seismotectonics at the trench trench trench triple junction off central honshu we study earthquakes in and near the ttt type triple junction off boso peninsula central honshu to elucidate the plate interaction in this area the pacific north america northeast japan and philippine sea plates meet at the junction of the japan and izu bonin trenches and the sagami trough we determine focal mechanisms using wwssn data we also determine accurate focal depths by modeling body waves there is no serious trade off between focal depth and source time function for the events treated in this study

p wave tomographic imaging of the crust and upper mantle beneath the japan islands more than 50 000 high accuracy arrival times from 1200 shallow and deep earthquakes selected from the japan university network earthquake catalog are used to investigate the three dimensional 3 d seismic velocity structure beneath japan travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique p wave tomographic images of the crust and upper mantle to a depth of 500 km beneath the japan islands are determined with spatial resolutions of 25 50 km the obtained p wave velocity in the crust and uppermost mantle is slightly higher and relatively homogeneous in western japan in contrast with an average lower velocity and a large heterogeneity in eastern japan where many active volcanoes exist this feature correlates well with the distributions of heat flow and gravity anomalies low velocity low v zones are imaged in the crust and uppermost mantle beneath active volcanoes the low v zones in the mantle wedge dip toward the west and extend continuously to a depth of about 200 km a high velocity high v zone corresponding to the subducted pacific plate is delineated the thickness of the high v zone is estimated to be 80 90 km in southwestern japan the descending philippine sea plate is also delineated its thickness is estimated to be 30 km

geology genesis and exploration implications of the footwall and hanging wall alteration associated with the hellyer volcanic hosted massive sulfide deposit tasmania australia hellyer is a large 16 2 million metric tons high grade 13 9 zn 7 1 pb 0 4 cu 168 g t ag 2 5 g t au sea floor mound style polymetallic volcanic hosted massive sulfide vhms deposit located in the mount read volcanics of western tasmania the deposit is hosted by the que hellyer volcanics a sequence of late middle cambrian mafic to felsic coherent volcanics and polymict volcaniclastics hydrothermal alteration occurs in the regional footwall immediate footwall and hanging wall alteration in the regional footwall is confined to patchy quartz albite and chlorite with minor sericite epidote and hematite underlying hellyer is a zoned alteration pipe with a central siliceous core quartz sericite which passes into zones of chlorite chlorite carbonate sericite chlorite and finally sericite quartz stringer envelope zone on the margin overlying the central part of the deposit within the hanging wall basalt is a distinctive and zoned alteration plume five alteration zones have been identified fuchsite chlorite carbonate quartzalbite and sericite fuchsite dominated alteration occupies the central portion of the hanging wall alteration plume chlorite and carbonate alteration surrounds the fuchsite zone with carbonate zones forming near to the ore deposit and chlorite zones extending above and lateral to the carbonate outward is quartz ablite alteration which extends laterally into distal sericite alteration mass change calculations for the footwall and hanging wall indicate that in general the footwall alteration zones are depleted in cao na2o la sr ni cr and v but have enrichments of fe2o3 mno mgo k2o s and most metals compared to the host basalt the hanging wall alteration has gained cao k2o na2o co2 s in rb ba ag as mo sb cs and tl while fe2o3 mno mgo p2o5 la sr pb zn th u cd and nd are depleted cao na2o cr v and ni are depleted in the footwall andesite but enriched in the hanging wall alteration plume this relationship suggests that these elements were sourced from the breakdown of feldspars pyroxenes and andesitic groundmass of the footwall lithologies and transported in the hydrothermal fluid into the overlying basalt and precipitated as albite calcite and white micas in the hanging wall alteration the development of alteration associated with the hellyer vhms deposit occurred in three stages stage 1 regional footwall alteration was formed by unfocused hydrothermal convection of seawater down into the recently deposited volcanic pile at temperatures between approximately 250 degrees celsius and 200 degrees celsius and at low to moderate water rock ratios stage 2 alteration formed by structurally controlled fluid flow from a deep intensifying hydrothermal convection system and created the footwall alteration pipe decreasing water rock ratios and temperatures over a range of 350 degrees celsius to less than 200 degrees celsius led to the development of the concentric alteration pipe mineral zones based on modeling of whole rock d18ofr values geochemical modeling and mineral assemblages the siliceous core is interpreted to have formed at temperatures near 350 degrees celsius the chlorite rich alteration zone at 300 degrees celsius to 250 degrees celsius and the outer sericite rich alteration at temperatures of 250 degrees celsius to 150 degrees celsius the final synmineralization fluid flow event stage 3 created the hanging wall alteration plume after rapid burial of the deposit by basalt continuation of upward hydrothermal fluid flow created the zoned hanging wall alteration distribution of hanging wall alteration assemblages suggests a temperature gradient from approximately 250 degrees celsius for the fuchsite zone to lower temperatures 150 degrees celsius for the distal quartz albite and sericite alteration zones alteration mineralogy mineral chemistry lithogeochemistry and stable isotope characteristics of the footwall and hanging wall alteration have been combined into a comprehensive set of vectors which can be used in exploration for vhms deposits in similar geologic settings

