a bump on the upper surface of the philippine sea plate beneath the boso peninsula japan inferred from seismic reflection surveys a possible asperity of the 1703 genroku earthquake abstract 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

complex slab structure and arc magmatism beneath the japanese islands abstract a dense nationwide seismic network recently constructed in japan has resulted in the production of a large amount of high quality data that have enabled the high resolution imaging of deep seismic structures in the japanese subduction zone seismic tomography precise locations of earthquakes and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath japan revealing that the subducting philippine sea slab underneath southwestern japan has an undulatory configuration down to a depth of 60 200 km and is continuous from kanto to kyushu without disruption or splitting even within areas north of the izu peninsula analysis of the geometry of the pacific and philippine sea slabs identified a broad contact zone beneath the kanto plain that causes anomalously deep interplate and intraslab earthquake activity seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the philippine sea slab beneath the japan sea north of chugoku and kyushu and beneath the east china sea west of kyushu down to a depth of 450 km seismic tomography also allowed the identification of an inclined sheet like seismic low velocity zone in the mantle wedge beneath tohoku a recent seismic tomography work further revealed clear images of similar inclined low velocity zones in the mantle wedge for almost all other areas of japan the presence of the inclined low velocity zones in the mantle wedge across the entirety of japan suggests that it is a common feature to all subduction zones these low velocity zones may correspond to the upwelling flow portion of subduction induced convection systems these upwelling flows reach the moho directly beneath active volcanic areas suggesting a link between volcanism and upwelling

fine fracture structures in the geothermal region of hakone volcano revealed by well resolved earthquake hypocenters and focal mechanisms abstract we investigated precise hypocentral distribution and the mechanisms of small earthquakes in the geothermal region of hakone volcano central japan where swarm activity has been frequently observed earthquake swarms were remarkably prevalent in 2001 and 2006 and were accompanied by crustal deformation we first determined initial hypocenters using station corrections and one dimensional velocity structures as estimated by the joint hypocenter determination method then we applied the double difference method to relocate the initial hypocenters using the differential arrival time obtained by both manual picking and waveform cross correlation analysis subsequently we determined the focal mechanisms from the absolute p and sh wave amplitudes and p wave polarities from the relocated hypocenter distribution we found that most swarm earthquakes are distributed on thin plane like zones with width length of 100 m to 1 km we found that these plane like hypocentral distributions range from the e w to n s strikes and are consistent with one of the nodal planes of the focal mechanism it is likely that most swarm earthquakes occurred on pre existing fracture planes shaping the plane like hypocenter distributions previous studies suggested that a highly permeable fracture system has developed in the fault interaction area and these highly permeable fractures affect the occurrence of swarm earthquakes and surface geothermal activity since hakone volcano is located in the interaction area of the active tanna and hirayama faults it is suggested that the fracture planes revealed by the relocated hypocenter distribution have been developed by the activity of the two faults the fracture planes in the caldera might be channels for the hydrothermal water of hot springs in the caldera of hakone volcano which are rich in sodium chloride and considered to be composed of fluid heated by a deep seated magma the strike of the fracture plane on which the swarm earthquakes of the 2001 activity occurred is inconsistent with the orientation of open crack models estimated from geodetic data it is therefore possible that the hydrothermal fluid aseismically intruded the open cracks during the 2001 activity

geometry and spatial variations of seismic reflection intensity of the upper surface of the philippine sea plate off the boso peninsula japan abstract 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

geodetic imaging of plate motions slip rates and partitioning of deformation in japan 1 interseismic deformation in japan results from the combined effects of tectonic processes including rotation of crustal blocks and the earthquake cycle process of elastic strain accumulation about upper plate faults and subduction zone interfaces we use spherical linear block theory constrained by geodetic observations from densely spaced global positioning system gps stations to estimate plate motions fault slip rates and spatially variable interplate coupling on the japan kuril sagami and nankai subduction zones the reference model developed in this paper consists of 20 blocks produces a mean residual velocity magnitude of 1 84 mm yr at 950 stations and accounts for 96 of the observed interseismic deformation signal we estimate fault slip rates in excess of 15 mm yr along the niigata kobe tectonic zone and itoigawa shizuoka tectonic line through central japan confirming their hypothesized roles as major tectonic boundaries oblique convergence across the nankai trough is partitioned with 3 4 of the 30 mm yr of trench parallel motion accommodated by strike slip motion on the subduction interface and the remaining 1 4 accommodated by right lateral slip on the median tectonic line in contrast our models suggest negligible slip partitioning in eastern hokkaido where oblique slip on the japan kuril subduction interface accommodates all of the trench parallel component of relative plate motion inferred spatial variations in the rake and magnitude of slip deficit on subduction zone interfaces reflect the influences of megathrust geometry and earthquake cycle processes such as enhanced elastic strain accumulation about seismic asperities and coseismic sense fault motion indicative of silent slip events or afterslip following large earthquakes

p wave anisotropic tomography of the nankai subduction zone in southwest japan 1 the active subduction of the young philippine sea phs plate and the old pacific plate has resulted in significant seismic heterogeneity and anisotropy in southwest sw japan in this work we determined a detailed 3 d p wave anisotropic tomography of the crust and upper mantle beneath sw japan using 540 000 p wave arrival times from 5 249 local earthquakes recorded by 1095 stations the phs slab is imaged clearly as a high velocity high v anomaly which exhibits considerable lateral variations significant low velocity low v anomalies are revealed above and below the phs slab the low v anomalies above the phs slab may reflect the upwelling flow in the mantle wedge and the phs slab dehydration and they form the source zone of the arc volcanoes in sw japan the low v zones under the phs slab may reflect the upwelling flow in the big mantle wedge above the pacific slab the anisotropy in the crust and upper mantle is complex in kyushu the p wave fast velocity direction fvd is generally trench normal in the mantle wedge under the back arc which is consistent with the corner flow driven by the phs slab subduction the fvd is trench parallel in the subducting phs slab under kyushu we think that the intraslab seismicity is a potential indicator to the slab anisotropy that is the phs slab with seismicity has kept its original fossil anisotropy formed at the mid ocean ridge while the aseismic phs slab has reproduced the anisotropy according to its current deformation

a proposal for a standard procedure of modeling 3 d velocity structures and its application to the tokyo metropolitan area japan abstract most metropolitan areas are located over large scale sedimentary basins since the sediments filling basins amplify ground motions and their velocity structures complicate the propagation of seismic waves it is important for the prediction of strong ground motion and seismic hazard to determine the three dimensional 3 d velocity structures of these urban basins this importance motivated various organizations to carry out extensive geophysical experiments and geological investigations and velocity structure models are being constructed all over japan however a study of a single dataset cannot completely define a 3 d velocity structure so that we propose a standard procedure for modeling the 3 d velocity structure of an urban basin in japan by simultaneously and sequentially using various kinds of datasets such as those from refraction reflection experiments gravity surveys surface geology borehole logging microtremor surveys and earthquake records we then apply the procedure to the tokyo metropolitan area tma over the kanto basin with an area of about 17 000 km2 and the maximum thickness of about 4 km as one of the steps in the procedure a joint inversion of refraction and gravity data has been formulated to determine the 3 d topography of interfaces of the sedimentary layers as well and lateral distribution of the basement slowness we validate the constructed velocity structure model by comparison of observed and synthetic waveforms since this modeling is carried out mainly for strong ground motion prediction the proposed procedure including the joint inversion and validation with ground motion simulations works well for tma and the applicability of the standard procedure has been confirmed for regions with substantial data of experiments and earthquake records in japan

