crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

earthquake prediction in northeast india a review a multidimensional geophysical approach is made to study earthquake precursors in northeast india resistivity gravity and seismological precursor anomalies for the two recent damaging earthquakes one on december 30 1984 magnitude 5 8 and the other on august 6 1988 magnitude 7 5 are very encouraging to intensify these studies in the region improved methods of magnetic and geohydrological precursor studies are suggested

seismic velocity ratio in the crust and uppermost mantle in southeast wellington province new zealand the regional variation of the seismic velocity ratio over a 200 km long traverse has been studied by means of microearthquake surveys the wadati plot method is used with a minimum of four p and s arrivals for each of 49 earthquakes the area as a whole is found to be characterized by a value of 1 74 1 76 for earthquakes of depth 12 40 km except in a 50 km long section near wellington where is low at 1 60 this low has been attributed to the fault zones in the region a small change of is observed between the upper crust 5 km and lower crust 12 km but there is no change of between the lower crust and uppermost mantle

joint spacing distribution of granites in hong kong abstract the structural stability of rock masses is dictated by the inherent geological structures present most notably joints joint spacings are one of crucial parameters to control the block volume permeability deformability as well as strength of rock masses while extensive rock engineering projects have been carried out in hong kong in the last century the distribution models of joint spacings in hong kong rocks have still rarely been studied systematically and published this research examines true joint spacings of granites in fourteen different slope outcrops at various localities of hong kong a total number of 1323 joint spacing measurements belonging to 44 joint sets spanning from 0 005 m to 4 5 m are obtained the mean joint spacings among 44 joint sets studied are computed in a range of 0 11 m to 1 33 m lognormal distribution is found to be the dominant joint spacing distribution of granites in hong kong a further in depth understanding of joint spacing distributions of hong kong granitic rocks which covers about 35 of the total land area is of profound practical value to the assessment of rock mass excavatability and numerical modelling of rock fracture network development

crustal structure variations in northeast india from converted phases 1 teleseismic receiver functions from a ten station network deployed in northeast india region sampling the shillong plateau mikir hills himalayan foredeep and the himalayan convergence zone are analyzed to obtain the crustal structure in this seismically active but less studied region the shillong plateau and mikir hills away from the convergent margins reveal remarkably simple crust with thickness 35 km and poisson s ratio 0 25 akin to the indian shield values a surprisingly thin crust for the uplifted shillong plateau may be explained invoking presence of an uncompensated crust that popped up in response to tectonic forces in contrast crustal signatures from assam valley suggest a thicker crust and higher poisson s ratio with evidences for a dipping moho predictably the crust is much thicker and complicated in the eastern himalaya further north with values in excess of 50 km

seismic b value and the assessment of ambient stress in northeast india seismicity data of northeast india recorded between 1986 and 1999 by a local network are analysed for estimation of b values based on the obtained values viz low b 0 5 moderate 0 5 0 7 the study area is classified into different seismic domains an assessment of stress level is also carried out in identifying seismic domains seismic activities though mostly confined in some sectors are presumably triggered by mutual interaction of the shillong plateau mikir hills indo burman ranges and the easternmost part of the himalayas and the contributions from deep seated fractures cannot be ignored the results resemble the seismic character of a foreland setting adjacent to a convergent margin the b values estimated for 240 square grids of dimension 0 6 0 6 over five seismic domains indicate wide variation an analysis of cumulative seismic moment release m o in different layers also indicates an anomaly in reference to the total seismic energy budget of the five zones the lower b value and higher m o recorded at relatively lower depth 30 km towards the southwest of the study area might be associated with upward bulging of a strong lithosphere the bulging is perhaps regionally compensated by the downward flexing of the descending indian lithosphere beneath the upper assam area features unequivocally observed in any foreland setup towards the north and east of the study area random variations of in both b value and m o along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas

scaling relationships in intraplate fracture systems related to red sea rifting abstract the geometry of fracture systems was analysed at various scales in the western arabian sedimentary cover rocks investigations covered a wide range of scales from regional surveys to outcrops in well exposed areas and were based on the field mapping of fractures and the interpretation and digitizing of fracture patterns on aerial and satellite images as a first step a ground structural analysis was carried out which provided a better understanding of the relationships of joint and fault systems related to red sea rifting secondly fracture datasets were subjected to statistical and geostatistical analyses of direction size spacing and density and the variability in these quantities with space and scale the main result of this combined approach is that fracturing is not a self similar process from the centimetre scale up to the 1000 km scale in this case the spatial organization of joints and faults appears to be highly controlled by both the red sea extensional regime and the thickness of the different mechanical layers that constitute the crust at different scales i e sedimentary beds basins and the seismogenic crust

