topography and tectonics of the central new madrid seismic zone results of numerical experiments using a three dimensional boundary element program we present results of a series of numerical experiments designed to test hypothetical mechanisms that drive deformation in the new madrid seismic zone experiments are constrained by subtle topography and the distribution of seismicity in the region we use a new boundary element algorithm that permits calculation of the three dimensional deformation field surface displacement fields are calculated for the new madrid zone under both far field plate tectonic scale and locally derived driving strains results demonstrate that surface displacement fields cannot distinguish between either a far field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults thus neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present day deformation we have also tested hypotheses about strain accommodation within the new madrid contractional step over by including linking faults two southwest dipping and one vertical recently inferred from microearthquake data only those models with step over faults are able to predict the observed topography surface displacement fields for long term relaxed deformation predict the distribution of uplift and subsidence in the contractional step over remarkably well generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step over region with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked these models suggest that the gently dipping central step over fault is a reverse fault and that the steeper fault extending to the southeast of the step over acts as a normal fault over the long term

source parameters and tectonic implications of aftershocks of the mw 7 6 bhuj earthquake of 26 january 2001 we present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period the data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock we have calculated high quality single event locations based on a 1d velocity model determined for the region for earthquakes with magnitudes between 2 and 5 aftershock locations reveal activity concentrated on a nearly east striking south dipping plane trapezoidal in outline the active zone tapers from about 45 km along strike at the shallow end which is about 5 km deep to no more than 25 km long at a depth of 35 km the total rupture area was about 1300 km 2 we estimate the static stress drop of the mainshock at 16 2 mpa aftershocks extend nearly through the entire crust with concentrations in the lower crust at about 26 km and in the upper crust at about 10 km the fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake the aftershock data are consistent with the bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift

rift basins in western margin of india and their hydrocarbon prospects with special reference to kutch basin the western continental margin of india can be classed as a divergent or passive margin the western continental shelf is an extensive carbonate bank bombay offshore basin passing into clastic sediments on the north and south three craton margin embayed basins kutch cambay and narmada in the northern part of the shelf are filled predominantly with clastic sediments these basins occupy grabens bounded by faults diverging seaward the grabens were formed by three rift systems along major precambrian tectonic trends the rifting developed sequentially from north to south around the saurashtra horst kutch basin was formed in the early jurassic followed by cambay basin in early cretaceous time and the narmada in the late cretaceous it appears that these rifting ev nts occurred at successive stages during the northward migration of the indian plate after its break from gondwanaland in late triassic or early jurassic it is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the indian craton bombay offshore and cambay are two major oil producing basins in the western margin these basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region oil has not been found in kutch basin which is mainly an onshore mesozoic basin the basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide

deep lower crustal earthquakes in central india inferences from analysis of regional broadband data of the 1997 may 21 jabalpur earthquake summary the only instance of a confirmed deep lower crustal earthquake occurrence in the indian shield region has been that of the 1938 satpura earthquake m 6 3 of central india reportedly at a depth of about 40 km moment tensor inversion of regional broadband waveform data of the 1997 may 21 jabalpur earthquake mw 5 7 confirms yet another such earthquake at about 35 km depth in the central part of the narmada son lineament nsl zone the study is based on a refined velocity model obtained using a traveltime grid search method a reverse fault mechanism is obtained which for a palaeo rift valley zone indicates the possibility of reactivation of a pre existing fault under the influence of the ambient stress field due to the india eurasia plate collision forces the occurrence of earthquakes at lower crustal depths quite unusual for the indian shield region indicates a possible causative mechanism related to crust mantle interaction based on the close proximity of the two deep earthquakes and their disposition with respect to the local trend of the central part of the nsl we suggest a model of stress accumulation due to horizontally elongated or elliptical possibly serpentinized mafic intrusives in the lower crust to explain the occurrence of deep earthquakes in the heart of the indian shield

seismic imaging of the aftershock zone of the 2001 mw 7 7 bhuj earthquake india 1 detailed seismic tomography reveals that the 2001 mw 7 7 bhuj earthquake was associated with a 10 14 increase in vp and vs and a 10 decrease in vp vs in the 10 to 35 km depth range covering a 2750 km2 area beneath the aftershock zone this anomaly could be attributed to the existence of a lithological heterogeneity or a pluton of mafic composition that might have intruded during the rifting in early jurassic 160 ma the banni basin and the wagad uplift are found to be associated with high velocity intrusive bodies extending from 5 to 35 km depth a few patches of low vp and vs and high vp vs between 10 to 30 km depth have also been detected on the causative 45 south dipping north wagad fault nwf for the 2001 mainshock which may be attributed to a fluid filled fractured rock matrix

