upper and mid crustal radial anisotropy beneath the central himalaya and southern tibet from seismic ambient noise tomography through analysis of the rayleigh wave and love wave empirical green s functions recovered from cross correlation of seismic ambient noise we image the radial anisotropy and shear wave velocity structure beneath southern tibet and the central himalaya dense ray path coverage from 22 broadband seismic stations deployed by the himalayan nepal tibet seismic experiment project provides the unprecedented opportunity to resolve the spatial distribution of the radial anisotropy within the crust of the central himalaya and southern tibet in the shallow subsurface the obtained results indicate significant radial anisotropy with negative magnitude vsv vsh mainly associated with the indus yarlung suture and central himalaya possibly related to the fossil microcracks or metamorphic foliations formed during the uplifting of the tibetan plateau with increasing depth the magnitude of radial anisotropy varies from predominantly negative to predominantly positive and a mid crustal layer with prominent positive radial anisotropy vsv vsh has been detected the top of the mid crustal anisotropic layer correlates nicely with the starting depth of the mid crustal lower velocity layers detected in our previous study the spatial correlation of the positive radial anisotropy layers and mid crustal lower velocity layers might suggest lateral crustal channel flow induced alignment of mineral grains most likely micas or amphiboles within the mid crust of the central himalaya and southern tibet this observation provides independent seismic evidence to support the thermo mechanical model which involves the southward extrusion of a low viscosity mid crustal channel driven by the denudation effect focused at the southern flank of the tibetan plateau to explain the tectonic evolution of the tibetanhimalayan orogen

evidence from earthquake data for a partially molten crustal layer in southern tibet earthquake data collected by the indepth ii passive source experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern tibet north of the zangbo suture beneath the southern lhasa block a midcrustal low velocity zone is revealed by inversion of receiver functions rayleigh wave phase velocities and modeling of the radial component of teleseismic p waveforms conversely to the south beneath the tethyan himalaya no low velocity zone was observed the presence of the midcrustal low velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern yadong gulu rift and possibly much of southern tibet

details of the doublet moho structure beneath lhasa tibet obtained by comparison of p and s receiver functions the collision of the indian and asian plates since about 55 ma has created the largest plateau with the thickest crust on earth there is however no general agreement on the modes of the crustal thickening the crustal thickness of the tibetan plateau still remains poorly determined it is generally accepted that the tibetan crust has roughly double normal thickness and that it thins somewhat toward the north but individual observations of moho depth vary spatially and for different techniques at the same place by greater than 20 km in this work we compare p and s receiver functions at station lsa located in the southern lhasa terrane to determine the crustal thickness beneath the station a doublet moho structure with two significant interfaces at about 60 and 80 km depth in the lower crust is seen clearly in the p receiver functions the deeper phase moho is however absent in the s receiver function data here we model the observed p and s receiver functions by a strong moho topography that dips to the east at an angle of 30 this result may indicate that the moho structure beneath tibet can be very complicated and has strong lateral variations and suggests that a detailed map of moho depth is only possible with 2d dense seismic experiments the moho dips locally perpendicular to the direction of the indian plate motion suggesting that the lower crustal deformation is decoupled from the underlying indian mantle lithosphere the extremely strong moho variation beneath and east of station lsa may also imply a crust mantle interaction specifically delamination or foundering of lower crust down to the upper mantle

shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

crustal structure and seismic anisotropy near the san andreas fault at parkfield california summary receiver functions rfs from station pkd located 3 km sw of the san andreas fault saf samples the salinian terrane near parkfield crustal multiples indicate a 26 km thick crust with avp vs of 1 88 which is slightly lower 1 83 for the upper and middle crust in the west for the mid crust arrivals are observed at times corresponding to recently imaged seismic reflectors and may correspond to a layer of metasedimentary rocks below the base of the granitic batholith exposed at the surface for the lower crust rfs display strong polarity reversals with backazimuth and a change in moho amplitude that require strong seismic anisotropy 15 per cent in a low velocity high vp vs possibly serpentinite or fluid filled schist layer that has a ene dipping 35 rock fabric similar patterns of amplitude variations and polarity reversal observed in rfs for some southern california stations located west of the saf support the hypothesis that the cause of these data characteristics is a regionally prevalent lower crustal anisotropy the orientation of this anisotropic fabric is inconsistent with the recent san andreas sense of shear and is most likely a fossilized fabric of past eastward directed farallon plate subduction

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

the tempo of climate change during dansgaard oeschger interstadials and its potential to affect the manifestation of the 1470 year climate cycle 1 the oxygen isotope record from the gisp2 ice core serves a air temperature proxy to estimate cooling rates during dansgaard oeschger interstadials between 15 53 thousand years before present the duration of individual dansgaard oeschger interstadials scales inversely with the cooling rate and high cooling rates are associated with large continental ice volume and vice versa cooling rates determine the extent to which the postulated 1470 year pacing cycle of the dansgaard oeschger interstadials is visible in paleoclimatological records only cooling rates associated with intermediate ice volume approx 84 20 m relative sea level equivalent allow the climate system to respond to succeeding trigger events of the 1470 year cycle and result in stadials interstadials of nearly the same duration

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

structure of the crust beneath the southeastern tibetan plateau from teleseismic receiver functions abstract southeastern tibet marks the site of presumed clockwise rotation of the crust due to the india eurasian collision and abutment against the stable sichuan basin and south china block knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region here we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method we find that the crustal thickness decreases gradually from the tibetan plateau proper to the sichuan basin and yangtze platform but that significant intra crustal heterogeneity exists on shorter lateral scales 0 30 are detected beneath the southeastern margin of tibet but in the sichuan basin and southeastern yangtze platform the values are close to the global average combined with high regional heat flow and independent evidence for mid crustal layers of high electric conductivity the large intra crustal s wave velocity reduction 12 19 and the intermediate to high average crustal poisson s ratios are consistent with partial melt in the crust beneath parts of southeastern tibet these results could be used in support of deformation models involving intra crustal flow with the caveat that significant lateral variation in location and strength of this flow may occur

