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In this study, the creep rate across the North Anatolian Fault was directly measured in the western Sea of Marmara using the seafloor acoustic ranging technique; the data reveal coupling conditions on the fault interface and stress accumulation with implications for regional seismic risk evaluation. Continuous measurements over a period of 3.5 years at a site in the Western High clearly indicate right‐lateral displacement at a rate of 10.7 ± 4.7 mm/year (95% confidence level); approximately half of the regional block motion at this location is released by this steady motion. A simple model of three elastic layers—a partially creeping sedimentary layer (8 km) at the top with the observed rate, a locked (3 km) and fully creeping layer in the middle, and a bottom layer—assumed from seismicity, reasonably explains onshore Global Navigation Satellite System data for the surrounding region. The North Anatolian Fault (NAF) cuts across the east and west of the Turkish mainland and has repeatedly generated destructive earthquakes (magnitude ~7‐class) to release accumulated strain originating from regional plate motion. The NAF beneath the Sea of Marmara has remained as a “seismic gap” for more than a century and the stress condition of the fault plane is thought to be nearly matured. An earthquake in this region is expected to cause severe damage in Istanbul, the most populous city in Turkey. However, the submarine environment prevents investigation of the fault movement using the traditional geodetic techniques applicable to onshore surveys. We applied a seafloor geodetic technique using continuous acoustic ranging for 3.5 years and found that the NAF in the western Sea of Marmara is creeping at nearly half of the regional plate motion. Despite the partial creep, strain accumulation on the fault has already reached the equivalent of a M7‐class earthquake. First direct evidence of creep in the submarine part of the North Anatolian Fault based on a seafloor geodetic surveyEstimated creep rate for the western part of the Sea of Marmara amounts to nearly half of the regional block motionA fault model incorporating the observed creep rate and the coupling condition estimated from microseismicity well explains GNSS data

The data provided by ten DONET deep-sea observatories, that on March 11, 2011, registered the Great East Japan Earthquake and tsunami, were used for investigation of the relationship between variations of the ocean bottom pressure and three-component accelerograms. Methods of cross-spectral analysis revealed the existence of a frequency range of “forced oscillations,” within which pressure variations are proportional to the vertical component of the acceleration. This proportionality is manifested by the magnitude-squared coherence (MSC) being close to unity and a phase lag (PL) practically equal to zero. The spectral analysis method showed the proportionality coefficient to be equal the mass of a water column of unit area at the installation point of the observatory or, approximately, to the product of the water density and the ocean depth. The observed boundaries of the frequency range of “forced oscillations” are revealed to correspond to the theoretical frequency values confining the manifestation of surface gravity and acoustic waves in pressure variations near the ocean bottom. The hypothesis is put forward that the small deviations of MSC from unity and of PL from zero observed by a number of stations within the range of “forced oscillations” are due to the contribution of horizontal movements of nearby submarine slopes. A theoretical analysis has been performed of the problem of forced oscillations of a water layer in a basin of varying depth. A formula is obtained that relates pressure variations at the ocean bottom to acceleration components of the bottom motion and the bottom slope. The pressure in the region of forced oscillations is shown to decrease exponentially with the distance from the moving segment of the ocean bed, so pressure variations, originating from movements of the bottom, are registered effectively by a gauge at the ocean bottom only within a radius less than 1–2 ocean depths. A cross-spectral analysis of pressure variations and of three-component accelerograms confirmed the hypothesis concerning the contribution of horizontal movements of nearby submarine slopes to pressure variations.

Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out a MCS reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection image reveals that beneath this topographic valley there exists a ~ 3-km-deep sedimentary basin atop the arc crust, suggesting that the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor which bifurcate at crustal depths and/or that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic front toward the back-arc (> 30 km away), these volcanic features vanish, and alternatively wide rift basins become predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur. This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the northern Okinawa Trough.

