anomalous seismograms generated by an intermediate depth earthquake unusual scattering sources in the upper mantle of central japan 1 strong scattering sources were detected in the upper mantle beneath central japan from seismic data recorded by the high sensitivity seismograph network hi net anomalous seismograms consisting of many strong later phases whose amplitudes did not decay significantly with time were observed from an intermediate depth earthquake a comparison of those anomalous seismograms with others generated by other deep earthquakes shows that major scattering sources may exist predominately in the upper mantle not in the crust three possible candidates have been considered as the sources of scattering in the upper mantle including magma conduits the previous collision zone and slab melting although seismic waves may be scattered strongly from each potential candidate a region associate with slab melting might be more suitable for generating the anomalous seismogram due to the existence of abundant melting spots within the subducted philippine sea slab that is extremely young and hot beneath central japan index terms 0669 electromagnetics scattering and diffraction 7218 seismology lithosphere and upper mantle 7230 seismology seismicityandseismotectonics citation lin c h m ando n fujii k yamaoka k tadokoro a s jin k obara and m ishida anomalous seismograms generated by an intermediate depth earthquake unusual scattering sources in the upper mantle of central japan geophys res lett 30 11 1586 doi 10 1029 2002gl016837 2003

microseismicity around rupture area of the 1944 tonankai earthquake from ocean bottom seismograph observations abstract along the nankai trough southwestern japan the philippine sea plate psp is subducting beneath the eurasian plate and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100 200 years the most recent large thrust event in the eastern nankai trough off kii peninsula was the 1944 tonankai earthquake in this region current seismicity is very low and hypocenters are not determined accurately by the land seismic network we conducted microseismicity observations around the rupture area of the 1944 tonankai earthquake using ocean bottom seismographs obss hypocenters were determined using a 2 d seismic velocity structure model based on an airgun obs seismic survey results obtained show that the seismicity was relatively active near the trough axis these earthquakes may relate to deformation of the subducting philippine sea plate on the other hand microseismicity in the rupture area of the 1944 tonankai earthquake was very low this low level seismic activity in the co seismic rupture area of the 1944 tonankai earthquake likely relates to a single large asperity off kii peninsula

stress perturbations associated with active faults penetrated by boreholes possible evidence for near complete stress drop and a new technique for stress magnitude measurement detailed studies of stress induced wellbore breakouts in wells drilled through active faults reveal stress field discontinuities that are apparently associated with recent fault movements these discontinuities are expressed as localized rotations in wellbore breakout orientation in the vicinity of the fault penetrated by the borehole this phenomenon is observed in a variety of tectonic environments and rock types utilizing cases where relatively complete knowledge of the horizontal principal stresses is available from in situ measurements we use three dimensional dislocation modeling to demonstrate that these discontinuities can be explained as the superposition of a reference stress state and a perturbation caused by movement on preexisting faults case studies from normal strike slip and reverse faulting stress states indicate that nearly complete stress drop is required to match the observed breakout orientation anomalies hydraulic fracturing data independently confirm the occurrence of near complete stress drop on some faults penetrated by drilling modeling of the observed interactions between breakouts and fractures can also be used to obtain information about the magnitude of in situ stress

three dimensional p and s wave velocity structures beneath japan abstract 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

splitting of the philippine sea plate and a magma chamber beneath mt fuji 1 mt fuji is located in a tectonically unique area but various aspects of the structure have not been fully explained here we show the results from a magnetotelluric survey across mt fuji along a 70 km observation line the profile shows that a conductive body is located between two resistive bodies at depths greater than 15 km low frequency earthquakes occur above the conductor we interpret these results in a model where beneath mt fuji the subducting philippine sea plate is split into two parts and a magma chamber is located in the gap due to this unique structure mt fuji may be able to sustain a high basalt magma flux throughout its entire history

subsurface structure physical properties fault zone characteristics and stress state in scientific drill holes of taiwan chelungpu fault drilling project continuous cores and a suite of geophysical measurements were collected in two scientific drill holes to under stand physical mechanisms involved in large displacements during the 1999 chi chi earthquake physical properties obtained from wire line logs including p and s wave sonic velocity gamma ray electrical resistivity density and temperature are primarily dependent on parameters such as lithology depth and fault zones the average dip of bedding identified from cores and fmi or fms logs is about 30 degrees towards se nevertheless local azimuthal variations and increasing or decreasing bedding dips appear across fault zones a prominent increase of structural dip to 60 80 below 1856 m could be due to deformation associated with propagation of the sanyi fault a total of twelve fault zones identified in hole a are located in the plio pleistocene cholan formation pliocene chinshui shale and miocene kueichulin formation the shallowest fault zone occurs at 1111 m depth fza1111 it is a 1 m gouge zone including 12 cm of thick indurate black material we interpreted this zone as the slip zone during the chi chi earthquake fza1111 was characterized by 1 beddingparallel thrust fault with 30 dip 2 the lowest resistivity 3 low density vp and vs 4 high vp vs ratio and poisson s ratio 5 low energy and velocity anisotropy and low permeability within the homogeneous gouge zone 6 increasing gas co2 and ch4 emissions and 7 richness in smectite within the primary slip zone in situ stresses at the drill site were inferred from leak off tests borehole breakouts and drilling induced tensile fractures from borehole fms fmi logs and shear seismic wave anisotropy from dsi logs the dominant fast shear wave polarization direction is in good agreement with regional maximum horizontal stress axis particularly within the strongly anisotropic kueichulin formation a drastic change in orientation of fast shear polarization across the sanyi thrust fault at 1712 m depth reflects the change of stratigraphy physical properties and structural geometry

current stress state and principal stress rotations in the vicinity of the chelungpu fault induced by the 1999 chi chi taiwan earthquake 1 we carried out various stress measurements in boreholes penetrating the northern segment of the chelungpu fault drilled about 5 years after the 1999 chi chi taiwan earthquake in the possible depth range of the chelungpu fault three major fault zones were encountered clearly recognizable principal stress rotations in the vicinity of the shallowest major fault zone at 1133 m depth in hole b suggest that this fault zone ruptured during the 1999 earthquake moreover the fault s rupturing altered the stress state in the area surrounding this fault zone in this paper we constrain the possible magnitudes of the current principal horizontal stresses around the fault zone and show that the current stress state belongs to a normal or strike slip fault regime therefore the stress state was changed from that of a reverse fault regime by the rupturing

representation of faults in reservoir scale geomechanical finite element models a comparison of different modelling approaches abstract different approaches exist to incorporate faults in reservoir scale geomechanical models challenges are the proper representation of the fault geometry as well as the small scale variations in the internal architecture and the mechanical properties of the fault zone regarding the typical size of such models i e kilometres to tens of kilometres the present study utilizes a simple generic fault zone model to compare three different possibilities commonly used to represent faults in finite element reservoir models two differ in the basic grid geometry and the arrangement of mechanically weak fault zone elements respectively the third uses a discontinuous grid and contact elements to represent the fault modelling results show remarkable differences in the calculated stress and strain patterns the relatively strongest perturbations result for a continuous curvilinear grid adapted to the fault geometry in contrast the fault implementation has the least impact on local stresses and strains if it is represented as a stair step structure contained in a rectangular grid the use of contact elements has an intermediate effect modelling results are used to infer some general recommendations concerning the appropriate approach of representing faults in a numerical geomechanical reservoir models depending on fault geometry model scale and scope of interest

recovery of stress during the interseismic period around the seismogenic fault of the 1995 m w 6 9 kobe earthquake japan

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

three dimensional p and s wave velocity structures beneath the ryukyu arc the three dimensional 3 d p and s wave velocity structures beneath the ryukyu arc are determined by seismic wave travel time tomography using recordings on temporary stations in the sea area as well as those on routine stations for the inversion we explicitly define the upper boundary of the philippine sea slab as a result we obtain 3 d p and s wave velocity structures beneath the ryukyu arc which are more accurate than previous ones p and s wave low velocity zones and high vp vs zones exist just beneath active volcanoes at about 10 km depth which show the influence of arc magmas beneath the volcanoes prominent p and s wave low velocity zones exist along the okinawa trough at about 50 km depth where vp vs values are high the low velocity and high vp vs anomalies will represent some mantle materials flow upward from the depth greater than 50 km along the trough low velocity anomalies are visible beneath the philippine sea slab which are possibly associated with small scale mantle convections d 2003 elsevier b v all rights reserved

micro seismicity around the seaward updip limit of the 1946 nankai earthquake dislocation area we observed micro seismicity around the seaward updip limit of the 1946 nankai earthquake dislocation area by using pop up type ocean bottom seismographs deployed for three months in 1998 at the subduction zone we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely we constructed 3 d p and s wave velocity structure models based on the results of airgun obs seismic surveys along the nankai trough we located 83 earthquakes using these 3 d models these hypocenters show two groups of seismicity one seismicity group locates in oceanic layer 3 within a subducting seamount these earthquakes may reflect the deformation of the seamount caused by its subduction the other seismicity group locates near the updip limit of the thermally modeled locked zone these hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

a new configuration and an aseismic slab of the descending philippine sea plate revealed by seismic tomography abstract we obtained the three dimensional seismic velocity structure of the uppermost mantle beneath the kanto tokai and chubu regions central japan where the philippine sea plate descends by means of travel time tomography using 18 years of p phase data recorded by a regional seismic network a distinct low velocity zone which was the oceanic crust of the descending philippine sea plate was found beneath the kanto region the low velocity zone revealed a new configuration of the philippine sea plate different from any models obtained through the analysis of hypocenter distribution and focal mechanisms we also found a high velocity area in the deep aseismic region below the volcanic front between the kanto and chubu regions it extends to the philippine sea plate in both regions it was likely to be an aseismic extended section of the philippine sea plate the existence of the aseismic philippine sea plate beneath the chubu region was confirmed there was another low velocity area beneath the active volcanoes high poisson s ratio areas were found both beneath the volcanoes and in the low velocity zone of the philippine sea plate near the volcanic front under the kanto region

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

an earthquake slip zone is a magnetic recorder during an earthquake the physical and the chemical transformations along a slip zone lead to an intense deformation within the gouge layer of a mature fault zone because the gouge contains ferromagnetic minerals it has the capacity to behave as a magnetic recorder during an earthquake this constitutes a conceivable way to identify earthquakes slip zones in this paper we investigate the magnetic record of the chelungpu fault gouge that hosts the principal slip zone of the chi chi earthquake mw 7 6 1999 taiwan using taiwan chelungpu fault drilling project core samples rock magnetic investigation pinpoints the location of the chi chi mm thick principal slip zone within the 16 cm thick gouge at 1 km depth a modern magnetic dipole of earth magnetic field is recovered throughout this gouge but not in the wall rocks nor in the two other adjacent fault zones this magnetic record resides essentially in two magnetic minerals magnetite in the principal slip zone and neoformed goethite elsewhere in the gouge we propose a model where magnetic record 1 is preserved during inter seismic time 2 is erased during co seismic time and 3 is imprinted during post seismic time when fluids cooled down we suggest that the identification of a stable magnetic record carried by neoformed goethite may be a signature of friction heating process in seismic slip zone