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

determination of earthquake focal depths and source time functions in central asia using teleseismic p waveforms we developed a new method to determine earthquake source time functions and focal depths it uses theoretical green s function and a time domain deconvolution with positivity constraint to estimate the source time function from the teleseismic p waveforms the earthquake focal depth is also determined in the process by using the time separations of the direct p and depth phases we applied this method to 606 earthquakes between 1990 and 2005 in central asia the results show that the centroid moment tensor solutions which are routinely computed for earthquake larger than m5 0 globally using very long period body and surface waves systematically over estimated the source depths and durations especially for shallow events away from the subduction zone most of the 606 earthquakes occurred within the top 20 km of crust this shallow distribution of earthquakes suggests a high geotherm and a weak ductile lower crust in the region

depth variant azimuthal anisotropy in tibet revealed by surface wave tomography azimuthal anisotropy derived from multimode rayleigh wave tomography in china exhibits depth dependent variations in tibet which can be explained as induced by the cenozoic india eurasian collision in west tibet the e w fast polarization direction at depths 100 km is consistent with the accumulated shear strain in the tibetan lithosphere whereas the n s fast direction at greater depths is aligned with indian plate motion in northeast tibet depth consistent nw se directions imply coupled deformation throughout the whole lithosphere possibly also involving the underlying asthenosphere significant anisotropy at depths of 225 km in southeast tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the himalaya and burma respectively the multilayer anisotropic surface wave model can explain some features of sks splitting measurements in tibet with differences probably attributable to the limited back azimuthal coverage of most sks studies in tibet and the limited horizontal resolution of the surface wave results

mica dominated seismic properties of mid crust beneath west yunnan china and geodynamic implications abstract measurements of crystallographic preferred orientations cpo and calculations of p and s wave velocities v p and v s and anisotropy were conducted on three quartz mica schists and one felsic mylonite which are representative of typical metamorphic rocks deformed in the middle crust beneath the southeastern tibetan plateau results show that the schists have v p anisotropy av p ranging from 16 4 to 25 5 and maximum v s anisotropy av s max between 21 6 and 37 8 the mylonite has lower av p and av s max but slightly higher foliation anisotropy which are 13 2 18 5 and 3 07 respectively due to the lower content and cpo strength of mica with increasing mica content the deformed rocks tend to form transverse isotropy ti with fast velocities in the foliation plane and slow velocities normal to the foliation however the presence of prismatic minerals e g amphibole and sillimanite forces the overall symmetry to deviate from ti an increase in feldspar content reduces the bulk anisotropy caused by mica or quartz because the fast axis of feldspar aligns parallel to the slow axis of mica and or quartz the effect of quartz on seismic properties of mica bearing rocks is complex depending on its content and prevailing slip system the greatest shear wave splitting and fastest v p both occur for propagation directions within the foliation plane consistent with the fast pms s wave converted from p wave at the moho polarization directions in the west yunnan where mica amphibole bearing rocks have developed pervasive subvertical foliation and subhorizontal lineation the fast pms directions are perpendicular to the approximately e w orienting fast sks s wave traversing the core as p wave directions indicating a decoupling at the moho interface between the crust and mantle beneath the region the seismic data are inconsistent with the model of crustal channel flow as the latter should produce a subhorizontal foliation where vertically incident shear waves suffer little splitting

radial anisotropy in the crust of se tibet and sw china from ambient noise interferometry 1 we use rayleigh and love wave green s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the sichuan basin and around the eastern himalaya syntaxis in agreement with previous studies low velocity zones are ubiquitous in the mid lower crust with substantial variations both laterally and vertically discrepancies between 3 ds hear velocity from either rayleigh vsv o r love vsh waves are examined both in view of non uniqueness of tomographic solutions and radial anisotropy low shear wave speed and radial anisotropy with vsh vsv are most prominent in mid lower crust in area northwest to the lijiang muli fault and around the red river and xiaojiang faults this anisotropy could be caused by sub horizontal mica fabric and its association with low velocity zones suggests mica alignment due to flow in deep crustal zones of relatively low mechanical strength citation huang h h yao andr d vanderhilst 2010 radialanisotropyinthecrust of se tibet and sw china from ambient noise interferometry geophys res lett 37 l21310 doi 10 1029 2010gl044981

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

evidence for mechanical coupling and strong indian lower crust beneath southern tibet how surface deformation within mountain ranges relates to tectonic processes at depth is not well understood the upper crust of the tibetan plateau is generally thought to be poorly coupled to the underthrusting indian crust because of an intervening low viscosity channel here however we show that the contrast in tectonic regime between primarily strike slip faulting in northern tibet and dominantly normal faulting in southern tibet requires mechanical coupling between the upper crust of southern tibet and the underthrusting indian crust such coupling is inconsistent with the presence of active channel flow beneath southern tibet and suggests that the indian crust retains its strength as it underthrusts the plateau these results shed new light on the debates regarding the mechanical properties of the continental lithosphere and the deformation of tibet

anisotropic effects of non axial seismic wave propagation in foliated crustal rocks 1 seismic wave propagation within the crust is usually in directions not parallel to inherent rock symmetry axes due to geologic dips of structures and or oblique to arcuate raypaths we use christoffel equations and petrophysically measured velocities of crustal metamorphic rocks to examine anisotropic velocities and traveltime effects as functions of non axial propagation p and s wave velocities are trigonometrically weighted between fast and slow axial velocities averaging over a range of non axial angles yields effective velocities as encountered by through going crustal raypaths axial velocities cannot be simply used to quantify crustal anisotropic effects the angular behavior of p wave delay and shear wave splitting are sensitive to vp measured diagonal to the material symmetry axes this diagonal velocity controls the presence or absence of non axial shear wave singularities these behavior help quantify the contributions of crustal rocks to observations of seismic anisotropy which can serve as proxies for crustal deformation

