surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

estimation of strain distribution using gps measurements in the kumaun region of lesser himalaya abstract the continuous process of continent continent collision between the indian and the eurasian plates has led to the formation of the himalayan range and continuously caused earthquakes in the region large earthquakes with magnitudes of 8 and above occur in this region infrequently releasing the elastic strain accumulated over years around the plate boundary geodetic measurements can help estimate the strain distribution along the fault system these measurements provide information on active deformations and associated potential seismic hazards along the himalayan arc in order to understand the present deformation around the plate boundary we collected gps data during three campaigns in the years of 2005 2007 at 16 sites in the kumaun region of the lesser himalaya horizontal velocity vectors estimated in itrf2000 are found to be in the range of 41 50 mm yr with an uncertainty level of the order of 1 mm yr the velocity field indicates that the present convergence of around 15 mm yr takes place in the kumaun himalaya further we estimate the strain components in the study area for understanding the currently active tectonic process in the region the estimated dilatational strain indicates that the northern part near the main central thrust mct is more compressional than the southern part maximum shear strain is mostly accommodated in the northern part too the maximum shear and dilatational strain rates are about 1 0 and 0 5 strain yr it is seen that the distribution of high shear strain spatially correlates with seismicity the maximum of extensional and compressional strains due to the force acting along the main central thrust mct in the nw se direction are found to be 0 4 and 0 1 strain yr respectively the maximum shear strain in the northern part of the himalaya appears to be associated with the convergence of the region proposed by other geophysical studies

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

cenozoic tectonics in the urumgi korla region of the chinese tien shan cenozoic deformation within the tien shan of central asia has accommodated part of the post collisional indentation of the indian plate into asia within the urumgi korla region of the chinese tien shan this occurred dominantly on thrusts with secondary strike slip faulting the gross pattern of deformation is of moderate to steeply dipping thrusts that have overthrust foreland basins to the north and south of the range the junggar and tarim basins respectively smaller foreland basins lie within the margins of the range itself turfan chai wo pu korla and qumishi basins these lie in the footwalls of local thrust systems both the turfan and the korla basins contain major thrusts within them they are complex foreland basins deformation has progressively affected regions further into the interior of the junggar basin and propagated into the interiors of the intermontane basins no unidirectional deformation front has passed across the tien shan in the neogene and quaternary an oligocene unconformity may indicate the time of the onset of the cenozoic deformation but most of the cenozoic molasse has been deposited after the palaeogene the rate of deposition in basins next to the uplifted ranges has increased since the onset of deformation there has been at least about 80 km of cenozoic shortening across this part of the tien shan cenozoic shortening is greater in sections of the range further west these are nearer to the northern margin of the indian indenter cenozoic compression has reactivated structures created by the two late palaeozoic collisions that created the ancestral tien shan these palaeozoic structures have exerted a strong control over the style and location of the cenozoic deformation

a new depression dominated delineation d cubed method for improved watershed modelling prior to hydrologic modelling topographic features of a surface are derived and the surface is divided into sub basins surface delineation can be described as a procedure which leads to the quantitative rendition of surface topography different approaches have been developed for surface delineation but most of them may not be applicable to depression dominated surfaces the main objective of this study is to introduce a new depression dominated delineation d cubed method and highlight its unique features by applying it to different topographic surfaces the d cubed method accounts for the hierarchical relationships of depressions and channels by introducing the concept of channel based unit cbu and its connection with the concept of puddle based unit pbu this new delineation method implements a set of new algorithms to determine flow directions and accumulations for puddle related flats the d cubed method creates a unique cascaded channel puddle drainage system based on the channel segmentation algorithm to demonstrate the capabilities of the d cubed method a small laboratory scale surface and 2 natural surfaces in north dakota were delineated the results indicated that the new method delineated different surfaces with and without the presence of depressional areas stepwise changes in depression storage and ponding area were observed for the 3 selected surfaces these stepwise changes highlighted the dynamic filling spilling and merging processes of depressions which need to be considered in hydrologic modelling for depression dominated areas comparisons between the d cubed method and other methods emphasized the potential consequences of use of artificial channels through the flats created by the depression filling process in the traditional approaches in contrast in the d cubed method sub basins were further divided into a number of smaller cbus and pbus creating a channel puddle drainage network the testing of the d cubed method also demonstrated its applicability to a wide range of digital elevation model resolutions consideration of cbus pbus and their connection provides the opportunity to incorporate the d cubed method into different hydrologic models and improve their simulation of topography controlled runoff processes especially for depression dominated areas

late quaternary to decadal velocity fields in asia 1 fields of crustal velocity and strain rate within asia averaged over horizontal distances of 300 km are calculated from estimates of quaternary slip rates on faults and rates of change of lengths of baselines between gps sites the calculated fields fit strain rate components estimated from quaternary slip rates within 95 confidence limits and fit the gps data with an rms residual of 2 5 mm yr 1 the strain rate field constrained only by quaternary slip rates of faults differs from the combined solution at the level of 20 nstrain yr 1 and the differences are explicable by the limited accuracy of geological estimates of slip rate therefore the strain rate fields determined by geodetic measurements taken on the 10 year timescale are consistent with geological observations on the 104 year timescale the combined solution for velocities shows plate or block like motion within deforming asia only in tarim south china and the amurian region elsewhere the kinematics of rigid blocks does not provide a useful description of the motion in particular velocities relative to eurasia point at high angles to contours of their magnitudes not parallel to them as they would if large regions moved as rigid blocks the axes of principal horizontal compressional strain align with topographic gradients in agreement with previous suggestions that asia deforms as a continuum under the influence of gravity

global detection and location of seismic sources by using surface waves we develop an algorithm for the detection and location of seismic sources using intermediate period 35 150 sec surface waves recorded on a global array of stations continuous vertical seismic waveforms from the global network are collected and a 4 4 global grid of target locations is defined for each target location and each station a surface wave propagation operator is deconvolved from the seismogram to restore any source pulse present the envelope of the seismogram is calculated and cross correlated with a theoretical source pulse shape the resulting waveforms are stacked to improve signal to noise characteristics and the quality strength and timing of the potential detection are determined when a successful event detection is made a finer grid is applied to locate the event with greater precision we apply the algorithm systematically for the period 1993 2003 and catalog the events approximately 2000 events are detected and located for each year and 98 of shallow events in the harvard centroid moment tensor cmt catalog are detected and located by the new algorithm a comparison of 9482 events common to the two catalogs allows the detection strength to be calibrated against the cmt moment magnitude all detected events have estimated moment magnitudes m w 4 5 in each year approximately 100 events not listed in other global seismicity catalogs are detected and located many of these events lie along the ridge transform plate boundaries in the southern hemisphere and may be regular earthquakes that have gone undetected because of poor station coverage other events located in areas where global and regional networks provide good coverage are potentially anomalous and may have escaped detection as a result of their unusual source properties

tectonics and seismicity of the toktogul reservoir region kirgizia ussr the 214 m high toktogul dam will impound a 19 5 km3 reservoir on the naryn river in central kirgizia the reservoir lies along the talas fergana fault a major tectonic feature of central asia the nature of cenozoic movements along the fault is in debate however there is considerable evidence of present day right lateral movement the region is one of moderate to high seismicity a magnitude 7 6 earthquake occurred in 1946 approximately 65 km from the dam near the intersection of the talas fergana fault and chatkal ranges construction of the dam was completed in 1975 and by late 1979 the water level in the reservoir reached three fourths its final planned depth low magnitude induced earthquakes began as the water level first exceeded 100 m in late 1977 a microearthquake survey using portable instruments was carried out in july august 1978 and a seven station telemetered network was installed in october 1978 the largest of the induced earthquakes has been of magnitude 2 5 the activity is in the immediate vicinity of the dam at very shallow depths 2 5 km and is related to water level preliminary analysis of data gathered during 1978 1979 shows a zone of activity extending to 5 km depth supporting geological evidence for an active reverse fault beneath the dam and parallel to the canyon of the naryn river where the reservoir crosses the talas fergana fault 15 km upstream from the dam the water level has not yet reached 100 m and seismicity along the talas fergana has not shown any significant change related to filling of the reservoir

