佐藤 利典

Abstract The Conrad Rise (CR), located midway between Antarctica and the Southwest Indian Ridge (SWIR), remains one of the least explored submarine large igneous provinces (LIPs) in the Indian Ocean to date. Relying on only seafloor paleomagnetic records, early studies hypothesized that the formation of the CR occurred during the Late Cretaceous. Here, we present new geochemical and geochronological data, including Sr‒Nd‒Pb‒Hf isotopes and ⁴⁰Ar/³⁹Ar data. Our results indicate that the uppermost part of the CR (Ob and Lena seamounts) unexpectedly formed later than previously predicted, at approximately 40 Ma in an intraplate setting. Another small seamount north of the Ob seamount formed later, at 8.5 Ma. The isotopic composition of lava from the small seamount north of the Ob seamount overlaps with that commonly defined by the Indian plume component. Overall, the isotopic variations defined by the volcanic suite from the CR could be accounted for by a three‐component mixing model involving the common component, lower continental crust, and depleted mantle endmembers. The newly obtained ⁴⁰Ar/³⁹Ar ages imply that the CR volcanism might have been triggered by major regional plate reorganizations during the middle to late Eocene and the late Miocene, inducing the release of a small upwelling rising from the African large low‐velocity province.

Abstract The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km³ DRE and is distributed in a depth range of 4–11 km. Graphical Abstract

We have developed a broadband ocean bottom seismometer (BBOBS) and its new generation (BBOBS-NX) with the penetrator sensor system since 1999. With them, we performed many practical observations to create a new research category of ocean bottom broadband seismology. As the next step in seafloor geophysical observation, the BBOBS and the BBOBS-NX can be a breakthrough in realizing a geodetic observation network on the seafloor. Although vertical displacement observation by the absolute pressure gauge has been widely conducted in recent years, other geodetic observations are rarely performed. A few trials to measure the seafloor tilt were performed, but those looked inadequate for practical observations. Note that the broadband sensor in our BBOBSs has a mass position signal output, which can be used to measure the tilt change. As the horizontal component noise level of the BBOBS-NX is good at a long period range, we expected it to be adequate for the tilt measurement. At the first evaluation, we performed a comparison with a water-tube tiltmeter. The result was comparable with a resolution of better than 1 mu radian. A practical observation at the south of Boso Peninsula (KAP3 site) was conducted as the in-situ study from April, 2013. In January, 2014, a slow slip event (SSE) occurred near this site. The tilt data were processed by removing steps, mechanical relaxation, and tides. The results show a clear peak started from late December 2013. Two more 2 year-long tilt observations began in 2015: one was at the KAP3 site and another was off the Miyagi Prefecture at the slope to the Japan Trench. The latter was recovered in 2017 with about 1.5 years of data, which indicate a large continuous tilt up to several tens of mu radian. This amount of tilt can be explained by a similar already estimated SSE. Mobile tilt measurement at the seafloor can be a powerful tool to study SSEs, as they can be located above the source area and also possible to build an observation array for a practical study because of its low cost and ease of deployment compared with a seafloor borehole site.

Boso Peninsula, Japan, was formed by the interaction of the Philippine Sea, Eurasian and Pacific plates around the trench-trench-trench Boso triple junction. Normal-type earthquakes are persistently observed in the subducting Philippine Sea slab under the peninsula at a depth of similar to 30 km, including a recent (2019) M-w 4.9 earthquake which caused shaking throughout the Kanto region (greater Tokyo). Such shallow intraplate earthquakes are potentially hazardous to this heavily populated region, yet their mechanism is poorly understood, especially in the context of a three-plate system. Here, we calculate stress rates in the Philippine Sea slab and the surrounding area, using a subduction model constructed in a previous study, to explain the generation of the regional stress field and its effect on earthquake occurrence. In general, the calculated stress rates under Boso Peninsula are horizontally extensional both above and below the Eurasian-Philippine Sea plate interface. We apply our calculated stress rates to the nodal planes of the observed earthquakes to calculate the Coulomb failure function (Delta CFF). These calculated Delta CFFs are generally positive on normal-type earthquakes under Boso. The Delta CFFs are also consistent with earthquakes in adjacent areas that are seismically active, for example, in the Philippine Sea plate to the south, in the collision zone around Izu Peninsula, and in the cluster in the Eurasian plate northeast of Boso Peninsula, which further supports our stress loading model. Calculation of the individual contributions of Philippine Sea plate and Pacific plate subduction shows that the development of the stress field around Boso is dependent upon contributions from both subducting plates. In contrast, the arc-arc collision at Izu Peninsula has little influence.

