seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

teleseismic waveform complexities caused by near trench structures and their impacts on earthquake source study application to the 2015 illapel aftershocks central chile

probing depth and lateral variations of upper mantle seismic anisotropy from full waveform inversion of teleseismic body waves we present an extension of the regional full waveform inversion method to image seismic anisotropy in the lithosphere and asthenosphere from teleseismic p and s waveforms the models are parameterized in terms of density and the 21 elastic coefficients of the fourth order elasticity tensor the inversion method makes no a priori assumptions on the symmetry class or on the orientation of the symmetry axes instead the elasticity tensors in the final models are decomposed with the projection method of browaeys chevrot 2004 this method allows us to determine the orientation of the symmetry axes and extract the contributions of each symmetry class e g hexagonal symmetry from simple synthetic experiments we demonstrate that our full waveform inversion method is able to image complex three dimensional anisotropic structures in particular the method is able to almost perfectly recover layered anisotropic models which produce very small apparent shear wave splitting an extremely challenging case we attribute this success to the joint exploitation of both p and s teleseismic waves which constrain different parts of the elasticity tensor another key ingredient is the preconditioning of the gradient with an approximate inverse hessian computed with scattering integrals the inverse hessian is crucial to correct for the effects of the uneven mostly vertical illumination of teleseismic acquisitions 2 beller et al

imaging the lithosphere beneath ne tibet teleseismic p and s body wave tomography incorporating surface wave starting models the northeastern margin of the tibetan plateau which includes the qiangtang and songpan ganzi terranes as well as the kunlun shan and the qaidam basin continues to deform in response to the ongoing india eurasia collision to test competing hypotheses concerning the mechanisms for this deformation we assembled a high quality data set of approximately 14 000 p and 4000 s wave arrival times from earthquakes at teleseismic distances from the international deep profiling of tibet and the himalaya phase iv broad band seismometer deployments we analyse these arrival times to determine tomographic images of p and s wave velocities in the upper mantle beneath this part of the plateau to account for the effects of major heterogeneity in crustal and uppermost mantle wave velocities in tibet we use recent surface wave models to construct a starting model for our teleseismic body wave inversion we compare the results from our model with those from simpler starting models and find that while the reduction in residuals and results for deep structure are similar between models the results for shallow structure are different checkerboard tests indicate that features of 125 km length scale are reliably imaged throughout the study region using synthetic tests we show that the best recovery is below 300 km and that broad variations in shallow structure can also be recovered we also find that significant smearing can occur especially at the edges of the model we observe a shallow dipping seismically fast structure at depths of 140 240 km which dies out gradually between 33 n and 35 n based on the lateral continuity of this structure from the surface waves we interpret it as indian lithosphere alternatively the entire area could be thickened by pure shear or the northern part could be an underthrust lhasa terrane lithospheric slab with only the southern part from india we see a deep fast wave velocity anomaly below 300 km that is consistent with receiver function observations of a thickened transition zone and could be a fragment of oceanic lithosphere in ne tibet it appears to be disconnected from faster wave velocities above i e it is not downwelling or subducting here our models corroborate results of previous work which imaged a relatively slow wave velocity region below the kunlun shan and northern songpan ganzi terrane which is difficult to reconcile with the hypothesis of southward directed continental subduction at the northern margin wave velocities in the shallow mantle beneath the qaidam basin are faster than normal and more so in the east than the west

multiple seismic reflectors in earth s lowermost mantle the modern view of earth s lowermost mantle considers a d region of enhanced seismologically inferred heterogeneity bounded by the core mantle boundary and an interface some 150 300 km above it with the latter often attributed to the postperovskite phase transition in mgsio3 seismic exploration of earth s deep interior suggests however that this view needs modification so called scs and skks waves which probe the lowermost mantle from above and below respectively reveal multiple reflectors beneath central america and east asia two areas known for subduction of oceanic plates deep into earth s mantle this observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the d region sensu stricto

