seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

is there a relationship between magmatic fabrics and brittle fractures in plutons a view based on structural analysis anisotropy of magnetic susceptibility and thermo mechanical modelling of the tanvald pluton bohemian massif abstract the present study focuses on the thermal and mechanical aspects of cooling of plutonic systems with respect to their internal structure analysis of the tanvald pluton and the nearby liberec granite the krkonose jizera plutonic complex bohemian massif using the anisotropy of magnetic susceptibility ams method indicates the decoupled magnetic fabrics formed by different processes in these two plutons two regional sets of vertical fractures cut across the plutons and their host rocks they are interpreted as having been formed as opening mode fractures joints after emplacement and cooling our thermo mechanical finite element modelling indicates that the thermal stresses within the tanvald pluton were compressive and that fracture criteria were not met during pluton cooling these results imply that the emplacement depth and confining pressure significantly affect the state of stress within a pluton during cooling and may play a crucial role in the formation of early fractures in plutons consequently the formation of early fractures during pluton cooling is driven by external conditions overriding the role of internal magmatic fabrics we argue that no genetic geometric or temporal correlation can exist between magmatic fabrics and fractures in granitoid plutons and thus that the original concept of hans cloos based on fabric joints relationships should be reconsidered instead an alternative classification of fractures in plutons should include cooling fractures syntectonic fractures uplift fractures and post uplift fractures

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

seismic structure of the lithosphere from teleseismic converted arrivals observed at small arrays in the southern sierra nevada and vicinity california 21 well distributed teleseisms 30 to 100 distance were recorded by mixed broad band bb and short period sp seismic arrays at mineral king mk and horseshoe meadow hm in the southern sierra nevada and at darwin plateau dp between the inyo and argus ranges these arrays permit identification and separation of direct p and s arrivals reflections from topography scattered energy and arrivals from different back azimuths multipath arrivals p to s conversions can be identified from beamed bb or sp seismograms least squares time domain processing recovered single event receiver functions from these beamed seismograms converted phases attributable to the moho are clear and uncomplicated at dp ps p times of 3 9 4 2 s correspond to crustal thicknesses of 31 33 km assuming a mean p crustal velocity of 6 2 km s and a poisson s ratio of 0 255 ps p times at hm eastern sierra are 3 9 4 6 s 32 37 km excepting some unusual seismograms from events at back azimuths of 225 239 mk western sierra times for ps p show a strong e w asymmetry 3 9 4 1 s 31 33 km from the east and 4 8 5 3 s 39 42 km from the west the mk arrivals from the east are multiple and substantially weaker by comparison with hm and dp these results confirm the absence of a thick crust under the southern high sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies at dp the moho event follows an intracrustal negative polarity event defining the top of an s wave low velocity zone this feature dips west under the sierran crest at hm but is absent farther west at mk this feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed sierra overlies a tectonized lower crust sub moho energy is absent under the basin and range dp but is conspicuous under the high sierra at positive arrivals 7 3 s mk and 9 s hm after the p these arrivals might be from the base of a low velocity low density upper mantle body supporting the sierra

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

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

velocity and anisotropy structure at the hikurangi subduction margin new zealand from receiver functions summary we examine seismic velocity and anisotropy at the interface between the australian and the subducting pacific plate in southern north island new zealand using receiver functions from teleseismic earthquakes recorded on two l shaped seismic arrays iris station snzo makes up the corner of the southern array velocity models are tested with a ray theoretical method that allows both dipping boundaries and anisotropy assuming a 2 d structure beneath each array the strongest arrival on the northern array consistently appears at 2 s the arrival is modelled by a jump in velocity at a flat boundary at 12 km which may be caused by a change in metamorphic grade this prominent arrival correlates with a weaker arrival at 1 5 s 7 km at station snzo isotropic layers dipping with the slab can explain many features in the first 20 s of the records but anisotropy is needed to explain both the transverse receiver functions and a change in the polarity of a prominent ps arrival at 3 s delay relative to p o nthe radial component this arrival is consistent with a change between an upper layer that is either isotropic or anisotropic with a horizontal symmetry axis and a supraslab metasediment layer having anisotropy with av ertical slow axis of symmetry there is tradeoff between boundary dip the plunge of the axis of symmetry layer thickness and the amount of anisotropy a dip of 20 shallowing to 15 under the tararua array thickness of 4 km and a vertical symmetry axis fits the backazimuth variation of the amplitudes and polarities of the arrivals with anisotropy of 4 6 per cent observed layer multiples are smaller than synthetic predictions to explain this discrepancy either layer boundaries are rough attenuation is strong or the layer boundaries are more gradational at the periods 1 s used in this study compared to previous studies 5 s