In the southern part of Boso Peninsula, central Japan, we can observe a series of well-developed Holocene marine terraces. We modeled the development of these marine terraces by considering sea-level fluctuation and steady land uplift. The evolution of coastal landform is generally described as follows: altitude change = − erosion + deposition − sea-level rise + land uplift. In this study, the erosion rate is supposed to be proportional to the dissipation rate of wave energy, and the deposition rate of eroded materials to decay exponentially as they are transported seaward. The rate of sea-level rise is given by the time derivative of a sea-level curve obtained from the sediment core records of oxygen isotope ratios. Steady plate subduction generally brings about steady crustal uplift/subsidence independently of earthquake occurrence, and so the land-uplift rate is regarded as time independent on a long-term average. Our simulation results show that a pair of sea cliff and abrasion platform is efficiently formed about a stationary point of the sea-level curve. The Holocene sea-level curve has four peaks and three troughs, and so basically seven terraces are formed one by one during the past 10,000 yr. However, when the land-uplift rate is low, most of the terraces formed at older times sink in the sea. When the land-uplift rate is high, the overlap and/or reverse of older and younger terraces occur frequently, and so the correspondence between the age and present altitude of terraces is not necessarily one-to-one. Taking the land-uplift rate to be 3–4 mm/yr, we can reproduce a series of well-developed Holocene marine terraces in Boso Peninsula independently of coseismic uplifts. From these simulation results, we may conclude that the Holocene marine terraces in Boso Peninsula were developed as a result of the composite process of sea-level fluctuation and steady coastal uplift.

In the region off the Boso Peninsula, Japan, the Pacific plate is subducting westward beneath both the Honshu island arc and Philippine Sea plate, while the Philippine Sea plate is subducting northwestward beneath the Honshu island arc. These complex tectonic interactions have caused numerous seismic events occurred in the past To better understand these seismic events, it is important to determine the geometry of the plate boundary, in particular the upper surface of the Philippine Sea plate. We conducted an active-source seismic refraction survey in July and August 2009 from which we obtained a 2-D P-wave velocity structure model along a 216-km profile. We used the velocity model and previously published data that indicate a P-wave velocity of 5.0 km/s for the upper surface of the subducting Philippine Sea plate to delineate its boundary with the overriding Honshu island arc. Our isodepth contours of the upper surface of the Philippine Sea plate show that its dip is shallow at depths of 10 to 15 km, far off the Boso Peninsula. This shallow dip may be a result of interference from the Pacific plate slab, which is subducting westward under the Philippine Sea plate. Within our survey data, we recognized numerous seismic reflections of variable intensity, some of which came from the upper surface of the Philippine Sea plate. An area of high seismic reflection intensity corresponds with the main slip area of the Boso slow slip events. Our modeling indicates that those reflections can be explained by an inhomogeneous layer close to the upper surface of the Philippine Sea plate. (C) 2017 Elsevier B.V. All rights reserved.

We determined the slip distribution of a slow-slip event (SSE) off the Boso Peninsula, Japan, from ocean bottom pressure gauge (OBP) observations. Most large SSEs occur under the ocean, and continuous OBP measurements have been used previously for their detection. These measurements require removing oceanic changes by subtracting averaged data of multiple stations from each OBP data set, a method that requires multiple stations. We developed a new method for extracting transient deformation due to a targeted event from data of a single OBP station. We fit a model that takes account of linear trends, seasonal variations, and other effects to the OBP data to detect transient deformation during the 2014 Boso SSE. We determined the OBP observation accuracy to be 10mm and detected 20mm of uplift which was objectively confirmed by using Akaike's information criterion. Our method improves the potential for measuring slip distributions of SSEs under the ocean.

In Northeast Japan, it remains a puzzle to reconcile the mismatch between long-term (geological) uplift and late-interseismic and coseismic subsidence associated with the 2011 Tohoku earthquake. To explain this mismatch between different periods, we modeled the entire megathrust earthquake cycle in the Northeast Japan arc using a simple dislocation model with a two-layered lithosphere-asthenosphere structure in which we account for viscoelastic relaxation in the asthenosphere and tectonic erosion. The model behaves differently when the rupture stops within the lithosphere and when it cuts through the lithosphere to reach the asthenosphere. It is possible to explain the mismatch in the case where the rupture stops within the lithosphere. In the early interseismic stage, the viscoelastic response to the megathrust earthquake dominates and can compensate for late-interseismic and coseismic subsidence. In contrast, the late-interseismic stage is dominated by the locking effect with the steady slip below the rupture area. Tectonic erosion explains up to about half of the long-term uplift by landward movement of arc topography. The rest of the long-term uplift may be attributed to indirect effects of internal deformation in the arc.

The Kanto Basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the repeated collision of the Izu-Bonin arc fragments with the Japanese island arc. Geomorphological, geological, and thermochronological data on vertical movements over the last 1 My suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modeled the tectonic evolution of the Kanto Basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the latest collision of the Izu Peninsula block has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0-0.5 Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following the shift in plate motion. Observed changes in the uplift/subsidence pattern are better explained by scenario (2), suggesting that recent (<1 My) deformation in the Kanto Basin shows a lag in crustal response to the plate motion shift. We also calculated stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions. (C) 2016 Elsevier B.V. All rights reserved.