focal depths and fault plane solutions of earthquakes and active tectonics of the himalaya synthetic seismograms were compared with long period body waves for nine earthquakes with epicenters in the himalayan arc to determine depths of foci and to improve fault plane solutions focal depths are shallow 10 20 km inferred slip vectors are locally perpendicular to the mountain range they plunge very gently about 10 deg in the eastern sections of the range and more steeply about 25 deg in western sections assuming india to be a rigid plate the radially oriented slip vectors imply that southern tibet extends at about half the rate of underthrusting in the himalaya and therefore probably at about 5 10 mm yr the shallow depths and gentle dips of the fault planes at least for the events in the eastern half of the range are consistent with coherent underthrusting of the indian plate beneath at least the lesser himalaya the steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the greater himalaya

validation of regional and teleseismic travel time models by relocating ground truth events recently developed 3d global seismic velocity models have demon strated location improvements through independent regional and teleseismic travel time calibration concurrently a large set of high quality ground truth gt events with location accuracies 10 km or better gt0 gt10 has been collected for europe the mediterranean north africa the middle east and western eurasia in this study we demonstrate event location improvements using this new data set by applying the regional and teleseismic model based travel time calibrations independently and jointly besides relocating events using all arrivals a subset of the gt events was also relocated using controlled station geometries generated from a constrained boot strapping technique this approach simulates sparse networks and reduces the effect of correlated errors to ensure valid 90 error ellipse coverage statistics with respect to the gt events we compared event relocations with and without travel time cal ibrations considering statistics of mislocation error ellipse area 90 coverage or igin time bias origin time errors and misfit relocations of over 1200 gt events show that pn and or p calibration reduced mislocation for 60 70 of the events joint regional pn and teleseismic p travel time calibration provided the largest lo cation improvements and achieved approximately gt5 accuracy levels due to cor related errors event locations using large numbers of stations have deficient 90 error ellipse coverage however the coverages derived from the model errors are appropriate for the simulated sparse regional and teleseismic networks our valida tion effort demonstrates that the global model based calibrations of pn and teleseis mic p travel times reduce both location bias and uncertainty over wide areas

ability of a global three dimensional model to locate regional events 1 we assess the ability of a global three dimensional 3d seismic model of the crust and upper mantle cub2 0 to locate seismic events using regional travel time data alone epicentral distances 20 assessments are based on a ground truth gt database comprising nearly 1000 uncommonly well located events which occur in 23 event clusters across eurasia and north africa groomed p and pn arrival time data observed at regional distances from individual events and 1000 empirical phase path anomalies produced from event clusters evidence is presented that supports the conversion of the shear velocities vs in cub2 0 to compressional velocities vp in the upper mantle using the thermoelastic properties of an assumed mantle composition rather than traditional empirical scaling relationships two principal results are presented first this 3d vp model fits the empirical p and pn phase path anomalies very well in both pattern and absolute level second location assessments demonstrate that intrinsic regional location accuracy is 5 km using the 3d vp model and 10 km using the 1d model ak135 these findings highlight the importance of gt databases in assessing regional travel time models and establish that a global 3d vs model together with vp inferred from it provides a solid basis on which to build improvements in regional location capabilities

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

source modeling of the hsingtai china earthquakes of march 1966 abstract the hsingtai china earthquakes of march 1966 were a series of destructive earthquakes associated with the shu lu graben five strong shocks of ms 6 occurred within a period of less than a month the largest of which was ms 7 2 body and surface waves over the period range from several to 100 s have been modeled for the four largest events using synthetic seismograms in the time domain and spectral analysis in the frequency domain data from ground deformation local geology regional seismic network and teleseismic joint epicenter determination have also been used to constrain the source model and the rupture process the fault mechanism of the hsingtai sequence was mainly strike slip with a small component of normal dip slip the strikes of the four largest shocks range from n26 to 30 e approximately along strike of the major faults of the shu lu graben and the aftershock distribution the source mechanisms can be explained with a nnw sse extensional stress and a nee sww compressional stress acting in the area the major shocks all had focal depths 10 km the four largest shocks in the sequence were characterized by a relatively simple and smooth dislocation time history the durations of the far field source time functions ranged from 3 5 to 5 s while the rise times were all 1 s the seismic moments of the four largest earthquakes ranged from 1 43 1025 to 1 51 1026 dyne cm 1 the fault sizes of the four events were very close assuming circular faults the diameters of the four events were determined to be between 10 and 14 km stress drops varied from 52 to 194 bars a trend of increasing stress drop with earthquake size was observed a survey of stress drop determinations for 15 major intraplate earthquakes shows that on the average the magnitude of stress drop of oceanic intraplate earthquakes and passive continental margin events is higher 200 to several hundred bars than that of continental intraplate earthquakes 100 bars or less

large paleoearthquake timing and displacement near damak in eastern nepal on the himalayan frontal thrust an excavation across the himalayan frontal thrust near damak in eastern nepal shows displacement on a fault plane dipping 22 has produced vertical separation across a scarp equal to 5 5 m stratigraphic structural geometrical and radiocarbon observations are interpreted to indicate the displacement is the result of a single earthquake of 11 3 3 5 m of dip slip displacement that occurred 1146 1256 ad empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 800 km and moment magnitudes mw of 8 6 to 9 sufficient strain has accumulated along this portion of the himalayan arc during the roughly 800 years since the 1146 1256 ad earthquake to produce another earthquake displacement of similar size

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

focal mechanisms and aftershock locations of the songpan earthquakes of august 1976 in sichuan china the precursory swarm three mainshocks m 7 2 6 7 7 2 and aftershocks of the songpan earthquakes have been reanalyzed using both local and teleseismic data the three mainshocks of this sequence occurred on the huya fault over a 7 day period relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence each mainshock occurred on a separate strand each one south of the strand activated in the previous mainshock and the aftershock zones of each mainshock appear to abut rather than overlap fault plane solutions determined by matching teleseismic p waveforms at world wide standard seismograph network stations with synthetic seismograms are consistent with the observed aftershock zones the first and third mainshocks m 0 1 3 10 19 and 8 4 10 18 n m respectively showed almost identical senses of motion a combination of reverse and left lateral strike slip motion on parallel strands striking n15 w that were separated by a large rightstepping en echelon offset the second mainshock m 0 4 0 10 18 n m occurred in this offset on a fault at a steep angle 125 to the other two strands and showed almost pure reverse motion differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock these changes in normal stresses may have given rise to the longer time between the first and second events 5 days as compared with the time between the second and third events 30 hours a precursory swarm that preceded the songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone the time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in soviet central asia

seismicity and the nature of plate movement along the himalayan arc northeast india and arakan yoma a review abstract the himalaya together with arakan yoma form a well defined seismic belt to the north and east of the indian peninsula the seismicity along this belt is attributed mostly to collision between the indian and the eurasian plates however the exact nature of activity along the major thrusts and faults is not well understood the seismicity along the entire himalaya and northern burma has been studied in detail it has been found that besides the main boundary fault and the main central thrust several transverse features are also very active some of these behave like steeply dipping fracture zones along the arakan yoma most of the seismicity appears to be due to subduction of the indian lithosphere to the east analysis of focal mechanism solutions for the himalaya shows that although thrust movements are predominant normal and strike slip faulting is taking place along some of the transverse features in addition to thrusting strike slip faulting is also taking place along the arakan yoma orientation of p axes for all thrust solutions show a sharp change from predominantly east west along the burmese arc to n s and ne sw along the himalaya the direction further changes to nw se along the baluchistan arc it appears that the indian lithosphere is under compression from practically all sides the present day seismicity of northeast india and northern burma can be explained in terms of a plate tectonics model after nandy 1976 no simple model appears to be applicable for the entire himalaya

