seismic stratigraphy and morphology of the holocene progradational system beneath bohai bay bohai sea lobate evolution of a multi sourced subaqueous fluviodeltaic complex abstract since 6 kyr bp the subaerial deltas of the yellow the yanshan and the taihang river systems evolved along the bohai bay coast forming several lobate progradational systems associated with frequent avulsions of these river systems their subaqueous parts consist of a multi sourced subaqueous deltaic complex in bohai bay but little is known about the complex s subsurface structure morphology and evolutionary history here we investigate these characteristics using a dense high resolution seismic survey using stratigraphic superposition seismic geometry and the results of previous studies we identified nine seismic units sus in the post last glacial maximum stratigraphy which are interpreted from bottom to top as 1 fluvial lacustrine sediments su0 2 marine sands combined with estuarine coastal deposits su1 and 3 a subaqueous fluviodeltaic complex sus2 8 these units correspond to the lowstand system tract the transgressive and or highstand system tracts and the normal regressive system tract respectively and evolved as a function of eustatic fluctuations the subaqueous fluviodeltaic complex is characterized by a discrete and spatially confined lobate architecture which is indicative of step wise switching of subaqueous lobes two sets of progradational systems ps a northern one sus2 3 and a southern one sus4 8 were observed within the complex tapering in opposite directions and bounded by an erosional unconformity r3 these characteristics indicate that the two ps were sourced from different river systems and prograded at different times r3 forms the base of the southern ps but cross cuts the northern ps indicating that it is a diachronous surface marks the preexisting seafloor which was further reworked eroded by the progradation of the southern ps in each ps the internal subaqueous lobes correlated well with the onshore lobes superlobes of the subaerial deltas suggesting the lobate evolution of the complex is driven predominantly by alternating erosion and deposition caused by the variation in fluvial supply which appears to be linked to the frequent avulsions of the major river systems in addition to the multi sourced supply based on the dipping geometries and the locations of the depocenters we hypothesize that 1 the northern ps is the seaward extension of the ancient haihe river s northern branch 2 the bulk of southern ps in the middle and southern bohai bay is correlated with the principal body of the yellow river subaqueous delta 3 the majority of the volume of the northern ps in northern bohai bay consists of the modern haihe river subaqueous delta

constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

mantle fabric at multiple scales across an archean proterozoic boundary grenville front canada abstract in eastern ontario and southwest quebec canada the proterozoic grenville province abuts against the archean superior province the complex tectonic history of the region is reflected in the pattern of electrical and seismic anisotropy within the lithosphere while asthenospheric anisotropy is expected to reflect current patterns of mantle flow magnetotelluric and teleseismic data from the polaris and fednor experiments and the lithoprobe abitibi grenville transect are examined for sks splitting and geoelectric strike and receiver functions are generated at selected stations in order to characterise both vertical and horizontal variations in anisotropy in the eastern ontario upper mantle the average shear wave split direction coincides with the direction of plate motion split times are found to be strongest in the southern part of the grenville province where asthenospheric flow is enhanced by the presence of a lithospheric divot the ottawa bonnechere graben and the immediate vicinity of the grenville front are regions of altered split direction indicative of a lithospheric component of anisotropy north of the grenville front there is a gradual reorientation of the split direction from ese to ene which is not easily attributable to crustal tectonics and may represent the northern limit of lithospheric deformation produced by the grenville orogen electrical anisotropy is pervasive in the study area the pattern of magnetotelluric strikes is more complex than the sks pattern though after correction for local distortion the geoelectric strikes correlate fairly well with sks measurements at nearby stations obliquity between sks and magnetotelluric results shows no consistent orientation across the study area receiver function analysis at three selected stations is indicative of a subcrustal anisotropic layer with a consistent sse fast direction underlain at station gac by a sequence of anisotropic layers with varying directions this sequence is not observed at station sado combining these results we interpret the strength and direction of anisotropic fabric in the grenville to vary strongly with depth the upper part of the lithosphere contains thin anisotropic layers perhaps related to eclogitization and relict slabs but which are insufficient to explain the observed sks splits the lower lithosphere is likely to be more ductile and uniformly anisotropic and may be an important control on magnetotelluric strike directions as well as a significant contributor to sks splitting the largest contributor to sks splitting in this region is interpreted to be asthenospheric anisotropy related to absolute plate motion

