late miocene early pliocene reactivation of the main boundary thrust evidence from the seismites in southeastern kumaun himalaya india abstract tectonic history of the himalaya is punctuated by successive development of the faults that run along the boundaries between different lithotectonic terrains the main boundary fault defining the southern limit of the lesser himalayan terrain is tectonically most active a review of published literature reveals that the nature and age of reactivation events on the main boundary fault is one of the poorly understood aspects of the himalayan orogen by systematic outcrop mapping of the seismites this study identifies a late miocene early pliocene reactivation on the main boundary thrust in southeast kumaun himalaya relatively friable and cohesionless neogene sedimentary sequences host abundant soft sediment deformation structures in the vicinity of the main boundary thrust among a large variety of structures deformed cross beds liquefaction pockets slump folds convolute laminations sand dykes mushroom structures fluid escape structures flame and load structures and synsedimentary faults are common the morphological attributes the structural association and the distribution pattern of the soft sediment deformation structures with respect to the main boundary fault strongly suggest their development by seismically triggered liquefaction and fluidization available magnetostratigraphic age data imply that the seismites were developed during a late miocene early pliocene slip on the main boundary thrust the hypocenter of the main seismic event may lie on the main boundary thrust or to the north of the study area on an unknown fault or the basal detachment thrust

simulated ground motions for hypothesized mw 8 subduction earthquakes in washington and oregon the amplitude and duration of strong ground motions from hypothesized mw 8 subduction zone thrust earthquakes in the puget sound portland region were estimated using a semi empirical method the simulation procedure assumes the rupture surface may be represented by a grid of fault elements finite difference wave simulation in a detailed 2 d velocity structure identifies direct s and the moho postcritical reflection as the primary components of the s wave field at horizontal distances of 0 to 150 km green s functions containing these two arrivals are computed with generalized ray theory in an equivalent 1 d structure for each source element receiver propagation path scattering and attenuation structure are empirically modeled by the use of corrected accelerograms from mw 7 michoacan mexico and valparaiso chile aftershocks for the fault element source functions different site conditions are empirically included in the simulation procedure by using empirical source functions recorded on rock and soil sites spatial variations in slip on the fault asperities are introduced by weighting the fault elements the technique has been validated for large subduction zone earthquakes by modeling acceleration time histories and response spectra from the 1985 michoacan mw 8 0 and valparaiso mw 8 0 main shocks somerville et al 1991 fault models for the puget sound and portland regions and seismic velocity structure models have been adapted from published regional studies uncertainty in the location of the asperities on the fault surface results in a large degree of uncertainty in the simulated ground motions at a given site if distance is defined as the distance to the closest asperity then the variability in the ground motions is reduced indicating that this uncertainty can be lessened by constraining the depth of the asperity the ground motion estimates are relatively insensitive to the difference in fault dip between the puget sound and portland fault models the attenuation of peak acceleration with distance r from the fault asperity for a mw 8 subduction earthquake is given by in pga 1 5 3 33 in r 128 0 79 s where s is a site term equal to 0 for rock and 1 for soil when distance r is the distance to the fault plane the attenuation relation is in pga 2 8 1 26 in r 0 79 s formal estimates of uncertainty in the calculated ground motions have been obtained by estimating both parametric uncertainty from the range of source models of hypothesized cascadia subduction earthquakes and modeling and random uncertainty from the misfit between recorded and simulated ground motions of the 1985 michoacan and valparaiso earthquakes for periods less than 1 sec the estimated response spectral velocities on soil sites in the seattle olympia region are about twice those recorded during the 1949 olympia and 1965 seattle wadati benioff zone earthquakes and the durations of strong motion on rock sites are significantly longer 45 to 60 sec versus 10 to 15 sec

active tectonics of the seattle fault and central puget sound washington implications for earthquake hazards we use an extensive network of marine high resolution and conventional industry seismic reflection data to constrain the location shallow structure and displacement rates of the seattle fault zone and crosscutting high angle faults in the puget lowland of western washington analysis of seismic profiles extending 50 km across the puget lowland from lake washington to hood canal indicates that the west trending seattle fault comprises a broad 4 6 km zone of three or more south dipping reverse faults quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault a the northernmost fault which forms the boundary between the seattle uplift and seattle basin analysis of growth strata deposited across fault a indicate minimum quaternary slip rates of about 0 6 mm yr slip rates across the entire zone are estimated to be 0 7 1 1 mm yr the seattle fault is cut into two main segments by an active north trending high angle strike slip fault zone with cumulative dextral displacement of about 2 4 km faults in this zone truncate and warp reflections in tertiary and quaternary strata and locally coincide with bathymetric lineaments cumulative slip rates on these faults may exceed 0 2 mm yr assuming no other crosscutting faults this north trending fault zone divides the seattle fault into 30 40 km long western and eastern segments although this geometry could limit the area ruptured in some seattle fault earthquakes a large event ca a d 900 appears to have involved both segments regional seismic hazard assessments must 1 incorporate new information on fault length geometry and displacement rates on the seattle fault and 2 consider the hazard presented by the previously unrecognized north trending fault zone

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

geometry and state of stress of the nazca plate beneath bolivia and its implication for the evolution of the bolivian orocline a series of deep earthquakes beneath bolivia in 1994 provides valuable constraints on the geometry and state of stress of the subducting nazca plate these earthquakes occurred northeast of the bolivian orocline which is bracketed between two regions of a flattened wadati benioff zone furthermore these events occurred in an area that experienced no seismicity in at least the past 30 years north and south of this zone the deep seismicity is abundant and indicates down dip compression collectively the seismicity defines the geometry of the deep nazca slab which is continuous and bent about a sharp arc this bending is a function of the dynamic evolution of the south america nazca plate system since the tertiary south america has incurred a maximum shortening along the bolivian orocline north and south of the orocline the south american crust has shortened less the differential shortening occurs in conjunction with the flattened and bent wadati benioff zone the upper nazca slab has undergone a tremendous amount of horizontal deformation that is controlled by the convergence direction relative to the trend of the trench in response the lower slab has been slowly bent about an axis parallel to the dip orientation of the deep nazca plate

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

the 23 19 aftershock of the 15 october 1979 imperial valley earthquake more evidence for an asperity the well recorded strong ground motion data for the 23 19 aftershock of the 15 october 1979 imperial valley earthquake provide a good opportunity to study the high frequency source characteristics and the path effects at near source distances the best fitting point source model has a strike slip mechanism n40 w which is nearly identical to the main event the estimated stress drop is extremely high roughly 500 bars with a triangular time history 0 1 0 1 sec but with a moment of 1 0 10 24 dyne cm a double source model found by inversion fits the high frequency data better but indicates complex faulting the first source with strike n319 e dip 42 ne and slip angle 165 has a moment of 0 7 10 24 dyne cm the second source with strike n324 e dip 82 sw and slip angle 181 lies about 0 5 km to the north and has a seismic moment twice that of the first source source dimensions appear very small for this amount of energy release many of the anomalous behaviors observed at certain stations for the main event are also present in the aftershock data these features are examined in terms of path effects

estimation of strong ground motions from hypothetical earthquakes on the cascadia subduction zone pacific northwest strong ground motions are estimated for the pacific northwest assuming that large shallow earthquakes similar to those experienced in southern chile southwestern japan and colombia may also occur on the cascadia subduction zone fifty six strong motion recordings for twenty five subduction earthquakes ofms 7 0 are used to estimate the response spectra that may result from earthquakesmw 81 4 large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude when compared with motions that have been observed in the western united states large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances an earthquake similar to the 22 may 1960 chilean earthquake mw 9 5 is the largest event that is considered to be plausible for the cascadia subduction zone this event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records the empirical green s function technique is used to synthesize strong ground motions for such giant earthquakes observed teleseismicp waveforms from giant earthquakes are also modeled using the empirical green s function technique in order to constrain model parameters the teleseismic modeling in the period range of 1 0 to 50 sec strongly suggests that fewer green s functions should be randomly summed than is required to match the long period moments of giant earthquakes it appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions nevertheless the occurrence of a giant earthquake in the pacific northwest may produce quite strong shaking over a very large region

the xigaze ophiolite tibet a peculiar oceanic lithosphere the xigaze ophiolite which outcrops along the yarlung zangbo river southern tibet locally displays a complete ophiolitic sequence from marine sedimentary cover over basaltic volcanics to the north to fresh cr diopside rich harzburgites to the south in contrast with other ophiolites the mafic part of the sequence is particularly thin it is almost devoid of cumulate gabbros consisting of a diabase sill complex covered by lava flows or pillow lavas the ultramafic unit is poor in residual harzburgites and dunites and consists dominantly of fresh cr diopside rich harzburgites the origin of this peculiar ophiolite of oceanic crust is discussed

