upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

fracture characterization using frequency dependent shear wave anisotropy analysis of microseismic data summary the presence of fractures in hydrocarbon reservoirs can enhance porosity and permeability and consequently increase production the use of seismic anisotropy to characterize fracture systems has gained much interest in the last two decades however estimating fracture sizes from observations of seismic anisotropy has not been possible recent work has shown that frequency dependent anisotropy fda is very sensitive to the length scale of the causative mechanism for the anisotropy in this study we observe fda in a microseismic data set acquired from a carbonate gas field in oman the frequency dependent shear wave anisotropy observations are modelled using a poroelastic model which considers fluid communication between grain size pore spaces and larger scale fractures a grid search is performed over fracture parameters radius density and strike to find the model that best fits the real data the results show that fracture size varies from the microscale within the shale cap rocks to the metre scale within the gas reservoir to the centimetre scale within the non producing part of the carbonate formation the lateral variation in fracture density agrees with previous conclusions from ordinary shear wave splitting sws analysis cumulatively the results show the potential for characterizing fracture systems using observations of fda

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

seismic anisotropy beneath the lau back arc basin inferred from sscs scs splitting data we determine upper mantle anisotropy just beneath active basins by comparing scs and sscs waveform splitting we have measured sscs scs shear wave splitting with data from the spanet and gsn seismic networks the observed sscs scs shear wave splitting suggests that there is a large lateral variation of anisotropy beneath the lau back arc basin scs and sscs waves which sample the southern part of the lau basin show small splitting in contrast the northern part of the lau basin which is characterized to be tectonically active is observed to have large seismic anisotropy with a wnw ese direction the cause of anisotropy may be related to the mantle dynamics of the active basin

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

delay times and shear wave splitting in the mediterranean region summary we measure sks splitting parameters and p and s delay times in the mediterranean region using broad band recordings from the temporary midsea array and permanent networks the midsea seismic array has substantially improved data coverage along the european mediterranean margins and in northern africa shear wave splitting is observed for 95 per cent of the stations with splitting times up to 2 s we propose five anisotropic domains based on the characteristics of the observations in the western part of the region we observe an alignment with plate motion suggesting an asthenospheric origin for anisotropy in the central and near eastern part the fast directions are less coherent laterally and influenced by extension and subduction processes along the dead sea fault the fast directions are parallel to the relative motion between arabia and africa which we attribute in part to coherent vertical deformation our measurements of teleseismic relative p and s wave delay times within the mediterranean region reach peak to peak values of 2 5 and 8 s respectively consistent with international seismological centre isc delay times we observe the earliest arrivals along the hellenic subduction zone and some of the latest along the dead sea fault we find that s wave delays are roughly three times as large as those of p waves from a slight frequency dependence of our cross correlation based delay time measurements we estimate mediterranean lateral heterogeneity to have a dominant size of one or more hundred kilometres we further determine that the anisotropy implied by our sks splitting measurements does not significantly bias the pattern of teleseismic delay times

upper mantle anisotropy beneath the african iris and geoscope stations summary upper mantle anisotropy beneath the african iris and geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as sks skks and pks phases seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station but each station is of potential interest since it can be used to retrieve source side seismic anisotropy at remote sites if one is able to perform station side anisotropy correction we performed systematic investigations of teleseismic shear wave splitting at 15 stations from the iris and geoscope global seismic networks which are located on both the oceanic and the continental parts of the african plate anisotropy is generally well observed at continental stations the patterns we present generally show much more complexity than the results previously published from smaller data sets despite this complexity the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting rift structures archaean craton panafrican belt although a deeper source of anisotropy asthenospheric may be present at the oceanic stations anisotropy measurements are much more difficult to perform because the signal is generally of poor quality msey in the seychelles indian ocean is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion as defined by hotspot tracks

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

frequency dependent shear wave splitting beneath the japan and izu bonin subduction zones abstract despite its importance for our understanding of physical processes associated with subduction the geometry of mantle flow in subduction zones remains poorly understood particularly in the mantle wedge above subducting slabs constraints on mantle flow and deformation can be obtained by measurements of shear wave splitting a valuable tool used to characterize the geometry and strength of seismic anisotropy a complete characterization of shear wave splitting is particularly important for understanding the mantle wedge beneath japan which overlies multiple subduction zones with complex slab morphologies previous studies indicate that the upper mantle beneath japan exhibits complicated anisotropy that manifests itself in complex splitting patterns to characterize better the geometry of mantle anisotropy beneath japan we analyzed direct s waves from local earthquakes originating in the subducting slabs for evidence of shear wave splitting using data from 54 broadband seismic stations in japan s f net array in addition both local s and teleseismic sks phases were examined using data from four f net stations in the izu bonin arc in order to characterize any frequency dependence of splitting parameters that may indicate the presence of complex anisotropy we carried out our splitting analysis in two different frequency bands 0 02 0 125 hz and 0 125 0 5 hz our measurements indicate that shear wave splitting due to upper mantle anisotropy beneath japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency

quantitative determination of hydraulic properties of fractured rock using seismic techniques there have been significant advances over the last ten years in the use of the seismic anisotropy concept to characterize subsurface fracture systems measurements of seismic anisotropy are now used to deduce quantitative information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon various equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume scale frequency independence and hence cannot distinguish between micro cracks and macrofractures the latter however control the fluid flow in many oil gas reservoirs as the fracture size and spacing hence fracture storability are essential parameters for reservoir engineers recently a new equivalent medium theory for modelling of wave propagation in media with multi scale fractures has been presented the model predicts velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale micro cracks and equant matrix porosity and formation scale fractures application of this model to field data shows that fracture density and fracture size can be inverted successfully from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data this paper presents the results of the latest development in the seismic characterization of natural fractures with an emphasis on the quantitative determination of fracture sizes