rayleigh wave phase velocity heterogeneity and multilayered azimuthal anisotropy of the superior craton ontario summary we study the azimuthally anisotropic upper mantle structure of the superior craton and grenville province in ontario canada using rayleigh wave phase velocity data in the period range 40 160 s 152 two station dispersion measurements are combined in a tomographic inversion that solves simultaneously for isotropic and anisotropic terms using a least squares technique we perform a series of tests to derive optimal regularization smoothing and damping and to assess the resolution of and trade offs between isotropic and anisotropic anomalies the tomographic inversion is able to resolve isotropic phase velocity anomalies on a scale of 200 300 km and to distinguish between different anisotropic regimes on a 500 km scale across the study region isotropic phase velocity anomalies in the tomographic model span a range of up to 2 per cent around a regional average which is similar to the canadian shield dispersion curve of brune dorman 1963 with phase velocities up to 3 per cent above global reference models the amplitude of azimuthal phase velocity anisotropy reaches a maximum of 1 2 per cent a clear east west division of the study area based on both isotropic phasevelocity anomalies and azimuthal anisotropy is apparent in the western superior isotropic phase velocities are generally higher than the regional average anisotropy is observed at all periods with ene wsw to ne sw fast propagation directions at periods 120 s the anisotropy likely results from frozen lithospheric fabric aligned with tectonic boundaries whereas the anisotropy at longer periods is interpreted to arise from present day sublithospheric flow the fast directions from published sks measurements are close to the fast rayleigh wave propagation directions throughout the period range sampled and the large sks splitting times may be accounted for by this near coincidence of fast propagation directions across most of eastern ontario phase velocities are lower than the regional average fast propagation directions rotate from nw se at 40 130 s period to wnw ese at periods 140 160 s the results suggest a difference in fast propagation directions between the anisotropic fabric frozen into the lithosphere and the fabric due to current and recent sublithospheric flow the superior craton and grenville province are characterized by large scale structural variations that reflect the complex tectonic history of the region this study highlights differences between the characteristics of eastern and western ontario and indicates the occurrence of multiple layers of anisotropy in the subcratonic upper mantle

surface wave tomography of the gulf of california 1 we measured interstation fundamental mode rayleigh wave phase velocities using data from the nars baja seismic network located around the gulf of california a region average shear velocity model and a set of azimuthally anisotropic phase velocity maps are obtained from these data the average shear velocity structure shows a strong low velocity zone underlying a thin lid and the data are suggestive of low velocities down into the transition zone the phase velocity maps display signatures of sedimentary layers crustal thickness variations upwelling under the plate boundary and the presence of the subducted farallon microplate remnants beneath the gulf the upper mantle features inferred from this study provide new seismic evidence on the tectonic evolution of the region

seismic anisotropy of the slave craton nw canada from joint interpretation of sks and rayleigh waves summary teleseismic events recorded at a 25 element array in nw canada between 2001 and 2006 provided sufficient distribution in back azimuth to demonstrate birefringence in sks and skks waves as well as directional dependence of rayleigh wave phase velocities typical delays between orthogonally polarized sks waves are 0 8 1 2 s and modelling of azimuthal dependence indicates two nearly horizontal layers of anisotropy within the mantle anisotropy of rayleigh waves is generally consistent with models of layered vs anisotropies that increase with depth from 1 per cent at the moho to 9 per cent at 200 km but vary between subarrays consistency between the sks and rayleigh wave anisotropies in one subarray suggests that the assumption of symmetry about a horizontal axis is valid there but is not fully valid in other parts of the craton the upper layer of anisotropy occupies approximately the uppermost 120 km in which the fast polarization direction strikes generally north south coinciding with regional scale fold axes mapped at the surface the fast polarization direction of the deeper layer aligns with current north america plate motion but its correlation with trends of coeval kimberlite eruptions within the lac de gras field suggests it can be at least partly attributed to structural preferred orientation of vertical dykes inferred to exist to depths of 200 km

