two dimensional modelling of subduction zone anisotropy with application to southwestern japan summary we present a series of 2 d numerical models of viscous flow in the mantle wedge induced by a subducting lithospheric plate we use a kinematically defined slab geometry approximating the subduction of the philippine sea plate beneath eurasia through finite element modelling we explore the effects of different rheological and thermal constraints e g a low viscosity region in the wedge corner power law versus newtonian rheology the inclusion of thermal buoyancy forces and a temperature dependent viscosity law on the velocity and finite strain field in the mantle wedge from the numerical flow models we construct models of anisotropy in the wedge by calculating the evolution of the finite strain ellipse and combining its geometry with appropriate elastic constants for effective transversely isotropic mantle material we then predict shear wave splitting for stations located above the model domain using expressions derived from anisotropic perturbation theory and compare the predictions to 500 previously published shear wave splitting measurements from seventeen stations of the broad band f net array located in southwestern japan although the use of different model parameters can have a substantial effect on the character of the finite strain field the effect on the average predicted splitting parameters is small however the variations with backazimuth and ray parameter of individual splitting intensity measurements at a given station for different models are often different and rigorous analysis of details in the splitting patterns allows us to discriminate among different rheological models for flow in the mantle wedge the splitting observed in southwestern japan agrees well with the predictions of trench perpendicular flow in the mantle wedge along with b type olivine fabric dominating in a region from the wedge corner to about 125 km from the trench

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

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

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

seismic signatures of a hydrated mantle wedge from antigorite crystal preferred orientation cpo abstract we report electron backscatter diffraction ebsd measurements of antigorite crystal preferred orientations cpos from 7 samples of antigorite schist from serpentinite melanges adjacent to the motagua fault system in central guatemala the cpos range from diffuse girdle to point distributions of 001 atg girdle distributions of 001 atg are not expected from deformation theory or experiments suggesting that they are a result of growth rather than deformation and may thus be topotactically related to the cpo of the olivine from which the antigorite grew the calculated seismic anisotropy ranges from 6 to 28 in v p and 5 to 33 in v s and is highest for samples with a point maximum of 001 atg for all samples the minimum v p corresponds to the pole to 001 atg and the maximum v p occurs within the foliation for samples with a clearly defined foliation trench parallel shear wave splitting observations for subduction zones can best be explained by a combination of olivine b type cpo and antigorite oriented with 001 parallel to the foliation only a relatively thin 20 km zone of hydrated mantle is required to explain the observed splitting times

p and s wave tomography and anisotropy in northwest pacific and east asia constraints on stagnant slab and intraplate volcanism we determined three dimensional p and s wave velocities and p wave azimuthal anisotropic tomography of the northwest pacific subduction zones by inverting 1 225 086 p wave and 335 117 s wave arrival times from 13 413 earthquakes our results show some differences between p and s wave images for the stagnant pacific slab in the mantle transition zone mtz beneath northeast china the stagnant slab looks thicker in the p wave image than that in the s wave image which may reflects the effects of both hydration and lower temperature in the mtz though differences in the resolution of p and s wave tomography may also have some effects the changbai intraplate volcanism is caused by hot and wet upwelling in the big mantle wedge above the stagnant pacific slab our p wave anisotropy tomography shows that the fast velocity direction fvd in the subducting philippine sea plate beneath the ryukyu arc is ne sw trench parallel which is consistent with the spreading direction of the west philippine basin during its initial opening stage suggesting that it may reflect the fossil anisotropy a striking variation of the fvd with depth is revealed in the subducting pacific slab beneath the northeast japan arc which may be caused by slab dehydration that changed elastic properties of the slab with depth the fvd in the mantle wedge beneath the northeast japan and ryukyu arcs is trench normal which reflects subduction induced convection beneath the kuril and izu bonin arcs where oblique subduction occurs the fvd in the mantle wedge is nearly normal to the moving direction of the downgoing pacific plate suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

