lateral heterogeneity in the upper mantle beneath the tibetan plateau and its surroundings from ss s travel time residuals 1 we use ss s differential travel times to map the upper mantle structure of the tibetan plateau and its surroundings new observations from analog stations in the former soviet union help to fill in gaps in the spatial coverage of previous studies providing new data for eastern tibet the kunlun range and the tarim basin the effects of crustal thickness variations anisotropy and global heterogeneity are evaluated and explicitly accounted for by incorporating results of previous studies and by ray tracing through a global 3d model the resulting residuals suggest that with the exception of the eastern part of the plateau tibet is underlain almost entirely by lithosphere of normal to greater than normal thickness in general there appear to be strong lateral gradients in wave speed between eastern and western tibet we find little evidence for low wave speeds under northern tibet to the west of about 87 e instead the average shear wave speeds in the upper 300 km of the mantle beneath this part of tibet are about 0 5 km s faster than those directly to the north beneath the tarim basin these results support models of tibetan dynamics in which the lithosphere underlying northern tibet underplates the crust as opposed to those in which large portions of this lithosphere disappear by convection subduction or detachment at the same time the dominant e w gradient in residuals implies that 2d models inadequately explain the dynamics of all of tibet

lateral variations in compressional velocities beneath the tibetan plateau from pn traveltime tomography summary a back projection algorithm is applied to 353 p traveltime measurements to image the lateral variations in compressional velocity in the uppermost mantle in and around tibet prior to the tomographic inversion we refined the apparent p slowness measurements based on a recently developed relocation procedure zhao helmberger 1991 and mathematical formulations of zhao 1993 these refinements are crucial in obtaining a high quality velocity image we obtained an average p velocity value for the uppermost mantle of 7 93 kms l and an average mantle p velocity gradient of 3 1 x lo s for tibet the final 2 d p velocity image in the uppermost mantle is characterized by a large 7 to 9 in dimension low velocity down to about 7 78 km s region in north central tibet and by large high velocity up to 8 11 km s i regions in the western and eastern flanks of tibet the crustal thickness is greater than 70 km inside the tibetan plateau it decreases toward the southern eastern and western boundaries of the plateau at and beyond these boundaries the crustal thickness is similar to those beneath the eurasian shield rigorous analyses of resolution and error suggest that 1 there is a pronounced difference between the tomographic velocity image obtained with and without the refinements to the p slowness measurements indicating a significant improvement in the quality of our inversion by these refinements 2 random errors in measuring earthquake origin times and locations and in the p arrival times should not cause errors of more than 0 1 km s in the velocity image 3 the resolution is generally within 6 to 10 the velocity image obtained in this study is consistent with previous s p traveltime studies and geothermal petrological studies and is consistent with a mantle convection model in which a convective upwelling is occurring in north central tibet and downwellings occurring in the surrounding regions molnar 1988 1990

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

constraining p wave velocity variations in the upper mantle beneath southeast asia we have produced a p wave model of the upper mantle beneath southeast se asia from reprocessed short period international seismological centre isc p and pp data short period p data of the annual bulletin of chinese earthquakes abce and long period pp p data we used 3d sensitivity kernels to combine the datasets and mantle structure was parameterized with an irregular grid in the best sampled region our data resolve structure on scale lengths less than 150 km the smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3d model our tomographic inversions reveal high velocity roots beneath the archean ordos plateau the sichuan basin and other continental blocks in se asia beneath the himalayan block we detect high seismic velocities which we associate with subduction of indian lithospheric mantle this structure is visible above the 410 km discontinuity and may not connect to the remnant of the neo tethys oceanic slab in the lower mantle our images suggest that only the southwestern part of the tibetan plateau is underlain by indian lithosphere and thus that the upper mantle beneath northeastern tibet is primarily of asian origin our imaging also reveals a large scale high velocity structure in the transition zone beneath the yangtze craton which could have been produced in multiple subduction episodes the low p wave velocities beneath the hainan island are most prominent in the upper mantle and transition zone they may represent counter flow from the surrounding subduction zones and may not be unrelated to processes beneath eastern tibet 2005 elsevier b v all rights reserved