In the southern Kanto region of Japan, where the Philippine Sea plate is descending at the Sagami trough, two different types of large interplate earthquakes have occurred repeatedly. The 1923 (Taisho) and 1703 (Genroku) Kanto earthquakes characterize the first and second types, respectively. A reliable source model has been obtained for the 1923 event from seismological and geodetical data, but not for the 1703 event because we have only historical records and paleo-shoreline data about it. We developed an inversion method to estimate fault slip distribution of interplate repeating earthquakes from paleo-shoreline data on the idea of crustal deformation cycles associated with subduction-zone earthquakes. By applying the inversion method to the present heights of the Genroku and Holocene marine terraces developed along the coasts of the southern Boso and Miura peninsulas, we estimated the fault slip distribution of the 1703 Genroku earthquake as follows. The source region extends along the Sagami trough from the Miura peninsula to the offing of the southern Boso peninsula, which covers the southern two thirds of the source region of the 1923 Kanto earthquake. The coseismic slip takes the maximum of 20 m at the southern tip of the Boso peninsula, and the moment magnitude (Mw) is calculated as 8.2. From the interseismic slip-deficit rates at the plate interface obtained by GPS data inversion, assuming that the total slip deficit is compensated by coseismic slip, we can roughly estimate the average recurrence interval as 350 years for large interplate events of any type and 1400 years for the Genroku-type events.

The southern limit of the 2011 Tohoku earthquake is considered to be located around off Ibaraki. However, it is not well constrained how far south the large slip extended. To give better constraints, we investigated seismicity including small earthquakes before and after the Tohoku earthquake around off Ibaraki using dense ocean bottom seismic array data. We automatically identified epicenters by backprojecting semblance values resulting in a considerable increase in the number of detected events compared with those listed in the catalog based on onshore observation. The results revealed a couple of seismicity-activated region. The largest aftershock also occurred similar to 30min after the main shock. Our detailed results suggest that this highly activated seismicity was initiated by the largest aftershock instead of the main shock. It, then, suggests that the large coseismic slip zone of the Tohoku earthquake may not have extended off Ibaraki.

The Southern Mariana Trough is an active back-arc basin with hydrothermal activity. We investigated relations between the back-arc spreading system and the hydrothermal system in this area by conducting a seismic reflection/refraction survey and a three-month campaign of seismic observations using ocean bottom seismometers. From a 3D seismic velocity structure analysis, we mapped a low-velocity structure just beneath the spreading axis, a high-velocity structure with convex upward beneath an off-axis knoll, and a thickening of layer 2 (to about 3 km) over the refraction survey area compared with normal mid-ocean ridges. We found very low seismicity in the hydrothermal area and high seismicity in areas of high topographic relief that probably represent arc volcanoes. The low-velocity structure at the axis suggests that there is some magmatic activity beneath the axis in the form of sheetlike mantle upwellings. These may constitute the hydrothermal heat source at this site. The high-velocity structure with convex upward at the off-axis knoll suggests the presence of off-axis volcanism there. The very low seismicity suggests that this volcanism may have ceased, thus residual heat of this off-axis volcanism may contribute the heat for hydrothermal activity at this site. A comparison of the velocity structure with other back-arc spreading zones and mid-ocean ridges shows that the Southern Mariana Trough has a relatively thick layer 2 with lower seismic velocities, suggesting that the crust was formed by magmas with high volatile contents, consistent with upwelling mantle influenced by subduction. The very low seismicity at the hydrothermal sites indicates that there are no faults or fractures related to the hydrothermal activity. This suggests that the activity is not related to tectonic stresses there.

2011年3月11日に，太平洋プレートと日本列島を乗せた陸側のプレートとの境界で2011年東北地方太平洋沖地震が発生した．この地震は，日本周辺では観測史上最大のマグニチュード9という巨大地震だった．本震発生後には多数の余震が発生するが，大地震発生のメカニズムを解明するためには，正確な余震分布を調べることが重要である．全国の6つの大学と海洋研究開発機構，気象庁気象研究所は，本震発生直後から共同で100台以上の海底地震計を用いて余震観測を行った．2011年6月中旬までのデータから，震源域全体で約3か月間の精度の良い震源分布が得られた．余震の震源の深さは，全体的に陸に近づくにつれて深くなっていた．震源分布からは，本震時に大きくすべったプレート境界では余震活動が低いことがわかった．上盤の陸側プレート内では余震活動が活発で，正断層型と横ずれ型が卓越していた．太平洋プレート内の余震も多くが正断層型か横ずれ型だった．このことから，日本海溝付近の太平洋プレート内の深部と上盤の陸側プレート内では，本震の発生によって応力場が圧縮場から伸張場に変化したことが示唆される．

Most studies of afterslip distribution consider only elastic media. However, the effects of poroelastic rebound in the upper crust and viscoelastic relaxation in the asthenosphere are part of the observed post-seismic deformation. Therefore, these effects should be removed to give a more reliable and correct afterslip distribution. We developed a method for calculating an afterslip distribution in elastic, poroelastic and viscoelastic media, and we applied this method to the case of the 2007 southern Sumatra earthquake (M-w 8.5). To estimate the coseismic slip and time evolution of the afterslip distribution, we applied Akaike's Bayesian Information Criterion (ABIC) inversion method of coseismic displacement, and analysed 15 months of GPS post-seismic deformation data in 3-month observation periods. To calculate afterslip in each period, we considered not only viscoelastic responses to coseismic slip but also viscoelastic responses to afterslip in the preceding periods. We used viscoelastic model to compute post-seismic deformation models every 3 months during the 15 months after the earthquake. The viscosity value for the asthenosphere layer is a crucial unknown parameter. To overcome this problem, we used a grid search method to determine the best-viscosity value, and we found that the best viscosity for the Sumatra subduction zone was 2.5 x 10(18) Pa.s. After removing the poroelastic and viscoelastic responses, we obtained maximum afterslip of 0.5 m during the 15-month investigation (the same as maximum afterslip estimated using the elastic medium only), but the poroelastic and viscoelastic responses brought the afterslip distribution to a shallower depth than the main coseismic rupture area. The results showed that the poroelastic and viscoelastic responses added significant corrections to the afterslip distribution. Compared with the traditional method, this method improved the determination of the afterslip distribution. We conclude that consideration of poroelastic and viscoelastic behaviours is essential for calculating the afterslip distribution. We propose that these parameters should be considered to obtain more reliable and correct afterslip distribution models following earthquakes.