shear wave splitting and the pattern of mantle flow beneath eastern oregon a r t i c l e i n f o article history the tectonic and geologic setting of eastern oregon includes the volcanically active high lava plains hlp province and the accreted terrains of the blue and wallowa mountains and is bounded by the columbia river ood basalts to the north basin and range extension to the south the cascade arc to the west and stable north america to the east several models have been proposed to explain the tectonic evolution of eastern oregon and in particular the voluminous volcanic activity in the hlp but a consensus on which model fully describes the complex range of processes remains elusive measurements of the seismic anisotropy that results from active mantle ow beneath the region can provide a crucial test of such models to constrain this anisotropy here we present new sks splitting results obtained at approximately 200 broadband seismic stations in eastern oregon and the surrounding region data come from the usarray transportable array ta and two temporary experiments carried out in the hlp and in the wallowa mountains our splitting data set includes 2900 individual splitting measurements from sks phases recorded between 2006 and 2008 stations in eastern oregon exhibit signi cant shear wave splitting with average delay times at individual stations between 0 8 s and 2 7 s in the hlp nearly all observed fast directions are approximately e w while to the north in the blue and wallowa mountains there is more variability in the splitting patterns the average delay time observed at stations located in the heart of the hlp province is 2 s well above the global average of 1 s for continental regions we infer from the large split times and homogeneous fast directions that there must be signi cant active ow in a roughly e w direction in the asthenosphere beneath the hlp this inferred ow eld places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region in the wallowa region the anisotropic signature is more complicated and there may be a signi cant contribution from fossil fabrics in the crust or mantle lithosphere

sks splitting and the seismic anisotropy of the mantle beneath the hikurangi subduction zone new zealand abstract the analysis of 12 sks phases recorded on broadband stations above the hikurangi subduction zone in new zealand shows clear evidence of mantle anisotropy with the fast direction 28 5 almost parallel to the strike of subduction and the dominant geology of the region the slow shear wave delay times show a systematic change with the azimuth of the arrivals which if hexagonal symmetry is assumed indicates that either the axis of symmetry of the anisotropic volume beneath the subduction zone is not horizontal or that more than one anisotropic layer is present the magnitude of the delays 1 5 0 4s suggests that the anisotropy is most probably confined to the top 300 km of the mantle

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

anisotropy repeating earthquakes and seismicity associated with the 2008 eruption of okmok volcano alaska 1 we use shear wave splitting sws analysis and double difference relocation to examine temporal variations in seismic properties prior to and accompanying magmatic activity associated with the 2008 eruption of okmok volcano alaska using bispectrum cross correlation a multiplet of 25 earthquakes is identified spanning five years leading up to the eruption each event having first motions compatible with a normal fault striking ne sw cross correlation differential times are used to relocate earthquakes occurring between january 2003 and february 2009 the bulk of the seismicity prior to the onset of the eruption on 12 july 2008 occurred southwest of the caldera beneath a geothermal field earthquakes associated with the onset of the eruption occurred beneath the northern portion of the caldera and started as deep as 13 km subsequent earthquakes occurred predominantly at 3 km depth coinciding with the depth at which the magma body has been modeled using geodetic data automated sws analysis of the okmok catalog reveals radial polarization outside the caldera and a northwest southeast polarization within we interpret these polarizations in terms of a magma reservoir near the center of the caldera which we model with a mogi point source sws analysis using the same input processing parameters for each event in the multiplet reveals no temporal changes in anisotropy over the duration of the multiplet suggesting either a short term or small increase in stress just before the eruption that was not detected by gps or eruption triggering by a mechanism other than a change of stress in the system

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

sks skks splitting and taiwan orogeny 1 we determine 58 sets of splitting parameters fast direction and delay time t from broadband sks skks waves recorded at temporary and permanent stations in taiwan these and earlier results from s and scs as well as sks show an island wide pattern of alignment with n nne structural trends in southern and central taiwan and then turn smoothly to ew following the structural bight in northern taiwan remarkable differences in t for eastern and southwestern events are found resulting probably from a more anisotropic upper mantle in the east under the central range with t an average of 1 3 seconds our observations are consistent with the hypothesis that the orogeny involves shear or flow in the upper mantle in central and southern taiwan in northern taiwan near the ends of the collision and subduction zones the extrusion and the hydration of olivine may contribute to the orientation of

strain modelling seismic anisotropy and coupling at strike slip boundaries applications in new zealand and the san andreas fault despite similar surface transform faulting behaviour observed shear wave splitting patterns in the california and new zealand plate boundary regions are markedly different to better understand the origin of the anisotropy in these regions we model mantle flow and strain for a variety of strike slip plate boundary scenarios simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths straincontrolled models fit the observations in new zealand and california better than simplified flow controlled models fast shear polarizations are progressively rotated toward the shear plane over time and even a constant viscosity model provides a good fit to the fast directions in new zealand and southern california the constant viscosity implies strong coupling between the surface and the deeper mantle to fit the lack of decrease in delay times with distance from the fault the relationship between delay time and strain must saturate at small strains if this is the case then southern new zealand and southern california both fit models in which strain is smaller than in nearby regions equivalent to that achieved along an infinite fault by about 3 10 my of their present motion stratified viscosity allows more rapid rotation of fast directions toward fault parallel than occurs in isoviscous models and can explain the nearly fault parallel fast directions in the central south island different aspects of the northern california results are fit with different models but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy noted in the area suggesting vertical decoupling

modelling frequency dependent seismic anisotropy in fluid saturated rock with aligned fractures implication of fracture size estimation from anisotropic measurements measurements of seismic anisotropy in fractured rock are used at present to deduce information about the fracture orientation and the spatial distribution of fracture intensity analysis of the data is based upon equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit conventional models assume frequency independence and cannot distinguish between microcracks and macrofractures the latter however control the fluid flow in many subsurface reservoirs therefore the fracture size is essential information for reservoir engineers in this study we apply a new equivalent medium theory that models frequency dependent anisotropy and is sensitive to the length scale of fractures the model considers velocity dispersion and attenuation due to a squirt flow mechanism at two different scales the grain scale microcracks and equant matrix porosity and formation scale fractures the theory is first tested and calibrated against published laboratory data then we present the analysis and modelling of frequency dependent shear wave splitting in multicomponent vsp data from a tight gas reservoir we invert for fracture density and fracture size from the frequency dependence of the time delay between split shear waves the derived fracture length matches independent observations from borehole data