The role of seamounts on interplate earthquakes has been debated. However, its impact on intraslab deformation is poorly understood. Here we present unexpected evidence for large normal‐fault earthquakes intersecting the slab just ahead of a subducting seamount. In 1995, a series of earthquakes with maximum magnitude of 7.1 occurred in northern Ryukyu where oceanic plateaus are subducting. The aftershock distribution shows that conjugate faults with an unusually high dip angle of 70–80° ruptured the entire subducting crust. Seismic reflection images reveal that the plate interface is displaced over 1 km along one of the fault planes of the 1995 events. These results suggest that a lateral variation in slab buoyancy can produce sufficient differential stress leading to near‐vertical normal‐fault earthquakes within the slab. On the contrary, the upper surface of the seamount (plate interface) may correspond to a weakly coupled region, reflecting the dual effects of seamounts/plateaus on subduction earthquakes. We find a subducting seamount and a displacement across the plate interface in the northern Ryukyu TrenchHigh‐angle normal‐fault earthquakes intersecting the slab occurred in front of the subducting seamountBuoyant seamounts/plateaus can produce differential stress leading to large normal‐fault earthquakes in the slab

Back‐arc basins are a primary target to understand lithospheric evolution in extension associated with plate subduction. Most of the currently active back‐arc basins formed in intraoceanic settings and host well‐developed spreading centers where seafloor spreading has occurred. However, rift structure at its initial stage, a key to understand how the continental lithosphere starts to break in a magma‐rich back‐arc setting, is poorly documented. Here we present seismological evidence for structure of the southern Okinawa Trough, an active rift zone behind the Ryukyu subduction zone. We find that the southern Okinawa Trough exhibits an almost symmetric rift system across the rift axis (Yaeyama Rift) and that the sedimentary layers are highly cut by inward dipping normal faults. The rift structure also accompanies a narrow (2–7 km wide) on‐axis intrusion resulted from passive upwelling of magma. On the other hand, an active submarine volcano is located ~10 km away from the rift axis. The Pwave velocity (Vp) model derived from seismic refraction data suggests that the crust has been significantly thinned from the original ~25 km thick arc crust and the thinnest part with 12 km thickness occurs directly beneath the rift axis. The velocity model also reveals that there exists a thick layer with Vpof 6.5–7.2 km/s at lower crustal levels and may indicate that mantle materials accreted at the bottom of the crust during the crustal stretching. The abrupt crustal thinning and the velocity‐depth profile suggest that the southern Okinawa Trough is at a transitional stage from continental rifting to seafloor spreading. We seismologically study a continental rift zone in the southern Okinawa TroughWe find pure‐shear structure accompanying on‐axis intrusion and numerous normal faultsMantle materials may have accreted at the bottom of the extended continental crust

Several recent large-scale earthquakes including the 2011 Tohoku earthquake (Mw9.0) in northeastern Japan and the 2014 Iquique earthquake (Mw8.1) in northern Chile were associated with foreshock activities (Mw> 6). The detailed mechanisms between these large earthquakes and the preceding smaller earthquakes are still unknown; however, to plan for disaster mitigation against the anticipated great Nankai Trough earthquakes, in this study, possible scenarios after Mw7-class earthquakes that frequently occur near the focal region of the Nankai Trough are examined through quasi-dynamic modeling of seismic cycles. By assuming that simulated Nankai Trough earthquakes recur as two alternative earthquakes with variations in magnitudes (Mw8.7–8.4) and recurrence intervals (178–143 years), we systematically examine the effect of the occurrence timing of the Mw7 Hyuga-nada earthquake on the western extension of the source region of Nankai Trough earthquakes on the assumed Nankai Trough seismic cycles. We find that in the latter half of a seismic cycle preceding a large Nankai Trough earthquake, an immature Nankai earthquake tends to be triggered within several years after the occurrence of a Hyuga-nada earthquake, then Tokai (Tonankai) earthquakes occur with maximum time lags of several years. The combined magnitudes of the triggered Nankai and subsequent Tokai (Tonankai) earthquakes become gradually larger with later occurrence of the Hyuga-nada earthquake, while the rupture timings between the Nankai and Tokai (Tonankai) earthquakes become smaller. The triggered occurrence of an immature Nankai Trough earthquake could delay the expected larger Nankai Trough earthquake to the next seismic cycle. Our results indicate that triggering can explain the variety and complexity of historical Nankai Trough earthquakes. Moreover, for the next anticipated event, countermeasures should include the possibility of a triggered occurrence of a Nankai Trough earthquake by an Mw7 Hyuga-nada earthquake.

The faults’ geometry and their seismic activity beneath the Marmara Sea have been under debate for a couple of decades. We used data recorded by three ocean bottom seismographs (OBSs) over a period of 3 months in 2014 to investigate the relationship of fault geometry to microseismicity under the western Marmara Sea in Turkey. We detected a seismic swarm at 13 to 20 km depth beneath the main Marmara fault (MMF), and the maximum depth of seismogenic zone was 25 km within the OBS observation area. These results provided evidence that the dip of the MMF is almost vertical and that the seismogenic zone in this region extends into the lower crust. Our analysis of past seismicity indicated that the seismic swarm we recorded is the most recent of an episodic series of seismic activity with an average recurrence interval of 2–3 years. The repetitive seismicity indicates that the MMF beneath the western Marmara Sea is coupled and that some of the accumulated strain is released every 2 to 3 years. Our study shows that OBS data can provide useful information about seismicity along the MMF, but more extensive studies using more OBSs deployed over a wider area are needed to fully understand the fault geometry and stick–slip behavior of faults under the Marmara Sea.

Change in the rates of aseismic deformation prior to large earthquakes is a major area of interest in earthquake physics. Here we present evidence that episodic tremor and slip occurred in the shallow subduction zone within the source region of the 2011 Tohoku‐Oki earthquake prior to the main shock. Ocean bottom seismometers near the Japan Trench recorded some excitations in amplitude of ambient noise level accompanying both the 2008 and 2011 slow slip events. The observed signals show that low frequencies of 5–8 Hz dominated, suggesting that the excitations were due to small low‐frequency tremors accompanying the slow slip events. The largest amplitude tremor was observed just before the 2011 event. The estimated sources of tremors were possibly distributed within the coseismic slip area of the 2011 event, suggesting the shallow plate‐boundary thrust near the trench is a general location of slow earthquakes. Tremor signal accompanied slow slip eventTremor activity accelerated before the 2011 Tohoku‐Oki earthquakePossible tremor source located near the trench within coseismic slip area

Motivated by the fact that temporal earthquake aperiodicity was observed off Kamaishi just after the 2011 Tohoku earthquake, we performed numerical simulations of chain reactions due to the postseismic slip of large earthquakes by applying rate‐ and state‐dependent friction laws. If the repeater is composed of single asperity, our results show that, (i) a mixture of partial and whole rupturing of a single asperity can explain some of the observed variability in timing and size of the repeating earthquakes off Kamaishi; (ii) the partial rupturing can be reproduced with moderate frictional instability with the aging‐law and not the slip or Nagata laws; (iii) the perturbation of the activated earthquake hypocenters observed mostly in the ESE‐WNW direction may reflect the fact that the large postseismic slip of the 2011 Tohoku earthquake propagated from ESE to WNW off Kamaishi; (iv) the observed region of repeating earthquake quiescence may reflect the strong plate coupling of megathrust earthquakes. Non‐similar earthquakes in and around the repeater should be counted for slipAging‐law can explain the aperiodicity off Kamaishi even for a single asperityThe large post‐seismic slip off Kamaishi may propagate from ESE to WNW

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.

We estimated the Swave attenuation structure in southwestern Japan and the western Nankai Trough by analyzing maximum Swave amplitudes at 4–8, 8–16, and 16–32 Hz with a correction term for apparent amplitude attenuation due to multiple forward scattering. Because the estimated attenuation (Q−1) in our tomographic study was much larger than Q−1due to wide‐angle scattering, our estimated Q−1was composed mainly of intrinsic attenuation. High‐attenuation areas (Q−1> 1/300 at 4–8 Hz) were imaged beneath Quaternary volcanoes and south off Shikoku. Low (<1/1500 at 4–8 Hz) or moderate Q−1(1/500–1/1000 at 4–8 Hz) was imaged beneath Shikoku and nonvolcanic areas of Chugoku. High and moderate Q−1in and around Shikoku are located near the top of subducting Philippine Sea Plate. This correspondence implies that these high and moderate Q−1reflect fluid in the subducting slab. By applying a theoretical model of attenuation in water‐saturated porous random media, we examined wave‐induced fluid flow induced by lower frequency (<1 Hz) seismic waves that may be related with triggering of nonvolcanic tremor by surface waves. Even though Q−1structure in this study cannot fully explain the tremor triggering by wave‐induced fluid flow, large uncertainties of Q−1in tremor zone suggest that high resolution imaging of Q−1and random inhomogeneities would give some constraints for the spatial variation of permeability and other medium properties. Attenuation structure is estimated by removing scattering attenuationA rift zone shows a peculiar combination of random inhomogeneity and 1/Q1/Q in subducting slab is examined in terms of fluid distribution and dynamics

