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Shallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities. [ABSTRACT FROM AUTHOR]

Feasibility studies on simulating seismic-wave propagation in media for a suboceanic earthquake, including both land and ocean-bottom topographies and a seawater layer, are scarce. Some of the conventional staggered-grid finite-difference method (FDM) simulations use a simplified structure model without a seawater layer and with a flat ocean-bottom topography. In this study, we apply our heterogeneity, oceanic layer, and topography scheme (HOT)-FDM and a 3D structure model including land and ocean-bottom topographies, a seawater layer, and a fluid-solid boundary condition to an aftershock (Mw 5.8) of the 2009 Suruga Bay earthquake. We attempt to reproduce observations at seismic stations near the coast and then simulate waveforms in the ocean-bottom stations. Our results show that a large difference between the cases with and without topographies can be seen in the coda part after the S wave in the simulated waveforms in terms of amplitudes and elongations. The synthetic waveforms in the model with topographies are in agreement with the observed waveforms. Our results also show that a significant difference in the amplification of the coda part between the cases with and without the seawater layer can be found at the ocean-bottom stations. This coda part is the S- and Rayleigh-wave propagation associated with the ocean and the underlying sediment layers. Our results show that a realistic model with topographies and a seawater layer is needed in FDM simulations in order to precisely reproduce observed waveforms or predict seismic motion for a suboceanic earthquake. [ABSTRACT FROM AUTHOR]

Abstract: Recently, the occurrence of slow earthquakes such as low-frequency earthquakes and very low-frequency earthquakes have been recognized at depths of about 30 km in southwest Japan and Cascadia. These slow earthquakes occur sometimes in isolation and sometimes break into chain-reaction, producing tremor that migrates at a speed of about 5–15 km/day and suggesting a strong interaction among nearby small asperities. In this study, we formulate a 3-D subduction plate boundary model with two types of small asperities chained along the trench at the depth of 30 km. Our simulation succeeds in representing various types of slow earthquakes including low-frequency earthquakes and rapid slip velocity in the same asperity, and indicates that interaction between asperities may cause the very low-frequency earthquakes. Our simulation also shows chain reaction along trench with propagation speed that can be made consistent with observations by adjusting model parameters, which suggests that the interactions also explain the observed migration of slow earthquakes. [Copyright &y& Elsevier]

The rupture areas of the large interplate thrust earthquakes along the Nankai trough, offshore southwestern Japan, are divided into several segments. The 1944 Tonankai earthquake ruptured one of the segments off of the Kii Peninsula. In 2005, we conducted an ocean-bottom seismograph experiment at the eastern end of the rupture area of the 1944 earthquake. Little seismic activity was observed on the plate interface; most earthquake activity was within the subducting and overriding plates. Aftershocks of the 2004 off Kii Peninsula earthquake, which was an intraplate earthquake in the subducting Philippine Sea plate, were mainly located in the subducting oceanic crust and uppermost mantle. However, several earthquakes at the eastern end of the rupture area of the 1944 earthquake were in the sedimentary wedge, with a focal mechanism indicating deformation by a subducting seamount. The earthquakes and faults in the sedimentary wedge show deformation related to the irregular topography of the subducting oceanic crust to the east of the rupture area of the 1944 Tonankai earthquake. In contrast, few earthquakes were observed in the sedimentary wedge in the rupture area of the 1944 earthquake. Difference in topography of the subducting oceanic crust in the two areas might have caused spatial variations of interplate coupling, which, in turn, caused the observed differences in the deformation of the sedimentary wedge. The spatial variation of interplate coupling may affect rupture propagation during large interplate earthquakes and cause the segmentation of large interplate earthquakes along the Nankai trough. [ABSTRACT FROM AUTHOR]