Large destructive interplate earthquakes, such as the 2011 Mw 9.0 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. © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS) The Seismological Society of Japan The Volcanological Society of Japan The Geodetic Society of Japan The Japanese Society for Planetary Sciences TERRAPUB.

The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive sea-floor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations. The aftershocks form a plane dipping landward over the whole area, nevertheless the epicenter distribution is not uniform. Comparing seismic velocity structures, there is no aftershock along the plate boundary where a large slip during the mainshock is estimated. Activity of aftershocks in the landward plate in the source region was high and normal fault-type, and strike-slip-type, mechanisms are dominant. Within the subducting oceanic plate, most earthquakes have also a normal fault-type, or strike-slip-type, mechanism. The stress fields in and around the source region change as a result of the mainshock.

We present the result of a seismic experiment conducted using ocean bottom seismometers and controlled sources in the region off Ibaraki and the Boso Peninsula. This region is the southern edge of the rupture zone of the 2011 off the Pacific coast of Tohoku Earthquake. We estimated the P-wave seismic velocity structure beneath the profile using a 2-D ray-tracing method. The crustal structure in the southern area is more heterogeneous than that of the northern area. This heterogeneity is thought to be related with subducting the Philippine Sea plate (PHS). The plate boundary between the landward plate and the Pacific plate (PAC) is positioned at depths of 20 km at a distance of 170 km from the southern end of the profile. The subducting PHS is imaged on the southern part of the profile. However, we could not obtain a distinct image of the contact zone of PHS and PAC. The contact zone of PHS and PAC is estimated to have a large heterogeneity resulting from strong deformation due to the collision of the two plates. We infer that the termination of the rupture, and the large afterslip in the collision region, are caused by this strong heterogeneity.

The focus of this study is investigation of land subsidence in Semarang city Indonesia with the use of Interferometry Synthetic Aperture Radar (InSAR) of ALOS-PALSAR satellite. We processed 22 ascending SAR images during January 2007 to January 2009 plus two descending SAR images acquired on 6 June 2006 and 17 June 2007. The time series analysis of interferometry was performed by using 12 pairs of interferogram relative to 21 January 2007 and 8 pairs of interferogram relative 24 January 2008. The topographic phase contribution was removed using the 3-arcsec (90 m) Shuttle Radar Topography Mission (SRTM), Digital Elevation Model (DEM). We performed precision baseline estimation to vanish the fringes from baseline effect between master and slave data. In order to investigate the contribution of horizontal movement in our analysis, we constructed two interferograms of ascending orbit and descending orbit. The time series results exhibited that the area is subsiding continuously without a significant seasonal effect during January 2007 to January 2009. The land subsidence observed from InSAR data is approximately up to 8 cm/year. Three cross sections on image displacement show the extreme land subsidence occurred especially along the coastal area and lowland area where this area is considered as industrial with high-density settlements, consuming a lot of groundwater, and land is changed from agriculture and cultivation purposes to industrial estates and house. Our result also shows a consistency with historical pattern of subsidence measured by leveling data. The results highlight the potential use of InSAR measurements to provide better constraints for land subsidence in Semarang city Indonesia. (C) 2010 Elsevier Ltd. All rights reserved.

The Sumatra-Andaman earthquake with moment magnitude (M-w = 9.2) was a thrust earthquake that occurred at 00:58:53 Coordinated Universal Time (UTC) on 26 December 2004, with an epicentre off the west coast of Sumatra Island, Indonesia. To estimate the crustal displacement associated with this earthquake, many scientists have been analysing the global positioning system (GPS) data. In this letter, we calculated the horizontal displacement associated with the earthquake using amplitude offset of synthetic aperture radar (SAR) data. Four C-band SAR images acquired by the European Remote Satellite (ERS)-2 SAR instrument were processed to produce single look complex (SLC) images and to obtain the offset displacement between master and slave images. The offset SAR displacement showed maximum displacement in the northern part of Sumatra Island of 4-6 m. We compared the results with GPS observation and synthetic displacement using a fault geometry model. Our result shows a good agreement with the long-span GPS observation and synthetic model deformation in the southern part of location 1 and in the western part of location 2. We show that the amplitude offset measurement can be an alternative method for measuring ground movements when the field observation data and/or GPS data are sparse or not available.

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 long-term 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.