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

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

seismic anisotropy in eastern tibet from shear wave splitting reveals changes in lithospheric deformation abstract knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions here we focus on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china we measured shear wave splitting of core refracted phases sks and skks at a temporary array of 25 iris passcal stations we calculated splitting parameters using a multi channel and a single record cross correlation method multiple layers of anisotropy cannot be ruled out but are not required by the data a fresnel zone analysis suggests that the shallow mantle between 60 and 160 km depth is the most likely source of anisotropy the polarization directions reveal a pronounced transition from primarily north south in the north sichuan to mostly east west orientations in the south yunnan in the southern part of the study region that is south of 26 n the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields whereas gps campaigns provide evidence suggesting north south crustal flow across the red river fault the pattern of anisotropy argues against such flow in the upper mantle these observations support models that allow differential movement of upper crust relative to lithospheric mantle in the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded the interpretation of the shear wave splitting results is non unique but we suggest that the observed n s transition reflects a fundamental change in deformation regime across our study region it may be related to lateral variations in lithospheric rheology or may mark a transition from the direct impact of the continental collision to dominance of the far field strain field associated with regional subduction processes understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of eastern tibet

seismic velocities and anisotropy of the lower continental crust a review seismic anisotropy is often neglected in seismic studies of the earth s crust since anisotropy is a common property of many typically deep crustal rocks its potential contribution to solving questions of the deep crust is evaluated the anisotropic seismic velocities obtained from laboratory measurements can be verified by computations based on the elastic constants and on numerical data pertaining to the texture of rock forming minerals for typical lower crustal rocks the influence of layering is significantly less important than the influence of rock texture surprisingly most natural lower crustal rocks show a hexagonal type of anisotropy maximum anisotropy is observed for rocks with a high content of aligned mica it seems possible to distinguish between layered intrusives and metasediments on the basis of in situ measurements of anisotropy which can thus be used to validate different scenarios of crustal evolution

right lateral shear along the northwest pacific margin and the india eurasia collision right lateral shear along the eastern margin of asia from the eocene to the present has led to the opening of pull apart basins intracontinental such as the bohai basin or oceanic such as the japan sea we suggest in this paper that this right lateral shear is a consequence of indentation of asia by india as in small scale analog experiments we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left lateral shear zones tien shan baikal stanovoy for the northern one and qin ling for the southern one we discuss the compatibility of this mechanism which involves a rather small amount of extrusion with the fast eastward expulsion described for southeast asia we re emphasize the role played in the opening of marginal basins by the pacific subduction as a free boundary to the east

complex and variable crustal and uppermost mantle seismic anisotropy in the western united states deformation in the crust and mantle measured using seismic anisotropy is poorly constrained in the western united states because of inconsistencies in the existing data a three dimensional model that reconciles these discrepancies reveals that seismic anisotropy in the crust correlates with different geological provinces but is unrelated to anisotropy in the underlying mantle

surface wave dispersion across tibet direct evidence for radial anisotropy in the crust recordings in western tibet of rayleigh and love waves at periods less than 70 s from aftershocks of the 2008 sichuan earthquake cannot be matched by an isotropic velocity model beneath tibet these intermediate period rayleigh and love waves require marked radial anisotropy in the middle crust of tibet with the vertically polarized s waves propagating more slowly than s waves with horizontal polarization the magnitude of anisotropy inferred using paths entirely within tibet is slightly greater than that obtained previously from a tomographic inversion of a dataset covering a larger region anisotropy in the middle crust likely reflects deformation of the middle crust and is consistent with the notion of mid crustal flow and thinning of the crust

automated detection extraction and measurement of regional surface waves our goal is to develop and test an effective method to detect identify extract and quantify surface wave signals for weak events observed at regional stations we describe an automated surface wave detector and extractor designed to work on weak surface wave signals across eurasia at intermediate periods 8 s 40 s the method is based on phase matched filters defined by the rayleigh wave group travel time predictions from the broadband group velocity maps presented by ritzwoller and levshin 1998 and ritzwoller et al 1998 and proceeds in three steps signal compression signal extraction or cleaning and measurement first the dispersed surface wave signals are compressed in time by applying an anti dispersion or phase matched filter defined from the group velocity maps we refer to this as the compressed signal second the surface wave is then extracted by filtering noise temporally isolated from the time compressed signal this filtered signal is then redispersed by applying the inverse of the phase matched filter finally we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal the method is naturally used as a detector by allowing origin time to slide along the time axis we describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the chinese lop nor test site from 1992 through 1996 and one explosion on the indian rajasthan test site that occurred in may of 1998

earthquake depth distributions in central asia and their relations with lithosphere thickness shortening and extension this paper examines the relationship between seismogenic thickness lithosphere structure and rheology in central and northeastern asia we accurately determine earthquake depth distributions which reveal important rheological variations in the lower crust these variations exert a fundamental control on the active tectonics and the morphological evolution of the continents we consider 323 earthquakes across the tibetan plateau the tien shan and their forelands as well as the baikal rift ne siberia and the laptev sea and present the source parameters of 94 of these here for the first time these parameters have been determined through body wave inversion the identification of depth phases or the modelling of regional waveforms lower crustal earthquakes are found to be restricted to the forelands in areas undergoing shortening and to locations where rifting coincides with abrupt changes in lithosphere thickness such as the ne baikal rift and w laptev sea the lower crust in these areas is seismogenic at temperatures that may be as high as 600 c suggesting that it is anhydrous and is likely to have great long term strength lower crustal earthquakes are therefore a useful proxy indicating strong lithosphere in places that are too small in areal extent for this to be confirmed independently by estimating effective elastic thickness from gravity topography relations the variation in crustal rheology indicated by the distribution of lower crustal earthquakes has many implications ranging from the support of mountain belts and the formation of steep mountain fronts to the localization and orientation of rifting in combination these processes can also be responsible for the separation of the front of the thin skinned mountain belts from their hinterlands when continents separate