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

a 3 d spectral element and frequency wave number hybrid method for high resolution seismic array imaging we present a three dimensional 3 d hybrid method that interfaces the spectral element method sem with the frequency wave number fk technique to model the propagation of teleseismic plane waves beneath seismic arrays the accuracy of the resulting 3 d sem fk hybrid method is benchmarked against semianalytical fk solutions for 1 d models the accuracy of 2 5 d modeling based on 2 d sem fk hybrid method is also investigated through comparisons to this 3 d hybrid method synthetic examples for structural models of the alaska subduction zone and the central tibet crust show that this method is capable of accurately capturing interactions between incident plane waves and local heterogeneities this hybrid method presents an essential tool for the receiver function and scattering imaging community to verify and further improve their techniques these numerical examples also show the promising future of the 3 d sem fk hybrid method in high resolution regional seismic imaging based on waveform inversions of converted scattered waves recorded by seismic array

regional mapping of the crustal structure in southern california from receiver functions lateral variations of the crustal structure in southern california are determined from receiver function rf studies using data from the southern california seismic network broadband stations and los angeles regional seismic experiment surveys the results include crustal thickness estimates at the stations themselves and where possible cross sections are drawn the large scale moho depth variation pattern generally correlates well with the current status of the mesozoic batholith deep moho of 35 39 km is observed beneath the western peninsula ranges sierra nevada and san bernardino mountains where the batholith is relatively intact and shallow moho of 26 32 km is observed in the mojave desert where the batholith is highly deformed and disrupted high resolution lateral variations of the crustal structure for individual geographic provinces are investigated and distinctive features are identified the crustal structure is strongly heterogeneous beneath the central transverse ranges and deep moho of 36 39 km is locally observed beneath several station groups in the western san gabriel mountains moho is relatively flat and smooth beneath the western mojave desert but gets shallower and complicated to the east anomalous rfs are observed at two stations in the eastern mojave desert where a moho step of 8 10 km is found between the nw and se back azimuthal groups of station dan in the fenner valley asymmetric extension of the salton trough is inferred from the moho geometry depth extension of several major faults such as the san andreas fault and san gabriel fault to the moho is inferred

imaging the upper mantle transition zone with a generalized radon transform of ss precursors we demonstrate the feasibility of using inverse scattering for high resolution imaging of discontinuities in the upper mantle beneath oceanic regions far from sources and receivers using broadband wavefield observations consisting of ss and its precursors the generalized radon transform grt that we developed for this purpose detects in the broadband data signals due to scattering from elasticity contrasts in earth s mantle synthetic tests with realistic source receiver distributions demonstrate that the grt is able to detect and image deep mantle interfaces even in the presence of noise depth phases ghosts phase conversions and multiples generated by reverberation within the transition zone as a proof of concept we apply the grt to 50 000 broadband seismograms to delineate interfaces in the depth range from 300 to 1000 km beneath the northwest pacific we account for smooth 3d mantle heterogeneity using first order perturbation theory and independently obtained global tomography models the preliminary results reveal laterally continuous but undulating scatter zones near 410 and 660 km depth and a weaker broader and more complex structure near 520 km depth the images also suggest the presence of multiple laterally intermittent interfaces near 350 km and between 800 and 1000 km depth that is above and below the transition zone sensu stricto filtering of the data we consider four pass bands 20 50 s 10 50 s 5 50 s and 2 50 s reveals a prominent frequency dependence of the magnitude width and complexity of the interfaces in particular of the scatter zone near 520 km depth such dependencies may put important constraints on the mineralogy and phase chemistry of the transition zone