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

configuration of subducting philippine sea plate beneath southwest japan revealed from receiver function analysis based on the multivariate autoregressive model 1 we construct a detailed image of velocity discontinuity beneath southwest japan by using receiver functions we first propose an improved receiver function estimation method based on the statistical multivariate autoregressive model then we apply the new method to the teleseismic waveform data recorded by the high density seismograph network in southwest japan the results show a clear velocity discontinuity at 30 km depth beneath the southern shikoku region this discontinuity corresponding to the boundary between the oceanic crust and the high velocity layer the oceanic moho of the philippine sea plate phs continues down to the north indicating that the aseismic phs extends to the central chugoku region the continental moho is also clearly imaged beneath the chugoku region the depth contour of the phs shows a rather complicated feature with ridges and valleys the most significant ridge is located around longitude 133 e west of which the contours are basically directed north south changing to west east to the east beneath the western part of this ridge hypocenters of microearthquakes are located above this velocity discontinuity while earthquakes mainly occur below the discontinuity within the oceanic mantle beneath the eastern part of the ridge

the ronda peridotite spain a natural template for seismic anisotropy in subduction wedges the ronda peridotite spain a natural template for seismic anisotropy in subduction wedges

crustal structure across the middle ryukyu trench obtained from ocean bottom seismographic data abstract in 1988 an extensive geophysical investigation was performed at the middle ryukyu island arc s japan an aim of the investigation is to obtain a detailed crustal structure at the trench arc back arc system by various geophysical surveys in this paper we present fine crustal models obtained from ocean bottom seismographic obs data along two profiles one profile was acquired from the northwest edge of the philippine sea plate to the okinawa island profile 2 a and the other profile which is perpendicular to profile 2 a was located on the ryukyu arc profile 3 a crustal model of profile 2 a is characterized by a sedimentary wedge and subducting oceanic crust the sedimentary wedge consists of four layers whose velocities are 1 8 2 8 2 9 3 5 and 4 7 5 0 km s at the top of each layer the maximum thickness of the wedge is 9 km at 50 km landward from the trench underlying the sedimentary wedge the model shows subducting oceanic layer 2 and 3 5 3 and 6 8 km s a crustal model of profile 3 indicates three layers of sediment whose velocities are 1 7 2 0 2 4 2 8 and 3 7 km s above a 5 6 6 0 km s layer a low q value layer at the bottom of the sedimentary layer is required to explain an observed amplitude reduction versus offsets three deep crustal reflection phases are observed along profile 3 these phases are interpreted as the reflection phases from the oceanic crust subducting beneath the okinawa island the depth of the subducting oceanic moho is deduced to be 26 km at 120 km landward from the trench axis by the reflection phases