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

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

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

new insights into the generation of the 2013 lushan earthquake ms 7 0 china global seismic waveform inversion can provide information on where the 2013 ms 7 0 lushan earthquake occurred in sichuan province china and how its aftershock sequence expanded to investigate the generation mechanism of the lushan earthquake and its relation to the 2008 wenchuan earthquake ms 8 0 50 temporary seismic stations were installed in the source area following the lushan earthquake crustal stress data were also collected along the longmen shan fault zone lmfz to reveal its influence on the generation of the lushan earthquake seismic imaging and crustal stress analysis have revealed that the lushan earthquake occurred in a distinct area with high velocity vp vs low poisson s ratio and high crustal stress the high velocity zone at the lushan source may reflect precambrian metasedimentary or igneous rock in the seismogenic layer which enables the accumulation of high crustal stress to generate large earthquakes however a sharply contrasting gap zone with low velocity high anomalies is clearly imaged in the upper crust under the concatenated area between the lushan and wenchuan earthquakes seismic images indicate that the low velocity gap zone is associated with fluid bearing ductile flow from the lower crustal materials of tibet being pushed into a weakened segment of the lmfz this study suggests that the 2013 lushan earthquake may have been triggered by high crustal stress accumulation together with high coseismic stress further increased by the wenchuan earthquake in the metamafic seismogenic layer contrasting rheological variations in the crust and crustal stress changes along the lmfz control the rupture processes that generated the lushan and wenchuan earthquakes as well as the generation of new earthquakes in the future

interplate coupling in the kanto district central japan and the boso peninsula silent earthquake in may 1996 i studied crustal deformation in the kanto district central japan based on continuous gps data horizontal as well as vertical displacement rate demonstrate significant interaction between the landward kanto block and the philippine sea plate although the subduction effect of the pacific plate is not apparent it is reasonable to consider the entire kanto district is displaced westward due to the interaction with the pacific plate the gps velocity data were inverted to estimate the slip deficit distribution on the sagami trough subduction zone the result delineates a strongly coupled region on the plate interface part of which corresponds to the 1923 kanto earthquake the strongly coupled region is located shallower than 20 km in addition the plate interaction is laterally heterogeneous even in the same depth range implying thermal structure is not the only factor controlling interplate coupling the gps data also detected a silent earthquake event on the interface of the philippine sea slab east of the boso peninsula in the middle of may 1996 the silent rupture propagated over a 50 km 50 km wide area during about a week the maximum slip was approximately 50 mm and the released seismic moment was 4 7 1018nm mw 6 4 there was a small seismicity triggered by this silent event the silent slip was located in the peripheral of the strongly coupled area suggesting that frictional properties and or stress conditions are inhomogeneous on the plate boundary interface

influence of fluids and magma on earthquakes seismological evidence abstract in this paper we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the japan islands the relationship between seismic tomography and large crustal earthquakes m 5 7 8 0 in japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity the low velocity zones represent weak sections of the seismogenic crust the crustal weakening is closely related to the subduction process in this region along the volcanic front and in back arc areas the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab in the forearc region of southwest japan fluids are suggested in the 1995 kobe earthquake source zone which have contributed to the rupture nucleation the fluids originate from the dehydration of the subducting philippine sea slab the recent 2001 geiyo earthquake m 6 8 occurred at 50 km depth within the subducting philippine sea slab and it was also related to the slab dehydration process a detailed 3d velocity structure is determined for the northeast japan forearc region using data from 598 earthquakes that occurred under the pacific ocean with hypocenters well located with sp depth phases the results show that strong lateral heterogeneities exist along the slab boundary which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling these results indicate that large earthquakes do not strike anywhere but only anomalous areas which can be detected with geophysical methods the generation of a large earthquake is not a pure mechanical process but is closely related to physical and chemical properties of materials in the crust and upper mantle such as magma fluids etc

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

focal depth determination of central aleutian earthquakes focal depths of suboceanic subduction zone earthquakes best determined by the careful identification and analysis of teleseismic depth phases on short period vertical component seismograms arc crustal structure can cause water surface reflections pwp and swp and sediment basement reflections defined here as pp and sp with amplitudes comparable to those of water sediment reflections pp and sp however changes in source mechanism and local variations in arc structure make the observation of these short period depth phases highly unpredictable digital and analog vertical component seismograms were examined from nearly 400 teleseismically recorded central aleutian earthquakes that occurred during the period 1964 to 1984 a data set consisting of the travel times relative to p of short period depth phases was assembled for 54 shallow depth earthquakes and 33 earthquakes deeper than 50 km across the arc in addition long period waveforms for the larger events were examined magnitude mb 4 8 was approximately the lower limit for useful depth phase arrivals short period reflected phases from shallow depth earthquakes were analyzed by matching observed travel times relative to p to theoretical values computed for a two dimensional model of arc crustal structure depths resolved in this fashion provide a sharper depth profile of the shallow seismic zone in the central aleutians than previously available outer rise and trench earthquakes occur over a narrow range of subcrustal depths north of the trench the plate dips gently beneath the terrace and appears to descend gradually into the mantle the thickness of the interplate thrust earthquake zone is no more than 10 to 15 km earthquakes near the volcanic arc and above the plate interface occur at depths beneath the sediment basement interface estimation of the focal depths of earthquakes deeper than 50 km poses a special problem because an accurate specification of slab structure is needed to correct for path effects the phases pwp and swp are ordinarily only weakly recorded on long period vertical component seismograms however long period waves reflected from boundaries with sharp long period impedance contrasts such as the sea bottom and the sediment basement interface are strongly recorded moment tensor depths for shallow earthquakes based on the inversion of long period waveforms using a standard earth model once transformed into true depths within a model of arc crustal structure for the central aleutians agree with depths determined from short period reflected phases alone to within a few kilometers reported depth phases are routinely used for depth determination by the preliminary determination of epicenters pde and the international seismological centre isc services however the phases pp and sp are often obscured in the p coda and for suboceanic earthquakes e g pwp is misidentified and reported as pp this often results in the overestimation of the focal depths of routinely determined hypocenters reported by these services