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

depth resolution of earthquakes in central asia by moment tensor inversion of long period rayleigh waves effects of phase velocity variations across eurasia and their calibration the moment tensor inversion method for long period rayleigh waves is discussed from the point of view of event depth resolution in this method depth is usually determined by finding the minima in appropriate least squares residuals versus depth curves we show that depth cannot be resolved if the data are sampled only at one frequency and a choice of frequencies spanning a wide range of values yields better resolution we also show that the variance reduction at the minimum depends strongly on biases in the data provided they are not simple periodic functions in azimuth from the source since such biases are introduced in particular when the propagation corrections are inaccurate this provides a means by which to study regional trends of phase velocities this method is then applied to obtain an estimate of lateral heterogeneity in central asia starting with a reference point in the pamir mountains and a set of accurately known phase velocities on paths to eurasian wwssn stations we study the rayleigh waves at these stations in the period range 30 to 90 s for eight events located in or near the tibet plateau at stances from the reference point heterogeneity in phase velocity is thus found to be about 1000 to 1500 km in this period range constraining the source parameters of the most distant events by body wave modeling we determine new phase velocities on paths to wwssn stations from the center of the tibet plateau the variation of phase velocity with source location reaches 1 to 2 and the low velocity within tibet clearly has a dominant effect we then discuss the regionalization of phase velocities as a possible way to determine a calibration curve for a given source to receiver path we find that although the regionalized phase velocities in eurasia are stable to first order they do not seem accurate enough to be used for moment tensor inversions with arbitrary source locations this may be due to non ray theoretical effects causing the breakdown of phase velocity additivity when higher order accuracy is needed on the other hand reference phase velocities on source receiver paths can be obtained reasonably fast for events large enough so that body wave modeling is possible to constrain depth and source mechanism we demonstrate that these phase velocity curves are accurate enough for the calibration of propagation effects for another event located as much as 400 to 500 km away this provides an encouraging approach to study both structure and source mechanisms of smaller events within a highly heterogeneous continental region the only constraint is to find several events with the right magnitude of both long period body waves and surface waves

characteristics of observed peak amplitude for strong ground motion from the 1995 hyogoken nanbu kobe earthquake over 200 peak amplitudes of strong motion were observed at distances of less than 250 km from the fault during the 1995 hyogo ken nanbu kobe earth quake we analyzed the attenuation of the peak ground acceleration and velocity as a function of distance and geological site conditions the observed peak amplitudes agree well with those predicted by an empirical attenuation relation that was devel oped for japanese earthquakes this demonstrates that on average the peak amplitude of the ground motion generated by this damaging earthquake did not exceed the level predicted by the empirical attenuation relation we found a significant effect of the surface geology on the observed ground motion peak amplitude in particular for soft soil sites located near the fault the peak horizontal acceleration decreases rap idly with distance as a result of the nonlinear response of soils in order to take into account the effect of the site conditions we introduced correction factors to the ex isting attenuation relation this resulted in a significant reduction of the residuals between the predicted and observed peak amplitudes based on the attenuation re lation corrected for the site condition effect we generated a map of horizontal peak ground acceleration in the kobe and osaka area for the kobe earthquake the area of simulated large ground motion agrees well with the severe damage zone of inten sity vii jma scale

importance of fracturing during retro metamorphism of eclogites presented textural and petrological data show that the deep to intermediate continental crust may fracture and that microfractures are the locus of fluid and mass transfer necessary for retrograde metamorphism kyanite eclogites from ulsteinvik norway underwent partial retrogression to granulite and amphibolite facies assemblages during near isothermal exhumation from depths equivalent to more than 2 0 gpa at temperatures of 700 800 c plagioclase bearing assemblages rich in hydrous phases formed along margins of eclogite lenses and along mesoscopic fracture systems hydrated zones are from 1 50 cm thick with adjacent wall rock eclogite replaced by symplectites at a low degree of reaction the secondary minerals in the wall rock are found along intra and intergranular microfractures typically 50 100 m wide minerals filling the microfractures include orthopyroxene plagioclase spinel in garnet plagioclase sapphirine plagioclase corundum and plagioclase spinel in kyanite and diopside plagioclase in omphacite the microfractures are often arranged en echelon and are connected through microfaults releasing bends filled with amphibole and spinel form along microfaults in garnet the faulting and fracturing caused localized chemical change in garnet the damage zones close to faults are enriched in feo and mno with steep compositional gradients 8 wt feo over 20 m these feo and mno enriched zones form wedge like structures around the tip of the faults horsetail structures and rose or flame like structures at sticking points along faults they may represent examples of stress induced chemical transport during fracture propagation the change from dry to amphibole bearing assemblages at the tip of the fracture and fractures ending in splays of fluid inclusions trails reflect the involvement of a fluid phase during fracture propagation this suggests that the dry granulite facies retrogression was also driven by fluid infiltration and that metamorphism at depth in collision zones may not be controlled by pressure and temperature alone

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

nonlinear ground motion amplification by sediments during the 1994 northridge earthquake it has been known since at least 1898 ref 1 that sediments can amplify earthquake ground motion relative to bedrock for the weak ground motion accompanying small earthquakes the amplification due to sediments is well understood in terms of linear elasticity hooke s law 2 but there has been a long standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes the view of geotechnical engineers based largely on laboratory studies is that hooke s law breaks down at larger strains causing a reduced nonlinear amplification seismologists on the other hand have tended to remain sceptical of this nonlinear effect mainly because the relatively few strong motion observations seemed to be consistent with linear elasticity although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances3 4 the significance of nonlinearity for the type of stiff soil sites found in the greater los angeles region remains unresolved5 here we report that ground motion amplification due to sediments for the main shock of the 1994 northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks these observations imply significant nonlinearity in such amplification and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites

site response shallow shear wave velocity and wave propagation at the san jose california dense seismic array ground motion records from a 52 element dense seismic array near san jose california are analyzed to obtain site response shallow shear wave velocity and plane wave propagation characteristics the array located on the eastern side of the santa clara valley south of the san francisco bay is sited over the evergreen basin a 7 km deep depression with miocene and younger deposits site response values below 4 hz are up to a factor of 2 greater when larger regional records are included in the analysis due to strong surface wave development within the santa clara valley the pattern of site amplification is the same however with local or regional events site amplification increases away from the eastern edge of the santa clara valley reaching a maximum over the western edge of the evergreen basin where the pre cenozoic basement shallows rapidly amplification then decreases further to the west this pattern may be caused by lower shallow shear wave velocities and thicker quaternary deposits further from the edge of the santa clara valley and generation trapping of surface waves above the shallowing basement of the western evergreen basin shear wave velocities from the inversion of site response spectra based on smaller local earthquakes compare well with those obtained independently from our seismic reflection refraction measurements velocities from the inversion of site spectra that include larger regional records do not compare well with these measurements a mix of local and regional events however is appropriate for determination of site response to be used in seismic hazard evaluation since large damaging events would excite both body and surface waves with a wide range in ray parameters frequency wavenumber plane wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array conventional high resolution and multiple signal characterization f k power spectra and stacked slowness power spectra are compared these spectra show surface waves generated scattered at the edges of the santa clara valley and possibly within the valley at the western edge of the evergreen basin

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

nonlinear and linear site response and basin effects in seattle for the m 6 8 nisqually washington earthquake we used recordings of the m 6 8 nisqually earthquake and its m l 3 4 aftershock to study site response and basin effects for 35 locations in seattle washington we determined site amplification from fourier spectral ratios of the recorded horizontal ground motions referenced to a soft rock site soft soil sites generally national earthquake hazard reduction program nehrp class e on artificial fill and young alluvium have the largest 1 hz amplifications factors of 3 7 for both the mainshock and aftershock these amplifications are correlated with areas of higher damage from the mainshock to major buildings and liquefaction there are several indications of nonlinear response at the soft soil sites for the mainshock ground motions despite relatively modest peak accelerations in the s waves of 15 22 g first the mainshock spectral ratios do not show amplification at 2 8 hz as do the aftershock spectral ratios spectral peaks at frequencies below 2 hz generally occur at lower frequencies for the mainshock spectral ratios than for the aftershock ratios at one soft soil site there is a clear shift of the resonant frequency to a lower frequency for the mainshock compared with the aftershock the frequency of this resonance increases in the coda of the mainshock record indicating that the site response during the weaker motions of the coda is more linear than that of the initial s wave three of the soft soil sites display cusped one sided mainshock accelerograms after the s wave these soft soil sites also show amplification at 10 20 hz in the s wave relative to the rock site that is not observed for the aftershock the cusped waveforms and 10 20 hz amplification are symptomatic of nonlinear response at the soft soil sites these sites had nearby liquefaction the largest amplifications for 0 5 hz occur at soft soil sites on the southern portion of the seattle basin stiff soil sites nehrp classes d and c on pleistocence age glacial deposits display similar spectral amplification for the mainshock and aftershock indicating approximately linear response the stiff soil sites generally have moderate amplification factors of 1 1 2 4 at 0 5 and 1 hz amplifications at 1 and 5 hz for all sites generally increase with decreasing shear wave velocity measured in the top 30 m v s 30 however larger amplifications at 0 5 and 1 hz for sites with similar v s 30 values are observed for sites in the seattle basin illustrating the amplification from the deeper 30 m sediments and the contribution from basin surface waves record sections for the mainshock and aftershock show that basin surface waves produce the peak velocities for many of the sites in the seattle basin and often dominate the amplitude at 1 hz and lower frequencies

seismic reflection images beneath puget sound western washington state the puget lowland thrust sheet hypothesis seismic reflection data show that the densely populated puget lowland of western washington state is underlain by subhorizontal paleogene and neogene sedimentary rocks deformed by west and northwest trending faults and folds from south to north beneath the lowland features seen on the seismic data include the horizontally stratified 3 5 km thick tacoma sedimentary basin the seattle uplift with south dipping 20 strata on its south flank and steeply 50 to 90 north dipping strata and the west trending seattle fault on its north flank the 7 5 km thick northward thinning seattle sedimentary basin the antiformal kingston arch and the northwest trending transpressional southern whidbey island fault zone swif interpreting the uplifts as fault bend and fault propagation folds leads to the hypothesis that the puget lowland lies on a north directed thrust sheet the base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic crescent formation its edges may be right lateral strike slip faults along the base of the cascade range on the east and the olympic mountains on the west our model suggests that the seattle fault has a long term slip rate of about 0 25 mm year and is large enough to generate a m7 6 to 7 7 earthquake

river profiles along the himalayan arc as indicators of active tectonics abstract longitudinal profiles along sixteen major transverse himalayan rivers add important constraints to models of active continental subduction and its evolution these profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases the base of the zone of increased gradients correlates with 1 the topographic front between the lesser and high himalayas 2 the narrow belt of intermediate magnitude thrust earthquakes 3 the main central thrust zone mct these features define a small circle in the central portion of the himalayan arc these correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising high himalayas and the lesser himalayas no sharp increases in gradient are observed near the main boundary thrust mbt except on a few rivers such as the jhelum or kundar where the mbt lies close to both the mct and the seismic belt thus it is unlikely that the mbt is a major tectonic boundary the diversion of river courses along the mbt and around anticlines in the sub himalayas has probably been caused by aggradation near the rosion deposition boundary upstream of uplifts in the mahabharat range and sub himalayas a parallel is drawn between the himalayas and new guinea based on the hypothesis that continent arc collision of the type occurring in northern australia preceded continent continent collision in the himalayas the present sedimentary tectonic phase in new guinea resembles the subathu paleocene eocene phase in the himalayas incipient counterparts of the major himalayan structures including the mct and the mbt are recognized in new guinea the drainage patterns in the himalayas and in new guinea bear a similar relation to major structures this suggests that 1 the tectonic evolution of the himalayas has been rather uniform since early stages of collision and 2 the himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the high himalayas

india asia convergence driven by the subduction of the greater indian continent the most spectacular example of plate convergence on earth was the motion of the indian plate towards eurasia and the subsequent collision density estimates of the greater indian continent after its upper crust is scraped off at the himalayan front suggest that this continental plate is readily subductable potentially explaining why the convergence did not halt on collision the most spectacular example of a plate convergence event on earth is the motion of the indian plate towards eurasia at speeds in excess of 18 cm yr 1 ref 1 and the subsequent collision continental buoyancy usually stalls subduction shortly after collision as is seen in most sections of the alpine himalayan chain however in the indian section of this chain plate velocities were merely reduced by a factor of about three when the indian continental margin impinged on the eurasian trench about 50 million years ago plate convergence accompanied by eurasian indentation persisted throughout the cenozoic era1 2 3 suggesting that the driving forces of convergence did not vanish on continental collision here we estimate the density of the greater indian continent after its upper crust is scraped off at the himalayan front and find that the continental plate is readily subductable using numerical models we show that subduction of such a dense continent reduces convergence by a factor similar to that observed in addition an imbalance between ridge push and slab pull can develop and cause trench advance and indentation we conclude that the subduction of the dense indian continental slab provides a significant driving force for the current india asia convergence and explains the documented evolution of plate velocities following continental collision

variability in nonlinear sediment response during the 1994 northridge california earthquake abstract spectral ratios between sediment sites and rock sites for strong and weak ground motion are compared to infer the extent of nonlinear sediment response in the los angeles area during the 1994 northridge earthquake northridge mainshock records form the strong motion database and aftershock records form the weak motion database the degree of nonlinearity is evaluated at 25 locations in and around the san fernando and los angeles basins based on the ratio of weak to strong spectral amplification factors wsaf values of wsaf vary greatly from site to site and for a given site from one frequency band to another over the range from 0 75 to 20 hz this complexity is attributed to differences in several factors material properties under the sites and the amplitude and frequency content of the input ground motions given these observations it is not possible to assign an acceleration or velocity level at which nonlinearity initiates for all the stiff soil sites analyzed however no nonlinearity is seen below a peak acceleration of about 200 to 300 cm sec 2 or peak velocity of 20 to 30 cm sec nonlinearity is seen at all sites in or near the san fernando valley nonlinearity in the los angeles basin is limited to a few isolated sites for the northridge earthquake even sites with strong nonlinearity still show amplification levels greater than 1 0 relative to rock sites for frequencies below 10 hz

influence of a sedimentary basin infilling description on the 2 d p sv wave propagation using linear and non linear constitutive models numerical modelling is a useful tool to understand the role of different parameters site geometry impedance contrast material properties constitutive model input motion governing site effects we focus here on the 2 d p sv seismic wave propagation in a simple shaped asymmetric model we consider two ways of describing the sedimentary infilling properties the basin is either composed of distinct homogeneous geological layers or is described through properties progressively changing as a function of depth smooth model in order to study the wave propagation in the two basins with similar soil properties p and s velocities of the layered basin are deduced from the smooth basin ones making both models compatible in a kinematic point of view p and s velocities globally increase with depth following epri non linear properties are considered as constant in layers for the two basins we assess the influence of the model choice on the wave propagation while taking into account linear and non linear constitutive models we test the influence of the input motion by propagating two different input motions the first being a simple impulsive one and the second a real complex accelerogram we find that transfer functions computed in each basin are quite similar irrespective of the soil constitutive behaviour and the input motion moreover we show that the maximum shear strain spatial distribution depends on the input motion frequency content and maximum values depend on the input motion strength and complexity maximum shear strains are generally higher for superficial layers than for deeper ones we show that the highest shear strain values are located above discontinuities where velocities are lower and the soil is more non linear than in the underlying layer regardless of the input motion indeed in a layered basin the shear strain is higher above interfaces for all the soil constitutive models that were used in the smooth visco elastoplastic model a kind of layering appears absent in a viscoelastic model due to the different non linear properties in each layer moreover no matter what the model structure and the input motion used maximum shear strain levels are higher in the visco elastoplastic soil than in the viscoelastic one such a numerical study helps to understand the non linearity location and therefore the consequences of a model choice when performing a site specific study

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

separation of intrinsic and scattering seismic wave attenuation in northeast india summary we have analysed the local earthquakes 2 0 ml 5 5 occurred in northeast ne india recorded by a temporary seismic network of 10 stations operated by national geophysical research institute ngri hyderabad to evaluate the relative contributions of scattering loss q 1 and intrinsic absorption q 1 to total attenuation q 1 using the multiple lapse time window analysis assuming multiple isotropic scattering in a medium of uniformly distributed scatterers the results show that q 1 is greater than q 1 at high frequencies f 3hz while the opposite is observed at low frequencies f 3hz the observed frequency dependence of q 1 corresponds to the scale length of lithospheric heterogeneity beneath ne india at least comparable with the wavelength for the lowest frequencies analysed of about 1km the observed q 1 for the study region obtained with single scattering theory is close to q 1 at high frequencies in agreement with theoretical prediction for an idealized case of uniform distribution of scatterers however a discrepancy exists between the two at low frequencies which can be explained by a depth dependent velocity and attenuation structure high value of q 1 is correlated with the geology and tectonic settings of the region characterized by himalayan and burman collision zones with a strong lateral heterogeneity the q 1 estimates obtained in this study can be used to infer the average temperature of the lower crust with an upper limit estimate of 800 c assuming a lower crustal gabbroic lithology

sedimentary basin effects in seattle washington ground motion observations and 3d simulations seismograms of local earthquakes recorded in seattle exhibit surface waves in the seattle basin and basin edge focusing of s waves spectral ratios of s waves and later arrivals at 1 hz for stiff soil sites in the seattle basin show a dependence on the direction to the earthquake with earthquakes to the south and southwest producing higher average amplification earthquakes to the southwest typically produce larger basin surface waves relative to s waves than earthquakes to the north and northwest probably because of the velocity contrast across the seattle fault along the southern margin of the seattle basin s to p conversions are observed for some events and are likely converted at the bottom of the seattle basin we model five earthquakes including the m 6 8 nisqually earthquake using 3d finite difference simulations accurate up to 1 hz the simulations reproduce the observed dependence of amplification on the direction to the earthquake the simulations generally match the timing and character of basin surface waves observed for many events the 3d simulation for the nisqually earthquake produces focusing of s waves along the southern margin of the seattle basin near the area in west seattle that experienced increased chimney damage from the earthquake similar to the results of the higher frequency 2d simulation reported by stephenson et al 2006 waveforms from the 3d simulations show reasonable agreement with the data at low frequencies 0 2 0 4 hz for the nisqually earthquake and an m 4 8 deep earthquake west of seattle

seismicity and the state of stress from investigations of local earthquakes in the kumaon himalaya abstract results of investigations of local earthquakes in the region of the main central thrust mct in the kumaon himalaya between and adjacent to the valleys of the bhagirathi and yamuna rivers are presented records of over 250 earthquakes were analysed and the following facts emerged 1 1 earthquakes in the himalayas occur in specific areas and belts one such belt has been identified in the region under investigation hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt crossing the yamuna river between the villages of barkot and syanachatti 2 2 all but a few epicentres in this middle segment lie to the southwest of the surface trace of the mct in a zone with a width of 10 30 km 3 3 most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface the maximum estimated depth being 32 km using observations of first motion for a composite focal mechanism solution the nodal planes were observed to be near vertical and the compression axis near horizontal and normal to the local strike of the seismic belt and of the mct we conclude that although the main central thrust itself is not seismically active in this region there is considerable activity immediately to the southwest of it furthermore the mode of faulting as inferred from the records of these earthquakes is strike slip