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

crustal and uppermost mantle shear velocity structure adjacent to the juan de fuca ridge from ambient seismic noise 1 based on 6 months of obs data from the cascadia initiative experiment near the juan de fuca ridge we obtain rayleigh wave group and phase speed curves from 6 s to about 20 s period from ambient noise cross correlations among all station pairs we confirm the hypothesis that the dispersion data can be fit by a simple age dependent formula which we invert using a bayesian monte carlo formalism for an age dependent shear wave speed model of the crust and uppermost mantle between crustal ages of 0 5 and 3 5 ma igneous crustal structure is age invariant with a thickness of 7 km water depth varies in a prescribed way and sedimentary thickness and mantle shear wave speeds are found to increase systematically with crustal age the mantle model possesses a shallow low shear velocity zone lvz with a velocity minimum at about 20 km depth at 0 5 ma with lithosphere thickening monotonically with age minimum mantle shear velocities at young ages are lower than predicted from a half space conductively cooling model hscm and the lithosphere thickens with age faster than the hscm providing evidence for nonconductive cooling in the young lithosphere the shallow lvz is consistent with expectations for a largely dehydrated depleted harzburgite mantle with a small retained near ridge partial melt fraction probably less than 1 with melt extending to a lithospheric age of approximately 1 ma i e 30 km from the ridge

lateral variations in the crustal structure of the indo eurasian collision zone we thank michael ritzwoller and two anonymous reviewers for constructive comments that have helped improve the manuscript the majority of the seismic data used in this study were downloaded from iris dmc data for the ngri stations in india were provided by s s rai and zahid ra provided the pmd pakistan data kajal borah provided the ambient noise cross correlations for the uttaranchal network figures were prepared using generic mapping tools gmt software wessel and smith 1998 we thank robert herrmann for making the computer programs in seismology freely available

thinning and flow of tibetan crust constrained by seismic anisotropy intermediate period rayleigh and love waves propagating across tibet indicate marked radial anisotropy within the middle to lower crust consistent with a thinning of the middle crust by about 30 the anisotropy is largest in the western part of the plateau where moment tensors of earthquakes indicate active crustal thinning the preferred orientation of mica crystals resulting from the crustal thinning can account for the observed anisotropy the middle to lower crust of tibet appears to have thinned more than the upper crust consistent with deformation of a mechanically weak layer that flows as if confined to a channel

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

paleo environment and flooding of the limpopo river plain mozambique between c ad 1200 2000 multi proxy analysis was performed on a radiocarbon dated core collected from a relic oxbow lake in the limpopo river plain mozambique with the aim to reconstruct paleo environment and past flooding of the lower river system over the past c 800 years an additional objective was to evaluate and investigate the potential use of different proxies as recorders of paleo flooding events and paleo environmental variability within the floodplain the proxies applied in this study were mineral magnetic properties grain size distribution organic carbon content and diatom microfossil assemblages we found that sediment grain size and mineral magnetic properties of the minerogenic fraction were the most sensitive proxies in terms of detecting signals from high intensity river discharge events in the 800 year long sequence variations in sand content magnetic susceptibility and saturation isothermal remnant magnetization suggest at least four major flooding events at the site during the reconstructed period in the mid 1200 late 1300 mid 1500 ad and during the last century the diatom proxy reflects the development of the site from an open oxbow lake to a mainly terrestrial area the diatom assemblage indicates that open lake conditions prevailed at the site between c ad 1200 1400 with periodic inundation by marine water most likely due to late holocene sea level changes from c ad 1400 and onwards diatoms were rarely deposited at the site which indicates drier conditions this was a result of soil formation and gradual in filling of the lake a process which possibly was accentuated by a regionally dry climate situation our study shows that oxbow lakes and the proxies used here have great potential for reconstructing flooding events a knowledge that is crucial for potential prediction and mitigation of flooding events in mozambique in the future although chronological uncertainties limit comparisons to other paleo environmental records it seems that the flooding events recorded at our site occurred both during regionally wet and dry periods our data infer however that flooding was probably more clearly recorded during the lake stages than during infilled stage probably as the terrestrial environment was more exposed to erosion c 2014 elsevier b v all rights reserved less

localized precipitation and runoff on mars 1 we use the mars regional atmospheric modeling system mrams to simulate lake storms on mars finding that intense localized precipitation will occur for lake size 103 km2 mars has a low density atmosphere so deep convection can be triggered by small amounts of latent heat release in our reference simulation the buoyant plume lifts vapor above condensation level forming a 20 km high optically thick cloud ice grains grow to 200 m radius and fall near or in the lake at mean rates up to 1 5 mm h 1 water equivalent maximum rates up to 6 mm h 1 water equivalent because atmospheric temperatures outside the surface layer are always well below 273 k supersaturation and condensation begin at low altitudes above lakes on mars in contrast to earth lake effect storms lake storms on mars involve continuous precipitation and their vertical velocities and plume heights exceed those of tropical thunderstorms on earth for lake sizes 102 5 to 103 5 km plume vertical velocity scales linearly with lake area convection does not reach above the planetary boundary layer for lakes 103 km2 or for atmospheric pressure o 102 mbar instead vapor is advected downwind with little cloud formation precipitation occurs as snow and the daytime radiative forcing at the land surface due to plume vapor and storm clouds is too small to melt snow directly 10 w m 2 however if orbital conditions are favorable then the snow may be seasonally unstable to melting and produce runoff to form channels we calculate the probability of melting by running thermal models over all possible orbital conditions and weighting their outcomes by probabilities given by long term integrations of the chaotic diffusion of solar system orbital elements with this approach we determine that for an equatorial vapor source sunlight 15 fainter than at present and snowpack with albedo 0 28 0 35 melting may occur with 4 0 1 probability this rises to 56 12 if the ancient greenhouse effect was modestly 6 k greater than today