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

mesozoic hotspot epeirogeny in eastern north america geologic data especially conodont paleotemperatures imply that southeastern canada and new england have been elevated at least 4 km relative to the central appalachian region apatite fission track ages suggest that the uplift and consequent erosion occurred in cretaceous early tertiary time consistent with the formation of a broad swell as north america moved northwestward over the great meteor hotspot

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

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

no net rotation model of current plate velocities incorporating plate motion model nuvel 1 we present nnr nuvel1 a model of plate velocities relative to the unique reference frame defined by requiring no net rotation of the lithosphere while constraining relative plate velocities to equal those in global plate motion model nuvel 1 demets et al 1990 differences between nnr nuvel1 and no net rotation plate motion model am0 2 minster and jordan 1978 are as large as 15 mm yr in nnr nuvel1 the pacific plate rotates in a right handed sense relative to the no net rotation reference frame at 0 67 m y about 63 s 107 e this rotation nearly parallels but is 32 slower than the pacific hotspot euler vector of hs2 nuvel1 which is a global model of plate hotspot velocities constrained to consistency with nuvel 1 gripp and gordon 1990 at hawaii the pacific plate moves relative to the no net rotation reference frame at 70 mm yr which is 25 mm yr slower than the pacific plate moves relative to the hotspots differences between nnr nuvel1 and hs2 nuvel1 are described completely by a right handed rotation of 0 33 m y about 49 s 65 e the 99 confidence ellipsoids for plate hotspot motion in model hs2 nuvel1 exclude the corresponding euler vectors from model nnr nuvell thus the no net rotation reference frame differs significantly from the hotspot reference frame if the difference between reference frames is caused by motion of the hotspots relative to a mean mantle reference frame then hotspots beneath the pacific plate move with coherent motion towards the east southeast alternatively the difference between reference frames may show that the uniform drag no net torque reference frame which is kinematically equivalent to the no net rotation reference frame is based on a dynamically incorrect premise possible exceptions to the assumption of uniform drag include a net torque on the lithosphere due to attached subducting slabs and greater resistance to plate motion beneath continents than beneath oceans

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

deep seismic structure of the northeastern south china sea origin of a high velocity layer in the lower crust we present a 2 d seismic tomographic image of the crustal structure along the obs2012 profile which delineates the moho morphology and magmatic features of the northeastern south china sea margin the image was created by forward modeling rayinvr and traveltime tomographic inversion tomo2d overall the continental crust thins seaward from 27 km to 21 km within the continental shelf across the zhu i depression and dongsha rise with slight local thickening beneath the dongsha rise accompanying the increase in the moho depth the dongsha rise is also characterized by 4 7 km thick high velocity layer hvl 7 0 7 6 km s in the lower crust and exhibits a relatively high velocity 5 5 6 4 km s in the upper crust with a velocity gradient lower than those of the zhu i depression and tainan basin across the continental slope and continent ocean transition cot which contain the tainan basin the crust sharply thins from 20 km to 10 km seaward and a 2 3 km thick hvl is imaged in the lower crust we observed that volcanoes are located only within the cot but none exist in the continental shelf the dongsha rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the cot based on those observations we conclude that the hvl underlying the cot is probably extension related resulting from the decompression melting in the cenozoic whereas the hvl beneath the dongsha rise is probably arc related and associated with the subduction of the paleo pacific plate these findings are inconsistent with those of some previous studies