sub slab anisotropy beneath the sumatra and circum pacific subduction zones from source side shear wave splitting observations understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid earth system observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs providing a tool for studying subduction dynamics while many observations of seismic anisotropy have been made in subduction systems our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub slab region recently the source side shear wave splitting technique has been refined and applied to several subduction systems worldwide making accurate and direct measurements of sub slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub slab mantle here we present source side shear wave splitting measurements for the central america alaska aleutians sumatra ryukyu and izu bonin japan kurile subduction systems we find that measured fast splitting directions in these regions generally fall into two broad categories aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate trench parallel fast splitting directions dominate beneath the izu bonin japan and southern kurile slabs and part of the sumatra system while fast directions that parallel the motion of the downgoing plate dominate in the ryukyu central america northern kurile western sumatra and alaska aleutian regions we find that plate motion parallel fast splitting directions in the sub slab mantle are more common than previously thought we observe a correlation between fast direction and age of the subducting lithosphere older lithosphere 95 ma is associated with trench parallel splitting while younger lithosphere 95 ma is associated with plate motion parallel fast splitting directions finally we observe source side splitting for deep earthquakes transition zone depths beneath japan and sumatra suggesting the presence of anisotropy at midmantle depths beneath these regions

shear wave anisotropy beneath nicaragua and costa rica implications for flow in the mantle wedge 1 we present new shear wave splitting data from local events in costa rica and nicaragua recorded by the temporary july 2004 to march 2006 48 station tucan broadband seismic array observed fast polarization directions in the fore arc arc and back arc range from arc parallel to arc normal over very short distances 5 km when plotted at raypath midpoints making the direct interpretation of individual splitting measurements in terms of flow tenuous even when considering variations in the relationship between lattice preferred orientation and deformation e g b type dislocation creep in olivine therefore we tomographically invert the splitting measurements to find a three dimensional model of crystallographic orientation in the wedge we assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped iterative least squares approach to account for the nonlinear behavior of splitting when considering three dimensional ray propagation and distributions of anisotropy the best fitting model contains roughly horizontal arc parallel olivine 100 axes in the mantle wedge down to at least 125 km beneath the back arc and arc which we interpret to indicate along arc flow in the mantle wedge pb and nd isotopic ratios in arc lavas provide additional evidence for arc parallel flow and also constrain the direction northwest from costa rica to nicaragua and minimum flow rate 63 190 mm a with only slightly oblique subduction at 85 mm a of the relatively planar cocos plate the most likely mechanism for driving along arc transport is toroidal flow around the edge of the slab in southern costa rica generated by greater slab rollback in nicaragua two important implications of this arc parallel flow are the progressive depletion of the mantle source for arc lavas from costa rica to nicaragua and the possible need for significant decoupling between the wedge and downgoing plate

p sh conversions in layered media with hexagonally symmetric anisotropy a cookbook reflectivity synthetic seismograms demonstrate that the type layering and orientation of 1 d anisotropy influences strongly the coda of teleseismic p waves at periods t 1 sec particularly p sh converted waves we assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis of arbitrary orientation the resulting phase velocities vary as cos 2 with respect to that axis using three families of simple crustal models we compare the effects of an anisotropic surface layer with reverberations caused by both thick and thin layers of anisotropy at depth if anisotropy in the surface layer is significant the polarization of direct p can be distorted to generate a transverse component followed by ps and a prominent shear reverberation converted from direct p at the free surface if the anisotropic layer is buried the first and often the most prominent arrival on the transverse component is the p to sh conversion at its upper surface if the anisotropic layer is sufficiently thin p to sh conversions from its boundaries interfere to form a derivative pulse shape on the transverse component which could be mistaken as the signature of shear wave splitting if is horizontal compressional p and shear s anisotropy both produce similar waveform perturbations with four lobed azimuthal patterns suggesting that a weighted stack of p coda from different back azimuths would improve signal to noise for tilted between the horizontal and vertical however the effects of p and s anisotropy differ greatly the influence of p anisotropy on p to s conversion is greatest for a symmetry axis tilted at 45 to the vertical where its azimuthal pattern has two lobes rather than four combinations of p and s anisotropy typically lead to a composite azimuthal dependence in the p coda reverberations