a no lid zone in the central chang thang platform of tibet evidence from pure path phase velocity measurements of long period rayleigh waves pure path phase velocities of fundamental mode rayleigh waves in the period range 30 80s are obtained for different paths across the tibet plateau using a two event method with events located in tibet and aligned with world wide standard seismograph network stations at teleseismic distance evidence is found for significant lateral variations of structure of the crust and uppermost mantle in tibet in particular phase velocities obtained on paths contained in the chang thang platform are higher than average in tibet at periods shorter than 60s while remaining very low at longer periods when interpreting these phase velocities in terms of simple layered s velocity models it is found that contrary to other areas in tibet they are incompatible with the existence of a high velocity lid s velocities of 4 4 4 5 km s are found immediately beneath the moho and the crust is somewhat thinner 50 60 km than average in tibet this indicates a possible upwelling of hot asthenospheric material compatible with landsat evidence for subrecent basaltic volcanism and independent seismic evidence such as inefficient transmission of sn waves on the other hand a 70 km thick crust and very high upper mantle s velocities are found on the western border of tibet karakorum hindu kush compatible with previous finer scale results concerning the associated zone of intermediate depth seismicity

s wave residuals from earthquakes in the tibetan region and lateral variations in the upper mantle abstract mean s wave residuals from 46 earthquakes within and on the margins of the tibetan plateau exhibit systematic lateral variations that do not correlate well with elevation or with simple aspects of the geologic history the earliest s waves come from earthquakes in western tibet the karakorum and the western himalaya and the latest come from earthquakes in north central tibet although s waves from earthquakes in the himalaya tend to be early the east west variation in residuals across tibet is at least as large as the north south difference between the himalaya and northern tibet if the variations in residuals are a reflection of temperature variations in the upper mantle associated with convection then upwelling beneath north central tibet seems to be flanked by downwelling in western eastern and probably southern tibet this convective flow might reflect the detachment and removal of thickened mantle lithosphere beneath tibet

lithospheric structure and its relationship to seismic and volcanic activity in southwest china 1 the sichuan yunnan region in southwest china is located in the boundary area between the active tibetan plateau to the west and the stable south china platform to the east this region is characterized by complex cenozoic structures and active seismotectonics in this study we have used over 30 000 arrival times from 1315 local earthquakes recorded by 172 seismic stations to determine a detailed three dimensional 3 d p wave velocity structure of the lithosphere down to 85 km depth in this region we have taken into account the complex morphology of the moho discontinuity to conduct the tomographic inversions which leads to a better result than that with a flat moho as in the previous studies our results show that large velocity variations of up to 7 exist in the crust and upper mantle in the sichuan yunnan region the velocity image of the upper crust correlates with the surface geological features the sichuan basin is imaged as a prominent low velocity zone while the panzhihua mining district is imaged as a high velocity feature velocity changes are visible across some of the large fault zones and the faults and some large crustal earthquakes seem to occur at the boundary areas between slow and fast velocity anomalies some of the faults such as the red river fault may have cut through the crust and reached up to the upper mantle under the tengchong volcanic area strong low velocity zones are visible down to 85 km depth with a lateral extent of about 100 km suggesting the existence of magma chambers under the volcano it is unclear how the tengchong intraplate volcanism was generated it may be related to the collision processes between the indian plate burma microplate and the eurasian plate and the possible subduction of the burma microplate under the eurasian plate another possibility is that it was caused by the extensional fractures of the lithosphere and the upward intrusion of the hot asthenospheric materials it is also possible that the tengchong volcanism represents a hot spot with a lower mantle origin

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

texture of mantle lithosphere along the dead sea rift recently imposed or inherited seismic anisotropy a property linked to the texture of the mantle rock should be distributed with depth along the trace of the dead sea rift dsr owing to a combination of present day and ancient tectonics using data from four permanent and one temporary seismic observatories we evaluate birefringence splitting of 91 teleseismic core refracted shear waves primarily sks phases we find significant levels of birefringence in the bulk of observed phases we also find that birefringence parameters fast directions and delays vary as a function of source receiver geometry notably the pattern of this directional variation in birefringence is quite similar at all sites we have examined we interpret observed birefringence in sks phases in terms of one and two layer models single layer models for all stations exhibit a fast polarization oriented 12 19 east of north with anisotropy sufficient to generate 1 3 s time delay we find strong evidence for at least two distinct anisotropic layers for the two layer models the upper layers resemble the single layer models showing near north fast polarizations and delays on the order of 1 s three out of four sites show fast polarizations in the lower layer that strike 50 80 cw from north with time delays 0 3 0 6 s one site at the northern end of the dsr displays a higher degree of anisotropy in the lower layer and a more northerly fast polarization overall the lower layers at all sites appear to be consistent with the deformation caused by plate motion relative to the asthenosphere the fabric in the upper layer is sub parallel to the present day transcurrent motion on the dsr but also matches the typical orientation of lithospheric seismic anisotropy in the arabian shield our overall conclusion is that the impact of the dsr on the rock fabric of the mantle lithosphere is probably quite weak