shear wave splitting observations in the lower great lakes region evidence for regional anisotropic domains and keel modified asthenospheric flow 1 fast axis directions obtained from shear wave splitting analysis of core refracted shear waves reveal several distinct anisotropic domains in the lower great lakes region we used data from 27 broadband seismograph stations extending across a low velocity anomaly in the lithospheric mantle observed splitting times vary from 0 4 to 1 4 s with a mean of 0 7 s a subset of the splitting vectors across a failed rift are oriented parallel to one arm of the rift zone most others show a coherent pattern of fast directions close to the direction of plate motion but oblique to surface tectonic belts within the area of the low velocity anomaly our observations are most simply explained by single layer anisotropy induced by asthenospheric flow spatial variability in the direction and magnitude of splitting are consistent with a flow regime influenced by basal topology of the lithospheric keel

shear wave splitting observations in the archean craton of western superior shear wave splitting observations in the archean superior province of the canadian shield show moderate to large delay times 1 1 2 1 s with azimuths suggesting upper mantle anisotropy is subparallel to the structural grain of the craton two regions with uniform anisotropy azimuth 62 7 and 87 3 are separated by a transitional zone showing strong dependence of observed azimuth on source direction suggesting that lateral structure is being observed no splitting was observed at stations near the coast of hudson bay continuing a pattern of weak splitting in surrounding proterozoic orogenic belts current absolute plate motion is consistent with the direction of anisotropy but would not explain the regional contrasts instead the anisotropy appears to be related to archean structure and tectonic history

global upper mantle tomography with the automated multimode inversion of surface and s wave forms summary we apply the automated multimode inversion of surface and s wave forms to a large global data set verify the accuracy of the method and assumptions behind it and compute an s vvelocity model of the upper mantle crust 660 km the model is constrained with 51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations structure of the mantle and crust is constrained by waveform information both from the fundamentalmode rayleigh waves periods from 20 to 400 s and from s and multiple s waves higher modes in order to enhance the validity of the path average approximation we implement the automated inversion of surface and s wave forms with a three dimensional 3 d reference model linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3 d reference in this study composed of a 3 d model of the crust and a one dimensional 1 d global average depth profile in the mantle below waveform information is related to shear and compressional velocity structure within approximate waveform sensitivity areas we use two global triangular grids of knots with approximately equal interknot spacing within each a finely spaced grid for integration over sensitivity areas and a rougher spaced one for the model parametrization for the tomographic inversion we use lsqr with horizontal and vertical smoothing and norm damping we invert for isotropic variations in s and p wave velocities but also allow for s wave azimuthal anisotropy in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity the lateral resolution of the resulting isotropic upper mantle images is a few hundred kilometres varying with data sampling we validate the imaging technique with a spectral element resolution test inverting a published global synthetic data set computed with the spectral element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately this test confirms both the accuracy of the implementation of the method and the validity of the jwkb and path average approximations as applied in it reviewing the tomographic model we observe that low s v velocity anomalies beneath mid ocean ridges and backarc basins extend down to 100 km depth only shallower than according to some previous tomographic models this presents a close match to published estimates of primary melt production depth ranges there in the seismic lithosphere beneath cratons unambiguous high velocity anomalies extend to 200 km pronounced low velocity zones beneath cratonic lithosphere are rare where present south america tanzania they are neighboured by volcanic areas near cratonic boundaries the images of these low velocity zones may indicate hot material possibly of mantle plume origin trapped or spreading beneath the thick cratonic lithosphere

teleseismic surface wave study for s wave velocity structure under an array southern california summary teleseismic surface wave data from the dense seismographic network in southern california cisn california integrated seismic network are used to construct s wave velocity maps for the region surface wave phase velocity measurements are achieved by way of a waveform matching spectral domain technique that was developed for use within the cisn array a two station phase velocity measurement technique was developed which takes advantage of the fact that for surface wave arrivals from any azimuth there are multiple pairs of cisn stations which are almost exactly on the same great circle paths to ensure the validity of this assumption particle motions were analysed for measured pairs within the network showing that the deviation between expected backazimuth from ray theory and observed backazimuths are small enough to be analysed by a simple distance correction this method was applied to 114 large earthquakes for time periods between 1999 and 2002 and led to the retrieval of phase velocity maps for rayleigh waves between 20 and 55 mhz and for love waves between 25 and 45 mhz finite frequency effect for propagating surface waves was approximately taken into consideration using a geometrical technique an ellipse determined by wavelength and great circle distance between the source and receiver can describe an influence zone for the finite frequency effect phase velocity map reconstructions using a method which incorporates this effect show minimal changes in phase velocity maps for our region but do extend solvable velocity regions inversion for s wave velocity distribution with depth shows that deep structural contrast across the san andreas extends into the upper mantle a slow velocity anomaly is evident under the southern sierras and the salton sea the latter region indicating the northward extension of tectonic activity in the gulf of california fast velocity roots that reach 60 km are found under the western part of transverse ranges and the northern peninsular ranges reconstruction of past location of the western transverse range microplate implies that these two ranges may have been adjacent to one another historically both the western transverse range fast velocity root and the peninsular range fast velocity root may be a remnant of old oceanic plate

how to relate body wave and surface wave anisotropy seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the earth upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave sks data we demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters l gc gs in the simplest case of a horizontal fast symmetry axis one application of this method is to display the sks delay time and the corresponding azimuth which can be derived from surface wave global with a lateral resolution of around 2000 km and regional lateral resolution of 350 km anisotropy tomography a global scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale same lateral resolution the anisotropy is well resolved in oceanic basins from surface waves whereas most measurements of sks splitting are available below continents however there is a good agreement between surface wave and body wave anisotropy in regions where large scale processes primarily tectonic are taking place such as the western united states and in central asia in particular we show that at high frequency 1hz the observed sks delay time and azimuth depend on the order in which s waves propagate through the layers a comparison between synthetic and observed sks phases in central asia highlights the varying sensitivity of sks waves at depth we can obtain a good correlation between synthetic and observed sks by considering anisotropy only between 80 and 200 km depth in the tomographic model rather than in the entire upper mantle however it is more difficult to explain the discrepancy between synthetic and observed sks under the eastern united states where a large scale coherent sks anisotropy is present such results call for more sophisticated sks waveform modeling and for enhanced lateral resolution in anisotropic tomographic models

lithospheric and sublithospheric anisotropy beneath the baltic shield from surface wave array analysis abstract we report measurements of radial and azimuthal anisotropy in the upper mantle beneath southern and central finland which we obtained by array analysis of fundamental mode rayleigh and love waves azimuthally averaged phase velocities were analysed in the period range 15 to 190 s for rayleigh waves and 15 to 100 s for love waves the azimuthal variation of the rayleigh wave phase velocities was obtained in the period range 20 to 100 s the limited depth resolution of fundamental mode surface waves necessitated strong damping constraints in the inversion for anisotropic parameters we investigated the effects of non unicity on the final model by experimenting with varying model geometries the radial anisotropy beneath finland can be explained by a lithosphere at least 200 km thick predominantly 50 by volume composed of olivine crystals having their a axes randomly distributed in the horizontal plane on the contrary the measured lithospheric azimuthal anisotropy is small this can be reconciled with body wave observations made in the area that indicate a complex pattern of rapidly varying anisotropy below 200 250 km depth that is below the petrologic lithosphere as revealed by xenolith analyses conducted in the area the magnitude of the azimuthal anisotropy increases and would be compatible with a mantle containing 15 20 by volume of olivine crystals whose a axes are coherently aligned in the n ne direction the alignment of the a axes is off the direction of present day absolute plate motion in either the no net rotation or hot spot reference frame currently n55 n60 we interpret this mismatch as evidence for a complex convective flow pattern of the mantle beneath the shield which by inference is decoupled from the overlying lithosphere