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

seismic imaging of arc magma and fluids under the central part of northeastern japan abstract a dense seismic network was temporarily deployed during 1997 1999 in the central part of northeastern japan by japanese university research groups using the local earthquake arrival time data recorded by the network we have determined high resolution three dimensional images of p wave velocity vp s wave velocity vs and vp vs ratio in this area our results show that low vp low vs and high vp vs areas are extensively distributed in the uppermost mantle along the volcanic front in the lower crust low vp low vs and high vp vs areas are visible but they are confined to the individual volcanic areas in contrast the upper crust of volcanic areas shows low vp low vs and low vp vs rather than high vp vs these observations suggest that partial melting materials exist in the uppermost mantle along the volcanic front and they spread up to the midcrust of the volcanic areas low vp low vs and low vp vs in the upper crust of the volcanic areas suggest the presence of h2o rather than melt there deep low frequency microearthquakes perhaps caused by the rapid movement of fluids are located mostly at the edge of partial melting zones particularly at their upper surface distinct s wave reflectors bright spots with large reflection coefficients are distributed in the midcrust with slightly low vp vs suggesting that most of them are filled with h2o

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

b type olivine fabric in the mantle wedge insights from high resolution non newtonian subduction zone models several hypotheses have been proposed to explain trench parallel shear wave splitting in the mantle wedge of subduction zones these include 3 d flow effects parallel melt filled cracks and b type olivine fabric we predict the distribution of btype and other fabrics with high resolution thermal and stress models of subduction zones a composite viscous rheology is used that incorporates wet diffusion creep wet dislocation creep and a stress limiting approximated peierls creep law rheological parameters for dislocation and diffusion creep are based on deformation experiments the effects of variable viscous coupling between the slab and mantle are explored using kinematic boundary conditions that change along the top of the slab two end member models are observed which depend on the depth of partial viscous coupling between the slab and mantle 1 deep partial coupling which gives rise to cold high stress conditions in the forearc mantle and high temperature low stress conditions in the arc and back arc mantle 2 full viscous coupling at the base of a 40 km conducting lid which is characterized by high temperature and low stress the case with deep partial coupling which produces a better match with attenuation tomography and heat flow shows a large region with suitable conditions for b type fabric in the forearc mantle and a rapid transition toward the back arc to conditions that are more suitable for a c or e type fabrics full viscous coupling gives rise to high temperature low stress conditions unsuitable for b type fabric our modeling predicts that the best candidates for regions with b type fabric are the forearc mantle and a cold 10 15 km layer above the slab d 2005 elsevier b v all rights reserved

shear wave polarization anisotropy and subduction induced flow in the mantle wedge of northeastern japan abstract shear wave polarization anisotropy in the central part of northeastern ne japan was investigated we analyzed s phases of 293 intermediate depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters the obtained results show the systematic variation in splitting parameters across the arc the leading shear wave polarization directions fast direction obtained at stations in the western side of the study area are oriented nearly e w which is sub parallel to the direction of relative plate motion between the pacific plate and the north american okhotsk plate in contrast fast directions obtained at stations in the eastern side of the study area are oriented approximately n s stations in the western side have a larger average delay time compared to those in the eastern side these variations suggest that the nature of anisotropy is quite different between the eastern and the western sides both experimental studies on the deformation of olivine and numerical simulations of plate driven flow predict that the lattice preferred orientation of olivine causes the fast direction sub parallel to the flow direction for the conditions in the mantle wedge of ne japan we thus infer that the lattice preferred orientation of olivine generated by flow induced strain is the most likely candidate for anisotropy that produces splitting with e w fast direction three possible causes of anisotropy in the eastern side are considered deformation of water rich olivine in the mantle wedge trench parallel flow in the mantle wedge due to along strike dip variations in the slab and anisotropy in the crust and the slab we could not however determine which causes are dominant further systematic study is required to reveal the cause of anisotropy in the eastern side

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

azimuthally anisotropic model of the oceanic upper mantle abstract a mixed form of forward and backward modelings of azimuthally anisotropic oceanic upper mantle is attempted for a data set of surface wave velocities of periods 30 100 s one of the basic configurations of the anisotropy postulated in the lid and lvz is axial symmetry of elastic properties with a horizontal axis parallel to the spreading direction as found in ultramafic rocks from ophiolite complexes of the bay of islands and oman basic features of the resulting model are a thin lithosphere of 45 km a decrease of s wave velocities and an increase of p wave velocities and of partial melting dislocation relaxation with depth in the lvz this single model explains the overall features of anisotropy observations in the pacific ocean p n velocity anisotropy sh sv anisotropy and azimuthal anisotropy of surface waves and scs polarization anisotropy this fact leads us to a conclusion that the ultramafic rocks from the ophiolite complexes can explain the seismic properties of the oceanic lid and lvz

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

multiscale seismic tomography and mantle dynamics abstract in this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs global and regional tomography shows that most of the slab materials under western pacific and east asia are stagnant in the mantle transition zone before finally collapsing down to the core mantle boundary as a result of large gravitational instability from phase transitions local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper mantle wedge indicating that geodynamic systems associated with arc magmatism and back arc spreading are related to deep processes such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab because most hotspots are located in poorly instrumented continental and oceanic regions 3 d crust and upper mantle structure is determined for only a few hotspots such as iceland yellowstone and eifel which are covered by seismic networks and plume like slow anomalies are revealed under those hotspots global tomography has revealed deep mantle plumes under the major hotspots such as hawaii iceland kerguelen south pacific and africa strong lateral heterogeneities are revealed at the bottom of the mantle which are associated with the deeply subducted slabs and the birth of mantle plumes a thorough understanding of the deep earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks particularly in the oceans

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

subduction of the philippine sea slab in view of p wave anisotropy abstract three dimensional 3d structure of isotropic and anisotropic velocities in the chugoku and shikoku districts southwest japan is retrieved from the first p arrival times from local earthquakes by assuming p wave azimuthal anisotropy with hexagonal symmetry axis the main result of isotropic velocity structure is that the philippine sea phs slab is represented by a high velocity body with the thickness of 20 30 km lying at the around depth of 40 km beneath the shikoku and southern chugoku districts the phs slab subduction appears to be interrupted by a small scale low velocity body under middle chugoku and is not clear under north chugoku these features of 3d isotropic velocity structure are basically similar to those obtained in previous tomographic studies using recent seismic data this result suggests that seismic anisotropy has no significant effect on estimation of isotropic velocity structure from the p arrival time data on the other hand fast propagation directions of p wave azimuthal anisotropy in the crust are basically in close agreement with fast polarization directions estimated from polarization anomaly of shear waves azimuthal anisotropy in the crust of shikoku district is characterized by the fast propagation direction with the e w orientation trend which is nearly parallel to trajectory of maximum principal stress acting on the southwest japan arc and strike directions of zonal geological structure crustal anisotropy in the chugoku district is consistent with the orientation of the geological structure and not with the maximum principal stress direction these results indicate that the crustal anisotropy is attributable not only to alignment of the stress induced cracks but also to lineament of geological structures we could also point out the difference in the upper mantle anisotropy between the shikoku and chugoku districts i e p wave velocity tends to be fast in the n s directions beneath shikoku and southern chugoku and in e w directions under northern chugoku since the n s axis trend of p wave anisotropy is highly correlated with high velocity anomaly of phs slab it is regarded as slab anisotropy the reason why the p wave anisotropy shows the high propagation velocity in the e w direction beneath northern chugoku is discussed on the example of two cases in which the phs slab is absent and present in the upper mantle beneath the region although we could not locate exactly the north leading edge of the phs slab in view of anisotropic velocity structure if the phs slab is defined as a high velocity body with fast propagation axes in the directions of n s system it turns out to exist at least beneath shikoku to south chugoku