a seismic hazard scenario in the sikkim himalaya from seismotectonics spectral amplification source parameterization and spectral attenuation laws using strong motion seismometry 1 in this paper we present a seismic hazard map of the indian himalayan state of sikkim lying between nepal and bhutan himalaya in terms of horizontal peak ground accelerations with 10 exceedance probability over the next 50 years these figures the first for the region were calculated through a stepwise process based on 1 an estimation of the maximum credible earthquake mce from the seismicity of the region and global seismic hazard assessment program considerations and 2 four seismotectonic parameters abstracted from accelerograms recorded at nine stations of the sikkim strong motion array specifically installed for this study the latter include 1 the frequency dependent power law for the shear wave quality factor qs 2 the site response function at each station using receiver function analysis and generalized inversion 3 source parameterization of various events recorded by the array and application of the resulting relationships between m0 and mw and corner frequency fc and mw to simulate spectral accelerations due to higher magnitude events corresponding to the estimated mce and 4 abstraction of regional as well as site specific local spectral attenuation laws at different geometrically central frequencies in low moderate and high frequency bands

mapping the crustal thickness in shillong mikir hills plateau and its adjoining region of northeastern india using moho reflected waves abstract in this study we have tried to detect and collect later phases associated with moho discontinuity and used them to study the lateral variations of the crustal thickness in shillong mikir hills plateau and its adjoining region of northeastern india we use the inversion algorithm by nakajima et al nakajima j matsuzawa t hasegawa a 2002 moho depth variation in the central part of northeastern japan estimated from reflected and converted waves physics of the earth and planetary interiors 130 31 47 having epicentral distance ranging from 60 km to 150 km taking the advantage of high quality broadband data now available in northeast india we have detected 1607 moho reflected phases pmp and sms from 300 numbers of shallow earthquake events depth 25 km in shillong mikir hills plateau and its adjoining region notably for pmp phase this could be identified within 0 5 2 3 s after the first p arrival in case of sms phase the arrival times are observed within 1 0 4 2 s after the first s arrival we estimated the crustal thickness in the study area using travel time difference between the later phases pmp and sms and the first p and s arrivals the results shows that the moho is thinner beneath the shillong plateau about 35 38 km and is the deepest beneath the brahmaputra valley to the north about 39 41 km deeper by 4 5 km compared to the shillong plateau with simultaneous observation of thinnest crust 33 km in the western part of the shillong plateau in the garo hills region

pn velocity study using a temporary seismographic network in the shillong plateau northeast india abstract seismic p wave velocity pn of the uppermost mantle beneath the shillong plateau has been studied using a temporary seismographic network refracted pn arrivals of shallow distant earthquakes m 3 0 recorded at pairs of stations lying within 2 azimuthal differences with respect to a given epicenter are used in this study since the method is based on relative travel time differences the measurement is independent of origin time and location errors the average pn velocity is found to be 8 57 0 02 km sec which is higher than the average mantle velocity 8 13 km sec under other areas of the indian subcontinent the observed high pn velocity beneath the plateau may be attributed to the presence of high density material

joint determination of seismic velocity ratios theory and application to an aftershock sequence abstract two different approaches are considered to the problem of determining seismic velocity ratios from p and s arrival data from a group of earthquakes each method arises naturally from a different assumption about the sources of error if the s reading error is the predominant source of error s versus p regression an extension of the wadati plot method to pooled data should be used if however s reading errors do not dominate a symmetric regression should be used a natural extension of the latter method allows the determination of station by station variation of the methods are illustrated with data from an aftershock sequence which occurred off the coast of northern california the average velocity ratio is not as well determined as the station by station variation about the average however the results show that the method has some utility in the study of regional variation of seismic velocities

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

3 d seismic structure of the northeast india region and its implications for local and regional tectonics abstract in this study we attempted to estimate 3 d p wave velocity structure of northeast india region using the first arrival data of local earthquakes that were recorded by about 77 temporary permanent local seismic stations the data set the published bulletins include 3494 p wave travel times and 3064 s p travel times from 980 local earthquakes that were located with a minimum of six observations the located earthquakes having a travel time root mean square rms residual 0 49 s and azimuthal gap 180 are selected to compute a 1 d velocity model for the region which is used as initial model for the subsequent 3 d inversions our results demonstrate that the computed 3 d velocity model has significantly improved hypocenter locations of the selected 980 earthquakes by reducing the rms error 0 06 by about 88 with respect to that by the 1 d velocity model the reconstructed p wave velocity v p structure with relocated events reveal strong heterogeneity in lateral as well as in vertical direction corresponding to the local and regional geology tectonics of the region high v p is mapped beneath the shillong plateau mikir hills and in the vicinity of indo burma ranges at shallower crust v p structure is imaged between the shillong plateau and mikir hills at 20 km depth which corresponds to the kopili fault the kopili fault system extends down to 30 km depth as evidenced by the low v p a high v p is imaged below the mikir hills at 40 km depth which is possibly the stress concentrator for high seismic activity along the kopili fault particularly at the fault end the bengal basin south of the shillong plateau is identified as a low v p zone extending down to a depth of about 20 km that indicates the thick alluvium sediments

earthquake source zones in northeast india seismic tomography fractal dimension and b value mapping we have imaged earthquake source zones beneath the northeast india region by seismic tomography fractal dimension and b value mapping 3d p wave velocity vp structure is imaged by the local earthquake tomography let method high precision p wave 3 494 and s wave 3 064 travel times of 980 selected earthquakes m d 2 5 are used the events were recorded by 77 temporary permanent seismic stations in the region during 1993 1999 by the let method simultaneous inversion is made for precise location of the events as well as for 3d seismic imaging of the velocity structure fractal dimension and seismic b value has been estimated using the 980 let relocated epicenters a prominent northwest southeast low vp structure is imaged between the shillong plateau and mikir hills that reflects the kopili fault at the fault end a high vp structure is imaged at a depth of 40 km this is inferred to be the source zone for high seismic activity along this fault a similar high vp seismic source zone is imaged beneath the shillong plateau at 30 km depth both of the source zones have high fractal dimension from 1 80 to 1 90 indicating that most of the earthquake associated fractures are approaching a 2d space the spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region the seismic b value in northeast india is found to vary from 0 6 to 1 0 higher b value contours are obtained along the kopili fault 1 0 and in the shillong plateau 0 9 the correlation coefficient between the fractal dimension and b value is found to be 0 79 indicating that the correlation is positive and significant to the south of shillong plateau a low vp structure is interpreted as thick 20 km sediments in the bengal basin with almost no seismic activity in the basin

deformation during the 12 november 1999 duzce turkey earthquake from gps and insar data only 87 days after the mw 7 5 17 august 1999 izmit earthquake the duzce earthquake ruptured a ca 40 km long adjoining strand of the north anatolian fault naf system to the east we used displacements of 50 global positioning system gps sites together with interferometric synthetic aperture radar insar range change data spanning the event to estimate the geometry and slip distribution of the coseismic rupture postseismic deformation transients from the duzce earth quake and the preceding i zmit event that are included in some of the measurements are corrected for using dislocation models fit to gps data spanning the various time periods nonlinear inversions for fault geometry indicate that the rupture occurred on a ca 54 north dipping oblique normal right lateral fault distributed slip inver sions indicate maximum strike slip near the center of the duzce fault close to the earthquake hypocenter slip magnitude and depth of faulting decrease to the west and east of the hypocenter both gps and insar data suggest that normal slip is restricted to the shallow portion of the rupture the duzce earthquake had the highest slip to rupture length ratio of any historic earthquake along the naf

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

mapping of b value beneath the shillong plateau abstract the seismic parameter b has been computed over rectangular grid of dimension 0 3 0 8 at four depths range 0 13 km first layer 13 1 26 km second layer 26 1 39 km third layer and 39 1 52 km fourth layer beneath the shillong plateau area the four depths were carefully selected based on the crustal structure and distribution of hypocentres the dimension of each grid was chosen so as to have enough events that can represent the b value at the respective layer finally two dimensional mapping was done at these depth levels considering the respective b value over each grid this analysis includes viz low b value all through the first layer and a trend of increasing b value which was initially towards north changes to northwest eastern and western parts of the second and third layers document almost moderate b values whereas the north south oriented central part of layer second is apparently dominated by low b values which seems to divide the area broadly into three parallel zones based on b values in the deeper part fourth layer beneath the shillong plateau a moderate b value that was initially trending towards north becomes high near the northeastern part this phenomenon may be associated with higher heterogeneity of the medium and interestingly this region lies between the lower crust and upper mantle possibly documents lower degree of seismic coupling where the shillong plateau is being supported by the strong indian lithosphere at these depths in addition minima were noted towards the southern parts of layers first second and third which may presumably be related with steeply bouguer gravity anomaly it is thus less clear that the occurrence of earthquakes beneath the shillong plateau whether is attributed to faults or lineaments at intermediate to deeper level however a correlation between high b values in few parts of each layer and deep seated minor faults cannot be ruled out

microseismicity and source mechanism study shillong plateau northeast india microearthquakes recorded by two surveys in the shillong plateau during december 1982 july 1983 and january february 1984 respectively have been analyzed for seismicity and tectonic study in the region of 1 000 events recorded during the surveys 180 events with reliable p and s readings which are observed within 50 km radius of the network are relocated by the homogeneous station method it is found that the events are mostly confined within a depth range of 10 to 35 km the seismicity in general may be correlated with the major geological structure lineaments in the area high rates of seismicity are observed in the shillong and tura areas on the other hand an aseismic zone is delineated to the east of shillong composite source mechanisms of two groups of microearthquakes show thrust fault mechanisms one group of microearthquakes 1982 to 1983 in the shillong area shows pure thrust faulting with a se nw compressive stress p axis whereas the other group of microearthquakes 1984 in the tura area shows thrust faulting with a strike slip component and a ene wsw compressive stress p axis the stress orientations are compatible with the known tectonics of the region collision of the indian plate with the eurasian plate at the himalaya and subduction of the indian plate beneath the burmese plate at the arakan yoma range

crustal structure beneath northeast india inferred from receiver function modeling abstract we estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast india by using h vp vs stacking method and a non linear direct search approach neighbourhood algorithm na technique followed by joint inversion of rayleigh wave group velocity and receiver function calculated from teleseismic earthquakes data results show significant variations of thickness shear velocities vs and vp vs ratio in the crust of the study region the inverted shear wave velocity models show crustal thickness variations of 32 36 km in shillong plateau north 36 40 in assam valley and 44 km in lesser himalaya south average vp vs ratio in shillong plateau is less 1 73 1 77 compared to assam valley and lesser himalaya 1 80 average crustal shear velocity beneath the study region varies from 3 4 to 3 5 km s sediment structure beneath shillong plateau and assam valley shows 1 2 km thick sediment layer with low vs 2 5 2 9 km s and high vp vs ratio 1 8 2 1 while it is observed to be of greater thickness 4 km with similar vs and high vp vs 2 5 in rup lesser himalaya both shillong plateau and assam valley show thick upper and middle crust 10 20 km and thin 4 9 km lower crust average vp vs ratio in assam valley and shillong plateau suggest that the crust is felsic to intermediate and intermediate to mafic beneath shillong plateau and assam valley respectively results show that lower crust rocks beneath the shillong plateau and assam valley lies between mafic granulite and mafic garnet granulite

geomechanical simulation of low order fracture of tight sandstone abstract the core factors in the study of fractures within tight sandstone reservoirs are tectonics geomechanics and fluid mobility which are not independent but are coupled to each other in this paper taking the upper paleozoic gas reservoir within the low amplitude tectonic zone in the sulige gas field of the ordos basin as an example the developmental characteristics and geomechanical model of fractures and their relationships with the productivity of tight gas sandstone reservoirs were systematically studied the results show that this low amplitude structural zone develops mainly low order ruptures the paleotectonic stress field of the target layer in the yanshanian period was restored using a 3d finite element method the horizontal principal stresses exhibited stress diffusion and were affected by local low amplitude structures there was a positive correlation between h and h and h h was distributed primarily between 1 35 and 1 95 with an average of 1 62 the strain energy density u was calculated to quantitatively characterize the microfracture distribution of the target layer based on the principle of energy conservation there are certain positive correlations between u and the horizontal principal stresses the geostresses of high u regions underwent a certain degree of stress torsion which manifested as a stress deflection reaching up to 30 during stress propagation finally we proposed the concept of u value geomechanical sweet spots in a tight gas sandstone based on this criterion commercial gas wells can be effectively identified

distribution of joints probabilistic modelling and case study near cardiff wales u k abstract in this paper a new probabilistic two dimensional model for simulating joint development in a single vertical set cutting horizontally layered rocks is presented the problem of considering the probability of joints nucleating within limestone beds and the probability of joints propagating across mudstone interbeds is solved using matrix analysis sources of dispersion relationships between joint spacing and bed thickness and the influence of pre existing joints on vertical propagation were taken into account simple assumptions were made concerning the sense of vertical joint propagation poorly constrained by field data in most cases we demonstrate that with the input of just a few geometrical and statistical parameters realistic cross sections can be constructed based on the probabilistic modelling our field analysis aimed to 1 provide real values for the statistical parameters and 2 compare actual and simulated patterns in order to check our results input parameters include bed thicknesses average numbers of joints and the proportion of joints cutting two adjacent limestone beds field data were collected from coastal exposures in liassic rocks at liantwit major wales u k at the sample sites tabular layers comprise alternating decimetric limestones and centimetric mudstone interbeds they are cut by two orthogonal sets of vertical joints we studied the distribution of the dominant joint set striking n170 both the models and the outcrops display similar characteristics e g a log normal distribution of joint spacing and a negative exponential distribution of the number of joints vs the number of beds crossed our probabilistic modelling is applicable to other situations with a limited number of numerical constraints obtained from field or sub surface observations

crustal shear wave velocity structure beneath northeast india from teleseismic receiver function analysis abstract we investigated the seismic shear wave velocity structure of the crust beneath nine broadband seismological stations of the shillong mikir plateau and its adjoining region using teleseismic p wave receiver function analysis the inverted shear wave velocity models show 34 38 km thick crust beneath the shillong plateau which increases to 37 38 km beneath the brahmaputra valley and 46 48 km beneath the himalayan foredeep region the gradual increase of crustal thickness from the shillong plateau to himalayan foredeep region is consistent with the underthrusting of indian plate beyond the surface collision boundary a strong azimuthal variation is observed beneath shl station the modeling of receiver functions of teleseismic earthquakes arriving the shl station from ne backazimuth baz shows a high velocity zone within depth range 2 8 km along with a low velocity zone within 8 13 km in contrast inversion of receiver functions from se baz shows high velocity zone in the upper crust within depth range 10 18 km and low velocity zone within 18 36 km the critical examination of ray piercing points at the depth of moho shows that the rays from se baz pierce mostly the southeast part of the plateau near dauki fault zone this observation suggests the effect of underthrusting bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the ew ne striking dauki fault dipping 300 toward northwest

microseismicity and tectonics in northeast india abstract three microearthquake surveys were carried out in shillong plateau mikir hills and assam valley areas during 1984 to 1986 some 422 events are relocated by the homogeneous station method the microseismicity map reveals intense crustal 10 to 40 km activity beneath the tura area of shillong plateau the areas to the west of shillong and the area around mikir hills also show high activity the microseismicity map further shows major tectonic lineaments that are compatible with the landsat imagery lineaments and the major faults composite focal mechanisms of the microearthquakes show spatial variation of the tectonic stresses in the region an ene wsw horizontal compressive stress is dominant in the tura area whereas a se nw horizontal compressive stress is dominant in the shillong and mikir hills areas

application of sar interferometry to a large thrust deformation the 1999 mw 7 6 chichi earthquake in central taiwan summary application of the interferometric method to four ers2 sar images acquired before and after the 1999 chichi earthquake has allowed determination of the coseismic surface displacement in the footwall area of the chelungpu fault the interferometric results revealed a relative shortening in the round trip distance between the radar antenna and the ground of the footwall side of chelungpu fault during the earthquake this shortening progressively increased from the west to the east and reaches the maximum amount of approximately 26 cm near the central segment of the chelungpu fault our interferometric results have been precisely examined using a dense gps network in the investigated area we mapped the gps coseismic measurements into the radar line of sight and implemented a forward simulation of sar interferogram from this synthesized result to control our unwrapping performance in this study these two observations are compared with a 3 d dislocation model of the fault finally a deformation analysis based on our interferometric result has indicated that a segment with irregular deformation behaviour can be distinguished in the footwall area of the chelungpu fault this segment may result from either the influence of inherited basement faults or the presence of a structural terrace that provide local opportunities for superficial deformation

co seismic displacements of the 1992 landers earthquake sequence we present co seismic displacement vectors derived from global positioning system gps measurements of 92 stations in southern california these gps results are combined with five well determined gps displacement vectors from continuously tracking stations of the permanent gps geodetic array as well as line length changes from usgs geodolite and two color laser trilateration observations to determine a self consistent set of geodetic data for the earthquake these combined displacements are modeled by an elastic dislocation representation of the primary fault rupture planes on average the model residuals are about twice the estimated measurement errors

three dimensional seismic structure beneath shillong plateau and assam valley northeast india abstract northeast india is bounded by the himalayan arc to the north and the burmese arc to the east the shillong plateau and assam valley lie at the boundary zone of the two arcs three microearthquake surveys were conducted in this area from 1984 to 1986 we have applied a tomographic method to about 2800 high quality p and s wave arrival times from 364 local earthquakes that were recorded in the magnitude range 2 0 to 4 0 by 22 temporary seismic stations during the surveys to determine the 3d velocity structure of the crust and upper mantle in this region the result reveals significant lateral heterogeneities in the study area the tomographic images obtained in this study are compatible with the major tectonic features such as active faults and seismicity trends

estimation of crustal discontinuities from reflected seismic waves recorded at shillong and mikir hills plateau northeast india in this study an attempt is made to determine seismic velocity structure of the crust and upper mantle beneath the shillong mikir hills plateau in northeast india region the principle of the technique is to relate seismic travel times with crustal thickness above the conrad and moho discontinuities broadband digital waveforms of the local earthquakes make a precise detection of the seismic phases possible that are reflected at these discontinuities the results show that the conrad discontinuity is at 18 20 0 5 km beneath the shillong mikir hills plateau and the moho discontinuity is at 30 1 0 km beneath the shillong plateau and at 35 1 0 km beneath the mikir hills

source parameters of the 1 october 1995 dinar turkey earthquake from sar interferometry and seismic bodywave modelling abstract the 1 october 1995 m s 6 1 dinar earthquake ruptured a 10 km section of the nw se dinar civril fault there are discrepancies between the published source parameters from seismic data with seismic moments in disagreement by over a factor of two we use both sar interferometry and seismic bodywave modelling to determine earthquake source parameters an interferogram generated from ers 1 2 sar imagery spanning the event and separated by 5 months is used to derive source parameters by a downhill simplex inversion with multiple monte carlo restarts we model the displacements in the satellite line of sight initially using uniform slip on a rectangular dislocation in an elastic half space the resultant model fault plane agrees in strike and location with the observed surface break but systematic residuals exist in the line of sight deformation field resulting in a r m s residual of 20 mm in the interferogram the residuals are reduced if the depth distribution of slip is allowed to vary spatially in four segments along a continuous fault plane our best fitting solution with a r m s misfit of 8 mm reveals two distinct areas of slip on the fault plane strike 145 dip 49 rake 270 a main rupture slipping by 1 44 m between depths of 1 and 8 km becoming deeper to the se and matching the observed surface rupture and an along strike continuation to the nw of the same fault plane but between depths of 8 and 13 km and not associated with a surface break the total geodetic moment 4 5 0 1 10 18 nm is more than twice as large as published seismic moments based on the inversion of p waveforms alone but close to the harvard cmt moment 4 7 10 18 nm we use sh waveforms in addition to the p waves used previously to determine an alternative seismic source mechanism sh waves constrain the depth to be shallower than solutions based on p waves alone agreeing with the depths from the interferometric inversion and resulting in a larger moment 3 1 0 4 10 18 nm than the previous bodywave estimates 2 2 1 10 18 nm the cmt moment reduces in magnitude to a similar size 3 3 10 18 nm if the centroid depth and fault dip are constrained to the values determined from bodywave modelling and interferometry thus the geodetic moment is 40 bigger than the moment determined from seismology

crustal p wave velocity and velocity ratio study in northeast india by a microearthquake survey the upper crustal 20 km p wave velocity beneath the shillong plateau and nowgong area has been studied by the time distance plot method thep arrival data of the shallow 20 km microearthquakes from three temporary networks are used and the average velocity is found to be 5 55 km s the velocity ratio v p v s for the upper crust 0 20 km as well as for the lower crust 21 40 km are determined by the wadati plot method and station by station method the average value obtained by the two methods is compatible thev p v s ranges between 1 74 to 1 76 a generalized seismic velocity model of the area is suggested by this study which has been very useful for microearthquake location

data files from the dry ships recordings of the chi chi earthquake seattle washington dry ships was a seismic refraction study of the seattle basin washington involving 29 chemical explosions recorded by 1008 seismographs conducted in september 1999 the e w trending dry ships line ran eastward from the olympic peninsula through the seattle basin to the foothills of the cascade mountains twenty eight refteks das model 07g 24 bit deployed along the dry ships line at 4 km intervals were programmed to record data continuously from three component sensors these 28 stations recorded the mainshock and several aftershocks of the m w 7 6 chi chi earthquake shin et al 2000 brocher et al 2000a the recordings were