anisotropic structure beneath central java from local earthquake tomography 1 in this study we present the new tomographic code anita which provides 3 d anisotropic p and isotropic s velocity distribution based on p and s traveltimes from local seismicity for the p anisotropic model we determine four parameters for each parameterization cell this represents an orthorhombic anisotropy with one predefined direction oriented vertically three of the parameters describe slowness variations along three horizontal orientations with azimuths of 0 60 and 120 and one is a perturbation along the vertical axis the nonlinear iterative inversion procedure is similar to that used in the lotos code we have implemented this algorithm for the updated data set of central java part of which was previously used for the isotropic inversion it was obtained that the crustal and uppermost mantle velocity structure beneath central java is strongly anisotropic with 7 10 of maximal difference between slow and fast velocity in different directions in the forearc area between southern coast and volcanoes the structure of both isotropic and anisotropic structure is strongly heterogeneous variety of anisotropy orientations and highly contrasted velocity patterns can be explained by a complex block structure of the crust beneath volcanoes we observe faster velocities in vertical direction which is probably an indicator for vertically oriented structures channels dykes in the crust beneath the middle part of central java north to merapi and lawu volcanoes we observe a large and very intense anomaly with a velocity decrease of up to 30 and 35 for p and s models respectively inside this anomaly e w orientation of fast velocity takes place probably caused by regional extension stress regime in a vertical section we observe faster horizontal velocities inside this anomaly that might be explained by layering of sediments and or penetration of quasi horizontal lenses with molten magma in the mantle trench parallel anisotropy is observed throughout the study area such anisotropy in the slab entrained corner flow may be due to presence of b type olivine having predominant axis parallel to the shear direction which appears in conditions of high water or and melting content

arc parallel shear deformation and escape flow in the mantle wedge of the central america subduction zone evidence from p wave anisotropy he p wave anisotropy of the crust and uppermost mantle has been determined in two separate areas of the continental margin of central america covering offshore and onshore parts of southern nicaragua and northern and central costa rica our study is based on traveltime data from local earthquakes recorded with three combined onshore and offshore seismic networks that had been deployed in three half year intervals between 2002 and 2006 the p wave traveltime data have been tomographically inverted in terms of an elliptical anisotropic p wave velocity distribution in the upper crust beneath costa rica the seismic anisotropy is of the order of 2 5 and locally strongly variable in orientation corresponding to the complicated tectonic structure in contrast the upper mantle beneath both study regions shows stronger anisotropy of the order of 5 10 and a more coherent orientation of the axis of fast p wave velocity that varies systematically on regional scales the upper mantle of the incoming oceanic plate is characterized by a trench normal orientation of the fast p wave velocity axis presumably corresponding to mineral alignment lattice preferred orientation lpo in transport direction this pattern is corroded in the uppermost part of the subduction zone possibly by the influence of bending related trench parallel faults and serpentinization that can overprint or annihilate the lpo anisotropy the upper mantle of the overriding plate and mantle wedge shows a clear trench parallel orientation of the fast p wave velocity axis the anisotropy is stronger in nicaragua than in southern central costa rica factor of 2 orders of magnitude the development of a stronger anisotropy in the nicaragua could be driven by a change in the stress regime from compressional in southern costa rica to transpressional in nicaragua corresponding to a change from near orthogonal subduction in the se to oblique subduction and slab retreat in the nw from a comparison with s sks wave gps and geochemical observations follows that the most likely explanation for the observed pattern of p wave anisotropy in the mantle wedge is lpo caused by a trench parallel shear deformation and or nw oriented escape flow originating in the compressional zone near the cocos ridge collision area

anisotropic tomography of hokkaido reveals delamination induced flow above a subducting slab we present a new 3 d anisotropic seismic model for the crust and upper mantle beneath hokkaido japan based on tomographic inversion of p and s arrival times from a regional seismic database the p model is parameterized with three parameters at each point that describe the azimuthal anisotropy the s model is represented isotropically the isotropic p and s velocity anomalies match nearly perfectly in the crust they show a prominent linear anomaly in central hokkaido along the kamuikotan and hidaka belts which represents the area of eastward underthrusting of the japan block underneath the kuril fore arc we interpret the high velocity anomaly beneath the hidaka zone as being delaminated mafic crust and entrained mantle lithosphere which developed due to crustal shortening in the collision zone one of our vertical sections shows a very unusual configuration for a subduction zone a low velocity slab is overlain by a high velocity body in the mantle wedge we propose that the high velocity delaminated material sinking along the slab surface prevents the escape of fluids and melts from the upper part of the slab where they are generated due to phase transitions as a result a large portion of the fluids is entrained downward and lowers the seismic velocities in the slab the azimuthal anisotropy in the crust clearly corresponds to the major tectonic units and delineates the major suture zones in the mantle the anisotropy has a fan shaped configuration and most likely represents the deviating of flows starting in southern hokkaido and splitting into three directions the western and eastern flows proceed toward the two volcanic groups on hokkaido and they may carry with them additional material to trigger the characteristic caldera forming eruptions in these groups