reverse time migration based reflection tomography using teleseismic free surface multiples converted and multiply reflected phases from teleseismic events are routinely used to create structural images of the crust mantle boundary moho and the elasticity contrasts within the crust and upper mantle the accuracy of these images is to a large extent determined by the background velocity model used to propagate these phases to depth in order to improve estimates of 3 d velocity variations and hence improve imaging we develop a method of reverse time migration based reflection tomography for use with wavefields from teleseismic earthquakes recorded at broad band seismograph arrays reflection tomography makes use of data redundancy that is the ability to generate numerous structural images of the subsurface with different parts of the wavefield in exploration seismology where it is known as migration velocity analysis reflection tomography typically involves the generation of an extended image e g offsetor angle gathers and the fitness of the background model is evaluated through the application of image domain annihilators in regional scale passive source seismology however annihilation based methods are inadequate because the sparse and irregular distribution of teleseismic sources is not likely to produce illumination over a sufficient range of angles to overcome this problem we turn towards a source indexed moveout scheme instead of extended image annihilation we determine the success of the tomographic velocity model by cross correlating images produced with multiply scattered waves from different teleseismic sources the optimal velocity model is the one that minimizes correlation power between windowed images away from zero depth shift we base our inversion scheme on the seismic adjoint method and a conjugate gradient solver for each image pair the update direction is determined by correlations between downgoing wavefields with upgoing adjoint wavefields for both images the sensitivity kernels used in this method is similar to those found in other forms of adjoint tomography but their shapes are controlled by the spatial distribution of the error function we present the method and a proof of concept with 2 d synthetic data

teleseismic shot profile migration the shot profile migration approach of wave equation migration generates subsurface images using the interferometric principle of crosscorrelating two passive wavefields these wavefields are typically a source wavefield containing energy from an excited source and a receiver wavefield comprised of scattered source wavefield energy by the discontinuous earth structure shot profile migration can be recast as a novel way of imaging the earth s lithosphere using teleseismic wavefield data where the source wavefield is the directly arriving wavefront and the receiver wavefield is the following wavefield coda we demonstrate that the shot profile technique can be tailored to suit teleseismic acquisition geometry and wavefields assuming an acoustic framework and 2 5d experimental geometry we develop procedures that enable kinematic and structural imaging migration using both transmission and free surface reflected passive wavefields experiments with synthetic data demonstrate the method s applicability a

seismic migration processing of p sv converted phases for mantle discontinuity structure beneath the snake river plain western united states we experiment with backprojection migration processing of teleseismic receiver functions from the snake river plain srp broadband seismic experiment previous analyses of data from this experiment have used a common midpoint cmp stacking approach a method widely applied for analysis of p sv converted phases receiver functions to obtain high resolution imaging of upper mantle discontinuities the cmp technique assumes that all p sv conversions are produced by flat lying structures and may not properly image dipping curved or laterally discontinuous interfaces in this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions we first perform synthetic experiments that illustrate the potential improvement of migration processing over cmp stacks application of the migration processing to the srp data set shows most of the major features as in the original cmp work but with a weaker 410 km discontinuity and a more intermittent discontinuity at 250 km apparent depth random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images these tests show that a 20 km elevation of the 660 km discontinuity directly beneath the snake river plain is robust but that the variations in 410 km discontinuity topography that we observe are not stable upon resampling bright spots near 250 km apparent depth are robust upon resampling but interpretation of these features is complicated by possible sidelobe artifacts from topside moho reverberations