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

using the pair correlation function as a tool to identify the location for shale gas oil reservoir based on well log data shale reservoirs as an unconventional resource are becoming an increasingly important exploration development and production target in the oil industry one of the main problems in these reservoirs is the answer to the question of where to drill the sweet spot location in other words one desires to detect the productive portion of the reservoir this involves finding data criteria that can be used to distinguish between productive and nonproductive portions of a reservoir as one of these criteria we have investigated the amplitude of the pair correlation function pcf for sonic data which reflected the contrast in elastic properties between inclusions oil and gas and background media based on various data from eight different wells we calculated the amplitudes of the pcf the obtained results showed that the productive layers were highly correlated and nonproductive layers had low values of the pcf amplitude hence the results of these calculations showed the potential applications of the pcf of seismic data and its amplitude in the detection of the productive layers therefore we could use this pcf and its amplitude as a tool to predict the location of shale gas or oil reservoirs and to estimate the thickness of productive zones in the well

seismic anisotropy under central alaska from sks splitting observations 1 seismic anisotropy under central alaska is studied using shear wave splitting observations of sks waves recorded on the broadband experiment across the alaska range beaar 1999 2001 splitting results can be divided into two distinct regions separated by the 70 km contour of the subducting pacific plate waves that travel through the thicker mantle wedge show fast directions that are parallel to the strike of the slab these slab parallel directions appear to indicate along strike flow in the mantle wedge and splitting delay times increase with path length in the mantle wedge suggesting anisotropy of 7 9 0 9 the region of along strike flow corresponds to high seismic attenuation and hence high temperatures along strike flow here may be driven by secular shallowing of the slab driven by subduction of buoyant yakutat terrane crust or by toroidal flow around the east end of the aleutian slab waves traveling southeast of the 70 km contour sample the pacific plate and the nose of the mantle wedge they show fast directions that parallel the direction of plate motion these fast directions are most likely due to flow under the subducting pacific plate and or anisotropy within the subducting pacific lithosphere the high splitting delay times 0 8 1 7 s associated with these convergence parallel directions cannot be produced in the mantle wedge which is 10 30 km thick here thus anisotropy shows a sharp 90 change in fabric associated with the onset of high temperature wedge flow

sks splitting measurements in the apenninic tyrrhenian domain italy and their relation with lithospheric subduction and mantle convection 1 we present a compilation of singular shear wave splitting measurements in the italian region we used the method of silver and chan 1991 to determine the splitting parameters of fast direction and delay time from broadband sks waveforms the stations analyzed were mainly deployed in temporary experiments a few are permanent mednet broadband stations the splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath italy the fast polarization directions define two anisotropic domains a tyrrhenian domain with e w prevalent fast directions and an apennines adriatic domain with nw se to nnw sse prevalent fast directions the stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths this variability is a sign of a complex upper mantle structure also shown by seismic tomography but allows us to constrain the depth of the anisotropic layer in the 100 250 km range at least in the boundary zone between the two domains data at station vsl sardinia suggest the presence of depth dependent anisotropic structure where the shallower signature is consistent with the tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670 km discontinuity we argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding tyrrhenian plate

boundary layer mantle flow under the dead sea transform fault inferred from seismic anisotropy lithospheric scale transform faults play an important role in the dynamics of global plate motion near surface deformation fields for such faults are relatively well documented by satellite geodesy strain measurements and earthquake source studies and deeper crustal structure has been imaged by seismic profiling relatively little is known however about deformation taking place in the subcrustal lithosphere that is the width and depth of the region associated with the deformation the transition between deformed and undeformed lithosphere and the interaction between lithospheric and asthenospheric mantle flow at the plate boundary here we present evidence for a narrow approximately 20 km wide subcrustal anisotropic zone of fault parallel mineral alignment beneath the dead sea transform obtained from an inversion of shear wave splitting observations along a dense receiver profile the geometry of this zone and the contrast between distinct anisotropic domains suggest subhorizontal mantle flow within a vertical boundary layer that extends through the entire lithosphere and accommodates the transform motion between the african and arabian plates within this relatively narrow zone

shear wave splitting variations across an array in the southern north island new zealand 1 we observe significant spatial variations in shear wave splitting from deep teleseismic s and regional scs recorded on an array in the fore arc region of the hikurangi subduction zone new zealand we also observe variations from different events recorded at the same station the fast polarization directions range between 23 and 72 and the delay times range between 0 75 s and 1 80 s these observations contrast with previous sks studies in new zealand where little spatial variation is seen and highlights the importance of using a variety of phases with different periods our observations are consistent with a model in which a lateral anisotropic boundary runs ne sw in a zone between kapiti island and 40 km east of the tararua mountain range the anisotropic boundary is possibly related to a prominent vertical velocity feature near kapiti island

seismic anisotropy in the lower mantle a comparison of waveform splitting of sks and skks 1 we have carried out a global investigation on seismic anisotropy in the lower mantle by comparing the waveform splitting of sks and skks found at the same seismogram the two shear waves have very similar ray paths in the upper mantle but different ones in the lower mantle we collected a total of 104 sks skks waveform data recorded at 76 stations the sks skks data are first matched with a single anisotropic model 2 parameters the fast polarization direction and delay time t and two independent anisotropic models 4 parameters we then applied an f test to examine whether the 4 parameter models are better than the 2 parameter ones based on error improvement we found that the data from most of the stations can be explained by the simple 2 parameter models while this observation provides compelling evidence that the vast part of the lower mantle below 1000 km including the d region is transversely isotropic it is still arguable that the uppermost lower mantle and the transition may partly contribute the sks skks splittings we also found that the 4 parameter models provide a better fitting to the sks and skks splitting at 8 stations suggesting the existence of transverse anisotropy or anomalous dipping structures in some part of the lower mantle

frequency dependent anisotropy in wellington new zealand shear wave splitting measurements are made using teleseismic s scs and sks waveforms recorded at the gdsn broadband station snzo situated in south karori new zealand the average s and sks delay times are around 2 to 3 s among the highest in the world the average scs delay time is significantly smaller around 1 s this discrepancy appears to be due to differences in dominant frequency the most likely cause of frequency dependent anisotropy is oriented heterogeneities with a scale length much smaller than the wavelength the fast polarizations range between 21 and 79 with an average ne sw direction which is sub parallel to the trend of the local geologic structure and the strike of the hikurangi subduction zone azimuthal variations in delay time which cannot be explained by differences in period may be due to a dipping axis of symmetry or laterally varying anisotropy or a more complicated symmetry system

characterization of fractures in shallow granite from the modeling of the anisotropy and attenuation of seismic waves abstract we try to characterize the fracturing of the anisotropic shallow granite et al 1992 is modeled by performing numerical simulations of propagation of p and sv waves in 2d fractured media the calculations are performed using a boundary integral equation method which takes into account multiple scattering and full crack diffraction the value of the time delay measured at garner valley between the fast s wave and the slow s wave is well reproduced for different models of fractured media in order to differentiate between these models we also measure and calculate the attenuation of the transmitted s waves from the comparison between the observations and the numerical simulations we conclude and infer the following 1 the presence of fractures in granite explains the seismic anisotropy and attenuation observed at garner valley 2 the cracks and fractures are nearly vertical and are oriented in a north south direction 3 there is no characteristic crack length but rather the crack length distribution seems to be fractal 4 the density of fractures present in the shallow granite at garner valley is about 10 4 representing the total volume of the fractures over the total volume of the fractured zone