suboceanic earthquake location and seismic structure in the kanto district central japan abstract we present a combined method using sp depth phase data and double difference arrival times to determine the precise hypocenter locations of earthquakes that occur under the pacific ocean outside of the area covered by the land based seismic network we assess the effectiveness of the combined method using a data set of p and s wave arrival times and sp depth phase from suboceanic earthquakes recorded by both land based seismic stations and offshore seismic stations ofs the hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and ofs data the differences in the hypocenters relocated by the two methods are less than 4 km we applied the method to the subduction region that underlies the kanto district central japan and located a large number of earthquakes that occurred beneath the pacific ocean we then determined the detailed 3d seismic velocity structure by inverting a large number of arrival times of p and s waves and sp depth phase from the relocated earthquakes in the study region high velocity anomalies related to the cold subducting pacific slab and low velocity anomalies related to the hot mantle wedge are clearly imaged beneath active volcanoes low velocity zones are visible from the surface to a depth of 100 km reflecting fluids released by dehydration of the subducting pacific slab strong lateral heterogeneities are revealed on the upper boundary of the pacific slab beneath the forearc region the low velocity areas under the offshore region are associated with low seismicity and weak interplate coupling a low velocity layer is imaged along the upper boundary of the philippine sea slab in the northern part of kanto district which may reflect dehydration of the slab our tomographic images indicate that the overlaying philippine sea plate has effects on the spatial distribution of active volcanoes related to the subducting pacific slab in the study region

back arc spreading of the northern izu ogasawara bonin islands arc clarified by gps data abstract we examined gps data in the northwestern pacific region which includes the izu ogasawara bonin mariana ibm arc and the japan arc gps velocity vectors on the izu islands including hachijo jima and aoga shima show systematic eastward movement deviating from that predicted by the rigid rotation of the philippine sea plate this deviation supports the active back arc spreading model suggested by previous geological studies the results of a statistical f test analysis with 99 confidence level showed that the forearc of the izu islands arc has an independent motion with respect to the rigid part of the philippine sea plate we developed a kinematic block fault model to estimate both rigid rotations of crustal blocks and elastic deformation due to locked faults on the block boundaries the model suggests that the back arc opening rate along the izu back arc rift zone ranges from 2 mm yr at its southern end to 9 mm yr near miyake jima its northern end it also predicts 23 28 mm yr of relative motion along the sagami trough in the direction of n25 w where the izu forearc subducts beneath central japan the orientation of this motion is supported by slip vectors of recent medium size earthquakes repeated slow slip events and the 1923 m 7 9 kanto earthquake

2d spatial distribution of reflection intensity on the upper surface of the philippine sea plate off the boso peninsula japan abstract the region off the boso peninsula japan is a tectonically complex area where the pacific plate is subducting beneath both the landward plate and the philippine sea plate phs from the japan trench and the izu bonin trench as the phs is subducting under the landward plate from the sagami trough it is important to better determine the structure of this region to deepen our understanding of its seismicity previous seismic reflection studies have shown that reflections from the upper surface of the phs vary with depth being stronger in the main slip area of the slow slip events beneath the boso peninsula boso sses however the spatial relationship between the reflective area and the sses is poorly constrained this study mapped the distribution of the reflective area using data recorded by ocean bottom seismometers during an active source seismic experiment we constructed a 3d p wave velocity structure by using traveltimes of first arrivals from 18 ocean bottom seismometer records we also adapted the traveltime mapping method to reflection traveltimes projecting them to the depth distance domain to map the 2d distribution of strong reflections from the top of the phs these reflections were concentrated in two areas one near the main slip area of the boso sses and the other about 60 km to the east in the first area the absence of strong velocity contrasts near the top of the phs suggests that the reflections were generated by a thin low velocity layer in contrast the structure of the second area has a convex shape of high velocity with a high velocity gradient near the top of the phs this structure may represent boninitic material of the outer arc high partially serpentinized peridotite or gabbro displaced by intraoceanic reverse faults

source process of the 1923 kanto earthquake considering subduction interface geometry and amplification effects caused by the large scale and 3d complex sedimentary basin abstract the 1923 kanto earthquake occurred along the sagami trough central japan causing severe damage in the tokyo metropolitan area this study was able to characterize the source process for this event using geodetic teleseismic and strong motion data the kanto region is located above a large scale sedimentary basin therefore 3d green s functions and a curved fault modeling the subduction interface geometry were used to account for 3d complex wave propagation inside the basin the later phases of the 3d green s functions with long paths inside the basin had large amplitude and long duration primarily because of amplification effects caused by the lateral heterogeneity of sedimentary layers however 3d static displacements were large mainly because of amplification effects caused by the presence of thick soft sedimentary layers in the basin the geodetic inversions with 3d and 1d green s functions had smaller seismic moments than half space ones this suggests that wave amplifications caused by large scale sedimentary basins exert significant effects on seismic moment determinations the joint inversion with 3d green s functions showed two large slip areas with a maximum slip of 6 m and an estimated total seismic moment of about 4 1 10 20 n m m w 7 7 strong motions were recovered very well even for the later phases by slips in shallow areas furthermore by comparing various source processes inverted by two fault models of 70 or 280 subfaults 3d or 1d green s functions and a curved or planar fault model we verified these assumptions based on the inversion results

heterogeneous structure across the source regions of the 1968 tokachi oki and the 1994 sanriku haruka oki earthquakes at the japan trench revealed by an ocean bottom seismic survey abstract 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 250 km 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 40 10 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 40 10 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

thickness variation of the descending philippine sea slab and its relationship to volcanism beneath the kanto tokai district central japan 1 we use an iterative nonlinear travel time tomography technique to determine three dimensional p wave and s wave seismic velocity models for the region beneath the kanto tokai district of central japan salient points of the models are the following 1 the subducting philippine sea slab has thickness variation with a stepwise offset east of the izu peninsula the eastern beneath the kanto district and western beneath the area north of izu peninsula and the tokai district regions have respective thicknesses of 60 and 25 km the slab clearly subducts continuously to a depth of 100 km from the sagami trough beneath the kanto district although the tokai slab is indistinct at depths of 50 70 km 2 the oceanic crust at the top of the descending philippine sea slab transforms into eclogite at a depth of about 70 km beneath the kanto district 3 low velocity anomalies are associated with present and past volcanism they are located on the west of a 110 km depth contour of the wadati benioff zone rather than only on the backarc side of the volcanic front deflection of the volcanic front might result from the subducting philippine sea slab which shifts the dehydration reaction deeper as it subducts deeper