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

3 d shear wave radially and azimuthally anisotropic velocity model of the north american upper mantle summary using a combination of long period seismic waveforms and sks splitting measurements we have developed a 3 d upper mantle model sawum na2 of north america that includes isotropic shear velocity with a lateral resolution of 250 km as well as radial and azimuthal anisotropy with a lateral resolution of 500 km combining these results we infer several key features of lithosphere and asthenosphere structure a rapid change from thin 70 80 km lithosphere in the western united states wus to thick lithosphere 200 km in the central cratonic part of the continent closely follows the rocky mountain front rmf changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere corresponding to the fast to slow discontinuity found in receiver functions below the lithosphere azimuthal anisotropy manifests a maximum stronger in the wus than under the craton and the fast axis of anisotropy aligns with the absolute plate motion as described in the hotspot reference frame hs3 nuvel 1a in the wus this zone is confined between 70 and 150 km decreasing in strengthwithdepthfromthetop fromthermftothesanandreasfaultsystemandthejuande fuca gorda ridges this result suggests that shear associated with lithosphere asthenosphere couplingdominatesmantledeformationdowntothisdepthinthewesternpartofthecontinent the depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study although it is peaked around 270 km a robust result radial anisotropy is such that predominantly 1 where vsh vsv 2 under the continent and its borders down to 200 km with stronger in the bordering oceanic regions across the continent and below 200 km alternating zones of weaker and stronger radial anisotropy with predominantly 1 correlate with zones of small lateral changes in the fast axis direction of anisotropy and faster than average vs below the lab suggesting the presence of small scale convection with a wavelength of 2000 km finally in the western united states complex 3 d patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region

3 d lithospheric structure beneath southern tibet from rayleigh wave tomography with a 2 d seismic array summary a 3 d shear wave velocity model has been established for the lithosphere of southern tibet through rayleigh wave tomography the teleseismic data are acquired from a 2 d seismic array selected out of the second phase of the hi climb project we first inverted for a 2 d phase velocity model using two plane wave tomography approach and then inverted for the 3 d shear wave velocity model with the 2 d phase velocity model and the map of crustal thickness derived from receiver functions the results reveal a pervasive low velocity anomaly in the middle crust of the lhasa block we interpret this low velocity anomaly as the presence of wholesale mid crustal channel flow prominent low velocity anomalies from the lower crust to the mantle lithosphere are observed along the north south trending rifts the tangra yum co rift and the pumqu xianza rift we propose a possible scenario for the formation of these rifts the entire lithosphere is involved in the early stage rifting while the late stage rifting is restricted in the upper crust in the non rift regions a slab like high velocity anomaly is traced beneath both the himalayas and lhasa block combining all the observations we suggest that the central part of southern tibet is currently underlain by the broken indian lower crust and mantle lithosphere

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

insight into ne tibetan plateau expansion from crustal and upper mantle anisotropy revealed by shear wave splitting abstract the northeastern tibetan plateau margin is the current expansion border where growth of the plateau is ongoing we analyze shear wave splitting at chinarray stations in the ne tibetan plateau and its margin with the stable north chine craton the measurements provide important information on the seismic anisotropy and deformations patterns in the crust and upper mantle which can be used to constrain the expansion mechanism of the plateau along the margin and within the craton the dominant nw se fast polarization direction fpd is nw se subparallel to the boundary between the plateau and the north china craton the shear wave splitting measurements on the ne tibetan plateau itself generally reflect two layer anisotropy the lower layer anisotropy with nw se fpds is consistent in the whole region and fpds are the same as those in the north china craton the upper layer fpds are parallel to crustal motion rather than surface structures within the high plateau the two layer anisotropy implies the presence of deformed tibetan lithosphere above the underthrusting north china craton the ne tibetan shows similar deformation patterns at the surface inferred from gps and within the mantle inferred from shear wave splitting but significant crustal anisotropy parallel to crustal motion requires mid lower crustal channel flow or detachment to drive further tectonic uplift of the plateau

layered crustal anisotropy and deformation in the se tibetan plateau revealed by markov chain monte carlo inversion of receiver functions abstract the southeastern se tibetan plateau is uplifting accompanying strong seismicity but the relationship between the crustal structure and tectonic process in this region is still unclear seismic anisotropy provides important clues for the deformation mechanism here we applied the harmonic decomposition approach and further conducted a markov chain monte carlo mcmc inversion to analyze the crustal anisotropy from receiver functions using waveforms recorded by dense stations in the se tibetan plateau more spatial details on the crustal anisotropy and interface are determined consequently while the anisotropies in the upper crust are relatively weak 2 the anisotropies in the mid lower crust are mostly stronger than 4 the fast polarization directions fpds in the mid lower crust show a dominant circular pattern in good coherence with the topographic contours indicating that the crustal deformations in the se tibetan plateau may be mainly controlled by the gravity potential because of the significant moho steps along the plateau margin the extruded deep crustal materials are blocked by the strong crust around so that the minerals are aligned to be sub parallel to the plateau margins

mountain building in the nepal himalaya thermal and kinematic model we model crustal deformation and the resulting thermal structure across the nepal himalaya assuming that since 20 ma shortening across the range has been primarily taken up by slip along a single thrust fault the main himalayan thrust mht fault and that the growth of the himalayan wedge has resulted mainly from underplating and to the development of a duplex at midcrustal depth we show that this process can account for the inverse thermal metamorphic gradient documented throughout the lesser himalaya lh the discontinuity of peak metamorphic temperatures across the mct as well as the distribution of age of exhumation across the range this study suggests that the metamorphic evolution of the range over about the last 20 million years is compatible with the kinematics of recent crustal deformation deduced from morphotectonic and geodetic studies

determination of the crustal structure in southern tibet by dispersion and amplitude analysis of rayleigh waves summary surface waves recorded by the nine broad band stations of the indepth ii experiment are analysed to study the crustal structure of southern tibet their frequency range is between approximately 0 015 and 0 050 hz i e between 20 and 60 s period phase velocity dispersion curves are calculated for the regions north and south of the tsangpo suture using wiener filtering of fundamental mode rayleigh waves incident upon the array with different backazimuths the two dispersion curves are inverted to obtain the s wave crustal models north and south of the suture they show that a low velocity layer is present in the lower crust north but not south of the tsangpo suture to confirm these findings variations in the amplitudes of the rayleigh waves across the suture are interpreted by numerical simulation the strong amplitude variations at a 30 s period are reproduced using a model with a low velocity layer in the lower crust north of the suture a normal high velocity layer to the south and a sharp transition between these layers

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

seismic evidence for widespread western us deep crustal deformation caused by extension morgan moschetti and colleagues use ambient noise tomography data from the western united states to show that strong radial seismic anisotropy exists in the deep crust confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since about 65 million years ago the coincidence of crustal radial anisotropy with these extensional provinces supports the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension here a method of seismic wave imaging known as ambient noise tomography is used to generate high resolution images of seismic wave speeds in the crust and uppermost mantle the observations reveal strong and uniform anisotropy where waves travel through rock at different speeds depending on direction in the deep crust in areas of the western united states that have undergone significant extension during the past 65 million years laboratory experiments have established that many of the materials comprising the earth are strongly anisotropic in terms of seismic wave speeds1 observations of azimuthal2 3 and radial4 5 anisotropy in the upper mantle are attributed to the lattice preferred orientation of olivine caused by the shear strains associated with deformation and provide some of the most direct evidence for deformation and flow within the earth s interior although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes large scale observations have been limited to regions of particularly thick crust6 here we show that observations from ambient noise tomography in the western united states reveal strong deep middle to lower crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since 65 myr ago 7 8 the coincidence of crustal radial anisotropy with the extensional provinces of the western united states suggests that the radial anisotropy results from the lattice preferred orientation of anisotropic crustal minerals caused by extensional deformation these observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension9 10 11

crustal structure and deformation under the longmenshan and its surroundings revealed by receiver function data abstract the convergence of india and eurasia and the obstruction from the rigid sichuan basin cause the longmenshan lms to have the steepest topographic gradient at the eastern margin of the tibetan plateau however the mechanisms of surface uplift are still controversial in this paper we estimate the crustal structure and deformation under the lms and its surroundings by analyzing a large amount of receiver function data recorded by regional seismic networks of the china earthquake administration we apply a comprehensive splitting measurement technique on ps conversion phase at the moho moho ps splitting to calculate crustal anisotropy from azimuthal variations of receiver functions our results show that most of the seismic stations beneath the lms area exhibit significant seismic anisotropy with the splitting time of 0 22 0 94 s and a fast polarization direction of nw se while less or even no crustal anisotropy has been observed under the sichuan basin comparing the fast polarization directions of moho ps splitting with the indicators of lithospheric deformation such as shear wave splitting absolute plate motion and global positioning system imply a consistent tendency of deformation between the lower crust and upper mantle but decoupling deformation in the crust beneath the lms area we further compare moho ps splitting time to that estimated from previous sks splitting indicating that crustal anisotropy is an important source of the sks splitting time in this study area in addition a thick crust 50 km with high vp vs values 1 74 1 86 is also observed using the h stacking method these seismic observations are consistent with the scenario that the lms area has been built by the lower crustal flow combined with the seismic reflection refraction profile and geology studies we further suggest that the lower crustal flow may extrude upward into the upper crust along the steeply dipping strike faults under the lms area resulting in the surface uplift of the lms

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

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

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

crustal structure of northern and southern tibet from surface wave dispersion analysis 1 group and phase velocities of fundamental mode rayleigh waves in the period range of 10 to 70 s are obtained for southern and northern tibet significant variations in crustal velocity structure are found the group velocity minimum for tibet occurs at 33 s and the minimum is 0 12 km s lower for southern tibet than for northern tibet at periods greater than 50 s however group velocities are up to 0 2 km s faster in southern tibet the group and phase velocities are inverted for layered s wave models the dispersion observations in southern tibet can only be fit with a low velocity layer in the middle crust in contrast the velocity models for northern tibet do not require any lowvelocity zone in the crust the s wave velocity of the lower crust of southern tibet is 0 2 km s faster than the lower crust of northern tibet in southern tibet the sub moho velocity increases with a positive gradient that is similar to a shield while there is no velocity gradient beneath northern tibet the high velocity lower crust of southern tibet is consistent with the underthrusting of indian continental lithosphere the most plausible explanation of the mid crustal low velocity zone is the presence of crustal melt resulting from h2o saturated melting of the interplate shear zone between the underthrusting indian crust and overflowing asian crust the lack of a pronounced crustal low velocity zone in northern tibet is an indication of a relatively dry crust the low s wave velocity in the lower crust of northern tibet is interpreted to be due to a combination of compositional differences high temperatures presumably caused by a high mantle heat flux and possibly small amounts of partial melt combined with all available observations in tibet the new surface wave results are consistent with a hot and weak upper mantle beneath northern tibet index terms 7205 seismology continental crust 1242 7218 seismology lithosphere and upper mantle 7255 seismology surface waves and free oscillations 8102 tectonophysics continental contractional orogenic belts keywords tibet crustal velocity structure surface wave rayleigh waves continental collision

multimode rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the australian upper mantle summary we present an azimuthally anisotropic 3 d shear wave speed model of the australian upper mantle obtained from the dispersion of fundamental and higher modes of rayleigh waves we compare two tomographic techniques to map path average earth models into a 3 d model for heterogeneity and azimuthal anisotropy method i uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure it performs an iterative inversion with norm damping and gradient regularization method ii uses a direct inversion of individual depth layers constrained by bayesian assumptions about the model covariance we recall that bayesian inversions and discrete regularization approaches are theoretically equivalent and with a synthetic example we show that they can give similar results the model we present here uses the discrete regularized inversion of independent path constraints of method i on an equal area grid with the exception of westernmost australia we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction to within 0 8 per cent for the velocity 20 per cent for the anisotropic magnitude and 20 for its direction on length scales of 1000 km and longer down to about 200 km there is a good correlation between velocity heterogeneity and geologic age at shorter length scales and at depths below 200 km however this relationship breaks down the observed magnitude and direction of maximum anisotropy do not in general appear to be correlated to surface geology the pattern of anisotropy appears to be rather complex in the upper 150 km whereas a smoother pattern of fast axes is obtained at larger depth if some of the deeper directions of anisotropy are aligned with the approximately n s direction of absolute plate motion this correspondence is not everywhere obvious despite the fast 7 cm yr 1 northward motion of the australian plate more research is needed to interpret our observations in terms of continental deformation predictions of sks splitting times and directions an integrated measure of anisotropy are poorly matched by observations of shear wave birefringence

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

group velocity tomography of the indo eurasian collision zone we present results of a rayleigh and love wave group velocity dispersion study of the indo eurasian collision zone group velocity dispersion curves are measured and combined to produce dispersion maps for 10 70 s period rayleigh waves from 4054 paths and for 15 70 s love waves from 1946 paths group velocity maps benefit from the inclusion of data recorded at a large number of stations within india an advantage over previous global studies this has the largest impact at short periods as a result of the improved path length distribution synthetic tests are used to estimate resolution which ranges from 3 to 5 on the continents for rayleigh wave maps and from 5 to 7 5 for love wave maps group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods the cratons of the indian shield can be distinguished in the short period and midperiod group velocities group velocities are slow across tibet until 70 s whereas the cratonic cores of the indian shield appear as a high velocity anomaly at 70 s dispersion curves extracted from the rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across india and tibet the relationship between shear velocity contours and the moho indicated by receiver function studies has been used to obtain a first order estimate of crustal thickness across the collision zone results suggest a slow tibetan midcrust and low sub moho velocities beneath the central and northeastern tibetan plateau

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

three dimensional radial anisotropic structure of the north american upper mantle from inversion of surface waveform data summary seismic anisotropy provides insight into palaeo and recent deformation processes and there fore mantle dynamics in a first step towards a model for the north american upper mantle with anisotropy characterized by a symmetry axis of arbitrary orientation aimed at filling the gap between global tomography and sks splitting studies we inverted long period waveform data simultaneously for perturbations in the isotropic s velocity structure and the anisotropic parameter v 2 sh v2 in the framework of normal mode asymptotic coupling theory nact the resulting 2 d broad band sensitivity kernels allow us to exploit the information contained in long period seismograms for fundamental and higher mode surface waves at the same time to ensure high quality of the retrieved regional upper mantle structure accurate crustal cor rections are essential here we follow an approach which goes beyond the linear perturbation approximation and split the correction into a linear and non linear part the inverted data set consists of more than 40 000 high quality three component fundamental and overtone sur face waveforms recorded at broad band seismic stations in north america from teleseismic events and provides a fairly homogeneous path and azimuthal coverage the isotropic part of our tomographic model shares the large scale features of previous regional studies for north america we confirm the pronounced difference in the isotropic velocity structure between the western active tectonic region and the central eastern stable shield as well as the presence of subducted material juan de fuca and farallon plate at transition zone depths the new regional 3 d radial anisotropic model indicates the presence of two distinct anisotropic layers beneath the cratonic part of the north american continent a deep asthenospheric layer con sistent with present day mantle flow and a shallower lithospheric layer possibly a record of ancient tectonic events

crustal structure beneath se tibet from joint analysis of receiver functions and rayleigh wave dispersion new constraints on the pattern of crustal flow in se tibet are obtained from joint analysis of receiver functions and rayleigh wave dispersion with a newly deployed seismic array the crust in the sichuan yunnan diamond block has an average thickness of 45 km and gradually thins toward the indo china block to the west and the yangtze block to the east high vp vs ratios are detected to the west of the xiaojiang fault but not in the yangtze block to the east the s wave velocity profile reveals that intra crustal low velocity zones ic lvzs are strongly heterogeneous with two lvzs in the middle and mid lower crust respectively in marked contrast to previous observations of a single lvz combined with other observations the two ic lvzs are interpreted as isolated channels of crustal flow at different depths beneath se tibet resulting in the observed complex pattern of radial anisotropy and further elucidating patterns of flow and deformation