strike slip tectonics within the northernmost philippine sea plate in an arc continent collisional setting abstract the geological processes in the northernmost philippine sea plate which is bounded by the suruga and sagami troughs are a typical example of an active collision zone we attempt to illustrate the stress field through seismic estimations and geodetic analysis and propose the kinematic mode of the northernmost tip of the philippine sea plate seven events m 4 0 are chosen for waveform inversion by the isola software to distinguish the stress field in particular six of the chosen events which exhibit strike slip motion are distributed in the eastern area where few focal mechanisms have been reported by previous studies according to the available focal mechanisms strike slip faults with similar p and t axes are widely distributed in the study area the stress inversion suggests that the northern area is characterized by a nw se compression and a ne sw extension stress regime although some spatial differences exist as indicated by an analysis of the geodesy epicenters focal mechanisms gravity anomalies and velocity structure the deformation in the northernmost tip is mainly accommodated by several conjugate strike slip fault systems with steep dips that center on the izu volcanic line generally the maximum principal stress of the kinematics is derived from the collision between the philippine sea plate and central japan because of the different subduction angles rates and directions of the down going plate diverging slab pull forces along the suruga and sagami troughs may be causing the ne nne extension in most of the areas that are bounded by the two troughs the extension propagates southwards along the izu volcanic line and reaches the area adjacent to miyake jima

spatial distribution of earthquakes associated with the pacific plate subduction off northeastern japan revealed by ocean bottom and land observation abstract precise earthquake locations in the vicinity of a trench axis elucidate the characteristics of the shallower part of a subducting plate we split the japan trench into two regions off sanriku in the north and off fukushima in the south on the basis of the seismic properties of the regions and the fine microearthquake structure investigated by ocean bottom seismographic surveys as well as recent observation of major seismic activity from ocean bottom seismograph data the hypocentral distributions along a profile perpendicular to the trench axis indicate that almost all earthquakes occur along the boundary separating the subducting pacific plate and the landward plate a difference of the hypocentral distribution between off fukushima and off sanriku is found at the shallow plate boundary level the seismic thrust zone begins at more than 100 km landwards from the trench axis off fukushima whereas it begins at 50 km off sanriku and is nearly flat between 50 and 100 km from the trench axis this reflects regional variations of the interplate coupling in spite of these variations a common characteristic in recent observations indicates that in both regions almost all the hypocenters are deeper than 10 km this suggests the plate boundary to be decoupled at least in the part shallower than 10 km

geothermal and rheological implications of intracontinental earthquakes beneath the kanto tokai region central japan abstract the kanto tokai region representing a boundary zone of three lithosphere plates has been the subject of intense geological geophysical studies for decades the increased attention can be explained partly by the complicated tectonic situation and even to a greater extent by the permanent threat of earthquakes in the region in this work the rheology of the lithosphere in the area is estimated on the basis of frictional failure in the brittle regime and power law steady state creep in the ductile regime rheological models are constructed for a three layered model involving a granitic upper crust a granulitic or gabbroic lower crust and a dunitic upper mantle results show a marked rheological stratification of the lithosphere controlled primarily by the thermal regime in zones of very low surface heat flow 40 mw m 2 the lithosphere shows brittle features down to depths of 50 60 km at least in areas of normal to moderately high heat flows 50 90 mw m 2 a soft ductile zone at the depth levels of the lower crust sandwiched between two brittle layers appears at locations with very high heat flows 100 120 mw m 2 only the upper crust remains brittle and plastic flow of rocks may start here from the depths of 10 15 km this rheological stratification of the lithosphere is compared with the depth distribution of earthquakes in the area and a generally reasonable confinement of seismicity to the brittle levels is indicated

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

double difference tomography the method and its application to the hayward fault california we have developed a double difference dd seismic tomography method that makes use of both absolute and relative arrival times by reducing systematic errors using the more accurate relative arrival times the method produces an improved velocity model simultaneously it yields event locations of a quality equivalent to those of the dd location method while avoiding simplifying assumptions of that method we test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography we also test this method on a hayward fault california earthquake dataset spanning 1984 1998 the earthquakes relocated by this method collapse to a thin line along the fault trace consistent with previous results the dd velocity model has sharper velocity contrasts near the source region than the standard tomography model

deep subduction of h2o and deflection of volcanic chain towards backarc near triple junction due to lower temperature in central japan near the triple junction of the pacific philippine sea and north american or ohotsuku plates the volcanic chain deflects towards backarc compared to the adjacent northeast japan and izu bonin arcs and lies 200 300 km above the wadati benioff zone numerical modeling shows that thermal recovery of the subducting pacific plate is slow due to the overlapping philippine sea plate which shifts the dehydration reactions to greater depths along the pacific plate and causes the magmatism above the deep wadati benioff zone the low geothermal gradient along the subducting pacific plate also implies that a considerable amount of h2o is carried further down by phase a which is formed just above the subducting plate without being released for magmatism central japan can be regarded as an entrance for h2o into the deep mantle

can the okhotsk plate be discriminated from the north american plate the plate geometry in northeast asia has been a long standing question with a major issue being whether the sea of okhotsk and northern japanese islands are better regarded as part of the north american plate or as a separate okhotsk plate this question has been difficult to resolve because earthquake slip vectors along the kuril and japan trenches are consistent with either pacific north america or pacific okhotsk plate motion to circumvent this difficulty we also use slip vectors of earthquakes along sakhalin island and the eastern margin of the japan sea and compare them to the predicted eurasia okhotsk and eurasia north america motions for a model with a separate okhotsk plate we invert 10 eurasia okhotsk and 255 pacific okhotsk slip vectors with pacific north america and eurasia north america nuvel 1 data alternatively for a model without an okhotsk plate those eurasia okhotsk and pacific okhotsk data are regarded as eurasia north america and pacific north america data respectively the model with an okhotsk plate fits the data better than one in which this region is treated as part of the north american plate because the improved fit exceeds that expected purely from the additional plate the data indicate that the okhotsk plate can be resolved from the north american plate the motions on the okhotsk plate s boundaries predicted by the best fitting euler vectors are generally consistent with the recent tectonics the eurasia okhotsk pole is located at northernmost sakhalin island and predicts right lateral strike slip motion on the nne striking fault plane of the may 27 1995 neftegorsk earthquake consistent with the centroid moment tensor focal mechanism and the surface faulting along the northern boundary of the okhotsk plate the north america okhotsk euler vector predicts left lateral strike slip consistent with the observed focal mechanisms on the nw boundary of the okhotsk plate the eurasia okhotsk euler vector predicts e w extension discordant with the limited focal mechanisms and geological data this misfit may imply that another plate is necessary west of the magadan region in southeast siberia but this possibility is hard to confirm without further data such as might be obtained from space based geodesy

role of fluids in the initiation of the 2008 iwate earthquake m7 2 in northeast japan 1 seismic velocity vp vs and poisson s ratio images show that the hypocenter of the 2008 iwate miyagi earthquake m7 2 is located in a distinctive zone with low vp slightly low vs and low within the upper 10 km depths where the mechanical strength might be weaker than the normal seismogenic layer with high velocity and low poisson s ratio immediate below a prominent low vp low vs and high zone is revealed in the lower crust and uppermost mantle under the iwate mainshock hypocenter which may reflect fluids resulting from the dehydration of the subducting pacific slab under northeast japan the aqueous component of fluids intruded into the source region may have reduced the mechanical strength of the rock matrix and so triggered the iwate earthquake our results indicate that crustal fluids beneath the seismogenic layer can play an important role in the initiation of large crustal earthquakes

three dimensional q structure beneath the kanto tokai district japan the q structure of the upper mantle beneath the kanto tokai district central japan where the eurasian pacific and philippine sea plates are converging is investigated from seismic p and s wave spectral ratios by means of a three dimensional 3 d inversion method it is found that the subducting philippine sea and pacific plates are characterized by high q zones a high q zone also exists between the philippine sea plate and the pacific plate to the east of the volcanic front on the other hand a low q zone in the uppermost mantle extends westward horizontally from the volcanic front under the philippine sea plate and appears to exist over the wedge mantle beneath the western tokai district a very low q value obtained here is roughly equal to the average qvalue in the low velocity layer derived from surface wave and free oscillation data a low q region that suggests a magma chamber under active volcanoes is not detected in the crust

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

early middle triassic trace fossils from the luoping biota southwestern china evidence of recovery from mass extinction trace fossils have proven useful for studying the timing and process of biotic recovery after the permian triassic mass extinction ptme recovery stages are defined by comparing successive ichnoassemblages from the latest permian to the early middle triassic lower triassic trace fossils have been explored in some detail but those of the lower middle triassic are less well known here well preserved fossil materials from the luoping biota from yunnan province south china suggest that a fully recovered shallow marine ecosystem was re established by the early middle triassic trace fossil assemblages of the luoping biota are characterized by high ichnodiversity with 14 ichnogenera in the shallow marine environment of an intra carbonate platform basin and nine ichnogenera in the subtidal environment such moderate to high ichnodiversity together with a marked increase in burrow sizes and the common occurrence of key ichnotaxa e g rhizocorallium and thalassinoides suggests that the ichnofauna had reached recovery stage four in contrast non turbiditic strata of the offshore setting record only three ichnogenera with bioturbation indices never exceeding one periodic anoxia in bottom waters was presumably the main control for such a protracted trace fossil recovery in an offshore setting which otherwise aided the fine preservation of body fossils of the luoping biota furthermore event sedimentation turbidite deposits in the offshore setting incorporated moderate ichnodiversity and moderate to high bioturbation indices both interpreted as a result of short term colonization by transported infaunal animals from proximal settings the occurrence of variable crustacean traces e g sinusichnus spongeliomorpha and thalassinoides at luoping and the locomotion traces of marine reptiles together with abundant fishes and fossil decapods highlights the value of trace fossils in ecosystem reconstruction after the ptme

seismic low velocity layer at the top of subducting slabs observations predictions and systematics abstract layered structure persists at the tops of subducting plates to depths in excess of 150 km a low velocity channel several km thick produces guided seismic guided waves which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to 150 km depth the surrounding medium has seismic velocities 3 6 faster than standard earth models as inferred from direct travel times presumably because the waveguide lies atop cold descending lithosphere however guided waves travel more slowly several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low velocity channel thus the frequent observation of dispersed body waves indicates that intermediate depth earthquakes tend to lie in waveguides also receivers must be favorably placed observations for seven circum pacific subduction zones show such signals which can be explained as low velocity channels 2 8 km thick velocity anomalies within low velocity channels are largest at shallowest depths up to 14 slower than surroundings at depths less than 150 km diminishing to

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

simultaneous travel time inversion for earthquake location and subduction zone structure in the central aleutian islands a combined location and velocity inversion technique is applied to travel time data from 151 well recorded central aleutian earthquakes the data include p and s wave arrivals at stations of a local network and p pp and sp wave arrivals at teleseismic stations after correction for crustal structure at the reflection points the depth phases pp and sp provide important constraints on subduction zone structure not ordinarily resolved by other types of data the structure is assumed to vary only across the arc and in depth it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section to avoid ray tracing it is assumed that the velocity part of the problem is linear preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab a slab thickness of 80 100 km and a downdip length of about 400 km well below the deepest seismic activity are indicated the slab is characterized by seismic velocities as much as 11 higher than the surrounding mantle in its upper portions and 4 6 higher at depth a sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab the slab anomaly appears to spread out and fall off very slowly with depth these are probably not real effects but consequences of the omission of three dimensional ray tracing

depth of the decoupling plate interface and thermal structure under arcs subduction proceeds as thrust movements at the boundary between the subducting plate and the rigid overlying plate at depth below this mechanically decoupling plate interface the subducting plate couples with the overlying mantle and flow is induced in the mantle wedge this induced flow is the main factor that controls thermal structure under arcs depth of this decoupling to coupling transition at the plate interface is estimated from observed surface heat flow in the northeastern japan arc using a two dimensional numerical simulation of thermal structure the estimated depth is about 70 km the termination depth of thrust earthquakes agrees well with the depth of the coupling transition which indicates that these earthquakes occur at the decoupling plate interface the mechanism of this decoupling plate interface is discussed in this paper it is believed that water associated with the subducting oceanic crust reduces the frictional stress at the plate interface and thrust movement thus becomes possible petrological studies show that dehydration reaction in the subducting oceanic crust terminate at about 2 0 gpa and have little temperature dependence termination depths of thrust earthquakes estimated from seismic studies are nearly the same 60 80 km for various arcs this indicates that depth of the decoupling to coupling transition is controlled by the termination depth of dehydration reaction in the subducting oceanic crust across arc heat flow profiles observed in various subduction zones show similar features heat flow is very low in the forearc region and increases steeply at the volcanic front considering that the depth of the slab is about 100 km at the volcanic front for various arcs this heat flow feature is closely related to the depth of the slab thus it is concluded that thermal structure under arcs is mainly controlled by the depth of the pressure dependent dehydration reactions in the subducting oceanic crust

tomographic imaging of the alaska subduction zone the alaska subduction zone is characterized by the pacific plate descending beneath the north american plate causing abundant seismic activity in the crust and along the wadati benioff zone down to a depth of approximately 200 km we have used 142 908 p wave arrival times from 12 237 shallow and intermediate depth earthquakes recorded by the alaska earthquake information center jointly run by the geophysical institute university of alaska fairbanks and u s geological survey in the period from january 1977 to november 1991 to investigate the three dimensional 3 d p wave velocity structure beneath central and southern alaska travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique the nonlinear tomographic problem is solved by iteratively conducting linear inversions and the velocity structure and hypocentral locations are simultaneously determined we conducted two types of inversions one is an inversion with a laterally homogeneous starting model the others are what we call slab inversions in which we introduce into the initial model the high velocity subducting pacific plate as a priori information we found that the slab inversions gave a seismologically more plausible result and a final root mean square travel time residual significantly smaller than that of the inversion with the homogeneous starting model detailed p wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30 60 km the tomographic image of the upper crust correlates well with the major surface geological features such as slow sedimentary basins and fast ultramafic bodies prominent low velocity anomalies exist in the crust and upper mantle beneath active volcanoes in the mantle wedge the low velocity anomalies dip toward the continental side and extend to a depth of about 150 km which are considered to be associated with the active volcanism in the alaska subduction zone the results suggest that the subducting pacific plate has a thickness of 45 55 km and a p wave velocity 3 6 higher than that of the surrounding mantle

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

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

detection of the subducting crust of oceanic plates beneath the kanto district japan abstract low velocity crustal layers on the top of the subducting slabs beneath the kanto district central japan were detected by a travel time inversion using 40 763 p wave arrival times from 3038 local earthquakes recorded by a local network of 41 seismic stations the inversion covered 188 228 110 km volume of the target region and the model was parameterized by 47 57 18 rectilinear blocks measuring 4 0 km per side horizontally and 4 10 km varying thickness in depth a prominent e w directed low velocity band of about 25 km width appeared in the central kanto region at a depth of 30 km moving north or northwestward gradually with depth this band can be traced to the depth of about 60 km at a depth of 60 km a slightly lower amplitude nearly n s directed low velocity band was observed at the east coast of the kanto region moving westward with depth these low velocity bands are thin layers attached to the top of the inclined thick layers with relatively high velocity in the kanto district the philippine sea phs plate subducts under the eurasian eur plate and the pacific pac plate also underthrusts beneath both the eur and phs plates some authors already suggested low velocity anomalies near the surface of those descending plates in this region mainly from the analyses of later phases and focal mechanisms of local earthquakes the shapes of the low velocity bands are similar to the depth distribution of the upper boundary of the phs and pac plates proposed by other authors the thrust earthquakes that occur at the boundaries between the eur and phs plates and between the phs and pac plates are almost located in the low velocity layers described above we conclude that these low velocity layers are the crusts that exist at the top of the subducting phs and pac plates while high velocity layers are subducting slabs themselves for the phs plate low density crust of the izu bonin arc is probably stacking at the range from 139 25 e to 139 55 e at 35 55 n which is observed as a thick low velocity region to the south of the range and in a steep inclination to the north of it this is also recognized from the gravity anomaly

metastable olivine wedge in the subducting pacific slab and its relation to deep earthquakes abstract a metastable olivine wedge mow is detected in the subducting pacific slab under the japan sea by applying a forward modeling approach with 3 d ray tracing to high quality arrival time data from 78 well located deep earthquakes the data were recorded by the dense and high quality seismic networks on the japan islands the double difference earthquake location technique is improved and used to relocate the 78 deep earthquakes precisely our results show that the mow does exist in the pacific slab and it has a p wave velocity 3 lower than the normal mantle or 4 lower than the surrounding slab velocity the top and bottom depths of the mow are 400 and 570 km respectively and the mow is located in the central portion of the slab after precise relocation all the 78 deep earthquakes are found to occur within the mow or along its edges within the slab the cut off depth of deep seismicity under the japan sea is consistent with the estimated mow bottom depth our results favor the transformational faulting as the mechanism of deep earthquakes in the pacific slab under the japan sea

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

variation of downdip limit of the seismogenic zone near the japanese islands implications for the serpentinization mechanism of the forearc mantle wedge abstract it has been proposed that the downdip limit of interplate thrust type earthquakes in subduction zones dlt is determined either by the brittle ductile transition around 350 450 c of the crustal material at the plate interface or the moho of the forearc of the upper plate the latter is due to possible serpentinization of the forearc mantle wedge above the subducting plate which makes the thrust zone slide in a stable manner i examine this criterion in subduction zones near japan where the pacific and philippine sea plates are subducting i show that in most of n honshu where the old pacific plate is subducting and in kanto and s kyushu where the izu bonin and kyushu palau ridges of the philippine sea plate are subducting the dlt is located at a depth of 60 70 km which is deeper than the moho of the upper plate this shows that the criterion does not hold and the mantle wedge is not likely to be serpentinized in these regions in contrast in the region off iwate in n honshu and in bonin where the old pacific plate is subducting the dlt is located at a depth of 20 30 km around the moho of the upper plate suggesting that the mantle wedge is serpentinized in these regions in sw japan where the young shikoku basin is subducting the dlt is around 30 km in depth which coincides with the moho but also with the 350 c temperature at the plate interface although seismic tomography indicates that the mantle wedge is serpentinized the dlt here does not help to examine the criterion assuming that hydration of the mantle wedge is conducted through hydro fracturing by water released from the dehydrating subducted crust i propose that the stress state of the wedge would determine the extent of serpentinization of the mantle wedge in the reverse fault type stress regime of n honshu kanto and s kyushu hydro fracturing is not favored and fractures in the thrust zone associated with repeated interplate earthquakes provide conduits for water to flow in the updip direction resulting in no serpentinization of the mantle wedge no serpentinization of the mantle wedge in turn causes interplate earthquakes in contrast in the normal fault type stress regime of the region off iwate and bonin and the strike slip fault type stress regime of sw japan hydro fracturing is favored resulting in serpentinization of the mantle wedge the frictional property at the thrust zone is governed by stable sliding resulting in no interplate earthquakes deeper than the moho thus the stress regime of the mantle wedge would determine bifurcation whether it is serpentinized or not and then the depth of dlt

relations between deep temperatures and other geophysical characteristics in central honshu japan abstract the area of interest central honshu represents a complicated knot of triple junction of the north american plate the eurasian plate and the izu bonin arc of the philippine sea plate due to complicated geological structural characteristics of the crust and specific plate tectonic features of the lithosphere this region has been extensively investigated by different geophysical methods for decades beside seismic studies yielding detailed information on the crustal structure and providing models for seismic velocity perturbations and seismic wave attenuation in the area particularly measurements of three component geomagnetic variations magnetic anomalies studies of seismicity etc were made this paper presents the results of a search for correlations between the thermal field in the region and other geophysical parameters to specify the thermal regime of the crust and the uppermost mantle a quantitative geothermal model of central honshu for six seismic profiles with a total length of some 1500 km was used no correlation is indicated between curie point depth from magnetic data and that inferred by thermal modeling moderate but significant correlations are found between heat flow and depths to the conrad and the moho discontinuities strong correlations were found between temperature and seismic wave attenuation and also between fractional velocity perturbations and temperature in the uppermost mantle again the correlation is strong and highly significant between electrical conductivity and temperature in the lower crust upper mantle

interplate coupling in the tokai district central japan deduced from continuous gps data i invert gps displacement rates in the tokai district to investigate strain accumulation associated with the anticipated tokai earthquake i apply a representation of interseismic strain accumulation at subduction zones by savage 1983 and estimate a distribution of virtual normal fault slip back slip on the plate boundary surface estimated back slip takes its maximum value of 30 40mm year under the sea of enshu this value is nearly equal to the relative plate motion there implying almost complete coupling back slip is smaller under suruga bay and its direction is different from the relative motion between the eurasian and the philippine sea plates localized deformation at the northern tip of the philippine sea plate may be responsible for this discrepancy partitioning tectonic strain due to the plate convergence into faults on both sides of the izu peninsula and also distributed strain inside the peninsula

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

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

deep structure of the northeastern japan arc and its implications for crustal deformation and shallow seismic activity abstract seismic tomography studies in the northeastern japan arc have revealed the existence of an inclined sheet like seismic low velocity and high attenuation zone in the mantle wedge at depths shallower than about 150 km this sheet like low velocity high attenuation zone is oriented sub parallel to the subducted slab and is considered to correspond to the upwelling flow portion of the subduction induced convection the low velocity high attenuation zone reaches the moho immediately beneath the volcanic front or the ou backbone range running through the middle of the arc nearly parallel to the trench axis which suggests that the volcanic front is formed by this hot upwelling flow aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the backbone range where they are expelled from solidified magma and migrate further upward the existence of aqueous fluids may weaken the surrounding crustal rocks resulting in local contractive deformation and uplift along the backbone range under the compressional stress field of the volcanic arc a strain rate distribution map generated from gps data reveals a notable concentration of east west contraction along the backbone range consistent with this interpretation shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone based on these observations a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern japan arc

interplate coupling and relative plate motion in the tokai district central japan deduced from geodetic data inversion using abic summary the spatial distribution of the strength of interplate coupling between the subducting philippine sea and overlying continental plates in the tokai district central japan was investigated in detail through the inversion analysis of geodetic data using akaike s bayesian information criterion abic the geodetic data used for the analysis are annual rates of level changes 1972 1984 and horizontal length changes 1977 1988 which presumably represent average crustal movements during the interseismic period the result of the inversion analysis shows the existence of a strongly coupled region extending from 10 to 30 km in depth the total seismic moment accumulated in this area since the last event the 1854 ansei earthquake is roughly estimated to be 5 5 1027 dyne cm which corresponds to ms 7 8 the interplate coupling becomes weaker toward the shallower and deeper portions this is consistent in general tendency with a rheological model inferred from petrological viewpoints the strength of coupling also tends to decrease toward northeast over the west coast of suruga bay the direction of plate convergence inferred from the inversion analysis is oriented n30 w this is significantly different from the direction of relative plate motion between the philippine sea and eurasian plates but concordant with that between the philippine sea and north american plates

aftershock investigation in the andaman nicobar islands of india and its seismotectonic implications six three component short period digital seismograph four reftek and two kinemetrics stations were established in different parts of the andaman nicobar islands following the 26 december 2004 devastating sumatra andaman mainshock m w 9 3 here we analyze about 18 000 aftershocks m 3 0 recorded from 6 january to 16 march 2005 to better understand the seismotectonics of the region a sudden burst of aftershock activity with irregular trend in the month of january 2005 was observed at almost all seismograph stations the estimate of p value 0 9532 from aftershocks m 4 5 is near to normal value of 1 0 which suggests a slow decay sequence of aftershock with complex and nonuniform stress change in a fault system creep effects and history dependent stress changes the frequency magnitude relation of the aftershocks followed the power law with average b value of 0 7723 and it varies from 0 49 to 1 03 indicating the compressive stress state of the region and its heterogeneous structure with possible variations in frictional conditions along the fault the distribution of the located aftershocks by a multistation method shows a north south trending aftershock cluster in an area of about 800 300 km 2 which reflects an approximate rupture dimension of the mainshock beneath the andaman nicobar islands most of aftershocks occurred in a depth range of 5 65 km the determination of composite fault plane solutions of the best located aftershock clusters at three different depth ranges 0 15 16 30 and 31 km in the ten blocks of the region suggests that the mainshock rupture propagated through normal reverse and strike slip earthquakes the areas in the vicinity of barren and narcondum volcanic zones show a dominantly normal fault mechanism due to predominant tensional forces suggesting the facilitation of brittle failure in the weakened crust by the process of underheating the zone near baratang mud volcano is associated predominantly with thrust fault at depth of 0 30 km suggesting a high compressive force beneath the baratang that ejected a huge amount of mud and slurry materials to the surface after the 26 december 2004 sumatra andaman earthquake m w 9 3 here we propose that the relocation of our large aftershock dataset using converted sp depth phase technique for precise depth control is needed and its sharing with global earthquake data can render more information on the subduction dynamics and geodynamical processes of the region

seismological evidence for the existence of serpentinized wedge mantle serpentinized peridotite is detected seismologically by mapping poisson s ratio beneath the kanto district central japan because serpentinite has a higher poisson s ratio than other rocks in the crust and upper mantle we determine three dimensional p and s wave velocity models using travel time tomography and then construct a three dimensional map of poisson s ratio the models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km in the low velocity region there is an area with poisson s ratio greater than 0 3 at depths of 20 45 km which we attribute to serpentinized peridotite the rest of the low velocity region is interpreted as descending oceanic crust with a poisson s ratio of about 0 25 the high poisson s ratio area is associated with low seismicity and weak interplate coupling which is consistent with the ductility of serpentinite

p wave velocity and gradient images beneath the okinawa trough abstract we determined the high resolution 3 d p wave velocity structure under the okinawa trough in the region between south taiwan and north kyushu japan by inverting 102 511 absolute travel time data from 5012 local earthquakes and 29 153 relative travel time residuals calculated from 1873 teleseismic events for the first time a combined data set recorded simultaneously by three seismographic networks located in kyushu okinawa trough taiwan and fujian province of china has been used which enables us to determine the high resolution p wave velocity structure in the crust and upper mantle under the entire arc system the resulting tomographic and gradient images provide a new insight into the structural heterogeneities related to the opening of the okinawa trough and dehydration of the philippine sea slab phs and confirm some of the features proposed by previous studies a high velocity zone is imaged clearly at depths of 50 300 km along the ryukyu arc system from south kyushu to north taiwan showing good agreement with the spatial distribution of seismicity under the subduction zone the relative residuals from different quadrants also exhibit early relative arrivals at the stations corresponding to the high vp regions and we therefore interpret the high vp zone as the subducting phs slab along the volcanic front from south kyushu to north taiwan low vp areas are visible which are mainly caused by the fluids liberated by the extensive dehydration process of the subducting phs slab this fluid related low velocity zone is located at depths of 20 120 km along the arc with a total length of 1200 km from north taiwan to south kyushu using the inverted p wave velocity model we also calculated the gradient images which are independent of the selection of reference model or color scale to provide further information and help us to understand and explain the tomoagraphic structures three sub blocks of the ryukyu arc system are revealed from the inverted velocity images and the calculated gradient maps these features of the velocity anomalies together with other parameters determined by the present study suggest that the okinawa trough extension arc magmatism and seismicity are mainly attributed to the dehydration process associated with the phs slab subduction and the oblique subduction direction of the plate

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

seismic images of the source area of the 2004 mid niigata prefecture earthquake in northeast japan to better understand the generation mechanism of the 23 october 2004 mid niigata prefecture earthquake m 6 8 we used 120 352 p wave and 95 391 s wave high quality arrival times from 5013 earthquakes to determine the three dimensional seismic velocity vp vs and poisson s ratio structures in and around the source area the hypocenter locations of the aftershocks are relocated accurately by using absolute and relative travel time double difference data our results demonstrate that the seismic velocity and poisson s ratio vary markedly around the source area most active faults are located along the margins of low velocity and high velocity zones an obvious change in seismic velocity and poisson s ratio occurs between the northwest and southeast parts of the study area and this boundary zone reflects the spatial distribution of active faults being well consistent with the surface geological features most of the large historic crustal earthquakes are located in or around the low seismic velocity zones the mainshock hypocenter is located near the margin of low velocity high poisson s ratio and high velocity low poisson s ratio anomalies along the active fault zone a zone with pronounced low velocity and high poisson s ratio is clearly imaged in the lower crust under the source area reflecting the existence of fluids that are released due to the dehydration of the subducting pacific slab such fluids might have reduced the mechanical strength of the fault zone and thus initiated the mid niigata prefecture earthquake