low velocity oceanic crust at the top of the philippine sea and pacific plates beneath the kanto region central japan imaged by seismic tomography 1 we construct fine scale three dimensional p and s wave velocity structures beneath the kanto region central japan by seismic tomography with a spatial correlation of velocities the philippine sea and pacific plates subduct beneath the eurasian plate in this area and are imaged with high velocities oceanic crust at the uppermost part of these subducting plates is a low velocity layer low velocity oceanic crust of the philippine sea plate subducts to a depth of approximately 80 km there are also two low velocity bodies with relatively high v p v s ratios of 1 80 1 90 in the mantle wedge above the oceanic crust of the philippine sea plate we speculate that the westernmost of these two low velocity bodies consists of 20 partially serpentinized peridotite continuous with gabbro in the oceanic crust of the uppermost philippine sea plate while the eastern body is composed of 30 partially serpentinized peridotite we trace the subducting oceanic crust of the pacific plate to a depth of 120 km the estimated v p v s ratio of this layer is 1 85 1 90 which indicates a low probability of molten rock the gabbroic oceanic crust may have been metamorphosed to garnet granulite

crustal block kinematics and seismic potential of the northernmost philippine sea plate and izu microplate central japan inferred from gps and leveling data 1 we clarify the contemporary deformation observed by gps and leveling for the greater tokyo kanto eastern tokai and the izu islands region where the izu ogasawara bonin arc is subducting and colliding with the central part of the japan arc from these data we develop a kinematic model of fault sources with variable components of seismic and aseismic slip under the assumption that the contemporary deformation during 1995 2000 is representative of the long term interseismic strain field the geodetic data are inverted to estimate the rotation poles of the crustal blocks the degree of elastic strain accumulation on faults and the volumetric inflation sources beneath volcanoes the present crustal movements are explained by four crustal blocks the izu microplate the central japan block the pacific plate and the philippine sea plate along the suruga and sagami margins of the philippine sea plate lie strongly coupled faults which include sites of the 1854 m 8 4 tokai 1923 m 7 9 kanto and 1703 m 8 2 genroku kanto earthquakes in contrast the philippine sea plate boundary immediately north of the izu peninsula site of m 7 5 collisional earthquakes is only weakly coupled the boundary between the izu microplate and the philippine sea plate experiences left lateral motion with the rate of 30 mm yr most of this boundary is locked and thus has a large potential of future earthquakes the izu microplate is found to rotate rapidly clockwise at 10 myr with a rotation pole relative to the central japan block located just north of its northern boundary the pole and angular velocity of the izu microplate depends however on the assumed location of the boundary with the exception of the site of the 1938 m 7 earthquake swarm which we infer has the current potential to produce a m 8 1 earthquake the pacific megathrust has a very low seismic potential although the geodetic data have little resolving power for much of the pacific plate interface east of tokyo

crustal structure of the izu collision zone in central japan from seismic refraction data 1 the izu bonin arc iba has been colliding with the more northerly honshu arc since the middle miocene forming a globally unique active izu arc arc collision zone icz in central japan to determine crustal structure of the icz we constrain p and s wave velocities from active source refraction data the velocity structures reveal that the style of collisions of the misaka tanzawa and izu blocks which were derived from the iba has been controlled by subduction of the iba high velocity rocks vp 6 2 km s observed in the icz continuously extend to deeper crust suggesting that plutonic rocks formed by partial melting of the subducted iba crust intruded into the honshu arc crust and the accreted crustal blocks of the iba the northernmost major fault in the icz dips southeastward which is in clear contrast with other major northwestward dipping faults farther southeast in the northernmost part of the icz the upper 5 km crust of the misaka block was delaminated from its lower part and obducted onto the honshu arc the tanzawa block southeast of the misaka block is bounded by northwestward dipping faults and is characterized by crustal stacking a low velocity sedimentary layer between the tanzawa and izu blocks dips northwestward along the collisional boundary and reaches 5 km depth suggesting that the izu block underthrust beneath the tanzawa block the overall geometry of the collisional boundaries exhibits a doubly vergent system which is characterized by inward dipping thrust faults on both sides

was the 1952 tokachi oki earthquake mw 8 1 a typical underthrust earthquake plate interface reflectivity measurement by an air gun ocean bottom seismometer experiment in the kuril trench 1 the kuril trench subduction zone is one of the most seismogenic regions where underthrust earthquakes with m 8 recur along the trench the seismic gap between the source areas of the 1973 nemuro oki and 2003 tokachi oki earthquakes which are typical underthrust earthquakes faulting with rupture velocities of 3 km s has been ruptured by the 1952 tokachi oki earthquake the seismic gap has also slipped incidental to neighboring asperities the difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface such as a reflectivity of the plate interface we estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun ocean bottom seismometer experiment on the along trench profile across the seismic gap strong reflections from the plate interface were observed in the 1952 tokachi oki source area including the seismic gap rather than in the 1973 nemuro oki source area the strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable the coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes such as the 2003 tokachi oki and 1973 nemuro oki earthquakes our results suggest that the 1952 tokachi oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake

fluid accumulation and channeling along the northern barbados ridge decollement thrust a volume of three dimensional seismic reflection data acquired in 1992 imaged the decollement beneath the northern barbados ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault we model the seismic reflection by inversion for seismic impedance the product of velocity and density throughout the 5 x 25 km survey area and thus map physical property variations in 1997 ocean drilling program leg 171a penetrated the protodecollement and decollement at five sites with a logging while drilling lwd tool to log density and other physical properties of the decollement we construct a regional map of density and inferred porosity within the decollement from seismic models calibrated with lwd density data in the sediments out in front of the trench the protodecollement forms in a radiolarian rich miocene mudstone with an anomalously high porosity 70 75 that appears as a pervasive inherent characteristic of this interval seaward of the deformation front in the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70 at the wedge toe to 50 4 km from the wedge toe a second distinct trend also forms along a 10 km long 1 to 2 km wide ne sw zone in which porosity is 70 as high as it is in the protodecollement this zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content we postulate that high fluid content is maintained by continuous recharge flowing into and along this channel this porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out of sequence thrusts above the decollement

geohazards in coastal areas near the northernmost sagami trough central japan review of neotectonic activity in onshore and offshore areas of the izu island arc collision subduction zone

configuration of the moho discontinuity beneath the japanese islands derived from three dimensional seismic tomography abstract the mohorovicic discontinuity moho is defined on the basis of an abrupt increase in seismic velocity in the lithosphere which has been observed using seismic refraction and receiver function analysis methods worldwide moho depth varies regionally and remains a fundamental parameter of crustal structure we present a new method of mapping the moho using a 3d seismic tomography model since the tomographic method cannot locate discontinuities we treat the moho as a zone of high velocity gradient maximum lower crust minimum upper mantle p wave velocities in japan are known to be 7 0 km s and 7 5 km s respectively we map the residual between isovelocity surfaces of 7 0 km s and 7 5 km s to find areas where the residual is small the separation between the surfaces is narrow and the velocity gradient is high the moho is best constrained where the isovelocity surfaces are close together and under much of japan they are 10 km apart we chose an isovelocity surface of 7 2 km s as a representative moho proxy in these areas our resulting moho map under japan compares favorably with existing regional moho models that were obtained from controlled source seismic investigations the moho varies from shallow 25 30 km to deep 30 km and this variability relates to the structural evolution of the japanese islands the opening of the sea of japan back arc ongoing arc arc collisions at the hidaka and izu collision zones ongoing back arc extension in kyushu and a possible failed back arc extensional event of mesozoic age it is apparent that the moho is less well constrained in areas where the crustal structure has been modified by magmatic activity or thickened due to arc arc collision