b type olivine fabric and mantle wedge serpentinization beneath the ryukyu arc 1 subduction is a fundamental earth process yet many aspects of subduction systems remain to be fully understood here we present anisotropic receiver function rf analysis to characterize the structure of the mantle wedge beneath the ryukyu subduction zone radial and transverse p to s rfs computed for eight broadband stations yield evidence for coherent p to sv conversions originating at the top of the subducting slab we also observe conversions on the transverse component rfs that are consistent with the presence of multiple anisotropic layers with structure varying significantly along strike forward modeling of observations at station zmm in the center of the ryukyu arc yields evidence for a 6 km thick layer of strongly anisotropic material most likely serpentinite directly above the subducting slab and likely b type olivine fabric in the shallow mantle wedge

elasticity of antigorite seismic detection of serpentinites and anisotropy in subduction zones abstract serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites which thus far has not been possible in the absence of single crystal elastic properties of antigorite the elastic constants of antigorite the dominant serpentine at high pressure in subduction zones were measured using brillouin spectroscopy under ambient conditions in addition antigorite lattice preferred orientations lpo were determined using an electron back scattering diffraction ebsd technique isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography the isotropic v p v s ratio is 1 76 in the voigt reuss hill average not very different from that of 1 73 in peridotite but may vary between 1 70 and 1 86 between the voigt and reuss bonds antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions v p varies between 8 9 km s 1 and 5 6 km s 1 46 anisotropy and 8 3 km s 1 and 5 8 km s 1 37 and v s between 5 1 km s 1 and 2 5 km s 1 66 and 4 7 km s 1 and 2 9 km s 1 50 for the single crystal and aggregate respectively the v p v s ratio and shear wave splitting also vary with orientation between 1 2 and 3 4 and 1 3 and 2 8 for the single crystal and aggregate respectively thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation these properties can be used to detect serpentinite quantify the amount of serpentinization and to discuss relationships between seismic anisotropy and deformation in the mantle wedge regions of high v p v s ratios and extremely low velocities in the mantle wedge of subduction zones down to about 6 and 3 km s 1 for v p and v s respectively are difficult to explain without strong preferred orientation of serpentine local variations of anisotropy may result from kilometer scale folding of serpentinites shear wave splittings up to 1 1 5 s can be explained with moderately thick 10 20 km serpentinite bodies

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

anisotropic uppermost mantle in young subducted slab underplating central mexico knowledge of the rate of plate spreading at mid ocean ridges is critical for estimating plate motions and the outward flux of heat from earth s interior magnetic lineations and anisotropy crystals that have aligned themselves with mantle flow preserved in oceanic lithosphere provide a means for estimating plate spreading rates up to 180 million years ago however reconstructions beyond this time are difficult because most older oceanic lithosphere has been subducted into the mantle here we use converted seismic waveforms to show that anisotropy is preserved in the subducted part of the cocos plate beneath central mexico we observe strong p and s wave anisotropy in the topmost 2 6 km of the subducted oceanic mantle the strength of the anisotropy is comparable to that measured in the surface portion of the cocos plate we also show that p wave azimuthal anisotropy and plate spreading rate at present day mid ocean ridges exhibit a linear relationship on the basis of this relationship we suggest that the subducted portion of the cocos plate formed at a half spreading rate of about 8 cm yr 1 at the east pacific rise about 15 million years ago our results imply that subducted oceanic lithosphere could preserve an archive of ancient plate spreading rates on earth

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

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

shear wave splitting from local events beneath the ryukyu arc trench parallel anisotropy in the mantle wedge we present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the japanese f net array located in the ryukyu arc we obtained high quality splitting measurements for 70 event station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction splitting times for individual measurements ranged from 0 25 to 2 s most values were between 0 75 and 1 25 s both the fast directions and the split times were similar to results for teleseismic s k ks and s wave splitting at the same stations which suggests that the anisotropy is located in the mantle wedge above the slab we considered several mantle deformation scenarios that would result in predominantly trench parallel fast directions and concluded that for the ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with b type olivine fabric in this model the flow direction in the wedge is perpendicular to the trench but the fast axes of olivine crystals tend to align perpendicular to the flow direction resulting in trench parallel shear wave splitting

