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

deep structure of the santos basin sao paulo plateau system se brazil the structure and nature of the crust underlying the santos basin sao paulo plateau system ssps in the se brazilian margin are discussed based on five wide angle seismic profiles acquired during the santos basin sanba experiment in 2011 velocity models allow us to precisely divide the ssps in six domains from unthinned continental crust domain cc to normal oceanic crust domain oc a seventh domain domain d a triangular shape region in the se of the ssps is discussed by klingelhoefer et al 2014 beneath the continental shelf a 100 km wide necking zone domain n is imaged where the continental crust thins abruptly from 40 km to less than 15 km toward the ocean most of the ssps domains a and c shows velocity ranges velocity gradients and a moho interface characteristic of the thinned continental crust the central domain domain b has however a very heterogeneous structure while its southwestern part still exhibits extremely thinned 7 km continental crust its northeastern part depicts a 2 4 km thick upper layer 6 0 6 5 km s overlying an anomalous velocity layer 7 0 7 8 km s and no evidence of a moho interface this structure is interpreted as atypical oceanic crust exhumed lower crust or upper continental crust intruded by mafic material overlying either altered mantle in the first two cases or intruded lower continental crust in the last case the deep structure and v shaped segmentation of the ssps confirm that an initial episode of rifting occurred there obliquely to the general opening direction of the south atlantic central segment

transdimensional inversion of scattered body waves for 1d s wave velocity structure application to the tengchong volcanic area southwestern china abstract inversion of receiver functions is commonly used to recover the s wave velocity structure beneath seismic stations traditional approaches are based on deconvolved waveforms where the horizontal component of p wave seismograms is deconvolved by the vertical component deconvolution of noisy seismograms is a numerically unstable process that needs to be stabilized by regularization parameters this biases noise statistics making it difficult to estimate uncertainties in observed receiver functions for bayesian inference this study proposes a method to directly invert observed radial waveforms and to better account for data noise in a bayesian formulation we illustrate its feasibility with two synthetic tests having different types of noises added to seismograms then a real site application is performed to obtain the 1 d s wave velocity structure beneath a seismic station located in the tengchong volcanic area southwestern china surface wave dispersion measurements spanning periods from 8 to 65 s are jointly inverted with p waveforms the results show a complex s wave velocity structure as two low velocity zones are observed in the crust and uppermost mantle suggesting the existence of magma chambers or zones of partial melt the upper magma chambers may be the heart source that cause the thermal activity on the surface

anisotropy in the oceanic lithosphere of the north western pacific basin summary a series of long range explosion seismological experiments has been conducted by the use of specially designed ocean bottom seismographs obss in the western pacific obs studies of apparent velocity measurements by the use of natural earthquakes have also been made the experiments have made clear that large scale p wave anisotropy exists in the entire thickness of the oceanic lithosphere the existence of the large scale anisotropy in the oceanic lithosphere has been demonstrated for the first time by seismic body wave studies previously anisotropy had been found only in the uppermost oceanic mantle in the eastern pacific the azimuth of the maximum velocity 8 6km s l is about 155 clockwise from north the direction is perpendicular to the magnetic lineation of the region however the direction differs from the direction of the present plate motion by about 30 so it appears that the anisotropy has been frozen at least since the change of the plate motion that occurred 40myr ago the frozen anisotropy should set important constraints on the mechanical properties of the lithosphere such as the viscosity and temperature of the lower lithosphere

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

structure of the mantle beneath montana lasa from analysis of long period mode converted phases abstract coherent precursors to shear phases s sks scs observed on long period seismograms recorded at lasa are interpreted as shear to p type mode conversions or sdp phases from mantle discontinuities at depth d polarization analysis on three component beam traces confirm that the precursors are compressional phases mode conversions are distinguished from multiple surface reflections on the basis of apparent slownesses constant lead times of mode converted phases relative to the shear phases for different focal depths and amplitude considerations the waveform shapes of mode conversions are modeled using the matrix propagator method two mode conversions from the top and bottom of a low velocity zone between 171 and 226 km depth explain the waveform shape immediately in front of the primary shear waves the strongest mode conversion is produced by a discontinuity at about 451 km and has a velocity contrast of about 10 per cent the variation in the measured s 451 p to primary s amplitude ratio recorded at the subarrays strongly correlates with the variation in the p to s amplitude ratios and is probably caused by focusing and defocusing effects in the lithosphere beneath the array a 556 km discontinuity is inferred from the partial interference of s 556 p with s 451 p interfering s 671 p and s 770 p phases seem to be present on one of the beam traces however the presence of these mode conversions is problematic because they do not appear on all the beam and subarray traces

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

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

constraints on the interpretation of s to p receiver functions summary we present results from forward modelling to study the feasibility of using s to p converted phases to image the seismic discontinuity structure of the crust and upper mantle we show that a significant level of p wave energy arriving before the direct s wave arrival can interfere with the s to p converted phases of interest and may result in sp receiver function phases that do not represent true earth structure the source of this p wave energy is attributable to a number of phases including those that have undergone multiple reflections off the earth s surface for deep focus earthquakes 300 600 km deep a significant amount of p wave energy is observed from pppp ppppp and spppp phases and arrives within the same time window as predicted for s to p converted phases from the direct s phase arrival furthermore for earthquakes at all depths interfering p wave energy arrives within the same time window as predicted for s to p converted phases from the sks phase arrival limiting the usefulness of sksp receiver functions for upper mantle imaging to isolate true sp receiver function phases from contamination due to other p wave phases we find it necessary to stack receiver functions from a range of epicentral distances and depths in order to aid the suppression of noise and other unwanted phases we provide constraints on the noise levels to be expected as a function of epicentral distance and earthquake depth we find that the lowest noise levels are achievable by restricting epicentral distance to less than 75 degrees and the depth of earthquakes used to less than 300 km

upper mantle seismic anisotropy and lithospheric decoupling the lithosphere underlying stable continental shield regions is thought to extend to a depth of 200 km compressional wave velocity models derived from a recent approximately ns refraction profile in northern australia include a pronounced high velocity zone at a depth of around 200 250 km conventional petrological interpretations in terms of mineralogical or compositional changes cannot readily explain this feature calculated velocities for the garnet pyrolite model using a derived geotherm fit the velocities above and below this high velocity zone but cannot account for the high velocity 8 6 km s 1 zone itself this feature is explained in terms of velocity anisotropy which might reflect mechanical decoupling of the continental lithosphere from the underlying mantle

fan delta uplift and mountain subsidence during the haiti 2010 earthquake the 2010 haiti earthquake was initially thought to have ruptured the enriquillo plantain garden fault but previously unrecognized faults were later implicated radar imaging of surface deformation caused by the earthquake reveals uplift of alluvial fans and subsidence of mountains consistent with reverse motion on a blind thrust fault

seismic anisotropy in northern and central gulf of california region mexico from teleseismic receiver functions and new evidence of possible plate capture 1 we present anisotropic models for the shallow lithosphere in the northern and central gulf of california obtained from receiver function rf analysis this region has undergone intense plate reorganization over the past 25 ma that led to the transfer of the plate boundary east of the modern baja california peninsula posterior to the cease of the subduction of the farallon plate beneath the north american plate our main interest in modeling the seismic anisotropy is to characterize rock fabric inherited from past and recent tectonism and to provide new information for understanding the tectonic processes involved in the evolution of the local plate boundary we calculate rfs from teleseismic p waves recorded by the network of autonomously recording seismographs nars baja array and we stack them in 20 wide back azimuthal bins to enhance coherent information these rfs yield significant transverse energy whose azimuthal variations are not consistent with that expected for a crust with dipping moho therefore we interpret the conspicuous rfs energy of the transverse component as convincing evidence of anisotropy we present forward models obtained by fitting the radial and transverse rfs with synthetic seismograms calculated with a reflectivity code our analysis suggests substantial variations in the structure of the shallow lithosphere along the baja california peninsula and the presence of stalled fragments of the extinct farallon plate beneath its central part the results at station ne81 in sonora are interpreted as frozen crustal anisotropy inherited from miocene basin and range extension consistent with previous observations of the upper mantle anisotropy beneath this station

upper mantle anisotropy in sw iberia from long range seismic profiles and teleseismic shear wave data abstract deep seismic profiles of the iliha experiment have provided evidence for the presence of anisotropy in the deeper levels of the lithosphere beneath the ossa morena zone of the hercynian domain of sw iberia allowing estimation of its main characteristics a n15 e fast p wave velocity direction and a degree of anisotropy of around 4 the consistency of these results is tested against an anisotropy analysis using teleseismic shear waves data recorded in a dense network of up to 13 digital three component stations deployed in the area for about 2 months are used to study the spatial coherence of wave fields in particular of shear wave splitting observations over an array significant small scale lateral variations are observed in the data suggesting that care should be taken in using single station results as representative for geodynamical interpretations in spite of the short duration of the test a few teleseismic events provide useful data the analysis of sks and s phases shows evidence of the presence of anisotropy the results from the different events depict a rather consistent anisotropic structure with roughly a ne sw to east west fast velocity direction the anisotropic patterns derived from dss crossing profiles and shear wave splitting over the ossa morena zone are compared mechanisms classically suggested to be responsible for the origin of the anisotropy such as past deformations of the lithosphere or relative motion between lithosphere and asthenosphere are discussed to test their consistency with the geophysical and geological data available in the area

testing five of the simplest upper mantle anisotropic velocity parameterizations using teleseismic s and sks data from the billings montana passcal array 1 five of the simplest parameterizations of upper mantle anisotropy are tested and ranked for a data set collected on a dense temporary passcal seismic array located 100 km ne of yellowstone these hexagonal symmetry anisotropy models possess either one or two layers with either flat or dipping fast velocity axis fva recordings from fifteen high quality direct s and sks arrivals are stacked to provide accurate waveform and error estimates source normalization is accomplished using the cross convolution technique a direct monte carlo neighborhood algorithm is used to map the posteriori model probability density ppd volume using the f test we find that models with purely flat fva can be rejected at 97 probability our best model p5 is a two layer dipping fva parameterization albeit the two layer model with one flat and one dipping fva can only be rejected at 80 probability the best model has a lower layer with a n65 e fva strike and a 12 dip down to the southwest and an upper layer with a n20 w fva strike and a 47 fva dip down to the southeast the bottom asthenospheric layer fva strikes parallel to north america s absolute plate motion direction and dips opposite to what passive plate shear of the asthenosphere would predict the upper lithospheric layer is consistent with lpo accretion associated with north directed drift of the north american plate during the mesozoic comparison between the sks and direct s wave data sets shows that the direct s waves improve resolution of the double layer anisotropic model parameters

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

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

upper mantle anisotropy beneath australia and tahiti from p wave polarization implications for real time earthquake location we report measurements of long period p wave polarization p pol in australia and tahiti made by combining modeling of the polarization deviation and harmonic analysis the analysis of the deviation of the horizontal polarization of the p wave as a function of event back azimuth may be used to obtain information about 1 sensor misorientation 2 dipping discontinuities 3 seismic anisotropy and 4 velocity heterogeneities beneath a seismic station the results from harmonic analysis and a grid search using snell s law suggest the presence of a dipping seismic discontinuity beneath stations ctao and can in australia these results are consistent with published receiver function studies for these stations the p pol fast axis orientation is close to the n s absolute plate motion direction at station tau tasmania which may be due to plate motion driven alignment of olivine crystals in the asthenosphere interestingly measurements of sks splitting at tahiti french polynesia show an apparent isotropy whereas an inversion of p pol observations at pptl seismic station located in tahiti suggests the presence of two anisotropic layers the fast axis azimuth is oriented e w in the upper layer and it is close to the nw se orientation in the lower layer since p pol orientations are used for real time earthquake locations especially in poorly instrumented areas such as the south pacific we show that the bias from anisotropy and sensor misorientation determined here can be corrected to improve the location accuracy which yields fundamental data for rapid location necessary for effective tsunami warning

origin of precursors to teleseismic s waves abstract in the literature there are descriptions of precursors to teleseismic s waves that are polarized as teleseimic p they are commonly regarded as phases converted from sv to p underneath the seismographic stations we present observations of precursors at the broadband digital geoscope stations in the distance range from 45 to 95 and at periods around 10 sec these precursors are polarized as the teleseismic p but cannot be interpreted in terms of conversion underneath the station we propose that they are formed by conversion and scattering from s to p at the free surface and scattering of the resulting p in the lithosphere of the region between the source and the receiver the apparent velocity of s in the region of scattering is usually around 7 km sec which implies a long wavepath of the converted p in the continental crust apparently s to p and p to p scattering play an important role in forming teleseismic wave fields even in the relatively long period band around 10 sec a possibility of observing true s p converted phases originating in the lithosphere underneath the station is certainly not ruled out however our analysis is a warning that scattered waves sometimes can be erroneously taken for converted phases

teleseismic shear wave splitting and deformations in central europe abstract we analyse the results of measurements of shear wave splitting due to azimuthal anisotropy in the upper mantle of central europe based on the sks technique the observations at seismograph stations in the eastern part of germany are combined with the published estimates of the parameters of azimuthal anisotropy at neighbouring locations these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the upper mantle directions of polarization of the fast wave counted clockwise from north change from 50 70 in the western part of the region to 100 120 in the eastern part these directions are close to the trend of the hercynian fold belt and might suggest that anisotropy is frozen in the subcrustal lithosphere however the observed traveltime delays between the split waves are generally too large to be easily reconciled with the thermal and seismic estimates of thickness of the subcrustal lithosphere underneath central europe

joint interpretation of upper mantle anisotropy based on teleseismic p travel time delays and inversion of shear wave splitting parameters abstract we inverted shear wave splitting parameters and simultaneously analysed delays of teleseismic longitudinal waves to obtain a self consistent three dimensional 3 d image of the anisotropic upper mantle beneath the continents efficiency of the simultaneous 3 d analyses of p residual spheres and shear wave polarizations is demonstrated on data from two regions southern sweden and the western usa the anisotropic inferences of the subcrustal lithosphere are to a first approximation represented by homogeneous hexagonal or orthorhombic media with plunging symmetry axes

upper mantle anisotropy a preliminary model seismic anisotropy in the upper mantle can be explained by crystallographic mineral alignment achieved through dislocation motion the physical mechanism of mineral alignment requires upper mantle shear flow which reorients and aligns minerals by dislocation glide and climb governed by the dominant glide system of each mineral the dominant glide systems are assumed to be 100 0l0 for olivine and 001 100 for the pyroxenes these yield a predominantly orthorhombic fabric with the olivine 100 and the pyroxene 001 axes aligned in the upper mantle flow direction and the olivine 010 and the pyroxene 100 axes aligned normal to the upper mantle flow plane these glide systems have a threshold temperature of enhanced mobility of 1100 1200 k which yields a solid state thermally defined lithosphere asthenosphere boundary in an olivine pyroxene mantle consistent with recent seismic determinations of the thickness of the lithosphere mantle anisotropy due to mineral alignment is then actively maintained below this boundary asthenosphere and mesosphere and is a fossil state above this boundary lithosphere we use the dominant glide systems to establish the crystallographic orientation of olivine and pyroxene in calculating the maximum seismic anisotropy of two petrologic models pyrolite and piclogite for the upper mantle by an extrapolation of single crystal anisotropic mineral elastic properties to a depth of 400 km the real earth seismic anisotropy will be bound by the limits of maximum anisotropy from perfect mineral alignment and minimum anisotropy isotropy from random mineral alignment the seismic anisotropy of the upper 220 km is best represented by the pyrolite model which reduces to quasi hexagonal symmetry with the unique axis in the direction of mantle flow the lehmann discontinuity is conjectured to be due to a change in composition from pyrolite to piclogite and therefore may represent a change in anisotropy the piclogite model has orthorhombic symmetry distinctly different from that of the pyrolite the piclogite anisotropy model can appear quasi isotropic when measured by transverse isotropy parameterization if the mantle flow is mainly horizontal with a horizontal shear plane that is the shear flow gradient duh dr dominates

neighbourhood inversion of teleseismic ps conversions for anisotropy and layer dip summary the inversion of teleseismic receiver functions for lithospheric structure is difficult due to the non linearity of the problem which is greatly increased in the presence of dipping interfaces and layer anisotropy given an efficient ray theoretical tool for forward modelling teleseismic seismograms we perform a directed monte carlo search technique using the neighbourhood algorithm of sambridge enabling us to search 20 30 parameters in a reasonable amount of computer time tests on synthetic data reveal inherent velocity depth trade offs in typical data sets due to the limited moveout present in teleseismic ps the azimuth of the anisotropic symmetry axis and the strike of a dipping interface prove to be well resolved given adequate backazimuthal coverage we apply this technique to two single station data sets the first from permanent station pgc vancouver island british columbia displays dipping low velocity sediment layers in the mid crust the second from a station at the northern end of the tibetan plateau operating in 1991 and 1992 requires a sequence of thick crustal anisotropic layers to explain the observed pattern of receiver function arrivals

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

simultaneous velocity and interface tomography of normal incidence and wide aperture seismic traveltime data summary a tomographic inversion technique that inverts traveltimes to obtain a model of the subsurface in terms of velocities and interfaces is presented it uses a combination of refraction wide angle reflection and normal incidence data it simultaneously inverts for velocities and interface depths and it is able to quantify the errors and trade offs in the final model the technique uses an iterative linearized approach to the non linear traveltime inversion problem the subsurface is represented as a set of layers separated by interfaces across which the velocity may be discontinuous within each layer the velocity varies in two dimensions and has a continuous first derivative rays are traced in this medium using a technique based on ray perturbation theory and two point ray tracing is avoided by interpolating the traveltimes to the receivers from a roughly equidistant fan of rays the calculated traveltimes are inverted by simultaneously minimizing the misfit between the data and calculated traveltimes and the roughness of the model this smoothing regularization stabilizes the solution of the inverse problem in practice the first iterations are performed with a high level of smoothing as the inversion proceeds the level of smoothing is gradually reduced until the traveltime residual is at the estimated level of noise in the data at this point a minimum feature solution is obtained which should contain only those features discernible over the noise the technique is tested on a synthetic data set demonstrating its accuracy and stability and also illustrating the desirability of including a large number of different ray types in an inversion

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

shear wave splitting at central tien shan evidence for rapid variation of anisotropic patterns at active collisional belts the fast polarization axis of shear wave splitting is generally aligned parallel to the strike of the belt which has been proposed to indicate mantle strain that is coherent with crustal deformation a notable exception is central tien shan where anomalous patterns of splitting have previously been observed we here analyze shear wave splitting of sks phases across north central tien shan using digital data from the kyrgyzstan broadband seismic network knet we find a pattern of short wavelength anisotropic heterogeneity that supports complex mantle flow due to small scale convection the along strike variations in mantle structure contrast with the coherent pattern of crustal shortening and indicate that mantle flow is not directly coupled to crustal deformation in this region

seismic constraints on dynamics of the mantle of the kaapvaal craton abstract shear wave splitting in the upper mantle of the kaapvaal craton in southern africa is measured by applying the sks technique to the records of a portable array of digital broad band seismographs the data are interpreted in terms of lattice preferred orientation in olivine fast wave polarization directions at all stations of the array are close to the absolute plate motion apm direction of southern africa since the end of the jurassic this alignment is similar to that previously reported for the north american craton and suggests that the large scale component of mantle anisotropy on both sides of the atlantic can be related to resistive drag at the base of the plates a likely depth range of the corresponding deformation is 150 400 km analysis of the mantle converted phases suggests that the high velocity mantle root of the kaapvaal craton which translates coherently with the plate resides in the same depth range the bottom of the root is found at a depth close to 380 km we conclude that the root is deformed by the recent plate motion but the deformations are not strong enough to be seen in the available tomographic models

velocity shift in heterogeneous media with anisotropic spatial correlation summary converted waves analysis is used to map the anisotropy as a function of depth in the upper mantle stacking of sv a ndt component records of several seismic events well distributed in azimuth helps to detect these converted waves weighting coe cients are used to extract the anisotropic signal in the records synthetic examples show that a harmonic analysis helps to separate azimuthal anisotropy eiects from the eiects of lateral heterogeneity such as those resultingfrom dipping structures a good azimuthal coverage of the data set is important in order to separate the diierent eiects in the stack sections obtained for diierent azimuthal orders the procedure is applied to data observed at geoscope station can and gives a model with two anisotropic layers in the upper mantle the upper layer located between the crust and a depth of140 km has its fast axis in an east west direction the lower layer 40 km thick has its fast axis in a north south direction the fast axis directions are in agreement with results obtained from surface wave analysis and tectonic information

seismic waves converted from velocity gradient anomalies in the earth s upper mantle summary modelling of elastic wave propagation in 1 d structures is frequently performed using reflectivity techniques in which the earth s velocity profile is approximated by stacks of homogeneous layers the complete reflection transmission r t response of a zone with arbitrary 1 d depth variation including both gradients and discontinuities in material properties can however be calculated using invariant embedding techniques results from earlier studies are here extended to derive exact expressions for r t matrices in arbitrary 1 d anisotropic media using a form of born approxi mation valid for thin scatterers and which does not assume small perturbations in material properties the r t matrices are solutions to a system of non linear ordinary differential equations of ricatti type and may be manipulated using standard r t matrix algebra in an equivalent description the wavefield within the heterogeneous zone is considered in terms of depth dependent contributions from up and downgoing waves propagating within the embedding reference medium this leads to efficient calculation of the internal wavefield using r t matrices of the heterogeneous stratification and portions thereof at minor additional expense mode conversion of teleseismic p and s phases from velocity gradients is examined by way of examples and comparison with three component data from broad band stations of the yellowknife seismic array the frequency dependence of such wave interactions depends on the differences in vertical slowness between incident and scattered modes it is shown that significant energy is converted from transition zones with extent l p 2 a broader interval than will generally produce intramode reflections a layer structure identified from ps conversions near 75 km depth below the slave craton is shown to be compatible with a 10 km thick gradient zone in which anisotropy increases from ambient levels to vp 5 per cent vs 2 5 per cent at a discontinuous upper boundary this characterization supports a previous interpretation as the upper strata of a former oceanic plate juxtaposed against overriding lithosphere during an ancient episode of shallow subduction

shear wave splitting around the eifel hotspot evidence for a mantle upwelling summary we present the results of a shear wave splitting analysis across the eifel hotspot in westcentral europe our data set includes 18 permanent european broadband stations and 102 temporary eifel broadband and short period stations we observe variations in splitting at most permanent stations but are not able to model them reliably with unique dipping axis or two layer anisotropy models we prefer instead a single layer model with a horizontal fast axis to approximate the first order anisotropy which varies smoothly between stations we observe a first order parabolic pattern in fast polarization azimuth around the hotspot which suggests that a lattice preferred orientation lpo of olivine fast axes exists in the asthenosphere as a result of the interaction between the slowly wsw moving eurasian plate and a mantle upwelling beneath the eifel volcanic fields the minority of the variation not explained by this model correlates with rapid lateral changes in splitting and is interpreted as a result of additional anisotropy and or the effects of dynamic recrystallization associated with lpo development in the region of corner flow near the conduit our parabolic asthenospheric flow paf interpretation is consistent with eifel geological tomographic receiver function global absolute plate motion electrical conductivity anisotropy and geochemical studies as well as with splitting studies in the great basin and around hawaii we suggest that the eifel upwelling is sporadic and a result of a low excess upwelling temperature and or varying crustal stresses that periodically shift and facilitate eruption the paf pattern we observe neither suggests nor rules out anisotropy in the conduit associated with a wet eifel upwelling we use our optimum pa f model parameters to calculate a eurasian plate speed of 12 km ma 1 w hich is consistent with the recent hs3 nuvel1a speed of 19 14 km ma 1

towards 3 d fabric in the continental lithosphere and asthenosphere the tien shan 1 as shown by previous observations of shear wave splitting of the sks phase the fast direction of azimuthal anisotropy beneath the tien shan tends to be aligned with the strike of the belt this direction implies that the anisotropy is related to the current orogenesis now we describe an attempt to constrain the distribution of the anisotropy with depth by combining observations of sks and p receiver functions the data are inverted for the parameters of anisotropy with the aid of a genetic algorithm the results of the joint inversion suggest that the actual distribution of anisotropy with depth can be more complicated than usually thought the deformed mantle layer can be a few hundred kilometres thick in the most reliable models fast direction of anisotropy in the shallow mantle is significantly different from that inferred from the observations of sks at some stations the data require anisotropy in the crust

shear wave anisotropy in the upper mantle beneath the aegean related to internal deformation seismic anisotropy deduced from sks splitting measured at 25 stations installed in the aegean does not show a homogeneous pattern it is not restricted to the north anatolian fault but is distributed over a region several hundreds kilometers wide little anisotropy is observed in continental greece or along the hellenic arc however significant anisotropy is observed in the north aegean sea large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy our results both in fast polarization directions and in values of delay time do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present day aegean motion relative to an absolute frame in contrast the direction of fast polarization and the magnitude of delay times correlate well with the present day strain rate observed at the surface deduced from both geodetic measurements and seismicity this anisotropy is not horizontally restricted to major surface faults but is spread over a wide region

shear wave splitting in the records of the german regional seismic network estimates of the parameters of shear wave splitting in the records of sks and skks of the new german regional seismograph network grsn for one layer model show large azimuthal variations at some stations of the network it is found that the variations are compatible with the presence of two anisotropic mantle layers in the region of the south german triangle sgt the fast direction in the upper layer is between n s and ne sw close to the previously reported estimates of the fast direction in the sub crustal lithosphere of the sgt the fast direction in the lower presumably asthenospheric layer is close to e w the estimates of the effective fast direction for one layer model at the stations of the grsn confirm the previously noted tendency of the fast direction of mantle anisotropy in central europe to change from 50 70 in the western part of the region to 100 120 in the eastern part a correlation of this change with a bending of the present day direction of maximum horizontal stress in the crust suggests that anisotropy in the asthenosphere of central europe can be related to the alpine orogeny

complex seismic anisotropic structure beneath the central tien shan revealed by shear wave splitting analyses summary shear wave splitting parameters fast polarization directions and splitting times obtained at the permanent broad band station wushi wus which is situated in central tien shan china demonstrate systematic azimuthal variations the fast polarization directions range from 26 to 85 and the delay times range from 0 4 to 1 6 s suggesting a complex anisotropic structure that cannot be adequately explained by a single anisotropic layer with horizontal axis of symmetry the azimuthal variations have an apparent 90 periodicity and thus can be best explained by a two layer anisotropic model with horizontal symmetry axes the optimal fast direction for the upper layer is in the 60 80 range that is subparallel to the strike of the tien shan orogenic belt and can consequently be attributed to the ongoing n s directed shortening the fast direction for the lower layer ranges from 85 to 105 and can be explained by lattice preferred orientation of olivine crystals in the asthenosphere associated with present day absolute plate motion of the eurasian plate

monte carlo approach to assess the uncertainty of wide angle layered models application to the santos basin brazil abstract in the santos basin brazil two parallel wide angle refraction profiles show different crustal structures one shows moderate crustal velocity gradient and a clear moho with topography the other has an anomalous velocity zone and no clear moho reflections this has large implications on the geological and geodynamical interpretation of the basin model uncertainties must be excluded as a source of these differences we developed vmontecarlo a tool to assess model uncertainty of layered velocity models using a monte carlo approach and simultaneous parameter perturbation using all picked refracted and reflected arrivals it gives insights into the acceptable geological interpretations allowed by data and model uncertainty through velocity depth plots that provide a the velocity depth profile range that is consistent with the travel times b the random model that provides the best fit keeping most of the observations covered by ray tracing c insight into valid models dispersion d main model features unequivocally required by the travel times e g first order versus second order discontinuities and velocity gradient magnitudes e parameter value probability distribution histograms vmontecarlo is seamlessly integrated into a rayinvr based modelling work flow and can be used to assess final models or sound the solution space for alternate models and is also capable of evaluating forward models without the need for inversion thus avoiding local minima that may trap the inversion algorithms and providing information for models still not well parametrised results for the brazilian models show that the imaged structures are indeed geologically different and are not due to different interpretations of the same features within the model uncertainty bounds these differences highlight the strong heterogeneity of the crust in the middle of the santos basin where the rift is supposed to have failed

shear wave splitting in the mantle ps phases distribution with depth of mantle anisotropy is uncertain to better constrain this distribution we analyse records of the mantle ps phases at the grf array in germany two components of these phases sv and t are detected by stacking teleseismic records of events well distributed in azimuth and distance the detected t component signals are generated not so much by splitting of sv as by conversion from p to sh at the mantle discontinuities to reconcile these observations with previously known observations of splitting in sks at least three anisotropic layers are required the data favour a model with nne oriented fast velocity at the top of the mantle this is consistent with the results of other seismic studies beneath this layer the fast direction of anisotropy changes by around 90 our analysis suggests as well that a layer at the base of the mantle transition zone mtz is anisotropic this anisotropy might arise in the convective boundary layer between the upper and the lower mantle

recent deformations of the deep continental root beneath southern africa seismic anisotropy manifested by the splitting of shear waves into two orthogonal polarizations provides evidence for deformation in the earth s upper mantle where such deformation has been inferred beneath precambrian cratons it has been interpreted either as related to recent plate motion1 2 or inherited from the precambrian3 4 here we present data from a portable seismograph array in the kaapvaal craton of south africa which indicate that the inferred direction of mantle flow underneath the array is close to the direction of absolute plate motion for southern africa since the end of the jurassic period such an alignment has also been reported for the north american craton2 suggesting that the flow in both regions is related mainly to shearing of the sublithospheric mantle by the plate above the old continental roots that are likely to exist in this depth range5 6 must therefore be deformed by the plate motion but the deformations are not strong enough to be seen in the presently available seismic tomography data

on the inversion of sd particle motion for seismic anisotropy in d in order to make accurate estimates of anisotropy in d as measured from observations of sdi f f sd it is necessary to correct the observed waveforms for anisotropy outside of the deepest mantle as well as side refraction along the path and to pick arrivals of svd and shd as accurately as possible we show that corrections based on estimates of upper mantle anisotropy inferred from sks skks splitting measurements are sometimes insufficient and propose a master event method to directly obtain more accurate corrections on specific source station paths our study illustrates the complexity of structure that needs to be taken into account when addressing issues of d properties using seismic body wave methods we also describe a method to accurately measure the arrival time of the corrected sv d wave

ellipticity of teleseismic s particle motion summary in the horizontal components of grf records particle motions of the teleseismic s waves in the period range between 5 and 20s are usually elliptic we correct the particle motions for the effect of azimuthal anisotropy at grf and explain the residual ellipticity by assuming that it is produced at the source side of the wave path by interference of two waves with orthogonal polarizations and differing traveltimes the estimates of polarization direction of the fast wave and time delay between the waves can be found by analysing the residual particle motion for groups of closely spaced seismic events the technique was applied to about 40 grf records of events from the northern and north western pacific in the source regions the s wave pulses propagate in subduction zones it is found that the surface projections of polarization of the fast wave of the shallow events coincide approximately with the strikes of the corresponding island arcs this regularity can be explained by anisotropy in the subduction zones provided that the a axis of olivine is parallel to the plane of subduction the fast direction for the deep events in the sea of japan and the sea of okhotsk is close to the strike of the kurile island arc the time delay between the fast and the slow waves for the deep events is smaller than for the shallow kurile events 0 9s versus 1 4s

lithospheric structure beneath the archaean slave province and proterozoic wopmay orogen northwestern canada from a lithoprobe refraction wide angle reflection survey the slave geological province a relatively small craton in nw canada that includes the oldest rocks in the world is one of five cratons that form the archaean continental core of north america the palaeoproterozoic wopmay orogen lying between the craton and the phanerozoic northern canadian cordillera represents crustal growth through addition of a series of north south oriented magmatic arcs and accreted terranes as part of lithoprobe s multidisciplinary studies line 11 of the snore 97 seismic refraction wide angle reflection survey investigated the structure of the lithosphere in this region interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling results show that crustal thickness at 32 35 km remains relatively constant from the archaean regions in the east to proterozoic domains in the west contrary to average values of crustal thicknesses from global compilations that suggest thicker proterozoic crust at the westernmost end of the line where crustal extension and basin formation are inferred the crust thins to 30 km upper crustal velocities to about 12 km depth lie in the range of 5 8 6 2 km s 1 the middle crust of thickness 8 10 km has velocities between 6 2 and 6 6 km s 1 average velocities for the lower crust differentiated as a 10 km thick layer range between 6 6 and 6 9 km s 1 with a higher velocity region 7 1 km s 1 in the transition zone between the fort simpson magmatic arc and hottah accreted terrane in the wopmay orogen where a palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data in the same area anomalously low upper mantle velocities 7 5 km s 1 have been found indicating that the effects of collision subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure may have been retained for 1800 ma data from most record sections with offset distances greater than 250 km show continuous wide angle reflections from upper mantle depths of about 75 km and almost 100 km although poorly constrained velocities in this depth range vary from 8 4 to 8 8 km s 1 interpretation of these phases and mantle reflections from equivalent depths in coincident near vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the palaeoproterozoic orogen

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

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

azimuthal anisotropy in the lithosphere from observations of long period s waves summary the paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by vinnik kosarev makeyeva 1984 and used by kind et ai 1985 the technique is based on the observations of long period converted phases like sks additionally we describe a generalization of this approach which makes use of long period s waves of arbitrary polarization both modifications were applied to records of the grf array in southern germany the results of this analysis if combined with the data on p velocities bamford 1977 suggest that the direction of the fast velocity in the lithosphere of the region varies with depth

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

anisotropic measurements in the rhinegraben area and the french massif central geodynamic implications as part of an integrated seismic study polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband semi permanent long and short period and temporary short period seismological stations located in two geodynamically important areas in western europe namely the rhinegraben urach area and the french massif central volcanic field

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

imaging faults and shear zones using receiver functions the geometry of faults at seismogenic depths and their continuation into the ductile zone is of interest for a number of applications ranging from earthquake hazard to modes of lithospheric deformation teleseismic passive source imaging of faults and shear zones can be useful particularly where faults are not outlined by local seismicity passive seismic signatures of faults may arise from abrupt changes in lithology or foliation orientation in the upper crust and from mylonitic shear zones at greater depths faults and shear zones with less than near vertical dip lend themselves to detection with teleseismic mode converted waves receiver functions provided that they have either a contrast in isotropic shear velocity vs or a contrast in orientation or strength of anisotropic compressional velocity vp we introduce a detection method for faults and shear zones based on receiver functions we use synthetic seismograms to demonstrate common features of dipping isotropic interfaces and contrasts in dipping foliation that allows determination of their strike and depth without making further assumptions about the model we proceed with two applications we first image a laramide thrust fault in the western u s the wind river thrust fault as a steeply dipping isotropic velocity contrast in the middle crust near the surface trace of the fault further downdip and across the range where basin geometry suggests the fault may sole into a subhorizontal shear zone we identify a candidate shear zone signal from midcrustal depths the second application is the use of microstructural data from exhumed ductile shear zones in scotland and in the western canadian shield to predict the character of seismic signatures of present day deep crustal shear zones realistic anisotropy in observed shear fabrics generates a signal in receiver functions that is comparable in amplitude to first order features like the moho observables that can be robustly constrained without significant tradeoffs are foliation strike and the depth of the foliation contrast we find that an anisotropy of only a few percent in the shear zone is sufficient to generate a strong signal but that the shear zone width is required to be 2 km for typical frequencies used in receiver function analysis to avoid destructive interference due to the signals from the boundaries of the shear zone

inversion of teleseismic s particle motion for azimuthal anisotropy in the upper mantle a feasibility study summary we have developed a technique for the inversion of teleseismic s waveforms in terms of azimuthal anisotropy in the upper mantle we test different models of the earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component the optimum model is found by minimizing the difference between the synthetic vertical component and the observed one using this method we explore the possibility of constraining the distribution of azimuthal anisotropy with depth we present examples of seismic observations where the data are clearly in favour of an anisotropic model these observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes

seismic receiver functions and the lithosphere asthenosphere boundary abstract the lower boundary of the lithospheric plates has remained as an enigmatic boundary for seismologists since it is relatively poorly observed by seismic means there is traditionally a broad consensus that the asthenosphere is observable as a low velocity zone by seismic surface waves seismic techniques which use shorter period p to s or s to p converted body waves are now far enough developed to be successful in observing such a low velocity zone with a higher resolution the principle of this technique the so called receiver function technique is that a strong teleseismic mother phase e g p s pp or sks incident from below on any seismic discontinuity beneath a station produces a converted phase ps or sp which indicates its depth and properties we discuss details of this technique a sufficient number of such observations exist already to indicate that the top of the low velocity zone is a globally observable discontinuity and it is sharper than previously thought an intriguing observation is that in some cratons the new seismic data indicate that the low velocity zone exists already at shallower depths than obtained from surface waves this confirms earlier results from controlled source observations thybo and perchuc 1997 we discuss possible interpretations of this shallow low velocity zone in cratonic regions

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

upper mantle stratification by p and s receiver functions summary seismic stratification of the upper mantle is investigated by applying two complementary techniques to the records of the graefenberg array in southern germany the anisotropic p receiver function technique kosarev et al 1984 vinnik montagner 1996 is modified by using summary seismic events instead of individual events and different weighting functions instead of the same function for the harmonic angular analysis of the sv and t components of the pds phases the summary events provide better separation of the second azimuthal harmonic than the individual events the parameters of the second harmonics of sv and t thus evaluated should be similar if they reflect the effects of azimuthal anisotropy this can be used as a criterion to identify the anisotropy to detect the sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique which can be termed the s receiver function technique it includes axis rotation to separate interfering p and s arrivals determination of the principal m component of the s wave motion deconvolution of the p components of many recordings by their respective m components and stacking of the deconvolved p components with weights depending on the level of noise and the angle between the m direction and the backazimuth of the event both techniques yield consistent results for the graefenberg array as indicated by the p receiver functions the upper layer of the mantle between the moho and 80 km depth is anisotropic with dvs vs around 0 03 and the fast direction close to 20 clockwise from north the fast direction of anisotropy below this layer is around 110 the boundary between the upper and the lower anisotropic layers is manifested by the detectable pds and sdp converted phases shear wave splitting in sks is strongly dominated by azimuthal anisotropy in the lower layer asthenosphere

shear wave splitting and small scale convection in the continental upper mantle the deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt but this deformation is difficult to observe seismic azimuthal anisotropy manifested by shear wave splitting provides the best geophysical evidence of deformation in the upper mantle here we use observations of shear wave splitting to investigate deformation in the upper mantle beneath the tien shan in central asia we find that underneath most of the tien shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt but it deviates by nearly 90 where the upper mantle is anomalously hot the former relationship previously observed in other collisional belts seems to be usual for this environment the anomalous directions however provide evidence of a rising plume beneath the range suggesting more generally that the along strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection

evidence for azimuthal anisotropy in southwest iberia from deep seismic sounding data abstract data from the iberian lithospheric heterogeneity and anisotropy iliha deep seismic sounding experiment have been used to study the seismic structure of the lower lithosphere beneath the ossa morena zone southwest iberia the interpretation of the mantle phases observed in the record sections evidences a layered lower lithosphere with alternating high and low velocity zones two to three main reflectors are observed in the uppermost mantle between 30 and 90 km depth a simple three layered crust with no significant lateral inhomogeneities over the area explored means that the mantle phases are unlikely to be affected by variations in crustal structure one reversed line and two intersecting unreversed lines define an area beneath southwest iberia where the same lithospheric reflectors segments are sampled along different azimuths the independent analysis of the different lines locates the reflectors at the same depths but significant variations appear between the velocities of the corresponding layers derived from each profile this fact is interpreted as evidence for azimuthal anisotropy in the lower lithosphere the anisotropy orientation deduced from our data is not consistent with results derived from other techniques such as sks splitting this fact already reported for other continental areas must be interpreted as evidence of the existence of changes with depth in the anisotropic properties of the mantle the different hypotheses suggested to explain the origin of the continental mantle anisotropy are discussed and related to the result of our study