40ar 39ar dating of the jiali and gaoligong shear zones implications for crustal deformation around the eastern himalayan syntaxis abstract we conducted a comprehensive 40ar 39ar geochronological study of the jiali and gaoligong shear zones to obtain a better understanding of crustal deformation and tectonic evolution around the eastern himalayan syntaxis ehs the new age data reveal that the main phase of deformation in the jiali and gaoligong shear zones occurred from 22 to 11 ma and from 18 to 13 ma respectively structural data collected during this study indicate that the jiali shear zone underwent a change in shear sense from sinistral to dextral during its movement history based on a comparison with the deformation histories of other major shear zones in the region we argue that the initial sinistral motion recorded by the jiali shear zone was coincident with that of the ailao shan red river shear zone which marked the northern boundary of the southeastward extrusion of the indochina block during the early miocene from the middle miocene 18 ma the jiali shear zone changed to dextral displacement becoming linked with the dextral gaoligong shear zone that developed as a consequence of continued northward indentation of the indian continent into asia since this time the jiali and gaoligong shear zones have been united defining the southwestern boundary of the ehs during clockwise rotation of the eastward extruding tibetan block as revealed by recent gps data the temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern himalayan drainage system around the ehs

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

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

validating accuracy of rayleigh wave dispersion extracted from ambient seismic noise via comparison with data from a ground truth earthquake abstract in order to validate the accuracy of estimated green s functions egfs which are widely used in ambient seismic noise tomography a broadband seismograph was installed in the epicentral area of an m 5 earthquake with ground truth location from interferometric synthetic aperture radar observations egfs between this station and permanent stations are compared with surface waves excited by the earthquake the group velocity dispersion measured from egfs at large interstation distances 1000 km or longer are consistent with measurements from the earthquake and the egfs from symmetric noise correlation functions ncfs are usually more accurate at shorter interstation distances 300 1000 km the match between noise and earthquake dispersion is good for most stations although we observed mismatch for a few stations the mismatch is probably caused by low signal to noise ratio of ncfs or nondiffusive noise wavefield at short distances online material figure showing mismatch of group velocity dispersion between earthquake and noise data and figure of phase velocity dispersion between earthquake data and noise correlation functions at station enh

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

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

seismicity of central tibet from project indepth iii seismic recordings as part of project international deep profiling of tibet and the himalaya iii a 400 km long densely spaced array of 57 broadband and short period seismic stations was deployed in central tibet from august 1998 through may 1999 although originally designed to image the lithosphere with teleseismic events the array also recorded numerous local and regional seismic events more than 900 local and regional events were detected on at least 10 stations during the 1 year deployment and we were able to locate 267 local earthquakes a substantial number of the events were found to cluster in or near large grabens and along known strike slip faults while other events show no obvious correlation with known structures in addition to spatial clustering at least one of the large clusters also exhibits temporal clustering that may be associated with magmatic or geothermal activity in the upper crust the average v p and v s are estimated to be 5 85 and 3 35 km sec for the upper crust and 7 0 and 3 9 km sec for the lower crust respectively the 50 focal mechanisms computed from this set of events are consistent with north south shortening and east west extension there are no clear indications of significant local perturbations in the regional stress field induced by the collision between india and eurasia the majority of the focal mechanisms indicate normal and strike slip faulting at least six of the newly computed focal mechanisms however indicate thrust faulting which is a phenomenon not well documented previously ninety nine percent of the local earthquakes have focal depths shallower than 25 km and the locations of the few deeper events are poorly constrained the shallow earthquake focal depths are consistent with high temperatures and proposed ductile or aseismic behavior in the middle to lower crust of central tibet figure 6 1 figure 6 a earthquake locations computed from this study shown on shaded topographic data triangles are indepth iii seismic stations circles are events and are scaled by magnitude graphic 2 a color version of this figure is available online at the ssa web site b earthquake locations shown with major geologic structures from taylor et al 2003 3 reproduced with permission m h taylor personal comm 2002 triangles are indepth iii seismic stations geologic structures are as in figure 2 4 1 pending yes 2 f1 embed inline graphic 1 gif 3 ref 46 4 f3

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

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

crustal structure and variability of the himalayan border of tibet reversed critical wide angle reflection profiles of the crust mantle boundary south of the yarlung zangbo suture in tibet reveal a deep 70 km moho extending north from the high himalayas whilst to the south the moho is 15 km higher even further south reflections from 35 km depth may correspond to the moho of peninsular india

quaternary extension in southern tibet field observations and tectonic implications we summarize evidence for quaternary and active faulting collected in the field during three sino french expeditions to southeastern tibet 1980 1982 detailed mapping of quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the himalayas in the last 2 0 5 m y the maximum horizontal principal stress in south tibet appears to be only the intermediate principal stress 2 1 being vertical south of the chord joining the eastern and western syntaxes of the himalayan arc extensional strains are principally localized within seven regularly spaced rift zones three of which have been studied in some detail the extension direction is determined to be n96 e 7 mainly from statistical averaging of strikes of newly formed normal faults throw rates on normal faults are evaluated for different time spans 2 0 5 m y 60 40 kyr and 10 2 kyr b p using structural and topographic reliefs as well as synglacial and postglacial vertical offsets the rate of quaternary extension is about 1 m y 1 along an 1100 km long ese traverse across south tibet this corresponds to a spreading rate of 1 0 6 cm yr this rate and the divergent horizontal projections of slip vectors of earthquakes along the himalayan front constrain the rate at which rigid india underthrusts southern tibet to be 2 1 cm yr although most of the normal faults appear to be independent of and nearly orthogonal to the e w mesozoic tertiary tectonic fabric the yadong gulu rift appears to be guided for over 130 km by the older oblique ne sw nyainqentanglha range and fault zones along it this reactivated zone is the most prominent left lateral strike slip fault system in se tibet excepting this zone and the vicinity of the se extremity of the karakorum fault quaternary strike slip faulting is rare in south tibet i e south of the chord between the syntaxes of the himalayan arc north of the chord the tectonic style is different there minor conjugate strike slip faulting is widespread and appears to control quaternary normal faulting which is more diffuse and subdued than in south tibet along the chord the presence of a major zone of active right lateral en echelon strike slip faults karakorum jiali fault zone probably reflects the greater facility of eastward extrusion in north central tibet in response to the northward push of india the eastern himalayan syntaxis may be an obstacle to such extrusion movements south of the chord

precise relative earthquake location using surface waves 1 earthquake locations provide a fundamental tool for seismological investigations while dense seismic networks can provide robust locations accuracy and precision of these locations suffer outside dense networks this is particularly true in offshore areas where location analysis relies heavily on distant seismic observations we present a method for estimating precise relative seismic source epicentroid locations using surface waves several reasons including lower velocities and strength of the signal at distance make use of surface waves for event location appealing we focus on the panama fracture zone region and relocate 81 strike slip earthquakes to produce tectonically consistent epicentroid locations the resulting pattern of earthquakes more clearly delineates recently active regional structures than original body wave locations the mean shift between the us geological survey national earthquake information center epicenter and our epicentroids is about 14 km the median is about 11 km and typical origin time changes are generally less than 2 s we find that north of 6 5 n the plate boundary motion is split across two roughly north south striking structures the panama and balboa fracture zones for the last 36 years slip along these two structures roughly matches slip along the panama fracture zone to the south from 4 5 n to 6 25 n but the balboa fracture zone has roughly three times the moment than the northern panama fracture zone our analyses show that observed rayleigh wave signal to noise ratios for moderate to large shallow earthquakes are suitable for applying the procedure and that rayleigh wave observations form a self consistent set of constraints on the relative location of earthquake centroids

the status of central seismic gap a perspective based on the spatial and temporal aspects of the large himalayan earthquakes abstract the central himalaya considered as a prominent seismic gap is generally believed to be the most vulnerable segment due for a great plate boundary earthquake m 8 two significant historical earthquakes are known to have occurred in this region in ad 1505 and 1803 interpreted by some workers as great earthquakes occurrences of these events question the validity of central himalaya being a seismic gap here we analyze the macroseismal data from the central himalaya and the gangetic plains and suggest that neither of these earthquakes can be qualified as a plate boundary type event and the size of at least the 1803 does not exceed mw 7 7 our studies further indicate that the central himalayan frontal thrusts may be undergoing a quiescence of 1000 years in terms of generation of great earthquakes assuming a steady elastic strain accumulation due to plate convergence and its simple translation as periodic great earthquakes such long quiescence may appear intriguing we suggest that the long term quiescence and the spatial and temporal clustering that characterize the distribution of large earthquakes in the himalaya is a manifestation of the tectonic deformation associated with a highly evolved fold and thrust belt our analyses also indicate that the large moderate earthquakes are mostly concentrated on higher level thrusts these structures probably accommodate most of the seismogenic slip in the himalaya and the rest of the strain could possibly be expended on passive folding consequently great earthquakes m 8 are relatively infrequent as in the case of central himalaya apparently leading to long periods of quiescence

nature of the upper crust beneath central tibet abstract a wide angle reflection refraction seismic line obtained through sino french joint research near the bangung nujiang suture in tibet 1 yielded crustal velocity estimates for both p and s waves avp vs ratio of 1 65 was derived fo waves propagating in the upper 50 c km near the surface a shallow low velocity zone is expected to form in a granitic upper crust for both p and s waves a two dimensional ray tracing study indicates that the observed first p and s waves 1 can be interpreted as diving waves above a low velocity zone where a positive gradient exists the low velocity zone commences at a depth of 10 km and extends possibly to 20 km or deeper

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

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

depressional storage for markov gaussian surfaces processes during rain events such as infiltration runoff soil erosion and crust formation are influenced in part by depressional storage and surface roughness if the surface topography is known its potential depressional storage can be calculated the objective of this paper was to relate the statistical parameters for quantifying surface roughness to depressional storage analysis of topographic data sets digitized at millimeter grids by a laser scanner showed that soil roughness can be quantified by a markov gaussian m g type random process a monte carlo simulation procedure was used to find the mean ponding characteristics from simulated m g surfaces depressional storages were found to be functions of two m g parameters two sample length scales and the slope steepness the markov parameters are the global variance 2 and the correlation length scale l and the sample length scales are grid spacing x and side length ls after proper scaling all storage functions collapsed into two nondimensional relationships 1 storage at zero slope as a function of relative sample length scale and 2 storage as a function of scaled slope when l 0 simulated surfaces followed the random gaussian model and the nondimensional storage was only a function of scaled slope storages calculated from digitized elevation data sets with m g type statistics agreed well with results obtained from simulated surfaces

average regional strain due to slip on numerous faults of different orientations if a region is cut by faults with different orientations and if the faults intersect the boundaries of the region of interest then the average finite rotational strain eij ui xj is given by eij mij v where is the shear modulus v is the volume of the region and mij represents the sum of asymmetric moment tensors mij for each fault mij m0 u n where m0 is the scalar seismic moment and u and n are the unit vectors parallel to the slip vector and perpendicular to the fault plane this result is applicable when the average regional strain occurs primarily by displacements of essentially rigid blocks that together compose the region of interest and hence when both elastic strains and inelastic strains due to folding and bending are small

on depressional storages the effect of dem spatial resolution depression storage can be a significant storage element on a watershed surface accounting for the retention of much water over the past decade many models were developed to derive landscape features however relatively little is known about the impact of changing the spatial and or temporal resolutions on subsequent features in this paper the surface topography was measured with the aid of a laser scanner with millimeter resolution emphasis was laid on quantifying the effect of changing spatial resolution upon geometric properties of depressional storages digital mapping allowed the visualization of the location and topology of potential water surfaces and the manner in which water might move from one depression to another from the pond analysis and associated spatial mapping it became clear that most geometric characteristics relating to size and spatial location of depression storage including area volume and depth are scale dependent the relationship between the contributing area of each depression and the surface area of the same depression at each pour depth was established and the organization inherent in such relationship was explored furthermore it was found that these geometric objects may best be described by resorting to fractal geometry a popular tool for quantifying variability across scales

the cretaceous tertiary deformation of the lhasa block and its implications for crustal thickening in tibet the strong tertiary deformation of the northern margin of the indian plate and of the tsangpo suture zone is not matched by comparable deformation within the lhasa block yet the southern margin of the block was the site of large scale intrusive activity the gangdese or transhimalayan batholith throughout the late cretaceous and was therefore presumably weaker than its surroundings this poses a problem for any approach that interprets the deformation of tibet in terms of diffuse crustal thickening however there is evidence that the lhasa block was in elevation as well as in magma type an andean margin by the end of the cretaceous and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern lhasa block this suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast when subjected to boundary conditions representing the collision between india and asia the sheet accommodates convergence by diffuse deformation over a region comparable in size with the tibetan plateau in the absence of an elevation contrast this deformation consists of approximately 100 thickening strain over the region a precollision elevation contrast of 3000 m in the lhasa block would have resulted in its experiencing roughly 40 rather than 100 postcollisional thickening smaller elevation contrasts would have resulted in greater thickening and vice versa these calculations have implications for the mode of crustal thickening in northern tibet as the available evidence suggests that the topographic expression of the earlier mesozoic tectonic activity in southern asia was erased before the india asia collision we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the lhasa block during the tertiary

refinements to the method of epicentral location based on surface waves from ambient seismic noise introducing love waves summary the purpose of this study is to develop and test a modification to a previous method of regional seismic event location based on empirical green s functions egfs produced from ambient seismic noise elastic egfs between pairs of seismic stations are determined by cross correlating long ambient noise time series recorded at the two stations the egfs principally contain rayleigh and love wave energy on the vertical and transverse components respectively and we utilize these signals between about 5 and 12 s period the previous method based exclusively on rayleigh waves may yield biased epicentral locations for certain event types with hypocentral depths between 2 and 5 km here we present theoretical arguments that show how love waves can be introduced to reduce or potentially eliminate the bias we also present applications of rayleigh and love wave egfs to locate 10 reference events in the western united states the separate rayleigh and love epicentral locations and the joint locations using a combination of the two waves agree to within 1 km distance on average but confidence ellipses are smallest when both types of waves are used

geomorphic evidence for active faulting in the altyn tagh and northern tibet and qualitative estimates of its contribution to the convergence of india and eurasia a brief traverse across the altyn tagh and kunlun in northern tibet revealed abundant evidence for recent crustal deformation there from ridges offset roughly 10 m and from fresh mole tracks 40 cm or more high along the altyn tagh strike slip fault we conclude that a large earthquake occurred in the past few hundred years accordingly the rate of slip could be relatively high 30 20 mm yr active reverse or thrust faults mark the bases of several mountain ranges in the altyn tagh and kunlun where the neighboring basins lie at elevations less than 4000 m therefore roughly north south crustal shortening appears to be occurring there probably at a rate of several millimetres per year the crustal shortening accommodated by strike slip and reverse or thrust faulting appears to account for a significant part 10 25 of india s convergence with eurasia the broad zone of crustal shortening in northern tibet indicates that the areal extent of the plateau is increasing northward by crustal thickening on its northern margin

phanerozoic plate boundaries in mainland se asia the himalayas and tibet recently available geological data on tibet yunnan and burma facilitate recognition of phanerozoic subduction systems and two continental fragments of gondwanaland which extend through mainland se asia a cambrian subduction system and 5 mesozoic early caenozoic collision belts are identified in the region indochina eastern thailand and central tibet accreted to china in the early triassic western se asia and s tibet separated from gondwanaland in the permian or early triassic and collided with asia in the late triassic and the western burma island arc system collided with asia in the jurassic upper triassic flysch and schist in the eastern indoburman ranges were accreted to western burma in the jurassic early cretaceous during eastward subduction and formation of a back arc thrust belt following collision of greater india with arc complexes to the n in the cretaceous and with tibet in the early eocene western burma moved northwards about 450 km along the sagaing namyin ridge thrust transform fault formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts

epicentral location based on rayleigh wave empirical green s functions from ambient seismic noise summary a new method to locate the epicentre of regional seismic events is developed with strengths and limitations complementary to existing location methods this new technique is based on applying empirical green s functions egfs for rayleigh waves between 7 and 15 s period that are determined by cross correlation of ambient noise time series recorded at pairs of seismic receivers the important advantage of this method in comparison with standard procedures based on use of body wave traveltimes is that it does not rely on an earth model rather it is based on interpolating the egfs to arbitrary hypothetical event locations the method is tested by locating well known ground truth crustal events in the western united states magnitude 4 earthquakes in california magnitude 3 earthquakes in utah and a mine collapse as well as locating seismic stations as virtual events in these applications location errors average less than 1 km but are expected to vary with event mechanism and depth numerical simulations show that the method optimizes when source depth is less than 1 km or more than 5 km and when the source mechanism is nearly purely strike slip thrust or normal

power law scaling of topographic depressions and their hydrologic connectivity topographic depressions areas of no lateral surface flow are ubiquitous characteristics of the land surface that control many ecosystem and biogeochemical processes high density of depressions increases the surface storage capacity whereas lower depression density increases runoff thus influencing soil moisture states hydrologic connectivity and the climate soil vegetation interactions with the widespread availability of high resolution lidar based digital elevation model ldem data it is now possible to identify and characterize the structure of the spatial distribution of topographic depressions for incorporation in ecohydrologic and biogeochemical studies here we use ldem data to document the prevalence and patterns of topographic depressions across five different landscapes in the united states and quantitatively characterize the probability distribution of attributes such as surface area storage volume and the distance to the nearest neighbor through the use of a depression identification algorithm we show that these probability distributions of attributes follow scaling laws indicative of a structure in which a large fraction of land surface areas can consist of high number of topographic depressions of all sizes and can account for 4 to 21 mm of depression storage this implies that the impacts of small scale topographic depressions in the landscapes on the redistribution of material fluxes evaporation and hydrologic connectivity are quite significant

the crustal evolution and uplift mechanism of the qinghai tibet plateau abstract the qinghai tibet plateau covers part of each of the following four plates the tarim qaidam the qiangtang yangtze the gandise and the indian plates containing four crustal suture zones the northern qilian the east kunlun the karakorum lancang and the yarlung zangbo tsangpo suture zones the tarim qaidam and qiangtang yangtze plates belong to part of the paleo eurasian continent and the gandise and indian plates are component parts of the old gondwana land a typical ancient trench arc basin system and ophiolite belt are developed in both the qilian mountains and the area of the yarlung zangbo river the crust of this particular plateau has an obvious layered structure and comprises three structural layers the upper sialic the middle simatic and the lower mixed crust mantle layers the crustal shortening and thickening since the late eocene has been brought about by deformations of these three structural layers in different ways the uplifting process of the qinghai tibet plateau probably experienced two stages a stage of thickening by compression and a stage of isostatic uplift from the late eocene to pliocene the deformation was mainly of a compressional nature resulting in a large scale crustal shortening and thickening a considerable depression of the upper mantle and formation of a broad and deep plateau mountain root from the early pleistocene to the present time along with the weakening of compression stress relaxation has occurred and with the operation of the mountain root a strong isostatic adjustment has taken place causing the plateau to uplift both rapidly and remarkably

precise relative locations for earthquakes in the northeast pacific region we report that double difference methods applied to cross correlation measured rayleigh wave time shifts are an effective tool to improve epicentroid locations and relative origin time shifts in remote regions we apply these methods to seismicity offshore of southwestern canada and the u s pacific northwest occurring along the boundaries of the pacific and juan de fuca including the explorer plate and gorda block plates the blanco mendocino revere dellwood nootka and sovanco fracture zones host the majority of this seismicity largely consisting of strike slip earthquakes the explorer juan de fuca and gorda spreading ridges join these fracture zones and host normal faulting earthquakes our results show that at least the moderate magnitude activity clusters along fault strike supporting suggestions of large variations in seismic coupling along oceanic transform faults our improved relative locations corroborate earlier interpretations of the internal deformation in the explorer and gorda plates north of the explorer plate improved locations support models that propose northern extension of the revere dellwood fault relocations also support interpretations that favor multiple parallel active faults along the blanco transform fault zone seismicity of the western half of the blanco appears more scattered and less collinear than the eastern half possibly related to fault maturity wemore use azimuthal variations in the rayleigh wave cross correlation amplitude to detect and model rupture directivity for a moderate size earthquake along the eastern blanco fault lastly the observations constrain the seismogenic zone geometry and suggest a relatively narrow seismogenic zone width of 2 to 4 km less

the tectonics of the western ordos plateau ningxia china slip rates on the luoshan and east helanshan faults analysis of the locus style and rate of faulting is fundamental to understanding the kinematics of continental deformation the ordos plateau lies to the northeast of tibet within the india eurasia collision zone previous studies have suggested that it behaves rigidly and rotates anticlockwise within a large scale zone of ene wsw left lateral shearing for this rotation to be accommodated the eastern and western margins of the ordos plateau should be undergoing right lateral shearing and yet the dominant faulting style appears to be extensional we focus specifically on the kinematics of the faults bounding the western margin of the ordos plateau and make new slip rate estimates for two of the major faults in the region the right lateral strike slip luoshan fault and the normal slip east helanshan fault we use a combination of infrared stimulated luminescence dating of offset landforms with high resolution imagery and topography from the pleiades satellites to determine an average right lateral slip rate of 4 3 0 4 mm a 1 uncertainty on the luoshan fault similarly we use 10be exposure dating to determine a vertical throw rate on the east helanshan fault of 0 6 0 1 mm a corresponding to an extension rate of 0 7 0 1 mm a 1 uncertainty both of these results agree well with slip rates determined from the latest campaign gps data we therefore conclude that right lateral shearing is the dominant motion occurring in the western ordos region supporting a kinematic model of large scale anticlockwise rotation of the whole ordos plateau

active faulting and seismicity of the dasht e bayaz region eastern iran summary the dasht e bayaz left lateral strike slip fault system in ne iran ruptured over a length of ca 120 km during two earthquakes in 1968 and 1979 we constrain source parameters of the 1968 august 31 dasht e bayaz and 1979 november 27 khuli buniabad earthquakes by analysing long period body wave seismograms both earthquakes involved complex rupture processes with at least two subevents in each case coseismic surface ruptures and cumulative scarps in alluvium indicate fault segmentation on numerous short ca 20 km long strands with small pull aparts between them the earthquake subevents seen in the seismograms probably relate to this segmentation the total cumulative offset on the fault system is estimated at ca 4 5 km this is small compared to the total amount of late tertiary deformation expected in this part of iran and indicates that the dasht e bayaz fault may be relatively young distributed strike slip faulting is widespread in the region and there are indications that the dasht e bayaz fault is evolving from several short faults that are coalescing these results are important not only for understanding of the regional tectonics but for the development and evolution of strike slip faults in general

improved hydrologic modeling for depression dominated areas abstract modeling of overland flow for a depression dominated area is a difficult task due to the spatial distribution of depressions particularly the hierarchical relationships of depressions and their variable contributing areas make the hydrologic modeling more complicated in traditional hydrologic modeling however digital elevation models dems that are used to represent surface topography are often edited by filling depressions before delineation and the impacts of depressions are considered by using certain simplified and lumped approaches due to such flaws these hydrologic models fail to account for the actual dynamic influences of surface depressions the objective of this study is to improve hydrologic modeling for depression dominated regions by developing a new modeling framework to quantify the influence of spatially distributed depressions to achieve this objective a new puddle based unit pbu probability distributed model pbu pdm is developed and coupled with the existing soil and water assessment tool swat the pbu pdm facilitates separate simulations for non depressional and depressional areas and accounts for the hierarchical relationships of depressions their filling and spilling processes and the dynamic variations in their contributing areas the pbu pdm enhanced swat was applied to the upper maple river watershed in north dakota and calibrated and validated using the observed data in particular it was also compared with the original swat model which demonstrated the abilities of pbu pdm to mimic the filling spilling overland flow dynamics and the threshold behavior the pbu pdm provides improved watershed scale hydrologic modeling especially for depression dominated areas

hillslope riparian stream connectivity and flow directions at the panola mountain research watershed to obtain a better understanding of water s journey from the hillslope to the stream water level time series from 26 groundwater wells in a hillslope riparian study area in the panola mountain res

global positioning system measurements of indian plate motion and convergence across the lesser himalaya we use global positioning system gps measurements acquired from 1991 to 1995 to constrain the motion of sites in bangalore in southern india and kathmandu nepal relative to a global gps network these measurements permit estimates of the northward motion of the indian plate and convergence between the southern himalaya and the indian subcontinent the velocities of bangalore and kathmandu in the itrf92 reference frame agrees with that predicted by the nnr nuvel1a plate motion model for indian plate motion and differ from that predicted for the australian plate confirming the independent motion of the indian and australian plate fragments no significant motion was detected between bangalore and kathmandu during the three years from 1991 1994 even though kathmandu is located in the hanging wall of the active himalayan thrust system the himalayan thrust system is thought to accommodate 18 7 mm yr of convergence and has been the source of several historic m 8 earthquakes the absence of motion of kathmandu relative to the indian plate can be explained if the thrust system is presently locked south of the greater himalaya our preferred model has no steady slip on the detachment south of the greater himalaya and steady slip at a rate greater than 6 mm yr 1 3 of the long term convergence rate can be ruled out at 95 confidence level

3 d upper mantle shear velocity model beneath the contiguous united states based on broadband surface wave from ambient seismic noise ambient noise seismic tomography has been widely used to study crustal and upper mantle shear velocity structures most studies however concentrate on short period 50 s surface wave from ambient noise while studies using long period surface wave from ambient noise are limited in this paper we demonstrate the feasibility of using long period surface wave from ambient noise to study the lithospheric structure on a continental scale we use broadband rayleigh wave phase velocities to obtain a 3 d vs structures beneath the contiguous united states at period band of 10 150 s during the inversion 1 d shear wave velocity profile is parameterized using b spline at each grid point and is inverted with nonlinear markov chain monte carlo method then a 3 d shear velocity model is constructed by assembling all the 1 d shear velocity profiles our model is overall consistent with existing models which are based on multiple datasets or data from earthquakes our model along with the other post usarray models reveal lithosphere structures in the upper mantle which are consistent with the geological tectonic background e g the craton root and regional upwelling provinces the model has comparable resolution on lithosphere structures compared with many published results and can be used for future detailed regional or continental studies and analysis

paleoseismology of the northern piedmont of tianshan mountains northwestern china the northern piedmont of the tianshan mountains consists of three rows of cenozoic ew striking fold and reverse fault zones with en echelon right lateral steps the southernmost row involves sediments as young as lower pleistocene but there is no evidence of activity along this row during the last 30 000 years the central row is composed of three linear anticlines houerguos manas and tugulu and associated reverse faults the northernmost row includes the dushanzi halaande and anjihai anticlines and their associated reverse faults abundant fault scarps and folds of late pleistocene to holocene river terraces across the anticlines within the central and the northernmost rows indicate recent folding and reverse faulting we divide the northern piedmont into the dushanzi and manas fold and reverse fault zone in the dushanzi zone we excavated 15 trenches across scarps controlled by reverse faults and back thrusts by comparing 11 trench logs among the 15 trenches we identify three paleoearthquakes since 13 000 years b p the first event occurred between 11 300 and 13 300 years b p and the second and the third events occurred 6300 8400 and 3000 5000 years b p respectively considering the uncertainties of the data the average recurrence interval for large earthquakes in the dushanzi zone is about 4000 years a large earthquake along this zone is expected in the near future because the elapsed time since the last surface rupturing event is already 3000 4000 years three large trenches and several small trenches excavated across the fault scarps along the manas fold and reverse fault zone reveal four events the first second and third events occurred at 18 000 13 000 years b p 11 300 10 500 years b p and 6900 3600 years b p respectively the fourth the latest one is the 1906 m 7 7 manas earthquake field investigation suggests that the 1906 manas earthquake occurred along a blind thrust fault this earthquake formed three discontinuous zones of fresh surface ruptures the longest of which is only 8 km long along the eastern segment of the tugulu reverse fault and was associated with a zone of uplift 130 km long related to the manas earthquake the average recurrence interval along this zone is probably 5000 6000 years therefore it is unlikely that a large earthquake will occur along this zone in the near future because the manas earthquake occurred only 89 years ago

postseismic relaxation theory on the spherical earth abstract postseismic displacements and strains observed at the earth9s surface can be explained through the relaxation of a viscoelastic asthenosphere underlying a purely elastic crust a theory is presented for modeling the effects of postseismic relaxation on a spherically symmetric earth for an earthquake point source located in the upper elastic layer the displacement field is decomposed into its toroidal and spheroidal components a linear maxwell rheology in the viscoelastic layers is assumed enabling the use of the correspondence principle for the solutions of the equations of static equilibrium displacement and strain fields are then calculated using normal mode summation computational results are presented for two simple earthquake sources a strike slip fault and a uniaxial thrust fault the patterns of postseismic displacements and strains are found to depend strongly on both the earth model and earthquake source geometry in particular the elastic plate thickness asthenosphere thickness fault type and fault length each play a major role in determining the spatial pattern of postseismic relaxation effects asthenospheric viscosity controls the temporal pattern of relaxation

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

extracting earth s elastic wave response from noise measurements recent research has shown that noise can be turned from a nuisance into a useful seismic source in seismology and other fields in science and engineering the estimation of the system response from noise measurements has proven to be a powerful technique to convey the essence of the method we first treat the simplest case of a homogeneous medium to show how noise measurements can be used to estimate waves that propagate between sensors we provide an overview of physics research dating back more than 100 years showing that random field fluctuations contain information about the system response this principle has found extensive use in surface wave seismology but can also be applied to the estimation of body waves because noise provides continuous illumination of the subsurface the extracted response is ideally suited for time lapse monitoring we present examples of time lapse monitoring as applied to the softening of soil after the 2011 tohoku oki earthquake the detection of a precursor to a landslide and temporal changes in the lunar soil

a plausible model for the present day seismicity and tectonic activity in the hindukush complex zone abstract the hindukush zone is situated on the northern boundary of the indian plate along its northwestern flanks fifteen years 1974 1988 of seismic data published by united states coast and geodetic surveys uscgs and focal mechanism solutions of the subducted lithosphere reported by different scientists have been used to model the seismic and tectonic characteristics of the hindukush complex zone the data indicate that the zone can be separated into two seismic segments op and pq with a v shaped pattern striking e w and ne sw respectively the two segments do not represent well defined subduction zones as the depth and frequency of events is not uniformly distributed the e w striking segment extends from 68 2 e to 70 5 e at about 36 5 n whereas the northeast southwest segment extends from 36 5 n 70 5 e to 38 n 73 e the frequency and depth of earthquakes are greatest at 36 5 n 70 5 e decreasing towards the w and ne focal mechanism solutions thrust and normal faulting for the hindukush zone suggest that the directions of stress may be grouped into 1 compressive forces acting ne sw for the east west striking segment for the ne sw striking seismic segment compressive forces are directed towards the nw 2 compressive forces are also acting along the strike of the two seismic segments and 3 the direction of the tensile stress is different in each segment and extends to a depth of up to 221 km a model is proposed to explain the distribution of earthquake foci in terms of the v shaped subduction of oceanic lithosphere with tensile and compressive stresses in the two seismic belts being due to the upper mantle flow of the indian plate and the tibetan plateau the sinking of the mantle in the hindukush zone is responsible for the stretching of the seismic slab in a vertical direction giving rise to the present earthquake frequency and focal depth distribution pattern

the field of crustal velocity in asia calculated from quaternary rates of slip on faults rates of quaternary faulting when treated as measures of strain rates within regions several times larger than the depths of brittle faulting yield a velocity field for asia that matches the average relative velocity between the indian and eurasian plates over the past 2 myr nuvel 1a and the velocity of shanghai with respect to eurasia measured with very long baseline interferometry vlbi by using rates of slip on discrete surfaces faults to define average strain rates we explicitly assume that the large scale deformation can be treated as that of a continuum depending on the jcales over which strain rates are averaged 87 to 93 per cent of the assumed strain ites are fit within assigned 1 sigma uncertainties and 97 to 99 per cent are fit within 2 sigma the internal consistency of these data and the agreement with independent rates nuvel 1a and vlbi test the validity of treating large scale deformation of continents as that of a continuum in the calculated velocity field a large fraction 85 per cent of india s convergence with eurasia is absorbed by crustal or lithospheric thickening among consistently misfit assumed strain rates are those dominated by high rates of strike slip faulting strain associated with 20 30 mm a 1 of slip on the altyn tagh karakax and karakorum faults is inconsistent with the distributions of strain in surrounding regions we conclude that these rates all of which are based on correlations of ages of offset landforms with changes in global climate and not on radiometric dating have been systematically overestimated lateral transport out of india s northward path is relatively minor south china is calculated to move east southeast at less than 10 mm a 1

faulting associated with large earthquakes and the average rate of deformation in central and eastern asia we determine approximate average rates of deformation in portions of eastern and central asia for the last 80 years these rates are based on estimates of source dimensions average displacements seismic moments and orientations of faults of all large earthquakes m 7 5 and as many other major earthquakes m 7 0 as possible in asia since 1900 a major part of this study is the compilation of these data for separate parts of asia we combine seismic moment tensors to estimate the average rotational strain or deformation tensors for these regions for the last 80 years we estimate that shortening across the tien shan has occurred at 11 mm yr between 5 and 19 mm yr left lateral shear on easterly trending planes in mongolia in northwestern tibet gansu and ningxia and on the xianshuihe fault in western sichuan has been very rapid more than 10 mm yr in each region and apparently as rapid as 40 mm yr in northwestern tibet gansu and ningxia slower rates of a few to 10 mm yr appear to characterize east west extension and conjugate north south shortening of tibet the earthquake history of the himalaya is consistent with rates of underthrusting of more than 10 mm yr but is probably less than the convergence rate of india with eurasia of about 50 mm yr the overall strain field is consistent with a large part of india s penetration into eurasia being absorbed by the extrusion of material out of india s way as a result southeast china moved at about 21 mm yr east southeast with respect to eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate

characteristics of earthquake sequence in northern himalayan region of south central tibet precursor search and location of potential area of future earthquake abstract this paper reports that three earthquake sequence occurred in 1996 m b 5 9 1998 m b 5 8 and 2004 2005 m b 6 2 6 3 were observed preceding the anomalous seismic activity the nature of anomalous seismicity associated with these earthquakes have been investigated considering earthquakes with m b 4 1 cut off magnitude for this region in four seismic episodes nagm normal background n anomalous swarm a precursory gap g and mainshock sequence m respectively it is established here that the anomalous seismicity swarm patterns follow episodes of relatively very low seismic activity and it is an important finding to visualize that an area might be preparing for the occurrence of a forthcoming mainshock such anomalous seismic patterns were observed prior to medium size mainshocks that occurred from 1963 to 2006 in south central tibet sct region to the north of central himalaya from these observations it may be inferred here that the patterns of anomalous seismicity earthquake swarms may be considered as an important parameter precursor for the forecasting of long range earthquake hazards in the sct region in consideration of spatial and the temporal clustering of swarm events that are prominent and confined in a vertical column of 10 45 km enabled to locate potential area enclosed by 29 6 30 1 n and 87 8 88 1 e where future earthquake of magnitude 6 0 and above in the depth range 25 15 km may be seated in view of this multi parameter short term precursory signals monitoring is suggested for the delineated preparatory area to make earthquake prediction forecasting programs more meaningful

ground truth location of earthquakes by use of ambient seismic noise from a sparse seismic network a case study in western australia the estimated green s function egf extracted from the ambient seismic noise cross correlation function ncf enables valuable calibration of surface wave propagation along the path connecting seismic stations such calibration is adopted in a new method for ground truth location of earthquakes achieved from the location relative to a seismic station the surface wave group travel times were obtained from the ncfs between a station near the earthquake and remote stations the differential travel times from the ncfs and the surface wave of the earthquake were used in a relative location procedure when this method was applied to earthquake location with only six seismic stations in western australia the location of the mw 4 1 kalannie september 21 2005 earthquake was found to be accurate to within 2 km compared with the ground truth location with insar for which azimuth coverage of seismic stations is preferable synthetic tests suggest that the group travel time is slightly affected by focal mechanism and focal depth thus unknown earthquake source parameters did not introduce substantial bias to earthquake location with the group travel time method