overriding plate mantle wedge slab and subslab contributions to seismic anisotropy beneath the northern central andean plateau the central andean plateau the second highest plateau on earth overlies the subduction of the nazca plate beneath the central portion of south america the origin of the high topography remains poorly understood and this puzzle is intimately tied to unanswered questions about processes in the upper mantle including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems the interpretation of seismic anisotropy measurements in subduction settings can be challenging however because different parts of the subduction system may contribute including the overriding plate the mantle wedge above the slab the slab itself and the deep upper mantle beneath the slab here we present measurements of shear wave splitting for core phases sks skks pks and ssks local s and source side teleseismic s phases that sample the upper mantle beneath southern peru and northern bolivia relying mostly on data from the caught experiment we find evidence for seismic anisotropy within most portions of the subduction system although the overriding plate itself likely makes only a small contribution to the observed delay times average fast orientations generally trend roughly trench parallel to trench oblique contradicting predictions from the simplest two dimensional flow models and olivine fabric scenarios our measurements suggest complex layered anisotropy beneath the northern portion of the central andean plateau with significant departures from a two dimensional mantle flow regime

elasticity of serpentines and extensive serpentinization in subduction zones 1 elastic constants of lizardite mg3si2o5 oh 4 were computed using first principles quantum mechanical calculations within the density functional theory the predicted c axis compressibility is much larger than measured modeling of the weak o h o interactions between layers must be improved in order to better predict layered hydrated mineral elastic properties the large computed bulk modulus range is consistent with equation of state and seismic velocities in chrysotile lizardite serpentinites but shear wave velocities are lower than the lowest theoretical estimate the low seismic velocities measured for chrysotile serpentinites can be attributed to specific contribution of the nanotube textured chrysotile for antigorite the data from acoustic and eos measurements are consistent if used for interpreting seismic velocities the inferred degrees of serpentinization of the mantle wedge are higher than with the commonly used calibrations using chrysotile serpentinite properties serpentine is likely a dominant phase in low velocity areas of the mantle wedge at 30 50 km depths

lattice preferred orientation of talc and implications for seismic anisotropy in subduction zones abstract strong seismic anisotropy is generally observed in subduction zones lattice preferred orientation lpo of olivine and elastically anisotropic hydrous minerals has been considered to be an important factor causing anomalous seismic anisotropy for the first time we report on measured lpos of polycrystalline talc the study comprises subduction related ultrahigh pressure metamorphic schists from the makbal complex in kyrgyzstan kazakhstan and amphibolite facies metasomatic schists from the valla field block in unst scotland the here studied talc revealed a strong alignment of 001 planes sub parallel to the foliation and a girdle distribution of 100 axes and 010 poles sub parallel to the foliation the lpos of polycrystalline talc produced a significant p wave anisotropy avp 72 and a high maximum s wave anisotropy max avs 24 the results imply that the lpo of talc influence both the strong trench parallel azimuthal anisotropy and positive negative radial anisotropy of p waves and the trench parallel seismic anisotropy of s waves in subduction zones

starch columns analog model for basalt columns desiccation of starch water mixtures produces tensile crack patterns which appear to be interesting but largely unknown study objects for fracture mechanics structural geology and volcanology this paper concentrates on columnar jointing and on columns in starch starch columns have polygonal cross sections and are very similar to basalt columns they are produced by lamp drying starch specimens with dimensions of several centimeters and have diameters in the millimeter range the columns develop behind a crack front which propagates from the surface into the interior the experiments supported by x ray tomograms show that polygonal regularity of the crack pattern is not present at the surface but develops during penetration this transition is steered by a minimum fracture energy principle the analogy between basalt cooling and starch desiccation is far reaching water concentration in starch is analogous to temperature in basalt both quantities obey diffusion equations water loss is equivalent to heat loss the resulting contraction stresses have similar dependences on depth and time and in both cases the material strength is exceeded the starch experiments show that column diameters are controlled by the depth gradient of water concentration at the crack front high low gradients are connected with thin thick columns by analogy a similar relation with the temperature gradient exists for basalt columns the normalized starch gradients are about 3 orders of magnitude larger than the normalized gradients in basalt this explains why starch columns are much thinner than basalt columns the gradients are so different because the crack front speeds differ by a factor of about 10 after 3 days the speed is about 10 mm d in starch but about 100 mm d in basalt peck 1978 the speed difference in turn results from the difference of the diffusion constants the hydraulic diffusivity of starch is 2 orders of magnitude lower than the thermal diffusivity of basalt