active blind thrusts beneath the tokyo metropolitan area seismic hazards and inversion tectonics 1 the tokyo metropolitan area underlain by neogene and quaternary sediments more than 5 km thick is currently deformed by blind thrusts that could generate hazardous earthquakes however their little geomorphic expression and dense urbanization make understanding of folds produced above them and recent deformation highly elusive here we show subsurface geometries of several active blind thrusts beneath this highly urbanized area based on tectonic landforms high resolution seismic reflection data and quaternary stratigraphy deep seismic reflection profiles corroborate the notion that steeply dipping blind thrusts are reactivated normal faults originally formed by middle miocene extensional tectonics despite very slow less than 0 1 mm yr late quaternary slip rates our work suggests the presence of previously unrecognized faults that pose seismic hazards to tokyo and outlying communities highlighting the need for additional information to define recent slip rates magnitude and recurrence of past earthquakes on them

high resolution 3d seismic reflection imaging across active faults and its impact on seismic hazard estimation in the tokyo metropolitan area abstract we collected and interpreted high resolution 3d seismic reflection data across a hypothesized fault scarp along the largest active fault that could generate hazardous earthquakes in the tokyo metropolitan area the processed and interpreted 3d seismic cube linked with nearby borehole stratigraphy suggests that a monocline that deforms lower pleistocene units is unconformably overlain by middle pleistocene conglomerates judging from structural patterns and vertical separation on the lower middle pleistocene units and the ground surface the hypothesized scarp was interpreted as a terrace riser rather than as a manifestation of late pleistocene structural growth resulting from repeated fault activity devastating earthquake scenarios had been predicted along the fault in question based on its proximity to the metropolitan area however our new results lead to a significant decrease in estimated fault length and consequently in the estimated magnitude of future earthquakes associated with reactivation this suggests a greatly reduced seismic hazard in the tokyo metropolitan area from earthquakes generated by active intraplate crustal faults

seismology breaking the slab two chains of seamounts on the pacific plate subduct beneath central japan in the process a fragment of the pacific slab has become wedged in the subduction zone and may be the source of recurring deep thrust earthquakes beneath tokyo

earthquake source fault beneath tokyo devastating earthquakes occur on a megathrust fault that underlies the tokyo metropolitan region we identify this fault with use of deep seismic reflection profiling to be the upper surface of the philippine sea plate the depth to the top of this plate 4 to 26 kilometers is much shallower than previous estimates based on the distribution of seismicity this shallower plate geometry changes the location of maximum finite slip of the 1923 kanto earthquake and will affect estimations of strong ground motion for seismic hazards analysis within the tokyo region

seismotectonics beneath the tokyo metropolitan area japan effect of slab slab contact and overlap on seismicity 1 we first determine the configuration of the upper surface of the pacific pac slab beneath kanto japan from the distribution of interplate earthquakes relocated by an appropriate 1 d velocity model then traveltime tomography is carried out to estimate three dimensional seismic velocity structures around kanto using 735 520 p wave and 444 049 s wave arrival times from 6508 local earthquakes the obtained results suggest that the philippine sea phs slab is subducting to depths of 130 140 km without a gap even to the northwest of the izu collision zone we subsequently define the lateral extent of the contact zone between the bottom of the phs slab and the upper surface of the pac slab phs pac interface and reveal that the slab contact zone underlies a wider area beneath kanto in harmony with the kanto plain the downdip limit of interplate thrust type earthquakes on the pac slab is deepened by 30 km locally under the slab contact zone this deepening is probably caused by a lower temperature environment in the pac slab resulting from the overlap with the phs slab subducting above and consequent thermal shielding by the phs slab from the hot mantle wedge we detect an extremely low velocity anomaly in the easternmost portion of the phs slab which is probably attributable to serpentinization of mantle peridotite interplate earthquakes are almost absent along the phs pac interface overlain by the serpentinized mantle in the phs slab suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine

a magma hydrothermal system beneath hakone volcano central japan revealed by highly resolved velocity structures high resolution images of subsurface structures are necessary to understand the transport processes of crustal fluids from deep magma sources and their relationship to earthquake swarms in active volcanic regions based on a seismic tomography approach we have developed a new model for the magma hydrothermal system beneath hakone volcano central japan where shallow earthquake swarms and crustal deformation associated with inflation of an open crack source are often observed by applying travel time data for local earthquakes to a tomographic inversion we obtained highly resolved seismic velocity structures that show a region of low p wave velocity vp low s wave velocity vs and high vp vs ratios at depths of 10 20 km beneath the volcano corresponding to the location of the open crack source we suggest that the high vp vs ratios represent a deep magma chamber with a high concentration of melt and or fluids deep low frequency earthquakes located just beneath this high vp vs zone may indicate that magmatic fluids are supplied from below above the high vp vs zone a region of low vp low vs and low vp vs ratios exists at depths of 3 10 km suggesting the presence of crack filled water or co2 supplied from the inferred deep magma chamber many earthquake swarms occur in this low vp vs zone indicating that crustal fluids play an important role in generating the swarms similar relationships between magma reservoirs overlying hydrothermal systems and swarm activity have been reported from other volcanic areas and thus may be a ubiquitous feature beneath active volcanoes

interaction between two subducting plates under tokyo and its possible effects on seismic hazards 1 underneath metropolitan tokyo the philippine sea plate phs subducts to the north on top of the westward subducting pacific plate pac new relatively high resolution tomography images the phs as a well defined subduction zone under western kanto plain as pac shoals under eastern kanto the phs lithosphere is being thrusted into an increasingly tighter space of the pac eurasian mantle wedge as a result zones of enhanced seismicity appear under eastern kanto at the top of phs internal to phs and also at its contact with pac these zones are located at depths greater than the causative fault of the disastrous 1923 great tokyo megathrust earthquake in the vicinity of several well located historical damaging m6 and m7 earthquakes thus a rather unique interaction between subducting plates under tokyo may account for additional seismic hazards in metropolitan tokyo