seismic anisotropy in the fore arc region of the hikurangi subduction zone new zealand abstract shear waves from local earthquakes recorded on an array of nine broad band seismograph stations have been used to investigate the anisotropic structure within the crust of the overlying australian plate and within and below the subducting slab underlying the southern north island new zealand earthquakes near the plate interface were used to image the crustal anisotropy whereas deeper down dip subduction zone events were used to sample the subducting slab and the mantle below apart from a small amount of variable near surface anisotropy top few kilometres of the crust the measured fast shear wave directions of both the crust of the overlying plate 51 18 and the mantle 41 15 are similar and are nearly parallel to the strike of the subduction zone and the dominant geological features of the region sks measurements which mainly sample the mantle beneath the subducting plate give a value of 28 11 and measurements made elsewhere in the new zealand region using sks and scs2 produce similar results the measurements of crustal anisotropy are similar to those found further south on the wellington peninsula and confirm the presence of near surface anisotropy the maximum delay times for crustal earthquakes indicate a pervasive shear wave velocity anisotropy of 4 in the crust of the overlying plate and the increase in the delay times with depth for the subduction zone earthquakes translates to a shear wave velocity anisotropy of 1 4 in the mantle below the subducting slab these results when combined with the sks measurements suggest trench parallel flow in the mantle below the subducted slab and a coherence of the deformation processes in the crust of the overlying plate and the mantle beneath the subducting slab

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

simultaneous inversion of shear wave splitting observations from seismic arrays 1 seismic waveforms recorded at high density receiver arrays facilitate the application of new inversion techniques which take advantage of the coherent nature of the observations we use measurements of shear wave splitting parameters from observed sks waveforms along a dense receiver profile and compare them with splitting parameters obtained from numerical waveform modeling through anisotropic earth models we use two different iterative approaches for the inversion of the observed splitting parameters 1 a local optimization technique the downhill simplex method and 2 a global genetic algorithm search in our forward modeling we calculate sks waveforms by a finite difference fd method solving the anisotropic wave equation instead of deriving individual anisotropic models for each station and combining them into one model by the comparison of fd modeling and observations we avoid a direct interpretation of the splitting parameters in terms of medium properties we apply these techniques to the analysis of sks phases recorded along a 100 km profile located at the dead sea transform fault the measured splitting parameters show gradual short scale variations along the profile which depend on frequency lateral and vertical variations of anisotropy are accounted for by two dimensional block structures for the simplex method the complexity of the models the number of anisotropic blocks is increased gradually whereas the number of anisotropic blocks is kept fixed for the genetic algorithm search the anisotropic structures of the best fitting models derived from the two inversion approaches agree well the results support earlier interpretations of the observations in terms of a narrow approximately 20 km wide vertical decoupling zone in the mantle that accommodates the transform motion between the african and arabian plates

lithospheric anisotropy beneath the pyrenees from shear wave splitting we investigate upper mantle anisotropy beneath the pyrenean range along three n s profiles across the mountain belt the results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere we present the results of two other profiles that ran in 1995 1996 and 1996 1997 in the eastern and western part of the belt respectively and propose an interpretation of the whole results teleseismic shear waves sks skks and pks are used to determine splitting parameters the fast polarization direction and the delay time t teleseismic shear wave splitting in the eastern pyrenees displays homogeneous values trending n100 e and t values in the range 1 1 to 1 5 s a station located in the southern massif central 100 km north of the range is characterized by different splitting parameters n70 e t 0 7 s in the western part of the belt anisotropy parameters are similar across the whole belt n110 e and t 1 3 to 1 5 s most of the measured delay times including those obtained in the central part of the range are above the global average of the sks splitting around 1 s at the belt scale is generally poorly correlated with recent estimations of the absolute plate motion which predicts a fast direction ranging between n50 e and n80 e instead the orientation of n100 e is parallel to the trend of the pyrenean belt but also to hercynian preexisting structures this parallelism supports an anisotropy primarily related to frozen or active lithospheric structures we show that a signature related to the pyrenean orogeny is likely for the stations located in the internal domains of the belt by contrast the anisotropy measured at the stations located on the external parts of the belt could reflect a pre pyrenean hercynian deformation we suggest that a late hercynian strike slip deformation is responsible for this frozen upper mantle anisotropy and that the pyrenean tectonic fabric developped parallel to this preexisting fabric finally no particularly strong splitting is related to the north pyrenean fault commonly believed to represent the plate boundary between iberia and eurasia

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

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

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

shear wave anisotropy in the mariana subduction zone to determine the location strength and orientation of seismic anisotropy in the mariana subduction zone beneath guam we evaluated shear wave splitting in local s regional s and scs and teleseismic core phases such as sks recorded at station gumo fast directions from the local s phases have an average azimuth of 45 and splitting times range from 0 1 s to 0 4 s for local s phases from events within the southeastern half of the subducting slab splitting parameters manifest minimal frequency dependence in both fast direction and splitting time however for the remaining local s phases in the data set fast directions vary with frequency content no well constrained splitting parameters were obtained for the regional and teleseismic phases but the particle motions of these unsplit phases are consistent with an average anisotropic fast direction of 45 anisotropy due solely to olivine oriented by slab entrained flow in the mantle wedge would produce local s fast directions at 66 and anisotropy due solely to fossil seafloor spreading in the subducting slab would yield fast directions at 20 to 30 neither of these predictions is consistent with the observed fast directions however the observed splitting including the frequency dependent fast directions can be explained by models containing anisotropy in both the slab and wedge and possibly although not necessarily anisotropy due to recent extension in the overriding philippine sea plate

short scale variations of shear wave splitting across the dead sea basin evidence for the effects of sedimentary fill 1 we examine shear wave splitting of sks waveforms collected by a temporary array of 68 stations in the region of the dead sea basin the observed splitting parameters exhibit systematic variations along a dense ew trending 60 km profile across the basin the delay times vary significantly between 1 0 and 2 8 seconds with smaller values in the very center of the profile the fast polarizations are oriented more or less parallel to the strike of the dead sea transform fault and vary between 10 and 20 degrees with respect to north finite frequency waveform modeling reveals that the source region of the small scale lateral variations is likely located within the crust the modeling further shows that purely isotropic velocity variations affect shear wave splitting to a large degree the observed variations of splitting parameters can be explained by the sedimentary fill of the basin and its low isotropic seismic velocities whereas the mantle is uniformly anisotropic our study indicates that precaution must be taken when interpreting short scale lateral variations of shear wave splitting in terms of anisotropic structures in the crust or upper mantle

synsedimentary deformation of pleistocene glaciolacustrine deposits in the albese con cassano area southern alps northern italy and possible implications for paleoseismicity abstract we describe and interpret the well exposed soft sediment deformations preserved in a mid pleistocene proglacial sequence outcropping at albese con cassano near como southern alps n italy the observed deformations affect six distinct stratigraphic intervals within the same depositional sequence the stratigraphy of this sequence the geomorphic setting and the careful geological mapping of the area clearly show that these deformations are not the result of glaciotectonics or cryoturbation but are due to six separate events of synsedimentary liquefaction in particular the 3 basal deformed units include sand and gravel volcanoes that formed at the bottom of a proglacial lake we examined several hypothesis of possible causative mechanism for the formation of these features taking into account all the information available on the local geology and sedimentary environment it is possible to assess that the liquefaction features at albese con cassano had a coseismic origin to our knowledge this would be the first report of similar evidence of paleoseismicity in the lombardia southern alps also there is no record of historical liquefaction triggered by seismic events in an area of ca 70 km of radius near albese con cassano including the town of como the large metropolitan area of milano and most of w lombardia that is one of the largest industrial districts in europe therefore the interpretation of the observed deformations can have critical impact for understanding the current seismicity level in the region and mitigating the associated risk in this paper we show that this interpretation is not a simple process liquefaction features must be treated with caution in order to derive suitable information for seismic hazard characterization even if their coseismic origin has been definitely proved it is necessary to understand the amount of seismic shaking for instance in terms of local macroseismic intensity that can have produced that specific feature at that specific site the location of the causative seismic event epicenter and focal depth and eventually its size in terms of epicentral intensity and or earthquake magnitude in the case of albese con cassano the implications in terms of seismic hazard of the observed possible earthquake paleoliquefactions can only be assessed through the definition of several scenarios taking into account a different thresholds of shaking for triggering the described synsedimentary structures and b different seismotectonic frameworks related to the mid pleistocene to holocene evolution of the s alps including seismicity induced by isostatic glacial rebound we conclude that it is worth checking these scenarios in the field with further paleoseismological research on other possible evidence of past earthquakes there are no unequivocal sedimentological indicators of paleoseismicity however when carefully interpreted within the proper local seismotectonic context and compared with other earthquake induced features soft sediment deformations can offer information of critical value for seismic hazard assessment

attenuation anisotropy and the relative frequency content of split shear waves summary the variation of frequency dependent seismic wave attenuation with direction attenuation anisotropy contains additional information to that contained in velocity anisotropy in particular it has the potential to distinguish between different mechanisms that can cause velocity anisotropy for example aligned fracturing might be expected to cause measurable velocity and attenuation anisotropy while preferred crystal orientation leads to significant velocity anisotropy but may cause only small amounts of attenuation attenuation anisotropy may also contain useful information about pore fluid content and properties we present a methodology for analysis of attenuation anisotropy and apply it to a microseismic data set previously analysed for shear wave splitting by teanby et al 2004 attenuation anisotropy values obtained show a temporal variation which appears to correlate with the temporal variation in the velocity anisotropy the comparison of the relative frequency content of fast s1 and slow s2 split shear waves is a convenient method for examining seismic attenuation anisotropy provided that s1 and s2 initially have the same spectral colouring that no spectral distortion is introduced by the differences between receiver responses of geophone components and that spectral distortion due to background noise can be ignored or corrected for we can attribute any differences in their frequency content to attenuation anisotropy attenuation anisotropy where present should be detected by the different approximately orthogonal polarizations of s1 and s2 as they pass through the anisotropic medium in the presence of attenuation anisotropy s1 and s2 should experience different levels of frequency dependent attenuation we quantify the differential attenuation of s1 and s2 using a scheme based on a spectral ratio method we present results from a microseismic data set acquired in an abandoned oil well at valhall a north sea oil field the results are surprising in that sometimes the slower arrival s2 is richer in high frequencies than the faster s1 this appears to be contrary to results predicted by theoretical crack models for attenuation anisotropy e g hudson 1981 the mechanism responsible for these observations is not clear our differential attenuation attribute correlates with the angle between the strike of the inferred initial shear wave source polarization and the fast shear wave polarization which suggests that the split shear wave with the larger amplitude is preferentially attenuated our attribute also correlates with the event backazimuth and the minimum percentage anisotropy

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

no regional anisotropic domains in the northeastern u s appalachians a fabric frozen within the continental lithosphere should lead to regional anisotropic domains with close correlation between tectonics seismic anisotropy and shear wave travel time delays alternatively seismic anisotropy could be maintained by active deformation in the sub lithospheric mantle this scenario would predict broad spatial consistency in shear wave splitting and weak correlation between the splitting surface tectonics and travel time delays a coherent pattern of shear wave splitting is observed in the northeastern united states covering large parts of new york and new england spanning the appalachian orogen and areas underlain by the grenvillian basement the direction of fast shear wave propagation as measured from core refracted sks skks and pks phases varies systematically with event back azimuth at all sites examined and is fit well by two layers of anisotropy with subhorizontal symmetry axes relative travel time delays of shear waves that sample the region vary over 100 km length scale and correlate with surface geology tomographic images suggest lateral shear velocity variations 3 including a slow anomaly beneath the grenvillian basement exposed in the adirondack mountains there is little correlation between this horizontally rough seismic velocity and the horizontally smooth anisotropic model consistent with shear wave splitting we therefore conclude that in the northeastern us the concept of regional anisotropic domains i e distinct regions within the lithosphere characterized by coherent anisotropic properties reflective of present or past deformation does not apply

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

experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles fractured rocks are known to exhibit seismic anisotropy and shear wave splitting sws sws is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type the presence of partial liquid gas saturation is also known to affect the elastic properties of rocks the combined effect of both fractures and partial liquid gas saturation is still unknown using synthetic silica cemented sandstones with aligned penny shaped voids we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on sws under partial liquid gas saturation conditions the result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model at high to full water saturation values sws decreases as a result of the fluid bulk modulus effect on the quasi shear wave this bulk modulus effect is frequency dependent as a result of wave induced fluid flow mechanisms which would in turn lead to frequency dependent sws this result suggests the possible use of sws for discriminating between full liquid saturation and partial liquid gas saturation

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

directional variations in travel time residuals of teleseismic p waves in the crust and mantle beneath northern tien shan we study the directional variation in travel time residuals using 13 820 p wave arrivals from 1 998 teleseismic events 15 d 98 4 1 mb 7 3 recorded in 1991 1997 by the kyrgyz digital seismic network knet based on a modified version of the iasp91 model that accounts for the kyrgyz crustal thickness beneath knet we convert p wave travel times to travel time residuals dt the de pendence of dt on backazimuth is modeled as one two and four lobed variations in a horizontal plane backus 1965 a least squares fit of the azimuthal variation of dt indicates that the crust in the northern tien shan is about 11 15 km thicker than it is in the kazakh shield and the chu depression from nine knet stations the one lobe model estimates that the slowest p wave travel time direction is 5 0 4 8 almost directly north and the magnitude of variation is 1 71 0 13 sec this result is consistent with an upwelling lower mantle plume for the two lobe model the slowest p wave travel time directions anisotropy term are 89 7 and 269 7 4 7 i e trending east west we find p wave velocity anisotropy of 2 0 2 9 associated with a layer with a thickness of 440 km at the top of the lower mantle the fast direction of the p wave travel time north south azimuthal anisotropy at the top of the lower mantle is 1 parallel to the absolute motion of the india plate and 2 close to the direction of the upwelling hot mantle flow the last result suggests that the azimuthal aniso tropy of the travel time residuals is due to the shape preferred orientation of middle mantle material that results from plume intrusion shear wave splitting studies mak eyeva et al 1992 wolfe and vernon 1998 estimated the fast polarization direction to be parallel to the strike of the geological structures of the northern tien shan 71 29 thus the fast polarization direction determined from these shear wave splitting studies using knet data contradicts differs by 90 the fast travel time direction 0 3 and 179 7 4 7 we determine here using p wave travel time residuals using knet data this suggests that the azimuthal anisotropy determined from p wave travel time variations and from shear wave splitting in sks and skks have different sources

detailed configuration of the underthrusting indian lithosphere beneath western tibet revealed by receiver function images we analyze the teleseismic waveform data recorded by 42 temporary stations from the y2 and antilope 1 arrays using the p and s receiver function techniques to investigate the lithospheric structure beneath western tibet the moho is reliably identified as a prominent feature at depths of 55 82 km in the stacked traces and in depth migrated images it has a concave shape and reaches the deepest location at about 80 km north of the indus yarlung suture iys an intra crustal discontinuity is observed at 55 km depth below the southern lhasa terrane which could represent the upper border of the eclogitized underthrusting indian lower crust underthrusting of the indian crust has been widely observed beneath the lhasa terrane and correlates well with the bouguer gravity low suggesting that the gravity anomalies in the lhasa terrane are induced by topography of the moho at 20 km depth a mid crustal low velocity zone lvz is observed beneath the tethyan himalaya and southern lhasa terrane suggesting a layer of partial melts that decouples the thrust fold deformation of the upper crust from the shortening and underthrusting in the lower crust the sp conversions at the lithosphere asthenosphere boundary lab can be recognized at depths of 130 200 km showing that the indian lithospheric mantle is underthrusting with a ramp flat shape beneath southern tibet and probably is detached from the lower crust immediately under the iys our observations reconstruct the configuration of the underthrusting indian lithosphere and indicate significant along strike variations

seismic anisotropy and mantle flow from the great basin to the great plains western united states shear wave splitting and p sks and s travel time residuals are calculated for teleseismic arrivals recorded on the colorado plateau great basin program for array seismic studies of the continental lithosphere passcal portable broadband seismic deployment and for permanent stations in the western united states little shear wave splitting is observed for broadband recordings in the northern colorado plateau the rocky mountains or the central great basin the transition between the colorado plateau and the great basin is marked by moderate shear wave splitting 1 0 s and unusually late teleseismic phase arrivals this suggests material with a higher content of mantle melt or volatiles than regions to either side splitting in the transition between the colorado plateau and great basin is part of a pattern of fast polarizations that align in a semicircle surrounding a central great basin region of null no splitting measurements away from the california plate boundary splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the north american plate no simple spatial relation of splitting with geological and geophysical features such as mountain ranges velocity anomalies gravity magnetics or heat flow is evident in most of the western united states however splitting in the great basin is compatible with asthenospheric flow the smallest shear wave splitting delay times coincide with the eureka low in heat flow also having low s velocity at 300 km depth we suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the great basin is caused by mantle flow by the interaction of mantle up welling and the absolute motion of the north american plate

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

strong variations in seismic anisotropy across the hikurangi subduction zone north island new zealand abstract shear wave splitting measurements from sks skks s and scs phases reveal the anisotropic structure of north island new zealand waveforms were recorded on temporary and permanent broadband seismographs strong anisotropy varies rapidly across the hikurangi subduction zone these changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow stations in the fore arc region eastern central north island record fast polarisations subparallel to the trench and moderate delay times of around 2 5 s consistent with earlier studies of north island this is interpreted as caused by trench parallel mantle flow beneath the subducted slab with a possible contribution of trench parallel fossil anisotropy within the subducted slab delay times increase towards the extending central volcanic region cvr with values up to 4 5 s one of the largest delay times measured worldwide strong s wave anisotropy 7 5 through 80 km in the mantle wedge in addition to the sub slab anisotropy can explain the observed high delay times one mechanism to create such high anisotropy is melt segregation of partially molten rock a frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements to the west delay times decrease systematically measurements in the far back arc region of western central north island show no apparent splitting this suggests that mantle wedge dynamics terminate beneath the western border of the cvr the apparent isotropy beneath the far back arc may be related to local small scale mantle convection or vertical mantle flow possibly caused by the detachment of lithosphere

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

shear wave anisotropy beneath the taiwan orogen abstract shear wave splitting in regional and teleseismic shear waves scs and s has been analyzed for fast polarization directions and delay times at 33 short period and 17 broad band permanent and temporary seismic stations in the taiwan region the orientations of anisotropy for the taiwan stations are generally parallel to the strike of the mountain belt with a clockwise rotation following the trend of regional geologic fabric in north taiwan the delay times on the other hand vary greatly from 0 5 to 2 1 s the largest values are from the southwestern taiwan foothills and an abrupt change in delay times is found in south taiwan the main source of shear wave splitting in taiwan is compatible with an anisotropic region between 25 and 230 km depths the observed splitting parameters are consistent with upper mantle anisotropy resulting from the collisional tectonics that built the taiwan orogen

anisotropy patterns in the subducting lithosphere and in the mantle wedge a case study the southern italy subduction system 1 in this study we present a collection of high quality s wave splitting measurements in the southern italy subduction system we analyzed 76 deep earthquakes located within the descending slab using the method of silver and chan 1991 to determine the splitting parameters and t the local deep earthquakes allow us to analyze raypaths primarily sampling the slab and the wedge above it mainland calabria is an outcropping forearc enabling us to sample rays that propagate up the slab s wave splitting parameters show a complex pattern of anisotropy with variable fast directions and with delay times ranging from 0 1 s to 2 2 s we compared local s wave splitting data with sks results at the same stations and we found that the average t is very different 1 8 s for sks and 0 5 s for s we found consistency between averaged s fast directions and the sks splitting measurements including a pattern of toroidal mantle flow at the sw edge of the slab the s wave splitting parameters show frequency dependent behavior that we attribute to the presence of small scale anisotropic heterogeneities comparison of s splitting measurements to p wave velocity anomaly at 100 200 km depth shows that where the rays primarily sample the slab the delay times are small in contrast where s rays sample the mantle wedge the delay times are quite high this t pattern depicts the slab as a weakly anisotropic region and suggests that the main source of anisotropy in the subduction zone is the surrounding asthenosphere

modelling shear wave splitting observations from wellington new zealand summary frequency dependent anisotropy was previously observed at the permanent broad band station snzo south karori wellington new zealand this has important implications for the interpretation of measurements in other subduction zones and hence for our understanding of mantle flow this motivated us to make further splitting measurements using events recorded since the previous study and to develop a new modelling technique thus in this study we have made 67 high quality shear wave splitting measurements using events recorded at the snzo station spanning a 10 yr period this station is the only one operating in new zealand for longer than 2 yr using a combination of teleseismic sks and s phases and regional scs phases provides good azimuthal coverage allowing us to undertake detailed modelling the splitting measurements indicate that in addition to the frequency dependence observed previously at this station there are also variations with propagation and initial polarization directions the fast polarization directions range between 2 and 103 and the delay times range between 0 75 s and 3 05 s these ranges are much larger than observed previously at snzo or elsewhere in new zealand because of the observed frequency dependence we measure the dominant frequency of the phase used to make the splitting measurement and take this into account in the modelling we fit the fast polarization directions fairly well with a two layer anisotropic model with horizontal axes of symmetry however such a model does not fit the delay times or explain the frequency dependence we have developed a new inversion method which allows for an inclined axis of symmetry in each of the two layers however applying this method to snzo does not significantly improve the fit over a two layer model with horizontal symmetry axes we are therefore unable to explain the frequency dependence or large variation in delay time values with multiple horizontal layers of anisotropy even allowing for dipping axes of symmetry for the upper layer of anisotropy the modelled fast polarization directions lie in the range 30 to 110 subparallel to the major geological features as well as the relative direction of plate motion we suggest that the upper layer of anisotropy consists of crust and subducting slab and that the lower layer consists of subslab mantle for the lower layer of anisotropy the modelled fast polarization directions lie in the range 50 to 30 which are intermediate

anisotropic structure of the hikurangi subduction zone new zealand integrated interpretation of surface wave and body wave observations summary a model of subduction zone anisotropy for south and central north island new zealand is presented by integrating the observations of shear wave anisotropy from anomalous surface wave polarizations local s phases at a range of hypocentral depths and sks splitting analyses anomalous particle motions of the equivalent to fundamentalmode love waves have been measured for propagation paths approximately along the strike of the hikurangi subduction zone in eastern north island new zealand these observations are modelled in terms of an anisotropic crust and subducted pacific slab with a pseudo hexagonal symmetry axis plunging at 30 to the vertical and a fast axis parallel to the strike of the margin a shear wave anisotropy of 4 per cent is assumed shear wave splitting analyses of regional phases in central north island show no increase in fast slow delay time 0 21 0 03 s with increased hypocentral depth unlike previous observations in southern north island which display an increase in delay time of about 0 5 s over 250 km depth fast polarization directions are again approximately parallel to the strike of subduction observed sks splitting parameters on the other hand do not vary significantly between southern and central north island and over the mantle wedge in western north island a mean fast slow delay time of 1 5 0 3 s and a fast polarization direction of 25 7 subparallel to the strike of the subduction zone were determined for the sks observations both the surface wave polarization and local shear wave splitting observations were modelled by an anisotropic new zealand crust with a pseudo hexagonal symmetry axis plunging at 15 30 and a horizontal fast axis parallel to the strike of the subduction the variation of the splitting delay times with hypocentral depth between central and southern north island implies a horizontal symmetry axis in the subducted slab in the south but an axis plunging parallel to the plate interface in central north island in the mantle wedge above the slab in western north island and the mantle beneath the slab in central and southern north island a horizontal anisotropic symmetry axis fits the observations 4 per cent anisotropy was assumed throughout the models within the new zealand crust the plunge of the symmetry axis of the anisotropic structure is compatible with the structural trends of the regional geology and fault orientations the variation in the plunge of the anisotropic symmetry axis within the subducted pacific lithosphere from central to southern north island is interpreted to reflect the change in intraplate stress southwards towards south island where the india australian and pacific plates are locked the orientation of olivine due to trenchparallel flow would explain the observations in the mantle wedge and pacific mantle below the subducted plate

stress orientations in a locked subduction zone at the southern hikurangi margin new zealand we analyse the orientation of the stress field in the southern hikurangi subduction zone new zealand using focal mechanism inversions s wave splitting fast directions and gravitational stresses here the oceanic pacific plate is being obliquely subducted beneath the continental australian plate in the new zealand plate boundary zone the study makes use of 399 earthquakes for focal mechanism inversion and 425 earthquakes for shear wave splitting analysis located with a network of seismic stations spanning southern north island we distinguish between stresses in the pacific plate from focal mechanism inversion and australian plate from s wave fast directions and gravitational stresses in three regions western central basin and eastern in the western region the principal axis of horizontal compression shmax is oriented ne sw parallel to the margin in the upper australian and lower pacific plate in the central basin shmax in the australian plate is oriented nw se perpendicular to the margin in the lower subducting pacific plate shmax is oriented ne sw in the eastern region shmax is oriented ne sw in the upper plate while in the lower plate there is a change in orientation to nnw sse we interpret the stress orientations of the lower plate in the western and central basin regions as a consequence of bending of the subducting plate sources of upper australian plate stresses are likely to be bending stresses gravitational stresses and tectonic loading with differing relative magnitudes across the study area

3d modeling of fracture induced shear wave splitting in the southern california basin splitting of shear waves generated by local earthquakes in the southern california basin and their polarization changes during propagation through layers with different orientations of anisotropy are investigated from multicomponent seismograms and their hodograms synthetic data for earthquakes in the southern california basin illustrate the effects on polarization caused by dry vertical microcracks with a crack density of 0 04 at the source and receiver locations hodograms for observed and synthetic data show that the polarization at the receiver depends only on the orientation of the anisotropy within two wavelengths of the receiver which qualitatively explains the frequency dependence of anisotropy in previous studies seismograms are simulated for a model with a range of station azimuths and distances if the anisotropy producing crack orientations do not change between the source and a receiver and the source motion is parallel to the cracks there is no shear wave splitting splitting occurs when the anisotropy orientation at the receiver or between the source and receiver is different from the source orientation

seismic anisotropy around subduction zones insights from three dimensional modeling of upper mantle deformation and sks splitting calculations 1 inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive to assist the interpretation of the seismic signal and its relation with the mantle circulation pattern we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3 d numerical models of earth like subduction this computational strategy accounts for the non steady state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered in the subduction models a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate the sub slab upper mantle is characterized by two domains with different fabrics at shallow depth the mantle entrained with the subducting slab develops trench perpendicular directed anisotropy due to simple shear deformation while in the deeper mantle slab rollback induces pure shear deformation causing trench parallel extension and fast seismic directions subducting plate advance favors the development of the fabric in the entrained mantle domain while slab retreat increases the trench parallel anisotropy in the deeper upper mantle in the deeper domain the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center as such strong trench parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates the synthetic sks splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain with trench perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges instead above relatively narrow retreating slabs 600 km and low subduction partitioning ratio spr azimuths are trench parallel due to the strong anisotropy in the lower sub slab domain in all models the anisotropy in the back arc and on the sides of the subducting plate is respectively trench perpendicular and sub parallel to the return flow at depth results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally

complex anisotropy beneath the peruvian flat slab from frequency dependent multiple phase shear wave splitting analysis flat or shallow subduction is a relatively widespread global occurrence but the dynamics remain poorly understood in particular the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail here we present measurements of seismic anisotropy to investigate mantle flow beneath the peruvian flat slab segment the largest present day region of flat subduction we conduct a detailed shear wave splitting analysis at a long running seismic station nna located near lima peru we present measurements of apparent splitting parameters fast direction and delay time t for sks scs and local s phases from 80 events we observe well defined frequency dependence and backazimuthal variability indicating the likely presence of complex anisotropy forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench normal fast direction underlying a layer with a more trench oblique to trench subparallel fast direction in order to further constrain the anisotropic geometry we analyzed the source side splitting from events originating within the slab measured at distant stations beneath the flat slab segment we found trench normal fast splitting directions in the subslab mantle while within the dipping portion of the slab further to the east likely trench subparallel anisotropy within the slab itself this subslab pattern contradicts observations from elsewhere in south america for normal i e more steeply dipping slab conditions it is similar however to inferences from other shallowly dipping subduction zones around the world while there is an apparent link between slab dip and the surrounding mantle flow at least beneath peru the precise nature of the relationship remains to be clarified

anisotropy in the oceanic lithosphere from the study of local intraplate earthquakes on the west flank of the southern east pacific rise shear wave splitting and waveform modeling shear wave splitting is observed on ocean bottom seismometer records from local intraplate microearthquakes on the west flank of the east pacific rise at 18 s split times reach a maximum of about 0 2 s for most of the incoming waves with long mantle paths 20 km the polarization direction of the fast arrival is subparallel to the spreading direction which we attribute to anisotropy caused by the strain induced preferential orientation of olivine in contrast for some records with short paths or shallower sources or propagation along the spreading direction the fast direction is nearly parallel to the ridge axis these polarizations are probably caused by seismic anisotropy from aligned cracks in the uppermost crust in addition for some events the apparent splitting is frequency dependent to explore the pattern of shear wave splitting that would be expected for nonvertical paths in an oceanic lithosphere with two distinct anisotropic layers we generate synthetic seismograms for a variety of source depths and mechanisms we employ a multidomain pseudospectral method to simulate the elastic wave fields from point sources in an inhomogeneous anisotropic medium splitting parameters measured from synthetic s waves demonstrate that the apparent fast direction is not always parallel to the symmetry axes and that in some cases fast directions at higher frequencies will be more characteristic of the shallower crustal anisotropy while fast directions at lower frequencies are dominated by the mantle portion of the path most of the observed characteristics of splitting can be reproduced if there is approximately 8 s wave anisotropy in the mantle and an average of about 6 s wave anisotropy in the upper crustal seismic layer 2

shear wave polarizations near the north anatolian fault ii interpretation in terms of crack induced anisotropy summary the polarizations of shear waves recorded by networks of digital thxee component seismometers immediately above small earthquakes near the north anatolian fault in turkey display shear wave splitting on almost all shear wave seismograms recorded within the shear wave window this splitting is incompatible with source radiation patterns propagating through simple isotropic structures but is compatible with effective anisotropy of the internal structure of the rock along the ray paths this paper interprets the phenomena in terms of widespread crack induced anisotropy distributions of stress induced cracks model many features of the observations and synthetic polarization diagrams calculated for propagation through simulated cracked rock are similar to the observed patterns this evidence for widespread crack induced anisotropy lends strong support to the hypothesis of extensive dilatancy anisotropy eda suggested by laboratory experiments in subcritical crack growth the crucial evidence confirming some form of eda would be observations of temporal changes in shear wave splitting as the stress field alters the crack density and crack geometry there is some weak evidence for such temporal changes at one site but further analysis of suitable digital three component seismometer networks in seismic areas is required to confirm eda

frequency dependent shear wave splitting and heterogeneous anisotropic structure beneath the gulf of california region abstract the gulf of california region has undergone a major evolution over the past 15 ma since subduction of the now extinct farallon plate ceased and transform motion began leading to the initiation of rifting and the opening of the gulf of california 5 ma ago the character of dynamic processes in the upper mantle beneath the region remains poorly understood but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes in order to characterize more fully the anisotropic structure of the upper mantle beneath the region i present measurements of sks splitting at 14 broadband stations of the nars baja array at periods between 8 and 50 s the measured splitting parameters t at nars baja stations exhibit a high degree of complexity with dramatic lateral variations significant backazimuthal variations at individual stations and a large number of well constrained null measurements over a range of backazimuths the dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal to noise sks arrivals i find that while fast directions are generally insensitive to the sks frequency content delay times are frequency dependent at several stations the measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the gulf of california region is complex and exhibits a high degree of both lateral and vertical heterogeneity the observed splitting is consistent with a scenario in which a large scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with farallon subduction the pattern of mantle flow appears to be controlled by more localized processes likely including small scale buoyant upwelling associated with partial melting beneath the central part of the gulf of california rift