anisotropic structures of oceanic slab and mantle wedge in a deep low frequency tremor zone beneath the kii peninsula sw japan 1 anisotropy is an important feature of elastic wave propagation in the earth and can arise from a variety of ordered architectures such as fractures with preferential alignments or preferred crystal orientations we studied the regional variations in shear wave anisotropy around a deep low frequency earthquake lfe zone beneath the kii peninsula sw japan using waveforms of local earthquakes observed by a dense linear array along the lfe zone the fast directions of polarization are subparallel to the strike of the margin for both crustal and intraslab earthquakes the delay time of the split shear waves in intraslab earthquakes is larger than that in crustal earthquakes and shows a down dip variation across the lfe zone this indicates that anisotropy exists in the mantle wedge and in the lower crust and or oceanic slab we explain the observed delay time of 0 015 0 045 s by suggesting that the mantle wedge consists of a deformed 1 15 km thick serpentine layer if the mantle wedge is completely serpentinized in addition to high fluid pressures within the oceanic crust the sheared serpentine layer may be a key factor driving lfes in subduction zones

experimental simulation of joint morphology abstract desiccation experiments with starch water mixtures whose dimensions and thickness are several centimeters produce a broad spectrum of tensile cracks their properties are remarkably similar to those of joints in rocks this note concentrates on crack morphology in the form of plumose structures and fringe zones moreover rupture velocity is determined from videos after an initial dynamic rupture phase with velocities of about 100 mm s which is restricted to a thin strongly dried surface layer the main rupture through the specimen is quasi static with velocities of 10 mm min and less this phase is driven by the low supply of strain energy due to the ongoing desiccation quasi static rupture produces plumose topographies which range from simple to complex from the similarity of plumose topography on joints in marine sedimentary rocks it is hypothesized that these joints are generated by quasi static rupture in an early stage of subsidence compaction and diagenesis of water bearing sediments rupture propagation in the fringe zone of cracks in starch where topography is stronger than on the main crack was slower than over the main crack the same applies probably to fringe zones along bedding planes in sediments

sks splitting beneath mount st helens constraints on subslab mantle entrainment the broadband seismic component of the imush project was supported by national science foundation grants ear 1144568 ear 1144351 ear 1460291 and ear 1444275 cme acknowledges support from the australian research council de190100062 we thank the 2017 iris undergraduate summer intern program for providing support to a w to work with e a w at the university of washington the facilities of iris data services and specifically the iris data management center were used for access to waveforms related metadata and or derived products used in this study iris data services are funded through the seismological facilities for the advancement of geoscience and earthscope sage proposal of the national science foundation under cooperative agreement ear 1261681

analogue modelling of fracturing in cooling plutonic bodies abstract fractures formed when igneous rocks cool below the surface of the earth are of considerable current interest in studies of hydrology mineral systems and radioactive waste disposal however little is known about the geometry and kinematics of such fractures conceptual models suggest that early fractures result from emplacement forces the cloos model while two dimensional numerical modelling suggests that fractures may result from the contraction of cooling bodies we use analogue modelling to investigate the geometry of fractures formed in a cylindrical static contracting body our experiments show that dehydration of buried starch flour is a workable analogue fractures in the analogue material result from drying and contraction of a cylindrical volume which differs from previous experiments in which drying was from a single planar surface and resulted in the formation of columnar joints in buried analogue models two key fracture sets form internally producing sub vertical radial fractures and concentric fractures that curve downwards with depth fracture density decreases towards the centre and bottom of the samples samples that were buried more deeply have fewer and less curved concentric fractures than those with shallow burials radial fractures have similar orientations to those predicted from numerical models and concentric fractures are comparable to marginal fissure orientations of the cloos model the analogue models suggest that both radial and concentric fractures may result from the contraction of plutonic bodies

