anisotropy and the splitting of ps waves the ps wave is radially polarized in an isotropic spherically symmetric earth owing to the p to sv conversion at the free surface if we choose stations with weak sks splitting to minimize the effect of the receiver we can measure the anisotropy beneath the bounce point of the ps wave we find that with a deep source and epicentral range of 90 125 the ps phase can be distinguished from other phases owing to the limitation of recent large deep events with epicentral range between 90 and 125 we analyze data from stations in north america and events beneath the bonin islands and the fiji islands for events in these two regions the bounce points of the ps wave are roughly located northwest of the kuril islands and near the nova canton trough in the central pacific respectively because the first fresnel zone for the ps wave at this distance range is roughly 700 km across each ps splitting observation must be interpreted as a local average of anisotropic properties the average direction of the fast axis of anisotropy below the nova canton trough region is n104 2 e with a standard deviation 12 7 and a typical delay time of 1 8 s landward of the kuril islands the average direction of the fast axis is n127 6 e clockwise from north and the standard deviation is 11 3 with a typical delay time of 1 2 s we have compared the fast axis with the direction of fossil seafloor spreading and with the present day absolute plate motion the fast axis of anisotropy at the nova canton trough agrees well with the direction of absolute plate motion and poorly with the complex fossil spreading pattern this suggests that significant upper mantle shear anisotropy exists in this part of the central pacific and is consistent with mineral alignment owing to strain associated with present day plate motion similarly the fast axis beneath the kuril islands parallels the convergent motion of the slab coupled mode synthetics constructed with the strong born approximation can be used to model the interaction of the ps waves with anisotropic structure that has a horizontal axis of symmetry we use all free oscillations up to 35 mhz in the calculation the coupled mode synthetics show shear wave splitting with delay time and fast axis consistent with the ray theory prediction

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

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

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

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

global anisotropy in the upper mantle inferred from the regionalization of phase velocities a data set of 2600 paths for rayleigh waves and 2170 paths for love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70 250 s the epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of earth a synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path to process such a large data set a new tomographic technique is designed a good correlation is found with surface tectonics in the whole period range for love waves and at shorter periods for rayleigh waves the directions of maximum velocities are also well correlated with plate tectonics motions for the 2 azimuthal term of rayleigh waves when the power spectrum of heterogeneities is computed it is found that for love waves the even orders are usually larger than the odd orders for rayleigh waves the degrees 2 and 6 dominate only at long periods suggesting a deep origin for these components

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

upper mantle flow beneath french polynesia from shear wave splitting summary upper mantle flow beneath the south pacific is investigated by analysing shear wave splitting parameters at eight permanent long period and broad band seismic stations and 10 broad band stations deployed in french polynesia from 2001 to 2005 in the framework of the polynesian lithosphere and upper mantle experiment plume despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity upper mantle seismic anisotropy has been detected at all stations except at tahiti where two permanent stations with 15 yr of data show an apparent isotropy the median value of fast polarization azimuths n67 5 w is parallel to the present pacific absolute plate motion direction in french polynesia apm n67 w this suggests that the observed sks fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving pacific plate and preserved in the lithosphere as the plate cools however analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper mantle structure more complex than a single anisotropic layer with horizontal fast axis a forward approach shows that a two layer structure of anisotropy beneath french polynesia better explains the splitting observations than a single anisotropic layer secondorder variations in the measurements may also indicate the presence of small scale lateral heterogeneities the influence of plumes or fracture zones within the studied area does not appear to dominate the large scale anisotropy pattern but may explain these second order splitting variations across the network

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

observations of teleseismic shear wave splitting in the basin and range from portable and permanent stations observations of shear wave splitting were obtained from temporary and permanent stations in western nevada and southern california measurements of fast polarization azimuth and delay time t were made on both sks and direct s phases for stations in the northern basin and range the results were consistent yielding an average of t 75 8 0 9 0 3 s one station in the southern basin and range gave 40 11 0 4 0 1 s and one in the mojave desert gave 54 3 1 2 0 1 s t 0 9s is consistent with a 100km thick mantle layer characterized by 4 anisotropy for the northern stations the anisotropy appears unrelated to the present day extension or absolute plate motion rather we suggest that it is fossil anisotropy associated with pre miocene extension whose direction is about 68 for the california station is nearly parallel to the strike of the san andreas fault system and is attributed to shear strain associated with relative plate motion

theory and observations of ekman flux in the chlorophyll distribution downstream of south georgia 1 a large phytoplankton bloom occurs downstream of south georgia an island on the northern edge of the scotia sea atlantic sector of the southern ocean this is due to natural iron fertilisation being advected downstream in the antarctic circumpolar current mapping of seawifs chl a in dynamic height longitude space reveals the trajectory of surface water under the dual influences of geostrophic flow and ekman flux a theoretical estimate of the trajectory shows good agreement with observations the trajectory is dependent on the wind stress mixed layer depth ratio but independent of the latitudinal separation of dynamic height contours the chl a distribution suggests that iron reaches 20 w below the mixed layer away from the effects of ekman flux but beyond the iron is confined to the surface layer chl a reduces monotonically east of south georgia with the signal detectable to 15 e over 3500 km downstream indicating the basin scale biogeochemical influence of the iron supply

anisotropy in kuril kamchatka subduction zone structure to constrain the orientation and extent of seismic velocity anisotropy in the kuril kamchatka subduction zone we have developed a technique that isolates anisotropy at shallow depths by differencing splitting observed in teleseismic shear wave phases olivine a axis orientations inferred from 29 ss s phase pairs and two local s phases vary between the major tectonic components of the trench back arc system although the ss paths do not uniquely constrain a single model of anisotropy a axes are consistent with finite strain patterns predicted by plate convergence and back arc spreading and shearing

anisotropy and shear velocity heterogeneities in the upper mantle long period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle the dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure the data are corrected for the effect of surface layers and both love and rayleigh waves are used shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km the shear wave velocity distribution to 200 km depth correlates with surface tectonics except in a few anomalous regions below that depth the correlation vanishes cold subducted material shows up weakly at 350 km as fast s wave anomalies in the transition region a large scale pattern appears with fast mantle in the south atlantic s anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones the pacific shows a nw se fabric

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

seismic structure of the upper mantle in a central pacific corridor the seismic structure of the tonga hawaii corridor has been investigated by combining two data sets revenaugh and jordan s reflectivity profile from scs reverberations which provides travel times to and impedance contrasts across the major mantle discontinuities and 1500 new observations of frequency dependent phase delays for the three component s ss and sss body waves and the r1 and g1 surface waves which constrain the velocity structure within this layered framework the shear waves turning in the upper mantle showed significant splitting of the sh and sv components indicative of shallow polarization anisotropy the data set was inverted in conjunction with attenuation and mineralogical constraints to obtain a complete spherically symmetric radially anisotropic structure the final model pa5 is characterized by a high velocity anisotropic lid bounded at 68 km depth by a large negative g discontinuity a low velocity anisotropic layer below g extending to a small l discontinuity at 166 km an isotropic steep gradient region between 166 km and 415 km and transition zone discontinuities at 415 507 and 651 km the depth of the radial anisotropy in pa5 is shallower than in most previous studies based on surface waves and higher modes the average value of radial shear anisotropy in the lid 3 7 is consistent with the magnitude expected from the spreading controlled models of olivine orientation while anisotropy in the low velocity zone which is required by our data set could be induced either by paleostrains that took place near the ridge crest or by shearing in the asthenosphere as a result of present day plate motions on the basis of recent work by hirth and kohlstedt we suggest that the g discontinuity is caused by a rapid increase in the water content of mantle minerals with depth marking the fossilized lower boundary of the melt separation zone active during crust formation the high gradient zone between 200 and 400 km is a characteristic feature of convecting oceanic upper mantle and is probably controlled by a steady decrease in the homologous temperature over this depth interval the average shear velocity gradient in the transition zone is lower than in most previous seismic models in better agreement with the predictions for a pyrolitic composition

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

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

a search for source side mantle anisotropy shear wave splitting due to source side mantle anisotropy is estimated from broadband teleseismic s waves using a suite of 19 subduction zone events more than 50km depth and recorded at several rstn stations we assume that there are at most two anisotropic regions along the ray path one on the source side and one on the receiver side and that receiver side splitting is known from analysis of sks it is found that splitting is not detectable below the slab in most cases there are however two specific regions one beneath peru and one beneath kamchatka that indicate significant source side anisotropy based on two nearby intermediate focus events in each region splitting delay times are found to be 1 0 1 5s with the fast polarization direction perpendicular to the trench assuming 4 intrinsic anisotropy this would correspond to path lengths of 100 170 km below the slab both regions are characterized by abrupt changes in along strike slab properties a change in slab dip in the case of peru and a cessation of deep focus seismicity in the case of kamchatka we speculate that the anisotropy is caused by an associated local disturbance in asthenospheric flow that the shear wave splitting parameters of s and sks are consistent with each other in most cases suggests that large scale anisotropy in the lower mantle is very small probably no more than a fraction of a percent

rayleigh wave phase velocities in the pacific with implications for azimuthal anisotropy and lateral heterogeneities summary the lateral distribution of fundamental mode rayleigh wave phase velocities in the pacific has been calculated in order to determine the variation in velocity as a function of the age of the oceanic plate the importance of azimuthal anisotropy and the presence of secondary lateral heterogeneities the data set used in this study consists of phase velocity measurements in the period range 20 125 s for 178 paths traversing the pacific both the pure path and spherical harmonic inversion techniques are used in this investigation with an emphasis placed on determining the effectiveness and resolving power of these methods in calculating the velocity distribution in an oceanic regime the first order velocity age relation which dominantly characterizes surface wave dispersion in an oceanic environment can be adequately modelled by either the pure path or spherical harmonic techniques azimuthal anisotropy is the dominant second order effect and is best developed in regions less than 80 myr in age in this age zone the faster direction of wave propagation is either in the direction of fossil seafloor spreading or in the direction of present day absolute plate motion in the western pacific there is a correlation with paleo relative motion fossil seafloor spreading for periods less than 50 s lateral velocity heterogeneities which are not related to the age of the oceanic plate are found by pure path inversions and are correlated with regions associated with shallow residual depth anomalies this effect slow velocities with respect to the velocity age model monotonically decreases as a function of period and is indicative of a shallow origin age independent anomalies were also modelled by the application of the pure path and spherical harmonic methods in a sequential inversion

quasi love phases between tonga and hawaii observations simulations and explanations seismograms of some shallow tonga earthquakes observed at hawaii contain sv polarized phases in the love wave time window most prominently on the vertical component given the geometry of the observations 40 45 such phases may be explained either as body waves or as mode converted surface waves detailed synthetic seismogram modeling of representative events reveals several instances where the body wave explanation is inadequate even when plausible uncertainties in the source mechanism are taken into account the observed sv polarized phase can instead be generated through love rayleigh scattering which requires laterally varying seismic anisotropy along the tonga hawaii path trial and error forward modeling with simple structures based on the transversely isotropic mid pacific velocity model pa5 of gaherty et al 1996 obtains velocity structure that yields synthetic seismograms matching the observations this model while non unique suggests first order constraints on the lateral variation in anisotropic properties and associated mantle flow along the tonga hawaii path by examining trade offs in model parameters we conclude that robust features of the model are 1 a transition from radial to mixed radial and azimuthal anisotropy 3 5 from hawaii 2 the nw se alignment of the axis of azimuthal anisotropy 3 higher degree of p anisotropy relative to s anisotropy and 4 the presence of azimuthal anisotropy within upper 200 250 km of the mantle taken together these features imply a disruption of mantle fabric by the processes forming hawaii emperor volcanic system a model with anisotropic gradients in both the lithospheric lid and shallow asthenosphere is the simplest extension of our starting model however an equivalent data fit can be obtained if the azimuthal anisotropy gradients are restricted to line beneath the high velocity lid of model pa5 so that mantle hot spot flow need not penetrate the lithospheric lid

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

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

where can seismic anisotropy be detected in the earth s mantle in boundary layers during the last 30 years considerable evidence of seismic anisotropy has accumulated demonstrating that it is present at all scales but not in all depth ranges we detail which conditions are necessary to detect large scale seismic anisotropy firstly minerals must display a strong anisotropy at the microscopic scale and or the medium must be finely layered secondly the relative orientations of symmetry axes in the different crystals must not counteract in destroying the intrinsic anisotropy of each mineral and there must be efficient mechanisms of orientation of minerals and aggregates finally the strain field must be coherent at large scale in order to preserve long wavelength anisotropy part of shallow anisotropy can be related to the past strain field frozen in anisotropy however the deep anisotropy is due to the present strain field all these conditions are fulfilled only in boundary layers of convective mantle we review in this paper the seismic data sets which provide insight into the location at depth of large scale anisotropy from the d layer up to the lithosphere in addition to the well documented seismic anisotropy in the lithosphere and asthenosphere there is new evidence of seismic anisotropy in the upper 400 660 km and lower 660 900 km transition zones and in the d layer nonetheless the bulk of the lower mantle seems close to isotropy if we assume the hypothesis that seismic anisotropy is associated with boundary layers in convective systems these observations strongly suggest that the transition zone is a boundary layer which makes the pasage of matter between the upper and the lower mantle difficult however this general statement does not rule out flow circulation between the upper and lower mantles finally the geophysical mineral physics and geological applications are briefly reviewed an intercomparison between surface wave anisotropy and body wave anisotropy data sets is presented we discuss the scientific potential of seismic anisotropy and how it makes it possible to gain more insight into continental root deformation and geodynamics processes

shear wave splitting and reservoir crack characterization the coso geothermal field abstract this paper aims to improve current understanding of the subsurface fracture system in the coso geothermal field located in east central california the coso reservoir is in active economic development so that knowledge of the subsurface fracture system is of vital importance for an accurate evaluation of its geothermal potential and day to day production to detect the geometry and density of fracture systems we applied the shear wave splitting technique to a large number of high quality seismograms from local microearthquakes recorded by a permanent 16 station down hole 3 component seismic array running at 480 samples s the analysis of shear wave splitting seismic birefringence provides parameters directly related to the strike of the subsurface fractures and their density number of cracks per unit volume and consequently is an important technique to outline zones of high permeability three major fracture directions n10 30w n0 20e and n40 50e of which the first and the second are the most prominent were identified from the seismograms recorded by the 16 station down hole array all orientations are consistent with the known strike of local sets of faults and fractures in local wells and at the surface as well as with previous analyses of seismic anisotropy in the region the high quality of the recordings has allowed us to launch an unprecedented investigation into the characteristics of the temporal variations in crack polarization and crack density in a producing geothermal environment preliminary results point to significant temporal changes in shear wave time delays probably influenced by temporal changes in crack density within a period of 5 years 1996 2000 they are tentatively interpreted as due to a local 3 increase in shear wave velocity in the southwestern part of the field during 1999

seismic determination of elastic anisotropy and mantle flow when deformed many rocks develop anisotropic elastic properties on many seismic records a long period 100 to 250 seconds quasi love wave with elliptical polarization arrives slightly after the love wave but before the rayleigh wave mantle anisotropy is sufficient to explain these observations qualitatively as long as the fast axis of symmetry is approximately horizontal quasi love observations for several propagation paths near pacific ocean subduction zones are consistent with either flow variations in the mantle within or beneath subducting plates or variations in the direction of fossil spreading in older parts of the pacific plate

hunting for azimuthal anisotropy beneath the pacific ocean region lateral variations in azimuthal anisotropy cause significant waveform anomalies in long period surface waves 8 mhz suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central pacific

sediment borne seeds from sand creek northwestern kansas taphonomic significance and paleoecological and paleoenvironmental implications abstract a study of seeds fruits and reproductive bracts collectively seeds collected during 1986 1988 from recent sediments along a small perennial stream in northwestern kansas was conducted to determine the taphonomic significance of sediment borne seeds and the implications for studies of similar accumulations of fossil plants the samples were collected from sediments in but at the margin of a flowing stream the abundant and diverse assemblage of aquatic and marginal aquatic plants recovered in the sediments shows that the sediment borne seeds accurately reflect the marginal aquatic habitat next to the channel and temperate climate of the region the sediments also contained a wide assortment of other taxa that represented native riparian floodplain and upland habitats as well as cultivated croplands overall these taxa are indicative of temperate grasslands disturbance of the study site by trampling by livestock and perphaps by lowered stream flow during the last year reduced the variety of taxa and numbers of specimens in samples but these influences did not change the marginal aquatic habitat suggested by sediment borne seeds comparison of a late miocene 7 8 m y b p assemblage of seeds from the minium quarry in northwestern kansas with the sand creek results suggests that the habitat nearest the deposition site of the fossils was a marginal aquatic setting similar to that sand creek the comparison alos suggests that the miocene landscape beyond the depositional site was dominated by warm temperate to subtropical grasslands an interpretation supported by faunal evidence