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

western nankai trough seismogenic zone results from a wide angle ocean bottom seismic survey the nankai trough southwestern japan is recognized as a vigorous seismogenic zone with well studied historic earthquakes this paper presents results of a wide angle ocean bottom seismographs obs study at the western nankai trough seismogenic zone the obs data used were acquired on a profile 250 km long across the presumed coseismic slip zone of the 1946 nankaido earthquake ms 8 2 the main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake the crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge p wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities vp 5 0 5 6 km s and 6 6 6 8 km s in oceanic layers 2 and 3 respectively the maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with vp 3 4 4 6 km s in the deeper part the subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward 3 2 and 7 2 at 0 50 km and 50 100 km from the trough axis respectively the oceanic crust is smooth to the hypocenter zone down to 40 km depth beneath shikoku island our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust island arc crust contact zone even though there is large uncertainty about the seaward limit of the coseismic slip zone the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism

the heidelberg basin upper rhine graben germany a unique archive of quaternary sediments in central europe abstract the heidelberg basin in the german part of the northern upper rhine graben hosts one of the thickest successions of plio pleistocene sediments in central europe from reflection seismic surveys and new research boreholes it is known that pleistocene sediments amount to more than 500 m thickness the basin sediments originated both from local source areas i e the highlands along the graben shoulders and from regional source areas i e the alps although temporal hiatuses probably exist the deposits are expected to be more or less complete because this part of the upper rhine graben has undergone continuous subsidence in the past currently the heidelberg basin is under investigation by a scientific drilling project new cored boreholes have been realized at three different locations spaced 15 km from each other this concept provides complementing information and enables a detailed characterization of the predominantly fluvial deposits the project aims to develop a stratigraphic reference profile of the quaternary for this region north of the alps to understand how the sedimentation was controlled by tectonics and past climate change and to derive proxy data of environmental changes the new core material demonstrates that the heidelberg basin would be a possible key location for quaternary stratigraphy in central europe information about pleistocene periods of cold and warm climate is found within this one sediment succession in superposition the temporal resolution of the sediment succession is high due to its mid continental location it links northern and southern europe as well as western and eastern europe based on a detailed analysis of the new drill cores a new lithostratigraphy for the northern part of the upper rhine graben is developed in a first step with the mannheim formation the ludwigshafen formation the viernheim formation and the iffezheim formation four new lithostratigraphic units of the northern upper rhine graben are officially introduced furthermore a new debate about identification of basal quaternary arises from the new research boreholes whereas during the last decades petrological markers were used to define basal quaternary i e the first deposition of alpine sediments or carbonate content this definition fails in the depocentre of the basin several rock physical properties e g the natural gamma radiation and the magnetic susceptibilities reveal significant changes at the basal quaternary the driving forces of these changes have to be studied in more detail in the future

seismic evidence for active underplating below the megathrust earthquake zone in japan determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes the peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate underplating at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging but direct seismic evidence of this process is lacking by comparing seismic reflection profiles with microearthquake distributions in central japan we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting philippine sea plate this region coincides with the location of slow slip events that may serve as signals for monitoring active underplating

history of ancient megathrust earthquakes beneath metropolitan tokyo inferred from coastal lowland deposits abstract metropolitan tokyo is located directly above a subduction zone that has generated two megathrust earthquakes in the past 300 years however the timing of older megathrusts on this margin is poorly understood in this study we aim to constrain the timings of past megathrust earthquakes using coastal stratigraphy paleoecology radiocarbon dating and archaeological records from coastal lowlands an investigation of 13 boreholes in the southern coastal area of metropolitan tokyo found evidence for 4 m of uplift in a 6000 year period however we found that net vertical displacement in the last 1000 years is approximately zero results suggest that preservation of usually ephemeral lagoon sediments occurred on three occasions in the past 1000 years and radiocarbon dating results show that the timings of these preservation episodes are close to that of major historical earthquakes we thus attribute the intermittent preservation of the ephemeral lagoon deposits to coseismic uplift caused by the megathrust earthquakes the candidates of the megathrust earthquakes are events that took place in 1703 ce the 13th century and 878 ce since these events produced no net vertical displacement due to inter seismic subsidence we propose that earthquakes responsible for long term uplift of this region took place prior to the 9th century this research also demonstrates the value of preserved intertidal sediments as paleoseismological archives where net tectonic displacement is neutral

simulation of tectonic evolution of the kanto basin of japan since 1 ma due to subduction of the pacific and philippine sea plates and the collision of the izu bonin arc abstract 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

seismic velocity discontinuities in the crust and uppermost mantle beneath the kanto district central japan identified from receiver function imaging and repeating earthquake activity abstract we constructed vertical cross sections of depth converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the kanto district central japan repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting philippine sea plate and the subducting pacific plate as a result we present images of some major seismic discontinuities the upper boundary of the pacific plate dips to the northwest in northern kanto and to the west southwest in southern kanto with some undulations on the other hand the upper boundary of the philippine sea plate as a whole dips to the northwest however it is concave to the northeast in the southern boso peninsula we suggest that the low velocity mantle wedge may be indicated on the top of both subducting plates plate thickness gradually decreases to the northeast the northeastern end of the philippine sea plate is interpreted to be at depths of 45 90 km the moho discontinuity in the overriding plate is deeper than 25 km in the northern kanto it contacts the subducting philippine sea plate in the southwestern part near 35 8 n

subduction of a wedge shaped philippine sea plate beneath kanto central japan estimated from converted waves and small repeating earthquakes 1 we estimate the configuration of the philippine sea plate phs subducted beneath kanto to understand the seismotectonics in the tokyo metropolitan area in addition to small repeating earthquakes that delineate both the upper and lower boundaries of the phs previously unrecognized converted waves at its upper boundary from lower boundary repeater sources provide a new constraint on the position of the boundary the results from 794 s to p and 212 p to s converted waves together with 200 repeater locations show that 1 the phs has a wedge like shape decreasing in thickness from 50 km beneath tokyo to zero at its northeastern limit and 2 the phs is relatively flat in the eastern part of kanto and bends upward near its northeastern limit from the fact that the phs in the izu bonin forearc also has a similar wedge like shape we infer that its shape was formed before its subduction on the other hand the relatively flat phs at its northeastern limit and its upward bend are probably a consequence of ongoing deformation from an interaction with the underlying thick pacific plate pac the history of the phs as a forearc before subduction and its contact with the pac after subduction suggests that the temperature of this part of the phs is colder than other regions which probably causes anomalously deep seismicity on the phs in kanto colder phs temperatures imply that the source regions of large earthquakes may extend deeper than expected an important fact when assessing the locations of disastrous earthquakes for tokyo

inversion analysis of site responses in the kanto basin using data from a dense strong motion seismograph array abstract we applied nonlinear inversion analysis to estimate site response in the kanto area using data from 19 moderate sized earthquakes observed at the 843 stations constituting strong motion accelerograph networks of the seismic kanto strong motion network sk net k net and kik net we determined the source parameters seismic moment m 0 corner frequency f c for each event and the quality factor q f for the propagation path based on borehole records we also determined the frequency independent and reference independent site responses the resulting seismic moments are in good agreement with those determined by the national research institute for earth science and disaster prevention nied and are proportional to we found a frequency dependent quality factor for the propagation path q f 107 f 0 52 the site responses averaged over 0 5 1 0 hz correlate well with shear wave velocities for the upper 30 m avs30 correlation coefficient of 0 63 and with depth to the seismic basement the shear wave velocity of which is around 3 0 km sec large site responses are found in areas of unconsolidated sediments such as swampy land the predominant frequencies of our site responses roughly agree with those of the horizontal to vertical h v spectral ratio this indicates that the h v ratio is valid for estimating predominant frequencies of s wave amplification

