shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

crustal anisotropy in northeastern tibetan plateau inferred from receiver functions rock textures caused by metamorphic fluids and lower crust flow abstract the crust of tibetan plateau may have formed via shortening thickening or large scale underthrusting and subsequently modified via lower crust channel flows and volatile mediated regional metamorphism the amplitude and distribution of crustal anisotropy record the history of continental deformation offering clues to its formation and later modification in this study we first investigate the back azimuth dependence of ps converted phases using multitaper receiver functions rfs we analyze teleseismic data for 35 temporary broadband stations in the ascent experiment located in northeastern tibet we stack receiver functions after a moving window moveout correction major features of rfs include 1 ps arrivals at 8 10 s on the radial components suggesting a 70 90 km crustal thickness in the study area 2 two lobed back azimuth variation for intra crustal ps phases in the upper crust

shear wave splitting in the upper mantle wedge above the tonga subduction zone summary intermediate period seismograms of shear waves from deep earthquakes are examined for shear wave splitting diagnostic of seismic velocity anisotropy in the upper mantle wedge between the subducting tonga slab and stations in the fiji islands earthquakes with near vertical incidence angles are selected to minimize complications of converted phases and the free surface horizontal component seismograms for 25 deep earthquakes recorded at station lak on the lau ridge show splitting of up to 0 9 s with an average of 0 45 0 25 s between fast and slow s wave components anisotropy in the upper mantle is the most likely cause of the splitting the ese orientation of the fast velocity direction at station lak corresponds to an axis of symmetry of the anisotropic mantle and is approximately perpendicular to magnetic lineations in the lau basin and the northeastern portion of the south iiji basin the observations are consistent with a model of flow alignment of olivine 100 and orthopyroxene 001 axes parallel to the direction of basin extension seismograms for eight deep earthquakes at station sva on viti levu show up to 1 0 s splitting with an average of 0 65 0 29 s and a weak nne preferred orientation of fast velocity polarizations of shear wave particle motions corrected for anisotropy are within 36 5 26 of polarixations predicted for nine moment tensor solutions the magnitude of shearwave splitting and estimates of the thickness of the lithosphere in the fiji region imply that the anisotropy cannot be contained in the lithosphere alone either the anisotropy is distributed in both the lithosphere and asthenosphere or the asthenosphere alone is anisotropic

passive seismological imaging of the narmada paleo rift central india abstract the narmada son lineament nsl is a prominent nearly e w trending tectonic feature dividing india into the peninsular and extra peninsular regions it marks the possible suture between two contrasting geological settings termed proto continents lying to the north and south of it various views prevail about the nature of the rift zone the present study delineates the seismic structure of the crust using p wave receiver functions at 10 broadband seismic stations installed along a profile across the nsl the results from common conversion point ccp stacking reveal a near continuous image of the moho boundary with the amplitudes of the ps conversions from the moho being weaker at stations within the rift modelling the receiver functions employing nearest neighborhood algorithm approach brings out a distinct 15 km thickening of the crust and a complex nature of the moho within the rifted zone we find that the backazimuthal variations of the sv and sh receiver functions at the permanent station jabalpur can be explained better by introducing azimuthal anisotropy in the middle and lower crust with the fast axes perpendicular to the rift axis also we find substantial evidence of post rifting magmatic underplating in the lower crust which could be reminiscence of deccan volcanism and or reunion plume path