p wave anisotropic tomography of the crust and upper mantle under hokkaido japan abstract we determined a 3 d anisotropic velocity structure beneath hokkaido in northern japan using a large number of first p wave 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 results show that low velocity zones exist beneath active arc volcanoes in the crust and in the central portion of mantle wedge above the high velocity pacific slab which are similar to the previous tomographic images p wave anisotropy is revealed in the crust mantle wedge and the subducting pacific slab in the upper crust the anisotropy is possibly caused by microcracks and cracks in the lower crust the anisotropic structure is different from that in the upper crust and it may be affected considerably by the plastic flow deformation in the mantle wedge the fast velocity direction fvd is generally trench normal in most of the study area while it becomes trench parallel in the pacific coast areas close to the trench which agrees with the shear wave splitting results beneath the volcanic front the fvd is also trench parallel which may suggest the existence of complex 3 d mantle flow in the mantle wedge the fvd in the subducting pacific slab is nearly north south which likely keeps the original fossil anisotropy of the pacific plate formed at the mid ocean ridge or is affected by the olivine fabric transition from a type to b type fabric due to the changes in water content stress and temperature

75 plus years of anisotropy in exploration and reservoir seismics a historical review of concepts and methods the idea that the propagation of elastic waves can be anisotropic i e that the velocity may depend on the direction is about 175 years old the first steps are connected with the top scientists of that time people such as cauchy fresnel green and kelvin for most of the 19th century anisotropic wave propagation was studied mainly by mathematical physicists and the only applications were in crystal optics and crystal elasticity during these years important steps in the formal description of the subject were made at the turn of the 20th century rudzki stressed the significance of seismic anisotropy he studied many of its aspects but his ideas were not applied research in seismic anisotropy became stagnant after his death in 1916 beginning about 1950 the significance of seismic anisotropy for exploration seismics was studied mainly in connection with thinly layered media and the resulting transverse isotropy very soon it became clear that the effect of layer induced anisotropy on data ac

microstructures and petro fabrics of lawsonite blueschist in the north qilian suture zone nw china implications for seismic anisotropy of subducting oceanic crust abstract we conducted a detailed study on the microstructures and petro fabrics of massive and foliated lawsonite blueschist lbs in north qilian suture zone nw china the lattice preferred orientation lpo of glaucophane and lawsonite in foliated lawsonite blueschist lbs is considered to be dominantly formed by the deformation mechanism of dislocation creep and rigid body rotation respectively the lpo of glaucophane is mainly characterized by the 001 axis aligning parallel to lineation and the 100 axis and 110 pole plunging perpendicular to foliation in contrast the lpo of lawsonite features the maximum 010 axis concentrated close to lineation and the 001 axis strongly clustered normal to foliation the preferred orientation of 010 axis of lawsonite parallel to lineation is supported by a two dimensional numerical modeling using the finite volume method fvm the mineral lpos are much stronger in foliated lbs than in massive lbs in addition a kinematic vorticity analysis suggests that both pure shear dominant wm 0 18 0 26 and simple shear dominant wm 0 86 0 93 deformation regimes are present in foliated lbs the 001 axis and 010 pole of glaucophane and the 100 and 010 axes of lawsonite tend to distribute in a foliation parallel girdle in the pure shear dominant samples but simple shear dominant samples display more lineation parallel concentrations of a 001 axis of glaucophane and a 010 axis of lawsonite because the whole rock seismic anisotropies in foliated lbs are significantly higher than those in massive lbs and a counteracting effect on seismic anisotropies occurs between glaucophane and lawsonite the delay time of fast s wave polarization anisotropy induced by an actual subducting oceanic crust with a high subducting angle 45 60 is expected to range from 0 03 to 0 09 s lower bound for massive lbs and from 0 1 to 0 3 s upper bound for foliated epidote blueschist

big mantle wedge anisotropy slabs and earthquakes beneath the japan sea abstract the japan sea is a part of the western pacific trench arc backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the japan sea which was determined using p and s wave arrival times of suboceanic earthquakes relocated precisely with sp depth phases ambient noise tomography revealed a thin crust and a thin lithosphere beneath the japan sea and significant low velocity low v anomalies in the shallow mantle beneath the western and eastern margins of the japan sea observations with ocean bottom seismometers and electromagnetometers revealed low v and high conductivity anomalies at depths of 200 300 km in the big mantle wedge bmw above the subducting pacific slab and the anomalies are connected with the low v zone in the normal mantle wedge beneath ne japan suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back arc magmatism the pacific slab has a simple geometry beneath the japan sea and earthquakes occur actively in the slab down to a depth of 600 km beneath the ne asian margin teleseismic p and s wave tomography has revealed that the philippine sea plate has subducted aseismically down to the mantle transition zone mtz 410 660 km depths beneath the southern japan sea and the tsushima strait and a slab window is revealed within the aseismic philippine sea slab seismic anisotropy tomography revealed a nw se fast velocity direction in the bmw which reflects corner flows induced by the fast deep subduction of the pacific slab large deep earthquakes m 7 0 depth 500 km occur frequently beneath the japan sea western margin which may be related to the formation of the changbai and ulleung intraplate volcanoes a metastable olivine wedge is revealed within the cold core of the pacific slab at the mtz depth which may be related to the deep seismicity however many of these results are still preliminary due to the lack of seismic stations in the japan sea the key to resolving these critical geoscientific issues is seismic instrumentation in the japan sea for which international cooperation of geoscience communities in the east asian countries is necessary

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

75 plus years of anisotropy in exploration and reservoir seismics a historical review of concepts and methods geophysics 70 9nd 23nd on page 18nd right column the phrase in the valhall oil field in norway was incorrect we apologize for any inconvenience caused by this error

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

effect of temperature on the b to c type olivine fabric transition and implication for flow pattern in subduction zones abstract the effect of temperature on the b to c type fabric transition in olivine was investigated by large strain shear deformation experiments a new series of deformation experiments was conducted at 1273 1373 k 2 0 gpa and strain rates ranging from 3 8 10 4 to 3 2 10 5 s 1 under water saturated conditions and combined with previous results at 1473 k the influence of temperature on deformation fabrics was investigated two types of olivine fabrics were identified one in which the olivine 0 0 1 axis is subparallel to the shear direction and the 0 1 0 plane is parallel to the shear plane b type and another one in which the 0 0 1 axis is parallel to the shear direction and the 1 0 0 plane is parallel to the shear plane c type the b type olivine fabric occurs at higher stress than the c type but the stress magnitude at which the b to c type fabric transition occurs decreases significantly with decreasing temperature the temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study when the experimental results are extrapolated to lower temperatures the fabric transition between b and c type is predicted to occur at 1 mpa and 1000 k this prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate convergent regions the present study indicates that when some amounts of water 200 ppm h si are present the b type fabric will dominate in low temperature regions of the upper mantle whereas the c type or e type fabric will dominate at high temperatures the complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables such as temperature and water content rather than by complexities in mantle wedge flow pattern

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

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

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

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

a global study of pn anisotropy beneath continents we apply a two station method of velocity determination to p n arrival times reported in the international seismological centre bulletin during the period 1964 1993 the azimuthal variation of these velocities provides evidence of p n anisotropy as large as 9 6 anisotropic regions are identified by examination of the azimuthal variation of the p n velocities within circular caps of radius 1 5 and 3 spaced at 1 intervals across the earth this procedure leads to 255 geographically distributed estimates of anisotropy the length scale over which the anisotropy varies indicates that uppermost mantle anisotropy beneath continents is controlled by regional orogenic strains and is inconsistent with a general interpretation in terms of plate scale mantle flow in addition the correlation between p n anisotropy and surficial tectonic features indicates that the anisotropy is primarily the result of the most recent deformation event we compare our p n anisotropy results fast directions and magnitudes with those of sks splitting studies to attempt to constrain the depth extent of the anisotropy this comparison suggests that in some areas where the fast directions agree sks splitting observations may arise from a layer beginning just below the moho whereas in others it must have a deeper origin in these areas the relative magnitudes of the measured anisotropy are consistent with a thin 100 150 km lithospheric layer of anisotropy

uppermost mantle velocities and anisotropy beneath europe pn data collected within southern europe and the mediterranean are used to tomographically image variations in both seismic velocity and seismic anisotropy seismic anisotropy is an essential part of the inversion and without it several low velocity features within the uppermost mantle could not be properly imaged the technically active mantle of southern europe has much lower seismic velocities 7 6 8 1 km s than the more stable mantle of the sub african plate of the adriatic sea 8 3 km s however the most dramatic features within europe s uppermost mantle relate to the apennine dinaride and hellenide arcs these arcs all have extremely low 5 amounts of arc parallel anisotropy the tyrrhenian and aegean back arc regions also show low velocities 7 7 7 9 km s but less anisotropy the same may be true for the pannonian basin but the tomography has poorer resolution there a model explaining these observations focuses on subducted water metasomatizing the mantle wedge the addition of water causes melting creates arc volcanism lowers the seismic velocity and enhances the formation of anisotropy due to preferential olivine orientation for collisional arcs of the northern mediterranean arc parallel anisotropy has formed in response to compression across them and extension along them within back arc regions water is no longer a major factor instead convection associated with subduction and back arc extension controls the anisotropy

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

stress strain and b type olivine fabric in the fore arc mantle sensitivity tests using high resolution steady state subduction zone models 1 shear wave splitting observations at several subduction systems show trench parallel fast directions in the fore arc mantle the presence of b type olivine fabric in the mantle wedge may provide an explanation for this pattern of anisotropy under low temperature and hydrated conditions sensitivity tests are shown that provide insights into the distribution and magnitude of b type fabric using two dimensional high resolution kinematic dynamic subduction zone models these models include a wet olivine rheology and a simulated aseismic creep zone in the fore arc mantle with parameterized viscous coupling between the slab and mantle wedge the calculated thermal structure is in reasonable agreement with heat flow observations and q tomography for small amounts of coupling and moderate amounts of shear heating in the seismogenic and aseismic creep zones these small amounts of viscous coupling give rise to a slowly flowing fore arc mantle with thermal stress and strain conditions suitable for significant b type fabric development the minimum shear stress necessary to drive geologically significant flow in the predicted b type region is sensitive to the magnitude of shear heating along the aseismic creep zone and ranges from less than 100 to 300 mpa the minimum time required to generate sufficient finite strain for fabric development in the b type region is around 10 myr depending on the amount of coupling and shear heating the predicted distribution of olivine fabric is consistent with a pattern of shear wave anisotropy with trench parallel fast polarization directions above the fore arc and a rapid rotation to trench perpendicular in the arc and back arc

seismic velocity structure of the crust beneath the japan islands abstract more than 13 000 arrival times of 562 local shallow earthquakes selected from the japan university network earthquake catalog are used to investigate the seismic velocity structure of the crust beneath the japan islands we simultaneously determined the hypocenter parameters p and s wave station corrections and depth distributions of the conrad and moho discontinuities beneath the whole of the japan islands by applying an inversion method the p and s wave station corrections show similar distribution patterns they are negative relatively late arrivals of seismic waves in hokkaido and positive relatively early arrivals of seismic waves in western japan in eastern honshu they are positive along the coast of the pacific ocean and negative in the land area and along the coast of the japan sea the conrad discontinuity is located at a depth range from 12 to 22 km in hokkaido the conrad is shallow in the eastern pan and becomes deep toward the west in ne honshu the conrad is deep beneath the land area and becomes shallow toward the surrounding seas in western japan the conrad is shallow in the northern part and becomes deep toward the south the moho is located in a depth range from 25 to 40 km in hokkaido the moho has the shape of a circular cone with a maximum depth of 36 km in most parts of the japan islands the moho is deep beneath the land area and becomes shallow toward the surrounding seas the moho is deepest up to 40 km beneath the central part of the chubu district

fine structure beneath the tohoku district northeastern japan arc as derived by an inversion of p wave arrival times from local earthquakes abstract three dimensional p wave velocity structure beneath the tohoku district northeastern japan arc is investigated by an inversion of arrival times from local earthquakes using the method originally due to aki and lee 1976 in the crust 0 32 km depth a low velocity region is found along the volcanic front and its vicinity velocities at depths of 32 65 km are low beneath the regions where many quaternary volcanoes and geothermal areas are distributed in the region deeper than 65 km the subduction of the pacific plate is clearly revealed and the mantle structure above the descending plate is rather uniform these features suggest that volcanic activities have relation to the upper mantle structure the results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone

outer slope faulting associated with the western kuril and japan trenches summary elongated fault escarpments on the outer slopes of the western kuril and japan trenches have been investigated through detailed swath bathymetric mapping numerous horsts and grabens formed by these escarpments were identified distinct n70 e linear alignment of the escarpments parallel to the magnetic anomaly lineations was revealed on the outer slope of the western kuril trench in the japan trench north of 39 00aen most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations a zig zag pattern of faulting exists south of 39 00aen each topographic profile was decomposed by computer analysis into two curves representing 1 the smoothed long wavelength slope of the subducting ocean crust surface and 2 the short wavelength 10 km roughness of plateaus and valleys edged by outwardand inward facing fault escarpments throughout the surveyed areas escarpment heights increase from the crest of the trench outer swell down to a depth of about 6000 m on the slope of the outer trench wall but with no distinct increase below that depth no significant diverence is recognized in fault throws towards and away from the trench it can be concluded that these elongated escarpments originate from normal faults on the upper layer of the oceanic crust under extensional stress in a direction perpendicular to the trench axis which is caused by downward bending of the subducting lithosphere the relationship of escarpment height to escarpment length is similar to that obtained from normal fault escarpments in the east pacific rise crest the maximum length and height of escarpments are small in the kuril trench compared with those in the japan trench implying a diverence in mechanical strength depending on the fault orientation the crust is weakest along the inherited spreading fabric second weakest probably along the non transform ovset direction and strongest in directions very oblique to these orientations seamounts appear to be more rigid than normal ocean crust with no particular weak orientations resulting in fewer but larger faults along the axis of plate bending as most clearly represented in the subducting daiichi kashima seamount

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

on the trade off between seismic anisotropy and heterogeneity numerical simulations and application to northeast japan p wave tomography has been recently used to study 3 d azimuthal and radial anisotropy of subduction zones and continental regions however the fundamental issue about the trade off between the isotropic and anisotropic structures is still unclear in this study we investigate this issue systematically with comprehensive synthetic tests our results indicate that good ray coverage in the azimuth for azimuthal anisotropy and incidence for radial anisotropy is required for determining reliable anisotropic models the isotropic and anisotropic structures are strongly coupled and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence we therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis i e the ratio of the short axis and long axis lengths for estimating the ray coverage quantitatively applying our novel approach to a large number of high quality arrival time data of local shallow and intermediate depth earthquakes we obtained new tomographic images of 3 d p wave azimuthal and radial anisotropy in northeast japan both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab our results show dominant trench normal and vertical fast anisotropy in the mantle wedge while trench parallel and horizontal fast anisotropy in the subducting slab which indicates different dynamics in different domains of the subduction zone

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

three dimensional distribution of seismic anisotropy in the hikurangi subduction zone beneath the central north island new zealand 1 we use earthquake arrival time data in the central north island new zealand in inversion for 3 d vp azimuthal anisotropy it is parameterized with an isotropic component and two azimuthal anisotropy parameters for each node and local earthquake shear wave splitting observations define the initial anisotropy model our results suggest that mantle flow either within the mantle wedge or below the subducted slab is not the primary source of anisotropy in the hikurangi subduction zone the largest region of anisotropy that we image is within the subducted slab where anisotropy is consistently trench parallel and 5 9 magnitude from 32 to 185 km depth bending induced yielding in the slab offshore and as it passes through the trench provides an explanation for this slab anisotropy beneath the taupo volcanic zone tvz there is 1 4 trench normal anisotropy within 30 km of the surface of the slab within a region entrained with motion of the slab in contrast only very weak 0 2 anisotropy is imaged in the region of high temperature partial melt arising from active corner flow in the mantle wedge in the crust geologic structure is important with the largest crustal anisotropy 14 related to schist we image significant anisotropy aligned with the margins of the tvz related to extensive fracturing on the margins as the tvz actively extends within the heavily intruded and underplated tvz lower crust the orientation of anisotropy switches to align with the extension direction this orientation is consistent with other geophysical data suggesting the presence of connected melt in this region

subducting plate boundary beneath the northeastern japan arc estimated from sp converted waves abstract in the tohoku district the northeastern part of honshu japan a conspicuous phase is frequently observed between p and s phases in seismograms for events in the lower seismic plane of the double planed deep seismic zone this phase is dominant in the vertical component and has an apparent velocity almost equal to or slightly smaller than the p phase we have never found this phase in seismograms for events in the upper seismic plane from these characteristics the phase is interpreted as sp converted from s to p at the upper boundary of the descending oceanic plate ubp using the travel times of the sp waves we estimated the location of the ubp beneath the tohoku district to be just above the upper seismic plane of the double planed deep seismic zone this position is consistent with those determined from the analyses of the travel times of the scsp and ps waves which are respectively the seismic wave converted from scs to p and that from p to s at the ubp

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

phosphorus in sediments of the amazon river and estuary implications for the global flux of phosphorus to the sea abstract the distribution of various forms of p in suspended and depositional bank sediments of the amazon river and bottom sediments of the mixing and freshwater portions of the amazon estuary and shelf have been determined using a modified sequential extraction method forms of p distinguished are p associated with ferric oxides hydroxides fe p organic p org p p extracted with acetate buffer at ph 4 acet p and detrital apatite detr p results indicate that there is a distinct drop in fe p org p and total p after deposition of suspended sediment on the banks of the river similar results are found for sediment deposited on the freshwater portion river channel bottom of the continental shelf sediment deposited on estuarine portions of the shelf also shows a significant drop in total p due mostly to a drop in org p no appreciable change in acet p was found between river suspended sediments and those of either the river banks or of the estuarine shelf an inverse correlation is found for all sediments between concentrations of fe p and detr p because of the association of detrital apatite with the coarser fractions of the sediments and fe p with the finer fractions these results suggest that considerable amounts of p are added to the oceans by the bacterial decomposition of river transported organic matter and by the desorption of p from iron oxides hydroxides some desorption from al silicates may also occur the organic p is released to interstitial solution during early diagenesis and this p may be adsorbed by fe oxides in the sediment during frequent resuspension events the fe oxides are carried to the overlying water where the p is desorbed and thereby released to solution based on our amazon results the total global p flux to the oceans by rivers may be more than three times that carried only in solution by the rivers further work is needed on other river systems to check these preliminary results

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

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

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

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

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

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 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

mapping p wave anisotropy of the honshu arc from japan trench to the back arc abstract we determined a 3 d p wave anisotropic tomography beneath the entire honshu arc using about 448 000 high quality p wave arrival times from 18 335 local earthquakes that occurred beneath the northeast ne japan land area and the fore arc area under the pacific ocean our results show that low velocity low v zones exist beneath the active arc volcanoes in the crust and in the central portion of the mantle wedge above the subducting pacific slab the low v anomalies are related to the arc magmatism low v zones are also revealed in the fore arc area which are probably caused by large volumes of water releasing upwards from dehydration of the subducting oceanic crust and sediments the anisotropic amplitude in the upper crust is weaker than that in other portions under ne japan in the mantle wedge the fast velocity direction fvd is generally trench normal in back arc area which may reflect that the olivine a axis aligns with the transport direction induced by the slab driven corner flow the fvd becomes trench parallel in the central portion of the fore arc mantle wedge which is possibly induced by the olivine of b type fabric in the slab driven corner flow the fvd shows trench parallel in the low v zones in the fore arc mantle wedge close to the upper boundary of the pacific slab which may reflect the b type olivine fabric dominating in those areas or it may be induced by the dextral shearing of the overlying crust the trench parallel fvd is also revealed beneath the volcanic front indicating complex 3 d mantle flows in the mantle wedge the fvd in the subducting pacific slab is mostly trench parallel which may reflect one or more of the following possibilities 1 the original fossil anisotropy of the pacific plate formed at the mid ocean ridge 2 cracks within the slab and 3 the olivine fabric transition due to the changes in water content stress and temperature

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3