three dimensional velocity structure in the bungo channel and shikoku area japan and its relationship to low frequency earthquakes 1 we used a tomographic inversion to determine p and s wave structures in the western shikoku and bungo channel region for comparison with the locations of low frequency earthquakes the velocity model clearly images the high velocity subducting slab and we can see the spatial relation to the low frequency earthquakes under western shikoku the low frequency earthquakes occur at depths close to the plate interface but under the adjacent bungo channel region to the west there is a clear depth separation our interpretation is that the low frequency events are occurring in a region of high vp vs that is located above the subducting slab the depth separation above the plate seems to vary depending on the region

p and s wave travel time inversions for subducting slab under the island arcs of the northwest pacific we have observed slablike high p and s velocity anomalies around the wadati benioff zone under island arcs of the northwest pacific through travel time tomographic inversions nineteen years of international seismological centre travel time residuals for events and stations in this large region are used analyses of resolution and noise show that the images are generally well resolved the images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth near the bottom of the upper mantle fingering of the slabs including segmenting and spreading is indicated the fast anomalies associated with the japan izu bonin and mariana subduction zones tend to flatten to subhorizontal at depth while downward spreading may occur under parts of the mariana and kurile arcs the fast anomalies below 700 km are not in the shape of a single coherent sheet the principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high velocity slab even when it bends to subhorizontal at depth the depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the 670 km discontinuity

low radiation efficiency of the intermediate depth earthquakes in the japan subduction zone robust determination of earthquake source parameters over a continuous depth range is central to inferring rupture mechanisms dominant at different depths we employed a cluster event method to constrain the source parameters as well as along path attenuation for earthquakes over 0 150 km depths and 4 orders of seismic moments in the japan subduction zone we found that corner frequency and stress drop increase with depth whereas the radiated energy scaled by seismic moment declines with depth slightly as a result the radiation efficiency exhibits a notable deficit for events deeper than 60 km together these suggest an increased energy dissipation during faulting in ductile deformation regime consistent with shear heating instability as an important faulting mechanism for intermediate depth earthquakes

crustal structure across the coseismic rupture zone of the 1944 tonankai earthquake the central nankai trough seismogenic zone 1 differences in the coseismic rupture process between the 1944 tonankai and the 1946 nankaido earthquakes have been studied by many fault models to understand what factors control coseismic rupture zones it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes the previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the neogene quaternary accretionary prism however little is known about the structure of the rupture zone of the 1944 earthquake to obtain a complete image of the seismogenic zone of the 1944 earthquake a wide angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land our model for the crustal structure is based on ocean bottom seismographic data the crustal structure appears characteristic for subducting oceanic crust and a neogene quaternary accretionary prism bounded by an island arc crust the neogene quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front the subducting oceanic crust can be traced down to 35 km the subduction angle becomes steeper landward reaching up to 11 beneath the island arc crust the depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km while the updip limit is located beneath the island arc upper crust similar structures of the updip and downdip limits are also published for several other subduction zones

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

whole mantle p wave travel time tomography abstract a method of tomographic inversion to obtain three dimensional velocity perturbations in the earth s whole mantle has been developed and applied to more than two million p wave arrival time data reported by international seismology center isc the model is parameterized with 32 768 blocks the divisions in latitude longitude and radius are 32 64 and 16 respectively horizontal cell size is 5 6 5 6 the layer thicknesses vary with depth 29 km just below the surface and 334 km just above the core mantle boundary starting from a spherically symmetric earth model we obtained a three dimensional model using the following iterative procedures first we relocated all the events second we backprojected the residual into the whole mantle third we refined the spherically symmetric earth model the solutions have been converged in five iterations we adopted the following techniques in the backprojection procedure the first order smoothness was introduced as a damping which makes the solution independent of the starting model and its apparent fluctuation minimal the basic equations for delay times and smoothness are solved using the conjugate gradient method an iterative method which guarantees the convergence of solution into the exact least squares solution the weight on the smoothness i e the damping factor was objectively determined by a simplified cross validation technique the final solution was obtained as an average of the ten solutions each of which was derived from one tenth of the total data set the reliability of the solution is examined in two ways 1 mapping the resolution given by the reconstruction of checkerboard patterns and 2 mapping the variance given from the gaussian noise input this is the first result which delineates the whole mantle structure with its reliability mapping the longest wavelength anomalies of the lower mantle are similar to those of previous studies by dziewonski 1984 and hager and clayton 1989 however the shorter wavelength patterns have many discrepancies the fast anomalies of the uppermost mantle beneath the shield and the slow anomalies beneath the active tectonic regions are clearly seen we also revealed the l 2 pattern at the transition zone reported by the previous surface wave and free oscillation studies the most striking feature is that of the fast anomalies lying along the extension of the large subducting plate up to depths of about 1000 km or more the resolving powers in these regions are generally good

clay mineral reactions caused by frictional heating during an earthquake an example from the taiwan chelungpu fault 1 to understand the chemical reactions of clay minerals in a fault zone during an earthquake we analyzed the clay minerals in the chelungpu fault which slipped during the 1999 chi chi earthquake x ray diffraction spectroscopy showed that kaolinite and smectite contents were lower in the black gouge zone than in the surrounding gray gouge breccia or fracture damaged zones we applied a chemical kinetics approach to examine whether dehydroxylation of kaolinite and dehydration of interlayer water dehydroxylation and illitization of smectite occurred during coseismic frictional heating and found that the first two reactions could complete under the temperature time profile of the chi chi earthquake reconstructed by a previous study because dehydration of smectite interlayer water and dehydroxylation of kaolinite would have completed 3 6 and 8 2 s after the beginning of slip the resulting release of water might have affected the frictional mechanism during the earthquake

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

geochemical variations of the quaternary daisen adakites southwest japan controlled by magma production rate abstract the late cenozoic southwest japan arc was formed by hot subduction of the young shikoku basin slab and daisen volcano has erupted exclusively adakitic products since ca 1 0 ma under this subduction regime although previous studies have revealed that the adakites of this arc were generated by melting of the subducted basaltic crust and incorporation of slab sediments this model cannot simply explain the observed geochemical variations of the daisen adakites we determined chronological variations in the major and trace element and sr nd pb isotopic compositions of the younger stage products of daisen

integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan article the paper describes the computation of the maximum horizontal stress h magnitude in the vicinity of the chelungpu fault taiwan host of the slip zone during the chi chi earthquake mw 7 6 1999 the scientific hole b intercepts the chelungpu fault at 1136 m at the depths of logged breakouts 940 1310 m the vertical stress v as estimated from density logs increases linearly with depth from 22 to 31 mpa a series of leak off tests yielded two reliable shut in pressures 23 7 mpa at 1085 m and 29 8 mpa at 1279 m which are lower than the estimated v albeit by only 2 1 and 0 6 mpa respectively in our analysis the shut in pressures were considered to represent estimates of the least horizontal principal stresses h at the respective depths and consequently the test induced fractures were assumed to have been vertical principal stress directions had been previously determined by others 105 155 for the maximum horizontal stress h

aseismic deep subduction of the philippine sea plate and slab window abstract we have made great efforts to collect and combine a large number of high quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both south korea and west japan this is the first time that a large number of korean and japanese seismic data sets are analyzed jointly as a result a high resolution 3 d p wave velocity model down to 700 km depth is determined which clearly shows that the philippine sea phs plate has subducted aseismically down to 460 km depth under the japan sea tsushima strait and east china sea the aseismic phs slab is visible in two areas one is under the japan sea off western honshu and the other is under east china sea off western kyushu however the aseismic phs slab is not visible between the two areas where a slab window has formed the slab window is located beneath the center of the present study region where many teleseismic rays crisscross detailed synthetic tests were conducted which indicate that both the aseismic phs slab and the slab window are robust features using the teleseismic data recorded by the japanese stations alone the aseismic phs slab and the slab window were also revealed zhao et al 2012 though the ray paths in the japanese data set crisscross less well offshore the slab window may be caused by the subducted kyushu palau ridge and kinan seamount chain where the phs slab may be segmented hot mantle upwelling is revealed in the big mantle wedge above the pacific slab under the present study region which may have facilitated the formation of the phs slab window these novel findings may shed new light on the subduction history of the phs plate and the dynamic evolution of the japan subduction zone

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

spatial distribution of scattering loss and intrinsic absorption of short period s waves in the lithosphere of japan on the basis of the multiple lapse time window analysis of hi net data summary propagation of short period s waves through the crust is strongly controlled by scattering loss and intrinsic absorption scattering loss controls the shape of the seismic envelope and the spatial distribution of the energy while intrinsic absorption produces an exponential decay as a function of time which is independent of the coordinates since both parameters have a different effect it is possible to compute them by means of the multiple lapse time window analysis mltwa in this paper high resolution maps of japan for both seismic s wave attenuation parameters are obtained by using the mltwa with data provided by the hi net seismic network the maps show strong regional variation of the parameters the variations depend mainly on the tectonic setting of each region and volcanism mechanisms in the scattering loss maps in the 1 2 hz map there is a very different behaviour between the northeast stronger scattering loss and the southwest much weaker scattering loss in the northeast of japan areas with strong scattering loss correspond mainly to the location of the volcanic arcs in southwest japan chugoku region and the west side of kii peninsula show strong scattering levels for the 4 8 hz and higher frequency bands in hokkaido strong scattering loss and intrinsic absorption can be observed for the lower frequency bands in the sendai plain we find strong scattering loss for all the frequency bands under all these areas low velocity anomalies have been detected in previous works in tohoku area the east side shows lower intrinsic attenuation than the west side this characteristic seems to extend to the southern part of hokkaido and to central japan in kanto area the values of intrinsic absorption are lower than in the surrounding areas but in the region around the itoigawa shizuoka tectonic line and in its western side strong absorption areas are observed the behaviour of each area for intrinsic absorption is similar to the behaviour observed in the q 1c maps the mltwa is based on the hypothesis of multiple isotropic scattering in a medium with the homogeneous distribution of scattering mean free path and intrinsic absorption although these hypotheses are very simple the results are very informative on the characteristics of each region and show the usefulness of studying the properties of scattered seismic waves in the understanding of the properties of the crust