high pressure elasticity of serpentine and seismic properties of the hydrated mantle wedge 1 a subset of the single crystal elastic moduli of natural antigorite has been measured using brillouin scattering at high pressure up to 9 gpa aggregate properties and axial compressibilities are in good agreement with equation of state results from x ray diffraction stiffness along the c axis increases becoming close to those within the silicate layer near 7 gpa a slight discontinuity in the evolution of the elastic moduli near 7 gpa is associated with a phase transition raman spectroscopy shows that the transition does not occur in subducting slabs except in the core of the coldest slabs where serpentine may be subducted to depths of about 200 km varying temperature and pressure has limited effects on the interpretation of seismic velocities principally because of the limited depth range of serpentine stability mostly above 100 km depth and also due to the compensating effects of pressure and temperature that maintain velocity variations well within the uncertainties and statistical variability of seismological studies serpentinites are excellent candidates for explaining low velocities in the hydrated mantle wedge inversion of the moho and thin anisotropic low velocity layers at the plate interface serpentinite layers provide an alternative explanation to fluid saturated oceanic crust for explaining these phenomena and account for a transition to an aseismic decoupled plate interface

stress and fracture evolution in a cooling pluton an example from the diamond joe stock western arizona u s a abstract an analytical model of fracturing in a crystallizing cylindrical pluton is applied to the natural fracture distribution in the diamond joe stock western arizona u s a the model accounts for three inputs to the stress during cooling thermal contraction magma pressure and regional tectonics fracture type shear or tensile orientation and density were determined at over 300 locations throughout the 8 km diameter stock a distinct pattern of fractures radiates outward from the center of the stock a deflection of these fracture trends toward the regional maximum principal stress direction provides a constraint on the magnitude and orientation of regional stress at the time of crystallization and cooling a calculated model of the fracture pattern suggests that regional maximum and minimum principal stress were 75 and 15 mpa with the orientation controlled by local tectonism the distribution of fractures appears to be insensitive to variations in magma pressure except near the wall of the chamber hence little constraint on magma pressure could be derived from the model thermal gradients during cooling and regional stress appeared to have a greater effect than magma pressure on the overall fracture distribution larger thermal gradients during the early stages of crystallization led to abundant early tensile fractures assuming that the crystalline portions of the pluton behaved elastically during cooling and that all strain was accommodated by fractures the variation of theoretical strain is similar to the distribution of fracture densities within the stock

testing models of sub slab anisotropy using a global compilation of source side shear wave splitting data recently a number of source side shear wave splitting measurements that directly constrain anisotropy in the upper mantle beneath subducting slabs have been published such measurements have yielded an observational foundation on which to base our understanding of the dynamics of the sub slab mantle here we compile measurements from recent studies of source side splitting beneath slabs that employ identical measurement and processing techniques we use this compilation to test the predictions of a number of recently proposed conceptual models for the dynamics of the sub slab mantle including those that invoke three dimensional return flow beneath slabs strong radial anisotropy in suboceanic asthenosphere that is entrained via subduction and a model based on the correlation between sub slab splitting behavior and the age of the subducting lithosphere we find that a model in which fast splitting directions are determined by slab age matches the observations better than either the 3 d return flow or radial anisotropic models based on this observation we propose that the sub slab mantle is characterized by two distinct anisotropic and mantle flow regimes beneath younger lithosphere 95 ma the entrained layer is thin and effectively serves as decoupling layer the dynamics of the sub slab region beneath old lithosphere is therefore dominated by three dimensional return flow this variation in the amount of mechanical coupling may be facilitated by the onset of small scale convection beneath older lithosphere

el chinchon eruption the dust cloud of the century on passe en revue les differentes methodes d observation du nuage de l eruption d el chichon satellite lidar observation aerienne