a parametric analysis of two dimensional elastic full waveform inversion of teleseismic data for lithospheric imaging the development of dense networks of broad band seismographs makes teleseismic data amenable to full waveform inversion fwi methods for high resolution lithospheric imaging compared to scattered field migration fwi seeks to involve the full seismic wavefield in the inversion we present a parametric analysis of 2 d frequency domain fwi in the framework of lithospheric imaging from teleseismic data to identify the main factors that impact on the quality of the reconstructed compressional p wave and shear s wave speed models compared to controlled source seismology the main adaptation of fwi to teleseismic configuration consists of the implementation with a scattered filed formulation of plane wave sources that impinge on the base of the lithospheric target located below the receiver network at an arbitrary incidence angle seismic modelling is performed with a hp adaptive discontinuous galerkin method on unstructured triangular mesh a quasi newton inversion algorithm provides an approximate accounting for the hessian operator which contributes to reduce the footprint of the coarse acquisition geometry in the imaging a versatile algorithm to compute the gradient of the misfit function with the adjoint state method allows for abstraction between the forward problem operators and the meshes that are during seismic modelling and inversion respectively an approximate correction for obliquity is derived for future application to real teleseismic data under the two dimension approximation comparisons between the characteristic scales involved in exploration geophysics and in teleseismic seismology suggest that the resolution gain provided by full waveform technologies should be of the same order of magnitude for both applications we first show the importance of the surface reflected wavefield to dramatically improve the resolving power of fwi by combining tomography like and migration like imaging through the incorporation of the forward scattered and the backscattered wavefields in the inversion the resolution of fwi is assessed through checkerboard tests and confirms a resolution of the order of the wavelength for both the p and s speeds when the full wavefield is incorporated in the inversion secondly we show that computationally efficient strategies which consist of decimating the number of frequency components involved in the inversion do not apply to teleseismic acquisitions because the scattering angle bandwidth sampled by plane wave sources can be narrow and coarsely sampled compared to that provided by dense profiles of point sources in exploration seismology the waveform inversion is less sensitive to the band of incidence angles spanned by the plane wave sources and to the sampling of this band however the deficit of vertically propagating plane waves hampers the vertical resolution of planar layers aliasing artefacts created by coarse arrays of receivers are illustrated we show how taking into account the hessian in the inversion and the suitable management of frequencies in the inversion help to mitigate these artefacts acceptable reconstructions are shown for both the p and s wave speeds for a receiver spacing of up to 20 km in the 0 1 0 4 hz frequency range building a reliable initial model for fwi is a highly non linear problem in exploration seismology we show how the low frequency content of the teleseismic sources allow us to build accurate p and s wave speed models starting from simple vertical gradient velocity models all of these results are derived using a 2 d realistic synthetic experiment that was performed with noise free data most of conclusions of this parametric study should apply in three dimensions the impact of noise and the footprint of other experimental parameters such as the estimation of the temporal plane wave signature and the estimation of the incidence angle of the impinging plane waves need however to be assessed in the future

a data adaptive multiscale approach of finite frequency traveltime tomography with special reference to p and s wave data from central tibet 1 we discuss an innovation in traveltime tomography that combines wavelet based multiscale parameterization and finite frequency theory to solve two outstanding issues that inevitably arise from uneven source station distributions and from the three dimensional 3 d nature of wavefront healing how to objectively address the intrinsically multiscale nature of data coverage while simultaneously maintain model resolution at each scale level we apply the new integrated methodology to investigate 3 d variations of p and s wave speeds lnvp and lnvs beneath the himalayan tibetan orogen in particular we are able to constrain variations in the poisson s ratio via ln vp vs the formulation is naturally data adaptive resolving features at each scale only if the required data converge is available the very first long wavelength feature that emerges is a clear anomaly of high lnv that extends over more than 500 km beyond the northern edge of the lhasa terrane at places farther northward a strong negative anomaly underlies the region where recent volcanism occurs in northern tibet regions of negative ln vp vs delineate a slab like subhorizontal feature concentrated between depths of 100 250 km such characteristics are consistent with the notion that chemically refractory and therefore buoyant mantle lithosphere of the indian shield greater india has advanced subhorizontally northward far beyond the surficial bangong nujiang suture in the crust two isolated regions of low lnv each extending to depths near 100 km occur along the lunggar and the yadong gulu active rifts in southern tibet deep penetrating rifts imply that only a limited amount of horizontal displacement is being accommodated on subvertical structures

