the seismic structure of the saurashtra crust in northwest india and its relationship with the reunion plume summary the saurashtra peninsula in northwest india lying at the northern boundary of the deccan volcanic province is almost entirely covered by these volcanics analogue seismic refraction wide angle reflection data along a 160 km long profile from navibandar to amreli were collected during 1977 to determine the crustal configuration reprocessing of these data after digitization has yielded a crustal model that is significantly different from the earlier model of kaila et al the model shows the upper crust down to a depth of 16 km in the west and 13 km in the east and underplating velocity of 7 20 km s 1 o fthe lower crust the moho is at a depth of 36 km in the western part and at 32 33 km in the eastern part the change of depth is quite sharp almost in the middle of the profile similar depth changes are seen in other crustal horizons indicating a deep fault that is in line with the extension of the proterozoic aravalli trend in to the saurashtra peninsula the crustal structure in the eastern part is similar to that in the cambay basin and indicates that the crust to the east of the proposed fault is uplifted the uplifted region extends as far as another arm of the aravalli trend that turns eastwards crustal underplating in large parts of western india is confined to the corridors affected

uppermost inner core seismic structure new insights from body waveform inversion abstract differential travel times and waveforms of pkikp and pkikp phases in the 129 141 distance range deconvolved for the effects of source time functions and average mantle attenuation operators are inverted for velocity and attenuation in the uppermost 80 km of the inner core and the velocity gradient and attenuation in the lowermost 200 km of the outer core f region results confirm degree one velocity structure in the inner core whose fast and slow regions appear to be separated by transitions with a velocity contrast of about 1 although attenuation in the inner core also exhibits degree one pattern a high attenuation region q p 1 0 002 0 0067 extends to the central pacific which has been previously considered a part of the western hemisphere characterized by slower velocity and lower attenuation the rest of the inner core exhibits a positive correlation between velocity and attenuation as observed previously an observed mantle like relationship between velocity and attenuation in the central pacific can be interpreted as originating from higher than average homologous temperature consistent with a model of inner core solidification driven by outer core convection coupled to core mantle boundary thermal heterogeneity alternative mechanisms including differences in grain size and impurity concentration and lateral differences in solidification rate might be required to explain the structure of the inner core the f region velocity and attenuation are best explained by the ak135 f velocity model and q p 1 0 0 001 respectively

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

a low seismic wavespeed anomaly beneath northwestern india a seismic signature of the deccan plume abstract seismic tomography of the indian region using p wave arrival times reveals an approximately cylindrical region of lowered seismic velocities in the upper mantle to the north of the gulf of cambay and the present exposure of the deccan flood basalts this anomaly with low velocities interrupts the characteristically high seismic velocities in the lithosphere beneath peninsular india and extends from shallow depth down to a more extensive low velocity zone beneath 200 km the centre of the low velocity anomaly lies just below the earliest alkaline magmatism which has been directly associated with the deccan plume in the northern portion of the cambay graben these magmatic events precede the main deccan flood eruptions by at least 3 myr the lowered seismic wavespeeds link into the ancient narmada lineament and to some extent to the south but here unfortunately the tomographic image runs out of resolution synthetic tests indicate that such a zone of lowered wavespeeds can be recovered with some loss of amplitude towards the south the association of the current seismic anomaly with the deccan traps is intriguing since the cambay graben remains a place of raised heat flow one explanation would be that the seismic anomaly represents the signature of the initial conduit through which plume material forced its way through the lithosphere beneath the cambay rift the zone of lowered seismic velocities beneath the indian lithosphere might then represent the remains of the initial plume head which would need to have moved with india over the 65 myr since the deccan eruptions

constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

electrical resistivity cross section across northern part of saurashtra region an insight to crystallized magma and fluids abstract saurashtra region forms the western part of the deccan volcanic province dvp in india and is bounded by the cambay rift basin crb in the east narmada rift basin nrb in the south and kachchh rift basin krb in the north this region is uplifted due to different stages of inter continental breakup madagascar india seychelles india different stages of rifting reactivation of fault zones and deccan volcanism the major portion of saurashtra region is covered by deccan basalts underlain by mesozoic sediments detailed magnetotelluric mt and long period magnetotelluric lmt soundings have been carried out in the northern part of saurashtra with an aim to bring out the geoelectrical configuration of different basins jamnagar jasdan and cambay along the profile 2d inversion carried out for different modes transverse electric te transverse magnetic mode tm and te tm independently the inversion of different modes shows distinct electrical signatures due to the 3d nature of the data we have considered the tm mode of interpretation because it is less prone to 3d distortions the final model represents large scale heterogeneities in the crust due to the presence of different resistive and conductive blocks high conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and or dehydration of minerals resulting in high conductivity anomalies a prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic ultra mafic intrusions related to the reunion plume activity different resistive blocks in conjunction with seismic tomography studies represent recrystallized older precambrian crust that forms the uplifted blocks of jamnagar jasdan and western part of cambay basins