global upper mantle structure from long period differential travel times we have made over ten thousand measurements of pp p and ss s differential travel times from long period global digital seismograph network recordings of all events with mb 5 5 which occurred during the years 1976 to 1986 we have obtained reasonable global coverage with very good sampling of several regions of the world the residuals range from 5 to 5 s for pp p and 12 to 12 s for ss s and reveal a large scale pattern of heterogeneity when plotted at the reflected phase s bounce point our experiments indicate that lower mantle structure and source receiver structure can each contribute approximately 0 5 s to our measured pp p residuals so there is considerable signal to be explained the pattern observed in the pp p measurements is similar to the pattern observed in the ss s measurements with the ss s residuals 2 to 4 times larger in magnitude comparisons of our measured residuals to those predicted by the upper mantle models of woodhouse and dziewonski show that the overall patterns are quite similar but the amplitude of the model residuals is roughly a factor of 2 too small comparisons with the predictions of a whole mantle model of tanimoto again shows that the predicted pattern of residuals is reasonably consistent with the observations but now the predicted residuals are too large by about a factor of 2 this variation in predicted amplitudes is probably mainly due to differences in the near surface structure of the models we have also binned our measurements according to the tectonic regionalization gtr1 of jordan and find a qualitative correlation of average residual with tectonic region in particular precambrian shields show a strong anomaly and there is a correlation of residual size with the age of oceanic crust at the bounce point for all tectonic regions the ratio of ss s to pp p residuals is approximately 2 this ratio is consistent with a thermal origin for the observed signal finally our measurements show no compelling evidence for azimuthal anisotropy which might be related to fossil spreading direction or the direction of absolute plate motion such a signal is probably overwhelmed by the large signal from three dimensional structure

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

teleseismic imaging of subducting lithosphere and moho offsets beneath western tibet teleseismic images suggest that the tarim plate plunges 45 s down to 300 km depth beneath nw tibet the 410 km discontinuity shallows by 10 km under the plateau implying 100 c cooler upper mantle the deepest moho on record 90 km lies under w qiangtang it rises abruptly by 20 and 10 km beneath the altyn tagh fault and bangong suture respectively vp vs ratios are normal except in the yecheng flexural basin and deep under the south karakax volcanics 1 92 w kunlun s neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward extruding thrust wedge of the tarim crust

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

seismic anisotropy indicators in western tibet shear wave splitting and receiver function analysis abstract using recently collected data from western tibet we find significant variation in the strength vertical distribution and attributes of seismic wave speed anisotropy constrained through a joint application of teleseismic shear wave splitting techniques and a study of p s mode converted waves receiver functions we find that the crust of tibet is characterized by anisotropy on the order of 5 15 concentrated in layers 10 20 km in thickness and with relatively steep 30 45 from the vertical slow symmetry axes of anisotropy these layers contribute no more than 0 3 s to the birefringence in teleseismic shear waves significantly smaller than splitting in many of the observations and much smaller than birefringence predicted by models developed through group inversions of shear wave recordings consequently we interpret models constrained with shear wave observations in terms of structures in the upper mantle near the altyn tagh fault our data favor a two layer model with the upper layer fast polarization approximately aligned with the strike of the fault near the karakorum fault our data are well fit with a single layer of relatively modest 0 5 s delay anisotropy fast polarization in this layer is 60 ne similar to that of the lower layer in the model for the altyn tagh fault site assuming that layers of similar anisotropic properties at these two sites reflect a common cause our finding favors a scenario where indian lithosphere under thrusts a significant fraction of the plateau data from a site at the southern edge of the tarim basin appear to be inconsistent with a common model of seismic anisotropy distribution we suspect that thick sediments underlying the site significantly distort observed waveforms our ability to resolve features of anisotropic structure in the crust and the upper mantle of western tibet is limited by the small amount of data collected in a 6 month observing period we stress the importance of future teleseismic data collection in tibet over extended periods to insure better directional distribution of observed seismic sources