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

exhumation history of the tatry mountains western carpathians constrained by low temperature thermochronology this study tests alternative models for the growth of the tatry mountains central western carpathians by the application of low temperature thermochronology zircon u th he ages from the north of the range are mostly between 48 to 37 ma and indicate cooling prior to the onset of forearc sedimentation in the region 42 39 ma in contrast zircon u th he ages in the south of the range are around 22 ma apatite fission track ages across the sampled sites range from 20 to 15 ma apatite u th he ages range from 18 to 14 ma with little variation with elevation or horizontal location based on thermal modelling and tectonic reconstructions these miocene ages are interpreted as cooling in the hanging wall of a northward dipping thrust ramp in the current location of the sub tatric fault with cooling rates of 20 c myr at 22 14 ma modeled cooling histories require an abrupt deceleration in cooling after 14 ma to 5 c myr this is associated with termination of deformation in the outer carpathians and is synchronous with the transition of the pannonian basin from a syn rift to a post rift stage and with termination of n s compression in the northern part of the central western carpathians overall the timing of shortening and exhumation is synchronous with the formation of the outer carpathian orogen and so the miocene exhumation of the tatry record retro vergent thrusting at the northern margin of the alcapa microplate

beyond receiver functions passive source reverse time migration and inverse scattering of converted waves 1 we present a wave equation prestack depth migration to image crust and mantle structures using multi component earthquake data recorded at dense seismograph arrays transmittedp and swaves recorded on the surface are back propagated using an elastic wave equation solver the wave modes are separated after the reverse time continuation of the wavefield from the surface and subjected to a cross correlation type imaging condition forming an inverse scattering transform reverse time migration rtm does not make assumptions about the presence or properties of interfaces notably it does not assume that interfaces are locally horizontal with synthetic experiments and different background models we show that passive source rtm can reconstruct dipping and vertically offset interfaces even in the presence of complex wave phenomena such as caustics and point diffraction and that its performance is superior to traditional receiver function analysis e g common conversion point ccp stacking in complex geological environments

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

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

shear wave birefringence and current configuration of converging lithosphere under tibet article i nfo new data from west central tibet show that birefringence of s waves has two pronounced increases in magnitude toward the hinterland null birefringence persists to about 75 km north of the indus yarlung suture iys between the indian shield and the lhasa terrane of southern tibet a second rapid increase occurs about 100 km farther north of the bangong nujiang sutures between the lhasa terrane and the qiangtang terrane in central tibet the latter feature is consistently observed along three long transects that collectively span a lateral orogen parallel distance of about 600 km and is likely to mark the northern leading edge of sub horizontally advancing mantle lithosphere of the indian shield the greater india an interpretation consistent with the latest results of finite frequency tomography using both p and s wave travel times previous results of modeling gravity anomalies and a host of other seismic observations similarly complementary constraints indicate that the sudden onset of significant birefringence north of the iys is likely to be the southern termination of eurasian mantle lithosphere curiously the shortest of three transects showed null birefringence through much of the lhasa terrane a pattern inconsistent with those of he isotopes and gravity

a scattered wave image of subduction beneath the transverse ranges over 5600 short period recordings of teleseismic events were used to create deterministic maps of p wave scatterers in the upper mantle beneath southern california between depths of 50 and 200 kilometers the southern flank of the slab subducting beneath the transverse ranges is marked by strong scattering the marked scattering indicates that the edge of the slab is a sharp thermal boundary such a boundary could be produced by slab shearing or small scale convection in the surrounding mantle the northern limb of the slab is not a strong scatterer consistent with thicker lithosphere north of the transverse ranges

small 660 km seismic discontinuity beneath tibet implies resting ground for detached lithosphere 1 using seismic profiles comprising of high resolution triplicate waveforms across apertures of over 1 000 km we show that because of high p wave speed vp near the bottom of the mantle transition zone the contrast in vp across the 660 km discontinuity beneath central tibet is small only about 70 of that beneath the northern indian shield this subhorizontal anomaly of high vp is most likely a remnant of detached mantle lithosphere that recently sank to depth thus providing key evidence for a direct connection between continental collision near the surface and deep seated dynamics in the mantle