a fluvial terrace record of late quaternary folding rate of the anjihai anticline in the northern piedmont of tian shan china abstract crustal shortening of the tian shan range in northwest china has largely been accommodated by fold and thrust belts that mark the northern and southern mountain boundaries which were formed during the basinward propagation of the deformation front of tian shan investigating the deformation rates of active folds anticlines in the mountain piedmonts within different timescales is important for understanding the mountain building and front propagation processes of the tian shan in this study we investigate the late quaternary folding rate of the anjihai anticline one of the major anticlines in the northern piedmont of tian shan our study is based on optically stimulated luminescence osl dating and topographic measurements of folded river terraces three well preserved fluvial terraces across the anjihai anticline were identified and dated to be 3 6 0 1 ka 9 0 0 6 ka and 53 3 2 2 ka respectively these terrace ages are combined with shortening and uplift of the terraces estimated using an area conservation method and the terrace height profiles to evaluate the average shortening and uplift rates our results show that the minimum shortening rate of the anjihai anticline is 0 4 mm a between 53 ka and 9 ka and 1 1 mm a over the past 9 ka and the corresponding minimum uplift rates are 0 5 mm a and 1 4 mm a respectively if a local sedimentation rate of 0 3 mm a is taken into consideration the shortening rate is 0 7 mm a between 53 ka and 9 ka and 1 3 mm a since 9 ka and the corresponding uplift rates are 0 8 mm a and 1 5 mm a respectively these data suggest that the anjihai anticline has accommodated 20 25 of the total shortening across the whole eastern tian shan 5 mm a in the holocene our results suggest that the holocene is a relatively tectonically active time interval since the late pleistocene the late quaternary shortening rate of the anjihai anticline is much higher than its long term average shortening rate 0 2 mm a since its formation at 7 4 ma a similar situation has also been observed for other active folds in the northern and southern piedmonts of tian shan the higher late quaternary folding rate is possibly attributed to the gradual propagation of the tian shan towards its northern and southern foreland basins in the quaternary

estimating the location of a tunnel using correlation and inversion of rayleigh wave scattering the investigation of near surface scatterers such as cavities tunnels abandoned mine shafts and buried objects is important to mitigate geohazards and environmental hazards by inversion of travel times of cross correlated scattered waves due to the incident rayleigh waves we estimate the location of a near surface tunnel from seismic field data the cross correlation eliminates the travel path between a source and a scatterer thus eliminating the need to know the position of the source making the estimation of the scatterers locations dependent only on properties between the receivers and the scatterer first time using a numerically verified method on seismic field data we show the potential of the method for estimating the location of a buried scatterer

dynamic puddle delineation and modeling of puddle to puddle filling spilling merging splitting overland flow processes 1 surface microtopography affects overland flow infiltration soil erosion pollutant transport and other fundamental hydrologic and environmental processes across scales under the influence of surface depressions overland flow essentially features a series of progressive puddle to puddle p2p filling spilling merging and splitting processes the objectives of this study are to characterize puddles and their hierarchical relationships and model the microtopography controlled p2p processes we proposed a new modeling framework for simulating the p2p overland flow dynamics through cell to cell c2c and p2p routing for a set of puddle based units pbus in a well delineated cascaded p2p drainage system testing of the p2p model demonstrated its potential to improve overland flow modeling and hydrologic connectivity analysis by explicitly incorporating the hydrologic roles of depressions and quantifying the real microtopography controlled p2p dynamics

stress evolution before and after the 2008 wenchuan china earthquake abstract we examine the distribution of coulomb stress before and following the m 7 9 wenchuan earthquake and model the time dependent components of the stress field into the future our modelling shows that the combined coseismic plus postseismic viscoelastic stress change due to the preceding large events m 6 7 at the hypocentral area of the wenchuan earthquake is negative which may have led to a delay on its occurrence between 16 and 95 yr depending on the rake of the rupture on the other hand coseismic stress modelling of the wenchuan earthquake shows strong interaction particularly with the faults close to its sw and ne ends such interactions have previously been shown to trigger further large earthquakes in addition redistribution of stresses due to continuing postseismic deformation will potentially increase the stress levels not only on these faults but also on the distant faults such as the xianshuihe kunlun and tazang faults postseismic viscoelastic stress load on these distant faults over the next 22 yr since the earthquake could be up to 2 5 times larger than that of the coseismic stress increase alone this would potentially advance these faults in their seismic cycle and thereby encourage future earthquakes resulting in clustering of large earthquakes in the region this behaviour however strongly depends on the stage that these faults are in their seismic cycle which is not known currently given the high density of population in the area with several cities chengdu guangyuan and hanzhong of more than 1 million inhabitants our results indicate a need for detailed paleoseismic studies to help elucidate the state of the stressed faults

geodynamical basis for crustal deformation under the tibetan plateau abstract this paper uses plate tectonics and satellite derived gravity data to further discussion of crustal deformation under the tibetan plateau the first of our three contributions is a spherical harmonic analysis of the global plate boundary system a distribution of 470 dirac delta functions is applied to describe the generating forces according to the rates of crustal creation and destruction on the plate boundaries analysis of the extensional and compressional forces in the spreading and subducting zones shows that the present global plate motion causes compressional stresses in the n s direction under the tibetan plateau the second contribution is the calculation of the crustal stresses in tibet as inferred from satellite gravity data by applying solutions to the problem of the spherical shells the satellite determined stresses indicate that the upwelling mantle material under tibet induce n s and e w extension finally a superimposed stress system is constructed this stress system shows that the present crustal deformation in tibet does not produce n s shortening but generates e w extension the results of this paper have provided geophysical explanations for geological field observations in tibet and fault plane solutions of earthquakes in the tibetan side of the india eurasia collison the stress patterns reveal that the cold downwelling mantle convection flow beneath southern tibet pulls the indian plate down but applies a bending moment on the end of the plate to uplift and support the mass of the himalayas

seismicity focal mechanisms and their correlation with the geological tectonic history of the tibetan plateau abstract the tibetan plateau appears to have been formed as a result of the welding together of several continental plates of asia these plates have originated during different geological times dating back from the palaeozoic to the tertiary and underwent folding in different orogenic cycles as a result several major faults of continental dimensions trending ene wsw to nw se have developed some examples of these are the siling kunlun altyn tagh kansu and kang ting faults in addition several major lineaments faults trending ne sw to n s were developed seismicity over the tibetan plateau can mostly be explained in terms of activity associated with these faults and transverse lineaments large magnitude m b 7 0 earthquakes have taken place along these faults since historic times strike slip faulting is mostly related to ene wsw to nw se trending faults while normal faulting is related to ne sw trending lineaments and faults the stress system prevailing over the tibetan plateau is quite different from that prevailing along the himalaya front tension is predominant over large parts of tibet at present compared with compression along the himalayas

uplift of tibetan plateau a history of the elevation and crustal thickness of the tibetan plateau since the continental collision at 45 ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume in our reconstructions the indochina block was attached to the southern margin of eurasia before being extruded to the southeast as evidenced by the opening of the south china sea this opening affected the mass input into tibet so that tibet did not uplift to an elevation higher than 1 km until early miocene when spreading of the south china sea ceased the model then gives a steady rise of tibet from middle miocene to its present elevation