We documented regional and local variations in basement relief, sediment thickness, and sediment type in the Shikoku Basin, northern Philippine Sea Plate, which is subducting at the Nankai Trough. Seismic reflection data, tied with ocean drilling program drill cores, reveal that variations in the incoming sediment sequences are correlated with basement topography. We mapped the three-dimensional seismic facies distribution and measured representative seismic sequences and units. Trench-parallel seismic profiles show three regional provinces in the Shikoku Basin that are distinguished by the magnitude of basement relief and sediment thickness: Western (<200–400 m basement relief, >600 m sediment thickness), Central (>1500 m relief, ∼2000 m sediments), and Eastern (<600 m relief, ∼1200 m sediments) provinces. The total thickness of sediment in basement lows is as much as six times greater than that over basement highs. Turbidite sedimentation in the Shikoku Basin reflects basement control on deposition, leading to the local presence or absence of turbidite units deposited during the middle Oligocene to the middle Miocene. During the first phase of sedimentation, most basement lows were filled with turbidites, resulting in smooth seafloor morphology that does not reflect basement relief. A second phase of turbidite deposition in the Eastern Province was accompanied by significant amounts of hemipelagic sediments interbedded with turbidite layers compared to the other provinces because of its close proximity to the Izu–Bonin Island Arc. Both regional and local variations in basement topography and sediment thickness/type have caused lateral heterogeneities on the underthrusting plate that will, in turn, influence lateral fluid flow along the Nankai accretionary prism. [ABSTRACT FROM AUTHOR]

When seamounts and other topographic highs on an oceanic plate are subducted, they cause significant deformation of the overriding plate and may act as asperities deeper in the seismogenic zone. Kashinosaki Knoll (KK) is an isolated basement high of volcanic origin on the subducting Philippine Sea Plate that will soon be subducted at the eastern Nankai Trough. Seismic reflection imaging reveals a thick accumulation of sediments (∼1200 m) over and around the knoll. The lower portion of the sedimentary section has a package of high-amplitude, continuous reflections, interpreted as turbidites, that lap onto steep basement slopes but are parallel to the gentler basement slopes. Total sediment thickness on the western and northern slopes is approximately 40–50% more than on the summit and southeastern slopes of KK. These characteristics imply that the basal sedimentary section northwest of KK was deposited by infrequent high-energy turbidity currents, whereas the area southeast of KK was dominated by hemipelagic sedimentation over asymmetric basement relief. From the sediment structure and magnetic anomalies, we estimate that the knoll likely formed near the spreading center of the Shikoku Basin in the early Miocene. Its origin differs from that of nearby Zenisu Ridge, which is a piece of the Shikoku Basin crust uplifted along a thrust fault related to the collision of the Izu–Bonin arc and Honshu. KK has been carried into the margin of the Nankai Trough, and its high topography is deflecting Quaternary trench turbidites to the south. When KK collides with the accretionary prism in about 1 My, the associated variations in sediment type and thickness around the knoll will likely result in complex local variations in prism deformation. [ABSTRACT FROM AUTHOR]

In 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeastern Japan (NEJ) arc. The objectives of this experiment were to clarify seismic structures of the forearc region off Sanriku, Japan using airgun–OBS data and to understand the nature of the Japan trench seismogenic zone. Fundamental features of the structure are as follows:(i)The subduction angle becomes steeper from east to west, from 3 to 8 to 11°, with the top of the plate located at depths of 11 km (40 km west from the trench), 21 km (120 km west from the trench) and 28 km (140 km west from the trench), respectively.(ii)The NEJ Moho beneath the forearc is defined at a depth of approximately 20 km.(iii)P-wave velocity of the island arc upper mantle is approximately 8 km s−1.(iv)TheP-wave velocity of the Cretaceous sedimentary layer exhibits significant lateral variability (4.0–5.5 km s−1) in the E–W direction.(v)The upper crust of NEJ forearc also has the significant lateral heterogeneity (5.0–6.3 km s−1) in the E–W direction, but not in the N–S direction.(vi)The middle crust defined on land extends in some locations 40 km east of the coastline.(vii)Lateral variability in theP-wave velocity of the lower crust (6.7–7.0 km s−1) is small compared with that of the upper crust.According to these results, it is suggested that the rupture areas of large earthquakes (>M7) in this region are limited within the contact zone between the NEJ arc crust and the subducting oceanic crust. Although a thin layer with low velocity is probably present within this contact zone, the serpentinization of uppermost mantle wedge of the NEJ arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited. This might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km. [ABSTRACT FROM AUTHOR]