seismic velocity and attenuation structures in the top 400 km of the earth s inner core along equatorial paths 1 we study seismic velocity and attenuation structures in the top 400 km of the earth s inner core based on modeling of differential traveltimes amplitude ratios and waveforms of the pkikp pkikp phases observed at the epicentral distance range of 120 141 and the pkpbc pkikp phases observed at the distance range of 146 160 along equatorial paths our data are selected from the seismograms recorded in the global seismographic network from 1990 to 2001 and many regional seismic networks the observed pkikp pkikp and pkpbc pkikp phases exhibit distinctive east west hemispheric patterns 1 at the distance ranges of 131 141 and 146 151 pkikp phases arrive about 0 3 s earlier than the theoretical arrivals based on the preliminary reference earth model prem for the pkikp phases sampling the eastern hemisphere 40 e 180 e of the inner core and about 0 4 s later for those sampling the western hemisphere 180 w 40 e at the distance range of 151 160 pkikp phases arrive about 0 7 s earlier than the predicted arrivals based on prem for those sampling the eastern hemisphere and about 0 1 s later for those sampling the western hemisphere 2 amplitude ratios of the pkikp pkikp phases at the distance range of 131 141 and of the pkikp pkpbc phases at the distance range of 146 151 are in general smaller for the pkikp phases sampling the eastern hemisphere than for those sampling the western hemisphere at distances greater than 151 the pkikp pkpbc amplitude ratios become indistinguishable for the two hemispheres these observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths one for each hemisphere in the top 400 km of the inner core for the eastern hemisphere the velocity structure has a velocity increase of 0 748 km s across the inner core boundary icb a small velocity gradient of 0 0042 km s 100 km in the top 235 km followed by a steeper velocity gradient of 0 1 km s 100 km extending from 235 km to 375 km and a velocity gradient of 0 01 km s 100 km in the deeper portion of the inner core the attenuation structure has an average q value of 300 in the top 300 km and an average q value of 600 in the deeper portion of the inner core for the western hemisphere the velocity structure has a velocity increase of 0 645 km s across the icb and a velocity gradient of 0 049 km s 100 km in the top 375 km the attenuation structure has an average q value of 600 in the top 375 km of the inner core our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the icb and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient high velocity and high attenuation

inner core freezing and melting constraints from seismic body waves abstract hypotheses for the mechanisms of growth of earth s inner core can be tested from observations of lateral variations of elastic and anelastic properties near the inner core boundary differential travel times of pkikp pkikp and the apparent travel time of p waves diffracted around the inner core are consistent with the existence of a stably stratified globally uniform region of reduced velocity gradient at the bottom of the outer core this layer may be a zone of iron enrichment relative to the bulk of the outer core representing a zone either of crystallization or melting of the inner core weak backscattered pkikp in the equatorial eastern hemisphere can be made consistent with either a hypothesis of freezing or melting in a melting hypothesis the eastern hemisphere is texturally older and smoother with larger crystals or smaller elastic contrasts across grain boundaries providing weaker backscatter to the coda of pkikp additional observations consistent with higher homologous temperature and melting in this region are the existence of a thin low velocity layer in the uppermost inner core inferred from multipathing of pkikp pkikp waves and a bright spot in the amplitude of pkiikp waves observed at antipodal distances alternatively a freezing hypothesis in the eastern hemisphere is an inner core that is growing radially outward with crystals stretched in the vertical direction this hypothesis can simultaneously explain weak backscattered pkikp coda and high attenuation of transmitted pkikp from the loss of backscattered energy at angles perpendicular to vertically oriented crystals strong backscattering observed in the coda of pkikp sampling the equatorial western hemisphere of the inner core agrees best with a heterogeneity having an isotropic distribution of scale lengths in the uppermost inner core