To reveal a subsurface structure beneath the southern part of the Boso Peninsula, Japan, where the Philippine Sea plate is subducting and great interplate earthquakes associated with the subduction occur repeatedly, we conducted a new seismic reflection survey from March to April 2005 (Boso05). We also reanalyzed old multi-channel seismic (MCS) survey data that had been collected off the Boso Peninsula in 1978 (SK78). We found clear strong reflectors beneath the southern coast of the Boso Peninsula. Since common mid points (CMPs) were distributed widely beneath the study area owing to the design of the receiver and shot lines of Boso05, we selected appropriate directions of stacking lines to get the best image of the dipping reflectors by optimum azimuth search (OAS) processing. We carefully checked the seismic profiles at the intersections of the survey lines to confirm the NNE-dipping configuration of the strong reflectors. These strong reflectors were interpreted as the upper surface of the subducting PHS plate from their locations and the estimated velocities beneath the reflectors. Furthermore, these reflectors revealed a topographic high (bump) beneath the southern coast of the Boso Peninsula where the source fault of the Genroku earthquake of 1703 is thought to be located. (C) 2008 Elsevier B.V. All rights reserved.

We performed 3-D seismic tomography in the forearc region of the northeastern Japan subduction zone using both onshore and offshore seismic station data. We obtained the Vp, Vs, and Vp/Vs structures around the plate boundary with high spatial resolution. The position of the plate boundary as defined by relocated hypocenters coincides with the sharp velocity boundary between the oceanic crust and the mantle wedge. The mantle wedge above the coseismic slip area of the 1978 and 2005 off-Miyagi interplate earthquakes (M > 7) is characterized by high Vp and Vs, but low Vp/Vs. Off Fukushima, however, where large earthquakes rarely occur, we found a high Vp/Vs anomaly at the tip of the mantle wedge. The spatial distribution of serpentinized mantle wedge limits the spatial extent of the strongly coupled area on the plate boundary, and thus can explain the difference in seismic activity between the off-Miyagi and off-Fukushima regions. Citation: Yamamoto, Y., et al. (2008), Spatial heterogeneity of the mantle wedge structure and interplate coupling in the NE Japan forearc region, Geophys. Res. Lett., 35, L23304, doi: 10.1029/2008GL036100.

We determined the three-dimensional Vp and Vs structures beneath Japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the Japan Sea and the Pacific Ocean, as well as those at permanent stations on the Japan Islands. As a result, we obtained more precise seismic images than previous studies. In the crust and the uppermost mantle, southwestern Honshu exhibited weaker heterogeneity than the other areas in Japan, corresponding to the distribution of active volcanoes. Stripe-like heterogeneities exist in the subducting Pacific slab. Relatively low-velocity zones correspond to low-seismicity areas in the Pacific slab, suggesting that the slab is possibly torn or thin around the areas. The fact that nonvolcanic deep tremors associated with the subducting Philippine Sea slab beneath Shikoku, Kii, and Tokai do not occur in zones of high Vp, high Vs, and low Vp/Vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab. Prominent and wide low Vp and Vs zones exist beneath central Honshu at the depth range of 30-60 km, where the volcanic front related to the subducting Pacific plate is located and seismicity around the Philippine Sea plate is very low. This condition may exist because magma genesis processes related to the subducting Pacific plate activate the same processes around the Philippine Sea plate. 2008 Elsevier B.V. All rights reserved.

Free-air gravity anomaly in plate subduction zones, characterized by island-arc high, trench low and outer-rise gentle high, reflects the cumulative effects of long-term crustal uplift and subsidence. In northeast Japan the island-arc high of observed free-air gravity anomaly takes its maximum about the eastern coastline. On the other hand, the current vertical crustal motion estimated from geological and geomorphological observations shows a gentle uplift in the land area and steep subsidence in the sea area with the neutral point near the eastern coastline. Such a discrepancy in spatial patterns between the free-air gravity anomaly and current vertical crustal motion can be ascribed to a change in the mode of crustal uplift and subsidence associated with the initiation of tectonic erosion at the North American-Pacific plate interface. We developed a realistic 3-D simulation model of steady plate subduction with tectonic erosion in northeast Japan on the basis of elastic/viscoelastic dislocation theory. Through numerical simulations with this model we found that simple steady plate subduction brings about the crustal uplift characterized by island-arc high with its maximum about the eastern coastline, while steady plate subduction with tectonic erosion, which is represented by the landward retreat of the plate interface, brings about gentle uplift in the land area and steep subsidence in the sea area with the neutral point near the eastern coastline. Therefore, if we suppose that tectonic erosion started 3-4 million years ago after the long duration of simple steady plate subduction, we can consistently explain both patterns of free-air gravity anomaly and current crustal uplift in northeast Japan.

We develop a systematic approach to the phase identification of late-arriving groups in 2-D seismic data. Waveforms in the same traveltime branch are grouped, and synthetic traveltimes for all phases are calculated using an initial approximation to the 2-D structure. For each group, we identify the two synthetic phases providing the smallest RMS residuals. If their ratio is less than some predetermined threshold, then the group's phase is ambiguous and both assignments must be tested by traveltime inversion. If there are n unidentified groups, we construct 2(n) phase tables and perform a traveltime inversion on every plausible phase assignment. The phase table that provides the highest value of the posterior probability density is taken as correct, and a 2-D velocity model is constructed from the data. This approach is shown to be effective and efficient on both simulated and real data. In addition, the residuals associated with late-arriving groups provide a means of identifying deficiencies in the initial model.