regional variations of the shear wave polarization anisotropy in the crust and mantle wedge beneath the tohoku district abstract we investigated the regional variations of the shear wave polarization anisotropy in the upper crust lower crust and mantle wedge beneath the tohoku district in the northeast japan arc the shear wave splitting parameters fast polarization direction fpd and split time for the upper and lower crusts are estimated by shear wave splitting analysis of the ps phases which are generated by the conversion of a p wave into an s wave at the conrad and moho discontinuity interfaces the splitting parameters for the mantle wedge are determined by splitting analysis of the slab ps phase converted at the oceanic crust on the subducting pacific slab and the direct s waves from deep focus earthquakes located just below seismic stations the ps phases are identified on the p wave receiver functions constructed from the teleseismic records to accurately determine the splitting parameters for the lower crust from the moho ps phase the ps phase should be corrected for the shear wave splitting effect in the upper crust similarly to estimate the mantle wedge anisotropy the direct s wave and slab ps phase must be corrected for the splitting effect in the crust the anisotropies of the crust and mantle wedge are estimated from the corrected ps phases and direct s waves the anisotropy of the upper crust exhibits a regional variation where fpd of the split shear wave is predominant in the n s direction in the pacific coast area and in the e w direction in the rest of the tohoku district the split time is less than 0 2 s the upper crustal anisotropy is attributed to the alignment of vertical cracks induced in the upper crust by tectonic stress in the lower crust fpd is predominant in the e w direction with a split time similar to that in the upper crust and the anisotropy of the lower crust is due to the lattice preferred orientation of rock forming minerals in the mantle wedge fpd is predominant in the n s direction trench parallel in the fore arc side of the volcanic front but in the e w direction trench perpendicular in the back arc side the split time in the mantle wedge is similar to that of the upper crust the most likely cause for the trench perpendicular anisotropy in the back arc mantle wedge is the lattice preferred orientation of a type olivine along flows of the secondary mantle convection for trench parallel anisotropy in the fore arc mantle wedge we considered two possible causes the preferred orientation of b type olivine along the secondary mantle convection and that of dry olivine along the trench parallel flow produced by the along strike dip variation of the pacific slab however we could not determine which one is the origin

three dimensional structure of p wave anisotropy beneath the tohoku district northeast japan 1 we retrieve three dimensional structures of isotropic and anisotropic velocities of p waves of the tohoku district from first p arrival time data assuming azimuthal anisotropy to be caused by hexagonal symmetry axes distributed horizontally in the earth the results show that the high velocity pacific slab is clearly imaged in the isotropic velocity structure even though the azimuthal anisotropy is taken into account in addition small scale low velocity regions and prominent low velocity anomalies are found just below the active volcanoes and in the mantle wedge above the high velocity pacific slab respectively the fast propagation axis of p waves is in mostly e w direction in the upper crust nearly n s and e w directions in the lower crust e w direction in the mantle wedge and n s direction in the descending pacific slab these features of the p wave anisotropy structure are consistent with those of lateral variations of the fast polarization directions measured previously by shear wave splitting observations the plausible factor that causes the crust anisotropy is interpreted as being alignment or preferred orientation of microcracks and crust minerals the mantle wedge anisotropy is attributed to lattice preferred orientation of the mantle minerals arising from present day mantle process such as the mantle wedge convection and the plate motion however the fast propagation axis of p waves in the slab is almost perpendicular to the magnetic lineation of the oceanic plate under the northwest pacific and thus the slab preserves the original anisotropic property that the pacific plate gained when it formed

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

shear wave polarization anisotropy beneath the north eastern part of honshu japan summary we have investigated the shear wave polarization anisotropy in the north eastern japan arc by using waveforms from many local earthquakes at various depths we used a cross correlation method to detect the shear wave splitting for intermediate depth and deep events the observed fast shear wave oscillation directions fsods are oriented parallel to the dip direction of the slab at the western stations while they are oriented perpendicular to the dip direction at the eastern stations these observations indicate that significant anisotropy exists in the mantle wedge and that its nature is quite different between the eastern and the western parts

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

olivine fabric transitions and shear wave anisotropy in the ryukyu subduction system recent shear wave splitting measurements from the fore arc region of the ryukyu subduction system show large magnitude 0 3 1 6 s trenchparallel splitting in both local and teleseismic phases the similarity of splitting parameters associated with shallow local s and teleseismic phases suggests that the source of anisotropy is located in the fore arc mantle one explanation for this pattern of shear wave splitting involves a transition from commonly observed high temperature olivine fabrics with flow parallel seismically fast directions to a flow normal b type olivine fabric in the cold fore arc mantle of the ryukyu wedge we test the b type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development the distribution of olivine fabric is calculated with high resolution thermomechanical models of the ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology we conclude that b type fabric can explain the magnitude and trench parallel orientation of deep local s phases that sample the core of the forearc mantle however our calculations show that b type fabric alone cannot account for large magnitude trench parallel splitting associated with teleseismic phases that sample the shallow tip of the fore arc mantle alternative models for trench parallel teleseismic splitting in the shallow tip of the fore arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite 2008 elsevier b v all rights reserved

frequency dependent shear wave splitting and multilayer anisotropy in northeast japan 1 we analyzed carefully shear wave splitting on 320 intermediate depth earthquakes occurring in the subducting pacific slab in different frequency bands to investigate the s wave anisotropy and subduction dynamics under northeast ne japan our results show that the differential time between the fast and slow shear waves t is definitely smaller 0 2 s in the high frequency band than that 0 3 0 4 s in the low frequency band and so the splitting parameters especially t are strongly frequency dependent although the t is indubitably smaller under ne japan than the other subduction zones regardless of the frequency band nine large t values 0 5 0 7 s are detected which indicates that the anisotropy is potentially strong in ne japan both the p and s wave anisotropy results in ne japan are consistent with a model of subduction driven back arc spreading and convection in the mantle wedge causing trench normal fast orientations in the wedge and aligned faults and cracks in the subducting pacific slab causing trench parallel fast orientations in the slab when an s wave travels through the area with the multilayer orthogonal anisotropies some of its splitting would be cancelled and thus small t is observed

shear wave anisotropy in the crust mantle wedge and subducting pacific slab under northeast japan to study the anisotropic structure beneath northeast ne japan we made 4366 shear wave splitting measurements using high quality seismograms of many earthquakes occurring in the crust and the subducting pacific slab our results provide important new information on the s wave anisotropy in the upper crust lower crust mantle wedge and subducting pacific slab in the upper crust the anisotropy is mainly caused by the stress aligned fluid saturated microcracks the measured delay times dts increase to 0 10 s at 10 11 km depth the fast velocity directions fvds are parallel to either the tectonic stress or the strike of active faults the maximum dts for the low frequency earthquakes near the moho are 0 15 0 17 s suggesting strong anisotropy at the base of the crust or in the uppermost mantle the measurements for the intermediate depth earthquakes in the pacific slab show dominant e w trench normal fvds in the back arc area and n s trench parallel fvds in the fore arc area the trench normal fvds in the back arc area are caused by the corner flow in the mantle wedge as a result of the subduction of the pacific plate the maximum dts for the slab earthquakes reach 0 30 0 32 s at 100 km depth but only half of the total dts are produced in the mantle wedge the small dts in the mantle wedge may result from an isotropic or weak anisotropic zone in the middle of the mantle wedge in the fore arc the dominant trench parallel fvds for the slab earthquakes are consistent with those for the upper crust earthquakes and 80 of the total dts can be accounted for by the anisotropy in the crust in the subducting pacific slab the trench parallel fvds may reflect either the original fossil anisotropy in the pacific plate when the plate was produced in the mid ocean ridge or the preferred orientations of the crystals and cracks in the upper part of the subducting slab

the late quaternary biogeographic histories of some great basin mammals western usa the great basin of arid western north america provides one of the most detailed late pleistocene and holocene mammal records available for any part of the world though the record is by far strongest for small mammals of the 35 genera of now extinct north american pleistocene mammals 19 are known to have occurred in the great basin a list that is likely to be complete or nearly so of these 19 seven can be shown to have survived beyond 12 000 radiocarbon years ago a proportion similar to that for north america as a whole horses camels mammoth and helmeted musk oxen appear to have been the most abundant of these genera pygmy rabbits brachylagus idahoensis yellow bellied marmots marmota flaviventris and bushy tailed woodrats neotoma cinerea declined in abundance at the end of the pleistocene at about the same time as populations south of their current arid western distributional boundary were extirpated subsequent declines occurred during the hot dry middle holocene pygmy rabbits also declined as modern pinyon juniper woodlands developed across the great basin the snake range of eastern nevada has seen the late pleistocene or holocene extinction of both northern pocket gophers thomomys talpoides and pikas ochotona princeps coupled with the rarity of yellow bellied marmots here these histories make the snake range a biogeographic oddity these and other great basin mammal histories provide significant insights into the possible responses of great basin small mammals to global warming

melting and seismic structure beneath the northeast japan arc transportation of h2o associated with subduction of the pacific plate beneath the northeast japan arc is modeled to predict distribution of aqueous solution and melt and the consequent p wave velocity structure the observed velocity structure coincides well with the model for equilibrium transport of h2o most of h2o subducted is brought down to 150 200 km depth along the slab then the aqueous fluid generated ascends to initiate melting which explains the low velocity regions 6 reduction observed beneath the backarc rather than beneath the volcanic front if equilibrium transport of h2o occurs initiation of melting beneath the backarc with deep subduction of h2o is likely to be the case also for other subduction zones with slabs older than several tens of m y cold enough to stabilize serpentine to a great depth

seismic tomography and anisotropy in the source area of the 2008 iwate miyagi earthquake m 7 2 in order to understand the generation mechanism of the iwate miyagi earthquake m 7 2 that occurred on 14 june 2008 in northeast japan we determined high resolution 3d seismic velocity vp vs and poisson s ratio structures as well as p wave azimuthal anisotropy in and around the source area using 246 623 p wave and 189 153 s wave high quality arrival times from 1941 aftershocks of the 2008 iwate miyagi earthquake as well as 4878 shallow and intermediate depth earthquakes in this area recorded by the dense high sensitivity seismic network hi net and the seismic stations of tohoku university our results show that hypocenters of the mainshock and three large aftershocks are located in a boundary zone where both seismic velocity and poisson s ratio change drastically in a short distance a zone with pronounced low velocity and high poisson s ratio is revealed in the lower crust and uppermost mantle under the source area which may reflect the arc magma and fluids ascending from the upper mantle wedge the p wave azimuthal anisotropy is complex under the study area which may also indicate the complex crustal structure there these results indicate that the generation of the 2008 iwate miyagi earthquake was influenced by the ascending arc magma and fluids associated with the dehydration reactions of the subducting pacific slab under northeast japan

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

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

centroid moment tensor solutions for january march 1993 abstract centroid moment tensor solutions are presented for 177 earthquakes that occurred during the first quarter for 1993 the solutions are obtained using corrections for aspherical earth structure represented by dziewonski and woodward s whole mantle shear velocity model sh8 u4l8

mantle flow in subduction systems the mantle wedge flow field and implications for wedge processes 1 the mantle wedge above subducting slabs is associated with many important processes including the transport of melt and volatiles our understanding of mantle wedge dynamics is incomplete as the mantle flow field above subducting slabs remains poorly understood because seismic anisotropy is a consequence of deformation measurements of shear wave splitting can constrain the geometry of mantle flow in order to identify processes that make first order contributions to the pattern of wedge flow we have compiled a data set of local s splitting measurements from mantle wedges worldwide there is a large amount of variability in splitting parameters with average delay times ranging from 0 1 to 0 3 s up to 1 0 1 5 s and large variations in fast directions we tested for relationships between splitting parameters and a variety of parameters related to subduction processes we also explicitly tested the predictions made by 10 different models that have been proposed to explain splitting patterns in the mantle wedge we find that no simple model can explain all of the trends observed in the global data set mantle wedge flow is likely controlled by a combination of downdip motion of the slab trench migration ambient mantle flow small scale convection proximity to slab edges and slab morphology with the relative contributions of these in any given subduction system controlled by the subduction kinematics and mantle rheology there is also a likely contribution from b type olivine and or serpentinite fabric in many subduction zones governed by the local thermal structure and volatile distribution

seismic anisotropy and heterogeneity in the alaska subduction zone summary we determined p and s wave tomography and p wave anisotropic structure of the alaska subduction zone using 259 283 p and 73 817 s wave arrival times from 7268 local shallow and intermediate depth earthquakes recorded by more than 400 seismic stations the results show strong velocity heterogeneities in the crust and upper mantle low velocity anomalies are revealed in the mantle wedge with significant along arc variations under the active volcanoes in the mantle wedge the low velocity zone extends down to 100 150 km depth under the backarc the results indicate that h2o and fluids brought downwards by the subducting pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge significant p wave anisotropic anomalies are revealed under alaska the predominant fast velocity direction fvd is trench parallel in the shallow part of the mantle wedge 90 km depth and in the subslab mantle whereas the fvd is trench normal within the subducting pacific slab the trench parallel fvds in the mantle wedge and subslab mantle may be caused by 3 d mantle flow that is induced by the complex geometry and strong curvature of the pacific slab under alaska the flat and oblique subduction of the pacific slab may play a key role in forming the trench parallel fvd under the slab the trench normal fvd in the subducting pacific slab may reflect the original fossil anisotropy when the pacific plate was produced at the mid ocean ridge

slab1 0 a three dimensional model of global subduction zone geometries 1 we describe and present a new model of global subduction zone geometries called slab1 0 an extension of previous efforts to constrain the two dimensional non planar geometry of subduction zones around the focus of large earthquakes slab1 0 describes the detailed non planar three dimensional geometry of approximately 85 of subduction zones worldwide while the model focuses on the detailed form of each slab from their trenches through the seismogenic zone where it combines data sets from active source and passive seismology it also continues to the limits of their seismic extent in the upper mid mantle providing a uniform approach to the definition of the entire seismically active slab geometry examples are shown for two well constrained global locations models for many other regions are available and can be freely downloaded in several formats from our new slab1 0 website http on doi gov d9arbs we describe improvements in our two dimensional geometry constraint inversion including the use of average active source seismic data profiles in the shallow trench regions where data are otherwise lacking derived from the interpolation between other active source seismic data along strike in the same subduction zone we include several analyses of the uncertainty and robustness of our three dimensional interpolation methods in addition we use the filtered subduction related earthquake data sets compiled to build slab1 0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far concluding that the width of these seismogenic zones is on average 30 larger than previous studies have suggested

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

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

3d upper mantle structure beneath japan and its surrounding region from inter station dispersion measurements of surface waves abstract three dimensional shear wave speed structure of the upper mantle beneath the japanese islands and the surrounding regions is constructed from the measurements of inter station phase speeds of surface waves we employ the dense broad band seismic network f net deployed throughout japan as well as permanent stations of global seismic network in east asia a temporary broad band seismic network in far east russia which has been constructed as a part of the stagnant slab project since 2004 is also used in combination with the permanent stations using seismic events from 2005 to 2007 with moment magnitude greater than 6 0 and depth shallower than 100 km we measure the phase speeds of the fundamental mode rayleigh and love waves working with the conventional two station technique phase speed maps of the japanese islands and the sea of japan are obtained from the inversions of inter station phase speeds that are measured for a large number of station pairs we have collected over 5300 paths for rayleigh waves and 3800 paths for love waves the phase speed maps of rayleigh waves in the period range from 25 to 140 s are inverted to obtain an isotropic 3d shear wave speed model in the depth range from 40 to 200 km the isotropic s wave model represents prominent fast wave speed anomalies in northeastern japan associated with the subducting pacific plate fast anomalies of the lithosphere beneath the sea of japan are also imaged implying that the average thickness of lithosphere beneath the yamato rise and japan basin is about 60 10 km immediately below this oceanic lithosphere slow anomalies are found in the asthenosphere beneath the entire region of the sea of japan in the depth down to about 200 km in southwestern japan the low angle subduction of the philippine sea plate is mapped clearly indicating that the northern end of the plate extends beyond the northern coast of the chugoku region reaching beneath the oki islands in the sea of japan a strong low velocity anomaly off the west coast of kyushu is found in the depth down to about 130 km which supports the existence of an upwelling flow beneath this area we also invert the phase speed maps of rayleigh and love waves in the period range from 30 to 80 s simultaneously to construct the radially anisotropic shear wave speed model a remarkable anisotropy with sh sv is found in the mantle wedge beneath northeastern japan while only weak anisotropy is seen beneath the sea of japan such anisotropic properties which cannot readily be obtained from body wave studies only will be an important key to understanding the dynamic processes in this tectonically actively region

tomographic evidence for the subducting oceanic crust and forearc mantle serpentinization under kyushu japan we determined high resolution three dimensional p and s wave velocity vp vs structures beneath kyushu in southwest japan using 177 500 p and 174 025 s wave arrival times from 8515 local earthquakes a poisson s ratio structure was derived from the obtained vp and vs values our results show that significant low vp low vs and high poisson s ratio zones are extensively distributed along the volcanic front in the uppermost mantle which extend and dip toward the back arc side in the mantle wedge in the crust low vp low vs and high poisson s ratio anomalies exist beneath the active volcanoes the subducting philippine sea slab is clearly imaged as a high vp high vs and low poisson s ratio zone from the nankai trough to the back arc a thin low velocity zone is detected above the subducting philippine sea slab in the mantle wedge and earthquakes in the upper mantle are distributed along the transition zone between this thin low velocity zone and the high velocity philippine sea slab which may imply that oceanic crust exists on the top of the slab and the forearc mantle wedge is serpentinized due to the slab dehydration the seismic velocity of the subducting oceanic crust with basaltic or gabbroic composition is lower than that of the mantle according to the previous studies the serpentinization process could also dramatically reduce the seismic velocity in the forearc mantle wedge c 2008 elsevier b v all rights reserved

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

multiple layers of seismic anisotropy and a low velocity region in the mantle wedge beneath japan evidence from teleseismic receiver functions 1 a complete characterization of seismic anisotropy can yield powerful constraints on mantle flow and deformation this is particularly important for the mantle wedge above subducting slabs where the geometry of mantle flow remains poorly understood we seek to better characterize the geometry and strength of anisotropy in the mantle wedge beneath northeast honshu and hokkaido both of which overlie the subducting pacific plate previous studies indicate that upper mantle anisotropy in the japan subduction zone is highly complex and exhibits dramatic spatial variations to provide complementary constraints on the along strike variations in anisotropy we analyze teleseismic receiver functions from stations of the broadband f net array using the multitaper correlation receiver function estimator backazimuthal variations in p to sh converted energy provide clear evidence for complicated anisotropic structure in the mantle wedge beneath northeast honshu and hokkaido in northeast honshu forward modeling of receiver functions using synthetic seismograms suggests the presence of an anisotropic layer in the forearc mantle wedge above the subducting slab and a second anisotropic layer beneath the crust of the overriding plate we also see evidence for a region of low isotropic velocity in the central part of the wedge beneath ne honshu comparisons between transverse component receiver functions at stations located in ne honshu and hokkaido reveal striking differences providing further evidence for along strike variation in anisotropic structure in the mantle wedge beneath japan

trench parallel anisotropy produced by serpentine deformation in the hydrated mantle wedge although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine the strong trench parallel anisotropy observed in several subduction systems is difficult to explain in terms of olivine anisotropy even if the entire mantle wedge were to act as an anisotropic source using high pressure deformation experiments ikuo katayama and colleagues show that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in such subduction systems although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine the strong trench parallel anisotropy observed in several subduction systems is difficult to explain in terms of olivine anisotropy using high pressure deformation experiments it is now shown that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in such subduction systems seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the earth s interior as it primarily reflects the deformation induced preferred orientation of anisotropic crystals1 2 although seismic anisotropy in the upper mantle is generally attributed to the crystal preferred orientation of olivine3 the strong trench parallel anisotropy delay time of one to two seconds observed in several subduction systems4 5 is difficult to explain in terms of olivine anisotropy even if the entire mantle wedge were to act as an anisotropic source here we show that the crystal preferred orientation of serpentine the main hydrous mineral in the upper mantle can produce the strong trench parallel seismic anisotropy observed in subduction systems high pressure deformation experiments reveal that the serpentine c axis tends to rotate to an orientation normal to the shear plane during deformation consequently seismic velocity propagating normal to the shear plane plate interface is much slower than that in other directions the seismic anisotropy estimated for deformed serpentine aggregates is an order of magnitude greater than that for olivine6 and therefore the alignment of serpentine in the hydrated mantle wedge results in a strong trench parallel seismic anisotropy in the case of a steeply subducting slab this hypothesis is also consistent with the presence of a hydrous phase in the mantle wedge as inferred from anomalously low seismic wave velocities7

seismic anisotropy of the pacific slab and mantle wedge beneath the japanese islands 1 the mantle beneath the japanese islands is complex because of subduction of the pacific and philippine sea plates and the deformation associated with it detection of seismic anisotropy should be useful for understanding the processes occurring in such mantle here we resolve seismic anisotropy of the mantle wedge from that of the underlying subducted pacific slab using a large number of measurements of shear wave splitting for a family of core reflected shear phases scs sscs scs2 and sscs2 the anisotropy of the mantle wedge changes sharply across the volcanic front on the pacific side of the volcanic front a vertically propagating shear wave is polarized with the fast direction approximately parallel to the trench whereas on the back arc side it is polarized with the fast direction approximately parallel to the plate convergence direction the pacific slab is uniformly anisotropic with the nnw fast direction consistent with the paleospreading direction of the northwestern pacific seafloor the preferential alignment of anisotropic crystals frozen in the plate at the time of its formation appears to be preserved down to depths beyond 400 km within the slab

p wave anisotropic tomography beneath northeast japan we determined a 3 d p wave anisotropic tomography beneath northeast japan by using first p arrival times the travel time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming p wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally the geometries of the conrad and moho discontinuities and the upper boundary of the subducting pacific plate are taken into account in the model our extensive synthetic tests show that the anisotropic velocity inversion method is reliable our results show that low velocity zones exist below active volcanoes in the crust in the fore arc mantle wedge and in the central portion of the mantle wedge above the high velocity pacific slab which are similar to the previous tomographic images anisotropic velocity variations are revealed in the crust mantle wedge and the subducting pacific slab the anisotropic patterns are complex and different in the upper and lower crust in the mantle wedge the fast direction of p velocity is generally trench parallel in the fore arc area while it becomes trench normal in the back arc area which agree well with the shear wave splitting results the trench parallel fast direction is revealed beneath the volcanic front which may suggest the existence of complex 3 d mantle flows in the mantle wedge the fast velocity direction in the subducting pacific slab is mostly n s which is likely due to the original fossil anisotropy of the pacific plate formed at the mid ocean ridge our result also suggests that the anisotropy is weak in the upper crust and the average amplitude of the velocity anisotropy in the mantle is no larger than 4

planform of small scale convection under the island arc we have proposed an existence of small scale convection in the mantle wedge in order to explain the alignment of group of volcanoes in the ne honshu subduction zone japan the roll type thermal convection whose axis is normal to the plate boundaries and flip flopping of rolls warmer and colder parts exchange their positions as time passes are reported however the subsequent study shows an existence of non flip flopping rolls also thus in this study i investigate the possible causes of these differences systematically i found that along arc wavelength of small scale convection is controlled by two length scales which may be associated with the inclined nature of the bottom of the low viscosity mantle wedge where the small scale convection emerges when the convection is in early stage of evolvement or the speed of subducting slab is small the long wavelength rolls become prominent as the convection evolves or the speed of subduction increases short wavelength rolls tend to take over the long wavelength rolls the transition from the long wavelength to the short wavelength rolls occurs in a several way it may occur through the splitting of rolls adjustment of roll patterns and flip flopping if the pattern becomes short wavelength roll it becomes stable the range of existence of flip flopping is limited and the existence of viscosity jump in the top thermal boundary layer may be required

mapping the crustal structure under active volcanoes in central tohoku japan using p and pmp data 1 we present high resolution tomographic images of the crust under the active arc volcanoes in the central part of northeast japan tohoku determined by using arrival times of first p waves and post critically reflected waves from the moho discontinuity pmp results of detailed resolution analyses show that the addition of pmp data can improve significantly the resolution of the lower crustal structure after the pmp data are included in the tomographic inversion the low velocity low v anomalies in the lower crust under the active volcanoes can be better imaged the low v zones are clearly visible in the entire crust beneath the volcanoes extending from the surface down to the moho discontinuity arc magma related deep low frequency microearthquakes are located around the low v zones in the lower crust under the volcanoes which occurrence is probably associated with the movement of fluid magma under the volcanoes

shear wave splitting beneath the southwestern kurile arc and northeastern japan arc a new insight into mantle return flow 1 shear wave splitting in the southwestern part of the kurile arc and the northeastern ne japan arc is investigated using the waveforms from local earthquakes for both arcs observed shear wave splitting shows clear evidence for a striking rotation of fast direction across the arc suggesting the different feature of anisotropy between the fore arc and back arc sides trench parallel fast directions are observed in the fore arc side which is consistent with the anisotropy expected from the deformation of the b type olivine fabric as well as the anisotropy in the slab fast directions observed in the back arc side show approximately e w and ese wnw in the ne japan arc and n s in the southwestern kurile arc which are characterized by the local maximum dip direction of the subducted pacific plate not by the direction of plate convergence this relationship suggests that the direction of the mantle return flow is governed by the local slab geometry