seismic velocity and attenuation structures in the top of the earth s inner core 1 we collect a global data set of pkikp and pkikp phases recorded by the global seismic network and many regional seismic arrays to study seismic structure in the top of the earth s inner core the pkikp and pkikp observations show different characteristics between those sampling the eastern hemisphere 40 e 180 e of the inner core and those sampling the western hemisphere 180 w 40 e pkikp phases 1 arrive about 0 4 s earlier than the theoretical arrivals based on preliminary reference earth model prem for those sampling the eastern hemisphere of the inner core and about 0 3 s later for those sampling the western hemisphere 131 141 2 bifurcate at smaller epicentral distances for those sampling the eastern hemisphere compared to those sampling the western hemisphere and 3 have smaller amplitudes for those sampling the eastern hemisphere waveform modeling of these observations suggests two different types of models for the two hemispheres of the top of the inner core with a model in the eastern hemisphere having a p velocity increase of 0 765 km s across the inner core boundary a small radial velocity gradient of 0 000055 km s km and an average q value of 250 and a model in the western hemisphere with a p velocity increase of 0 633 km s across the inner core boundary a radial velocity gradient of 0 000533 km s km and an average q value of 600 the hemispherical difference of seismic structures may be explained by different geometric inclusions of melt and or different alignments of iron crystals with anisotropic properties in both velocity and attenuation we speculate that this large scale pattern of seismic heterogeneities may be caused by a large scale heat flow anomaly at the bottom of the outer core and or different vigorousness of convection in the top of the inner core between the two hemispheres

mantle fabric at multiple scales across an archean proterozoic boundary grenville front canada abstract in eastern ontario and southwest quebec canada the proterozoic grenville province abuts against the archean superior province the complex tectonic history of the region is reflected in the pattern of electrical and seismic anisotropy within the lithosphere while asthenospheric anisotropy is expected to reflect current patterns of mantle flow magnetotelluric and teleseismic data from the polaris and fednor experiments and the lithoprobe abitibi grenville transect are examined for sks splitting and geoelectric strike and receiver functions are generated at selected stations in order to characterise both vertical and horizontal variations in anisotropy in the eastern ontario upper mantle the average shear wave split direction coincides with the direction of plate motion split times are found to be strongest in the southern part of the grenville province where asthenospheric flow is enhanced by the presence of a lithospheric divot the ottawa bonnechere graben and the immediate vicinity of the grenville front are regions of altered split direction indicative of a lithospheric component of anisotropy north of the grenville front there is a gradual reorientation of the split direction from ese to ene which is not easily attributable to crustal tectonics and may represent the northern limit of lithospheric deformation produced by the grenville orogen electrical anisotropy is pervasive in the study area the pattern of magnetotelluric strikes is more complex than the sks pattern though after correction for local distortion the geoelectric strikes correlate fairly well with sks measurements at nearby stations obliquity between sks and magnetotelluric results shows no consistent orientation across the study area receiver function analysis at three selected stations is indicative of a subcrustal anisotropic layer with a consistent sse fast direction underlain at station gac by a sequence of anisotropic layers with varying directions this sequence is not observed at station sado combining these results we interpret the strength and direction of anisotropic fabric in the grenville to vary strongly with depth the upper part of the lithosphere contains thin anisotropic layers perhaps related to eclogitization and relict slabs but which are insufficient to explain the observed sks splits the lower lithosphere is likely to be more ductile and uniformly anisotropic and may be an important control on magnetotelluric strike directions as well as a significant contributor to sks splitting the largest contributor to sks splitting in this region is interpreted to be asthenospheric anisotropy related to absolute plate motion

thermochemical flows couple the earth s inner core growth to mantle heterogeneity the uppermost 100 km of the earth s inner core is subject to an east west divide seismic waves travel more rapidly and are attenuated more severely in the eastern than in the western hemisphere and the west is more anisotropic that is seismic waves travel at different speeds in different directions than the eastern hemisphere the origin of this hemispherical dichotomy has remained enigmatic aubert et al now show that a model incorporating thermochemical convection and dynamo action can account for these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core have revealed that the eastern hemisphere is seismically faster more isotropic and more attenuating than the western hemisphere it is now shown that a single model of thermo chemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle seismic waves sampling the top 100 km of the earth s inner core reveal that the eastern hemisphere 40 e 180 e is seismically faster1 2 more isotropic2 3 and more attenuating4 than the western hemisphere the origin of this hemispherical dichotomy is a challenging problem for our understanding of the earth as a system of dynamically coupled layers previously laboratory experiments have established that thermal control from the lower mantle can drastically affect fluid flow in the outer core5 which in turn can induce textural heterogeneity on the inner core solidification front6 the resulting texture should be consistent with other expected manifestations of thermal mantle control on the geodynamo specifically magnetic flux concentrations7 8 in the time average palaeomagnetic field9 10 over the past 5 myr and preferred eddy locations11 in flows imaged below the core mantle boundary by the analysis of historical geomagnetic secular variation12 here we show that a single model of thermochemical convection and dynamo action can account for all these effects by producing a large scale long term outer core flow that couples the heterogeneity of the inner core with that of the lower mantle the main feature of this thermochemical wind is a cyclonic circulation below asia which concentrates magnetic field on the core mantle boundary at the observed location and locally agrees with core flow images this wind also causes anomalously high rates of light element release in the eastern hemisphere of the inner core boundary suggesting that lateral seismic anomalies at the top of the inner core result from mantle induced variations in its freezing rate

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

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

evidence for small scale heterogeneity in earth s inner core from a global study of pkikp coda waves abstract recent seismic observations have provided evidence that the inner core contains strong heterogeneity at a scale length of tens of kilometers the corresponding lateral variations in elastic properties could be caused by pockets of partial melt alignment of iron crystals or variations in chemistry however the relevant seismic observations precritical pkikp coda waves were subtle and were made using historic seismic data furthermore it has been suggested that the seismic data might be explainable by scatterers in the lower mantle or by a complex inner core boundary to address these issues we investigate a preexisting global database of precritical pkikp waveforms at distances of 10 50 and a second newly generated global data base of pkikp waveforms at distances of 50 90 we analyze the data using standard array processing techniques and identify pkikp coda waves based on travel time ray parameter amplitude and coherence although it remains unclear whether the scattered energy is being created within the inner core or along its boundary we find three lines of evidence which support the idea that it is in fact related to the inner core at smaller distances the decay rate of pkikp coda is significantly lower than the decay rates of the corresponding pcp and scp codas at larger distances we find examples of emergent spindle shaped pkikp coda waves that exist without the parent pkikp phase and at larger distances we infer a pkikp coda decay rate similar to that determined from the data at the smaller distances it is likely that many more pkikp coda observations can be made with existing data sets and hence seismologists possess a new extraordinarily fine probe for inferring inner core structure

shear wave splitting beneath the snake river plain suggests a mantle upwelling beneath eastern nevada usa the snake river plain srp a 90 km wide topographic depression in southern idaho is a topographically anomalous feature in the western u s previous seismic studies focused on the northeastern srp to study its relationship with the yellowstone hotspot we present new teleseismic shear wave splitting data from six broadband seismic stations deployed along the axis of the srp from june 2000 to september 2001 we also analyze splitting at hlid a permanent station of the national seismic network located f100 km north of the plain splitting of individual teleseismic phases is consistent at all stations within 2r errors and we favor the interpretation of anisotropy with a single horizontal fast axis although a dipping axis interpretation is statistically permitted at two of the stations our station fast directions as well as shear wave splitting data from numerous other stations throughout the basin and range are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume like upwelling material beneath eastern nevada into a southwestward flowing asthenosphere with respect to a fixed hotspot reference frame this parabolic asthenospheric flow paf model for the great basin is attractive because it explains the observed high elevations high mantle buoyancy low velocity anomaly beneath eastern nevada high heat flow and depleted geochemistry of some erupted basalts the lack of pliocene recent major volcanism in eastern nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy our splitting station delay times vary in a way not predicted by the paf model and can be explained by a zone of aligned magma filled lenses and or partially molten dikes beneath the srp lithosphere a depleted olivine rich residuum underneath the sides of the eastern srp and or the effect of lateral lower crustal flow from beneath the esrp toward its adjacent flanks d 2004 elsevier b v all rights reserved

using pkikp coda to determine inner core structure 2 determination of qc 1 despite the similar and rather uniform composition of two components of the core the solid inner part presents a much larger attenuation compared to the liquid outer part more over several authors have reported evidence of scattered seismic energy coming from the inner core ic this implies that the ic has lateral variations in structure or composition with a scale length from one to tens of kilometers these seismic scatterers could be caused by changes in crystal grain size variations in the orientation of anisotropy or the presence of partial melt and or impurities this work pursues the characterization of inner core scattering ics using a global data set of high frequency pkikp waveforms recorded at short period small aperture seismic arrays we apply a methodology used in the study of heterogeneities in the crust and upper mantle that consists of curve fitting of the observed scattered energy with a standard model along with a stripping technique to isolate the pkikp coda from the rest e g p pcp and pp codas from the observed pkikp codas we found an average qc 500 for the inner core reflecting a scattering attenuation at least comparable to the intrinsic attenuation we found a geographical dependence in the observations of pkikp coda with most of them coming from the pacific ocean and asia and relatively few observations coming from the atlantic ocean the simplest explanation is that the ics has a quasi hemispherical variation similar to other properties such as p wave velocity jump at the inner core boundary attenuation in the upper ic strength of anisotropy and or thickness of an overlaying isotropic layer this phenomenon is probably related to the solidification texturing of iron crystals showing hemispherical differences in the growth of the inner core

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

shape preferred orientation of iron grains compatible with earth s uppermost inner core hemisphericity abstract constraining the possible patterns of iron fabrics in the earth s uppermost inner core uic is key to unravel the mechanisms controlling its growth and dynamics in the framework of crystalline micro structures composed of ellipsoidal aligned grains we discuss possible textural models of uic compatible with observations of p wave attenuation and velocity dispersion using recent results from multiple scattering theory in textured heterogeneous materials we compute the p wave phase velocity and scattering attenuation as a function of grain volume shape and orientation wrt to the propagation direction of seismic p waves assuming no variations of the grain volume between the eastern and western hemisphere we show that two families of texture are compatible with the degree one structure of the inner core as revealed by the positive correlation between seismic velocity and attenuation 1 strong flattening of grains parallel to the inner core boundary in the western hemisphere and weak anisometry in the eastern hemisphere 2 strong radial elongation of grains in the western hemisphere and again weak anisometry in the eastern hemisphere both textures can quantitatively explain the seismic data in a limited range of grain volumes furthermore the velocity and attenuation anisotropy locally observed under africa demands that the grains be locally elongated in the direction of earth s meridians our study demonstrates that the hemispherical seismic structure of uic can be entirely explained by changes in the shape and orientation of grains thereby offering an alternative to changes in grain volumes in the future our theoretical toolbox could be used to systematically test the compatibility of textures predicted by geodynamical models with seismic observations

two distinct shear wave splitting directions in the northwestern deccan volcanic province 1 the northwestern deccan volcanic province in india has witnessed several tectonic episodes resulting in the formation of rift zones wide spread magmatism and deep seated faults that are host to some deadly intraplate earthquakes in this study we attempt to decipher the mantle deformation beneath the region using the sks splitting technique applied to high quality data from a regional network comprising 44 broadband seismic stations sited on diverse tectonic environments the first measurements of 461 312 sks and 149 skks splitting parameters from 117 earthquake sources reveal two major trends one coinciding with the absolute plate motion apm of the indian plate and the other roughly oriented in the e w direction 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 study region with the fast axis azimuths oriented n22 e in the bottom layer and n77 e in the top layer while stronger anisotropy t 1 2 s in the bottom layer is related to asthenospheric flow a weaker anisotropy t 0 5 s in the upper layer could represent anisotropy frozen in the lithosphere due to deformation related to widespread magmatism in this plume affected region

shear wave splitting around the eifel hotspot evidence for a mantle upwelling summary we present the results of a shear wave splitting analysis across the eifel hotspot in westcentral europe our data set includes 18 permanent european broadband stations and 102 temporary eifel broadband and short period stations we observe variations in splitting at most permanent stations but are not able to model them reliably with unique dipping axis or two layer anisotropy models we prefer instead a single layer model with a horizontal fast axis to approximate the first order anisotropy which varies smoothly between stations we observe a first order parabolic pattern in fast polarization azimuth around the hotspot which suggests that a lattice preferred orientation lpo of olivine fast axes exists in the asthenosphere as a result of the interaction between the slowly wsw moving eurasian plate and a mantle upwelling beneath the eifel volcanic fields the minority of the variation not explained by this model correlates with rapid lateral changes in splitting and is interpreted as a result of additional anisotropy and or the effects of dynamic recrystallization associated with lpo development in the region of corner flow near the conduit our parabolic asthenospheric flow paf interpretation is consistent with eifel geological tomographic receiver function global absolute plate motion electrical conductivity anisotropy and geochemical studies as well as with splitting studies in the great basin and around hawaii we suggest that the eifel upwelling is sporadic and a result of a low excess upwelling temperature and or varying crustal stresses that periodically shift and facilitate eruption the paf pattern we observe neither suggests nor rules out anisotropy in the conduit associated with a wet eifel upwelling we use our optimum pa f model parameters to calculate a eurasian plate speed of 12 km ma 1 w hich is consistent with the recent hs3 nuvel1a speed of 19 14 km ma 1

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

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

surface heat flow and probable evolution of deccan volcanism abstract in the light of surface heat flow observations as well as other related geological and geophysical data the origin of the deccan basalts has been examined the indian lithosphere after its detachment from gondwanaland apparently traversed a rising plume at la reunion which virtually bored through the lithosphere to emerge as the deccan trap volcanism on the surface subsequent volcanic and plutonic activity appears to have continued not only up to the oligocene as is indicated by the alkaline magmatic activity observed near the junction of the three prominent features the west coast faults the narmada son tapti lineament and the cambay graben but also up to the mio pliocene as indicated by the heat flow and gravity data over the cambay graben the dyke swarms and sills which are mostly post trappean evolved from the lithosphere after the indian plate moved away from the hot spot

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

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

gravity and magnetic signatures of volcanic plugs related to deccan volcanism in saurashtra india and their physical and geochemical properties abstract the bouguer anomaly and the total intensity magnetic maps of saurashtra have delineated six circular gravity highs and magnetic anomalies of 40 60 mgal 10 5 m s 2 and 800 1000 nt respectively three of them in western saurashtra coincide with known volcanic plugs associated with deccan volcanic province dvp while the other three in se saurashtra coincide with rather concealed plugs exposed partially the dvp represents different phases of eruption during 65 5 2 5 ma from the reunion plume the geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition which varies from ultramafic mafic to felsic composition of volcanic plugs in western saurashtra and an alkaline composition for those in se saurashtra detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc alkaline differentiation trend and a continental tectonic setting of emplacement the alkaline plugs of se saurashtra are associated with ne sw oriented structural trends related to the gulf of cambay and the cambay rift basin along the track of the reunion plume this indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma the junagadh plug with well differentiated ring complexes in western saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude this implies that other plugs are also associated with mafic ultramafic components which may not be differentiated and may be present at subsurface levels paleomagnetic measurements on surface rock samples from dvp in saurashtra suggest a susceptibility of 5 5 10 2 si units with an average koenigsberger ratio q n of almost one and average direction of remanent magnetization of d 147 4 and i 56 1 the virtual geomagnetic pole vgp position computed from the mean direction of magnetization for the volcanic plugs and deccan basalt of saurashtra is 30 n and 74 w which is close to the vgp position corresponding to the early phases of deccan eruption modeling of gravity and magnetic anomalies along two representative profiles across junagadh and barda volcanic plugs suggest a bulk density of 2900 and 2880 kg m 3 respectively and susceptibility of 3 14 10 2 si units with a q n ratio of 0 56 which are within the range of their values obtained from laboratory measurements on exposed rock samples the same order of gravity and magnetic anomalies observed over the volcanic plugs of saurashtra indicates almost similar bulk physical properties for them the inferred directions of magnetization from magnetic anomalies however are d 337 and 340 and i 38 and 50 which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs some of these plugs are associated with seismic activities of magnitude 4 at their contacts based on this analysis other circular semi circular gravity highs of nw india can be qualitatively attributed to similar subsurface volcanic plugs

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

inversions for earthquake focal mechanisms and regional stress in the kachchh rift basin western india tectonic implications abstract more than a decade after the 2001 m w 7 7 bhuj earthquake in western india aftershocks up to m w 5 0 are still continuing around the rupture zone in the kachchh rift basin over the years some surrounding faults in the region have been activated and a transverse fault generated an m w 5 1 earthquake in 2012 most of the earthquakes occur in the lower crust at depths between 15 and 35 km we have determined focal mechanism solutions of 47 earthquakes m w 3 2 5 1 that were recorded by a 60 station broadband network during 2007 2014 within an area of 50 km radius of the 2001 main shock south dipping nodal planes in most of the solutions correlate well with the active faults the earthquakes near the epicenter of the 2001 main shock primarily show reverse faulting mechanisms the surrounding earthquakes in the area however show predominantly strike slip mechanisms the p axes of the earthquakes mostly oriented in north south and the t axes in east west however the orientations of the p and t axes exhibit more complexity near the source area of the main shock stress field inversion of the solutions yields a dominant north south compression which is consistent with the ambient tectonic stress field owing to the northward movement of the indian plate with respect to the eurasian plate the geodetic measurements are in reasonable agreement with our results

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

strong variations in seismic anisotropy across the hikurangi subduction zone north island new zealand abstract shear wave splitting measurements from sks skks s and scs phases reveal the anisotropic structure of north island new zealand waveforms were recorded on temporary and permanent broadband seismographs strong anisotropy varies rapidly across the hikurangi subduction zone these changes constrain the width of the deformation associated with the plate boundary and define three different regions of mantle flow stations in the fore arc region eastern central north island record fast polarisations subparallel to the trench and moderate delay times of around 2 5 s consistent with earlier studies of north island this is interpreted as caused by trench parallel mantle flow beneath the subducted slab with a possible contribution of trench parallel fossil anisotropy within the subducted slab delay times increase towards the extending central volcanic region cvr with values up to 4 5 s one of the largest delay times measured worldwide strong s wave anisotropy 7 5 through 80 km in the mantle wedge in addition to the sub slab anisotropy can explain the observed high delay times one mechanism to create such high anisotropy is melt segregation of partially molten rock a frequency dependence of this mechanism could also explain persisting discrepancies between local and teleseismic shear wave splitting measurements to the west delay times decrease systematically measurements in the far back arc region of western central north island show no apparent splitting this suggests that mantle wedge dynamics terminate beneath the western border of the cvr the apparent isotropy beneath the far back arc may be related to local small scale mantle convection or vertical mantle flow possibly caused by the detachment of lithosphere

hemispherical structure in inner core velocity anisotropy 1 we investigate the hemispherical pattern in inner core velocity anisotropy using a new and independent high quality data set of pkpbc pkpdf and pkpab pkpdf body wave observations our data show no evidence for a tilted anisotropy axis with respect to the earth s rotation axis and is significantly better fit by a model with 4 4 anisotropy in the western hemisphere and only 1 0 in the eastern hemisphere than by a model of uniform anisotropy we carry out variance minimization and find the boundaries between the hemispheres at lines of constant longitude at 14 e and 151 w variance minimization enables us to extract the imprint of hemispherical structure from all of the data and not just polar paths resulting in boundaries which are nearly 30 from the results of previous studies the high quality of the data allows us to provide robust evidence that the isotropic velocity in the eastern hemisphere is 0 2 lower than in the western hemisphere in the top 660 km of the inner core our data set also suggests an increase in inner core anisotropy in the eastern hemisphere from 1 to around 6 at depths deeper than 660 km indicating that the hemispherical pattern in anisotropy may disappear at greater depths the presence of hemispherical structure rules out mechanisms for creating anisotropy which are unable to sustain longitudinal variations in the inner core furthermore steady inner core superrotation of the order of 0 1 year would eradicate the hemispherical differences though inner core oscillation would still be permissible

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

intricate heterogeneous structures of the top 300 km of the earth s inner core inferred from global array data i regional 1d attenuation and velocity profiles abstract we apply a waveform inversion method based on simulated annealing to complex core phase data observed by globally deployed seismic arrays and present regional variation of depth profiles of attenuation and velocity for the top half of the inner core whereas measured attenuation parameters exhibit consistent trends for data sampling the eastern hemisphere of the inner core for the western hemisphere there is a remarkable difference between data sampling the inner core beneath africa w1 and beneath north america w2 obtained attenuation profiles suggest that intricate heterogeneities appear to be confined in the top 300 km the profile for the eastern hemisphere has a high attenuation zone in the top 150 km that gradually diminishes with depth conversely for the western hemisphere the profile for w1 shows constant low attenuation and that for w2 represents a gradual increase from the inner core boundary to a peak at around 200 km depth velocity profiles obtained from differential traveltimes between pkp df and pkp cd bc phases for the eastern and western hemispheres are respectively about 0 8 faster and 0 6 slower than the reference model at the top of the inner core and the difference nearly disappears at about 200 km depth our result suggests the presence of intricate quasi hemispherical structures in the top 200 300 km of the inner core

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

shear wave splitting to test mantle deformation models around hawaii seismic anisotropy allows us to study mantle deformation and it can thus help to constrain mantle flow in the vicinity of hotspots hypotheses for the cause of seismic anisotropy in this environment include the parabolic asthenospheric flow paf model radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion in map view this gives a parabolic pattern of flow opening in the direction of plate motion we present new shear wave splitting observations from land and ocean stations around the hawaiian islands that can be explained by the parabolic flow model the observations suggest asthenospheric anisotropy under the hawaiian islands which may be explained if dislocation creep persists to deeper depths there than in other regions perhaps due to the higher temperatures near hotspots

teleseismic arrivals at a mid ocean ridge effects of mantle melt and anisotropy recent observational evidence of upwelling mantle anisotropy at a slow spreading center has motivated the modeling of teleseismic arrivals at mid ocean ridges the models consider a variety of types of anisotropy and heterogeneity where the emphasis is to ascertain whether or not travel times can be used to discriminate between the existence of partial melt and anisotropy two mechanisms for anisotropy in the upwelling asthenosphere are considered one due to the preferential alignment of the fast axes of olivine crystals in the direction of mantle flow and the other due to the preferential alignment of cracks that feed melt towards the spreading axis the results indicate that p waves are most sensitive to even modest amounts of flowinduced asthenospheric anisotropy while s waves are most sensitive to the presence of mantle melt multiple s wave arrivals are predicted for many models most notably the ones with anisotropy due to crack alignment where very large s wave separations develop the model which best fits existing data requires a higher degree of crystal alignment anisotropy in the upwelling asthenosphere than in the lithosphere this effect has been predicted in studies of the evolution of crystal alignment anisotropy in polycrystalline aggregates

fine scale heterogeneity in the earth s inner core the seismological properties of the earth s inner core have become of particular interest as we understand more about its composition and thermal state1 2 observations of anisotropy and velocity heterogeneity in the inner core are beginning to reveal how it has grown and whether it convects3 4 the attenuation of seismic waves in the inner core is strong and studies of seismic body waves5 6 have found that this high attenuation is consistent with either scattering or intrinsic attenuation5 the outermost portion of the inner core has been inferred to possess layering and to be less anisotropic than at greater depths7 8 9 10 here we present observations of seismic waves scattered in the inner core which follow the expected arrival time of the body wave reflection from the inner core boundary the amplitude of these scattered waves can be explained by stiffness variations of 1 2 with a scale length of 2 kilometres across the outermost 300 km of the inner core these variations might be caused by variations in composition by pods of partial melt in a mostly solid matrix or by variations in the orientation or strength of seismic anisotropy

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

hemispherical variations in seismic velocity at the top of the earth s inner core knowledge of the seismic velocity structure at the top of the earth s inner core is important for deciphering the physical processes responsible for inner core growth1 2 3 previous global seismic studies4 5 6 7 8 9 have focused on structures found 100 km or deeper within the inner core with results for the uppermost 100 km available for only isolated regions10 11 12 here we present constraints on seismic velocity variations just beneath the inner core boundary determined from the difference in travel time between waves reflected at the inner core boundary and those transmitted through the inner core we found that these travel time residuals observed on both global seismograph stations and several regional seismic networks are systematically larger by about 0 8 s for waves that sample the eastern hemisphere of the inner core 40 e to 180 e compared to those that sample the western hemisphere 180 w to 40 e these residuals show no correlation with the angle at which the waves traverse the inner core this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0 8 higher than that of the western hemisphere

structure and evolution of the western continental margin of india deduced from gravity seismic geomagnetic and geochronological studies abstract available geochronological gravity seismic and geomagnetic data on the western continental margin of india between 8 20 n are presented based on the data three models describing the evolution of the crustal structure are developed model 1 describes the evolution of the crustal structure along 19 n the main basic igneous activity that accompanied the tectonic events since the late cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure the acid volcanic event that preceded the above activity at about 93 ma ago is responsible for the observed crustal structure along 13 30 n shown in model 2 this event resulted in the formation of the st mary group of islands along this latitude counterclockwise rotation of the indian plate during the tertiary period caused rifting of the continental blocks giving rise to the laccadive kerala graben bounded by the chagos laccadive ridge on the west and the west coast fault on the east vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault this tectonic event was accompanied by upwarping of the mantle crust thus giving rise to the present day structure shown in model 3

fault geometry of the mw 7 7 western india intraplate earthquake constrained from double difference tomography and fault plane solutions abstract globally one of the largest intraplate earthquakes of m w 7 7 occurred on 26 january 2001 in the kachchh rift basin krb western india the continuing long aftershock sequence over decades has generated much debate on the seismogenic fault s we have analyzed more than 10 000 aftershocks m w 1 0 recorded by a 50 station broadband network in the region during 2006 2014 a total of 834 aftershocks m w 2 4 each recorded by at least eight broadband seismic stations with a minimum of eight p and six s phases are relocated in this study by double difference tomography tomodd method the relocated aftershocks and velocity images reveal a near vertical or steeply south dipping deeper structure as the source zone of the mainshock and aftershocks the structure correlates well with the geologically mapped south wagad fault swf among the other geologically known faults the kachchh mainland fault kmf and the gedi fault gf are also well identified in the seismic sections further fault plane solutions of 109 aftershocks having m w 3 5 corroborate well with the known faults the geological model and seismological observations suggest that the swf overstepped the kmf and intersects it at depth the intersecting fault zone is the source area for the deeper 10 35 km reverse faulting earthquakes in the krb at the fault end of the swf a heterogeneous velocity structure is imaged which is attributed to a fluid filled rock matrix that triggered the mainshock on the other hand the gf is reported to be a later activated fault to the north of swf it generated some shallower aftershocks 0 20 km mostly by strike slip mechanisms