crust and uppermost mantle beneath the north china craton northeastern china and the sea of japan from ambient noise tomography 1 a 3 d shear velocity model of the crust and uppermost mantle to a depth of 100 km is presented beneath the north china craton ncc northeastern china the korean peninsula and the sea of japan ambient noise rayleigh wave tomography is applied to data from more than 300 broadband seismic stations from chinese provincial networks cearray the japanese f net and the iris global seismic network continuous data from 2007 to 2009 are used to produce group and phase velocity maps from 8 s to 45 s periods the model is motivated to constrain the distributed intraplate volcanism crustal extension cratonic rejuvenation and lithospheric thinning that are hypothesized for the study region numerous robust features are observed that impose new constraints on the geometry of these processes but discussion concentrates only on four 1 the north south gravity lineament follows the 40 km contour in crustal thickness and crustal thickness is anticorrelated with water depth beneath the sea of japan consistent with crustal isostasy for a crust with laterally variable composition 2 the lithosphere is thin 70 km beneath the songliao bohai graben but seismically fast 3 even thinner more attenuated lithosphere bounds three sides of the eastern ncc in a horseshoe shape identifying a region of particularly intense tectonothermal modification where lithospheric rejuvenation may have reached nearly to the base of the crust 4 low velocity anomalies reach upward in a y shape in the mantle beneath the eastern and western borders of the sea of japan extending well into continental east asia in the west and are separated by a 60 km thick lithosphere beneath the central sea of japan this anomaly may reflect relatively shallow slab dehydration in the east and in the west may reflect deeper dehydration and convective circulation in the mantle wedge overlying the stagnant slab

the structure of the crust and uppermost mantle beneath south china from ambient noise and earthquake tomography summary two years of continuous recordings of ambient seismic noise observed at 354 stations in south china from 2009 to 2010 are used to estimate rayleigh wave group and phase velocity maps from 6 to 40 s period these results are merged with rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame eikonal tomography generates the dispersion maps which by monte carlo inversion are used to estimate a 3 d vsv model of the crust and upper mantle down to a depth of 150 km across all of south china with attendant uncertainties a clear image emerges of the west yangtze block a region of the western yangtze craton characterized by relatively thick crust 40 km overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the yangtze craton in the archean and may present a present day obstacle to the eastward expansion of tibet the west yangtze block contrasts with the thinner crust 30 km and mantle lithosphere 70 80 km beneath the eastern yangtze craton and south china foldbelt these observed differences are consistent with processes associated with flat slab subduction in the mesozoic that may have eroded the lithosphere of the eastern yangtze craton and the south china foldbelt

crustal anisotropy and ductile flow beneath the eastern tibetan plateau and adjacent areas abstract crustal anisotropy beneath 71 broadband seismic stations situated at the eastern tibetan plateau and the sichuan basin is investigated based on the sinusoidal moveout of p to s conversions from the moho and an intra crustal discontinuity significant crustal anisotropy is pervasively detected beneath the study area with an average splitting time of 0 39 0 18 s the resulting fast orientations are mostly parallel to the major shear zones in the songpan ganzi terrane and can be explained by fluid filled fractures favoring the model of rigid block motion with deformations concentrated on the block boundaries in the vicinity of the xianshuihe xiaojiang fault zone in the southern songpan ganzi terrane our results when combined with previously revealed high crustal poisson s ratio in the area support the existence of mid lower crustal flow the longmenshan fault zone and adjacent areas are dominated by strike orthogonal fast orientations which are consistent with alignments of cracks associated with compressional stress between the plateau and the sichuan basin the observations suggest that crustal thickening is the main cause of the high topographic relief across the longmenshan fault zone

midcrustal low velocity layer beneath the central himalaya and southern tibet revealed by ambient noise array tomography 1 ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the earth using 2 months of ambient noise data from 20 stations of the himalayan nepal tibet seismic experiment we investigate the upper and middle crustal structure in the central himalaya and southern tibet about 120 interstation rayleigh wave empirical green s functions with sufficient signal to noise ratio are obtained and used for group velocity dispersion analysis in the period range 6 25 s using frequency time analysis technique the obtained dispersion data are then used to construct 2 d group velocity maps at the short periods from 9 to 15 s the distribution of rayleigh wave velocities delineates several distinct low and high velocity zones separated mainly by geological boundaries the high group velocity zone is located mainly around regions with plutonic rocks and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks finally we invert for the shear velocity structure in the upper and middle crust along a n s trending cross section at the longitude 86 5 e we observe a clear low velocity layer in the middle crust about 10 25 km depth distributed on both sides of the indus yarlung suture zone the existence of this midcrustal low velocity zone suggests a mechanically weaker middle crust beneath the central himalaya and southern tibet which might decouple the upper crustal deformation from that of the lower crust in the tibetan himalayan orogenic processes

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

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

oblique high angle listric reverse faulting and associated development of strain the wenchuan earthquake of may 12 2008 sichuan china the 2008 wenchuan earthquake occurred on imbricate oblique steeply dipping slowly slipping listric reverse faults measurements of coseismic slip the distribution of aftershocks and fault plane solution of the mainshock all confirm this style of deformation and indicate cascading earthquake rupture of multiple segments each with coseismic slip occurring in the shallow crust above a depth range of 10 to 12 km interactions among three geological units eastern tibet the longmen shan and the sichuan basin caused slow strain accumulation in the longmen shan so that measurable preearthquake slip was minor coseismic deformation however took place mostly within the interseismically locked longmen shan fault zone the earthquake may have initiated from slip on a fault plane dipping 30 40 northwest in a depth range from 15 to 20 km and triggered oblique slip on the high angle faults at depths shallower than 15 km to form the great wenchuan earthquake

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

crustal seismic anisotropy in central tibet implications for deformational style and flow in the crust 1 receiver functions obtained at indepth iii stations located near the bangong nujiang suture in central tibet display a weak moho signal and strong p to s conversions within the first 5 s that vary systematically with back azimuth a single station with representative azimuthal variations located at the sharp onset of strong sks splitting is modeled for both dipping layers and seismic anisotropy by using a global minimization technique inversion results indicate strong anisotropy 10 near the surface and in the middle crust separated by a south dipping 25 layer possibly related to the earlier phase of crustal shortening near surface anisotropy has a fabric dipping steeply southward and trending wnw ese that correlates with the suture and younger strike slip faults in contrast mid crustal anisotropy occurs in a low velocity zone and has a fabric dipping gently 18 northward that might be related to a well developed near horizontal rock fabric induced by crustal flow

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet