, suggesting that crustal fluids separating and upwelling from deeper portion may be closely related with the occurrence of both the active seismic zones and the 2008 earthquake.

The devastating Mw 9.0 earthquake on March 11, 2011 is one of the most complex earthquakes of all the recent large events. Its source region is compact for an earthquake of this size, but it has highly variable amount of energy release from different segments. These conditions prevent conventional back-projection analysis to reveal the details of the rupture process. We incorporate a new metric to assess coherency as a part of back-projection analysis to ensure identification of these weak features. The main features obtained with this new back-projection approach are consistent with previous back-projection results, with strongest energy release downdip and close to the epicentral location. The main rupture propagation is along strike, in southwestern direction giving rise to the length extent of the earthquake. The new coherency function also allows us to investigate rupture characteristics at the beginning of the earthquake, resolving initial updip propagation from the epicentral location. Furthermore, some of very weak energy in the stacks are identified with high coherency. These additional source regions extend the area of the earthquake farther south and north than the region that has been imaged by other back-projection studies.

Detailed magnetic surveys carried out in the Suruga trough area, where the Philippine Sea plate subducts beneath Japan, revealed some local magnetic anomalies on the overriding plate side. We first show that these anomalies reflect seamounts sitting on and subducting with the Philippine Sea plate, since no magnetic sources corresponding to the anomalies are identified in the seafloor topography. An important issue to be addressed is how these seamounts affect the generation of a great interplate earthquake at the Suruga trough (often referred to as the Tokai earthquake). So we made a numerical simulation of earthquake generation, applying the rate- and state-variable friction law to the plate interface. Here we incorporated the seamounts in the simulation as an additional normal stress exerted locally to the interface. We then found how the seamounts affect the generation of an interplate earthquake depends on whether or not they are located in the seismogenic zone. The most notable effect is that the amount of slip immediately before the main shock, often called a pre-slip, tends to be enhanced if they are located inside the zone.

A large earthquake (Mw=7.7) along a plate boundary occurred in the south of Java Island on July 17, 2006, and caused a significant tsunami. We made GPS observations and tsunami heights measurements during the period from July 24 to August 1, 2006. The earthquake seems to be due to an interplate low angle reverse faulting, though there might be a possibility of high angle faulting within the subducting lithosphere. Crustal deformation distribution due to the earthquake, aided by tsunami heights measurements, might clarify which would be the case. We occupied 29 sites by GPS in the area of southern Java encompassing the area from 107.8 E to 109.50 E. These sites were occupied once before the earthquake. However, we were not able to detect significant coseismic displacements. The obtained displacements, most of which span several years, show ESE direction in ITRF2000 frame. This represents the direction of Sunda block motion. The tsunami heights measured at 11 sites were 6–7 m along the southern coast of Java and indicate that the observed heights are systematically higher than those estimated from numerical simulations that are based on seismic data analysis. This might suggest that fault offsets might have been larger—nearly double—than those estimated using seismic analysis. These results lead us to an idea that the rupture was very slow. If this is the case, the earthquake might have been a “tsunami earthquake” that is similar to the one that occurred on June 2, 1994 in the east of the present earthquake.

We propose a source fault model for the 2011, April 7, earthquake (M 7.1) deduced from a dense GPS network. The coseismic displacements obtained by GPS data analysis clearly show the spatial pattern specific to intraslab earthquakes, not only in the horizontal components but also the vertical ones. A rectangular fault with uniform slip was estimated by a non-linear inversion approach. The results indicate that the simple rectangular fault model can explain the overall features of the observations. The amount of moment released is equivalent to Mw 7.17. The hypocenter depth of the main shock estimated by the Japan Meteorological Agency is slightly deeper than the neutral plane between down-dip compression (DC) and down-dip extension (DE) stress zones of the double-planed seismic zone. This suggests that the depth of the neutral plane was deepened by the huge slip of the 2011, M 9.0, Tohoku Earthquake, and the rupture of the thrust M 7.1 earthquake was initiated at that depth, although more investigations are required to confirm this idea. The estimated fault plane has an angle of ∼60 degrees from the surface of the subducting Pacific plate. It is consistent with the hypothesis that intraslab earthquakes are thought to be a reactivation of the pre-existing hydrated weak zones produced in the bending process of oceanic plates around outer-rise regions.

The recurring devastating earthquake that occurs in the Nankai Trough subduction zone between the Philippine Sea plate and the Eurasian plate has the potential to cause an extremely dangerous natural disaster in the foreseeable future. Many previous studies have assumed interplate-coupling ratios for this region along the trench axis using onshore geodetic data in order to understand this recursive event. However, the offshore region that has the potential to drive a devastating tsunami cannot be resolved sufficiently because the observation network is biased to the land area. Therefore, the Hydrographic and Oceanographic Department of Japan constructed a geodetic observation network on the seafloor along the Nankai Trough using a GPS-acoustic combination technique and has used it to observe seafloor crustal movements directly above the Nankai Trough subduction zone. We have set six seafloor sites and cumulated enough data to determine the displacement rate from 2006 to January 2011. Our seafloor geodetic observations at these sites revealed a heterogeneous interplate coupling that has three particular features. The fast displacement rates observed in the easternmost area indicate strong interplate coupling (>75%) around not only the future Tokai earthquake source region but also the Paleo-Zenisu ridge. The slow displacement rates near the trench axis in the Kumano-nada Sea, a shallow part of the 1944 Tonankai earthquake source region, show a lower coupling ratio (50% to 75%). The slow displacement rate observed in the area shallower than the 1946 Nankaido earthquake source region off Cape Muroto-zaki reflects weakening interplate coupling (about 50%) probably due to a subducting seamount. Our observations above the subducting ridge and seamount indicate that the effect of a subducting seamount on an interplate-coupling region depends on various conditions such as the geometry of the seamount and the friction parameters on the plate boundary.

An Mw7.1 Kumamoto earthquake occurred at 01:25:05 on April 16, 2016 (JST). The earthquake involved a rupture at a shallow depth along a strike-slip fault with surface breaks. Near-fault ground motion records, especially those of a strike-slip earthquake, can provide us with direct information on the earthquake source process. During the earthquake, near-fault seismograms were obtained at KMMH16 station located about 500 m off the fault. The ground displacements were well recovered from the double numerical integration of accelerograms at KMMH16 both on the surface and at the bottom of the 252-m-deep borehole. Fault-parallel static displacement was estimated to be about 1.1 m from the acceleration waveforms. The Dc″ value, which is defined as double the fault-parallel displacement at peak velocity time, was proposed as a proxy of the slip-weakening distance. Using both the velocity and displacement fault-parallel waveforms, the Dc″ value was estimated at about 1 m. This value was between 30 and 50% of the total slip on the fault, which is consistent with previous observations. Graphical abstract GRAPHICAL ABSTRACT CAPTION

We test the static Coulomb stress triggering hypothesis for three recent megathrust earthquakes (the 2004 Sumatra–Andaman earthquake, the 2010 Maule earthquake, and the 2011 Tohoku-Oki earthquake) using focal mechanism solutions for actual earthquakes as receiver faults to calculate Coulomb stress changes. For the 2004 Sumatra–Andaman and 2011 Tohoku-Oki earthquakes, the median values of the Coulomb stress changes for 100 consecutive earthquakes revealed temporal changes from approximately zero before the megathrust earthquake to significant positive values following the mainshock, followed by decay over time. Furthermore, the ratio of the number of positively to negatively stressed receiver faults increased after the megathrust. These results support the triggering hypothesis that the static stress changes imparted by megathrust earthquakes cause seismicity changes. This is in contrast to the results of a previous study that used optimally orientated receiver faults to calculate Coulomb stress changes, and this difference indicates the importance of considering the spatial and temporal heterogeneities of receiver fault distributions. For the 2010 Maule earthquake, however, the results are strongly dependent on fault-slip models. Since most receiver faults are concentrated in the mainshock source region, slip models significantly affect the computed Coulomb stress changes and sometimes cause anomalous stress concentrations along the edge of each sub-fault.

A strength profile across the NE Japan interplate megathrust was constructed in the source region of the 2011 Tohoku-oki earthquake (Mw9.0) using friction, fracturing, and ductile flow data of the oceanic crustal materials obtained from laboratory experiments. The depth-dependent changes in pressure, temperature, and pore fluid pressure were incorporated into a model. The large tsunamigenic slips during the M9 event can be explained by a large gradient in fault strength on the up-dip side of the M9 hypocenter, which was located 17 to 18 km beneath sea level. A large stress drop (approximately 80 MPa) induced by the collapse of a subducted seamount possibly triggered the M9 earthquake. In the deep (>35 km) part of the thrust fault, where M7-class Miyagi-oki earthquakes have repeatedly occurred, plastic deformation occurs in siliceous rocks but not in gabbroic rocks. Thus, the asperity associated with the M7-class earthquakes was most likely a gabbroic body, such as a broken seamount, surrounded by siliceous sedimentary rocks. The conditionally stable nature of the surrounding region can be explained by the frictional behavior of wet quartz in the brittle-ductile transition zone. In contrast to the deep M7-class asperity, the M9 asperity (i.e., a region that was strongly coupled before the M9 Tohoku-oki earthquake) extended to a large area of the plate interface because shear strength is relatively insensitive to lithological variation at intermediate depths. However, the along-arc extension of the M9 asperity was constrained by fluid-rich regions on the plate interface.

To understand the effect of plate coupling on very low-frequency event (VLFE) activity resulting from megathrust earthquakes, we performed long-term multiscale earthquake cycle simulations (including a megathrust earthquake and slow earthquakes) on a 3-D subduction plate boundary model, based on a rate- and state-dependent friction law. Our simulation suggests that quiescence of shallow VLFEs off Miyagi may be explained by the location in the shallow central part of the 2011 Tohoku earthquake because of the locally strong coupling, while observed activation of VLFEs off Iwate (northern part of Tohoku district), Fukushima (southern part of Tohoku district), and Ibaraki (northern part of Kanto district) is explained by the location on the outer rim. The area and duration of the quiescence off Miyagi may be a new clue to evaluate the potential for plate coupling strong enough to cause the next megathrust earthquake.