Seismic waves at high frequencies (>1?Hz) show collapsed and broadened wave trains caused by multiple scattering in the lithosphere. This study analyzed the envelopes of direct Swaves in southwestern Japan and on the western side of the Nankai trough and estimated the spatial distribution of random inhomogeneities by assuming a von Kármán type power spectral density function (PSDF). Strongly inhomogeneous media have been mostly imaged at shallow depth (0–20?km depth) in the onshore area of southwestern Japan, and their PSDF is represented as P(m)?˜?0.05m-3.7?km3, with mbeing the spatial wave number, whereas most of the other area shows weak inhomogeneities of which PSDF is P(m)?˜?0.005m-4.5?km3. At Hyuga-nada in Nankai trough, there is an anomaly of inhomogeneity of which PSDF is estimated as P(m)?˜?0.01m-4.5?km3. This PSDF has the similar spectral gradient with the weakly inhomogeneous media, but has larger power spectral density than other offshore areas. This anomalous region is broadly located in the subducted Kyushu Palau ridge, which was identified by using velocity structures and bathymetry, and it shows no clear correlation with the fault zones of large earthquakes in past decades. These spatial correlations suggest that possible origins of inhomogeneities at Hyuga-nada are ancient volcanic activity in the oceanic plate or deformed structures due to the subduction of the Kyushu Palau ridge. Random inhomogeneities in SW Japan are estimated by the peak delay time analysisPresence of volcanic rocks is a possible origin of strong inhomogeneitiesSubducted Kyushu Palau ridge shows anomalous random inhomogeneity

Large destructive interplate earthquakes, such as the 2011 Mw9.0 Tohoku-oki earthquake, have occurred repeatedly in the northern Japan subduction zone. The spatial distribution of large interplate earthquakes shows distinct along-trench variations, implying regional variations in interplate coupling. We conducted an extensive wide-angle seismic survey to elucidate the along-trench variation in the seismic structure of the forearc and to examine structural factors affecting the interplate coupling beneath the forearc mantle wedge. Seismic structure models derived from wide-angle traveltimes showed significant along-trench variation within the overlying plate. In a weakly coupled segment, (i) the sediment layer was thick and flat, (ii) the forearc upper crust was extremely thin, (iii) the forearc Moho was remarkably shallow (about 5 km), and (iv) the P-wave velocity within the forearc mantle wedge was low, whereas in the strongly coupled segments, opposite conditions were found. The good correlation between the seismic structure and the segmentation of the interplate coupling implies that variations in the forearc structure are closely related to those in the interplate coupling.

Megathrust earthquakes anticipated in the Nankai trough will likely cause severe damage in central and western Japan. The Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET), a network of permanent ocean-bottom seismic stations for the early detection of earthquakes and tsunamis developed by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), is in place above the expected source region of such earthquakes. Data from DONET sensors are transmitted in real time to our laboratory at JAMSTEC. Intensive ongoing seismic activity is being detected off the Kii Peninsula by DONET, mainly distributed in three clusters that overlap with the aftershock distribution of the 2004 off the Kii Peninsula earthquakes (MJMA= 7.1 and 7.4), and most of them are also aftershocks of the 2004 earthquakes. Some are linearly distributed on a different trend from the strike of the 2004 foreshock and mainshock fault planes. This result implies that the 2004 events triggered seismic activity on different faults on the subducting Philippine Sea plate. We also observed changes in seismic activity caused by the 2011 Tohoku-oki earthquake. These results could not have been obtained with on-land observations alone, which indicates the importance of DONET for monitoring seismic activity along the Nankai trough.

