upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

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

seismic azimuthal anisotropy beneath the pakistan himalayas teleseismic s sks and skks data collected from a temporary broadband array across the himalayan front in pakistan are analyzed for shear wave splitting parameters the sks and skks phases have ray paths originating from both the south pacific and colombia which have azimuths approximately 40 apart with respect to the pakistan array if significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths no evidence was seen for any shear wave splitting beneath any of the stations in the array teleseismic s waves were also used in order to provide better azimuthal coverage for the shear wave splitting measurements we were able to correct for any source side anisotropy when needed no receiver side splitting was observed in any of the s wave data the lack of shear wave splitting beneath the pakistan array indicates that there is no appreciable large scale azimuthal anisotropy beneath this part of the himalayas therefore if there is any significant strain in the upper mantle beneath this area it must either be vertically oriented or if horizontal vertically vary in such a way that the integrated effect on s wave splitting is null

upper mantle beneath foothills of the western himalaya subducted lithospheric slab or a keel of the indian shield summary the fate of the mantle lithosphere of the indian plate in the india eurasia collision zone is not well understood tomographic studies reveal high p velocity in the uppermost mantle to the south of the western himalaya and these high velocities are sometimes interpreted as an image of subducting indian lithosphere we suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near horizontal mantle keel of the indian shield in the south of the indian shield upper mantle velocities are anomalously low and relatively high velocities may signify a recovery of the normal shield structure in the north our analysis is based on the recordings of seismograph station nil in the foothills of the western himalaya the t component of the p receiver functions is weak relative to the q component which is indicative of a subhorizontally layered structure joint inversion of the p and s receiver functions favours high uppermost mantle velocities typical of the lithosphere of archean cratons the arrival of the ps converted phase from 410 km discontinuity at nil is 2 2 s earlier than in iasp91 global model this can be an effect of remnants of tethys subduction in the mantle transition zone and of high velocities in the keel of the indian shield joint inversion of sks particle motions and p receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60 at 80 160 km depths to 150 at 160 220 km the fast direction in the lower layer is parallel to the trend of the himalaya the change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the indian shield a similar boundary was found with similar techniques in central europe and the tien shan region but the base of the lithosphere in these regions is relatively shallow in agreement with the higher upper mantle temperatures the ongoing continental collision is expressed in crustal structure the crust beneath nil is very thick 58 2 km and the s velocity in the intermediate and lower crust is around 4 0 km s 1 this anomalously large velocity and thickness can be explained by scraping off the lower crust when the indian lithosphere underthrusts the himalaya

correlation of change in magnetotelluric parameters before and after sikkim himalayan earthquakes associated with stress variation abstract mt stations that were recorded before the moderate 5 3 mw 2006 sikkim himalaya earthquake had been reoccupied after 2007 mw 5 0 and 2011 mw 6 9 seismic activities we analyzed the broadband mt data 0 001 1000 s within the 60 km radius from epicenters location and within the focal depth range of 9 30 km the observed change in apparent resistivity in the period range of 0 001 10 s has shown a considerable post seismic decrease of the order 10 30 the 2d geoelectric model constructed with post seismic acquired data also reveals the feature of a decrease in resistivity c1 the relative change decrease of observed apparent resistivity obs after 2006 2007 and 2011 earthquakes find a correlation with combined stress drop 11 18 2 mpa values estimated by arms approach the correlation is further strengthened by a similar change in relative apparent resistivities 2d model derived from 2d model with stress drop integrated stress drop values attribute to the accumulated apparent stresses a 0 23 that reveals the area might be subjugated to 0 6 1 2 mpa stress before 2006 1 1 5 mpa stress before 2007 and 1 7 2 9 mpa before 2011 earthquakes occurrence development of micro cracks and generation of aqueous fluids is possible due to such stressed and slow rate of relentless thrust shear environment within the area of crust therefore resistivity could have been decreased with a fluid transition from compression to dilatancy zones at different depth levels before each earthquake and further decreased upon subsequent stress drops associated with strong ground motions we interpreted that maximum stress might be dropped in dilatational zones oblique to the maximum compression stress directions along n200e n s and nnw sse before 2006 2007 and 2011 earthquakes and resulted in a considerable decrease in apparent resistivity along e w te mode i e yx component

seismotectonic model of the sikkim himalaya constraint from microearthquake surveys the seismotectonic model in the sikkim himalaya does not fit well with the proposed steady state or evolutionary models the main boundary thrust mbt is seismogenic and is a mantle reaching fault the earthquakes are not confined to shallower depths 25 km above the plane of detachment as proposed in the models the seismic activity continues from surface to the lower crust 0 45 km to the north of mbt and earthquakes are produced by a thrust mechanism the earthquakes to the east of sikkim in the bhutan himalaya on the other hand are produced along a 200 km long northwest southeast trending lineament by transverse tectonics the seismogenic lineament cuts across the himalayan major thrusts and extends to the goalpara wedge in the southeast the earthquakes occur by strike slip mechanism in the midcrust at a depth range of 10 25 km along this long active lineament

evidence for upper crustal anisotropy in the songpan ganze northeastern tibet terrane 1 the radial and transverse receiver functions obtained at several stations located on the songpan ganze terrane tibetan plateau are characterized by strong p to s conversions in the first 4 s after the direct p wave we show that the amplitude variations of these phases with back azimuth can be well explained by a simple model with one or two anisotropic layers the anisotropy we obtained is quite strong about 15 and might be related to the presence of highly deformed flyschs and schists in the first 25 km of the crust this study shows that anisotropy in the crust could be locally high and has to be taken into account when analyzing teleseismic data

tomographic evidence for wholesale underthrusting of india beneath the entire tibetan plateau we analyzed a global tomographic model for the tibet himalayan collision zone which indicates that the indian lithospheric slab has been subducted subhorizontally beneath nearly the entire tibetan plateau to depths of 165 260 km tibetan velocity structure is low in its crust and high in its lithospheric mantle at depths between 75 and 120 km we interpret an asthenospheric layer positioned above the subducted indian slab at depths between 120 and 165 km beneath the tibetan plateau beneath the central portion of the plateau a lowvelocity anomaly exists from the crust down to 310 km depth indicating mantle upwelling through a weakened part of the subducted slab we present a model which explains that the uplift history and low relief of the tibetan plateau is a result of subhorizontal subduction and heating of indian lithosphere that is separated from tibetan lithosphere by a thin channel of asthenosphere two predictions made by our model are 1 the amount of shortening in the himalayas is equivalent to the amount of underthrusted indian mantle lithosphere and 2 a young mantle geochemical signature should be present along the entire southern portion of the tibetan plateau q 2004 elsevier ltd all rights reserved

seismicity and one dimensional velocity structure of the himalayan collision zone earthquakes in the crust and upper mantle 1 earthquakes beneath the himalayan collision zone occur at depths between near surface and around 100 km below sea level after relocating earthquakes with two one dimensional 1 d velocity models we found a clear bimodal depth distribution for earthquakes in the himalayas of eastern nepal and the southern tibetan plateau and evidence that some earthquakes originate at upper mantle depths seismicity in nepal shows an accumulation of earthquakes along the front of the himalayan arc with a seismic gap between longitudes 87 3 e and 87 7 e although upper crustal seismicity along the topographic front of the high himalaya is consistent with a region of high strain accumulation associated with convergence on the main himalayan thrust fault microearthquakes do not necessarily occur on this fault instead they concentrate in the hanging wall seismic activity in the sub himalaya and the terai plains is almost exclusively limited to the vicinity of the location of the magnitude 6 5 20 august 1988 udayapur earthquake with most of the earthquakes in the lower crust and the upper mantle clusters of earthquakes in the lesser and high himalayas and south tibet tethyan himalayas mark very well defined zones of seismicity at depths between 50 and 100 km confirming the presence of earthquakes in the upper mantle in the region of continental collision the occurrence of earthquakes at sub moho depths favors the idea that the continental upper mantle deforms by brittle processes

anisotropy of the indian continental lithospheric mantle summary due to the paucity of seismological data available in the public domain the structure of the indian lithosphere is still little known we investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the himalayan arc and underneath peninsular india using seismic anisotropy shear wave splitting measurements are performed on core refracted phases for each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast s wave phi assumed to be a proxy for the orientation of the a axis of olivine and the delay dt between the arrival time of the fast and slow s waves we present a very comprehensive data set recorded at 86 seismological stations deployed from the himalayas to the southern tip of the indian peninsula in a joint effort by the national geophysical research institute hyderabad india the university of cambridge and the indian institute of astrophysics the unprecedented data set we present sheds light on the mechanisms involved in the india eurasia continental collision in a region along the himalayan arc south of the indus tsangpo suture zone at the scale of the indian plate the majority of the stations show a nne ssw orientation of phi over hundreds of kilometres from sri lanka to the northern part of the dharwar craton this direction closely parallels the trend of the indian plate motion with respect to a fixed eurasian plate as defined through the nuvel1a plate model along the himalayan arc from ladakh in the northwest to bhutan and the shillong plateau in the east the orientation of phi rotates to become ew perpendicular to the plate motion as defined through nuvel1a unlike previous studies we do find strong evidence for seismic anisotropy south of the indus tsangpo suture zone a large number of null results have been computed with consistent orientation of the two fast polarization directions phi across the subcontinent we demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy from these results we infer the dominance beneath the indian lithosphere of the asthenospheric flow in aligning minerals in the sheared lithosphere asthenosphere boundary layer masking any compression induced anisotropy expected to be normal to this direction closer to the collision front in northern india the anisotropy may in part be due to the foliation planes of the himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting indian lithosphere acting as a barrier the continent wide consistency of results strengthens the understanding that the indian lithosphere has distinct anisotropic signatures contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of indo eurasian convergence

disparate behaviour of deformation patterns beneath northeast indian lithosphere inferred from shear wave splitting analysis abstract new shear wave splitting measurements are obtained from the north east indian region utilizing the core refracted pks sks skks phases suggesting the region as more complex and anisotropic in nature the splitting parameters namely time delays t and fast polarization directions are computed at nine stations and the results are found to be more consistent with the geologic tectonic structures and present mantle deformation patterns under this particular region the result streamlines the deformation patterns strictly into three categories significant anisotropy in the western fringe of shillong plateau and surrounding regions implies an asthenospheric flow related strain dominating the splitting direction inline the direction of absolute plate motion apm of the indian plate in a no net reference frame and discards the effects from lithospheric strain the stations in himalayan collision zone redefines an actual deformation pattern in ene wsw direction in line with the maximum shear plane construing the himalayan arc at the northeast corner as a consequence of n s indo eurasian collisional derived lithospheric strain the anisotropic effect at the himalayan foredeep section signifies the lithospheric strain induced e w deformation from n s continental collision mapping the anisotropic layer depth under these stations also strengthens our anisotropic observations and geodynamics understanding of this region with a similar thought of ideas where the deformation patterns are supposed to be dominated by huge forces like apm and lithospheric strains with negligible effects from the local geological structures

4 d evolution of se asia s mantle from geological reconstructions and seismic tomography abstract how the collision between india and asia is related to processes deeper in the mantle is unclear here we compare geological reconstructions of block motions within asia since 50 ma with the tomographically imaged three dimensional 3 d morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure past positions of the convergent margin show remarkable similarities with slab geometry at specific depths the striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision the slab contours match the progressive deformation of asia s margin including india s indentation and sundaland s extrusion ever since the onset of collision the indian plate appears to have overridden its own sinking mantle and it does not seem at present to underthrust tibet significantly north of the zangbo suture if correct this observation would provide further evidence against models of plateau build up involving indian lithosphere the tomographic images beneath india confirm that asian deformation has absorbed at least 1500 km of convergence since collision began from the match between the southeastward motion of sundaland between 40 and 20 ma and the principal change in slab structure between 700 and 1100 km depths we infer that lateral advection in the mantle is small and that the sinking rate beneath sunda was 2 cm yr in the lower mantle and 5 cm yr above the transition zone

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

a review of crust and upper mantle structure beneath the indian subcontinent this study has been supported by a grant from the ministry of earth sciences moes iitkgp ckh

correlated crust and mantle strain fields in tibet vertical to subvertical planes of shear within the active crustal deformation field in tibet align with fast directions of shear wave polarization this observation suggests that the present day velocity gradient tensor field within the crust of tibet correlates with the lithospheric mantle velocity gradient tensor field beneath tibet this inference requires the following 1 the 100 axes of olivine are aligned within established zones of lithospheric mantle fabric or anisotropy built up through finite strain 2 present day shear in the mantle lithosphere is occurring within these established zones of fabric parallel to the direction of olivine 100 axis alignment 3 the zones of mantle lithospheric fabric or zones of mantle shear are aligned with zones of crustal shear faults the correlation of crustal and mantle strain fields most simply results from the fact that both crust and mantle lithosphere are under the influence of similar velocity boundary conditions furthermore the observations confirm the distributed nature of lithospheric mantle deformation beneath east central tibet and suggest that left lateral shear has been the dominant component of finite shear there

seismic anisotropy beneath the indian continent from splitting of direct s waves 1 in this study we investigate the shear wave splitting beneath 39 broadband seismic stations located over various tectonic units of the indian shield using direct s waves from deep to intermediate depth earthquakes the delay times between fast and slow axes of anisotropy do not reveal anomalous values that reflect contributions from midmantle or the mantle transition zone the fast polarization azimuths are generally consistent with the results from sks splitting that indicate a predominance of plate motion related strain in the indian continent splitting results at station hyb which was hitherto considered isotropic indicate significant anisotropy with a ne oriented fast axis although a general trend of sh leading sv is indicated by most of the measurements invoking radial anisotropy to explain this phenomenon appears farfetched since the near vertical incidence of the s waves in the usable distance range of 40 80 makes them more sensitive to azimuthal anisotropy underneath the station forward modeling of the dependence of the splitting parameters on incoming polarization assuming horizontal axis of symmetry brings out two layers of anisotropy beneath the indian shield with fast axis azimuths oriented n15 e in the bottom layer and n60 e in the top layer while stronger anisotropy t 1 s in the bottom layer is related to asthenospheric flow a weaker one t 0 4 s in the upper layer could represent anisotropy frozen in the lithosphere

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

geodetic analysis of reservoir depletion at the geysers steam field in northern california reservoir depletion at the geysers from 1974 to 1977 is evident in measured changes in gravity surface strain and pore pressure drainage the drainage area increased about 20 the maximum gravity decrease was about 120 gal and the maximum elevation change was about 6 cm during this period since the net mass withdrawal is known it may be combined with the gravity change to estimate a drainage volume the maximum drainage volume is 25 km3 because the depth of the caprock and extent of the pore pressure drainage is known this volume limit implies that no significant mass withdrawal occurs below a depth of 4 km the ratios of surface elevation changes to horizontal contraction coinciding with the drainage area imply an equant drainage geometry assuming that fluid production produces negative dilatation using the same cylindrical geometry used to model the gravity negative dilatational strain rates of 4 to 5 10 5 yr were found to produce the observed surface displacements the likelihood of boiling in the system to produce steam combined with apparent large bulk moduli from seismic velocities and small pore pressure declines suggest that most of the reservoir contraction is due to cooling

seismic anisotropy beneath the southern himalayas tibet collision zone shear wave phases scs sks and skks collected from the project international deep seismic sounding of tibet and himalayas ii indepth ii array in the eastern himalayas and tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern himalayas tibet collision zone no evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the indus tsangpo suture the lack of shear wave splitting beneath the stations in the vicinity of suture and the tethyan himalayas indicates that there is no appreciable large scale mantle polarization seismic anisotropy beneath this part of the collision zone one explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened indian lower continental lithosphere north of the gandese belt the fast directions of the split shear waves are oriented in a northeast southwest direction approximately parallel to the trend of a major left lateral strike slip fault zone that bounds the gulu rift the coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric wide coherent deformation

volume strain within the geysers geothermal field during the 1970s and 1980s the geysers geothermal region was rapidly developed as a site of geothermal power production the likelihood that this could cause significant strain within the reservoir with corresponding surface displacements led to a series of deformation monitoring surveys in 1973 1975 1977 and 1980 the geysers region was surveyed using first order class i spirit leveling in 1994 1995 and 1996 many of the leveling control monuments were resurveyed using high precision global positioning system receivers the two survey methods are reconciled using the geoid96 geoid model the displacements are inverted to determine volume strain within the reservoir for the period 1980 1994 peak volume strains in excess of 5x10 sup 4 are imaged there is an excellent correlation between the observed changes in reservoir steam pressures and the imaged volume strain if reservoir pressure changes are inducing volume strain then the reservoir quasi static bulk modulus k must be 4 6x10 sup 9 pa however seismic velocities indicate a much stiffer reservoir with k 3 4x10 sup 10 pa this apparent discrepancy is shown to be consistent with predicted frequency dependence in k for fractured and water saturated rock inversion of surface deformation data therefore appears to be a powerful method for imaging pressure change withinmore the body of the reservoir correlation between induced seismicity at the geysers and volume strain is observed however earthquake distribution does not appear to have a simple relationship with volume strain rate c 1999 american geophysical union less

significant seismic anisotropy beneath southern tibet inferred from splitting of direct s waves abstract this study presents a total of 12008 shear wave splitting measurements obtained using the reference station technique applied to direct s waves from 106 earthquakes recorded at 143 seismic stations of the hi climb seismic network the results reveal significant anisotropy in regions of southern tibet where null or negligible anisotropy has been hitherto reported from sk k s measurements while the individual fast polarization direction fpd at each station are found to be consistent the splitting time delays tds exhibit deviations particularly at stations located south of the indus tsangpo suture zone the fast polarization directions fpds are oriented a ne sw to e w to the south of the indus tsangpo suture zone b ne sw to ene ssw between bangong nujiang suture zone and the indus tsangpo suture zone itsz and c e w to the extreme north of the profile the splitting time delays t vary between 0 45 and 1 3 s south of the itsz 0 8 s observed beneath himalaya reveals a complex deformation pattern in the region and can be best explained by the combined effects of deformation related to shear at the base of the lithosphere and subduction related flows with possible contributions from the crust additional measurements obtained using direct s waves provide new constraints in regions with complex anisotropy

upper mantle anisotropy beneath northeast india asia collision zone from shear wave splitting analysis teleseismic earthquake data recorded by 11 broadband digital seismic stations deployed in the india asia collision zone in the eastern extremity of the himalayan orogen tidding suture are analyzed to investigate the seismic anisotropy in the upper mantle shear wave splitting parameters and t derived from the analysis of core refracted sks phases provide first hand information about seismic anisotropy and deformation in the upper mantle beneath the region the analysis shows considerable strength of anisotropy delay time 0 85 1 9 s with average ene wsw oriented fast polarization direction fpd at most of the stations the fpd observed at stations close to the tidding suture aligns parallel to the strike of local geological faults and orthogonal to absolute plate motion direction of the indian plate the average trend of fpd at each station indicates that the anisotropy is primarily originated by lithospheric deformation due to india asia collision the splitting data analyzed at closely spaced stations suggest a shallow source of anisotropy originated in the crust and upper mantle the observed delay times indicate that the primary source of anisotropy is located in the upper mantle the shear wave splitting analysis in the eastern himalayan syntaxis ehs and surrounding regions suggests complex strain partitioning in the mantle which is accountable for evolution of the ehs and complicated syntaxial tectonics

induced seismicity in the geysers geothermal area california a simultaneous inversion for hypocenters velocities and station delays was performed from the p arrival times of 55 microearthquakes and four explosions distributed throughout a 90 by 100 km region centered at the geysers geothermal area in northern california the resulting one dimensional layered model shows velocity increasing with depth from 4 4 km s at the surface to 5 9 km s at 8 km and deeper we relocated 7215 earthquakes which occurred between may 1975 and february 1982 using the new velocity model these relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies we infer from the spatial and temporal pattern of seismicity that the seismicity at the geysers is induced by geothermal production activities with the expansion of power production by approximately 70 in 1979 1980 seismic activity has developed near the new production areas seismicity has also expanded northwestward of the present production area although there are no active steam wells in this area statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated for the wells studied we find no consistent pattern of correlation between injection and seismicity likewise we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years two mechanisms remain plausible for inducing seismicity at the geysers volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field alternatively aseismic deformation due to regional tectonic stress may be converted to stick slip seismic deformation due to an increase of coefficient of friction along fault surfaces such as could occur from exsolution of dissolved silica into fractures neither mechanism would be likely to show strong correlation between production and seismicity rates however we expect seismicity to continue in the geysers and for seismicity to appear in areas where new production occurs

seismic structure of the underthrusting indian crust in sikkim himalaya 1 this study presents the first results of the seismic character of the underthrusting indian crust in the sikkim himalaya deduced through an analysis of 3600 receiver functions rfs abstracted from waveforms registered at 11 broadband stations spanning a 110 km long n s profile from the foothills to the higher himalaya common conversion point stacks of receiver functions prominently trace the northward dipping geometry of the indian moho beneath the himalaya monte carlo inversion of the azimuthal variations of the rfs at individual stations adopting the nearest neighborhood algorithm approach reveals that the crustal thickness varies from 40 km to 61 km from south to north with a dip varying between 4 and 10 among stations a moho doublet prominently seen at a depth of 40 km in the higher himalaya to the north of main boundary thrust has been interpreted in terms of possible partial eclogitization of a granulitic indian lower crust akin to the finding just north of the study region beneath southern tibet a strong layer of anisotropy 17 localized within a low velocity layer between 20 and 30 km has a nw se oriented fast polarization direction counterintuitive to the convergence parallel and range perpendicular alignment expected in a convergent setting due to shear processes midcrustal transcurrent deformation in sikkim and bhutan evidenced by a conjugate system of strike slip faulting with nw to ne trending p axis orientations is the most feasible mechanism for causing a near strike parallel oriented fast axis of anisotropy in this segment of himalaya

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

on the use of satellite altimetry to infer the earthquake rupture characteristics application to the 2004 sumatra event summary standard data and methods such as the inversion of seismic and gps data have been used extensively to infer the details of the 2004 december 26 earthquake the unprecedented large size of this event gave the opportunity to modern altimeters to provide the first clear records of a tsunami in deep ocean therefore allowing us to study the rupture history from an independent perspective we invert the jason 1 and topex poseidon altimetry records considering the new constraints available on the geometry of the fault plane and taking them into account in a 3 d rupture model the data are corrected for the non negligible effect of satellite motion during measurements our results show that the rupture propagated over the 1500 km of subduction zone initially identified by the aftershock distribution with a magnitude of mw 9 1 our solution compares well with the latitudinal distribution of slip inferred from other data sets with a maximum of energy release north of sumatra and two other slip patches near the nicobar and andaman islands based on waveform comparison we assert that the shallow portion of the megathrust offshore banda aceh had slip amplitudes of more than 20 m also we find that significant amounts of slip about 10 m concentrated below the andaman islands and did not propagate on the shallow portion of the interface although synthetic tests tend to show less resolution in the northern part of the rupture this solution is compatible with the near field data gps coral heads and imagery and would allow one to explain the apparent paradox between the large local displacements and the moderate tsunami observed locally finally we demonstrate the rapidly dominating effect of propagation and slip distribution over the rupture velocity and how it precludes the direct estimate of this latter parameter

shear wave splitting at central tien shan evidence for rapid variation of anisotropic patterns at active collisional belts the fast polarization axis of shear wave splitting is generally aligned parallel to the strike of the belt which has been proposed to indicate mantle strain that is coherent with crustal deformation a notable exception is central tien shan where anomalous patterns of splitting have previously been observed we here analyze shear wave splitting of sks phases across north central tien shan using digital data from the kyrgyzstan broadband seismic network knet we find a pattern of short wavelength anisotropic heterogeneity that supports complex mantle flow due to small scale convection the along strike variations in mantle structure contrast with the coherent pattern of crustal shortening and indicate that mantle flow is not directly coupled to crustal deformation in this region

shear wave anisotropy of the northeast indian lithosphere 1 analysis of sks skks phases in the northeast india region provides the first results using digital data from the southern side of the india asia collision zone our analysis reveals detectable anisotropy in the study region contrary to negligible anisotropy reported in southern tibet the direction of anisotropy is e w within the himalaya and its foredeep n s in the indo burma convergence zone and ne sw close to the shillong plateau while lithospheric deformation due to finite strain induced by collision seems to be the source of anisotropy in the himalaya and its foredeep an asthenospheric source is invoked as a causative for the mantle strain in the plate interior

seismic waves converted from velocity gradient anomalies in the earth s upper mantle summary modelling of elastic wave propagation in 1 d structures is frequently performed using reflectivity techniques in which the earth s velocity profile is approximated by stacks of homogeneous layers the complete reflection transmission r t response of a zone with arbitrary 1 d depth variation including both gradients and discontinuities in material properties can however be calculated using invariant embedding techniques results from earlier studies are here extended to derive exact expressions for r t matrices in arbitrary 1 d anisotropic media using a form of born approxi mation valid for thin scatterers and which does not assume small perturbations in material properties the r t matrices are solutions to a system of non linear ordinary differential equations of ricatti type and may be manipulated using standard r t matrix algebra in an equivalent description the wavefield within the heterogeneous zone is considered in terms of depth dependent contributions from up and downgoing waves propagating within the embedding reference medium this leads to efficient calculation of the internal wavefield using r t matrices of the heterogeneous stratification and portions thereof at minor additional expense mode conversion of teleseismic p and s phases from velocity gradients is examined by way of examples and comparison with three component data from broad band stations of the yellowknife seismic array the frequency dependence of such wave interactions depends on the differences in vertical slowness between incident and scattered modes it is shown that significant energy is converted from transition zones with extent l p 2 a broader interval than will generally produce intramode reflections a layer structure identified from ps conversions near 75 km depth below the slave craton is shown to be compatible with a 10 km thick gradient zone in which anisotropy increases from ambient levels to vp 5 per cent vs 2 5 per cent at a discontinuous upper boundary this characterization supports a previous interpretation as the upper strata of a former oceanic plate juxtaposed against overriding lithosphere during an ancient episode of shallow subduction

towards 3 d fabric in the continental lithosphere and asthenosphere the tien shan 1 as shown by previous observations of shear wave splitting of the sks phase the fast direction of azimuthal anisotropy beneath the tien shan tends to be aligned with the strike of the belt this direction implies that the anisotropy is related to the current orogenesis now we describe an attempt to constrain the distribution of the anisotropy with depth by combining observations of sks and p receiver functions the data are inverted for the parameters of anisotropy with the aid of a genetic algorithm the results of the joint inversion suggest that the actual distribution of anisotropy with depth can be more complicated than usually thought the deformed mantle layer can be a few hundred kilometres thick in the most reliable models fast direction of anisotropy in the shallow mantle is significantly different from that inferred from the observations of sks at some stations the data require anisotropy in the crust

evidence for plate motion related strain in the indian shield from shear wave splitting measurements 1 the predominantly precambrian indian lithospheric plate experienced a long history of deformation ever since its breakup from gondwanaland and its passage over four hot spots the nature of mantle deformation of this fast moving plate has hitherto remained elusive in view of the paucity of measurements of shear wave anisotropy in this paper we present the first results of 182 measurements of azimuthal ansiotropy obtained by analyzing the sks and skks waveforms from 85 earthquakes recorded at 35 broadband seismic stations located on various geological units of the indian shield results show that the delay times t between the fast and slow axes of anisotropy are close to 1s which is the global average for continental shield regions the fast axis azimuths do not reveal coherent patterns that suggest dominance of asthenospheric flow where a basally flat indian lithosphere is dragged over an inactive sublithospheric mantle the northern part of the indian shield where the lithosphere is thick shows a strong correlation between plate motion and azimuths of fast polarization due to the effect of the basal cratonic keel plowing through the sublithospheric mantle most of the fast axes azimuths can be explained by the absolute plate motion apm related strain considering the basal topography of the lithosphere the consistent ns orientations along the west coast may be due to edge flow associated with a transition from a thicker to a thinner lithosphere the southern granulite terrain and the cuddapah basin shows evidences for fossilized anisotropy particularly in regions close to continental shear zones

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

improved stability of a deeply anchored geodetic monument for deformation monitoring a test of two different monument designs used in geodetic networks shows that monuments installed to depths of 5 to 10 meters and laterally braced exhibit less environmentally caused displacement than those installed to 2 meters depth at parkfield california we have been monitoring the lengths of 17 baselines over the past decade with a 2 to 3 day interval between measurements using a two color geodimeter with a nominal precision of 0 5 mm over 5 km long baselines significant variations are observed on many of these baselines which appear to be related to the seasonally occurring rainfall with the larger variations approaching 10 mm over the past decade to test whether we could improve upon the measurements on some of the more susceptible lines at two sites we installed new monuments within about 30 meters of the original monuments after 1 5 years of measurements it is evident that the new monuments significantly attenuate the seasonal displacements to less than 1 mm the use of deeply anchored monuments should improve the ability of fault scale geodetic monitoring arrays to detect small tectonic displacements

crustal structure across sikkim ne himalaya from new gravity and magnetic data abstract the new gravity and magnetic data recorded along a profile in the sikkim ne himalaya are combined with the existing data from tibet bangladesh and india to delineate the crustal structure in this part of himalaya modelling of gravity data constrained from seismic results suggests that long wavelength gravity anomalies arise due to variations in the depth of moho 36 to 74 km which are caused by flexed lithosphere of effective elastic thickness of 50 10 km simultaneous modelling of magnetic anomalies and short wave length gravity anomalies reveals that a the magnetic anomalies observed over the lesser himalaya and the higher himalaya crystalline rocks might be caused by remnant magnetisation with inclination i 18 8 and declination d 147 10 which is in conformity with the palaeomagnetic results these magnetic parameters correspond to 35 10 ma age of magnetic direction and suggest that the rocks might have acquired magnetisation during cooling period of metamorphism b low grade meta sediments of the lesser himalaya extend up to 12 km depth and thins on either sides forming a bowl shaped geometry and c relative gravity high in the bengal basin might be caused by intrusion of the rajmahal volcanics modelling has also provided constraint on the geometry of the north dipping thrusts

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

upper and lower mantle anisotropy inferred from comprehensive sks and skks splitting measurements from india abstract in this study we investigate the upper mantle anisotropy beneath india using high quality sks and skks waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations in addition we present evidence for anisotropy in the d layer beneath southeast asia using sks and skks splitting discrepancies on the same seismogram during this exercise we obtain 200 new splitting measurements from 35 stations recently deployed in the indo gangetic plains igp central india and northeast india while the delay times between the fast and slow axes of anisotropy t range from 0 3 to 1 7 s the fast polarization azimuths at a majority of stations in the igp and central india coincide with the absolute plate motion of india implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy however stations in ne india reveal fast polarization azimuths mainly in the ene wsw direction suggestive of lithospheric strain induced by the ongoing indo eurasian collision our analysis for d anisotropy yielded a total of 100 sks skks pairs which can be categorized into those exhibiting i null measurements for one phase and significant splitting for the other phase ii null measurement for both the phases iii significant splitting for both the phases a pair is considered to be anomalous if the splitting difference between sks and skks is 0 5 s and the individual split time is 0 5 s using this criterion we obtain 12 measurements under category iii and 9 under category i that show a null measurement for sks and large splitting for the skks phase further we quantify the strength of the lower mantle anisotropy by correcting the skks measurement for the upper mantle anisotropy obtained by the sks phase on the same seismogram the sks delay times are found to be consistently less than skks times suggesting that the sks phases do not capture the lower mantle anisotropy in comparison to their skks counterparts seven coherent measurements thus obtained reveal measurable d anisotropy with fast polarization azimuths oriented mainly in the ene wsw direction these results suggest presence of a large region of deformation in the lowermost mantle beneath southeast asia a possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary cmb and is experiencing large shear deformation resulting in lattice preferred orientation lpo of the lower mantle van der hilst and karason 1999 long 2009 the other possibility is the phase transformation from mgsio3 perovskite to a more stable post perovskite phase under favorable conditions which results in lpo of the lower mantle

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

depth localized azimuthal anisotropy from sks and p receiver functions the tien shan summary shear wave splitting in the seismic sks phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations fast s wave polarization direction in collisional belts is often parallel to the trend of the belt although deformations of the mantle lithosphere in low angle thrusts would lead to the fast polarization direction normal to the trend of the belt these considerations suggested that the upper mantle in collisional belts is decoupled from the crust however sks technique is notable by a poor depth resolution and usually it assumes that the fast polarization direction is the same at any depth which is hard to justify here to investigate depth dependent azimuthal anisotropy in the mantle we invert jointly p receiver functions and sks particle motions at a number of seismograph stations the technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium a search for the optimum models is conducted with a technique similar to simulated annealing testing with synthetics demonstrates that this approach is robust the results for 10 seismograph stations in the tien shan the world s most active intracontinental collisional belt in central asia reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km in the mantle lithosphere at depths less than 100 km anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range this layer is not necessarily decoupled from the crust its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past in the lower layer asthenosphere the average azimuth of fast wave polarization is close to the trend of the belt whereas magnitude of s wave anisotropy is stable and large between 5 and 6 per cent this anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt at some stations the data require anisotropy in the crust there is no evidence for anisotropy at depths exceeding 150 250 km

imprints of diverse mantle deformational episodes in the cauvery suture zone south india abstract this study attempts to understand the upper mantle deformation pattern beneath the northern part of southern granulite terrain sgt located to the south of archean dharwar craton in south india the study region comprises the tectonic blocks nilgiri biligiri rangan shevaroy and madras blocks separated from the northern part of madurai block by the cauvery suture zone csz associated with subduction accretion collision tectonics shear wave splitting observed on high quality xk k s sks skks pks waveforms from 119 teleseismic events recorded by 19 broadband seismic stations mostly within the csz and the northern part of madurai block is utilized in this study the 122 splitting measurements obtained show an average delay time of 0 8 s between the fast and slow waves strong lateral variations in the fast polarization azimuths fpa are observed with the splitting parameters at the majority of stations showing a dependence on back azimuth importantly excluding the madras block a poor correlation between the fpa and the present day absolute plate motion apm direction is observed this dominance of non apm strain reflects complex anisotropy preserved in the lithosphere by plotting the fast polarization trends based on the ray piercing points at a depth of 150 km three distinct splitting patterns associated with the i biligiri rangan block and the eastern part of the nilgiri block ii eastern part of the csz and the northern madurai block nmb and iii madras block and its vicinity within the csz are observed the strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions two episodes of ocean closure and the collision that followed plate motion related strain or a combination of these similar deformation patterns in the northern part of madurai block might be related to remnants of subducted oceanic lithosphere

shear wave splitting and small scale convection in the continental upper mantle the deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt but this deformation is difficult to observe seismic azimuthal anisotropy manifested by shear wave splitting provides the best geophysical evidence of deformation in the upper mantle here we use observations of shear wave splitting to investigate deformation in the upper mantle beneath the tien shan in central asia we find that underneath most of the tien shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt but it deviates by nearly 90 where the upper mantle is anomalously hot the former relationship previously observed in other collisional belts seems to be usual for this environment the anomalous directions however provide evidence of a rising plume beneath the range suggesting more generally that the along strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection

p wave velocity structure below india and tibet incorporating anisotropic delay time effects we incorporate the effects of anisotropy to refine the continental scale 3 d isotropic velocity model previously produced for india and tibet by inverting 52 050 teleseismic p wave residuals we have exploited a total of 1648 individual sks splitting parameters to calculate the p wave travel time corrections due to azimuthal anisotropy our results suggest that anisotropy affects the p wave delays significantly 0 3 to 0 5 s integration of these corrections into the 3 d modeling is achieved in two ways a a priori adjustment to the delay time vector and b inverting only for anisotropic delays by introducing strong damping above 80 km and below 360 km depths and then subtracting the obtained anisotropic artifact image from the isotropic image to get the corrected image under the assumption of azimuthal anisotropy resulting from lattice preferred orientation lpo alignment due to horizontal flow the bias in isotropic p wave tomographic images is clear the anisotropy corrected velocity perturbations are in the range of 1 2 at depths of around 150 km and reduced further at deeper levels although the bias due to anisotropy does not affect the gross features it does introduce certain artifacts at deeper levels

correlation between seismic anisotropy and bouguer gravity anomalies in tibet and its implications for lithospheric structures summary as a baseline measurement for understanding the himalayan tibetan orogen a product of continent continent collision between india and eurasia we analyse digital seismic data in order to constrain the seismic anisotropy of the indian shield based on spatially sparse data that are currently available in the public domain there is little shear wave birefringence for sks phases under the indian shield even though it is part of a fast moving plate in the hotspot frame of reference if most of the northern indian mantle has little transverse anisotropy the onset of significant anisotropy under tibet marks the northern terminus of intact indian lithosphere that is thrusting under the himalayan tibetan orogen beyond this terminus tectonic fabric such as that associated with the deforming lithospheric mantle of eurasia must be present in the upper mantle along the profile from yadong to golmud the only profile in tibet where a number of shear wave birefringence data are available the amount of birefringence shows two marked increases near 30 and 33 n between which a local high in bouguer gravity anomaly is observed such a correlation between patterns of shear wave birefringence and gravity anomalies is explained by the juxtaposition of indian lithosphere against the overlying eurasian lithosphere while the eurasian lithospheric mantle appears only to the north of 30 n the indian lithospheric mantle extends northwards to near 33 n

subsidence derived volumetric strain models for mapping extensional fissures and constraining rock mechanical properties in the san joaquin valley california large scale subsidence due to aquifer overdraft is an ongoing hazard in the san joaquin valley subsidence continues to cause damage to infrastructure and increases the risk of extensional fissures here we use insar derived vertical land motion vlm to model the volumetric strain rate due to groundwater storage change during the 2007 2010 drought in the san joaquin valley central valley california we then use this volumetric strain rate model to calculate surface tensile stress in order to predict regions that are at the highest risk for hazardous tensile surface fissures we find a maximum volumetric strain rate of 232 microstrain yr at a depth of 0 to 200 m in tulare and kings county california the highest risk of tensile fissure development occurs at the periphery of the largest subsiding zones particularly in tulare county and merced county finally we assume that subsidence is likely due to aquifer pressure change which is calculated using groundwater level changes observed at 300 wells during this drought we combine pressure data from selected wells with our volumetric strain maps to estimate the quasi static bulk modulus k a poroelastic parameter applicable when pressure change within the aquifer is inducing volumetric strain this parameter is reflective of a slow deformation process and is one to two orders of magnitude lower than typical values for the bulk modulus found using seismic velocity data the results of this study highlight the importance of large scale high resolution vlm measurements in evaluating aquifer system dynamics hazards associated with overdraft and in estimating important poroelastic parameters

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

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

complex multilayered azimuthal anisotropy beneath tibet evidence for co existing channel flow and pure shear crustal thickening of the two debated end member models for the late cenozoic thickening of tibetan crust one invokes channel flow rapid viscous flow of the mid lower crust driven by topography induced pressure gradients and transporting crustal rocks eastward and the other pure shear faulting and folding in the upper crust with viscous shortening in the mid lower crust deep crustal deformation implied by each model is different and would produce different anisotropic rock fabric observations of seismic anisotropy can thus offer a discriminant we use broadband phase velocity curves each a robust average of tens to hundreds of measurements to determine azimuthal anisotropy in the entire lithosphere asthenosphere depth range and constrain its amplitude inversions of the differential dispersion from path pairs region average inversions and phase velocity tomography yield mutually consistent results defining two highly anisotropic layers with different fast propagation directions within each the middle crust and the asthenosphere in the asthenosphere beneath central and eastern tibet anisotropy is 2 4 per cent and has a nne ssw fast propagation azimuth indicating flow probably driven by the nne ward shallow angle subduction of india the distribution and complexity of published shear wave splitting measurements can be accounted for by the different anisotropy in the mid lower crust and asthenosphere the estimated splitting times that would be accumulated in the crust alone are 0 25 0 8 s in the upper mantle 0 5 1 2 s depending on location in the middle crust 20 45 km depth beneath southern and central tibet azimuthal anisotropy is 3 5 and 4 6 per cent respectively and its e w fast propagation directions are parallel to the current extension at the surface the rate of the extension is relatively low however whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals implying large finite strain and consistent with high rate horizontal flow together radial and azimuthal anisotropy suggest eastward mid crustal channel flow in central tibet along the regional topography gradient in ne high tibet mid crustal azimuthal anisotropy is 4 8 per cent and has wnw ese and nw se fast propagation directions parallel to the net extension at the surface these fast directions are inconsistent with channel flow following the sw ne regional topography gradient instead they suggest similar net deformation in the decoupled shallow and deep crust in the brittle upper crust it is accommodated by strike slip faulting in the ductile mid lower crust by shear oriented at 45 to the faults although mid crustal flow beneath ne tibet may transport some material towards the plateau periphery at a low region average rate the dominant mid crust deformation pattern is shear parallel to the plateau boundary this implies that channel flow from central tibet is not the main cause of the on going crustal thickening farther northeast

estimation of depth dependent fault slip from measured surface deformation with application to the 1906 san francisco earthquake we estimate the depth distribution of slip in the 1906 san francisco earthquake by applying new inversion methods to triangulation data collected near point arena california modeling crustal deformation from antiplane slip in a half space we define three regularizing functionals to produce minimum norm inverse problems these functionals measure components of the coseismic change in elastic strain energy stress magnitude and stress variability and are all quadratic in slip orthogonality conditions define finite dimensional representations of slip estimates coefficients of basis functions in these representations are estimated by damped least squares with damping parameter and faulting depth chosen by cross validation the best resulting estimates exhibit right lateral slip to depths of 15 to 20 km with about 6 m of surface slip this is consistent with directly observed offsets of cultural landmarks at the surface and indicates deeper faulting than has been previously inferred for the 1906 earthquake

long wavelength satellite gravity and geoid anomalies over himalaya and tibet lithospheric structures and seismotectonics of deep focus earthquakes of hindu kush pamir and burmese arc abstract free air anomaly map of himalaya tibet and adjoining regions derived from global models including satellite observations reflect most of the tectonic elements of this region fold and thrust belts and suture zones are characterized by small wave length free air high anomalies representing shallow high density mafic ultramafic rocks including ophiolitic melange on the other hand medium to large wavelength negative gravity anomalies which are related to crustal thickening due to isostatic compensation are better visualized in the bouguer anomaly map this emphasizes the importance of both free air and bouguer anomaly maps of complex orogenic belts spectral analysis of the bouguer and the geoid anomaly maps and their low pass filtered maps and modeling along a n s profile 90 e constrained from various seismic studies suggest low density rocks 3250 kg m3 in the lithospheric mantle between depths of 130 320 km under tibet the low pass filtered geoid anomaly shows a prominent geoid low in the same depth range oriented ne sw in the direction of motion of the indian plate that is near perpendicular to the prominent tectonics features it coincides with the ne sw oriented seismic anisotropy in the central part of the himalayan collision zone low density rocks in the lithospheric mantle suggest under thrusted indian and asian lithospheres in this section that constitutes the thick lithosphere under tibet the moho in general lies at a depth of 50 75 km under tibet with deepest being under south tibet the low density rocks in the upper mantle make this region buoyant that explains large scale uplifts the prominent negative free air bouguer and geoid anomalies are observed over the hindu kush pamir seismic zone between the kohistan arc towards the south and pamir towards the north suggesting predominance of low density rocks approximately at a depth of 120 km in the lithospheric mantle gravity anomalies and its modeling along a profile 73 e constrained from the hypocenters of deep focus earthquakes in this region and results from receiver function analysis in adjoining regions suggest following characteristics of the subduction model in this section i the subducting plates from two sides viz the indian kohistan arc and the asian plates show relatively high angle subductions compared to the east of it ii the subducting plates are affected by the strike slip chamman panjshir faults towards the west and karakoram strike slip fault towards the east iii the subducting plates from the two sides interact with each other in the lithospheric mantle iv the subducted rocks in this section are generally comprised of high density mafic ultramafic rocks of the kohistan arc and tien shan combination of these factors would induce fast subduction and slab break off that usually facilitate deep focus earthquakes as observed in case of mariana type subductions similar is the case along the burmese arc that is the only other section characterized by deep focus earthquakes in the long himalayan collision zone 5000 km modeling of the bouguer anomaly along 23 5 n in this case also shows relatively high angle subduction of the indian plate which is affected by the sagaing strike slip fault towards the east that would facilitate slab break off and generation of deep focus earthquakes as described above gravity modeling across tibet hindu kush and burmese arc suggest the presence of high density rocks 3100 kg m3 in the lower crust and low density rocks 3250 kg m3 in the upper mantle that may be attributed to partial eclogitization and serpentinization of rocks respectively in these sections that are typical of several orogenic belts such conditions are expected to facilitate delamination and detachment

significant seismic anisotropy beneath the southern lhasa terrane tibetan plateau 1 shear wave splitting measurements using teleseismic pks skks and sks phases recorded by station lsa on the southern part of the lhasa terrane of the tibetan plateau reveal significant azimuthal anisotropy with a splitting time of up to 1 5 s a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy in addition systematic variations of the splitting parameters fast polarization direction and splitting time with the arriving azimuth of the seismic ray path are observed suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry the measurements are consistent with a model with two layers of anisotropy the top layer has a ne sw fast direction which is consistent with gps revealed direction of surface movement and can be associated with lattice preferred orientation of middle lower crustal minerals such as amphibole the lower layer has a nearly e w fast direction and can be the consequence of either the n s directed compressional stress originated from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia this study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy

mantle deformation in sikkim and adjoining himalaya evidences for a complex flow pattern abstract new measurements of azimuthal anisotropy within the himalayan collision zone and southern tibet obtained by applying the sks splitting technique to waveforms from 31 broadband seismic stations reveal significant variations in the strength and orientation of the fast axes stations in the sikkim himalaya show strong anisotropy south of main central thrust mct with delay times 1 s the fast polarization directions are nearly parallel to the strike of the himalaya mountain chain and vary consistently along the profile from 105 in the south to 130 in the north however in the adjacent nepal himalaya a predominance of plate motion related strain is observed at least up to the mct the western portion of southern tibet south of the indus tsangpo suture zone reveals characteristics of e w oriented anisotropy contrary to null anisotropy found in the eastern part the complex flow pattern revealed in the present study may be due a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression

upper mantle dynamics of bangladesh by splitting analysis of core mantle refracted sks pks and skks phases abstract new shear wave splitting measurements are obtained from the bengal basin using core mantle refracted sks pks and skks phases the splitting parameters namely time delays t and fast polarization directions were estimated through analysis of 54 high quality waveforms 2 5 signal to noise ratio from 30 earthquakes with magnitude 5 5 recorded at ten seismic stations deployed over bangladesh no evidence of splitting was found which indicates azimuthal isotropy beneath the region these null measurements can be explained by either vertically dipping anisotropic fast axes or by the presence of multiple horizontal anisotropic layers with different fast polarization directions where the combined effect results in a null characterization the anisotropic fabric preserved from rifting episodes of antarctica and india subduction related dynamics of the indo burmese convergence zone and northward movement of the indian plate creating shear at the base of the lithosphere can explain the observed null measurements the combined effect of all these most likely results in a strong vertical anisotropic heterogeneity creating the observed null results

southern california permanent gps geodetic array error analysis of daily position estimates and site velocities we analyze time series of daily positions estimated from data collected by 10 continuously monitoring global positioning system gps sites in southern california during the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes each time series exhibits a linear tectonic signal and significant colored noise spectral power at frequencies in the range 5 yr 1 to 0 5 d 1 is dominated by white noise or possibly fractal white noise and is several orders of magnitude higher than what would be expected from random walk noise in this short period range attributed by others to geodetic monument motions estimating a single slope for the time series power spectra suggests fractal white noise processes with spectral indices of about 0 4 site velocity uncertainties assuming this fractal white noise model are 2 4 times larger than uncertainties obtained assuming a purely white noise model a combination white noise plus flicker noise spectral index of 1 model also fits the data and suggests that the velocity uncertainties should be 3 6 times larger than for the white noise model we cannot adequately distinguish between these two noise models nor can we rule out the possibility of a random walk signal at the lowest frequencies these questions await the analysis of longer time series in any case reducing the magnitude of low frequency colored noise is critical and appears to be best accomplished by building sites with deeply anchored and braced monuments otherwise rate uncertainties estimated from continuous gps measurements may not be improved significantly compared to those estimated from infrequent campaign mode measurements

seismic anisotropy beneath tibet evidence for eastward extrusion of the tibetan lithosphere strong seismic anisotropy beneath tibet has recently been reported from the study of sks shear wave splitting the fast split waves are generally polarized in an easterly direction close to the present day direction of motion of the tibetan crust relative to stable eurasia as deduced from holocene slip rates on the major active faults in and around tibet this correlation may be taken to suggest that the whole tibetan lithosphere is being extruded in front of indenting india and that the anisotropic layer is the deforming asthenosphere that accommodates the motion of the tibetan lithosphere relative to the fixed mantle at depth uncertainties about this motion are at present too large to bring unambiguous support to that view assuming that this view is correct however a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer the observed delay times would require a 50 100 km thick anisotropic layer beneath south central tibet and an over 200 km thick layer beneath north central tibet where particularly hot asthenosphere has been inferred this study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents

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

lithospheric variations across the superior province ontario canada evidence from tomography and shear wave splitting 1 the superior province of the canadian shield is the largest contiguous region of the archean crust a combination of data from multiple experiments is used to obtain shear wave splitting parameters and a three dimensional tomographic velocity model beneath a large portion of the superior corresponding approximately to the province of ontario shear wave splits are obtained at 24 sites across the superior displaying strong averaging 1 34 s ene wsw splitting at stations west of 86 w and weaker 0 67 s e w splitting in the east the fast direction is subparallel to both absolute plate motion and tectonic belt boundaries the recovered tomographic velocity model shows a major boundary oriented nnw sse separating high velocities in the western superior ws from low velocities in the east and coinciding with the divide between weak and strong shear wave splits other features include a 200 km thick low velocity anomaly corresponding to the nipigon embayment a 1 0 ga failed rift branch and a linear low velocity anomaly in the east attributed to the great meteor hot spot track the nipigon anomaly in the western portion of the model is probably in situ while the great meteor track is displaced from crustal features associated with the hot spot we interpret this displacement as evidence that the eastern lithosphere has been deformed by basal drag while the western lithosphere has remained stable and propose that the east west lithospheric boundary we have detected represents a change in mechanical properties between stronger higher velocity western material with consistent anisotropic fabric and weaker eastern material with more variable fabric

progressive inversion for hypocenters and p wave and s wave velocity structure application to the geysers california geothermal field seismicity at the geysers is induced by some aspect of steam production accurate earthquake locations particularly hypocentral depth are needed to determine the relationship between geothermal energy production and seismicity progressive p and s wave velocity hypocenter inversions were done using data from 39 microearthquakes at the geysers to estimate microearthquake locations and determine if the geothermal field has a distinctive seismic signature comparable final vp vs and vp vs velocity models were obtained from four different starting velocity models the elevation interval of maximum steam production coincides with minimum observed vp vs and vp vs increases below the primary production zone suggesting that reservoir rock becomes more fluid saturated with increasing depth a vp vs peak at the condensation zone production zone elevation delineates the top of the stream reservoir earthquake locations are confined to two depth intervals a zone of shallow seismicity occurs immediately below the condensation zone and above maximum depths of steam production a more arealy restricted zone of seismicity is located below maximum production depths and is abruptly terminated at an elevation of 3 7 km

subsidence at the geysers geothermal field n california from a comparison of gps and leveling surveys between 1994 and 1996 three gps surveys were conducted in the geysers region of northern california our aim was to constrain models of the stresses and strains induced by geothermal power production in that region each survey spanned the geysers geothermal field and consisted of typically 40 monuments these monuments had been previously employed in a series of first order leveling surveys during the 1970 s this earlier study had determined that the geysers region was subsiding with a maximum rate of 0 048 0 0055 m yr between 1973 and 1977 in order to be able to directly compare the leveling and gps surveys we transform them to the same reference frame using the geoid 96 geoid model for the period 1977 1996 we determine a maximum susbsidence rate of 0 047 0 002 m yr we then model this subsidence using a series of point sources of contraction and find their optimal configuration by applying the random cost method and the f test the minimum volume strain that we find consistent with this subsidence is approximately 5 10 4 such a strain cannot be explained by a thermoelastic mechanism but does seem to be consistent with poroelastic deformation and a quasi static reservoir bulk modulus k of 3 6 109 pa