a wave equation migration method for receiver function imaging 2 application to the japan subduction zone 1 the newly developed wave equation poststack depth migration method for receiver function imaging is applied to study the subsurface structures of the japan subduction zone using the fundamental research on earthquakes and earth s interior anomalies freesia broadband data three profiles are chosen in the subsurface imaging two in northeast ne japan to study the subducting pacific plate and one in southwest sw japan to study the philippine sea plate the descending pacific plate in ne japan is well imaged within a depth range of 50 150 km the slab image exhibits a little more steeply dipping angle 32 in the south than in the north 27 although the general characteristics between the two profiles in ne japan are similar the imaged philippine sea plate in eastern sw japan in contrast exhibits a much shallower subduction angle 19 and is only identifiable at the uppermost depths of no more than 60 km synthetic tests indicate that the top 150 km of the migrated images of the pacific plate is well resolved by our seismic data but the resolution of deep part of the slab images becomes poor due to the limited data coverage synthetic tests also suggest that the breakdown of the philippine sea plate at shallow depths reflects the real structural features of the subduction zone rather than caused by insufficient coverage of data comparative studies on both synthetics and real data images show the possibility of retrieval of fine scale structures from high frequency contributions if high frequency noise can be effectively suppressed and a small bin size can be used in future studies the derived slab geometry and image feature also appear to have relatively weak dependence on overlying velocity structure the observed seismicity in the region confirms the geometries inferred from the migrated images for both subducting plates moreover the deep extent of the pacific plate image and the shallow breakdown of the philippine sea plate image are observed to correlate well with the depth extent of the seismicity beneath ne and sw japan such a correlation supports the inference that the specific appearance of slabs and intermediate depth earthquakes are a consequence of temperature dependent dehydration induced metamorphism occurring in the hydrated descending oceanic crust

lithospheric imaging via teleseismic scattering tomography summary the coda of the teleseismic p phase consists largely of energy scattered by small inhomogeneities in the receiver side lithosphere given large collections of teleseismic data from dense permanent networks previous workers have successfully back propagated coda energy back to scattering source points using various kinematic migration schemes as well as by inverting using an inverse scattering radon transform approach under the born approximation seismic scattering is a linear process therefore it is possible to approach coda scattering as a linear waveform inversion problem mathematically similar to transmission based tomography assuming ray theoretical propagation and rayleigh scattering we pose the inverse scattering problem in tomographic form and recover perturbations in density and p and s velocities from pp and ps scattered data the method is applied to data from the southern california seismic network scsn covering the san jacinto anza region the results show a considerable correlation between seismicity and velocity perturbation structure particularly in the region between the mission creek and banning fault branches features connecting the coyote creek and elsinore faults at right angles are correlated with seismicity lineations and may represent conjugate faulting with no surface expression

northward thinning of tibetan crust revealed by virtual seismic profiles 1 a new approach of constructing deep penetrating seismic profiles reveals significant regional variations in crustal thickness under near constant elevation of tibet over distances of hundreds of kilometers the crust is as thick as 75 km in southern tibet but shoals to just over 60 km under the qiangtang terrane in central tibet where the deviation from airy isostasy is equivalent to a thickness of over 10 km in missing crust northward thinning of crust occurs gradually over a distance of about 200 km where mechanical deformation instead of pervasive magmatism also seems to have disrupted the crust mantle interface

high resolution seismic array imaging based on an sem fk hybrid method we demonstrate the feasibility of high resolution seismic array imaging based on teleseismic recordings using full numerical wave simulations we develop a hybrid method that interfaces a frequency wavenumber fk calculation which provides analytical solutions to 1 d layered background models with a spectral element sem numerical solver to calculate synthetic responses of local media to plane wave incidence this hybrid method accurately deals with local heterogeneities and discontinuity undulations and represents an efficient tool for the forward modelling of teleseismic coda including converted and scattered waves we benchmark the accuracy of the sem fk hybrid method against fk solutions for 1 d media we then compute sensitivity kernels for teleseismic coda waves by interacting the forward teleseismic waves with an adjoint wavefield produced by injecting coda waves as adjoint sources based on adjoint techniques these sensitivity kernels provide the basis for mapping variations in subsurface discontinuities density and velocity structures through non linear conjugate gradient methods we illustrate various synthetic imaging experiments including discontinuity characterization volumetric structural inversion for the crust or subduction zones these tests show that using pre conditioners based upon the scaled product of sensitivity kernels for different phases combining finite frequency traveltime and waveform inversion and or adopting hierarchical inversions from longto short period waveforms could reduce the non linearity of the seismic inverse problem and speed up its convergence the encouraging results of these synthetic examples suggest that inversion of teleseismic coda phases based on the sem fk hybrid method and adjoint techniques is a promising tool for structural imaging beneath dense seismic arrays