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

receiver function analysis reveals layered anisotropy in the crust and upper mantle beneath southern peru and northern bolivia abstract subduction systems play a key role in plate tectonics but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems the subduction zone beneath peru and bolivia where the nazca plate subducts beneath south america represents a particularly interesting location to study subduction related deformation given the along strike transition from flat to normally dipping subduction in this study we constrain seismic anisotropy within and above the subducting slab including the overriding plate beneath peru and bolivia by examining azimuthal variations in radial and transverse component receiver functions because anisotropy aware receiver function analysis has good lateral resolution and depth constraints it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography we examine data from long running permanent stations nna near lima peru and lpaz near la paz bolivia and two dense lines of seismometers from the pulse and caught deployments in peru and bolivia respectively the northern line overlies the peru flat slab while the southern line overlies the normally dipping slab beneath bolivia we applied harmonic decomposition modeling to constrain the presence depth and characteristics of dipping and or anisotropic interfaces within the crust and upper mantle we found evidence for varying multi layer anisotropy in some cases with dipping symmetry axes underneath both regions the presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the andes in particular our identification of depth dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry dominant mineralogy and or rheology with depth shedding light on the nature of deep crustal deformation during orogenesis

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

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

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

structural features of the subducting slab beneath the kii peninsula central japan seismic evidence of slab segmentation dehydration and anisotropy 1 seismic evidence of lithospheric slab segmentation dehydration and anisotropic rocks beneath the kii peninsula central japan is detected with receiver function rf analyses by stacking radial and transverse rfs with moveout perturbations we estimate the downdip azimuth dip angle and depth of the slab moho beneath each seismic station comparing the downdip azimuth to the slab convergence azimuth we can classify the slab into three segments the segments correspond to both the fault zones of past megathrust earthquakes and to the spatial distribution of intraslab seismicity the slab moho lies just above the intraslab seismicity in the southwestern kii peninsula slab moho lies deeper at the eastern part of the peninsula at the bottom of seismicity this suggests that earthquakes occur only within the slab mantle under the southern kii peninsula the dip angle of slab moho becomes steeper at 40 to 50 km depth perhaps caused by dehydration transformation of basalt to eclogite at some stations located in the western part of the kii peninsula rf estimated dip angle of the interface is much steeper than that estimated by spatial trends in seismicity since the local seismicity distribution reflects well the larger scale geometry of the slab the rf estimated dip angle may imply anisotropic wave speeds within strongly sheared and or hydrated media around the slab moho these detailed features of the slab may be one key in distinguishing the source regions of megathrust earthquakes in the nankai region

seismic anisotropy produced by serpentine in mantle wedge abstract trench parallel seismic anisotropy has been observed in many subduction zones in the upper mantle in this study the crystal preferred orientation cpo and seismic anisotropy of natural serpentine from val malenco in northern italy and punta bettolina in western italy are studied it is found that the 001 axes are aligned subnormal to the foliation but the 010 axes of the serpentine are aligned subparallel to the lineation which is significantly different from that produced in a recent high pressure experiment cpos of serpentine found herein can be used to explain the trench parallel seismic anisotropy in a cold mantle wedge for serpentine deformed not only at angles greater than 45 but also at low angles such as in horizontal shear from the surface indicating the broader implications of the cpo of serpentine for interpreting seismic anisotropy than previously thought it is also found that seismic anisotropy caused by the cpo of serpentine depends on the degree of serpentinization and flow geometry the seismic anisotropy increases with antigorite volume fraction the seismic anisotropy of shear waves is large i e 23 36 for samples with an antigorite volume fraction of 40 93 current data suggest that the trench parallel seismic anisotropy with a delay time of 1 2 s in the forearc mantle wedge can be attributed to the cpo of antigorite having 010 axes aligned subparallel to the flow direction and 001 axes subnormal to the flow plane