Megathrust earthquakes have occurred repeatedly at intervals of 100 to 150 years along the Nankai Trough, situated in the southwest of Japan. Given that it has been 70 years since the last event, the occurrence of the next devastating earthquake is anticipated in the near future. On April 1, 2016, a moderate earthquake (Mw 5.9, MJMA 6.5) occurred off the southeastern coast of Mie Prefecture in the source region of the 1944 Tonankai earthquake (Mw 8.2). In this study, we investigated the influence of the 2016 earthquake on future megathrust earthquakes. We first determined the hypocenter distributions using a precise velocity structure obtained from seismic surveys in the source region. Using data obtained from the DONET ocean-bottom observation network, we found that this earthquake occurred along the plate boundary fault, which is also believed to have slipped during past megathrust earthquakes. We then performed a preliminary numerical simulation to reproduce the occurrence of a moderate earthquake in the middle of a megathrust earthquake cycle. We used a hierarchical asperity model, in which smaller asperities causing moderate earthquakes are embedded in a hyper-asperity that serves as the source region of megathrust earthquakes. The simulation shows that moderate earthquakes, caused by ruptures of a smaller asperity, occur as a result of shrinkage of strongly coupled areas in the hyper-asperity. This result is consistent with the observation that the hypocenter of the 2016 earthquake was located at the edge of a strongly coupled region along the plate boundary. The simulation also reproduced postseismic slip (afterslip/slow slip) along the plate boundary updip of the hyper-asperity, which is consistent with the observations of slow slip events and very-low-frequency earthquakes after the 2016 earthquake. Thus, the occurrence of the moderate earthquake offshore southeastern Mie Prefecture in the middle of the megathrust earthquake cycle implies that the shrinkage of the strongly coupled area along the plate boundary is occurring as a preparatory process for the next megathrust earthquake in the region. [ABSTRACT FROM AUTHOR]

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100–200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula. [Copyright &y& Elsevier]

We detected long-term hydrostatic pressure changes at ocean-bottom stations of the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) along the Nankai trough, off southwestern Japan. We detected these changes after removing the contributions of ocean mass variations and sensor drift from the records. In addition, we detected a decrease in the background seismicity rate of a nearby earthquake cluster that was synchronous with the hydrostatic pressure changes. We interpreted these observed hydrostatic pressure changes to reflect vertical deformation of the ocean floor of 3–8 cm, and we consider the cause of the seafloor crustal deformation to be a slow slip event (SSE) beneath the stations. Because the pressure changes were observed at stations with distances less than 20 km to each other, we inferred that the SSE occurred in the shallow part of the sedimentary wedge, such as on a splay fault system. The synchronous observation of an SSE and a seismicity rate change suggests that both were triggered by a change in the regional stress that may be associated with stress accumulation and release processes occurring along the Nankai trough. These data show that continuous and careful monitoring of crustal activities by DONET stations provides an effective way to detect seismic and geodetic signals related to the occurrence of megathrust or other types of large earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: The Nankai and Hyuga-nada seismogenic segments, in the western part of the Nankai subduction zone off southwestern Japan, have sometimes ruptured separately and sometimes simultaneously. To investigate the relationships among heterogeneities of seismic structure, spatial variation of the incoming plate, and the seismogenic segments, we carried out seismic observations in the western Nankai subduction zone and modeled the area with 3D seismic tomography using both onshore and offshore seismic data. Our seismic observations suggested that the pattern of seismicity is related to heterogeneities within the subducted plate rather than the seismogenic segments. The up-dip depth limit of seismicity along the plate boundary and in the oceanic crust is typically around 15 km, corresponding to the depth of dehydration of the oceanic crust. In addition, the seaward-extended seismicity observed where the subducted plate was considered to have rough internal structures. In the resulting velocity model, the up-dip limit of the area where the P-wave velocity just above the plate boundary exceeds 6 km/s corresponds to the up-dip limit of coseismic slip in the 1968 Hyuga-nada and 1946 Nankai earthquakes. Between the two coseismic rupture zones is an area of lower P-wave velocity about 40 km wide that is evidence of lateral heterogeneities in the upper plate along the trough-parallel direction. Structural heterogeneities in the upper plate may explain the variety of coseismic slip patterns in this region. [Copyright &y& Elsevier]