stress tensor inversion for the chi chi earthquake sequence and its implications on regional collision we study the stress regime associated with the 1999 chi chi taiwan earthquake sequence broadband waveforms of 115 events recorded by the broadband array in taiwan for seismology bats are analyzed to determine their focal mechanisms depths and seismic moments these solutions are then used to invert for the reduced stress tensor using the algorithm of angelier 1998 we perform the stress inversion in three different ways first we aim at defining in the first approximation a uniform stress field by taking the entire dataset as a whole the inversion result shows a stress regime with the maximum compression along the direction of n119 e remarkably consistent with the direction of the relative motion between the philippine sea plate and eurasia this solution is stable and varies little in terms of the orientations of stress axes within 8 and the ratio of stress differences between 0 26 and 0 44 in the second approximation we aim at explaining more subtle features of our observations by considering differently built subsets we divide the dataset according to different seismogenic structures as identified by kao and chen 2000 our results yield rather similar results axes of maximum compressive stress fall in the range of n99 138 e in terms of trends and in the range of 1 17 in plunges for all subsets except a small group in the footwall furthermore we observe a systematic clockwise change in the direction of compression from south to north representing the more general fan shaped pattern of stress trajectories that characterizes the whole collision zone finally the inversion is performed for the three main types of faulting that is reverse strike slip and normal types the axis of maximum compression for the reverse mechanisms has the same direction as that for the strike slip whereas the normal type subset yields a n54 e extension that trends obliquely to the compression the fact that the stress regime associated with the chi chi aftershock sequence is similar to that derived from geological and seismic data in the region implies that the stress regime corresponding to the regional collision in taiwan is responsible for the occurrence of the chi chi earthquake moreover systematic secondary variation in the state of stress is observed for different types of focal mechanisms and seismogenic structures such variation may play an important role in producing local stress concentration that leads to the nucleation of disastrous earthquakes in central taiwan manuscript received 12 january 2001

evidence for the location and cause of large crustal earthquakes in japan large crustal earthquakes magnitude 5 7 8 0 depth 0 20 km during a period of 115 years from 1885 to 1999 in japan occurred in or around zones of low seismic velocity revealed by seismic tomography the low velocity zones may represent weak sections of the seismogenic crust the crustal weakening is considered to be closely related to the subduction of the oceanic pacific and philippine sea plates in this region along the volcanic front and in back arc areas the crustal weakening may be caused by active volcanoes and magma chambers in the forearc areas fluids are detected in the earthquake source zones which may have contributed to the crustal weakening and the rupture nucleation the fluids may be related to the dehydration of the subducting philippine sea slab these results indicate that large crustal earthquakes do not strike anywhere but only anomalous areas that may be detected with geophysical methods

seismic and geodetic evidence for the existence of hot materials beneath the wakayama swarm activity southwestern japan abstract a remarkable seismic swarm area exists amid the wakayama plain on the western coast of the kii peninsula in southwestern japan close to the median tectonic line mtl the hypocenters of these swarm earthquakes are very shallow with a median of 6 km this is exceptionally shallow for seismic activity in plain areas where the seismogenic layer is usually deep compared to that in mountainous regions takayama et al 2006 also the b value below 8 km in the eastern part of this swarm region is anomalously high in addition to these seismic features geonet data and leveling survey data obtained by the geospatial information authority of japan indicate that land around the swarm region are expanded and upheaved areas of the dilatation and upheaval almost overlap and their centers correspond to the site where the high b value anomaly is observed these facts strongly suggest that there are hot materials beneath the source region in the kii peninsula acid volcanic rocks outcrop widely and the wakayama swarm area is located on the southern side of the mtl in the metamorphic belt below which the relatively young hot philippine sea plate is subducting and dehydration from a surpentinized forearc mantle is expected to occur based on the seismic and geodetic observations and the tectonic background of the area in which the swarm region is situated we propose a hypothesis that a diorite or a serpentine diapir with plentiful hot fluids might be intruding beneath the source area

seismic observations at a seismic gap in the eastern margin of the japan sea using ocean bottom seismometers abstract the eastern margin of the japan sea is a nascent convergent plate boundary previous studies proposed the existence of a seismic gap along this boundary between 39 n and 40 n the trend of this gap is reported by ohtake island arc 4 156 165 1995 to be north northwest to south southeast but by ishikawa gekkan kaiyo suppl 7 102 107 1994 and matsuzawa prog abstr seismol soc jpn 2 b92 1995 to be north northeast to south southwest during one month ocean bottom seismic observations were conducted using nine ocean bottom seismometers to investigate seismicity in and around the seismic gap area in detail the observations revealed that the earthquake epicentral distribution had an echelon shape and could be divided into three groups these groups have a north northeast to south southwest trend this trend is consistent with the fault system in this area which was formed by the back arc spreading in the early to middle miocene this suggests that previously formed tectonic structures affect the present seismo tectonics and that this area has weak planes with a north northeast to south southwest trend

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

crustal heterogeneity beneath southwest japan estimated from direct and moho reflected waves abstract in this study we have used later phases reflected at the moho discontinuity in addition to direct arrival time data to study the 3 d crustal velocity structure beneath southwest sw japan we have collected more than 6000 direct and moho reflected phases and used them to obtain p and s wave velocity structures in the study area our results revealed significant low velocity anomalies beneath the active and quaternary volcanoes in sw japan results obtained using each data set p pmp s and sms are generally consistent with each other and all showed low velocity anomalies beneath most of the active and quaternary volcanoes western kii peninsula and eastern shikoku ray path analysis showed that the data set we used has a good ray criss crossing that is sufficient to accurately locate the velocity anomalies in the study area in addition results of the checkerboard resolution test indicate that these structures are generally well recovered and that adding later phases could significantly improve the resolution scale of the obtained tomographic images in comparison with those from first arrivals alone