lateral heterogeneity in the upper mantle and ss s traveltime intervals for ss rays reflected from the tibetan plateau and its surroundings abstract we report large differences between measured and calculated intervals between arrival times of s waves and ss phases whose bounce points lie beneath different parts of the tibetan plateau and its surroundings when compared with traveltimes calculated from the laterally homogeneous preliminary reference earth model prem the shortest intervals between ss and s correspond to paths with ss reflected from the karakorum and westernmost parts of the tibetan plateau and himalaya comparable but more scattered ss s residuals characterize paths with ss reflected beneath southern tibet the longest such intervals typify paths with ss reflected beneath north central tibet if upper mantle structure near the bounce points of ss causes these average apparent advances of 7 s and delays of up to 6 s then these data suggest that material with high s wave speeds underlie the karakorum and low s wave speeds underlie north central tibet a result that concurs with other seismological observations if such high or low speed regions were confined to the upper 250 km of the mantle they would imply a difference on the order of 13 in s wave speeds between these two regions heterogeneity in the lower mantle inferred in part from these data can however account for part of the large variation in residuals that we observe after correcting for large scale heterogeneity in the upper and lower mantle roughly 9 s of difference persists which would correspond to average differences of 9 or 0 4 km s in s wave speeds in the upper 250 km of the mantle beneath these regions only at subduction zones can one find such a difference in average s wave speeds in the mantle beneath regions only 500 1000 km apart suggesting that dynamic flow in the mantle presently occurs in the upper mantle beneath the tibetan plateau this difference supports the contention that small scale convection occurs beneath the tibetan plateau

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

deformation from 1973 to 1987 in the epicentral area of the 1959 hebgen lake montana earthquake ms 7 5 a 40 km aperture trilateration network centered on the 1959 hebgen lake earthquake epicenter has been surveyed in 1973 1974 1976 1978 1981 1984 and 1987 the deformation inferred from those surveys is described roughly by a uniaxial 0 266 0 014 strain yr n15 e 1 extension that is uniform in both time and space that extension is orthogonal to the strike n78 w 5 of the 1959 rupture plane inferred from focal mechanism solutions the absence of strain accumulation in the n75 w direction suggests that the source of deformation must be greatly elongated in that direction looked at in greater detail the deformation is found to be concentrated in the northern part of the network the zone of concentrated deformation coincides with a part of a well defined trend in seismicity that extends at least 100 km n75 w from the sour creek resurgent dome in the yellowstone caldera extension perpendicular to that trend is indicated by focal mechanism solutions thus the zone of rifting identified within the hebgen lake network apparently extends east southeast to the yellowstone caldera about 8 mm yr extension is observed across a 20 km width within the zone

on the relation between seismic anisotropy and finite strain because seismic anisotropy in the upper mantle is due primarily to lattice preferred orientation lpo of olivine crystals induced by deformation observations of anisotropy can provide constraints on mantle deformation and flow in this paper we use a previously published theory for deformation induced lpo ribe and yu 1991 to quantify the relation between seismic anisotropy and deformation for two model compositions a pure olivine dunite aggregate and a harzburgite comprising 70 olivine and 30 enstatite analytical and numerical solutions of the governing equations show that the predicted lpo and hence also the seismic anisotropy depends only on the finite strain and not on the deformation path by which that strain is produced the magnitude of the anisotropy depends only on the ratios c1 c2 and c2 c3 where c1 c2 c3 are the lengths of the principle axes of the finite strain ellipsoid and the directions of maximum and minimum compressional wave velocity vp coincide with the axes c1 and c3 respectively we calculate the percent anisotropies for compresional and shear waves propagating along the three principle axes of the finite strain ellipsoid as functions of the axial ratios c1 c2 and c2 c3 for our two model compositions however inferences of the magnitude of finite strain obtained using these results are likely to be minimum estimates because our theory neglects the effects of dynamic recrystallization finally we present as a byproduct of this study a new direct method for calculating the finite strain produced by an arbitiary deformation history