crustal and upper mantle structure of peninsular india and source parameters of the 21 may 1997 jabalpur earthquake mw 5 8 results from a new regional broadband network abstract in the past two years a permanent network of 10 broadband seismographs has become operational in the indian peninsular shield region in this article we analyze the data recorded by this network with two goals 1 to lay ground for a quick estimation of source parameters of future earthquakes in the region and 2 to estimate the source parameters of the destructive jabalpur earthquake of 21 may 1997 mw 5 8 which occurred within the network toward these goals we measure rayleigh and love wave group velocity dispersion curves in the period range of 10 to 75 sec and invert these curves to estimate the crustal and the upper mantle structure below the indian shield region our best model consists of a two layer crust the upper layer is 14 km thick with a shear wave velocity v s of 3 55 km sec the corresponding values for the lower layer are 25 km and 3 85 km sec v s for the upper mantle is 4 65 km sec based on this structure we perform a moment tensor mt inversion of the bandpassed 0 05 0 02 hz seismograms of the jabalpur earthquake the best fit is obtained for a source located at a depth of 42 km with a seismic moment m 0 of 7 7 10 24 dyne cm and a focal mechanism with strike 61 dip 64 and rake 74 these source parameters are similar to those previously reported in the literature demonstrating that a routine mt inversion of moderate and large indian penisular earthquakes is now possible a careful examination of the seismograms and modeling of the two closest records reveal that the jabalpur earthquake nucleated in the lower crust at a depth of 36 km it consisted of two subevents separated in time by 0 65 sec with the second subevent located 2 km above the first one the total source duration was 1 4 sec

intensity estimation for the 2001 bhuj earthquake india on esi 07 scale and comparison with historical 16th june 1819 allah bund earthquake a test of esi 07 application for intraplate earthquakes abstract on 26th january 2001 an earthquake of magnitude mw 7 7 occurred near bhuj in northwestern india resulting in severe environmental effects no unequivocal primary surface rupture was observed for the earthquake but it caused widespread liquefaction and lateral spreading in the rann of kachchh and little rann after the earthquake several researchers collected field evidence of secondary surface rupture rockfall dry craters and surface manifestations of liquefaction including the formation of mud volcanoes and lateral spreads in the meizoseismal area analysis of pre and post earthquake satellite images suggests that several dry streams in the rann of kachchh began to flow due to extensive liquefaction induced by the earthquake in this present study the macroseismic intensity of the bhuj earthquake is evaluated by considering these environmental effects and applying the esi 07 intensity scale to the affected area as an outcome the epicentral intensity of the 2001 bhuj earthquake was determined to be xi according to historical records and seismic catalogs 16th june 1819 allah bund earthquake caused prominent surface rupture which was not so clear in the case of 2001 bhuj earthquake but the secondary effects were similar for both earthquakes considering the environmental effects caused by the 1819 allah bund earthquake an intensity of xi was estimated for the epicentral area for both earthquakes the esi scale yields a significant difference of one to two degrees with the traditional intensity scales the 2001 bhuj earthquake and 1819 allah bund earthquake shows similar esi 07 intensity of xi despite of different epicentral locations this implies the reliability of esi 07 scale application for different earthquakes of similar dimensions in the same geological setting this study contributes to the application of esi 07 scale for indian earthquakes especially reverse faulting events and to the future improvement of the esi scale with emphasis on its applicability to historical earthquakes on the indian subcontinent also this study may help in future land use planning in the meizoseismal area of 1919 allah bund and 2001 bhuj earthquakes

high dead live mismatch in richness of molluscan assemblages from carbonate tidal flats in the persian arabian gulf live dead ld fidelity studies are a fundamental tool for assessing the quality of the paleobiological information preserved in the fossil record an important feature of death assemblages das is that they are time averaged to some degree previous studies found that subtidal carbonate environments are characterized by lower time averaging in comparison to siliciclastic settings primarily because of a high rate of shell loss due to bioerosion and dissolution these factors lead to increased temporal resolution in subtidal carbonate death assemblages das but more intense bias in species composition due to differential loss of vulnerable shells a consequence of lower time averaging is a better match in live dead ld richness we evaluated ld fidelity of molluscan assemblages from soft sediments in subtropical carbonate tidal flats along the coast of the united arab emirates persian arabian gulf to test these predictions a total of 7193 mollusks from six sites were analyzed we found that das are on average over five times richer than living assemblages las at site scale and nearly five times at habitat scale we found that early cementation lateral mixing intense bioturbation and moderate sedimentation rates can account for large ld differences in richness differential intensity in lateral mixing by post mortem transport inflated alpha beta and gamma diversity by 28 10 and 42 however beta diversity is strongly underestimated in das when compared to las evenness was larger in das both at site and at habitat scales but there was only a high mismatch at sites with patchy las fidelity of feeding guild structure was high for rank abundance with secondary consumers well represented in das the occurrence of out of habitat taxa did not affect fidelity in rank abundance of feeding guild structure our results suggest that time averaging in carbonate tidal flats can potentially be higher compared with subtidal carbonate environments

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

structural controls on localized intraplate deformation and seismicity in southern australia insights from local earthquake tomography of the flinders ranges data from an array of 24 seismometers are used to image the crust beneath the flinders ranges southeast australia with the goal of improving our understanding of crustal structure rheology and the mechanism responsible for the localized intraplate deformation that characterizes this region a subset of p and s wave traveltimes is inverted to jointly recover earthquake hypocenters p wave velocity structure and v p v s anomalies the p wave velocity model reveals a spatial correlation between major negative velocity perturbations and concentrations of seismicity in particular a cluster of seismicity is observed within a distinct low velocity region between the archean mesoproterozoic gawler craton and the palaeo mesoproterozoic curnamona province from 7 to 20km depth we postulate that this may be associated with a pre existing structural weakness in the crust that arises primarily from rifting between the curnamona province and the gawler craton another area characterized by a high level of seismicity overlies a major sequence of n s trending ross delamerian thrust faults which correspond to a band of low v p and particularly v p v s the lack of evidence for elevated heat flows in both of these seismogenic regions suggests that thermally induced weakness is unlikely to play a dominant role instead the dynamic behavior of this intraplate region appears to be caused by a serendipitously oriented regional stress field provided by far field forces that originate from the boundary between the pacific and australian plates which acts upon preexisting structural weaknesses in the lithosphere

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

gis based approach for the measurement of variability in tectonomorphic signatures using dem s data a case study from the habo dome in the kachchh area india the kachchh region of western india is located in a plate interior region but falls under one of the highest seismic zones of india having very active crustal deformation and erosion the drainage system as well as the geomorphic evolution of the region under a semi arid climate zone with high to very high strength of rocks in areas with high relief is dominantly controlled by the seismic instability associated with continual tectonics in the present paper we have analysed a block close to the epicentre of the 2001 bhuj earthquake to understand the role of relative active tectonics in the evolution of domal landscape morphology shuttle radar topographic mission digital elevation model data were used to extract drainage network in the gis platform and evaluate active tectonics using different geomorphic indices of the habo hill the second largest domal structure situated in northern hill range of the kachchh region the average cumulative weightage of different geomorphic indices provides a quantitative tool index of relative active tectonics irat which shows significant variations in and around the habo dome activation of earlier lineaments and generation of new lineaments controlled the morphometric evolution of the fault bound block of the region to form the habo dome the undulating topography in addition to the domal shape of the study area resulted from the differential movements along several active transverse faults striking ne sw n s and nw se and the regional faults striking e w the kachchh mainland fault the kas fault and the jikadi fault stress propagations from the himalayan range in the northeast and suleiman range in the northwest are identified to be the causative factors for active tectonics and drainage anomalies in the area

infrasound data inversion for atmospheric sounding summary the international monitoring system ims of the comprehensive nuclear test ban treaty ctbt continuously records acoustic waves in the 0 01 10 hz frequency band known as infrasound these waves propagate through the layered structure of the atmosphere coherent infrasonic waves are produced by a variety of anthropogenic and natural sources and their propagation is controlled by spatiotemporal variations of temperature and wind velocity natural stratification of atmospheric properties e g temperature density and winds forms waveguides allowing long range propagation of infrasound waves however atmospheric specifications used in infrasound propagation modelling suffer from lack and sparsity of available data above an altitude of 50 km as infrasound can propagate in the upper atmosphere up to 120 km we assume that infrasonic data could be used for sounding the atmosphere analogous to the use of seismic data to infer solid earth structure and the use of hydroacoustic data to infer oceanic structure we therefore develop an inversion scheme for vertical atmospheric wind profiles in the framework of an iterative linear inversion the forward problem is treated in the high frequency approximation using a hamiltonian formulation and complete first order ray perturbation theory is developed to construct the frechet derivatives matrix we introduce a specific parametrization for the unknown model parameters based on principal component analysis finally our algorithm is tested on synthetic data cases spanning different seasonal periods and network configurations the results show that our approach is suitable for infrasound atmospheric sounding on a regional scale

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

rupture process of large earthquakes in the northern mexico subduction zone the cocos plate subducts beneath north america at the mexico trench the northernmost segment of this trench between the orozco and rivera fracture zones has ruptured in a sequence of five large earthquakes from 1973 to 1985 the jan 30 1973 colima event m s 7 5 at the northern end of the segment near rivera fracture zone the mar 14 1979 petatlan event m s 7 6 at the southern end of the segment on the orozco fracture zone the oct 25 1981 playa azul event m s 7 3 in the middle of the michoacan gap the sept 19 1985 michoacan mainshock m s 8 1 and the sept 21 1985 michoacan aftershock m s 7 6 that reruptured part of the petatlan zone body wave inversion for the rupture process of these earthquakes finds the best earthquake depth focal mechanism overall source time function and seismic moment for each earthquake in addition we have determined spatial concentrations of seismic moment release for the colima earthquake and the michoacan mainshock and aftershock these spatial concentrations of slip are interpreted as asperities and the resultant asperity distribution for mexico is compared to other subduction zones the body wave inversion technique also determines the moment tensor rate functions but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes an appendix describes the moment tensor rate functions methodology in detail

characteristics of deformation and past seismicity associated with the 1819 kutch earthquake northwestern india the 1819 earthquake in kutch northwestern india is one of the most significant events to have occurred in a plate interior setting despite being the second largest among the stable continental region scr earthquakes this event has not been analyzed within the context of present day understanding of earthquake seismology coseismic changes related to this earthquake include massive ground deformation in a wide low lying tidal flat area although detailed historic accounts of this earthquake exist many questions regarding the mode of deformation and the seismic history of the region remain unresolved we explored the region nearly 180 years after the earthquake and the information gathered adds to our understanding of this event and provides a fresh perspective on this unique intraplate seismogenic zone a 90 km long tract of elevated land with a peak height of 4 3 m is the most visible surface expression of this earthquake we surveyed and analyzed the morphological features of this scarp and also carried out exploratory trenching in this region the scarp morphology is suggestive of a growing fold related to a buried north dipping thrust rather than a discrete fault that could have resulted from a surface rupture the extensive liquefaction field associated with the earthquake offered an ideal setting to explore the paleoearthquake history age data of liquefaction features suggest that a previous event of comparable size must have occurred 800 1000 years ago seismic activity appears to be related to the reactivation of an ancient rift in a stress regime that is dominated by nearly north south compression

tectonic stress field in the indian subcontinent a map of maximum horizontal compressive stress orientation in the indian subcontinent has been prepared using orientations derived from three different stress indicators borehole elongation breakouts in situ hydraulic fracturing measurements and earthquake focal mechanisms most part of the subcontinent appears to be characterized by a compressional stress regime thrust and strikeslip faulting imposed by plate boundary forces although shmax orientations do not in general show clear correlation with the direction of motion of the indian plate four provinces are recognized on the basis of regionally consistent orientations these are the midcontinent stress province the southern shield the bengal basin and the assam wedge their boundaries have been determined taking into consideration regional tectonics and seismicity central and northern india including the shillong plateau stretching up to the great himalaya pakistan and nepal are included in a broad midcontinent stress province characterized by nne ene oriented shmax the mean orientation of shmax in this province is n23 e subparallel to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone suggesting that it is largely determined by the tectonic collision processes much of southern india mysore plateau and the high grade metamorphic terrain south of the plateau appears to be part of a second stress province characterized by nw oriented shmax these appear close to those of the intraplate stress field prevailing in the central indian ocean a third stress province was recognized in the bengal basin including parts of west bengal tripura manipur and mizoram in northeastern india and most of bangladesh this province extends eastward from the marginal fault in the western margin of the bengal basin to the indo burma subduction zone and is bounded on the north by the e w striking dauki fault shmax within the sedimentary pile of the bengal basin is oriented in e w direction while p axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the indian plate beneath the indo burman ranges generally trend north n30 e shmax orientations within the sedimentary pile of the basin are parallel to the local approximately e w direction of the convergence of indian and burmese plates suggesting a casual relation to the resistive forces at the subduction zone in the indo burma region interestingly the stress field in the basement and the crust beneath the bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province assam wedge stress region occupies the northeastern corner of the indian plate including upper assam arunachal pradesh and much of nagaland this region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the himalayan and northern limb of the indo burman fold belts as a result the stress field in this province is depth differentiated and most likely responsible for the absence of consistent shmax directions

the 26 january 2001 m 7 6 bhuj india earthquake observed and predicted ground motions although local and regional instrumental recordings of the devastating 26 january 2001 bhuj earthquake are sparse the distribution of macroseismic effects can provide important constraints on the mainshock ground motions we compiled available news accounts describing damage and other effects and interpreted them to obtain modified mercalli intensities mmis at 200 locations throughout the indian subcontinent these values are then used to map the intensity distribution throughout the subcontinent using a simple mathematical interpolation method although preliminary the maps reveal several interesting features within the kachchh region the most heavily damaged villages are concentrated toward the western edge of the inferred fault consistent with western directivity significant sediment induced amplification is also suggested at a number of locations around the gulf of kachchh to the south of the epicenter away from the kachchh region intensities were clearly amplified significantly in areas that are along rivers within deltas or on coastal alluvium such as mudflats and salt pans in addition we use fault rupture parameters inferred from teleseismic data to predict shaking intensity at distances of 0 1000 km we then convert the predicted hard rock ground motion parameters to mmi by using a relationship derived from internet based intensity surveys that assigns mmi based on the average effects in a region the predicted mmis are typically lower by 1 3 units than those estimated from news accounts although they do predict near field ground motions of approximately 80 g and potentially damaging ground motions on hard rock sites to distances of approximately 300 km for the most part this discrepancy is consistent with the expected effect of sediment response but it could also reflect other factors such as unusually high building vulnerability in the bhuj region and a tendency for media accounts to focus on the most dramatic damage rather than the average effects the discrepancy may also be partly attributable to the inadequacy of the empirical relationship between mmi and peak ground acceleration pga when applied to india the mmi pga relationship was developed using data from california earthquakes which might have a systematically different stress drop and therefore a different frequency content than intraplate events when a relationship between response spectra and mmi is used we obtain larger predicted mmi values in better agreement with the observations

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

occurrences of large magnitude earthquakes in the kachchh region gujarat western india tectonic implications abstract moderate to large damaging earthquakes in the peninsular part of the indian plate do not support the long standing belief of the seismic stability of this region the historical record shows that about 15 damaging earthquakes with magnitudes from 5 5 to 8 0 occurred in the indian peninsula most of these events were associated with the old rift systems our analysis of the 2001 bhuj earthquake and its 12 year aftershock sequence indicates a seismic zone bound by two linear trends nnw and nne that intersect an e w trending graben the bouguer gravity values near the epicentre of the bhuj earthquake are relatively low 2 mgal the gravity anomaly maps the distribution of earthquake epicentres and the crustal strain rate patterns indicate that the 2001 bhuj earthquake occurred along a fault within strain hardened mid crustal rocks the collision resistance between the indian plate and the eurasian plate along the himalayas and anticlockwise rotation of the indian plate provide the far field stresses that concentrate within a fault bounded block close to the western margin of the indian plate and is periodically released during earthquakes such as the 2001 m w 7 7 bhuj earthquake we propose that the moderate to large magnitude earthquakes in the deeper crust in this area occur along faults associated with old rift systems that are reactivated in a strain hardened environment

3d crustal resistivity structure beneath the wagad aftershock zone of the 2001 bhuj earthquake kutch india heterogeneous resistivity structure controlled by widespread fluid infiltration and clues to aftershocks pattern abstract the epicentral region of the 26 january 2001 bhuj earthquake 7 7 m w in the kutch region of india has been experiencing a vigorous and unique intraplate aftershock activity which shows a v shaped pattern and n ne migration along the western margin of the wagad uplift located to the northeast of the mainshock this study presents 3d resistivity structure of the wagad aftershock zone imaged using magnetotelluric mt data to understand the crustal geoelectric structure and subsurface geometry of faults lineaments which could provide critical inputs to evaluate and strengthen the earthquake generation mechanism and aftershocks pattern the obtained 3d conductivity structure showed a thick 3 km and homogeneous conductive 1 30 m layer of tertiary mesozoic sedimentary cover in the area a highly heterogeneous upper and middle crust characterized by a mix of conductive 300 m structures is imaged below the sedimentary cover enhanced conductivity zones showed spatial correlation with the surface trace of various faults lineaments and are interpreted as fluid enriched zones in the crust which play a critical role in earthquake triggering significantly higher fluid enrichment noticed in the crust below two minor faults khadir transverse fault ktf and manfara fault mf which are hitherto considered to have an insignificant role in the seismogenesis of the area the aftershock events seem to occur mostly on the resistive parts of the crust and its pattern shows good alignment with the anomalous conductive zones associated to the above two faults the results provide a strong clue to infer that the ktf and mf have a vital role in the observed aftershock activity and control the v shaped aftershocks pattern

cenozoic epeirogeny of the indian peninsula peninsular india is a cratonic region with asymmetric relief manifest by eastward tilting from the 1 5 km high western ghats escarpment toward the floodplains of eastward draining rivers oceanic residual depth measurements on either side of india show that this west east asymmetry is broader scale occurring over distances of 2 000 km admittance analysis of free air gravity and topography shows that the elastic thickness is 10 3 km suggesting that regional uplift is not solely caused by flexural loading to investigate how indian physiography is generated we have jointly inverted 530 river profiles to determine rock uplift rate as a function of space and time key erosional parameters are calibrated using independent geologic constraints e g emergent marine deposits elevated paleosurfaces uplifted lignite deposits our results suggest that regional tilt grew at rates of up to 0 1 mm a 1 between 25 ma and the present day neogene uplift initiated in the south and propagated northward along the western margin this calculated history is corroborated by low temperature thermochronologic observations by sedimentary flux of clastic deposits into the krishna godavari delta and by sequence stratigraphic architecture along adjacent rifted margins onset of regional uplift predates intensification of the indian monsoon at 8 ma suggesting that rock uplift rather than climatic change is responsible for modern day relief a positive correlation between residual depth measurements and shear wave velocities beneath the lithosphere suggests that regional uplift is generated and maintained by temperature anomalies of 100 c within a 200 25 km thick asthenospheric channel this article is protected by copyright all rights reserved

evidence for right lateral strike slip environment in the kutch basin of northwestern india from moment tensor inversion studies abstract the kutch region located in northwestern part of india is an ancient rift basin that was active until cretaceous period the region falls close to the india arabia and the india eurasia plate boundaries and has experienced devastating earthquakes in the past namely the 1819 allah bund earthquake the 1956 anjar earthquake and the 2001 bhuj earthquake to understand the tectonics of this region with respect to the adjacent plate boundaries we invert seismic waveform data of 11 earthquakes in this region recorded by a network of the institute of seismological research isr during 2007 2009 the study yields focal mechanism solutions of reverse fault and strike slip type mechanism the inferred fault planes correlate well with the local trends of the known tectonic faults while the principal stress directions derived from stress inversion based on a linearized least squares approach trend agreeably with the ambient stress field directions a consistently right lateral sense of shear is found on all the local faults as derived from each of the matching planes of the focal mechanism solutions computed in the present study it is inferred that in the kutch region a right lateral strike slip environment prevails along predominantly ew to nw se oriented deep seated pre existing faults in an otherwise compressive stress regime this in conjunction with the left lateral movements along the girnar mountain in southern saurashtra inferred from previous studies indicates a westward escape of the kutch saurashtra block as a consequence of the northward collision of the indian plate with respect to the eurasian landmass

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

characterization of earthquake induced ground motion from the l aquila seismic sequence of 2009 italy summary based only on weak motion data we carried out a combined study on region specific source scaling and crustal attenuation in the central apennines italy our goal was to obtain a reappraisal of the existing predictive relationships for the ground motion and to test them against the strong motion data peak ground acceleration pga peak ground velocity pgv and spectral acceleration sa gathered during the mw 6 15 l aquila earthquake 2009 april 6 01 32 utc the l aquila main shock was not part of the predictive study and the validation test was an extrapolation to one magnitude unit above the largest earthquake of the calibration data set the regional attenuation was determined through a set of regressions on a data set of 12 777 high quality high gain waveforms with excellent s n ratios 4259 vertical and 8518 horizontal time histories seismograms were selected from the recordings of 170 foreshocks and aftershocks of the sequence the complete set of all earthquakes with ml 3 0 from 2008 october 1 to 2010 may 10 all waveforms were downloaded from the iside web page http iside rm ingv it iside standard index jsp a web site maintained by the istituto nazionale di geofisica e vulcanologia ingv weak motion data were used to obtain a moment tensor solution as well as a coda based moment rate source spectrum for each one of the 170 events of the l aquila sequence 2 8 mw 6 15 source spectra were used to verify the good agreement with the source scaling of the colfiorito seismic sequence of 1997 1998 recently described by malagnini et al 2008 finally results on source excitation and crustal attenuation were used to produce the absolute site terms for the 23 stations located within 80 km of the epicentral area the complete set of spectral corrections crustal attenuation and absolute site effects was used to implement a fast and accurate tool for the automatic computation of moment magnitudes in the central apennines

crustal structure of the gujarat region india new constraints from the analysis of teleseismic receiver functions abstract in this study receiver function analysis is carried out at 32 broadband stations spread all over the gujarat region located in the western part of india to image the sedimentary structure and investigate the crustal composition for the entire region the powerful genetic algorithm technique is applied to the receiver functions to derive s velocity structure beneath each site a detail image in terms of basement depths and moho thickness for the entire gujarat region is obtained for the first time gujarat comprises of three distinct regions kachchh saurashtra and mainland in kachchh region depth of the basement varies from around 1 5 km in the eastern part to 6 km in the western part and around 2 3 km in the northern part to 4 5 km in the southern part in the saurashtra region there is not much variation in the depth of the basement and is between 3 km and 4 km in gujarat mainland part the basement depth is 5 8 km in the cambay basin and western edge of narmada basin in other parts of the mainland it is 3 4 km the depth of moho beneath each site is obtained using stacking algorithm approach the moho is at shallower depth 26 30 km in the western part of kachchh region in the eastern part and epicentral zone of the 2001 bhuj earthquake large variation in the moho depths is noticed 36 46 km in the saurashtra region the crust is more thick in the northern part it varies from 36 38 km in the southern part to 42 44 km in the northern part in the mainland region the crust is more thick 40 44 km in the northern and southern part and is shallow in cambay and narmada basins 32 36 km the large variations of poisson s ratio across gujarat region may be interpreted as heterogeneity in crustal composition high values of 0 30 at many sites in kachchh and few sites in saurashtra and mainland regions may be related to the existence of high velocity lower crust with a mafic ultramafic composition and locally to the presence of partial melt the existing tectono sedimentary models proposed by various researchers were also examined

paleontological view of the ages of the deccan traps the cretaceous tertiary boundary and the india asia collision volcanism in the deccan traps of india occurred over at most three magnetic chrons centered on a main reversed chron paleontological data indicate that this reversed chron must coincide with 29r which contains the cretaceous tertiary k t boundary recent 40 ar 39 ar data may therefore provide one of the best estimates of the absolute age of the k t boundary 65 7 2 0 ma moreover the eurasian character of k t boundary terrestrial faunas in india suggests that this was also close to the time of india asia collision this is significantly earlier than generally recognized and implies about 4000 km of continental shortening in the collision zone probably absorbed jointly by crustal thickening and strike slip extrusion

rupture process of the 26 january 2001 mw 7 6 bhuj india earthquake from teleseismic broadband data we investigate the rupture process of the 26 january 2001 bhuj india m w 7 6 earthquake through inversion of teleseismic broadband body waves this earthquake ranks as one of the most important recent events due to its occurrence within a stable continental interior where such events are rare the bhuj earthquake occurred on a moderately dipping blind thrust fault within an ancient failed rift about 70 of the seismic moment released in the earthquake was confined to a very small area 375 km2 surrounding the hypocenter and at depths below 12 km the static stress drop of the bhuj earthquake is anomalously high 20 mpa the source time history of the event indicates very rapid onset to the moment release and most likely high slip velocities within the deep asperity this suggests that some of the damage near the epicenter may have been caused by anomalously high frequency ground motions the teleseismic data also indicate the presence of a second area of large slip in the shallow part of the bhuj fault although the depth extent of this shallow large slip area is not resolved comparisons of the predicted ground motions with observed intensities suggest that substantial slip occurred in the upper 10 km of the fault in order to explain the distribution of high intensities to the west and northwest of the fault the upper surface layers near the bhuj fault consist of unconsolidated low rigidity sediments and alluvium the upper 10 km of the bhuj fault may therefore be in a conditionally stable region that normally deforms through aseismic creep and can sustain seismic rupture only when dynamically stressed by rupture of the high strength deep asperity we suggest that this deep asperity may be related to a lithologic anomaly of ultramafic composition

source parameters of the anjar earthquake of july 21 1956 india and its seismotectonic implications for the kutch rift basin abstract the kutch rift basin is a major seismic region in india characterized by a low earthquake occurrence rate but a very high maximum intensity the largest event in the region is the great kutch earthquake of 1819 estimated m w 7 8 however lacking instrumental data its exact source parameters are not well known the anjar or rann of kutch earthquake of july 21 1956 m w 6 0 took place very close to the 1819 kutch event and caused considerable damage and casualties our modeled waveforms of teleseismic p and sh waves and p wave first motions determine a reverse faulting mechanism strike 235 dip 46 5 and rake 85 as well as source parameters for the anjar earthquake a previous strike slip mechanism for this event has been eliminated on the basis of waveform constraints the two nodal planes of our solution both strike northeast sub parallel to the trend of the kutch rift very close to the trend of the isoseismals the seismic moment is 1 19 10 25 dyne cm the focal depth and the stress drop are 15 km and 162 bar respectively the anjar reverse fault may represent a reactivated normal fault undergoing tectonic inversion in the kutch rift zone under present day compressional stress two larger earthquakes adjacent to anjar but outside of the kutch rift are also found to be mainly reverse faulting with a minor strike slip component judging from available information the great kutch event of 1819 is most likely a reverse faulting event

a spectral analysis of the 21 may 1997 jabalpur india earthquake mw 5 8 and estimation of ground motion from future earthquakes in the indian shield region the jabalpur earthquake 23 08 n 80 06 e h 36 km is the first event in the indian peninsular shield region to be well recorded by a newly installed 10 station broadband seismographic network using these data we estimate q of lg wave in the indian shield region as q 508 0 48 1 20 hz the corrected source spectrum with m 5 4 1024 dyne cm reported in the harvard cmt catalog and an 2 source model requires a stress parameter p of 420 bar to explain high frequency spectral level the computed seismic energy from the records is 7 4 1020 erg which yields an apparent stress and brune stress drop of 62 and 270 bars respectively the analysis of the jabalpur earthquake provides some elements for the estimation of ground motions during future earthquakes in the indian shield region based on the results of the jabalpur earthquake and on studies of tectonically similar region of eastern north america we assume that 1 the sources follow an 2 model 2 s waves dominate at r 100 km and lg waves dominate at r 100 km 3 q f 508 0 48 4 the ground motion is a bandlimited finite duration gaussian white noise and 5 the effective duration of the ground motion equals 1 c 0 05 r where fc is the corner frequency we apply random vibration theory rvt to compute various measures of ground motion such as a max and v max at near source distances the source finiteness is approximately taken into account the attenuation curves for 5 5 mw 7 and for p of 100 400 bar are presented as expected the predicted values with p 420 bar agree reasonably well with the limited jabalpur data an a max of 150 gal is predicted in the epicentral region of this earthquake the predicted curves imply p 100 bar for the latur earthquake of 1993 mw 6 1 h 2 5 km to explain a max 1 g and the reported isoseismal intensities in the epicentral region for koyna earthquake of 1967 mw 6 3 the inferred a max and v max from isoseismal intensities and the recorded strong motions at the koyna dam site are in agreement with the prediction curves for p 100 bar the rvt predictions seem reasonable but need validation from more strong motion data which is presently lacking

size and orientation of the fault plane for the 2001 gujarat india earthquake mw7 7 from aftershock observations a high stress drop event 1 we used a small array of portable seismographs to determine aftershock locations of the 2001 gujarat earthquake our aftershock locations show a trend that dips toward the south at about 50 which is interpreted as the fault plane of the mainshock the depth range of the aftershocks is from 10 to 35 km which is somewhat deeper than other crustal earthquakes and indicates that the faulting did not reach the surface the area of the fault is about 40 40 km2 which is small for a mw7 7 earthquake and results in a high static stress drop of 13 to 25 mpa there are no mapped faults or obvious topographic features along the surface projection of this fault these findings show that very large damaging earthquakes can occur without producing surface faulting which is an important issue for earthquake hazard assessments in continental regions