The Tsushima Basin is located in the southwestern Japan Sea, which is a back-arc basin in the northwestern Pacific. Although some geophysical surveys had been conducted to investigate the formation process of the Tsushima Basin, it remains unclear. In 2000, to clarify the formation process of the Tsushima Basin, the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern Tsushima Basin and its margin, which are presumed to be the transition zone of the crustal structure of the southwestern Japan Island Arc. The crustal thickness under the southeastern Tsushima Basin is about 17 km including a 5 km thick sedimentary layer, and 20 km including a 1.5 km, thick sedimentary layer under its margin. The whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern Japan Island Arc. On the other hand, thickness of the lower part of the crust seems more uniform than that of the upper part. The crust in the southeastern Tsushima Basin has about 6 km/s layer with the large velocity gradient. Shallow structures of the continental bank show that the accumulation of the sediments started from lower Miocene in the southeastern Tsushima Basin. The crustal structure in southeastern Tsushima Basin is not the oceanic crust, which is formed ocean floor spreading or affected by mantle plume, but the rifted/extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern Japan Island Arc. In the margin of the southeastern Tsushima Basin, high velocity material does not exist in the lowermost crust. For that reason, the margin is inferred to be a nonvolcanic rifted margin. The asymmetric structure in the both margins of the southeastern and Korean Peninsula of the Tsushima Basin indicates that the formation process of the Tsushima Basin may be simple shear style rather than pure shear style. (c) 2005 Elsevier B.V. All rights reserved.

The Hahajima Seamount, located at the junction between the Izu-Bonin and Mariana forearc slopes, is a notable rectangular shape and consists of various kinds of rocks. An elaborated bathymetric swath mapping with geophysical measurements and dredge hauls showed the Hahajima Seamount is cut by two predominating lineaments, northeast-southwest and northwest-southeast. These lineaments are of faults based on the topographic cross-sections and a 3-D view (whale's eye view). The former lineament is parallel to the transform faults of the Parece Vela Basin, whereas the latter is parallel to the nearby transform fault on the subducting Pacific Plate. The rocks constituting the seamount are ultramafic rocks (mostly harzburgite), boninite, basalt, andesite, gabbro, breccia and sedimentary rocks, which characterize an island arc and an ocean basin. Gravity measurement and seismic reflection survey offer neither a definite gravity anomaly at the seamount nor definite internal structures beneath the seamount. A northwest-southeast-trending fault and small-scale serpentine flows were observed during submersible dives at the Hahajima Seamount. The rectangular shape, size of the seamount, various kinds of rocks and geophysical measurements strongly suggest that the Hahajima Seamount is not a simple serpentine seamount controlled by various tectonic movements, as previously believed, but a tectonic block.

Strong anticorrelation between intensity of plate boundary PP reflection and seismicity had been revealed by a seismic reflection-refraction survey conducted in 1996 in a seismic-aseismic boundary region on the forearc slope of the Japan Trench. Amplitude of the strong reflection was explained by the presence of a thin layer (similar to200 m) of low P wave velocity (3-4 km/s) at the top of the plate boundary. We conducted another seismic survey in 2001 in the same region as that of 1996 and verified the strong anticorrelation with planer extension along the plate boundary. Therefore existence of the thin layer of low P wave velocity along the plate boundary is expected within the aseismic regions. Extremely low P wave velocities along the plate interface at depths of around 10-20 km suggest that the layer may include fluid, clay minerals, and/or serpentine-chlorite. Because these materials have low mechanical strength or cause low friction between the overriding and subducting plates, large strain may not be accumulated and aseismic slip may be dominant. As a result of the above speculation, the intense reflection observed within the aseismic regions may imply aseismic slip between the plates. Amplitude of the plate boundary PP reflection phase was found maximum about reflections from deeper plate interface, although the reflected waves propagate for longer distances. One possible explanation to this phenomenon is the presence of serpentinized wedge mantle material that is buoyancy-driven to move along the top of the slab.

Microearthquake observation using Ocean Bottom Seismometers (OBSs) was carried out to obtain a detailed distribution of microearthquakes beneath the area off Fukushima, in the middle section of the Japan Trench in the summer of 1997. The observation period spanned approximately one month. Almost all of the well-determined hypocenters occurred in the vicinity of the plate boundary in this region (approximately 100 km landward of the trench axis), while seismicity is markedly lower between this area and the trench itself. The seaward limit of the high seismicity region is close to the western end of the zone of direct contact between the oceanic crust and the overriding landward crust. Twenty-nine earthquakes were recorded within the overriding landward plate. Twelve earthquakes were recorded about 30 km below the plate boundary, and form a landward dipping plane that appears to be an up-dip continuation of the lower plane of the double seismic zone. The microseismicity characteristics of the plate boundary region are interpreted to be controlled by the geometry and physical properties of the plate interface. The seismic activity in the lower seismic plane near the trench is considered to relate to bending of the subducting plate and to dehydration of serpentinized Pacific Plate mantle.

We have carried out seismological observations within the Sea of Marmara (NW Turkey) in order to investigate the seismicity induced after Golcuk-Izmit (Kocaeli) earthquake (M-w 7.4) of August 17, 1999, using ocean bottom seismometers (OBSs). High-resolution hypocenters and focal mechanisms of microearthquakes have been investigated during this Marmara Sea OBS project involving deployment of 10 OBSs within the Cinarcik (eastern Marmara Sea) and Central-Tekirdag (western Marmara Sea) basins during April-July 2000. Little was known about microearthquake activity and their source mechanisms in the Marmara Sea. We have detected numerous microearthquakes within the main basins of the Sea of Marmara along the imaged strands of the North Anatolian Fault (NAF). We obtained more than 350 well-constrained hypocenters and nine composite focal mechanisms during 70 days of observation. Microseismicity mainly occurred along the Main Marmara Fault (MMF) in the Marmara Sea. There are a few events along the Southern Shelf. Seismic activity along the Main Marmara Fault is quite high, and focal depth distribution was shallower than 20 km along the western part of this fault, and shallower than 15 km along its eastern part. From high-resolution relative relocation studies of some of the microearthquake clusters, we suggest that the western Main Marmara Fault is subvertical and the eastern Main Marmara Fault dips to south at similar to45degrees. Composite focal mechanisms show a strike-slip regime on the western Main Marmara Fault and complex faulting (strike-slip and normal faulting) on the eastern Main Marmara Fault. (C) 2004 Elsevier B.V. All rights reserved.

The Izu-Bonin arc system is the subduction zone forming the plate boundary between the downgoing Pacific plate and the overriding Philippine Sea plate. Seismicity along the Izu-Bonin subduction zone is very different in character from other western Pacific subduction zones. Few large earthquakes have occurred at shallow depths (0-100 km), but many large earthquakes have occurred at greater depths (>400 km). Other unique characteristics of this subduction zone include the existence of serpentine seamounts exposed along the forearc slope and the presence of low-velocity (<7.3 km/s) material between the two plates in a zone extending from the forearc seamounts to the mantle wedge. To investigate these unique characteristics, we carried out an ocean-bottom seismic experiment in 1999 to estimate the hypocenter distribution and the structure of the mantle wedge simultaneously by performing 3D event locations employed within different velocity models. We obtained the following results: (1) No earthquake occurred on the upper surface of the subducting plate and some were located as far as 20 km away from the upper surface. Most events occurred within the mantle of the subducting slab. (2) There were no earthquakes in the mantle wedge above the subducting slab. (3) The low mantle velocity area in the mantle wedge terminates ∼140 km west of the trench axis. (4) The subducting slab has a dip of 55° to the west. From these results we suggest that the low-velocity material between the plates is chrysotile, a low-temperature, low-strength, low friction phase of serpentine, which may act as a lubricant on the plate boundary. The western boundary of the low mantle velocity region in the mantle wedge, which is about 140 km west of the trench axis and at a depth of about 20 km, coincides with the temperature-controlled transition from chrysotile to antigorite (the high-temperature phase of serpentine) along the plate boundary. Our results suggest that chrysotile may migrate upward and eastward along the plate boundary, while antigorite may move downward with the subducting slab. © 2004 Elsevier B.V. All rights reserved.

To study earthquake generation mechanisms, we carried out two seismic experiments in the Izu-Bonin Trench and the Japan Trench subduction zones using OBS and controlled sources in 1996 and 1998, respectively. In the Izu-Bonin subduction zone, we inferred the presence of serpentinized rocks along the top of the slab (Kamimura et al., 2002). Between 38-39° N along the Japan Trench forearc slope, we observed a good correlation between aseismicity and PP reflection phase from the top of the slab (the plate boundary) (Fujie et al., 2002).<BR>To confirm the results of the survey in the Izu-Bonin Trench subduction zone, we calculated synthetic waveforms from the velocity structure obtained by travel-time tomography and seismic attenuation (Q model). A comparison of observed and synthetic waveforms suggests the existence of a low Qp ( 10) layer beneath the Torishima serpentine seamount and extending from the seamount 80 km from the west of the seamount along the top of the slab. This modeling supports the presence of serpentinized peridotite at the top of the slab. We suggest that such a layer may allow an aseismic slip during subduction of the oceanic plate, primarily due to the low frictional coefficient of the crysotile (a phase of low temperature serpentine).<BR>In the Japan Trench subduction zone, we carried out a more extensive OBS-controlled source seismic experiment to confirm the results from 1996. We shot airguns along 7100 kmlong path that runs nearly parallel to the trench axis, and this paper presents the results from the western 5 lines. We observed very intense PP reflections from the top of the slab over the aseismic zone. By including a thin low velocity layer just above the slab, we obtained similar strong PP reflection phases in synthetic waveforms from the top of the slab. These observations and models suggest that a layer with a P velocity as low as 2-4 km/s and a thickness of 100-400 m is needed at the top of the slab to explain the observed PP reflection intensity. Such a layer can comprise aqueous fluid, clay, and/or serpentine-chlorite generated by hydration/dehydration of the subducting oceanic crust and the wedge mantle peridotite. The top of the slab at 18 a path of km exhibits stronger PP reflections than it does at 12 a depth of km depth, and it suggests upwelling of serpentinized and/or clay-rich material from the wedge mantle to the shallower plate boundary.<BR>In the Izu-Bonin Trench and the Japan Trench regions, we conclude the presence of low P wave velocity and low Q materials along the top of the slab. This result strongly suggests aseismic slip during the subduction of the Pacific Plate in the shallow part of the Izu-Bonin Trench subduction zone and in the aseismic zone between 38-39° N over the Japan Trench forearc slope.

To study the physical properties along the subducting plate boundary at the Japan Trench, a seismic study was carried out using ocean bottom seismometers (OBSs) and artificial sources was carried out. The 250km long survey line with an almost N-S strike crosses the two major focal zones of the 1968 Tokachi-Oki earthquake and the 1994 Sanriku-Haruka-Oki earthquake. Ray tracing, and non-linear inversion were used to analyze the data obtained by the OBSs. P-wave velocity structure down to the island-arc-Moho was obtained. The result shows distinct heterogeneities along the plate boundary at 40degrees10'N, which are located exactly at the southern boundary of major moment release regions of the above two earthquakes. P-wave velocities for the crust north of 40degrees10'N are approximately 7% slower than those for the south. The depths of the island-arc-Moho vary from 15 km in the south to 21 km in the north below the sea surface. Three explanations of the velocity heterogeneities are presented in terms of the migration of water. (C) 2002 Elsevier Science B.V. All rights reserved.

An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138degreesE, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 kin landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu-Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough. (C) 2002 Elsevier Science B.V. All rights reserved.

[1] There is a regional variation of seismicity in the Japan trench region characterized by cluster distribution of seismically active zones which persist for at least a few decades. We conducted seismic refraction-reflection experiments there to clarify the relationship between seismicity and crustal structure. A velocity structure model was obtained by travel-time inversion. Although there does not seem to be distinct relationship between seismicity and the bulk velocity structure, we found a good relationship between seismicity and variation of reflective amplitude. Large amplitude reflected waves generated at the plate boundary were observed at low seismicity region and vice versa.

The northeastern Japan Arc is a typical arc in the northwestern Pacific. In this area, many geophysical and geological studies have been conducted to resolve the formation of the Japan Sea and the northeastern Japan Arc. Therefore the northeastern Japan Arc is one of the most well investigated trench-arc systems in the world. However, the origin and evolution of the northeastern Japan Arc and the Japan Sea have not been revealed yet. Also, the data for detailed seismic structure in the boundary area are not enough. To understand the formation of the Arc and the Sea, we need data of the seismic structure of the whole arc-trench system from the trench to back-arc basin over the arc. In the autumn of 1997, seismic experiments were carried out in sea and land both on a profile from forearc to back-arc across the northeastern Japan Arc. We obtained a detailed seismic structure beneath the northern Yamato Basin and the eastern margin of the Japan Sea using twenty-six ocean bottom seismographs (OBSs), airguns and explosives as controlled sources on the profile in the Japan Sea. The profile in the Japan Sea is a part of the whole profile to obtain the seismic structure of the northeastern Japan arc-trench system. The Yamato Basin has a layer with a P-wave velocity of about 6km/s. The 6-km/s layer has a thickness of 3-4km. A layer with a P-wave velocity of 6.7-7.1km/s underlies the 6-km/s layer and has a thickness of approximately 10km. The entire crust is about 15-16km thick, and the Moho interface is approximately 18km deep below the sea surface in the Basin. The thickness of the 6-km/s layer and the depth of the Moho interface gradually increase with approaching to the northeastern Japan Arc. The velocities of the lower crust with 6.7-7.1km/s are diminished with increase of a thickness of the 6-km/s layer. The P-wave velocity in the uppermost mantle is 8.0km/s. In the northern Yamato Basin, the crust is twice thicker than usual oceanic crust and we can not confirm a high velocity layer in the lower crust, which were found in the southern Yamato Basin. Beneath some continental margins (volcanic margin), high velocity layers (>7km/s) interpreted as the result of a large igneous activity were found at the bottom of the crust. However, in the boundary region between the Yamato Basin and the northeastern Japan Arc, there is no seismic evidence of such a high velocity layer in a lower crust. Thus, the seismic velocity structure in this area is similar to those of non-volcanic margins rather than the velocity structure in volcanic margins.

Hypocenters and focal mechanisms of microearthquakes have been investigated at the Rodriguez Triple Junction in the Indian Ocean. Little was known on microearthquake activity in this region. We deployed 18 ocean bottom seismographs during the KH93-3 cruise of the RN Hakuho-Maru (Ocean Research Institute, University of Tokyo) from July 30 to August 20, 1993. We obtained 579 well-constrained hypocenters and 13 focal mechanisms. Microearthquakes were found to be active along all of the three ridges: the Central Indian Ridge, the Southeastern Indian Ridge, and the Southwestern Indian Ridge. Especially at the triple junction there was an earthquake swarm within narrow area of approximately 15 X 5 km(2). All of the 13 focal mechanisms showed normal or strike-slip faultings, which means that the extensional stress field characterizes this region.