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

constraints on the seismic wave velocity structure beneath the tibetan plateau and their tectonic implications we combine observations of group and phase velocity dispersion of rayleigh waves of the waveform of a long period pl phase of pn and sn velocities from unreversed refraction profiles using earthquakes and of teleseismic s p travel time residuals to place bounds on the seismic wave velocity structure of the crust and upper mantle under tibet from surface wave measurements alone the tibetan crustal thickness can be from 55 to 85 km with corresponding uppermost mantle shear wave velocities of about 4 4 to 4 9 km s respectively the pn and sn velocities were determined to be 8 12 0 06 and 4 8 0 1 km s respectively using travel time data at lhasa from earthquakes in and on the margins of tibet combining these results the crustal thickness is most likely to be between 65 80 km with an average shear wave velocity in the upper crust less than 3 5 km s a synthesis of one pl waveform does not provide an additional constraint on the velocity structure but is compatible with the range of models given above in contrast to observations obtained for eight earthquakes in the himalaya measurements of both teleseismic s and p wave arrival times for nine earthquakes within tibet show unusually large intervals between p and s compared with the jeffreys bullen tables thus the pn and sn velocities apparently do not reflect high velocities in the mantle to a great depth beneath tibet from the dependence of the seismic velocities of olivine on pressure and temperature and from the similarity of the measured pn and sn velocities beneath tibet and beneath shields and platforms the velocities at the moho beneath tibet are compatible with the temperature being 250 300 higher than beneath shields and platforms i e 750 c if the temperature beneath the platforms is close to 500 c such a temperature could reach or exceed the solidus of the lower crust simple one dimensional heat conduction calculations suggest that the volcanic activity could be explained by the recovery of the geotherm maintained by a mantle heat flux of about 0 9 hfu at the base of the crust if the distribution of radioactive heat production elements were not concentrated at the top of the crust radioactive heating could also contribute significantly to the recovery of the geotherm and thus lower the required mantle heat flow thus the idea of a thickened crust in response to horizontal shortening is compatible both with these data and with these calculations appendices are available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 004 1 00 payment must accompany order

microearthquake activity in some parts of the himalaya and the tectonic model abstract microearthquake data from temporary permanent networks in different parts of the himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region the microearthquake activity in arunachal pradesh northeastern himalaya is found to be pronounced at the main boundary thrust mbt and to its south the subcrustal earthquakes depth 50 80 km occur much below the plane of detachment of the tectonic models proposed by seeber et al 1981 ni and barazangi 1984 the mbt is not the seismogenic fault the earthquakes are generated by strike slip movement on deep seated hidden faults transverse to the mbt the high seismic activity in the shillong plateau about 200 km south of the mbt in the northeast region is due to the influence of himalayan collision tectonics to the north and burmese arc subduction tectonics to the east the activity in the plateau is not directly related to the himalayan thrust belt or seismic belt these are mostly crustal earthquakes depth 10 30 km and are caused by local active faults lineaments in the eastern himalaya in the sikkim and darjeeling area the seismic activity is found to be clustered mostly to the north of the mbt the earthquakes occur at a depth range 0 50 km the majority of them occur below the detachment plane by thrust faulting in the central part in the nepal himalaya lateral variations of the seismic activity are observed which represent lateral segmentation of the mbt by transverse tectonic features in the western himalaya however the tectonic models fit well with the microearthquake data in the himachal pradesh of the western himalaya the microearthquakes are mostly recorded in the mbt zone and the hypocentres depth 0 20 km are confined above the plane of detachment or on the basement thrust the earthquakes mostly occur to the south of the main central thrust mct which suggests that the mct is not seismogenic it is rather a dormant fault no single tectonic model explains the himalayan earthquakes

active tectonics in eastern lunana nw bhutan implications for the seismic and glacial hazard potential of the bhutan himalaya 1 paleoseismological investigations brittle fault analysis and paleostrain calculations combined with the interpretation of satellite imagery and flood wave modeling were used to investigate the seismic and associated glacial hazard potential in eastern lunana a remote area in nw bhutan seismically induced liquefaction features cracked pebbles and a surface rupture of about 6 8 km length constrain the occurrence of m 6 earthquakes within this high altitude periglacial environment which are the strongest earthquakes ever been reported for the kingdom of bhutan seismicity occurs along conjugate sets of faults trending ne sw to nnw sse by strike slip and normal faulting mechanism indicating e w extension and n s shortening the strain field for these conjugate sets of active faults is consistent with widespread observations of young e w expansion throughout southern tibet and the north himalaya we expect however that n s trending active strike slip faults may even reach much farther to the south at least into southern bhutan numerous glacial lakes exist in the investigation area and today more than 100 106 m3 of water are stored in moraine dammed and supraglacial lakes which are crosscut by active faults strong earthquakes may trigger glacial lake outburst floods and the impact of such flash floods may be worst 80 km downstream where the valley is broad and densely populated consequently tectonic models of active deformation have to be closely linked with glacial hazard evaluation and require rethinking and modification

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

structure and tectonics of the himalaya constraints and implications of geophysical data the himalayan range is a product of continental collision between india and eurasia and for several reasons it is a logical place to examine the physical causes and mechanisms of mountain building the exceptionally high mountains and the great length of the range alone single the himalaya out as a major structural feature the convergence rate between india and eurasia of about 50 mm yr 1 minster jordan 1978 requires rapid rates of slip and the occurrence of four major earthquakes with magnitudes in excess of 8 4 in the last 100 years attests to a high level of seismicity and a rapid rate of active deformation e g richter 1958 seeber armbruster 1981 the exposures of high grade metamorphic rocks in the greater himalaya may suggest large vertical transport of lower crustal rocks e g valdiya 1980a b and the occurrences of young postcollision granites imply recent large perturbations of the temperature structure of the range e g lefort 1981 moreover as sediments derived from the himalaya fill the broad ganga basin south of the range the mountain belt simul taneously seems to override the basin e g g d johnson et al 1979 n m johnson et al 1982 as such the processes occurring now and since the collision began in the early tertiary as well as the present structure of the himalayan belt are probably similar to those at different stages of collision and mountain building in other older orogenic belts such as the appalachians the scottish and scandinavian caledonides the alps and

aftershocks of the march 1999 chamoli earthquake and seismotectonic structure of the garhwal himalaya the aftershocks of the chamoli earthquake m b 6 3 of 28 march 1999 are analyzed to understand the seismotectonic structure of the garhwal himalaya the permanent as well as temporary microearthquake network data are used the mainshock occurred on the basement thrust by thrust faulting the aftershocks on the other hand are triggered at the seismogenic faults to the south of the main central thrust mct by thrust faulting as well as by strike slip faulting and all the events occurred above the plane of detachment basement thrust the mct is not the seismogenic fault

site amplification and attenuation via downhole array seismogram inversion a comparative study of the 2003 miyagi oki aftershock sequence weak motion geotechnical array recordings at 38 stations of the japanese strong motion network kik net from the 2003 m w 7 0 miyagi oki aftershock sequence are used here to quantify the amplification and attenuation effects of near surface formations to incident seismic motion initially a seismic waveform optimization algorithm is implemented for the evaluation of high resolution low strain velocity v s attenuation q s and density profiles at the sites of interest based on the inversion results v s versus q s correlations are developed and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation surface to downhole traditional spectral ratios ssr cross spectral ratios c ssr and horizontal to vertical h v site response estimates are next evaluated and compared while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average vs of the upper 30 m v s30 of the formations single and reference station site response estimates are successively compared to surface to rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results comparison of the observed ssr and estimated surface to rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class site amplification factors are successively computed in reference to the national earthquake hazards reduction program nehrp b c boundary site conditions v s30 760 m sec and results are compared to published values developed on the basis of strong motion data and site response analyses finally weak motion ssr estimates are compared to the mainshock spectra and conclusions are drawn for the implications of soil nonlinearity in the near surface results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near surficial soil formations

the 1994 northridge california earthquake investigation of rupture velocity risetime and high frequency radiation a hybrid global search algorithm is used to solve the nonlinear problem of calculating slip amplitude rake risetime and rupture time on a finite fault thirty five strong motion velocity records are inverted by this method over the frequency band from 0 1 to 1 0 hz for the northridge earthquake four regions of larger amplitude slip are identified one near the hypocenter at a depth of 17 km a second west of the hypocenter at about the same depth a third updip from the hypocenter at a depth of 10 km and a fourth updip from the hypocenter and to the northwest the results further show an initial fast rupture with a velocity of 2 8 to 3 0 km s followed by a slow termination of the rupture with velocities of 2 0 to 2 5 km s the initial energetic rupture phase lasts for 3 s extending out 10 km from the hypocenter slip near the hypocenter has a short risetime of 0 5 s which increases to 1 5 s for the major slip areas removed from the hypocentral region the energetic rupture phase is also shown to be the primary source of high frequency radiation 1 15 hz by an inversion of acceleration envelopes the same global search algorithm is used in the envelope inversion to calculate high frequency radiation intensity on the fault and rupture time the rupture timing from the low and high frequency inversions is similar indicating that the high frequencies are produced primarily at the mainshock rupture front two major sources of high frequency radiation are identified within the energetic rupture phase one at the hypocenter and another deep source to the west of the hypocenter the source at the hypocenter is associated with the initiation of rupture and the breaking of a high stress drop asperity and the second is associated with stopping of the rupture in a westerly direction

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

active transverse features in the central portion of the himalaya abstract many transverse lineaments or faults across the himalaya are already known interpretation of satellite images of the central portion of the himalaya given here adds to that number and suggests their continuity from the himalaya to its foredeep several such faults appear to be seismically active their shear motion recognized from geologic data and focal mechanism solutions compares well with a classical stress model when applied to northward progress of india under the himalayan collision front

latitudinal variation of denudation in the evolution of the bolivian andes latitudinal gradients in topography relief climate and deformation have been used to suggest that climate driven erosion has exerted a first order control on the development of the central andes we synthesize the spatial and temporal variations in denudation across the eastern bolivian andes 14 22 s from new and existing estimates to test whether physical evidence exists to support the hypothesis that erosion influences thrust belt evolution basin morphometry channel network indices climate and longitudinal river profiles indicate a northward increase in relative relief fluvial incision and denudation short term denudation rate averages from landslide mapping and sediment flux data range from 1 to 9 mm yr in the north compared to 0 3 to 0 4 mm yr in the south long term denudation rate estimates from thermochronology cosmogenic radionuclides foreland basin sediment volumes stra tigraphy paleoerosion surface degradation and balanced cross sections range from 0 04 to 1 6 mm yr with rates up to more than twice as fast in the north when comparing estimates from the same method the shorter term denudation rates exhibit the greatest variance our denudation synthesis shows that an along strike disparity in denudation greater in the north has existed throughout the holocene and perhaps existed since as early as the late miocene our denudation synthesis also suggests that the disparity and denudation rates have increased to the present correlations between the thrust belt geology geometry geomorphology climate and kinematics of the orogenic wedge provides a case study in observing the regional scale interactions between uplift climate and erosion we conclude that the denudation history uplift history and tectonic geomorphic correlations suggest that models of the evolution of the bolivian andes should incorporate a latitudinal erosion gradient for the last 10 kyrs to perhaps 10 myrs

site response for seattle and source parameters of earthquakes in the puget sound region we analyzed seismograms from 21 earthquakes ml 2 0 4 9 recorded by digital seismographs we deployed in urban seattle to determine site response and earthquake stress drops the seismometers were situated on a wide variety of geologic units including artificial fill e g kingdome harbor island pleistocene age soils glacial till and outwash deposits of seattle s hills modified land downtown seattle space needle and tertiary sedimentary rock two mainshock aftershock sequences were recorded the june 1997 bremerton sequence mainshock ml 4 9 and the february 1997 south seattle sequence mainshock ml 3 5 along with other events in the puget sound region we developed a new inversion procedure to estimate site response source corner frequencies and seismic moments from the s wave spectra this inversion uses corner frequencies determined from spectral ratios of mainshock aftershock pairs as constraints the site responses found from the inversion are not relative to the rock site but are relative to an idealized site with a flat frequency response the response of the rock site is also found from the inversion the inversion results show high response for the sites on artificial fill more moderate amplification for most sites on stiff pleistocene soils or modified land and low response for the rock site some sites display resonances such as a strong 2 hz resonance at our site near the kingdome which is caused by the surficial layers of fill and younger alluvium the sites in west seattle exhibit high amplification even though they are on relatively stiff soils of glacial outwash this may be partly caused by basin surface waves produced by conversion of incident s waves this high response in west seattle is consistent with damage reports from the 1949 mb 7 1 and 1965 mb 6 5 earthquakes stress drop estimates for the events we recorded were generally low between 0 4 and 25 bars although some of the events may have had higher stress drops that could not be resolved because of the limited passband we calculated a stress drop of 24 bars for the bremerton mainshock and 10 bars for the south seattle mainshock

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

state of tectonic stress in northeast india and adjoining south asia region an appraisal an attempt is made to map the spatial variation of the tectonic stress pattern in northeast india and its adjoining south asia region using stress tensor inversion of some 516 fault plane solutions the bhutan himalaya and the arunachal himalaya are mapped with north south to north northwest south southeast compression the eastern himalaya syntaxis zone on the other hand shows a clockwise rotation a north northeast compression is dominant to the south in the intraplate part of the region the shillong plateau assam valley bengal basin bangladesh and tripura fold belt exhibit north northwest to north northeast compression orthogonal horizontal extension is dominant in southern tibet bhutan and partly in the syntaxis zone and the same is also observed in the shillong plateau and assam valley area of the intraplate region the indo burma ranges and the sagaing fault in the myanmar region show a northeast southwest compression an orthogonal horizontal northwest southeast extension is also observed in the sagaing fault zone a depth variation of the tectonic stress is observed below the indo burma ranges it changes from north south to northeast southwest in the southern part and from northeast southwest to north northeast south southwest in the northern part in the deeper seismogenic zone the stress inversion results of clusters of events in individual zones though mostly conformable with the average observations indicate a variation in the shillong plateau due to heterogeneity and tectonic complexity

amplification of seismic waves by the seattle basin washington state recordings of the 1999 m w 7 6 chi chi taiwan earthquake two local earthquakes and five blasts show seismic wave amplification over a large sedimentary basin in the u s pacific northwest for weak ground motions from the chi chi earthquake the seattle basin amplified 0 2 to 0 8 hz waves by factors of 8 to 16 relative to bedrock sites west of the basin the amplification and peak frequency change during the chi chi coda the initial s wave arrivals 0 30 sec had maximum amplifications of 12 at 0 5 0 8 hz whereas later arrivals 35 65 sec reached amplifications of 16 at 0 3 0 5 hz analysis of local events in the 1 0 to 10 0 hz frequency range show fourfold amplifications for 1 0 hz weak ground motion over the seattle basin amplifications decrease as frequencies increase above 1 0 hz with frequencies above 7 hz showing lower amplitudes over the basin than at bedrock sites modeling shows that resonance in low impedance deposits forming the upper 550 m of the basin beneath our profile could cause most of the observed amplification and the larger amplification at later arrival times suggests surface waves also play a substantial role these results emphasize the importance of shallow deposits in determining ground motions over large basins

orogen transverse structures in the eastern himalaya dextral riedel shear along the main boundary thrust in the garu gensi area arunachal pradesh india implication in hydrocarbon geoscience abstract structural mapping and analyses reveal deformation in the rocks of the lesser himalayan miri and gondwana and lower siwalik dafla successions across the miri thrust mt and the main boundary thrust mbt in the garu gensi area arunachal himalaya india the north dipping gondwana sequence is sandwiched between the miri group of rocks in the north and the lower siwalik dafla sandstones in the south the gondwana rocks folded four times d1 d4 detail outcrop mapping and structural analyses substantiated by satellite images elucidate that the miri the gondwana and the lower siwalik rock sequences in the area are traversed by two sets of nw and ne trending cross fault zones with 70 100 m thickness mappable for 70 km the cross faults trend nw around the locality daring while in the western part around gensi they trend ne the westernmost fault slipped dextrally and the easternmost one at daring sinistrally these transverse faults offset the mbt 0 52 0 59 km for the western and 0 58 1 41 km for the eastern faults the deformation in the gondwana sequence may be constrained as syn himalayan i e india asia collisional event this was followed by a compression from e ese due to the post eocene indo burmese collision the culminating phase of combined india asia and indo burmese compression is manifested in field with a strong ne directed compression with dextral reidal shear developing in the transverse conjugate fault zones in the area with maximum displacement towards se cluster of the earthquake data indicates record of the events m 6 within 30 80 km depth both n and s of the mbt in the study area further the southerly continuation of the transverse faults below the alluvium can provide vital clue in studying the potential hydrocarbon trap in the northern fringe of the brahmaputra river

seismotectonics of transverse lineaments in the eastern himalaya and its foredeep abstract the himalayan collision zone and the burmese subduction zone lie in rather close vicinity across northeast india their possible interaction produces complex tectonics and a high level of seismicity in the region several prominent transverse lineaments across the eastern himalaya and its foredeep appear to be seismically active quite a few of the active lineaments are regionally extensive even transgressing the bengal basin a focal mechanism study indicates that active lineaments are either normal or strike slip faults for the highly active shillong mikir massif a fragmented portion of the indian shield this correlation is less clear although it appears that some of the activity may be associated with the northeasterly lineaments crossing the massif into the eastern himalayan foredeep this presumably results due to drag experienced by the indian lithosphere near its margins under the himalayan and burmese arcs the assam valley which is the common foredeep for both the arcs is remarkably aseismic though it is surrounded by active regions mainly thrusting mechanisms characterize the earthquakes which originate from the himalayan and burmese arcs adjoining the valley a model of basement reactivation below the valley is proposed in order to explain the style of tectonic deformation and current seismicity in the himalayan and burmese orogens adjoining the valley

an intermediate depth earthquake beneath tibet source characteristics of the event of september 14 1976 this paper presents the results of a detailed study of an intermediate depth earthquake beneath southern tibet september 14 1976 0643 51 6 ut 29 8l n 89 57 e focal depth 90 10 km mb 5 4 from the differences in arrival times of p pp and sp and from rayleigh wave spectral densities the focus is estimated to be at depth of 90 10 km and therefore in the uppermost mantle from the polarities of the first motions of p waves inversion of the relative amplitudes of p pp and sp and synthesis of their waveforms the source mechanism is constrained to be predominantly normal faulting with the t axis approximately horizontal and trending east west this mechanism is similar to those of shallow earthquakes in tibet the occurrence of an earthquake at depth of 90 km is consistent with rapid deformation at a temperature of about 700 to 800 c in the mantle beneath the tibetan crust the seismic moment of an equivalent double couple point source is estimated to be about 9 1024 dyne cm with an uncertainty factor of 1 3 from the sp amplitudes in the period range near 10 s and 5 1024 dyne cm also with an uncertainty factor of 1 3 from the spectral density of rayleigh waves with periods of 50 s

earthquake ground motion and 3d georgia basin amplification in southwest british columbia shallow blind thrust scenario earthquakes abstract finite difference modeling of 3d long period 2 s ground motions for large m w 6 8 scenario earthquakes is conducted to investigate the effects of the georgia basin structure on ground shaking in greater vancouver british columbia canada scenario earthquakes include shallow blind thrust north america na plate earthquakes simulated in locations congruent with linear clusters of shallow seismicity that is potential active faults a slip distribution model of the m w 6 7 northridge california blind thrust earthquake with the hypocenter modified to 5 km depth is used to characterize the source rupture process two sets of simulations are performed for a given scenario earthquake using models with and without georgia basin sediments the ratio of predicted peak ground velocity pgv for the two simulations is applied here as a quantitative measure of amplification due to 3d basin structure a total of eight shallow blind thrust na plate scenario earthquakes are simulated within 100 km of greater vancouver overall predicted ground motions are higher in the down dip direction of each epicenter due to the source radiation pattern hence scenario earthquakes located south of vancouver produce the highest motions in the city the average maximum pgv at stiff soil sites across greater vancouver considering all eight scenario earthquakes is 17 8 cm s modified mercalli intensity vii the average increase in peak motion due to the presence of georgia basin sediments is a factor of 4 1 the effective duration of moderate level 3 4 cm s shaking within greater vancouver is an average of 22 s longer when georgia basin sediments are included in the 3d structure model online material snapshots and videos of wave propagation and peak ground velocity maps

ground motion simulation in the lower tagus valley basin throughout history the lower tagus valley ltv region has been shaken by several earthquakes including some with moderate to large magnitudes and with sources located inside the basin for example the 1344 m6 0 and 1909 m6 0 earthquakes previous simulations bezzeghoudet al natural hazard 69 1229 1245 2011 have revealed strong amplification of the amplitude waves in the region because of the effect of the low velocity sediments that fill the basin the model used in those simulations was updated in this work by including new high resolution geophysical and geotechnical data available for the area seismic reflection aeromagnetic gravimetric deep wells standard penetration tests and geological data to contribute to improved assessment of seismic hazard in the ltv we simulated propagation of seismic waves produced by moderate earthquakes in a 3d heterogeneous medium by using elastic finite difference wave propagation code the method successfully used by grandinet al geophys j int 171 1144 1161 2007 involves evaluation of the seismic potential of known faults in the area studied and three dimensional seismic ground motion modelling by use of finite difference methods on the basis of this methodology we calculated the ground motion for the april 23 1909 benavente portugal earthquake mw 6 0 in dense grid points then computed the synthetic isoseismic map of the area by use of appropriate relationships between seismic intensity mmi and peak ground velocity pgv the synthetic results in contrast with available macroseismic and instrumental data enable validation of the source models proposed for the area identification of the sources of historical earthquakes and could also indicate which areas are more exposed to seismic ground motion

tectonic implication of drainage set up in the sub himalaya a case study of papumpare district arunachal himalaya india abstract the sub himalaya deformed between the main boundary thrust and or bomdila thrust towards north and the himalayan frontal thrust hft towards south is characterized by the presence of fault propagated folds and an imbricate thrust system along the hft the mountain front is offset for about 10 km sinistrally along a nw se trending transverse fault the banderdewa fault which runs through the dikrang river valley that seems to have followed a pull apart basin developed along the banderdewa fault the western part of study area associated with a major folding faulting and the development of fault bounded quaternary basins with normal faulting along their northern margins is interpreted to be tectonically more transported towards southeast than the eastern part where no quaternary basins are developed the channel forms and landforms have been studied in relation to the neotectonics of the area the watersheds exhibit their structurally controlled nature with drainages following tectonic alignments rivers streams experienced compressed meandering narrowing widening and deflection of channels longitudinal profiles of river stream channels exhibit knick points unpaired terraces are observed at higher levels factors like the unconsolidated siwalik rocks tectonic instability along faults and high rainfall with the advent of the holocene led to the development of the present day drainage set up a tectonic model is proposed for development of tectonic geomorphology of the area influenced by ultra tremor to low magnitude earthquakes in seismically very active northeastern india as moderate to big earthquakes m 6 0 are scarce in this region

variability of site response in seattle washington ground motion from local earthquakes and the ships seismic hazards investigation in puget sound experiment is used to estimate site amplification factors in seattle earthquake and ships records are analyzed by two methods 1 spectral ratios relative to a nearby site on tertiary sandstone and 2 a source site spectral inversion technique our results show site amplifications between 3 and 4 below 5 hz for west seattle relative to tertiary rock these values are approximately 30 lower than amplification in the duwamish valley on artificial fill but significantly higher than the calculated range of 2 to 2 5 below 5 hz for the till covered hills east of downtown seattle although spectral amplitudes are only 30 higher in the duwamish valley compared to west seattle the duration of long period ground motion is significantly greater on the artificial fill sites using a three dimensional displacement response spectrum measure that includes the effects of ground motion duration values in the duwamish valley are 2 to 3 times greater than west seattle these calculations and estimates of site response as a function of receiver azimuth point out the importance of trapped surface wave energy within the shallow low velocity sedimentary layers of the duwamish valley one dimensional velocity models yield spectral amplification factors close to the observations for till sites east of downtown seattle and the duwamish valley but underpredict amplifications by a factor of 2 in west seattle a two dimensional finite difference model does equally well for the till sites and the duwamish valley and also yields duration estimates consistent with the observations for the duwamish valley the two dimensional model however still underpredicts amplification in west seattle by up to a factor of 2 this discrepancy is attributed to 3d effects including basin edge induced surface waves and basin geometry focusing effects caused by the proximity of the seattle thrust fault and the sediment filled seattle basin

attenuation and source parameters of earthquakes in the cascadia region digital ground motion data recorded by the western canada telemetered network wctn are used to examine the attenuation and source parameters of earthquakes in the cascadia region of southwestern british columbia and northwestern washington the data base is comprised of over 1000 vertical component fourier spectra from earthquakes of magnitude 3 to 7 at distances from 10 to 500 km regression analyses determine the shape and coefficients of the regional attenuation curve and source spectra for 68 earthquakes seismic moment and stress drop are inferred from the amplitudes of the source spectra shallow h 10 km earthquakes have an attenuation curve with a complex shape exhibiting significant flattening no apparent geometric spreading in the distance range from 75 to 230 km this shape is probably a result of strong reflected phases from mid crustal discontinuities and the subducting slab events within the subducting slab and the lower crust exhibit a simple r 1 attenuation curve the anelastic attenuation coefficient for the region as a whole is given by q 380 f 0 39 the duration of motion for each wctn record is determined as the value that yields the observed relationship between time domain and spectral domain amplitudes according to random process theory these durations are approximately constant within 50 km of the source then increase with distance as 0 07 r the high frequency ground motions from cascadia earthquakes are relatively weak cascadia source spectra are characterized by an average brune stress drop of about 30 bars this is significantly lower than the average california stress drop of 70 to 100 bars and dramatically lower than the average eastern stress drop of 150 to 200 bars it is concluded that there are significant regional variations in source parameters hazard estimates for the cascadia region based on california groundmotion relations may be overly conservative

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process

simulation of broadband ground motion including nonlinear soil effects for a magnitude 6 5 earthquake on the seattle fault seattle washington the seattle fault poses a significant seismic hazard to the city of seattle washington a hybrid low frequency high frequency method is used to calculate broadband 0 20 hz ground motion time histories for a m 6 5 earthquake on the seattle fault low frequencies 1 hz are calculated by a stochastic method that uses a fractal subevent size distribution to give an 2 displacement spectrum time histories are calculated for a grid of stations and then corrected for the local site response using a classification scheme based on the surficial geology average shear wave velocity profiles are developed for six surficial geologic units artificial fill modified land esperance sand lawton clay till and tertiary sandstone these profiles together with other soil parameters are used to compare linear equivalent linear and nonlinear predictions of ground motion in the frequency band 0 15 hz linear site response corrections are found to yield unreasonably large ground motions equivalent linear and nonlinear calculations give peak values similar to the 1994 northridge california earthquake and those predicted by regression relationships ground motion variance is estimated for 1 randomization of the velocity profiles 2 variation in source parameters and 3 choice of nonlinear model within the limits of the models tested the results are found to be most sensitive to the nonlinear model and soil parameters notably the overconsolidation ratio

response of high rise and base isolated buildings to a hypothetical mw 7 0 blind thrust earthquake high rise flexible frame buildings are commonly considered to be resistant to shaking from the largest earthquakes in addition base isolation has become increasingly popular for critical buildings that should still function after an earthquake how will these two types of buildings perform if a large earthquake occurs beneath a metropolitan area to answer this question we simulated the near source ground motions of a m w 7 0 thrust earthquake and then mathematically modeled the response of a 20 story steel frame building and a 3 story base isolated building the synthesized ground motions were characterized by large displacement pulses up to 2 meters and large ground velocities these ground motions caused large deformation and possible collapse of the frame building and they required exceptional measures in the design of the base isolated building if it was to remain functional

focal depths and fault plane solutions of earthquakes under the tibetan plateau we compare synthetic and recorded p wave forms to place constraints on the focal depths and fault plane solutions of 16 crustal earthquakes beneath the highest parts 4000 m of the tibetan plateau fault plane solutions for all 16 events show combinations of normal and strike slip faulting with t axes oriented approximately east west none of these solutions show thrust faulting thus the data corroborate previous inferences that the active tectonics are dominated by east west extension focal depths for all 16 events are less than 15 km and appear to be between 5 and 10 km this style of deformation and these depths of faulting are similar to those in the basin and range province of the western united states two intermediate depth events below the crust of southern tibet also show primarily normal faulting with east west t axes the solution for one discussed by chen et al 1981 is unambiguous the solution for the other the event of august 1 1973 27 59 n 89 17 e 85 10 km mb 4 9 is less certain both apparently occurred in the mantle beneath a thick aseismic lower crust and their occurrence suggests that brittle deformation occurs there in response to a stress field similar to that operating at shallow depths beneath tibet

microearthquakes at the main boundary thrust in eastern himalaya and the present day tectonic model abstract two microearthquake surveys were carried out at the main boundary thrust mbt eastern himalaya temporal as well as spatial variation in microseismicity was observed the vertical section of the hypocentres shows subcrustal earthquakes down to a depth of 80 km to the south of the mbt and composite focal mechanisms show strike slip solutions or transverse tectonics which do not agree with the conceptual tectonic model

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

basement control on oblique thrust sheet evolution seismic imaging of the active deformation front of the central andes in bolivia abstract in the area of the bolivian orocline we examine the deformation pattern associated with the active development of a new thrust sheet a dense grid of reprocessed 2 d seismic lines from hydrocarbon exploration industry is interpreted and a 3 d simplified structural and kinematic model is deduced in the boomerang hills onlapping paleozoic and foredeep sediments are detached from the underlying s dipping basement they are thrust northeastwards by less than 2 km two zones can be differentiated along the andean deformation front 1 a w e to nw se striking frontal segment of predominantly orthogonal shortening comprising a thrust and anticline system 2 a wsw ene striking lateral zone of oblique shortening within a complex system of thin skinned strike slip faults and minor folds the deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the paleozoic basin the observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography which provides a near oblique ramp for the onlapping wedge of sediments the sw ne thrusting direction is divided into orthogonal and tangential components these are accommodated by convergent and strike slip structures respectively which sole into a common detachment horizon the structural evolution of the new thrust sheet in the bolivian orocline is primarily controlled by the paleorelief of the brazilian shield because 1 the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and 2 small asperities in close to the top of the basement promote fault localization the coincidence of a relatively high basement position and a structural high of the eastern cordillera leads to the conclusion that the shape of the brazilian shield also controls the structural evolution of the pronounced eastern border of the bolivian orocline

abrupt uplift within the past 1700 years at southern puget sound washington shorelines rose as much as 7 meters along southern puget sound and hood canal between 500 and 1700 years ago evidence for this uplift consists of elevated wave cut shore platforms near seattle and emerged peat covered tidal flats as much as 60 kilometers to the southwest the uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform the tidal flats also emerged abruptly they changed into freshwater swamps and meadows without first becoming tidal marshes where uplift was greatest it adjoined an inferred fault that crosses puget sound at seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago the uplift and probable fault slip show that the crust of the north america plate contains potential sources of damaging earthquakes in the puget sound region

pop up tectonics of the shillong plateau in the great 1897 earthquake ms 8 7 insights from the gravity in conjunction with the recent seismological results 1 it was reported that a hidden 110 km long south dipping fault named oldham fault was responsible for the 1897 great earthquake ms 8 7 in the shillong plateau by reverse faulting due to pop up tectonics of the plateau in northeast india here we report the results of our geophysical investigations where we critically examine the crustal structure of the plateau on the basis of gravity modeling and attempt to shed light on the hitherto debated hidden oldham fault at the northern boundary of the shillong plateau our gravity model constrained by broadband seismological data suggests that the moho beneath the shillong plateau is at a shallower depth of about 35 km when compared to the bengal basin to the south and the assam valley to the north which is about 42 km thinning of the crust under the plateau may be a consequence of the pop up mechanism to examine the possible pop up of the plateau during the great 1897 earthquake we have estimated the energy released by this earthquake and compared it with the energy required for the pop up of the plateau using a simplistic rigid model it is found that the shillong plateau between the oldham fault and dauki dapsi thrust would require an energy of 4 5 1019 j which does match well with the energy released by the great earthquake of ms 8 7

a three dimensional simulation of seismic waves in the santa clara valley california from a loma prieta aftershock abstract the finite difference method is used to propagate elastic waves through a 3 d model of the santa clara valley an alluvium filled basin that underlies the city of san jose california the model was based on depth to bedrock information from water wells in the area the simulation corresponded to a region 30 east west by 22 north south by 6 depth km and contained about 4 million grid points synthetic seismograms from the simulation are accurate at frequencies up to 1 hz motions from a magnitude 4 4 aftershock of the loma prieta earthquake were modeled snapshots of ground motion and synthetic seismograms from the simulation are presented the simulation illustrates s to surface wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics because of the slow group velocity of the love waves sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges sites near the center of the basin also show larger peak amplitudes on the transverse component array analysis of the synthetic seismograms indicates that love waves tend to propagate parallel to the eastern and western edges of the valley rayleigh waves are produced along the southern margin of the basin from incident s waves large radial motions occur where a rayleigh wave impinges on the northeast margin of the valley some rayleigh waves travel westward across the basin after being scattered from the eastern edge of the valley synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the sunnyvale dense array for this aftershock although the duration of the tranverse component is not matched by the synthetic seismogram using this basin model the simulation indicates that the love waves observed on the actual seismograms were produced by conversion of incident s waves at the southern margin of the santa clara valley 2 d simulations show how the s to love wave conversion is affected by the angle of incidence of the s wave and the sharpness of the velocity transition between the alluvium and bedrock as more accurate basin models are developed 3 d simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes

nonlinear attenuation and rock damage during strong seismic ground motions strong seismic waves cause nonlinear behavior in the shallow subsurface in fractured rocks seismologists use low amplitude signals from small repeating earthquakes to measure s wave velocity decrease after strong motion the 2004 parkfield california earthquake provides examples of such velocity changes in fractured sandstone with an s wave velocity of 300 m s 1 this nonlinear behavior occurred around the wave number depth of the incident waves 30 m for the 10 s 1 dominant angular frequency on a velocity seismogram the low amplitude s wave velocity gradually recovered with the logarithm of time the attenuation of strong waves in general depends nonlinearly on their amplitude high dynamic stress triggered small very shallow earthquakes at sites including parkfield the theoretical frictional behavior of a fractured medium with heterogeneous prestress relates these phenomena failure occurs in the highly prestressed domains causing small earthquakes and opening mode cracks the energy to dilate the cracks dissipates a significant fraction of the incoming seismic energy the local high porosity domains close with the logarithm of time as expected from the aging law of rate and state friction increasing the s wave velocity the domain model indicates that nonlinear effects increase gradually over a range of dynamic coulomb stresses as observed and as included in the widely used masing rules the linker and dieterich 1992 relationship provides the maximum sustainable dynamic coefficient of friction needed to utilize the masing rules this parameter is the coefficient of friction at a laboratory normal traction plus a constant 0 15 times the logarithm of the ratio of field normal traction to the laboratory normal traction it is helpful to relate s wave velocity to starting frictional strength as coefficient of friction near the quarter wavelength depth determines nonlinear behavior then the dynamic coefficient of friction and equivalently the maximum sustainable acceleration at the dominant frequency depend weakly on s wave velocity for example the coefficient of friction at an angular frequency of 10 s 1 is less than 1 5 for rocks ranging from parkfield sandstone to intact granite

simulation of strong ground motion for an mw 7 0 earthquake beneath the bhutan himalaya in ne india and its trans boundary seismic hazard implications abstract we simulate the strong ground motion for an expected major earthquake mw 7 0 beneath the bhutan himalayan region with an empirical green s function method using waveforms recorded from the 2009 bhutan mainshock mw 6 1 and its largest aftershock mw 5 1 fault orientation and location of the simulated event extend the length of the fault plane determined from the 2009 bhutan mainshock and aftershock along its strike the simulated pga values are compared with the results derived from a ground motion prediction equation gmpe for the himalayan region and it is found that different levels of accelerations are associated with different rupture initiation points on the fault plane it is observed that the ne indian region is capable of generating peak ground acceleration pga in exceedance to 121 cm s2 for simulated earthquake mw 7 0 the maximum impacts of shaking will be on the sites located near the rupture initiation points that are poised to generate higher values of ground acceleration to validate our simulation we also estimate the extent of rupture directivity of the simulated earthquake with respect to four initiation points indicating that higher value of pga and shaking duration exist either to south or to south west azimuths from the imitated locations which are corroborated with respective geology of the sites this study suggests that areas of maximum ground shaking would occur in the vicinity of the source initiation where possibility of relatively stronger earthquake hazards does exist which in turn requires attention for adoption of earthquake risk mitigation plans in view of impacts of trans boundary earthquakes in the region

a study of source parameters site amplification functions and average effective shear wave quality factor qseff from analysis of accelerograms of the 1999 chamoli earthquake himalaya the accelerograms of the 1999 chamoli earthquake and nine of its aftershocks which occurred in uttaranchal himalaya have been analyzed to investigate their source parameters the site amplification functions and the average effective shear wave quality factor qseff in the region the fault plane solution of the main shock is obtained using the spectral amplitudes of sh waves approximated by transverse components of accelerograms of the high energy packets observed in the accelerograms of the main shock it is found to be comparable with the reported solutions in other studies similarly the other source parameters viz seismic moment 5 03 1 7 1025 dyne cm stress drop 65 bars source duration 5 2 s and moment magnitude 6 4 estimated for the main shock are consistent with the values obtained in other studies the stress drops estimated for the aftershocks vary from 23 bars to 153 bars and the seismic moment from 1 4 1023 dyne cm to 2 9 1023 dyne cm the average estimated values of the effective shear wave quality factor qseff vary from 655 359 in the uttaranchal sector of himalaya and 1475 130 in the delhi region in general the qseff value increases with an increase in the epicentral distance reflecting the penetration of the waves into deeper layers of the crust as the epicentral distance of the observation point increases these values of qseff indicate that in general the curst is at low temperatures that will promote brittle behavior and conditions for episodic failure as compared to creep under the accumulated strains from plate collision at the himalaya plate boundary the site amplification characteristics at sites have been identified from the frequency bands of significant amplification observed in the spectral ratios of the horizontal to the vertical component records the decay of peak ground acceleration pga values with distance has been investigated using the empirical regression curves vis a vis the site amplification factors

seismic structure and evidence for eclogitization during the himalayan convergence abstract reflections on the deep moho of the tethyan himalayas have been obtained from vertical seismics at the time predicted by wide angle reflection interpretations supporting the previous inference of a low average crustal velocity a mostly felsic composition is then required through the 70 km of the thickened crust the moho appears as a heterogenous transition layer more than 10 km thick the high velocity measured beneath the moho from pn refractions cannot be attributed any more to e w oriented olivine since sks observations do not confirm such an anisotropy instead eclogite just under the seismic moho could account for the observations these three results from independent parts of the seismic data set support a model of the himalayan convergence in which the indian crust has undergone phase transformation in the granulite eclogite system upon burial the intermediate and basic composition rocks of the middle and lower layers of an initial crust of average platform type can be transformed to such high velocities that they are lost across the himalayas into the level of the moho or just under the moho defined by seismic velocity mostly upper crustal type material then remains to form the thickened crust above consistent with the observed low velocity the variation in moho depth across the himalayas as suggested from wide angle seismics accounts for the first order variation in teleseismic transit times variations in the velocity depth structure of the mantle hence do not dominate but a change in the orientation of sks splitting anisotropy would suggest deformation within the lithosphere to change across the high himalayas and the indus tsangpo suture

three dimensional simulations of ground motions in the seattle region for earthquakes in the seattle fault zone we used the 3d finite difference method to model observed seismograms of two earthquakes m l 4 9 and 3 5 in the seattle region and to simulate ground motions for hypothetical m 6 5 and m 5 0 earthquakes on the seattle fault for periods greater than 2 sec a 3d velocity model of the seattle basin was constructed from studies that analyzed seismic reflection surveys borehole logs and gravity and aeromagnetic data the observations and the simulations highlight the importance of the seattle basin on long period ground motions for earthquakes occurring just south of the basin the edge of the basin and the variation of the thickness of the quaternary deposits in the basin produce much larger surface waves than expected from flat layered models the data consist of seismograms recorded by instruments deployed in seattle by the usgs and the university of washington uw the 3d simulation reproduces the peak amplitude and duration of most of the seismograms of the june 1997 bremerton event m l 4 9 recorded in seattle we found the focal mechanism for this event that best fits the observed seismograms in seattle by combining green9s functions determined from the 3d simulations for the six fundamental moment couples the february 1997 event m l 3 5 to the south of the seattle basin exhibits a large surface wave arrival at uw whose amplitude is matched by the synthetics in our 3d velocity model for a source depth of 9 km the m 6 5 simulations incorporated a fractal slip distribution on the fault plane these simulations produced the largest ground motions in an area that includes downtown seattle this is mainly caused by rupture directed up dip toward downtown radiation pattern of the source and the turning of s waves by the velocity gradient in the seattle basin another area of high ground motion is located about 13 km north of the fault and is caused by an increase in the amplitude of higher mode rayleigh waves caused by the thinning of the quaternary deposits

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

himalayan main central thrust and its implications for himalayan inverted metamorphism abstract within the scheme of south directed orogenic polarity in the himalayan region the main central thrust mct developed at an intermediate stage between the collisional tectonics at the indus tsangpo suture in the north and the latest underthrusting at the main boundary thrust mbt in the south the proposition that the mct marks the base of the high himalayan central crystalline zone against the inner sedimentary belt of the lower himalayas creates confusion in its definition and its location in kashmir and especially in the eastern himalayas the problem can be resolved by defining the mct as the basal thrust of the crystalline nappe sequences either rooted at or detached as klippe from the central crystalline zone characteristically the nappe sequences show inverted metamorphism which generally starts at the basal thrust the redefined mct from the chlorite biotite grades and reaches the kyanite sillimanite grades with migmatites and anatectic granites in the highest tectonic levels the metamorphic rank of both the substrate and the overthrust rock units in the vicinity of the mct is generally low the inverted metamorphism may be explained by intracontinental underthrusting and resultant downbowing of the isotherms the redefined mct does not appear to represent the thrust zone along which the continental underthrusting was initiated it developed to the south of this zone as a possible sole thrust of the nappe stack of the crystalline rocks that overrode the parautochthonous sedimentary sequences of the lower himalayas the sequence of events associated with the mct would probably be repeated in the mbt

nonlinear behavior of strong surface waves trapped in sedimentary basins abstract numerical calculations and observations indicate that surface waves reverberate within sedimentary basins a site response approach is inapplicable to the nonlinear attenuation of these waves as the energy passes repeatedly through the shallow subsurface computed dynamic stresses obtained by using published shakeout calculations graves et al 2008 indicate that nonlinear attenuation is modest but not negligible in the los angeles california basin for large earthquakes on the san andreas fault the dominant surface waves have velocity amplitudes of 1 5 m sec 1 and periods 3 5 sec around 20 of the total elastic strain energy resides above dynamic lithostatic stress ratios of 0 2 for both rayleigh and love waves where prestressed rock is highly likely to fail somewhat in friction

predictions of shear wave velocities in southern california using surface geology a new model of the average shear wave velocity in the uppermost 30 m has been generated by extrapolation of discrete velocity profiles using surface geology at several scales statistical methods have been applied to create a map that is no more complicated than is supported by the velocity data several geologic units with similar responses are grouped together the resulting map is simpler than previous ones and yet fits the observed velocity profiles better than earlier more complicated maps analysis within a geographic information system will permit updates and modification of the map as new velocity data are added

high frequency seismic wave propagation beneath the indian shield himalayan arc tibetan plateau and surrounding regions high uppermost mantle velocities and efficient sn propagation beneath tibet summary this paper reports the regional propagation characteristics of high frequhcy p p s and l waves 0 5 to 2hz and p pg and s velocities for several regions in central asia tibet and india an important result is that s waves propagate efficiently in the uppermost mantle beneath most of the tibetan plateau also s transmission in tibet is not affected by crossing many physiographic and tectonic boundaries such as the indus tsangpo suture these observations are similar to observations of s propagation in stable continental and shield regions efficient s propagation is also observed across the stable tarim basin along the tien shan and himalayan mountains and across the indian shield s is strongly attenuated across the northern tibetan plateau chang thang terrane such strong attenuation suggests the existence of a low q zone in the uppermost mantle beneath that region this interpretation is supported by recent geological observations of widespread quaternary to recent basaltic volcanism in the chang thang terrane l propagates efficiently across the tarim basin the indian shield and along the strike of the himalayan mountains l is also observed although less pronounced for paths crossing large portions of the complicated structures of tien shan karakoram and pamir l is not observed for paths crossing the tibetan plateau the southern boundary of the region of inefficient l propagation in tibet lies in the vicinity of the indus tsangpo suture zone the lack of l can be attributed to attenuation within the tibetan crust however other possibilities including scattering by structural discontinuities at the margins of tibet are equally plausible pg and s are the most prominent features on most short period wwssn seismograms at epicentral distances less than about 4 apparent pg velocity determined for the area of the himalayan mountains is 6 0 0 2 km s l apparent velocities of p and s waves that have at least 50 per cent of their paths in the uppermost mantle beneath the tibetan plateau are 8 42 it 0 10 and 4 73 f 0 06 km s respectively these velocities are quite similar to those determined for the himalayan mountains p 8 45 f 0 08 km s i s 4 74 0 05 km s and the indian shield p 8 40 0 08 km s s 4 69 k 0 04 km s p