vp and vs tomography of kyushu japan new insight into arc magmatism and forearc seismotectonics high resolution 3d tomographic images of the crust and upper mantle are determined under the entire arc region of kyushu by inverting a large number of arrival time data of p wave s wave and sp depth phases the hypocenters of the offshore earthquakes are accurately located by applying a combined method that makes use of sp depth phases and double difference of the p wave and s wave arrival times our tomographic results confirmed the major features delineated by previous studies and revealed some new features of structural heterogeneity related to arc magmatism beneath the kyushu arc high velocity anomalies correspond to the subducting philippine sea slab whilst low velocity zones in the mantle wedge are clearly imaged coinciding well with the velocity heterogeneities determined by previous studies strong low velocity anomalies are distributed extensively along the volcanic front and extend to the back arc side in the crust and upper mantle our present seismic images together with the previous results suggest that the fluids supplied by the dehydration processes of the descending philippine sea slab and the hot upwelling materials related to the opening of the okinawa trough might have contributed to the unzen volcanism under the forearc region of the kyushu subduction zone prominent low velocity anomalies are visible at depths of 10 40 km along the upper boundary of the subducting philippine sea slab and large interplate earthquakes are located around the low velocity zones which is similar to the features revealed in the northeast japan subduction zone the spatial distribution of large thrust earthquakes below the offshore region reflects the existence of weak coupled or decoupled portions on the interface of the subducting philippine sea slab

earthquake distribution in the subduction zone off eastern hokkaido japan deduced from ocean bottom seismographic and land observations summary in 1986 observations of microearthquakes were conducted with the use of five ocean bottom seismometers obss on the continental slope east of the kushiro submarine canyon ksc off hokkaido island japan the observation area is located about 50 100 km landward of the southern kuril trench beneath which the pacific plate is subducted toward hokkaido island the obs data were analysed together with land data on hokkaido island to determine the precise earthquake distribution and the geometry of the subducted plate the obs data revealed for the first time the seaward seismicity east of ksc high and shallow seismicity is concentrated in an area between the coast line of hokkaido island and the 2000 m isobath a region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 the nemuro hanto oki earthquake m 7 4 the obs array detected more than 10 events in this region most of them however are classified as earthquakes occurring within the subducted plate and the remarkable aftershock activity of the 1973 event is not recognized the seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones the seismicity is also very low beneath the trench in contrast with the adjacent trench areas in a vertical section taken perpendicular to the trench axis most of the located events form a seismic plane dipping landward the dip angle of the plane is 20d 25d beneath the continental slope while 35d 40d under hokkaido island the change in dip angle occurs 150 km landward of the trench axis this seismic plane is located 30 40 km below the plate boundary defined from the aftershock distribution of the nemuro hanto oki event the seismicity on the plate boundary is correlated with the geometry of the subducted plate the high activity occurs in a restricted region where the subducted plate shows a remarkable bend the seismic activity obtained is different from those in the adjacent regions along the southern kuril trench in particular the regional difference in seismicity is remarkable between the western and eastern sides of ksc our result suggests that the subduction zone along the kuril trench is divided into several tectonic blocks and ksc is one of their boundaries

the seismicity and structure of izu bonin arc mantle wedge at 31 n revealed by ocean bottom seismographic observation abstract 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

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

stress orientations of taiwan chelungpu fault drilling project tcdp hole a as observed from geophysical logs 1 the taiwan chelungpu fault drilling project tcdp drilled a 2 km deep research borehole to investigate the structure and mechanics of the chelungpu fault that ruptured in the 1999 mw 7 6 chi chi earthquake geophysical logs of the tcdp were carried out over depths of 500 1900 m including dipole sonic imager dsi logs and formation micro imager fmi logs in order to identify bedding planes fractures and shear zones from the continuous core obtained from the borehole a shear zone at a depth of 1110 meters is interpreted to be the chelungpu fault located within the chinshui shale which extends from 1013 to 1300 meters depth stress induced borehole breakouts were observed over nearly the entire length of the wellbore these data show an overall stress direction n115e that is essentially parallel to the regional stress field and parallel to the convergence direction of the philippine sea plate with respect to the eurasian plate variability in the average stress direction is seen at various depths in particular there is a major stress orientation anomaly in the vicinity of the chelungpu fault abrupt stress rotations at depths of 1000 m and 1310 m are close to the chinshui shale s upper and lower boundaries suggesting the possibility that bedding plane slip occurred during the chi chi earthquake citation wu h y k f ma m zoback n boness h ito j h hung and s hickman 2007 stress orientations of taiwan chelungpu fault drilling project tcdp hole a as observed from geophysical logs geophys res lett 34 l01303 doi 10 1029 2006gl028050

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

characterization of slip zone associated with the 1999 taiwan chi chi earthquake x ray ct image analyses and microstructural observations of the taiwan chelungpu fault abstract to characterize the fault related rocks within the chelungpu fault we performed x ray computed tomography ct image analyses and microstructural observations of hole b core samples from the taiwan chelungpu fault drilling project we identified the slip zone associated with the 1999 chi chi earthquake within the black gouge zone in the shallowest major fault zone by comparison with previous reports the slip zone was characterized by low ct number cataclastic or ultracataclastic texture and high possibility to have experienced a mechanically fluidized state taking these characteristics and previous reports of frictional heating in the slip zone into consideration we suggested that thermal pressurization was the most likely dynamic weakening mechanism during the earthquake

low frequency continuous tremor around the moho discontinuity away from volcanoes in the southwest japan 1 recent enhancement of seismic networks in the japan islands revealed occurrence of low frequency continuous tremors of a beltlike distribution in the southwest japan where the subducting philippine sea plate reaches depths of 30 40 km source depth of the tremor is estimated by selecting tremor segments with relatively clear p wave onsets the source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust mantle wedge and the subducting slab the long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor the most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments the northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water

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

magnetic inference of in situ open microcracks in sandstone samples from the taiwan chelungpu fault drilling project tcdp abstract this paper presents a thorough magnetic study of centimeter sized rock samples from hole b of the taiwan chelungpu fault drilling project tcdp fabrics were obtained from magnetic susceptibility and anhysteretic remanent magnetization are interpreted using whole rock and magnetic mineralogy characterization we infer the presence in the sandstone lithology of secondary magnetite coating a dense network of microcracks using other evidence from ferrofluid saturation experiments we conclude that this network of microcracks is not sealed but open in situ and should therefore be incorporated in the interpretation of seismic and borehole scale studies in the tcdp area

crustal and upper mantle structure in the ryukyu island arc deduced from deep seismic sounding summary in 1984 an extensive geophysical investigation was conducted in the northernmost part of the ryukyu island arc south of kyushu island japan the aim of this investigation was to obtain new information on the nature of crust and upper mantle at the continental margin through various geophysical measurements seismic profiling gravity magnetism sea beam and heat flow this paper presents the crust and upper mantle structure deduced from ocean bottom seismographic profiling conducted along two lines one line 190 km long was taken behind the northernmost part of the ryukyu island arc parallel to the major tectonic units i e the okinawa trough ryukyu arc and ryukyu trench the other 295 km long was perpendicular to the tectonic units along these profiles we deployed ocean bottom seismograms spaced 10 20km apart as a controlled source we used both explosives and an airgun array the experiment was successful and provided quite important information on the tectonics of a trenchisland arc back arc system the velocity structure obtained for the profile behind the ryukyu islands has continental properties the sediment thickness is 3 4 km although it shows lateral variations due to basement undulation the velocities of the upper and lower crust are 5 8 6 2 and 6 6 6 8 km s respectively the total crustal thickness decreases southwestward from 27 30 to 23 24 km this is direct evidence for crustal thinning associated with the process of back arc spreading the velocity structure along the profile from the trench to the island arc clearly shows the subduction accretion and deformation at this margin the crust beneath the trench has oceanic properties the sediment thickness is 1 3 3 0 km beneath which the igneous basement shows severe undulation the total crustal thickness is 7 8 km the p velocity is slightly less than 8 0 km s our seismic data revealed a huge sedimentary wedge located 50 150 km landward of the trench the maximum thickness of this wedge exceeds 12 km the origin of this wedge may be oceanic because the p wave velocity in its eastern half is almost comparable to that obtained for the trench area the velocity structure landward of the wedge is continental and similar to that obtained for the profile behind the island arc

localized rotation of principal stress around faults and fractures determined from borehole breakouts in hole b of the taiwan chelungpu fault drilling project tcdp abstract to reveal details of stress perturbations associated with faults and fractures we investigated the faults and large fractures accompanied by stress induced borehole breakouts or drilling induced tensile fractures in hole b of the taiwan chelungpu fault drilling project tcdp then we determined the relationship between the faults and fractures and stress orientation changes we identified faults and fractures from electrical images of the borehole wall obtained by downhole logging but also from photographs and descriptions of retrieved core samples and measured the variations in the principal horizontal stress orientation ascertained from borehole breakouts observed on the electrical images in the vicinity of the faults and fractures identification of geological structures faults fractures and lithologic boundaries by electrical images only is difficult and may sometimes yield incorrect results in a novel approach therefore we used both the electrical images and core photographs to identify geological structures we found four patterns of stress orientation change or no change in the vicinity of faults and fractures in tcdp hole b i abrupt discontinuous rotation in the vicinity of faults or fractures ii gradual rotation iii suppression of breakouts at faults fractures or lithologic boundaries and iv no change in the stress orientation we recognized stress fluctuations that is heterogeneous mesoscale 10 cm stress distributions with respect to both stress orientation and magnitude in addition we found that stress state changes occurred frequently in the vicinity of faults fractures and lithologic boundaries

deep structure of the japan subduction zone abstract we determined 3 d p wave velocity structure down to 700 km depth under the japan islands using a large number of arrival time data from local and teleseismic events simultaneously we collected 207 000 arrival times from 7743 shallow and deep earthquakes occurred in and around japan and 34 148 data from 333 teleseismic events which were recorded by over 1000 seismic stations on the japan islands our tomographic model revealed some new features the philippine sea slab is found to subduct down to 500 km depth under southwest japan though the seismicity within the slab ends at 150 200 km depth significant low velocity anomalies are found to exist in the deep portion of the mantle wedge above the pacific slab which may be caused by the deep dehydration process of the slab slow anomalies are detected in the mantle beneath the pacific slab which may be caused by a mantle plume or upwelling of hot mantle materials associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

imaging the subducting slabs and mantle upwelling under the japan islands summary a high resolution p wave tomography of the crust and mantle down to 700 km depth beneath the japan islands is determined using a large number of high quality arrival time data from local earthquakes and teleseismic events simultaneously the tomography shows that the philippine sea slab is subducting aseismically down to 430 km depth under southwest japan though the seismicity within the slab ends at 180 km depth a low velocity low v zone in the mantle wedge under tohoku and kyushu is found to extend westward from the volcanic front to the backarc under the japan sea and east china sea significant low v anomalies are revealed in the deep portion of the mantle wedge 400 500 km depth above the pacific slab under southwest japan which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the pacific slab low v anomalies appear at 420 700 km depths beneath the pacific slab under eastern japan which may reflect hot mantle upwelling associated with the deep subduction of the pacific slab and its collapsing down to the lower mantle

structural heterogeneity and its implications for the low frequency tremors in southwest japan abstract to better understand the generation of low frequency tremors lfts that occurred in the nankai subduction zone detailed 3 d seismic velocity vp and vs and poisson s ratio structures are determined by inverting 181 576 p wave and 159 517 s wave arrival time data from 7572 earthquakes we also estimated the crack density e saturation rate and porosity structures along the lft belt using the inverted velocity data our results provide geophysical evidence for that the long duration tremors are closely related to the dehydration reactions of the subducting philippine sea slab and in the lowermost crust low vp low vs high high e high and high porosity anomalies are imaged at depths of 25 45 km along the lft belt which may indicate the existence of fluids liberated by the dehydration of the descending slab and or by the metamorphism in the lowermost crust the interpretation accords well with the behavior of the subducted hydrous minerals which are expected to dehydrate at the temperature of 350 500 c at depths of 35 50 km in southwest japan two prominent lft gaps between shikoku and kii peninsula and between kii peninsula and tokai along the tremor belt are imaged with low and low anomalies due to the lack of dehydration reactions within those gaps we conclude that the lfts are caused by the dehydrated fluids migrating in the faults and or by crack opening closing and extending in the corner of mantle wedge along the subducting philippine sea slab possibly due to the high pore fluid pressure and high crack density under the forearc region of the nankai subduction zone

moho depth variation beneath southwestern japan revealed from the velocity structure based on receiver function inversion abstract the philippine sea plate is subducting under the eurasian plate beneath the chugoku shikoku region southwestern japan we have constructed depth contours for the continental and oceanic mohos derived from the velocity structure based on receiver function inversion receiver functions were calculated using teleseismic waveforms recorded by the high density seismograph network in southwestern japan in order to determine crustal velocity structure we first improved the linearized time domain receiver function inversion method the continental moho is relatively shallow 30 km at the coastline of the sea of japan and at the seto inland sea and becomes deeper greater than 40 km around 35 n and 133 8 e near the seto inland sea a low velocity layer of thickness 10 km lies under the continental moho this low velocity layer corresponds to the subducting oceanic crust of the philippine sea plate the oceanic moho continues to descend from south to northwest and exhibits complicated ridge and valley features the oceanic moho runs around 25 km beneath the pacific coast and 45 km beneath the seto inland sea and it extends to at least to 34 5 n the depth variation of the moho discontinuities is in good qualitative agreement with the concept of isostasy from the configurations of both the continental and oceanic mohos we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of shikoku and that a mantle wedge may exist beneath the western and eastern part of shikoku the southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake

along arc variation in seismic velocity structure related to variable growth of arc crust in northern izu bonin intraoceanic arc the izu bonin arc is an intraoceanic island arc where the pacific plate subducts beneath the philippine sea plate along arc variation in thickness of the arc crust has been observed by previous active seismic surveys beneath the izu bonin arc we have imaged three dimensional 3 d seismic velocity structures in the northern izu bonin arc using arrival time data of the earthquakes during 80 days of observations by ocean bottom seismographs and three permanent island seismic stations our 3 d velocity model indicates heterogeneous structure in the mantle wedge along the arc low velocity anomalies related to upwelling flow in the mantle wedge are not uniform beneath the volcanic front low velocity anomalies extending down to the subducting slab beneath the volcanic front coincide with thicker parts of the arc crust north of aoga shima and south of sumisu jima this coincidence suggests that heterogeneous mantle upwelling flow fundamentally controls the growth of the arc crust along topographic highs in the forearc low velocity anomalies in the crust and uppermost mantle coincide with positive magnetic anomalies suggesting the presence of a remnant arc providing further evidence of this view of arc crust formation

delamination structure imaged in the source area of the 1982 urakawa oki earthquake 1 the kuril arc collides with the northeast japan arc in the southern part of hokkaido japan 3 d tomographic inversion of data from a dense network of sensitive ocean bottom seismographs and land stations has allowed imaging of previously unseen details of the arc arc collision structure a low velocity body dips gently southwestward at depths of 35 to 45 km from east of the hidaka mountains to the source area of the 1982 urakawa oki destructive earthquake ms 6 8 the low velocity body is the lower half of the lower crust of the kuril arc which must have been delaminated by the collision we believe that the continuing collision of the delaminated lower crust with the northeast japan arc resulted as an episode of aseismic slow slip prior to the 1982 urakawa oki earthquake as well being the reason for the high seismic activity in this region

mapping the northern portion of the chelungpu fault taiwan by shallow reflection seismics 1 the chelungpu fault was activated by the 1999 chi chi earthquake mw 7 6 taiwan this fault exhibited extraordinarily large surface ruptures up to 9 8 m as well as underground fault slippages up to 12 m during the earthquake these large displacements were concentrated along the northern portion of the fault 40 km north of the epicenter to prepare data for the future drilling of deep wells in this area many shallow seismic reflection surveys were conducted to investigate the sites an approximate 3d structure of the fault surface can be deduced by this cost effective approach although the depth penetration may be limited e g 3 km the method still provides reliable information to study large ruptures and to better plan future deep wells