We relocated hypocenters of the foreshock, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku earthquake (M9.0) in the middle part of the Japan Trench where the earthquake rupture initiated. Ocean Bottom Seismographs (OBSs), deployed in the area, recorded the earthquakes and these data provide improved images of the hypocenter distribution. The mainshock hypocenter was relocated slightly westward from that reported by Japan Meteorological Agency (JMA), placing it near the intersection between the plate boundary and the Moho of the overriding plate. The foreshock seismicity mainly occurred on the trenchward side of the mainshock hypocenter, where the Pacific slab contacts the island arc crust. The foreshocks were initially activated at the up-dip limit of the seismogenic zone ~30 km trenchward of the largest foreshock (M7.3, two days before the mainshock). After the M-7.3 earthquake, intense interplate seismicity, accompanied by epicenters migrating toward the mainshock hypocenter, was observed. The focal depth distribution changed significantly in response to the M-9 mainshock. Earthquakes along the plate boundary were almost non-existent in the area of huge coseismic slip, whereas earthquakes off the boundary increased in numbers in both the upper and the lower plates.

In order to reproduce slow earthquakes with short duration such as very low frequency events (VLFs) migrating along the trench direction as swarms, we apply a 3-D subduction plate boundary model based on the slowness law of rate- and state-dependent friction, introducing close-set numerous small asperities (rate-weakening regions) at a depth of 30 km under high pore pressure condition, in addition to a large asperity. Our simulation indicates that swarms of slip events occur repeatedly at the small asperities, and these events are similar to the observed slow earthquake group, especially to VLF, on the basis of the relation between characteristic duration and seismic moment. No slip events occur there without the small asperities, which mean that the close-set numerous small asperities may be one of the necessary conditions for generating the short-duration slow earthquakes such as VLFs. In the preseismic stage of the megathrust earthquakes that occur at the large asperity, the swarms of VLFs have higher migration speeds and higher moment release rate as well as shorter recurrence interval. Thus, monitoring the migration of slow earthquakes may be useful in imaging the preseismic slip of megathrust earthquakes.

The 2011 off the Pacific coast of Tohoku Earthquake occurred offshore of northeast Japan region on March 11th, 2011. In order to study the aftershock activity of this event, we started deployment of seventy-two ocean bottom seismometers (OBSs) four days after the mainshock. In the south of the source region, thirty-four longterm OBSs (LT-OBSs) had been deployed before the occurrence of the mainshock, and we recovered three LT-OBSs to clarify the depth distribution of aftershocks. Using the data of OBSs, ninety-nine aftershocks were located. Most of the aftershocks were located in a depth range of 5–30 km and concentrate in the plate boundary region. In addition, aftershocks occurred within the subducting oceanic crust and the 6.2-km/s layer of the landward plate. No aftershocks were found in the mantle of the subducting plate. From the results of a previous seismic survey using OBSs and controlled sources, the subducting Philippine Sea plate is estimated to be in contact with the subducting Pacific plate. The southern end of the seismic activity region of the aftershocks corresponds to the contact region of two subducting plates. We infer that the rupture of the mainshock sequence was terminated at the oceanic plate contact region.

We derive the 1-yr afterslip distribution following the 2003 Tokachi-oki (Hokkaido, northeastern Japan) earthquake (Mw8.0)by inverting geodetic data, i.e., horizontal and vertical displacements, at 142 land stations of the Global Positioning System (GPS) in Hokkaido and northernmost Tohoku districts, together with vertical displacements at two offshore stations of pressure gauge (PG) off the Pacific coast of Hokkaido. We use the Green’s functions (GFs), calculated with a finite element method, for an inhomogeneous elastic (IE) model incorporating subsurface structure. Obtained results show a striking feature of the distribution pattern of significant afterslip, namely, a U-shaped afterslip zone encircling the co-seismic rupture zone of the 2003 event. Amounts of the 1-yr afterslip reach up to 0.9 m, and total seismic moments released from all afterslip zones are of the order of 1021 N m, corresponding to an earthquake of MW 8.0. For comparison, we also estimate the 1-yr afterslips based on GFs for the homogeneous elastic (HE) model to find that the total seismic moment with GFs for the IE model is larger than that with GFs for the HE model by ∼33% (when the most probable values are compared) if we assume a rigidity of 40 GPa. This result implies that inhomogeneities due to subsurface structure have an important role in geodetic inversions.

A multi-channel seismic reflection (MCS) survey was conducted to investigate the tectonic structure off Niigata, which caused the 2007 Niigata-ken Chuetsu-oki earthquake, using the research vessel (R/V) KAIREI of the Japan Agency for Marine-Earth Science and Technology. Based on the results of data processing and interpretation of available data, three areas are identified according to seismic characteristics. The most deformed area is located on the continental shelf near the source region of the 2007 Niigata-ken Chuetsu-oki earthquake, i.e., the area east of the Yoneyama-Ogi Uplifts. A remarkable growth of folds, including fault-related folds, and a strong reflector dipping east is identifiable by localized strain concentrations. The second area is located between the Yoneyama-Ogi Uplifts and the Jouetsu Knoll in the Toyama Trough. Although the deformation of deposits in the second area was smaller than in the first area, folds are identified. The third area is located toward the west of the Jouetsu Knoll in the Toyama Trough. No significant deformed structures developed in this area. Based on the interpretation of stratigraphy obtained in previous studies, seismic characteristics, and well data, the development of an anticline was initiated by a compression field after about 3.6 Ma. In particular, the deformation of sedimentary layers by the compression field occurred rapidly after about 1.3 Ma. Folds have grown larger toward the east after about 1.3 Ma. In addition, subsidence of about 0.2 s in sedimentary layers can be seen at the western margin of the Yoneyama-Ogi Uplifts, suggesting that tectonic movement related to reverse faulting has advanced there very recently. From aftershock distribution on a depth section on line S-2 near the hypocentral region, most hypocenters were determined to be below the strong reflector. This result suggests that fold growth has accompanied past large earthquakes, such as the 2007 Niigata-ken Chuetsu-oki earthquake.

The Kyushu-Palau ridge, a remnant arc on the Philippine Sea Plate, subducts beneath the Eurasian Plate along the westernmost part of the Nankai Trough. A seismic reflection profile on strike line images the ~70-km-wide Kyushu-Palau ridge where it subducts beneath the toe of the forearc accretionary wedge. The geomagnetic anomaly signature, seafloor topographic features, wide-angle refraction data, and on-land geomorphologic evidence enable us to trace the forearc extension of the subducted ridge up to the east Kyushu. The subducted Kyushu-Palau ridge with excess mass may be relatively buoyant, and thus is more likely to resist subduction upon collision with the overriding plate at depth, leading us to speculate that there is locally large tectonic stress at the contact zone between the subducted ridge and base of the overriding plate. The large stress zone is marked by historic thrust-type intermediate-class (magnitude 6 or 7) earthquakes. The flank regions of the subducted buoyant Kyushu-Palau ridge are more likely to tear and result in slab fracturing when the ridge subducts deeper. We propose that the subducted Kyushu-Palau ridge may serve not only as a seismic asperity at depth but also produce the slab fracture as a seismic barrier inhibiting the rupture propagation of the adjacent megathrust earthquakes in the Hyuga segment.

The 2007 Chuetsu-oki earthquake occurred in the offshore area of Niigata Prefecture, central Japan, on 16 July 2007. The aftershock distribution shows two fault planes, dipping to the northwest and southeast, respectively, which form a cross-sectional “V” pattern. We analyze the kinematic source process for the two fault planes consistent with the aftershock distribution using teleseismic body waveform data. Our results indicate that the mainshock initiated rupture on both faults northeast of their intersection and that the rupture propagated unilaterally southwestward along the southeast-dipping fault. The maximum slip is 1.6 m on the southeastdipping fault, and the seismic moments of northwest- and southeast-dipping faults are 8.0 × 1017N m (Mw5.9) and 7.5 × 1018N m (Mw6.5), respectively. The forward modeling of geodetic displacement from the obtained fault model is consistent with the GPS and InSAR measurements. Successive conjugate reverse faults around the source region are identified from the reflection seismic sections. We presume that stress changes and complicated structures in property and geometry around the source region are the dominant causal factors for generating the complexity of coseismic fault system.

The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake generation, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four aftershocks were recorded with high spatial resolution during the observation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the aftershocks occurred in a depth range of 6–15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the aftershocks occurred along a plane dipping to the southeast in the whole aftershock region. The dip angle of this plane is approximately 40°. This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.

The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P-wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.