a slab fragment wedged under tokyo and its tectonic and seismic implications a three dimensional evaluation of earthquake hypocentres beneath the kanto basin in japan reveals the presence of a distinct 25 km thick and 100 km wide body its fast seismic velocity and the presence of a double seismic zone suggest that it is a fragment of the pacific slab rather than an extension of the philippine sea slab this implies that the penetration of the philippine sea slab is much shallower beneath the kanto basin than was previously thought the philippine sea and pacific plate slabs both subduct beneath tokyo and so their configuration and seismic potential have been subject to intensive study1 2 3 4 5 6 7 8 previous work suggests that the philippine sea slab extends up to 100 km northwest of tokyo and subducts to a depth of 90 km beneath the kanto basin where it is folded against the underlying pacific slab here we evaluate seismic data in three dimensions and delineate a distinct 25 km thick and 100 km wide body beneath the kanto basin that has hitherto been considered to be part of the philippine sea slab we find that several of its characteristics such as its high seismic velocity and the presence of a double seismic zone point instead to an affinity with the pacific slab implying that the philippine sea slab penetrates to depths of only 35 40 km beneath the kanto basin we propose that the body is a fragment of pacific lithosphere that formed by the collision of two intersecting seamount chains with the japan trench 2 3 million years ago we suggest that such slab fragments may not be uncommon especially where seamount subduction deforms trenches and near triple junctions

high resolution subduction zone seismicity and velocity structure beneath ibaraki prefecture japan 1 we use double difference tomography and waveform derived cross correlation differential times to estimate earthquake locations and p and s wave velocity structure in the subduction zone under ibaraki and neighboring prefectures of north central honshu we find evidence in both earthquake hypocenters and the velocity structure that the philippine sea plate or perhaps a microplate fragment may be caught between the subducting pacific plate and overriding okhotsk plate in this region in the southern part of the study area we find a zone of interplate events extending as deep as 60 km forming a distinct lineation in cross section focal mechanisms support the interpretation that these are low angle subduction interface events we infer that these events probably occur on the interface between the pacific and philippine sea plates rather than between the pacific and okhotsk plates in the upper part of the downgoing pacific plate we find a zone of high vp vs ratio 1 95 which may be explained by high pore fluid pressures within the subducting crust at a depth of 60 80 km this region of high vp vs appears to diffuse into the overlying mantle wedge possibly indicating the upward release of fluids from the slab

characteristic silent earthquakes in the eastern part of the boso peninsula central japan 1 analysis of global positioning system gps data shows transient crustal deformation in the boso peninsula central japan for about fifty days from october 2002 the southeastward horizontal displacements detected suggest the occurrence of interplate aseismic slip between the philippine sea plate and the north american plate six years after a similar event in 1996 we estimate an area of aseismic slip with moment magnitude mw 6 6 centered off the boso peninsula adjacent to associated seismic activity slip evolution of the 2002 aeismic slip started on october 4 and migrated from north to south for about ten days followed by gradual subsidence and restarting of slip which lasted until december 2 propagating more southward similarities in focal area and slip process between the 1996 and the 2002 events suggest the boso aseismic slips as characteristic silent earthquakes together with the repetitive occurrence of seismic swarm in a similar region every six to seven years a more tightly coupled southern area that did not release enough energy in the 1996 event may have caused the differences in magnitude central slip area and duration between the 1996 and the 2002 events if the hypothesized recurrence interval for strain accumulation is true based on swarm seismicity

slow slip events and seismic tremor at circum pacific subduction zones 1 it has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements however only recently has a fuller spectrum of strain release processes including normal slow and silent earthquakes or slow slip events and continuous and episodic slip been observed and generated by numerical simulations of the earthquake cycle despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive the concurrence of seismic tremor with slow slip episodes in cascadia and southwestern japan provides insight into the process of slow slip a perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface alternatively evidence is accumulating to support the notion that tremor results from shear failure during slow slip global observations of the location spatial extent magnitude duration slip rate and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation most slow slip events occur just downdip of the seismogenic zone consistent with rate and state dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation at a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault bringing it closer to or farther from earthquake failure respectively this paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide with a focus on circum pacific subduction zones trends in global observations of slow slip events suggest that 1 slow slip is a common phenomena observed at almost all subduction zones with instrumentation capable of recording it 2 different frictional properties likely control fast versus slow slip 3 the depth range of slow slip may be related to the thermal properties of the plate interface and 4 the equivalent seismic moment of slow slip events is proportional to their duration mo different from the mo 3 scaling observed for earthquakes

reflection signature of seismic and aseismic slip on the northern cascadia subduction interface at the northern cascadia margin the juan de fuca plate is underthrusting north america at about 45 mm yr 1 ref 1 resulting in the potential for destructive great earthquakes2 3 the downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake thought to have been in 1700 4 occurred before instrumental recording thermal and deformation studies5 indicate that off southern vancouver island the interplate interface is presently fully locked for a distance of 60 km downdip from the deformation front great thrust earthquakes on this section of the interface with magnitudes of up to 9 4 5 have been estimated to occur at an average interval of about 590 yr ref 3 further downdip there is a transition from fully locked behaviour to aseismic sliding where high temperatures allow ductile deformation with the deep aseismic zone exhibiting slow slip thrust events6 here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked to a broad reflection band at greater depth where aseismic slip is thought to be occurring this change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

spatial changes of inter plate coupling inferred from sequences of small repeating earthquakes in japan 1 we extract sequences of small repeating earthquakes to clarify inter plate coupling of subducting plates over a large area of the japanese islands as a result many sequences arc detected at the philippine sea plate subducting from the ryukyu trench and pacific plate subducting from the kuril japan trench the average slip rates and standard deviations estimated from the sequences show substantial spatial changes of inter plate coupling the large deviations of slip rates correspond to the occurrence of episodic slips such as after slips following large earthquakes constant slip rates approaching the relative plate motion indicate weak coupling areas slip deficits and sparse distributions of repeating groups suggest locked areas in the nankai trough deep low frequency earthquakes in the transition zone and burst type repeating sequences within plates have not been located in the downdip direction of groups with slow slip rates this suggests that the space time characteristics of inter plate coupling affected these seismic events

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

subduction process of the philippine sea plate off the kanto district central japan as revealed by plate structure and repeating earthquakes abstract the structure of the philippine sea plate in the offshore region adjacent to the boso peninsula in central japan is resolved by comparison of recent oceanic seismic survey data with natural repeating earthquake data this region is characterized by a range of seismic and aseismic phenomena associated with subduction motion of the philippine sea plate including slow slip events sses large backslip events and repeating earthquakes this offshore region is also the site of the largest aftershock of the 1923 kanto earthquake m7 9 the detailed structure of the philippine sea plate is successfully resolved and the upper boundary of plate is clearly traceable to depths of 20 km the reflection intensity of the upper boundary varies spatially in accordance with the depth of the reflector the upper boundary continues smoothly with a line of repeating earthquake hypocenters regarded as an indicator of subduction shear a continuous model extending into the offshore region can thus be proposed the variation in reflection intensity on the upper boundary of the philippine sea plate also correlates well with variation in event types a weak reflection zone overlaps the area of large backslip while a moderately strong reflection zone encroaches into the large slip area of the boso sse the reflectivity of the upper boundary is thus considered to be dependent on the physical properties and conditions of the reflector which in turn affect the event types on the plate boundary a source fault model is proposed for the largest aftershock of the 1923 kanto earthquake based on geodetic data the source fault is located eastward of the mainshock with dimensions of 40 50 km and moment magnitude of 7 5 the source fault lies within the region of large backslip and the large slip area of the boso sse

subduction of the philippine sea plate beneath southwestern japan slab geometry and its relationship to arc magmatism 1 we carry out high resolution three dimensional seismic tomography of the crust and upper mantle beneath southwestern sw japan using arrival time data obtained from the nationwide seismograph network the tomographic images provide new insights into the configuration of the philippine sea slab and arc magmatism the results confirm the existence of an aseismic portion of the philippine sea slab at greater depths beyond the seismic portion the philippine sea slab is subducting aseismically down to at least a depth of 200 km in chubu 60 80 km in kinki and 60 km in chugoku and is subducting seismically to depths of 150 200 km in kyushu in the chubu district it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area whereas in the kinki district it is subducting subhorizontally as far as the japan sea the differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity suggesting the importance of the geometry of the philippine sea slab on arc magmatism our tomographic images further imply subduction of the philippine sea slab down to a depth of around 70 km northwest of the izu peninsula the philippine sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the pacific slab at depths of 150 200 km

geometry and relative motion of the philippine sea plate and pacific plate beneath the kanto tokai district japan the recent acquisition of high density and improved seismic data by the kanto tokai k t observational network of the national research institute of earth science and disaster prevention requires the revision of conventional plate configuration models for the k t district we propose a new interpretation of the configuration and relative motion of the philippine sea phs plate pacific pac plate and eurasian eur plate on the basis of the distribution of hypocenters velocity structure and focal mechanisms in the k t district the new model for the configuration of the phs and pac slabs clearly delineates the phs slab subducting northwestward beneath the k t district from the sagami and suruga troughs and the pac slab subducting westward beneath the phs slab from the japan trench the thickness of the descending phs slab is estimated to be 30 5 km the proposed configuration of the phs slab is significantly different from those obtained by other researchers including our own previous study the new model is fully consistent with global plate motion data the new model leads to an interpretation of the tectonic processes taking place beneath the k t district for the sagami trough wing of the phs slab the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the pac slab the southwestern portion carrying the izu peninsula has been bent upward because of its buoyancy producing the observed contorted shape for the suruga trough wing the dip direction of the phs slab gradually changes from northwestward at the eastern portion to northward at the western portion the new model also indicates the presence of offset within the phs slab between the sagami and suruga trough wings this area is characterized by a low level of seismic activity the absence of a high velocity structure and the presence of quaternary volcanoes it seems reasonable to speculate that the distribution of quaternary volcanic edifice construction is controlled by the location of the offset within the phs slab in view of this new model the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined

the 1923 kanto earthquake reevaluated using a newly augmented geodetic data set 1 this study revisits the mechanism of the 1923 ms 7 9 kanto earthquake in japan we derive a new source model and use it to assess quantitative and qualitative aspects of the accommodation of plate motion in the kanto region we use a new geodetic data set that consists of displacements from leveling and angle changes from triangulation measurements obtained in surveys between 1883 and 1927 two unique aspects of our analysis are the inclusion of a large number of second order triangulation measurements and the application of a correction to remove interseismic deformation the geometry of the fault planes is adopted from a recent seismic reflection study of the kanto region we evaluate the minimum complexity necessary in the model to fit the data optimally our final uniform slip elastic dislocation model consists of two adjacent 20 dipping low angle planes accommodating reverse dextral slip of 6 0 m on the larger eastern plane and 9 5 m on the smaller western plane with azimuths of 163 and 121 respectively the earthquake was located in the sagami trough where the philippine sea plate subducts under honshu compared to the highly oblique angle of plate convergence the coseismic slip on the large fault plane has a more orthogonal orientation to the strike of the plate boundary suggesting that slip partitioning plays a role in accommodation of plate motion what other structure is involved in the partitioning is unclear uplift records of marine coastal terraces in sagami bay document 7500 years of earthquake activity and predict average recurrence intervals of 400 years for events with vertical displacement profiles similar to those of the 1923 earthquake this means that the average slip deficit per recurrence interval is 50 of the relative plate convergence these findings of plate motion partitioning and slip deficit lead us to suggest that instead of a simple recurrence model with characteristic earthquakes additional mechanisms are necessary to describe the accommodation of deformation in the kanto region so far obvious candidates for these alternative mechanisms have not been discovered

collision and subduction structure of the izu bonin arc central japan revealed by refraction wide angle reflection analysis abstract since the middle miocene the izu bonin arc iba has been colliding from the south with the honshu arc in central japan associated with subduction of the philippine sea plate psp this process is responsible for forming a complex crustal structure called the izu collision zone icz to obtain direct evidence of the deep structure dominated by collision and subduction an intensive seismic experiment using explosive and vibroseis sources was conducted in the eastern part of the icz cmp reflection and refraction wide angle reflection data were acquired on a 130 km long seismic line crossing the kanto mountains km the tonoki aikawa tectonic line tatl and the tanzawa mountains tm from north to south the tatl is considered to be a collision boundary separating the honshu arc km from the iba tm the structural model constructed by refraction tomography and forward ray tracing shows remarkable lateral velocity variation across the tatl and some clear reflectors in the deep crust a north dipping reflector beneath the km was interpreted to be the deeper extension of the tatl from the geometry of reflectors we interpret the tanzawa block is delaminated from the subducting slab due to the collision to form a wedge like body thrusting between the upper and lower crust of honshu the tm itself shows a velocity structure almost consistent with the upper part of the crust of the izu intraoceanic arc south of our study area the relocated hypocenter distribution using our velocity model shows high seismic activity concentrated around the collision boundary which is in a marked contrast of low seismicity within the tanzawa block these features of seismicity are strongly dominated by the collision of the iba with the honshu arc

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

variable slip rupture model of the great 1923 kanto japan earthquake geodetic and body waveform analysis abstract the available geodetic and teleseismic data sets for the 1923 kanto earthquake m s 8 1 have been combined into a joint inversion for both temporal and spatial slip variations we assumed an initial faulting model to be consistent with the geometry determined by kanamori 1971 on the basis of first motion data aftershock area and the amplitude of surface waves at teleseismic distances and also to enclose the slipped area estimated by matsu9ura et al 1980 from the geodetic data employed here we then inverted for a heterogeneous distribution of slip of the fault plane the leveling routes and triangulation stations used consisting of 225 bench marks and 31 triangulation points are from matsu9ura et al 1980 we chose to first determine the overall static slip distribution by inverting the geodetic data alone we then proceeded to gradually increase the importance of the teleseismic data always requiring a good fit to the geodetic leveling and horizontal displacements in this way we could provide a constraint on the overall static slip characteristics from the geodetic data and provide stability for the teleseismic inversion yet determine the degree of slip heterogeneity and time history most suitable for matching the waveform data and for simulating strong ground motions our analysis yields a seismic moment of 7 to 8 10 27 dyne cm m w 7 8 to 7 9 with a maximum slip of approximately 8 m the most concentrated slip is in the shallow central and western portion of the fault the location of the concentrated slip on the fault plane has important consequences for the amplitude duration and frequency content of the resulting ground motions as documented by takeo and kanamori 1993

deep seismic reflection profiling across active reverse faults in the kinki triangle central japan abstract the kinki triangle central japan is marked by a dense distribution of active faults for regional characterization of the crust in the kinki metropolitan area to be used for seismic hazard mitigation seismic reflection profiling was carried out using vibroseis trucks and explosive sources along a 135 km long seismic line from metropolitan osaka to the ise basin across several active reverse faults the seismic reflection section portrays the west dipping upper surface of the down going philippine sea plate at 35 to 40 km depth lower crustal reflectors at 27 km and mid crustal reflectors at 15 to 16 km depth dipping reflections are dominant in the seismogenic upper crust and some of them can be traced as deeper extensions of active faults a 60 km wide uplifted zone between osaka and ise basins were formed as a pop up structure bounded by double vergent reverse fault systems the mid crustal reflectors beneath the uplifted zone form a slight convex shape and the lower limit of the hypocentral distribution of crustal earthquakes lies a few km above the reflectors the dipping reflectors in the upper crust merged to the sub horizontal mid crustal reflectors and are interpreted as a possible mid crustal detachment the hanging wall of active faults in the osaka basin show back tilting associated with reverse faults suggesting a curved fault geometry and shallower detachment in the upper crust

evidence of paleoprocesses in some poorly developed soils on consolidated material in the sierra de carrascoy se spain abstract four representative profiles of poorly developed soils on consolidated material were studied in the sierra de carrascoy se spain the soils with a a r or a c r type profiles have medium low contents of organic carbon and nitrogen and calcium carbonate occurs in only one of the profiles the ph in aqueous suspension is slightly alkaline the cation exchange capacity is medium low but varies slightly between profiles and the electrical conductivity of the saturation extract is low there are wide textural differences the clay fractions contain illite kaolinite vermiculite smectites chlorite quartz and feldspars the heavy fraction of the fine sand contains mainly opaque minerals and alterites with some resistant minerals of metamorphic origin the profiles are classified by f a o i s r i c i s s s world reference base for soil resources world soil resources reports 84 f a o rome 1998 as calcaric eutric and hyperskeletic leptosols according to soil survey staff soil taxonomy a basic system of soil classification for making and interpreting soil surveys 2nd edn united states department of agriculture natural resources conservation service us government printing office washington dc 1999 they are all lithic and typic xerorthents and lithic xeric and xeric torriorthents the principal present day pedogenetic processes in these soils are humification and clay formation however profile 1 has evolved from a fersiallitic soil with clay accumulation although these features have almost disappeared because of erosion and modern humification pedoturbation and recalcification its relict features consist of the remains of a paleoargic horizon inherited from earlier climatic conditions which was later truncated and is now preserved only in more stable physiographic positions

dual subduction tectonics and plate dynamics of central japan shown by three dimensional p wave anisotropic structure abstract the central japanese subduction zone is characterized by a complex tectonic setting affected by the dual subduction of oceanic plates and collisions between the island arcs to better understand of the subduction system we performed an anisotropic tomography analysis using p wave arrival times from local earthquakes to determine the three dimensional structure of p wave azimuthal anisotropy in the overriding plate and the pacific and philippine sea phs slabs the principal characteristics of anisotropy in the subducted and subducting plates are 1 in the overriding plate the distribution pattern of fast direction of crustal anisotropy coincides with that of the strike of geological structure 2 in the two oceanic plates fast propagation directions of p wave were sub parallel to the directions of seafloor spreading additionally our tomographic images demonstrate that 1 the bottom of the median tectonic line the longest fault zone in japan reaches to the lower crust and seems to link to the source region of an inter plate earthquake along the phs slab 2 the segmentation of the phs slab the izu islands arc the nishi shichito ridge and the shikoku basin due to the formation history is reflected in the regional variation of anisotropy the tomographic study further implies that there might be a fragment of the pacific slab suggested by a previous study beneath the tokyo metropolitan area the overall findings strongly indicate that seismic anisotropy analysis provide potentially useful information to understand a subduction zone

broadband ground motion reconstruction for the kanto basin during the 1923 kanto earthquake ground motions of the 1923 kanto earthquake inside the kanto basin are numerically simulated in a wide frequency range 0 10 hz based on new knowledge of the earthquake s source processes the sedimentary structure of the basin and techniques for generating broadband source models of great earthquakes the kanto earthquake remains one of the most important exemplars for ground motion prediction in japan due to its size faulting geometry and location beneath the densely populated kanto sedimentary basin we reconstruct a broadband source model of the 1923 kanto earthquake from inversion results by introducing small scale heterogeneities the corresponding ground motions are simulated using a hybrid technique comprising the following four calculations 1 low frequency ground motion of the engineering basement modeled using a finite difference method 2 high frequency ground motion of the engineering basement modeled using a stochastic green s function method 3 total ground motion of the engineering basement i e 1 2 and 4 ground motion at the surface in response to the total basement ground motion we employ a recently developed three dimensional 3d velocity structure model of the kanto basin that incorporates prospecting data microtremor observations and measurements derived from strong ground motion records our calculations reveal peak ground velocities pgv exceeding 50 cm s in the area above the fault plane to the south where the fault plane is shallowest pgv reaches 150 200 cm s at the engineering basement and 200 250 cm s at the surface intensity 7 the maximum value in the japan meteorological agency s intensity scale is calculated to have occurred widely in sagami bay which corresponds well with observed house collapse rates due to the 1923 event the modeling reveals a pronounced forward directivity effect for the area lying above the southern shallow part of the fault plane the high pgv and intensity seen above the southeastern corner of the fault plane and further east are largely due to this effect waveforms above the fault plane contain both short and long period components but the short period components are not observed further afield away from the fault long period waves 2 s dominate the ground motion and in areas where the base of the third layer is relatively deep the predominant period is 5 s levels of long period ground motion in the southern part of the study area around sagami bay and the southern parts of boso peninsula and tokyo bay exceed that recorded at tomakomai during the 2003 tokachi oki earthquake when large oil storage tanks collapsed in response to sloshing generated by strong long period motions