Abstract: We performed 3D seismic tomography of the Hyuga-nada region, western Nankai subduction zone, to investigate the relationship of the subducted part of Kyushu-Palau Ridge (KPR) to coseismic rupture propagation, seismicity, and shallow very low frequency earthquakes. Combining active-source and passive-source data recorded both onshore and offshore, we imaged the deep slab from near the trough axis to the coastal area. Our results show the subducted KPR as a low-velocity belt oriented NW–SE extending down the plate boundary to around 30km depth. At this depth, we suggest that the subducted KPR detaches from the slab and becomes underplated on the overriding continental plate. As the coseismic slip areas of past large earthquakes do not extend into the subducted KPR, we suggest that it may inhibit rupture propagation. The interior of the subducted KPR shows active intraslab seismicity with a wide depth distribution. Shallow very low frequency earthquakes are continuously active above the location of the subducted KPR, whereas they are intermittent to the northeast of the subducted KPR. Thus, the subducted KPR appears to be an important factor in coseismic rupture propagation and seismic phenomena in this region. [Copyright &y& Elsevier]

Abstract: The southwestern Kuril Arc, which is not affected by a series of accretion and collision processes, is one of the most important experimental sites among the world arc–trench systems for investigating the evolution of arc crust. Published petrological and seismic studies have shown that, in south-central Hokkaido, the upper part of the Kuril Arc crust is thrusting upward toward the west while the lower part is descending downward in the same direction. Among several hypotheses for formation of continental crust, we discuss the role of delamination of the island arc lower crust based on geochemical and seismic velocity structure models. Seismic velocity structure is obtained from an onshore–offshore seismic survey across the southwestern Kuril Arc–Trench system. The structure in the back-arc area shows a thick accretionary complex formed by Paleogene accretion and collision. Based on published seismic results, this accretionary complex is considered to continue on land to the Cretaceous accretionary complex observed in the Hidaka Belt. The structure of the Kuril Arc appears characteristic of upper crustal andesitic rocks (6.0–6.2 km/s) and thick middle to lower crust (6.5–7.3 km/s). Based on published geochemical study, we consider a geochemical model of simple crustal evolution explaining the mechanism of generation of andesitic rocks on the Kuril Arc. Each volume of andesitic, basaltic, and mafic layers estimated from the geochemical model is consistent with those obtained from the seismic velocity model. Crustal composition estimated from the geochemical model and major-element chemistry of the volcanic rocks in the Kuril Arc indicate that mechanical delamination is insufficient to cause the oceanic island arc crust to evolve into continental crust. [Copyright &y& Elsevier]

Abstract: A multi-purpose seismic experiment named the 2002 integrated seismic experiment Southwest Japan was conducted in 2002 along a more-than-240-km-long line across southwest Japan from the Pacific coast to the Japan Sea coast. Its profile provides the first crustal-scale cross section across the Japanese island arc, which highlights a number of significant points related to the structural development of the arc. Major outstanding points are that the Japanese island arc is composed of two completely different crusts juxtaposed by the Median Tectonic Line (MTL), and that the MTL started its activity associated with lower crustal thinning and formation of an upper crustal-scale half-graben in Late Cretaceous. [Copyright &y& Elsevier]

Abstract: Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw=8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6–7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc–arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock. [Copyright &y& Elsevier]

Abstract: We have developed a seafloor acoustic ranging system as a possible future application to monitoring seafloor crustal movement with the DONET (Development of Dense Ocean-floor Network System for Earthquake and Tsunami) cabled observatory system. In 2007 we carried out an experiment for the seafloor acoustic ranging system. We deployed two precision acoustic transponders (PXPs) with about 750m spacing in Kumano-nada at a water depth of about 2035m. We collected 660 ranging data in this one-day experiment. The round-trip travel time shows a variation with peak-to-peak amplitude of about 25mm in the range. It was confirmed that most of the variation could be explained by the change in sound speed estimated from measured temperature and pressure. The remaining fluctuation in the acoustic measurements is ±2mm. [Copyright &y& Elsevier]

Abstract: Inversion of tsunami waveforms is a well-established technique for estimating the slip distributions of subduction zone earthquakes, with some of the most detailed results having been obtained for earthquakes in the Nankai Trough, SW Japan. The present study, although it uses a method and tsunami waveform data set almost identical to previous study, aims to improve on previous work by using a more precise specification of initial conditions for the calculation of tsunami Green''s functions. Specifically, we incorporated four improvements in the present study: (1) we used a realistic plate model based only on seismic survey results, and assumed it to be the fault plane of the 1944 Tonankai earthquake; (2) the smallest subfaults consistent with the long wavelength approximation were used in the tsunami inversion analysis; (3) we included the effect of horizontal displacement of the ocean bottom on tsunami generation; and (4) we performed a checkerboard resolution test. As obtained in previous studies, a zone of high slip (>2.0 m) was resolved off the Shima Peninsula. However, the more precise calculation of tsunami Green''s functions has revealed additional detail that was not evident in previous studies, which we demonstrate is resolvable and correlates with the position of known faults in the accretionary prism. While there was little or no slip near the trench axis in the eastern part of the rupture zone, there was up to 1.5 m of slip resolved within 30 km of the trough axis in the western part, along the coast of the Kii Peninsula. This troughward slip zone coincides with the position of a large splay fault mapped in multichannel reflection surveys. Furthermore, it is also clear that the upper edge of the Enshu fault off Shima and Atsumi peninsulas is consistent with the up-dip limit of slip in the eastern part of our model. We tested the possibility that slip occurred on the former splay fault instead of on the plate interface during the 1944 Tonankai earthquake, and find that slip on this splay fault is also consistent with the data, although we cannot distinguish whether slip was dominant on the splay fault or on the plate interface. We further suggest that the position of the Enshu fault may be determined by the subduction of topographic highs, and that such faults may have an important influence on the up-dip rupture limit of the 1944 Tonankai and, potentially, other subduction zone earthquakes. [Copyright &y& Elsevier]

A seismic experiment with six explosive sources and 391 seismic stations was conducted in August 2001 in the central Japan region. The crustal velocity structure for the central part of Japan and configuration of the subducting Philippine Sea plate were revealed. A large lateral variation of the thickness of the sedimentary layer was observed, and the P-wave velocity values below the sedimentary layer obtained were 5.3–5.8 km/s. P-wave velocity values for the lower part of upper crust and lower crust were estimated to be 6.0–6.4 and 6.6–6.8 km/s, respectively. The reflected wave from the upper boundary of the subducting Philippine Sea plate was observed on the record sections of several shots. The configuration of the subducting Philippine Sea slab was revealed for depths of 20–35 km. The dip angle of the Philippine Sea plate was estimated to be 26° for a depth range of about 20–26 km. Below this depth, the upper boundary of the subducting Philippine Sea plate is distorted over a depth range of 26–33 km. A large variation of the reflected-wave amplitude with depth along the subducting plate was observed. At a depth of about 20–26 km, the amplitude of the reflected wave is not large, and is explained by the reflected wave at the upper boundary of the subducting oceanic crust. However, the reflected wave from reflection points deeper than 26 km showed a large amplitude that cannot be explained by several reliable velocity models. Some unique seismic structures have to be considered to explain the observed data. Such unique structures will provide important information to know the mechanism of inter-plate earthquakes. [Copyright &y& Elsevier]

We present the results of a seismic refraction experiment conducted off eastern Aomori, northeastern Japan forearc region, where the detailed crustal structure was poorly known. An objective analysis using high-quality survey data allowed us to obtain the seismic velocity structure to about 30 km depth, and in particular, to obtain a well-constrained geometry of the plate boundary in the source region of the 1994 Sanriku-oki earthquake. Our results show that the subducting Pacific oceanic plate increases its dip from about 5° to approximately 15° around the longitude of 143.5° E, 80 km landward of the trench axis near the latitude of 40° N. The location of this bend in the plate is 40 km east of the bending point previously proposed on the basis of hypocenter distribution. A cluster of aftershocks of the 1994 Sanriku-oki earthquake occurred around the bending point. This is because the bending of the subducting plate appears to affect the stress state in the vicinity of the plate boundary. Furthermore, the plate geometry is also considered to be related to the coupling or slip features along the plate boundary as the bending point correlates with the spatial distribution of repeating earthquakes. Our results will lead to more reliable determinations of asperity areas because an accurate dip angle of the fault plane is necessary for slip distribution studies. [Copyright &y& Elsevier]

Presents information on a study which investigated the rupture process of the 1946 Nankaido earthquake. Information on zone earthquakes along the Nankai Trough in Southwest Japan; Topography of the Western part of Nankai; Proposed rupture of the earthquake.

The article discusses research investigating the cause of the tsunami that followed the March 11, 2011 Tohoku-Oki earthquake, which is believed to be the result of a fault rupture that extends to a shallow part of the subduction zone at the Japan Trench. The team conducted repeated multibeam bathymetric surveys across the trench in the rupture zone before and after the earthquake. Results regarding the trench's up-dip limit and sea-floor displacement are discussed.