shdiff svdiff splitting in an isotropic earth 1 the vertically polarized svdiff seismic phase sometimes arrives later than the horizontally polarized shdiff both phases propagate in the d layer at the base of the mantle and the difference in their arrival times is usually interpreted in terms of seismic anisotropy in d using numerical simulations we demonstrate that a significant 1 s delay of svdiff relative to shdiff is present even in the iasp91 isotropic reference earth model and in a model with a high s velocity in d relative to iasp91 the delay is accompanied by strong amplitude decay of svdiff with distance this relationship resembles the effect of anelasticity in a model with a low s velocity in d the waveform of svdiff can be very different from shdiff and this difference can be mistaken for a delay of shdiff relative to svdiff in a laterally heterogeneous isotropic d svdiff can also be delayed because unlike shdiff svdiff has strong amplitude decay along fast paths its traveltime is thus dominated by slow propagation paths and tends to be positively biased for d models with an s velocity difference of 3 between two hemispheres there are regions where shdiff arrives up to 15 s earlier than the first clearly visible svdiff this is observed in a corridor several hundred kilometers wide if these effects are misinterpreted in terms of d anisotropy in actual seismic data estimates of anisotropy are biased and may contribute to the idea of seismic anisotropy being different in high and low s velocity regions of d

first multi scale finite frequency tomography illuminates 3 d anatomy of the tibetan plateau 1 with a new multi scale parameterization 3 d images from finite frequency seismic tomography including the very first images beneath central tibet from s waves reveal that regions of low electric resistivity in the crust previously observed along active rifts in southern tibet correlate well with regions of low p and s wave speeds v however such regions are not interconnected indicating that a prevailing south directed channel like crustal flow seems inactive or this popular geodynamic model needs modification in the upper mantle there is no clear indication of regional down welling between depths of 100 to 400 km instead a strong lateral boundary between high and low v extends up to 33 n marking the northern limit of sub horizontally advancing indian lithospheric mantle

time evolving seismic tomography the method and its application to the 1989 loma prieta and 2014 south napa earthquake area california we propose a time evolving approach to conduct traveltime seismic tomography in the 1989 mw 6 9 loma prieta earthquake and 2014 mw 6 0 south napa earthquake area california the recording period of the chosen seismic data between january 1 1967 and the day before the 2014 south napa earthquake is divided into two time windows separated by the 1989 loma prieta earthquake in each time window the subsurface velocity structure is iteratively updated starting from the final model of the first time window the velocity model has been successively improved throughout iterations in the second time window indicating that the traveltime data of later time windows have provided extra information to refine the subsurface images strong heterogeneities are observed in the final p wave velocity model both of the two large earthquakes occurred at transition zones in between high vp and low vp anomalies in all this study shows the effectiveness of the time evolving seismic tomography method

a hybrid method to compute short period synthetic seismograms of teleseismic body waves in a 3 d regional model we present a hybrid method to simulate the propagation of short period teleseismic body waves through 3 d regional models the incident wavefield is computed in an spherically symmetric reference earth model based on the direct solution method the global and regional wavefields are matched at the boundaries of the regional mesh in the regional domain we implement a spectral element method with absorbing boundaries to cancel the outgoing scattered wavefield the hybrid method is successfully benchmarked against the direct solution method in the reference earth model iasp91 the potential of the method is illustrated by computing short period p wave synthetic seismograms in a 3 d model with a 20 km moho offset and or topography on the free surface focusing on the actual example of the pyrenees

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

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma