an insight into crack density saturation rate and porosity model of the 2001 bhuj earthquake in the stable continental region of western india abstract the 2001 bhuj earthquake mw 7 6 source zone is examined in the light of crack density e saturation rate and porosity parameter using new data set derived from a large aftershock sequence recorded by the gujarat seismic network gsnet during november 2006 december 2009 processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset pattern of crustal heterogeneities associated with high e high and high anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by mishra and zhao 2003 the anomalous zone is found extended distinctly by 50 60 km in the lateral direction indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 bhuj earthquake in the source area it is inferred that the presence of a fluid filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone anomalous pattern of high e with wider distribution of high indicates the existence of micro cracks in the lower crust while high suggests the cementation of cracks through permeation of residual magma metamorphic fluids into the hypocenter zone the results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences

crust beneath the northwestern deccan volcanic province india evidence for uplift and magmatic underplating the northwestern deccan volcanic province of india and its pericratonic rift basins were reactivated during different stages after the breakup of india from the gondwanaland and collision with the asian plate in this study we present results of crustal thickness and average crustal vp vs ratios beneath this plume affected region using common conversion point imaging and h k stacking analysis of 6893 receiver functions using data from a network comprising 58 broadband seismic stations sited on diverse tectonic terrains we find large variations in crustal thickness with the moho depths varying from 28 to 43 km in the kachchh rift 28 to 38 km in the cambay rift 39 5 41 5 km in the north and eastern parts of the cambay rift and 29 to 39 km in the saurashtra region and south gujarat a moho upwarp of 6 to 7 km in the saurashtra region can be attributed to positive buoyancy and uplift due to thermal influx affected by the reunion plume high crustal vp vs ratios beneath the kachchh rift 1 8 to 2 05 coastal areas of saurashtra 1 75 to 2 06 and north gujarat 1 81 to 1 85 indicate dominance of a mafic ultramafic crust high regional heat flow high electrical conductivity large intracrustal s wave velocity reduction and high average crustal poisson s ratios are consistent with partial melt related to the process of magmatic underplating in the lower crust at other stations the crust appears to be felsic with vp vs ratios in the range of 1 57 to 1 76

structural attributes and paleostress analysis of quaternary landforms along the vigodi fault vf in western kachchh region abstract in this paper we elucidated deformational and kinematic characteristics of the nw se trending vigodi fault vf and its associated subsidiary faults in the western kachchh basin we applied paleostress inversion technique to understand the past stress regime from the acquired fault slip data the subsidiary faults strike nw se and ne sw and slickensided surfaces are well developed on the fault planes we present slip analysis and changes in the stress reorientation using the plane solutions to identify the principal stress axis 1 position regionally to obtain pressure tension and null axis pbt stress ratio and principal 1 axis we used the right dihedron method using the win tensor and t tecto software s respectively geomorphologically the vf zone comprises of bedrock strath terraces towards the upthrown block and fill terraces on the downthrown block of the fault development of fault scarps warped quaternary sediments slit canyons within quaternary sediments river nick points and deflected rivers are the major striking features observed along the strike length of the vigodi fault the analyses indicate that ne sw compressional stress dominates the regional area and the local area is affected by transpressional stress exhibiting oblique slip motion the combined observations made from the present investigation can be used to estimate seismic hazard and mitigation planning of the study area

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

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

self organized fractal seismicity and b value of aftershocks of the 2001 bhuj earthquake in kutch india the devastating intraplate earthquake of m w 7 7 of 26 january 2001 took place along the south dipping reverse fault in the lower crust 23 km of kutch gujarat india obliterating some 14 000 people the aftershock activity has ensued for three years we analyzed 997 aftershocks of m 3 0 to study the b value and fractal correlation dimensions in time and space the b value is found to be 0 8 0 03 from the gutenberg richter relation and 0 76 0 02 from the maximum likelihood suggesting a typical value for the intraplate region the spatial correlation is 1 71 0 02 indicating that events are approaching a two dimensional region further the temporal correlation dimension is estimated to be 0 78 0 02 confirming the structure is mono fractal in time domain the depth section of b value shows a peak at 15 35 km depth range coinciding with the maximum occurrence of aftershocks 47 which is inferred as a fluid filled highly fractured rock matrix with fractures of high density it will be important to note that tomographic results also suggest a low vp low vs and a large poisson s ratio for the same depth range further confirming this inference additionally we have studied the variation of d2 s and b value with time during the first two months of aftershock activity the results show a marked negative correlation between spatial correlation dimension d2 large and b value low indicating the predominance of large events associated with weak clustering the negative correlation means the stress release along faults of a larger surface area after two months the fractal dimension d2 s and b value suggests a positive correlation implying more numerous smaller shocks with stress release along faults of a smaller surface area this would indicate a reduced probability of large magnitude earthquakes due to fragmentation of the fault zone

seismic imaging of the 2001 bhuj mw7 7 earthquake source zone b value fractal dimension and seismic velocity tomography studies abstract we use precisely located aftershocks of the 2001 mw7 7 bhuj earthquake 2001 2009 to explore the structure of the kachchh seismic zone by mapping the 3 d distributions of b value fractal dimension d and seismic velocities from frequency magnitude analysis we find that the catalog is complete above mw 3 0 thus we analyze 2159 aftershocks with mw 3 0 to estimate the 3 d distribution of b value and fractal dimensions using maximum likelihood and spatial correlation dimension approaches respectively our results show an area of high b d and vp vs ratio values at 15 35 km depth in the main rupture zone mrz while relatively low b and d values characterize the surrounding rigid regions and gedi fault gf zone we propose that higher material heterogeneities in the vicinity of the mrz and or circulation of deep aqueous fluid volatile co2 is the main cause of the increased b d and vp vs ratio values at 15 35 km depth seismic velocity images also show some low velocity zones continuing in to the deep lower crust supporting the existence of circulation of deep aqueous fluid volatile co2 in the region probably released from the eclogitasation of olivine rich lower crustal rocks the presence of number of high and low velocity patches further reveals the heterogeneous and fractured nature of the mrz interestingly we observe that aki 1981 s relation d 2b is not valid for the spatial b d correlation of the events in the gf d2 1 2b zone however the events in the mrz d2 1 7b show a fair agreement with the d 2b relationship while the earthquakes associated with the remaining parts of the aftershock zone d2 1 95b show a strong correlation with the aki 1981 s relationship thus we infer that the remaining parts of the aftershock zone are probably behaving like locked un ruptured zones where larger stresses accumulate we also propose that deep fluid involvement may play a key role in generating seismic activity in the mrz therefore a similar episode of deep fluid inflow could play a crucial role in causing the next strong earthquakes in the surrounding blocks of nwf

magnetotelluric studies in the epicenter zone of 2001 bhuj earthquake abstract some earthquakes to mw5 level are occurring along two faults viz kachchh mainland fault and south wagad fault near the epicenter of mw7 7 bhuj earthquake of 2001 these faults are not the causative fault of the 2001 earthquake and suspect for future large earthquake in absence of detail geophysical surveys the sub surface orientation of these faults was not known and mt profiles have proved useful in delineating the subsurface trends two mt profiles having lengths of 24 km and 15 km with an inter station spacing of 1 3 km are taken about 10 km east and west of the epicenter of mw7 7 2001 bhuj earthquake respectively the profiles are in nearly n s direction to detect the faults which trend e w in the area from 2 d inversion of mt data of 23 stations it is found that there are two distinctive conductive features matching with the south wagad fault and kachchh mainland fault south of it both being about 10 km apart the south wagad fault is seen extending to banni low lying area 30 km west of the present trace

low coda qc in the epicentral region of the 2001 bhuj earthquake of mw 7 7 on 26 january 2001 03 46 55 ut a devastating intraplate earthquake of m w 7 7 occurred in a region about 5 km nw of bhachau gujarat 23 42 n 70 23 e the epicentral distribution of aftershocks defines a marked concentration along an e w trending and southerly dipping 45 zone covering an area of 60 40 km2 the presence of high seismicity including two earthquakes of magnitudes exceeding 7 7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the kutch area a site lying in the seismic zone v zone of the highest seismicity for potentially m8 earthquakes on the seismic zoning map of india attenuation property of the medium around the epicentral area of the bhuj earthquake covering a circular area of 61 500 km2 with a radius of 140 km is studied by estimating the coda q c from 200 local earthquakes of magnitudes varying from 3 0 4 6 the estimated q 0 values at locations in the aftershock zone high seismicity are found to be low in comparison to areas at a distance from it this can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone distribution of q 0 values suggests that the local variation in q 0 values is probably controlled by local geology the estimated q 0 values at different stations suggest a low value of q 102 0 80 f 0 98 0 02 indicating an attenuative crust beneath the entire region the frequency dependent relation indicates a relatively low q c at lower frequencies 1 3 hz that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and or intrinsic attenuation due to fluid movement in the fault zone and fluid filled cracks the large q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected based on the attenuation curve estimated for q 0 102 the ground acceleration at 240 km distance is 13 of 1 g i e 0 13 g agreeing well with the ground acceleration recorded by an accelerograph at ahmedabad 0 11 g hence it is inferred that the q 0 value obtained from this study seems to be apt for prediction of ground motion for the region

sinking mafic body in a reactivated lower crust a mechanism for stress concentration at the new madrid seismic zone we propose a geodynamic model for stress concentration in the new madrid seismic zone nmsz the model postulates that a high density mafic body situated in the deep crust directly beneath the most seismically active part of the nmsz began sinking several thousands of years ago when the lower crust was suddenly weakened based on the fact that deformation rates in the nmsz have accelerated over the past 9 k y we envision the source of this perturbation to be related to the last north american deglaciation excess mass of the mafic body exerts a downward pull on the elastic upper crust leading to a cycle of primary thrust faulting with secondary strike slip faulting after which continued sinking of the mafic body reloads the upper crust and renews the process this model is consistent with the youth of activity the generation of a sequence of earthquakes and the velocity evolution during interseismic periods which depend upon the density contrast of the mafic body with respect to the surrounding crust its volume and the viscosity of the lower crust

results from local earthquake velocity tomography implications toward the source process involved in generating the 2001 bhuj earthquake in the lower crust beneath kachchh india to comprehend the source processes of the 26 january 2001 bhuj earthquake sequence of m w 7 7 and its influence on the seismic hazard of the kachchh we estimate various seismological parameters using reliable and accurate aftershock data the estimated parameters led to several important findings including the delineation of an east west trending south dipping 45 fault north wagad fault nwf which touches the surface about 25 km north of kachchh mainland fault kmf the aftershock zone is confined to a 60 km long and 40 km wide region lying between the kmf to the south and nwf to the north extending from 10 to 45 km depth focal mechanism solutions of the mainshock and 25 significant aftershocks of m w 3 0 obtained from waveform inversion of broadband data and local earthquake moment tensor inversion suggest that the region between the kmf and nwf is mainly characterized by reverse faults with east west trend and southerly dip matching with the geological faults in the region the tomographic inversion technique is used to invert 5516 p wave travel times and 4061 s p travel time differences from 600 aftershocks recorded at 8 18 stations tomographic results suggest a regional high velocity body characterized by high v p 7 0 8 5 km sec high v s 4 0 4 8 km sec and low 0 24 0 26 with a head extending 60 km in north south and 40 km in east west at 10 40 km depths this high velocity anomaly is inferred to be a mafic pluton rift pillow which might have intruded during the rifting time 135 ma this crustal mafic pluton must be contributing significantly in accumulating large crustal stresses resulting in the generation of large earthquakes in this intraplate area another important result of our study is the detection of a low velocity zone low v p 6 5 7 0 km sec low v s 3 6 4 0 km sec large 0 26 0 265 within the mafic body at the hypocentral depth of the mainshock 18 25 km which is inferred to be a fluid filled trapped aqueous fluid resulting from metamorphism fractured rock mass the analysis of depth distribution of b values suggests a high b value zone between 15 and 25 km depths which further supports this contention hence the presence of fluids at the hypocenter might have facilitated the occurrence of the 2001 bhuj earthquake within the inferred mafic body in the lower crust

upper mantle seismic anisotropy in the intra continental kachchh rift zone gujarat india abstract shear wave splitting study of 411 sks skks phases covering backazimuth range of 13 to 305 recorded by 12 broadband stations in the kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements the average vector mean of fast axis orientation 86 14 corresponds to the e w axis of the kachchh rift and the delay time 1 6 s is attributed to the 184 km thick upper mantle layer with 4 anisotropy the anisotropic character observed for the kachchh rift kr is comparable to other continental rifts and these are related to the high temperature lattice preferred orientation fabric of olivine inherited from the mantle flows the source of the rift axis parallel anisotropy is traced to the rift parallel flows within the 76 6 km thick lithosphere additionally the rift parallel pockets of partial melts also induce anisotropy within the asthenosphere both these are inherited from the plume lithosphere interaction during the deccan reunion plume episode 65 ma

magnetotellurics study to identify subsurface resistivity structure in the eastern part of kachchh little rann area of gujarat india a magnetotelluric mt survey has been conducted in the eastern part of the kachchh region of gujarat state crossing little rann starting from adesar village in the north to enjar village in the south where limited geophysical data is available the profile has been acquired in the n s direction having a total length of 52 km with the interstation spacing of 3 4 km from two dimensional 2d inversion of the mt data four conductive zones are identified the analysis reveals that the depth of the basement varies from 1 2 km in the south to 1 8 km in the north the first conductive zone is found at a distance of 3 km in the south of adesar and is identified at the location of the kanmer fault kf the eastern extension of south wagad fault swf and might be the kf the second conductive zone is found at a distance of 42 km in the south of adesar and 13 km north of enjar village and is located at the contact zone of kachchh and saurashtra peninsula at the location of the north kathiawar fault nkf it is inferred as the nkf the third conductive zone at a distance of 27 km in the south of adesar is found between the first and second conductor this is interpreted as a step fault formed during the rifting process of kachchh the lower crust below 20 km is found conductive 120 m and might indicate the presence of fluid filled mafic ultramafic rocks in the lower crust

estimation of site response in kachchh gujarat india region using h v spectral ratios of aftershocks of the 2001 mw 7 7 bhuj earthquake site response in the aftershock zone of 2001 bhuj mw 7 7 earthquake has been studied using the h v spectral ratio method using 454 aftershocks mw 2 5 4 7 recorded at twelve three component digital strong motion and eight three component digital seismograph sites the mean amplification factor obtained for soft sediment sites quaternary tertiary varies from 0 75 6 03 times for 1 3 hz and 0 49 3 27 times for 3 10 hz the mean amplification factors obtained for hard sediment sites hard jurassic mesozoic sediments range from 0 32 3 24 times for 1 3 hz and 0 37 2 18 times for 310 hz the upper bounds of the larger mean amplification factors for 1 3 hz are found to be of the order of 3 13 6 03 at chopadwa vadawa kavada vondh adhoi jahwarnagar and gadhada whereas the upper bounds of the higher mean amplification factors at 3 10 hz are estimated to be of the order of 2 00 3 27 at tapar chopadwa adhoi jahwarnagar gandhidham and khingarpur the site response estimated at bhuj suggests a typical hard rock site behavior preliminary site response maps for 1 3 hz and 310 hz frequency ranges have been prepared for the area extending from 23 23 85 n and 69 65 70 85 e these frequency ranges are considered on the basis of the fact that the natural frequencies of multi story buildings 3 to 10 floor range between 1 3 hz while the natural frequencies for 1 to 3 story buildings vary from 3 10 hz the 1 3 hz map delineates two distinct zones of maximum site amplification 3 times one lying in the nw quadrant of the study area covering jahwarnagar kavada and gadadha and the other in the se quadrant of the study area with a peak of 6 03 at chopadwa covering an area of 70 km 50 km while the 3 10 hz map shows more than 2 times site amplification value over the entire study area except ne quadrant two patches in the southwest corner covering bhuj and anjar and one patch at the center covering vondh manfara and sikara the zones for large site amplification values 3 times are found at tapar chopadwa adhoi and chobari the estimated site response values show a good correlation with the distribution of geological formations as well as observed ground deformation in the epicentral zone

crustal structure of the indian shield new constraints from teleseismic receiver functions teleseismic earthquake waveform data from 10 broadband stations spread over the indian shield and in operation since 1997 were analyzed to infer the crustal structure using the receiver function technique the south indian shield is characterized by a 33 39 km thick and remarkably simple crust with an average poisson s ratio close to 0 25 the archaean crust is devoid of any prominent intra crustal discontinuities the velocity contrast at the well developed moho is large resulting in very clear p to s conversions as well as first order multiples in contrast the predominantly proterozoic crust in the northern part of the shield exhibits a complex character due to the presence of additional seismic discontinuities moho conversions which are considerably weaker compared to the archaean terrains indicate crustal thicknesses of more than 40 km

mapping active magma chambers by b values beneath the off ito volcano japan the b value of the frequency magnitude relation and thus the mean magnitude in the off ito volcanic area is not uniform based on detailed b value tomography using about 10 000 events in an area of 10 km radius and in the upper 15 km of the crust a high b value anomaly b 15 below 7 km depth and with a radius of about 2 km located below the coast southeast of ito contrasts with lower values of typically b 0 7 north of it and at shallower depths on the basis of surface deformations a tensile crack dislocation source was located above 7 km depth thus we surmise that the magma chamber in the off ito area was located below 7 km depth the correlation of a high b value anomaly with this model supports our hypothesis that active magma chambers may be mapped by high b value anomalies the magma body may be described as consisting of two parts a lower well established chamber below 11 km depth and a shallower part 11 7 km depth the b value in the volume surrounding the magma chamber increased from 0 7 during the 1980s to 1 5 during the earthquake swarms of the 1990s this may reflect a trend of increasing crack density due to the intrusion activity

estimation of source parameters for the aftershocks of the 2001 mw 7 7 bhuj earthquake india the source parameters for 213 bhuj aftershocks of moment magnitude varying from 2 16 to 5 74 have been estimated using the spectral analysis of the sh waveform on the transverse component of the three componnet digital seismograms as well as accelerograms the estimated stress drop values for bhuj aftershocks show more scatter mo0 5 to 1 toward the larger seismic moment values log mo 1014 5 n m larger aftershocks whereas they show a more systematic nature mo3 for smaller seismic moment log mo 15 mpa lying in 22 26km depth range beneath the region we suggest that the concentration of large stress drop values at 10 36km depth may be related to the large stress strain associvated with a brittle competent intrusive body of mafic nature

coulomb static stress variations in the kachchh gujarat india implications for the occurrences of two recent earthquakes mw 5 6 in the 2001 bhuj earthquake region summary double difference algorithm was used to relocate aftershock sequences of two mw 5 6 kachchh earthquakes india which have occurred on 2006 march 7 and april 6 along almost vertical strike slip gedi fault gf and south dipping reverse north wagad fault nwf respectively the relocated focal depths delineate a marked variation of 4 and 7 km in the brittle ductile transition depths beneath gf and nwf respectively we modelled the coulomb failure stress change cfs produced by the 2001 mainshock and 2006 gedi events on an optimally oriented plane a strong correlation with occurrences of aftershocks and regions of increased cfs is obtained on both the faults the predicted cfs on the gf increased by 0 14 mpa at 3 km depth where the 2006 march 7 mw 5 6 event occurred the variation in brittle ductile transition depths may be related to the perturbation in geothermal gradient resulted from the presence of intrusives or varying saturation state of fluid filled fractured rock matrix beneath both the fault zones

crustal seismic structure beneath the deccan traps area gujarat india from local travel time tomography abstract the gujarat region in western india is known for its intra plate seismic activity including the mw 7 7 bhuj earthquake a reverse faulting event that reactivated normal faults of the mesozoic kachchh rift zone the late cretaceous deccan traps one of the largest igneous provinces on the earth cover the southern part of gujarat this study is aimed at bringing light to the crustal rift zone structure and likely origin of the traps based on the velocity structure of the crust beneath gujarat tomographic inversion of the gujarat region was done using the non linear passive source tomographic algorithm lotos we use high quality arrival times of 22 280 p and 22 040 s waves from 3555 events recorded from august 2006 to may 2011 at 83 permanent and temporary stations installed in gujarat state by the institute of seismological research isr we conclude that the resulting high velocity anomalies which reach down to the moho are most likely related to intrusives associated with the deccan traps low velocity anomalies are found in sediment filled mesozoic rift basins and are related to weakened zones of faults and fracturing a low velocity anomaly in the north of the region coincides with the seismogenic zone of the reactivated kachchh rift system which is apparently associated with the channel of the outpouring of deccan basalt

three dimensional gravity modelling of kutch region india abstract understanding the crustal architecture of kutch rift region india is important due to the presence of thick mesozoic sediments and also due to occurrence of earthquakes we present here three dimensional gravity models the major features of our shallow crustal model are 1 thickness of deccan traps is almost negligible towards northern side of vigodi fault and katrol hill fault 2 mesozoic sediment thickness is about 3 km on the southern side and 3 several undulations in the basement on the wagad uplift the model is consistent with the geological observation that the basin slopes towards southwest the observed bouguer gravity anomalies are explained by 10 km thick granitic upper crust of 2 7 g cm 3 10 km thick intermediate mid crust of 2 82 g cm 3 15 km thick mafic lower crust of 2 91 g cm 3 density crustal thickness is about 40 45 km on wagad uplift and near katrol hill fault the modelled gravity data do not require presence of anomalous amount of fluids melts and or magma chambers in the deep crust of kutch region

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

electrical conductance map for the kachchh rift basin constraint on tectonic evolution and seismotectonic implications geomagnetic field variations recorded by an array of magnetometers spread across the kachchh rift basin are reduced to a set of induction arrows as a diagnostic of lateral electrical conductivity variations a non uniform thin sheet electrical conductance model is developed to account for the salient induction patterns it indicates that the imaged conductivity anomalies can be related to the sediment filled structural lows in between the fault bounded uplifts it is suggested that sagging structural lows preserved the marine sediments deposited during the mesozoic sea transgression and later developed into first order embayment basins for the deposition of sediments in association with late eocene transgression depth integrated electrical conductance helped in mapping two depo centres along the ene wsw trending banni half graben bounded by the kachchh main fault on the south and second along the vinjan depression formed in response to the subsidence between the vigodi fault and westward extension of the katrol hill fault together with the westward bending of the median high presence of metamorphosed graphite schist clasts in shale dominated mesozoic sequence and or thin films of carbon resulting from the thermal influence of deccan activity on carbonate rich formations can account for the high electrical conductivity anomalies seen in the depo centres of thick mesozoic and tertiary sediments additionally two high conductivity zones are imaged encompassing a block defined by the 2001 bhuj earthquake and its aftershocks in agreement with gravity magnetic and seismic velocity signatures aqueous fluids released by recrystallizing magmatic bodies intruded in association with deccan trap activity account for mapped high conductivity zones high fluid pressure in such a fractured domain surrounding the intruded magmatic plugs perturb the regional stress concentrations to produce frequent and low magnitude aftershocks in the shallow section of the epicentral track of the 2001 bhuj earthquake

damage due to the mw 7 7 kutch india earthquake of 2001 abstract this paper presents a study of the damage due to the m w 7 6 7 7 intraplate kutch earthquake of 26 january 2001 it was a powerful earthquake with a high stress drop of about 20 mpa aftershocks up to m 4 have continued for 2 5 years the distribution of early aftershocks indicates a rupture plane of 20 25 km radius at depths of 10 45 km along an e w trending and south dipping hidden fault situated approximately 25 km north of the kutch mainland fault the moment tensor solution determined from regional broadband data indicates reverse motion along a south dipping by 47 fault the earthquake is the largest event in india in the last 50 years and the most destructive in the recorded history in terms of socioeconomic losses with 13 819 deaths including 14 in pakistan collapse severe damage of over a million houses and us 10 billion economic loss surface faulting was not observed however intense land deformations have been observed in a 40 20 km meizoseismal area these include lateral spreading ground uplifts about a meter ground slumping and deep cracks liquefaction with ejection of sand and copious water was widespread in the banni grassland rann areas salt plains along rivers and also in the coastal areas up to 200 km distance from the epicenter in areas of intensity vii to x stray incidences of liquefaction have occurred up to distances of at least 300 km for the first time in india multistory buildings have been destroyed damaged by an earthquake the maximum acceleration is inferred to be 700 cm s 2 and intensities are 1 3 units higher in soil covered areas than expected from the decay rate of acceleration for hard rock

imaging upper mantle discontinuity topography using underside reflection data this paper presents a method to invert underside reflection p d p or s d s arrivals data for lateral depth variations of upper mantle discontinuities combining traveltime and amplitude data the method greatly improves the resolution of small scale undulations obtained by existing imaging methods and does not suffer from the long wavelength biases that are likely to be present in currently available models existing inversion methods account for the large size of the fresnel zone of underside reflections but not for its complexity arising from the mini max traveltime nature of pp and ss related waves this neglect results in long wavelength artefacts from small scale undulations of the discontinuities obscurring true long wavelength depth variations the inversion method presented in this paper uses a complex valued sensitivity kernel derived from the representation of underside reflections through a kirchhoff integral formulation the sensitivity kernel accounts for the varying sensitivity of the waveforms to discontinuity structure over the fresnel zone the method is applied to a large synthetic data set the data set consists of p d p amplitudes and traveltimes the results show that the new inversion method resolves depth variations on a lateral scale that is smaller than the size of the fresnel zone of individual underside reflections but larger than the dominant wavelength retaining the resolution of large scale variations the results presented here suggest that the discontinuity depth variations induced by slab penetration of the 670 discontinuity could be resolved by current broad band p670p data sets

numerical modeling of seismicity and geodynamics of the kachchh rift zone gujarat india abstract the numerical block and fault model of lithosphere dynamics and seismicity bafd is used to understand crustal motion and features of the observed seismicity in the kachchh rift zone gujarat western india the block model allows simulating seismicity and geodynamics simultaneously unlike other modeling approaches for studying seismicity or geodynamics the model structure of kachchh rift zone is composed of seven major crustal blocks separated by fault planes based on the orientation of boundary crustal block movements we develop a set of numerical experiments to analyze the spatial distribution of earthquakes frequency to magnitude relationships earthquake focal mechanisms velocity field and fault slip rates in the model the main results of our modeling suggest that an nnw sse trending compression is a principal driving force in the kachchh rift zone that explains basic features of the regional seismicity direction of block motions and the presence of an extensional stress regime associated with the cambay rift zone large synthetic events occur on the fault segments associated with the allah bund fault katrol hill fault and north wagad fault which have been causative faults for the 1819 mw7 7 allah bund 1956 mw6 0 anjar and 2001 mw7 7 bhuj earthquakes the frequency magnitude distribution for both synthetic seismicity and observed seismicity shows a similar slope the focal mechanisms of the synthetic events are found to be consistent with those of earthquakes in the region a special attention has been paid to study long term and post seismic deformations our results are in a qualitative agreement with the gps post seismic observations in the kachchh rift zone we infer that the observed seismicity and crustal block motions are a consequence of the dynamics of the entire regional fault and block system rather than that of a single causative fault only

sediment thicknesses and qs vs qp relations in the kachchh rift basin gujarat india using sp converted phases delineation of the top sedimentary structure and its qs vs qp relationship using the travel time difference of direct s and converted sp phase is key to understanding the seismic hazard of any sedimentary basin area we constructed filtered displacement waveforms from local etna episensor acceleration recordings as well as local velocity recordings of aftershocks of the 2001 bhuj earthquake recorded by the kachchh seismological network of the national geophysical research institute ngri hyderabad india during 2001 2004 stations are within 15 70km of epicenters and the resulting displacement waveforms are generally simple displaying prominent p sp and s wave pulses particle motion of p and s waves suggest near vertical raypaths consistent with preliminary depth estimates the direct s wave on the horizontal component is characterized by lower frequency content than the converted sp phase on the vertical component this difference in frequency content between s and sp phases can be explained in terms of different attenuation effects for p and s waves in the unconsolidated sediments the sp phase is generated by s to p phase conversion at the base of mesozoic sediments of the kachchh basin travel time inversion velest of 2565 p and 2380 s arrivals from 658 well located aftershocks recorded at 8 14 three component local seismic stations led to 1 d velocity models indicated very slow sediments in the upper 0 2 km depth range vp 2 92 km s and vs 0 90 km s and an increasing trend of velocities with depth at 2 40 km depth the estimated sediment thicknesses beneath 12 accelerograph and 6 seismograph sites from the estimated velocity model and the travel time difference between s and converted sp phases reaches a maximum of 1 534 0 117 km beneath bandri near the location of 2001 bhuj mainshock and attains a minimum sediment thickness of 0 858 0 104 km beneath ramvav and burudia the spectral ratios between sp and s from 159 three component accelerograms have been used to study seismic wave attenuation in the kachchh rift basin the estimated qs vs qp relations for 12 accelerograph sites vary from qs 0 184 qp at chobari to qs 0 505 qp at dudhai for stations chobari chopdwa jahawarnagar vondh and tapar the spectral ratio slopes and hence the calculated qs vs qp relations are effectively the same and the correlation coefficients are quite high 0 91 0 93 stations adhoi manfara new dudhai dudhai and sikara have similar qs vs qp relationships to each other and also have high correlation coefficients 0 78 0 87 the spectral ratios for stations anjar and ramvav are small and poorly constrained resulting in less reliable qs vs qp relations this could be due to noisy data fewer available waveforms or scattering due to velocity heterogeneities and or interface irregularities

a local magnitude scale ml for the kachchh rift basin an active intraplate region gujarat indiaa local magnitude scale ml for the kachchh rift basin

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

crustal heterogeneity in bulk velocity beneath the 2001 bhuj earthquake source zone and its implications in order to resolve an enigmatic issue relating to the existence of fluid related or temperature related anomalies at the mainshock hypocenter of the 2001 bhuj india earthquake m w 7 6 an estimate of the 3d bulk sound velocity structure is made from the inverted high quality p and s arrival times from a total of 368 aftershocks recorded by 12 temporary seismic stations installed following the 2001 bhuj earthquake results reveal strong lateral and vertical heterogeneity in bulk velocity graphic 1 beneath the source zone the 2001 bhuj mainshock and its aftershock source zones are associated with anomalously high bulk sound velocity high graphic 2 indicating high bulk elastic strength of the source rocks at the mainshock hypocenter due to high pore pressures of the fully saturated cracked rocks associated with solute precipitation through the processes of acoustic fluidization and cementation the interpretation of bulk velocity tomograms suggests that the processes of mineral dehydration and permeation of sea surface water through several active quaternary faults down to the deep crust might have contributed to in situ fluid related material heterogeneity in bulk velocity within the fluid filled fractured rock matrix in the paleorift zone at the 2001 bhuj mainshock hypocenter which in turn increased pore pressure lowered the effective stress and brought the system into a brittle failure high graphic 3 in the intersecting fault geometry in the fractured rock matrix at the mainshock hypocenter can be taken as evidence for the strong role of fluids in association with the intraplate earthquake of the indian peninsula 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif

tomography of the source area of the 1995 kobe earthquake evidence for fluids at the hypocenter seismic tomography revealed a low seismic velocity 5 and high poisson s ratio 6 anomaly covering about 300 square kilometers at the hypocenter of the 17 january 1995 magnitude 7 2 kobe earthquake in japan this anomaly may be due to an overpressurized fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake

seismicity analysis of the kachchh aftershock zone and tectonic implication for 26 jan 2001 bhuj earthquake abstract we have carried out a detailed analysis of seismicity in the vicinity of the 26 jan 2001 bhuj earthquake mw 7 7 from the depth sections of 24 parallel profiles and from the b value cross section we claim the existence of a hidden fault which conjugates to the major rupture fault i e north wagad fault of the 2001 bhuj earthquake the proximity of the intersection of these faults to the focus suggests a close association bhuj main shock generation a circular pattern in the profiles also provides evidence for the existence of an intrusive consistent with earlier findings from gravity magnetic modeling and tomography studies the location of the fault intersection within the intrusive support a model where both play a significant role in the earthquake generation the intersection of the conjugate faults acts as a stress concentrator while their presence within a big pluton possibly will facilitate the stress amplification this mechanism might explain the occurrence of two mw 7 7 earthquakes in a relatively short time span of 182 years in the kachchh rift the b value cross section displays high and low b value patches along the two intersecting faults this suggests a model of a faulted block that consists of two kinds of segment the locked and the unlocked locked segments do not easily participate in creeping and therefore generate strong magnitude aftershocks m 3 while unlocked segments easily creep and result in only weak aftershocks m

a new insight into crustal heterogeneity beneath the 2001 bhuj earthquake region of northwest india and its implications for rupture initiations the seismic characteristics of the 2001 bhuj earthquake mw 7 6 has been examined from the proxy indicators relative size distribution 3d b value mapping and seismic tomography using a new data set to understand the role of crustal heterogeneities in rupture initiations of the 2001 bhuj earthquake of the gujarat india one of the disastrous indian earthquakes of the new millennium the aftershocks sequence recorded by 22 seismograph stations of gujarat seismic network gsnet during the period from 2006 to 2009 encompassing approximately 80 km 70 km rupture area had revealed clustering of aftershocks at depth of 5 35 km which is seismogenic layer responsible for the occurrence of continued aftershocks activity in the study region the 3d b value mapping estimated from a total of 3850 precisely located aftershocks with magnitude of completeness mc 2 7 shows that a high b value region is sandwiched within the main shock hypocenter at the depth of 20 25 km and low b value region above and below of the 2001 bhuj main shock hypocenter estimates of 3 d seismic velocity vp vs and poisson s ratio structure beneath the region demonstrated a very close correspondence with the b value mapping that supports the similar physicochemical processes of retaining fluids within the fractured rock matrix beneath the 2001 bhuj mainshock hypocenter the overall b value is estimated close to 1 0 which reveals that seismogenesis is related to crustal heterogeneity which in turn also supported by low vs and high structures the high b value and high anomaly at the depth of 20 25 km indicate the presence of highly fractured heterogeneous rock matrix with fluid intrusions into it at deeper depth beneath the main shock hypocenter region low b value and high vp in the region is observed towards the north east and north west of the main shock that might be an indication of the existence of relatively competent rock masses with negligible volume of cracks that may have contained over pressurized fluids without molten rocks

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

relocation of early and late aftershocks of the 2001 bhuj earthquake using joint hypocentral determination jhd technique implication toward the continued aftershock activity for more than four years we employed layered model joint hypocentral determination jhd with station corrections to improve location identification for the 26 january 2001 mw 7 7 bhuj early and late aftershock sequence we relocated 999 early aftershocks using the data from a close combined network national geophysical research institute india and center for earthquake research institute usa of 8 18 digital seismographs during 12 28 february 2001 additionally 350 late aftershocks were also relocated using the data from 4 10 digital seismographs accelerographs during august 2002 to december 2004 these precisely relocated aftershocks error in the epicentral location 30 meter error in the focal depth estimation 50 meter delineate an east west trending blind thrust north wagad fault nwf dipping 45 southward about 25 km north of kachchh main land fault kmf as the causative fault for the 2001 bhuj earthquake the aftershock zone is confined to a 60 km long and 40 km wide region lying between the kmf to the south and nwf to the north extending from 2 to 45 km depth estimated focal depths suggest that the aftershock zone became deeper with the passage of time the p and s wave station corrections determined from the jhd technique indicate that the larger values both ve and ve characterize the central aftershock zone which is surrounded by the zones of smaller values the station corrections vary from 0 9 to 1 1 sec for the p waves and from 0 7 to 1 4 sec for the s waves the b value and p value of the whole aftershock 2001 2004 sequences of mw 3 are estimated to be 0 77 0 02 and 0 99 0 02 respectively the p value indicates a smaller value than the global median of 1 1 suggesting a relatively slow decay of aftershocks whereas the relatively lower b value less than the average b value of 1 0 for stable continental region earthquakes of india suggests a relatively higher probability for larger earthquakes in kachchh in comparison to other stable continental regions of the indian peninsula further based on the b value mainshock magnitude and maximum aftershock magnitude the bhuj aftershock sequence is categorized as the mogi s type ii sequence indicating the region to be of intermediate level of stresses and heterogeneous rocks it is inferred that the decrease in p value and increase in aftershock zone both spatially as well as depth over the passage of time suggests that the decay of aftershocks perhaps could be controlled by visco elastic creep in the lower crust

relocations and local earthquake tomography implications toward the mafic pluton induced crustal seismicity in kachchh gujarat india for last 18 years abstract the study presents new probable crustal model based on precise double difference relocations of 2420 aftershocks of the 2001 bhuj earthquake mw7 7 during 2001 16 and combining other geophysical and geological studies in the region which will lead to a better understanding of the cause of uninterrupted aftershock activities in the region relocations of aftershocks delineate two main seismic clusters in the central aftershock zone caz i e eastern a1 and western a2 the alignments of relocated hypocenters of the aftershocks delineate three main fault segments viz the south dipping north as well as south wagad faults and north dipping kachchh mainland fault relocations also detect some zones of devoid of aftershocks with a typical dimension of 3 9 km within the caz which could be related to locales of future earthquakes seismic velocity tomograms depict that both seismic clusters a1 and a2 are associated with high velocity anomalies inferred to be mafic plutons below the wagad and banni regions respectively the e w depth section of relocated hypocenters delineates an 18 km variation of the seismogenic base which could be attributed to the increase in crustal maficness due to the intrusion of mafic plutons below the caz stress inversions of selected focal mechanisms depict a rotation of 44 in the 1 orientation within the caz which could be attributed to large local stresses induced by mafic plutons the continued seismicity at kachchh is proposed to be caused by the presence of large stresses associated with mafic plutons owing to the n s plate compression associated with northward movement of the indian plate wherein crustal fluid flows aqueous metamorphic fluids or volatile co2 might provide the triggering to generate continued aftershock activities at kachchh india

geophysics going with the mantle flow measurements of directional travel speed of seismic waves constrain flow in the upper mantle laboratory experiments suggest that high pressure can change the mantle s mineral alignment leading to a 90 offset in the direction of the fastest seismic waves

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

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

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

ground motion attenuation relation from strong motion records of the 2001 mw 7 7 bhuj earthquake sequence 2001 2006 gujarat india predictive relations are developed for peak ground acceleration pga from the engineering seismoscope srr records of the 2001 mw 7 7 bhuj earthquake and 239 strong motion records of 32 significant aftershocks of 3 1 mw 5 6 at epicentral distances of 1 r 288 km we have taken advantage of the recent increase in strong motion data at close distances to derive new attenuation relation for peak horizontal acceleration in the kachchh seismic zone gujarat this new analysis uses the joyner boore s method for a magnitude independent shape based on geometrical spreading and anelastic attenuation for the attenuation curve the resulting attenuation equation is eqalign ln hbox y 7 9527 1 4043 hbox m rm w ln left hbox r rm jb 2 19 82 2 right 1 2 0 0682 hbox s hfill cr hbox for 3 1 hbox m rm w le 7 7 quad quad hbox std hbox dev left sigma right pm 0 8243 hfill cr where y is peak horizontal acceleration in g mw is moment magnitude rjb is the closest distance to the surface projection of the fault rupture in kilometers and s is a variable taking the values of 0 and 1 according to the local site geology s is 0 for a rock site and s is 1 for a soil site the relation differs from previous work in the improved reliability of input parameters and large numbers of strong motion pga data recorded at short distances 0 50 km from the source the relation is in demonstrable agreement with the recorded strong ground motion data from earthquakes of mw 3 5 4 1 4 5 5 6 and 7 7 there are insufficient data from the kachchh region to adequately judge the relation for the magnitude range 5 7 mw 7 7 but our ground motion prediction model shows a reasonable correlation with the pga data of the 29 march 1999 chamoli main shock mw 6 5 validating our ground motion attenuation model for an mw6 5 event however our ground motion prediction shows no correlation with the pga data of the 10 december 1967 koyna main shock mw 6 3 our ground motion predictions show more scatter in estimated residual for the distance range 0 30 km which could be due to the amplification noise at near stations situated in the kachchh sedimentary basin we also noticed smaller residuals for the distance range 30 300 km which could be due to less amplification noise at sites distant from the kachchh basin however the observed less residuals for the longer distance range 100 300 km are less reliable due to the lack of available pga values in the same distance range