AbstractThe analysis of wide-angle seismic reflection and refraction data plays an important role in lithospheric-scale crustal structure study. However, it is extremely difficult to develop an appropriate velocity structure model directly from the observed data, and we have to improve the structure model step by step, because the crustal structure analysis is an intrinsically non-linear problem. There are several subjective processes in wide-angle crustal structure modelling, such as phase identification and trial-and-error forward modelling. Because these subjective processes in wide-angle data analysis reduce the uniqueness and credibility of the resultant models, it is important to reduce subjectivity in the analysis procedure. From this point of view, we describe two software tools, PASTEUP and MODELING, to be used for developing crustal structure models.PASTEUP is an interactive application that facilitates the plotting of record sections, analysis of wide-angle seismic data, and picking of phases. PASTEUP is equipped with various filters and analysis functions to enhance signal-to-noise ratio and to help phase identification. MODELING is an interactive application for editing velocity models, and ray-tracing. Synthetic traveltimes computed by the MODELING application can be directly compared with the observed waveforms in the PASTEUP application. This reduces subjectivity in crustal structure modelling because traveltime picking, which is one of the most subjective process in the crustal structure analysis, is not required. MODELING can convert an editable layered structure model into two-way traveltimes which can be compared with time-sections of Multi Channel Seismic (MCS) reflection data. Direct comparison between the structure model of wide-angle data with the reflection data will give the model more credibility. In addition, both PASTEUP and MODELING are efficient tools for handling a large dataset. These software tools help us develop more plausible lithospheric-scale structure models using wide-angle seismic data.

In geodetic inversions such as estimation of coseismic slip and/or afterslip distribution on faults, the displacements on the surface calculated under an assumption of homogeneous elastic half space have been mostly used as the Green’s functions (GF’s). However, this seems not adequate for better estimations of such slip distribution, because the subsurface structures are more or less inhomogeneous, especially those in and around Japan where the structure must be much complicated. In this study, to examine how much the inhomogeneous subsurface structure affects on the surface displacements, we conduct some 3-D finite element calculations with a grid for the region of 1400 km (EW) × 1200 km (NS) × 200 km (depth) including the Tohoku and Hokkaido, northeastern Japan. Assuming homogeneous and inhomogeneous elastic models with various values for the Young’s modulus and Poisson’s ratio, we calculated the surface displacements due to a dip-slip type dislocation of 1 m on many cell-like subfaults assumed on the interface between the Pacific and land side plates. Comparing the results, we find a large discrepancy in the surface displacements between the homogeneous and inhomogeneous elastic models and less dependency of the surface displacements on the Poisson’s ratio. The discrepancy is found to be more than 20% and can be as large as ~40% in some cases. Such a large discrepancy indicates that the surface displacements calculated for inhomogeneous elastic medium with realistic subsurface structure, unlike as in usual cases, should be used as the GF’s for better geodetic inversions.

Hokkaido Island in northern Japan is located at the intersection of the Kuril and Northeast Japan arcs. Eastern Hokkaido represents the southwestern margin of the Kuril arc that evolved as an oceanic island arc, but its crustal structure has remained unclarified. In the summer of 2000, a highly dense onshore-offshore integrated seismic experiment was conducted in order to reveal the entire crustal section covering from the Kuril Trench to the back-arc basin of the Okhotsk Sea crossing eastern Hokkaido. High-quality refraction/wide-angle reflection data collected for the onshore survey line delineated a detailed crustal structure of easternmost part of Hokkaido Island. Our seismic velocity model shows a good correlation with the surface geology along the profile. The notable feature of the velocity model is the existence of a 6.0-km/s layer beneath the onshore survey line. A reflective middle to lower crust is also found beneath eastern Hokkaido. These results indicate that the southwestern margin of the Kuril arc is in a matured state of oceanic island-arc crust.

The preliminary hypocenter distribution of the 2005 Off Miyagi Prefecture earthquake and its aftershocks is estimated using data from five ocean bottom and six onshore seismic stations located around the rupture area of the earthquake. The epicenter of the mainshock is relocated at 38.17°N, 142.18°E, and the focal depth is estimated to be 37.5 km. The aftershocks surrounding the mainshock hypocenter form several clusters that are concentrated along a distinct landward dipping plane corresponding to the plate boundary imaged by the previous seismic experiment. The strike and dip angles of the plane agree well with those of the focal mechanism solution of the mainshock. The size of the plane is about 20×25 km2in the strike and dip directions, which is similar to that of the large coseismic slip area. The up-dip end of the planar distribution of the aftershocks corresponds to the bending point of the subducting oceanic plate, suggesting that the geometry of the plate boundary affects the spatial extent of the asperity of the 2005 earthquake

We deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.

In the summer of 2001, an onshore-offshore integrated seismic survey was carried out along a 485 km long profile crossing the eastern Nankai Trough and central Japan from the western edge of the Izu island arc. Seismic velocity and reflectivity images showongoing subduction of repeated ridges. Two ridges are clearly resolved, the Paleo Zenisu-south ridge and -north ridge located between the Nankai Trough and central Japan. The two subducting ridges are 12 km thick by 40 km wide, and 20 km thick by 60 km wide, respectively. We propose that the north ridge may have prevented the eastward propagation of co-seismic slip during the 1944 earthquake due to strong coupling. Highly reflective subducting oceanic crust is imaged at 25–45 km depth beneath central Japan, and has a 2 km high dome structure at 30–40 km depth, indicating an even deeper ridge structure. This highly reflective subducted oceanic crust overlaps with the high Poisson’s ratio zone imaged in a seismic tomography study. From these relations, we propose that the Tokai silent slip can be attributed to a high pore pressure zone, which significantly extends the conditionally stable region at the top of the subducted crust.

The 2004 off the Kii Peninsula earthquake occurred on September 5, 2004. Knowing the precise aftershock distribution is important for understanding the mechanism of this earthquake. However, the hypocenter of the main shock was located more than 100 km offshore from the nearest station of the land observation network. In the three days after the main shock, we started ocean bottom seismometer (OBS) observation in order to determine the precise distribution of the aftershocks. We assumed a seismic velocity structure for the hypocenter calculation, based on the results of previous seismic refraction study. The station corrections were incorporated to locate the hypocenter precisely. The hypocenters located within an area covered by five OBSs show relatively small errors. It is found that the OBS-located hypocenters are located about 5.5 km east-southeast from those by JMA and the depth range of the aftershocks is about 5–25 km just beneath the Nankai trough axis. The aftershock hypocenters can be grouped into two clusters at different depths of about 10 km and about 20 km. It is inferred that the main shock also has a depth of 5–25 km. Since this extent of the main shock was larger than one of the oceanic crust of the Philippine Sea plate, the fault plane of the main shock extended at the upper most mantle of the Philippine Sea plate. Although we cannot assign the actual fault plane of the main shock form our observation results, it is clarified that intra-plate earthquakes occurred near the trench region. Our OBS result supports that the main shock was the earthquake not at the plate boundary but within the bending Philippine Sea plate near the trough axis.

A new criterion is introduced to judge if the vicinity of the source region of a great interplate earthquake is in an ‘active period’. It is based on the stress change caused by the great earthquake. A region is regarded as being in an active period of seismicity if the occurrence rate of earthquakes on faults in the stress shadow of the great earthquake is significantly higher than in the early stage of the seismic cycle, and if the stressing rate of these faults is sufficiently low. This criterion was applied to the seismicity in the central part of southwest Japan before and after the 1944 Tonankai and 1946 Nankai earthquakes. The results show that before the 1944 Tonankai earthquake, the region was in an active period from at least 1927.The region was in a quiet period for almost50 years after the 1946 Nankai earthquake.Data after 1995 show that the region is once more in an active period of seismicity preceding the next great interplate earthquakes along the Nankai trough,although the total number of earthquakes has not yet significantly increased. Our results indicate that earthquake probability in the central part of southwest Japan will become high in the coming decades until the next great interplate earthquakes along the Nankai trough.

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the aftershock distribution.

We consider whether splay fault slip may be a factor influencing the behavior of megathrust earthquakes in the Nankai Trough. Consideration of tsunami inversion results from other studies indicates that slip on one or more splay faults may be particularly important for the segment of the Nankai Trough offshore western Shikoku. These results suggest that during at least two megathrust earthquakes substantial slip may have occurred on one or more splay faults which cut the island arc crust over 100 km landward of the trench axis. In contrast to smaller subsidiary thrusts in the semi-consolidated and unconsolidated sediments very near the trough axis, the crustal faults considered here could accumulate and release considerable tectonic stress. Through a simple finite element calculation we demonstrate that slip on such faults can release some of the shear stress on the megathrust accumulated through plate subduction, and therefore may have some influence on the behavior of megathrust earthquakes.