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

crustal structure and deformation of the se tibetan plateau revealed by receiver function data we analyze a large amount of receiver function data recorded by regional seismic networks of the china earthquake administration to estimate crustal structure and deformation beneath the southeast margin of the tibetan plateau and its surrounding areas we develop a comprehensive analysis method that facilitates robust extraction of azimuthal seismic anisotropy from receiver function data the method includes an estimate of fast polarization direction and splitting time by a joint analysis of radial and transverse receiver function data and an evaluation of measurement reliability by statistical and harmonic analysis we find significant seismic anisotropy with a splitting time of 0 5 0 9 s beneath the se margin of the tibetan plateau both the splitting time and fast polarization direction are comparable to those estimated from sks skks data suggesting that crustal anisotropy is the main cause of shear wave splitting of the sks skks wave this also suggests that deformation in the upper mantle is either weak or predominantly vertical and is obviously different from the one in the crust a vertical flow in the upper mantle combined with the observation of a thin lithosphere beneath the study area leads to the inference that part of the mantle lithosphere may have been removed and is descending into deep mantle stations located in the surrounding areas on the other hand exhibit very little to no crustal anisotropy the estimated moho depth and vp vs ratio also show a distinct difference between the se tibetan plateau and the surrounding regions stations on the tibetan plateau have a vp vs ratio of 1 79 which is substantially higher than those measured at the yunnan guizhou yungui plateau 1 69 our observations here are consistent with the scenario that the se tibet has been built by lower crustal flow they also suggest that the mantle lithosphere beneath the margin may have been mechanically decoupled from the upper crust

a new seismic data set on the depth of the moho in the alps we present the results from receiver function analysis applied to a comprehensive data set in the eastern alps teleseismic events were recorded at 70 stations with an average deployment of 1 year the investigated area includes the eastern part of the eastern alps and their transition to the bohemian massif the pannonian domain and the southern alps the crustal structure at each station is examined with the zhu kanamori zk method which yields well resolved interface depths in laterally homogeneous media with limited layering the application of the zk technique is challenged because of the complex tectonic setting therefore we include additional constraints from recent active source seismic studies in particular the well known crustal p wave velocity and where available the vp vs ratio are kept fixed thus reducing the ambiguity in determining moho depths individual depth values vary strongly between adjacent stations showing that the employment of the zk technique in tectonically complex settings is limited we therefore avoid interpreting the results in detail but rather compare them to existing crustal models of the eastern alps we regard this receiver function study in the easternmost part of the alps as a documentation of a data set that has potential to be exploited in the future

estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data summary we developed an integrated technique for estimating crustal anisotropy with a horizontal axis using radial r and transverse t receiver functions the technique includes computing three individual and one joint objective function jof and a reliability analysis of the estimated anisotropy the individual objective functions iofs are designed to 1 maximize the peak energy of the stacked r receiver function after a cosine moveout correction in the ps arrival time 2 to maximize the correlation of the radial receiver functions after a full correction of anisotropy or 3 to minimize the total energy of transverse receiver functions stacked after a removal of crustal anisotropy the jof was computed by a weighted average of the three iofs and the reliability analysis uses the principle that stacking coherent signals can lead to an increase of signal to noise ratio we applied the technique to synthetic receiver functions generated with 30 60 per cent white noise from a variety of anisotropic and heterogeneous models the synthetic tests indicate that the proposed technique has good capability to recover the input models despite the presence of random and other coherent noises such as those caused by inhomogeneous structures in the data the technique can always provide accurate estimates of crustal anisotropy we applied the technique to two permanent seismic stations in western china and found significant crustal anisotropy beneath one station located at the northern edge of the tibetan plateau the observed fast polarization direction at this station follows the direction of the maximum horizontal tensile stress suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust the station situated in the sichuan basin on the other hand shows little to no seismic anisotropy which may suggest that the crust beneath the basin is nearly rigid with very little deformation the developed technique can be applied to any broadband seismic stations that have a good backazimuthal coverage of teleseismic events

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

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

distinct lateral variations of upper mantle anisotropy beneath eastern china revealed by shear wave splitting 1 a dense seismic array consisting of 28 temporary stations was deployed to study the crustal and upper mantle deformations beneath eastern china we measured the splitting parameters in the crust and mantle by analyzing p to s phases converted at the moho discontinuity called pms phases and the core mantle boundary i e core phases respectively the splitting parameters of core phases are retrieved at most stations while that of the pms phases are retrieved at only a few stations distinct lateral variations of the fast polarization directions analyzed with the core phases are found in different tectonic blocks in eastern china the delay times in the mantle and crust are moderately large 1 s and averagely smaller than 0 3 s respectively by the fresnel zone analysis the laterally variant lithospheric anisotropy is revealed between the two sub blocks southeast china orogenic belt and yangtze craton of the eastern south china block in contrast in the southeastern north china craton the anisotropy in a relatively deep layer contributes to the splitting observations

receiver functions of seismic waves in layered anisotropic media application to the estimate of seismic anisotropy abstract we investigate the effect of seismic anisotropy on p wave receiver functions calculating synthetic seismograms for p wave incidence on multilayered anisotropic structure with hexagonal symmetry the main characteristics of the receiver functions affected by the anisotropy are summarized as 1 appearance of seismic energy on radial and transverse receiver functions 2 systematic change of p to s ps converted waveforms on receiver functions as ray back azimuth increases and 3 reversal of the ps phase polarity on the radial receiver function in a range of the back azimuth another important influence is shear wave splitting of the ps converted waves and other later phases reverberated as s wave by numerical experiments using synthetic receiver functions we demonstrate that the waveform cross correlation analysis is applicable to splitting ps phases on receiver functions to estimate the seismic anisotropy of layer structure advantages to utilizing the ps phases are 1 they appear more clearly on receiver functions than on seismograms and 2 they inform us about what place along the seismic ray path is anisotropic real analysis of shear wave splitting is executed to the moho generated ps phases that are identified on receiver functions at six seismic stations in the chugoku district southwest japan the time lags between the two arrivals of the split ps phases are estimated at 0 2 0 7 sec and the polarization directions of the fast arrival components are from north south to northeast southwest this result is consistent with recent results of shear wave splitting measurements and the trend of linear epicenter distributions of crustal earthquakes and active fault strikes in the chugoku district

complicated crustal deformation beneath the ne margin of the tibetan plateau and its adjacent areas revealed by multi station receiver function gathering abstract we use a multi station stacking method to analyze receiver function data recorded by a large scale dense seismic array covering the northeastern ne margin of the tibetan plateau and its adjacent regions to estimate crustal thickness h vp vs ratio and crustal anisotropy fast polarization direction and splitting time for each station of the array we gathered all the receiver functions recorded by the station and its nearby stations located inside a circle with a radius of 0 5 we applied the h stacking technique and the joint inversion scheme to a total of 654 station clusters to measure the h and respectively the measured moho depth varies from a peak value of 67 km beneath the northern central plateau to 39 45 km beneath the surrounding blocks in the northeast the front of the depressed moho beneath the plateau margin exhibits a complicated geometry which suggests that when the tibetan plateau encroached upon the surrounding terranes the front expanded unevenly depending on the strength of the encountering terranes the thickened crust beneath the margin also has a very low vp vs ratio indicating that the crust is composed largely of felsic minerals more than one third of the station clusters 221 have a splitting time significantly larger than 0 2 s the average splitting times of the 221 measurements is 0 68 s which is comparable to those measured from the sks skks xks phases 0 94 s the fast directions estimated from the moho ps and xks phases recorded by the station clusters within the margin are very similar and are also approximately parallel to the strikes of major faults sutures and thrust fronts in the area the low vp vs ratio together with the observed fast polarization directions from the moho ps and xks phases suggests that shortening of the entire lithosphere orthogonal to the compressional direction is likely the main cause for the observed crustal thickening occurring at the margin we also found that the moho ps and xks recorded by stations inside the western hetao basin and the yinchuan basin present very different anisotropy which may suggest that the crust and mantle beneath the two basins have very different deformation patterns

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

seismic anisotropy in the lower mantle a comparison of waveform splitting of sks and skks 1 we have carried out a global investigation on seismic anisotropy in the lower mantle by comparing the waveform splitting of sks and skks found at the same seismogram the two shear waves have very similar ray paths in the upper mantle but different ones in the lower mantle we collected a total of 104 sks skks waveform data recorded at 76 stations the sks skks data are first matched with a single anisotropic model 2 parameters the fast polarization direction and delay time t and two independent anisotropic models 4 parameters we then applied an f test to examine whether the 4 parameter models are better than the 2 parameter ones based on error improvement we found that the data from most of the stations can be explained by the simple 2 parameter models while this observation provides compelling evidence that the vast part of the lower mantle below 1000 km including the d region is transversely isotropic it is still arguable that the uppermost lower mantle and the transition may partly contribute the sks skks splittings we also found that the 4 parameter models provide a better fitting to the sks and skks splitting at 8 stations suggesting the existence of transverse anisotropy or anomalous dipping structures in some part of the lower mantle

layered crustal anisotropy around the san andreas fault near parkfield california the rheology of the earth s crust controls the long term and short term strength and stability of plate boundary faults and depends on the architecture and physical properties of crustal materials in this paper we examine the seismic structure and anisotropy of the crust around the san andreas fault saf near parkfield california using teleseismic receiver functions these data indicate that the crust is characterized by spatially variable and strongly anisotropic upper and middle crustal layers with a moho at 35 km depth the upper layer is 5 10 km thick and is characterized by strong 30 anisotropy with a slow axis of hexagonal symmetry where the plane of fast velocity has a strike parallel to that of the saf and a dip of 40 we interpret this layer as pervasive fluid filled microcracks within the brittle deformation regime the 10 15 km thick midcrustal layer is also characterized by a weak axis of hexagonal symmetry with 20 anisotropy but the dip direction of the plane of fast velocity is reversed the midcrustal anisotropic layer is more prominent to the northeast of the san andreas fault we interpret the mid crustal anisotropic layer as fossilized fabric within fluid rich foliated mica schists when combined with various other geophysical observations our results suggest that fault creep behavior around parkfield is favored by intrinsically weak and overpressured crustal fabric

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

azimuthal shear wave velocity anisotropy in the basin and range province using moho ps converted phases an eight station large aperture seismometer array was deployed in the basin and range province of west central nevada for the 1988 1989 program for array seismic studies of the continental lithosphere passcal passive source seismic experiment during the 10 months of data collection a total of 100 teleseismic events were recorded 30 source equalized p wave receiver functions show waveform variations in timing amplitude and polarity in the moho ps phase that is diagnostic of shear wave velocity anisotropy information regarding the crustal anisotropic component can be recovered through the ps phase since it is generated at the moho and is sensitive to shear wave splitting in the crust only we utilize single event stacked and array beam formed receiver functions to minimize the influence of structural variations and scattering on the observed converted phases analysis of shear wave splitting of the ps conversion from the crust mantle boundary indicates a fast azimuth of anisotropy oriented approximately nw se with observed time delays of 0 20 s this strongly suggests coherent crustal anisotropy with a fast direction perpendicular to the strike of fault block mountain ranges this fabric is most likely due to the preferred crystallographic orientation of seismically anisotropic minerals in the middle to lower crust rather than the distribution of basin and range upper crustal fracture systems

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

crustal anisotropy and deformation of the southeastern margin of the tibetan plateau revealed by pms splitting abstract the crustal seismic anisotropy in the southeastern margin of the tibetan plateau was obtained by analyzing receiver functions of a newly deployed dense seismic array the measured splitting time of the pms phase varies from 0 02 to 0 88 s with an average value of approximately 0 28 s which is much larger than the splitting time in the upper crust top 15 km implying that the crustal anisotropy is mainly generated within the middle and lower crust below 15 km the fast polarization directions of pms are in good agreement with the directions of principle compression stress in the sub blocks and are parallel with the strikes of the faults near the xiaojiang fault the red river fault and the lancangjiang fault the splitting time shows a conspicuous contrast with a small value in the east of the xiaojiang fault zone and a large value in the west we assume that the differential movement in the ductile middle to lower crust caused the alignment of materials along the movement directions and developed the crustal anisotropy in the sub blocks near the major faults the low velocity zone high poisson s ratio and high heat flow in the crust provide evidence for the existence of partial melt which may be related to the strong crustal anisotropy in this area the crustal deformation near these faults is mainly caused by the strike slip motion resulting in the alignment of minerals and partial melts along the strikes of these faults analyzing the anisotropy pattern from sks and pms splitting we speculate that the crustal deformation is mechanically decoupled from the upper mantle in our study region

mantle flow beneath northwestern venezuela seismic evidence for a deep origin of the merida andes abstract we measured shear wave splitting from sks data recorded by the national seismic network of venezuela and a linear broadband passcal rice seismic array across the merida andes the linear array was installed in the second phase of the passive seismic component of the bolivar project to better understand the complicated regional tectonics in western venezuela using the method proposed by wolfe and silver 1998 sks waveforms from 2 to 36 earthquakes mostly from the tonga subduction zone were selected for each of the 23 stations in the region in order to do splitting analysis the fast polarization directions can be divided into 3 zones all in agreement with local gps data the first zone comprises the stations north of the dextral strike slip oca ancon fault these stations show the largest split times 1 6 3 2 s oriented in a roughly ew direction and are similar to splitting observations made further to the east along the strike slip plate boundary growdon et al 2009 we attribute this to trench parallel mantle flow that passes around the northwest corner of the subducting caribbean plate and along the northern edge of south america as proposed by russo and silver 1994 forming an eastward flow beneath the southern caribbean plate zone two is the merida andes with the right lateral bocono fault in the center where split orientations are at n45 e suggesting that the observed seismic anisotropy is likely caused by lithospheric deformation parallel to the bocono fault zone three is east of the bocono fault inside the barinas apure basin where the measured split times are smaller 0 8 s with an ew fast direction that is consistent with those observed at the guarico basin maturin basin and the guayana shield in the east and are interpreted as orientation with the motion of the continent

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

fine crustal and uppermost mantle s wave velocity structure beneath the tengchong volcanic area inferred from receiver function and surface wave dispersion constraints on magma chamber distribution the tengchong volcanic area is located in the southeastern margin of the collision zone between the indian and eurasian plates it is one of the youngest intraplate volcano groups in mainland china imaging the s wave velocity structure of the crustal and uppermost mantle beneath the tengchong volcanic area is an important means of improving our understanding of its volcanic activity and seismicity in this study we analyze teleseismic data from nine broadband seismic stations in the tengchong earthquake monitoring network we then image the crustal and uppermost mantle s wave velocity structure by joint analysis of receiver functions and surface wave dispersion the results reveal widely distributed low velocity zones we find four possible magma chambers in the upper to middle crust and one in the uppermost mantle the chamber in the uppermost mantle locates in the depth range from 55 to 70 km the four magma chambers in the crust occur at different depths ranging from the depth of 7 to 25 km in general they may be the heat sources for the high geothermal activity at the surface based on the fine crustal and uppermost mantle s wave velocity structure we propose a model for the distribution of the magma chambers

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

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

characteristics of deep crustal seismic anisotropy from a compilation of rock elasticity tensors and their expression in receiver functions rocks in the continental crust are long lived and have the potential to record a wide span of tectonic history in rock fabric mapping rock fabric in situ at depth requires the application of seismic methods below depths of microcrack closure seismic anisotropy presumably reflects the shape and crystallographic preferred orientations cpos influenced by deformation processes interpretation of seismic observables relevant for anisotropy requires assumptions on the symmetry and orientation of the bulk elastic tensor we compare commonly made assumptions against a compilation of 95 bulk elastic tensors from laboratory measurements including electron backscatter diffraction ebsd and ultrasound on crustal rocks the majority of samples developed fabric at pressures corresponding to mid lower crustal depths tensor symmetry is a function of mineral modal composition with mica rich samples trending towards hexagonal symmetry amphibole rich samples trending towards an increased orthorhombic symmetry component and quartz feldspar rich samples showing a larger component of lower symmetries 77 of samples have a best fit hexagonal tensor with slow axis symmetry as opposed to mantle deformation fabric that usually has fast axis symmetry the best fit hexagonal approximation for crustal tensors is not elliptical but deviates systematically from elliptical symmetry with increasing anisotropy an observation that affects the magnitude and orientation of anisotropy inferred from receiver function and surface wave observations we present empirical linear relationships between anisotropy and ellipticity for crustal rocks the maximum out of plane conversion amplitudes in receiver functions scale linearly with degree of anisotropy for non elliptical symmetry the elliptical assumption results in a bias of up to 1 4 times true anisotropy

structural features of the subducting slab beneath the kii peninsula central japan seismic evidence of slab segmentation dehydration and anisotropy 1 seismic evidence of lithospheric slab segmentation dehydration and anisotropic rocks beneath the kii peninsula central japan is detected with receiver function rf analyses by stacking radial and transverse rfs with moveout perturbations we estimate the downdip azimuth dip angle and depth of the slab moho beneath each seismic station comparing the downdip azimuth to the slab convergence azimuth we can classify the slab into three segments the segments correspond to both the fault zones of past megathrust earthquakes and to the spatial distribution of intraslab seismicity the slab moho lies just above the intraslab seismicity in the southwestern kii peninsula slab moho lies deeper at the eastern part of the peninsula at the bottom of seismicity this suggests that earthquakes occur only within the slab mantle under the southern kii peninsula the dip angle of slab moho becomes steeper at 40 to 50 km depth perhaps caused by dehydration transformation of basalt to eclogite at some stations located in the western part of the kii peninsula rf estimated dip angle of the interface is much steeper than that estimated by spatial trends in seismicity since the local seismicity distribution reflects well the larger scale geometry of the slab the rf estimated dip angle may imply anisotropic wave speeds within strongly sheared and or hydrated media around the slab moho these detailed features of the slab may be one key in distinguishing the source regions of megathrust earthquakes in the nankai region

mapping seismic anisotropy using harmonic decomposition of receiver functions an application to northern apennines italy 1 isotropic and anisotropic seismic structures across the northern apennines italy subduction zone are imaged using a new method for the analysis of teleseismic receiver functions rfs more than 13 000 p wave coda of teleseismic records from the 2003 2007 retreating trench extension and accretion tectonics retreat experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of mediterranean evolution a new methodology for the analysis of receiver functions is developed which combines both migration and harmonic decomposition of the receiver function data set the migration technique follows a classical common conversion point scheme and helps to focus on a crucial depth range 20 70 km where the mantle wedge develops harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of p to s converted phases the separation of the back azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures the application of our method to the retreat data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the northern apennines chain where previous studies failed to clearly retrieve the geometry of the subducted interfaces we present our results in closely spaced profiles across and along the northern apennines chain to highlight the convergence of the tyrrhenian and the adriatic microplates which differ in their crustal structure where the adriatic microplate subducts beneath tuscany and the tyrrhenian sea a signature of the dipping adriatic moho is clearly observed beneath the tyrrhenian moho in a large portion of the forearc region in the area where the two mohos overlap our new analysis reveals the presence of an anisotropic body above the subducted moho there is a strong ps converted phase with anisotropic characteristics from the top of the adriatic plate to a depth of at least 80 km because the ps conversion occurs much deeper than similar ps phases in cascadia and japan dehydration of oceanic crust seems unlikely as a causative factor rather the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting

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

shallow crustal radial anisotropy beneath the tehran basin of iran from seismic ambient noise tomography abstract we studied the shear wave velocity structure and radial anisotropy beneath the tehran basin by analyzing the rayleigh wave and love wave empirical green s functions obtained from cross correlation of seismic ambient noise approximately 199 inter station rayleigh and love wave empirical green s functions with sufficient signal to noise ratios extracted from 30 stations with various sensor types were used for phase velocity dispersion analysis of periods ranging from 1 to 7 s using an image transformation analysis technique dispersion curves extracted from the phase velocity maps were inverted based on non linear damped least squares inversion method to obtain a quasi 3d model of crustal shear wave velocities the data used in this study provide an unprecedented opportunity to resolve the spatial distribution of radial anisotropy within the uppermost crust beneath the tehran basin the quasi 3d shear wave velocity model obtained in this analysis delineates several distinct low and high velocity zones that are generally separated by geological boundaries high shear velocity zones are located primarily around the mountain ranges and extend to depths of 2 0 km while the low shear velocity zone is located near regions with sedimentary layers in the shallow subsurface our results indicate strong radial anisotropy with negative magnitude vsv vsh primarily associated with thick sedimentary deposits reflecting vertical alignment of cracks with increasing depth the magnitude of the radial anisotropy shifts from predominantly negative less than 10 to predominantly positive greater than 5 our results show a distinct change in radial anisotropy between the uppermost sedimentary layer and the bedrock

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust