calibration of mb pn mb lg scales and transportability of the m0 mb discriminant to new tectonic regions we have developed mo mb relationships for earthquakes and explosions in central asia using regional waveform data from the chinese broadband digital station wmq regional magnitude scales mb pn and mb lg were established by 1 determining the attenuation rates of pn and lg waves 2 calibrating the mb pn against the teleseismic rob and 3 applying nuttli s method to determine m b from the lg waves seismic moments for earthquakes and explosions were obtained by applying the bolt and herraiz 1983 method calibrated using cmt moments for earthquakes and regional moment estimates of the soviet joint verification experi ment jve explosion the results show that mo mb pn relationships for central asian earthquakes and explosions are in excellent agreement with the relationships for the western united states this demonstrates that the mo r discriminant is transportable to other regions and supports theoretical claims patton and walter 1993 that mo m b are directly comparable for explosions detonated in different emplacement media successful application depends upon correcting the mb pn for the magnitude bias known to exist between the shagan river test site and the nevada test site nts in general region specific mb bias must be accounted for whenever regional mag nitude scales are calibrated against teleseismic rob as pointed out by douglas and marshall 1996 transporting mo mb lg can be problematic because attenuation rates based on coda waves or lg waves from earthquakes may not necessarily apply to lg waves from explosions and vice versa this is demonstrated for the western united states where the coda q measured from explosion data is much lower than the coda q from earthquake data we show that mb lg for earthquakes located close to nts is greatly overestimated using the q values determined from the coda of nts explosions this is apparently not a problem for lg calibration in central asia perhaps because of differences in velocity and or q structures for the two regions other important considerations are the character of lg propagation on short paths 400 km for nts explosions versus long paths 1000 kin for east kazakh explosions and the properties of coda wave spreading as a function of distance and velocity structures for different tectonic regions

extent of power law scaling for natural fractures in rock new data sets from natural faults and extension fractures exhibit simple power law scaling across 3 4 4 9 orders of magnitude regardless of rock type or movement mode the data show no evidence of natural gaps or scaling changes each data set consists of independent measurements made at different observational scales a power law regression to the subset of smaller fractures in each case provides an extrapolation that accurately predicts associated larger fractures consequently data representing a limited range of fracture sizes may be used to characterize a much broader spectrum of fracture sizes

gabor frame based gaussian packet migration we present a gaussian packet migration method based on gabor frame decomposition and asymptotic propagation of gaussian packets a gaussian packet has both gaussian shaped time frequency localization and space direction localization its evolution can be obtained by ray tracing and dynamic ray tracing in this paper we first briefly review the concept of gaussian packets after discussing how initial parameters affect the shape of a gaussian packet we then propose two gabor frame based gaussian packet decomposition methods that can sparsely and accurately represent seismic data one method is the dreamlet gaussian packet method dreamlets are physical wavelets defined on an observation plane and can represent seismic data efficiently in the local time frequency space wavenumber domain after decomposition dreamlet coefficients can be easily converted to the corresponding gaussian packet coefficients the other method is the gabor frame gaussian beam method in this method a local slant stack which is widely used in gaussian beam migration is combined with the gabor frame decomposition to obtain uniform sampled horizontal slowness for each local frequency based on these decomposition methods we derive a poststack depth migration method through the summation of the backpropagated gaussian packets and the application of the imaging condition to demonstrate the gaussian packet evolution and migration imaging in complex models we show several numerical examples we first use the evolution of a single gaussian packet in media with different complexities to show the accuracy of gaussian packet propagation then we test the point source responses in smoothed varying velocity models to show the accuracy of gaussian packet summation finally using poststack synthetic data sets of a four layer model and the two dimensional seg eage model we demonstrate the validity and accuracy of the migration method compared with the more accurate but more time consuming one way wave equation based migration such as beamlet migration the gaussian packet method proposed in this paper can correctly image the major structures of the complex model especially in subsalt areas with much higher efficiency this shows the application potential of gaussian packet migration in complicated areas

evidence of nonlinear elasticity of the crust from the mw7 6 manyi tibet earthquake satellite synthetic aperture radar sar interferometry shows that the magnitude 7 6 manyi earthquake of 8 november 1997 produced a 170 kilometer long surface break with up to 7 meters of left lateral slip reactivating a n76 e quaternary fault in western tibet the radar interferometric map reveals asymmetric along strike displacement profiles between the two sides of the surface rupture a pattern that cannot be explained with linear elastic theory this observation suggests that the elastic moduli of the crust in tension and in compression are different because of the presence of cracks in the crust at shallow depth a model indicates that a ratio of 2 between compressive and tensile elastic moduli can account for the observed asymmetry a ratio that is consistent with laboratory and borehole measurements

evaluation of anisotropy by shear wave splitting the polarizations of three component shear wavetrains carry unique information about the internal structure of the rock through which they pass specifically commonly observed shear wave splitting may contain information about the orientation of crack distributions this information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface even for nearly vertical incidence however shear wave splitting in synthetic three component vertical seismic profiles in some cases may be interpreted directly in terms of the direction of strike of vertical cracks because the human eye is not skilled at identifying the phase relationships between three component seismograms played out conventionally as parallel time series the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear wave splitting estimating the orientations of cracks an

bias in the centroid moment tensor for central asian earthquakes evidence from regional surface wave data surface waves recorded on the chinese digital seismic network cdsn station urumqi wmq are analyzed for 26 centroid moment tensor cmt earthquakes located within an epicentral distance of 1300 km applying a simple grid search method for source parameter retrieval i obtain revised estimates of the focal depth fault strike and seismic moment m sub 0 from amplitude spectra of love and rayleigh waves and compare these estimates with the cmt best double couple solutions the comparisons show that residuals are unbiased for focal depth and fault strike where population spreads are plus minus 13 km and plus minus 17 degree respectively on the other hand m sub 0 estimates are found to be biased with cmt m sub 0 larger than regional estimates by an average of 0 27 plus minus 0 04 log units the results for focal depth and strike are consistent with previous global and regional studies comparing independent estimates with cmt results recent full waveform modeling studies for central asian earthquakes support the findings of bias in m sub 0 estimates i suggest that causes for m sub 0 bias may be related to great thicknesses of continental crust in hindu kush and tien shan regions and to data censoring practices at small magnitudes m sub 0 m sub b scaling relationships for central asianmore earthquakes show better agreement with western u s scaling when m sub 0 estimates determined in this study are used copyright 1998 american geophysical union less

reservoir fracture characterization from seismic scattered waves the measurements of fracture parameters such as fracture orientation fracture density and fracture compliance in a reservoir is very important for field development and exploration traditional seismic methods for fracture characterization include shear wave birefringence gaiser and dok 2001 dok et al 2001 angerer et al 2002 vetri et al 2003 and amplitude variations with offset and azimuth avoa ruger 1998 shen et al 2002 hall et al 2003 liu et al 2010 lynn et al 2010 these methods are based on the equivalent medium theory with the assumption that fracture dimension and spacing are small relative to the seismic wave length so a fracture zone behaves like an equivalent anisotropic medium but fractures on the order of seismic wave length are also very important for enhanced oil recovery and they are one of the important subsurface scattering sources that generate scattered seismic waves willis et al 2006 developed the scattering index method to extract the fracture scattering characteristics by calculating the transfer funtion of a fracture zone fang et al 2011 proposed a modification of the si method the fracture transfer function ftf method that leads to a more robust fracture characterization in this paper we use both laboratory data and field data to explore the capability of the ftf method

poststack velocity analysis by separation and imaging of seismic diffractions small geologic features manifest themselves in seismic data in the form of diffracted waves which are fundamentally different from seismic reflections using two field data examples and one synthetic example we demonstrate the possibility of separating seismic diffractions in the data and imaging them with optimally chosen migration velocities our criteria for separating reflection and diffraction events are the smoothness and continuity of local event slopes that correspond to reflection events for optimal focusing we develop the local varimax measure the objectives of this work are velocity analysis implemented in the poststack domain and high resolution imaging of small scale heterogeneities our examples demonstrate the effectiveness of the proposed method for high resolution imaging of such geologic features as faults channels and salt boundaries

fracture clustering effect on amplitude variation with offset and azimuth analyses abstracttraditional amplitude variation with offset and azimuth avoaz analysis for fracture characterization extracts fracture properties through analysis of reflection avoaz to determine anisotropic parameters e g thomsen s parameters that are then related to fracture properties the validity of this method relies on the basic assumption that a fractured unit can be viewed as an equivalent anisotropic medium as a rule of thumb this assumption is taken to be valid when the fracture spacing is less than 10 under the effective medium assumption diffractions from individual fractures destructively interfere and only specular reflections from boundaries of a fractured layer can be observed in seismic data the effective medium theory has been widely used in fracture characterization and its applicability has been validated through many field applications however through numerical simulations we find that diffractions from fracture clusters can significantly distort the avoaz signatures when a

gaussian beam migration just as synthetic seismic data can be created by expressing the wave field radiating from a seismic source as a set of gaussian beams recorded data can be downward continued by expressing the recorded wave field as a set of gaussian beams emerging at the earth s surface in both cases the gaussian beam description of the seismic wave propagation can be advantageous when there are lateral variations in the seismic velocities gaussian beam downward continuation enables wave equation calculation of seismic propagation while it retains the interpretive raypath description of this propagation this paper describes a zero offset depth migration method that employs gaussian beam downward continuation of the recorded wave field the gaussian beam migration method has advantages for imaging complex structures like finite difference migration it is especially compatible with lateral variations in velocity but gaussian beam migration can image steeply dipping reflectors and will not produce unwanted reflectio

borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock1 this paper discusses the borehole radar system ramac developed within the framework of the international stripa project which can be used in three different measuring modes single hole reflection cross hole reflection and cross hole tomography the reflection modes basically provide geometrical data on features located at some distance from the borehole in addition the strength of the reflections indicate the contrast in electrical properties single hole reflection data are cylindrically symmetrical with respect to the borehole which means that a unique fracture orientation cannot be obtained a method has been devised where absolute orientation of fracture zones is obtained by combining single hole reflection data from adjacent holes similar methods for the analysis of cross hole reflection data have also been developed and found to be efficient the radar operates in the frequency range 20 60 mhz which gives a resolution of 1 3 m in crystalline rock the investigation range obtained in the stripa granite is approximately 100 m in the single hole mode and 200 300 m in the cross hole model variations in the arrival time and amplitude of the direct wave between transmitter and receiver have been used for cross hole tomographic imaging to yield maps of radar velocity and attenuation the cross hole measurement configuration coupled withmore tomographic inversion has less resolution than the reflection methods but provides better quantitative estimates of the values of measured properties the analysis of the radar data has provided a consistent description of the fracture zones at the stripa cross hole site in agreement with both geological and geophysical observations less

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

fault slip in the 1997 manyi tibet earthquake from linear elastic modelling of insar displacements summary the mw 7 6 1997 manyi earthquake occurred in an area of central northern tibet where sparse vegetation coverage and a lack of human habitation provide excellent conditions for interferometric synthetic aperture radar insar studies we use coseismic pairs of radar images acquired by the esa ers 2 satellite to construct interferograms of the surface displacement field due to the earthquake the location and extent of the coseismic fault rupture are mapped using a combination of optical satellite imagery high resolution digital topography interferometric correlation and azimuth offset measurements in so doing we are able to relate prominent geomorphic features in the fault zone to bends in the fault using elastic dislocation models consistent with this mapped fault trace we then test a range of fault geometries and slip conditions to find the combination which best explains the insar displacements our favoured model contains a reversal in fault dip approximately halfway along its length occurring at the location of a restraining bend slip on this model fault is heterogeneous with two areas of peak slip of 7 m or greater and components of dip slip displacement which vary significantly along strike the success of this model in fitting the data implies that an observed asymmetry in the coseismic interferograms can be explained in terms of the local fault geometry rather than by using non linear elastic rheologies as suggested by earlier authors

3d seismic characterization of fractures in a dipping layer using the double beam method the characterization of natural and induced fractures in terms of fracture orientation fracture spacing or density and fracture compliance is critical in reservoir development given the multiscale nature of the fracture distribution the commonly used effective anisotropy assumption may not be valid the recently proposed double beam method to characterize the fractured reservoir has the potential to invert for the spatially dependent fracture network information for a horizontal reservoir layer however the inverted results can be biased if the fractured reservoir layer is dipping as a result it is essential to estimate and include the dip angle information of reservoir layers in applying the double beam method we used forward modeling to demonstrate the bias and proposed a new method to correct for the error caused by the reservoir layer dip angle for a dipping layer our new method can correctly invert for the fracture parameters including the fracture orientation and fracture spacing

equivalent medium representation of fractured rock a similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks fractures can be modeled using an empirical linear slip condition however natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip cracks model 1 or conversely as planar distributions of imperfect interfacial contacts model 2 an alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material model 3 we present analytic expressions for the fracture compliances for these three models and using these analytic results compute the effective compliances and stiffnesses of the fractured material as a result it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture given appropriate a priori information on the fracture surfaces the results for model 1 are equivalent to a volume distribution of cracks as studied by hudson 1980 1981 for small crack density the results for model 2 are basically the same as those given by white 1983 for a packing of spheres and finally the results for model 3 are in agreement with those given by backus 1962 for combinations of two constituent layers these results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock finally it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill

when do fractured media become seismically anisotropic some implications on quantifying fracture properties fractures are pervasive features within the earth s crust and they have a significant influence on the multi physical response of the subsurface the presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems in this study we confirm the general scale dependence of seismic anisotropy and provide new results specific to shear wave splitting sws we find that sws develops under conditions when the ratio of wavelength to fracture size s d is greater than 3 where rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model emm theory is qualitatively valid when 1 s d 3 there is a transition from rayleigh to mie scattering where no effective anisotropy develops and hence the sws measurements are unstable when s d 1 we observe geometric scattering and begin to see behaviour similar to transverse isotropy we find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio more importantly we observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 and 2 wavelengths depending on the fracture density and compliance ratio the existence of a transition zone means that inversion of seismic anisotropy parameters based on emm will be fundamentally biased more importantly we observe that linear slip emm commonly used in inverting fracture properties is inconsistent with our results and leads to errors of approximately 400 in fracture spacing equivalent to fracture density and 60 in fracture compliance although emm representations can yield reliable estimates of fracture orientation and spatial location our results show that emm representations will systematically fail in providing quantitatively accurate estimates of other physical fracture properties such as fracture density and compliance thus more robust and accurate quantitative estimates of in situ fracture properties will require improvements to effective medium models as well as the incorporation of full waveform inversion techniques

seismic characterization of fractured reservoirs by focusing gaussian beams abstractnaturally fractured reservoirs occur worldwide and they account for the bulk of global oil production the most important impact of fractures is their influence on fluid flow to maximize oil production the characterization of a fractured reservoir at the scale of an oil field is very important for fluid transport the critical parameters are connectivity and transmittivity plus orientation these can be related to fracture spacing compliance and orientation which are the critical seismic parameters of rock physics models we discovered a new seismic technique that can invert for the spatially dependent fracture orientation spacing and compliance using surface seismic data unlike most seismic methods that rely on using singly scattered diffracted waves whose signal to noise ratios are usually very low we found that waves multiply scattered by fractures can be energetic the direction information of the fracture multiply scattered waves contains fracture orientation and spacing informati

crustal structure of china from deep seismic sounding profiles abstract more than 36 000 km of deep seismic sounding dss profiles have been collected in china since 1958 however the results of these profiles are not well known in the west due to the language barrier in this paper we summarize the crustal structure of china with a new contour map of crustal thickness nine representative crustal columns and maps showing profile locations average crustal velocity and pn velocity the most remarkable aspect of the crustal structure of china is the well known 70 km thickness of the crust of the tibetan plateau the thick 45 70 km crust of western china is separated from the thinner 30 45 km crust of eastern china by the north south trending seismic belt 105 e the average crustal velocity of china ranges from 6 15 to 6 45 km s indicating a felsic to intermediate bulk crustal composition upper mantle pn velocities are 8 0 0 2 km s equal to the global continental average we interpret these results in terms of the most recent thermo tectonic events that have modified the crust in much of eastern china cenozoic crustal extension has produced a thin crust with a low average crustal velocity similar to western europe and the basin and range province western usa in western china mesozoic and cenozoic arc continent and continent continent collisions have led to crustal growth and thickening inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by dss profiles and other seismological studies a high velocity 7 0 7 4 km s lower crustal layer has been reported in western china only beneath the southernmost tibetan plateau we identify this high velocity layer as the cold lower crust of the subducting indian plate as the indian crust is injected northward into the tibetan lower crust it heats and assimilates by partial melting a process that results in a reduction in the seismic velocity of the lower crust in the central and northern tibetan plateau

source parameters of moderate sized earthquakes in the tien shan central asia from regional moment tensor inversion we estimate faulting geometry depth and seismic moment of 32 moderate sized earthquakes in the tien shan by matching observed and synthetic seismograms recorded at regional distances the moment tensor compression axes are oriented in the direction 170 350 consistent with the orientation of india eurasia convergence this compression is manifested in reverse faulting on roughly east west trending fault planes faults within the mountain system have moderate to steep dips but range bounding faults appear to dip more gently earthquake source depths are in the range 5 25 km consistent with those determined from local earthquake studies the distribution and mechanism of earthquakes in the tien shan suggests that a significant proportion of the active deformation in the tien shan is absorbed by shortening across intramontane basins the absence of strike slip earthquake mechanisms associated with the talas ferghana fault suggests that the fault is presently locked with deformation accommodated on neighboring compressional structures

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

broadband source modeling of the november 8 1997 tibet mw 7 5 earthquake and its tectonic implications we studied the source process of a large ms 7 9 intraplate earthquake that occurred on november 8 1997 at 1002 ut in a remote region of northern tibet we used four distinct methods to investigate the broadband source process and thereby better understand the tectonic implications of this event we relocated aftershocks using a master event technique and found that the distribution of aftershocks covers a region of 200 km in lateral extent we also employed a surface wave spectral inversion technique to estimate the mainshock moment depth centroid location and centroid time and utilized an empirical green s function technique to extract rupture directivity information and a detailed source time function from observed seismograms we also inverted body waves to estimate the moment release along the fault and the source time function the 1997 earthquake ruptured a strike slip fault that appears to be an extension or splay of the kun lun fault system this fault is one of the most seismically active strike slip faults within the tibetan plateau and has had events with surface wave magnitudes of 6 1 7 4 and 7 9 in this region since 1973 the rupture released most of the energy within the first 20 s and propagated bilaterally initially with the later rupture propagating westward for 20 30 s the absence of large aftershocks suggests that the earthquake efficiently released the stored strain comparing mainshock to the largest aftershock energy ratios for this event and other large strike slip events shows that faulting within the plateau has the characteristics of weak faults e g fracture zone faulting

precise relative location of 25 ton chemical explosions at balapan using ims stations wetest how well low magnitude m blg 1 8 to 2 6 25 ton chemical explosions at balapan kazakhstan can be located using ims stations and standard earth models relying on precisely determined relative arrival times of nearly similar regional and teleseismic waveforms three 1997 balapan explosions were recorded by a number of currently reporting and surrogate ims stations three regional stations and two teleseismic arrays yielded consistent waveforms appropriate for relative picking master event locations based on the ak135 model and ground truth information from the first shallowest and best recorded explosion fell under 1 km from known locations for depths constrained to that of the master event the resulting 90 confidence ellipses covered 12 13 km2and contained the true locations however results for depth constrained to true depth were slightly less satisfactory from predictions based on ground truth we found a pgcoda phase at makanchi kazakhstan to be misidentified and poorly modeled after accounting for this 90 ellipses shrank to 2 3 km2and true depth mislocation vectors became more consistent with confidence ellipse orientations these results suggest that a high level of precision could be provided by a tripartite array of calibration shots in cases where models are poorly known we hope that the successful relocation of these small balapan shots will support the role of calibration explosions in verification monitoring and special event studies including on site inspection

lateral variations in compressional velocities beneath the tibetan plateau from pn traveltime tomography summary a back projection algorithm is applied to 353 p traveltime measurements to image the lateral variations in compressional velocity in the uppermost mantle in and around tibet prior to the tomographic inversion we refined the apparent p slowness measurements based on a recently developed relocation procedure zhao helmberger 1991 and mathematical formulations of zhao 1993 these refinements are crucial in obtaining a high quality velocity image we obtained an average p velocity value for the uppermost mantle of 7 93 kms l and an average mantle p velocity gradient of 3 1 x lo s for tibet the final 2 d p velocity image in the uppermost mantle is characterized by a large 7 to 9 in dimension low velocity down to about 7 78 km s region in north central tibet and by large high velocity up to 8 11 km s i regions in the western and eastern flanks of tibet the crustal thickness is greater than 70 km inside the tibetan plateau it decreases toward the southern eastern and western boundaries of the plateau at and beyond these boundaries the crustal thickness is similar to those beneath the eurasian shield rigorous analyses of resolution and error suggest that 1 there is a pronounced difference between the tomographic velocity image obtained with and without the refinements to the p slowness measurements indicating a significant improvement in the quality of our inversion by these refinements 2 random errors in measuring earthquake origin times and locations and in the p arrival times should not cause errors of more than 0 1 km s in the velocity image 3 the resolution is generally within 6 to 10 the velocity image obtained in this study is consistent with previous s p traveltime studies and geothermal petrological studies and is consistent with a mantle convection model in which a convective upwelling is occurring in north central tibet and downwellings occurring in the surrounding regions molnar 1988 1990

locating oceanic earthquakes the influence of regional models and location criteria summary earthquakes in oceanic areas are normally located using traveltime tables which are representative of continental paths since most seismic stations lie on continents it should therefore be possible to improve such locations by employing a set of traveltimes more appropriate to paths from oceanic events to continental stations a comparison has therefore been made between locations for a number of oceanic events using the recent iasp91 global traveltimes and the times for the pac91 model derived from observations of events in the pacific although there were often significant differences in the location estimates for the two models these were often no larger than the shifts induced by changing the misfit criterion used for determining the location for events in purely oceanic regions such as tonga and the marianas with little nearby continent the results from the pac91 model either provided a significantly better fit to the data or produced depth estimates in close accord with independent constraints e g centroid moment tensor locations in these cases the use of a specific set of oceanic traveltimes can be recommended however for marginal zones and island arcs the situation is less clear and it is probably best to employ the global traveltime set with the use of additional phases to improve depth estimates

accurate locations of nuclear explosions in balapan kazakhstan 1987 to 1989 locations of 20 nuclear explosions from 1987 to 1989 at the balapan test site kazakhstan are derived with a precision of about 100 m from time sequence spot satellite images combined with teleseismic epicenter estimates for most events there is little or no ambiguity in the association between seismic events and image features the locations determined by jed are associated with formal error estimates 95 confidence ellipses that are significantly too small in 16 out of 19 cases the events are found from satellite imagery to lie outside these ellipses possible causes are nonuniform observation poor azimuthal coverage and source region heterogeneity our results should prove valuable for studies of seismic waveforms arrival time and amplitude tomography event location tectonic release and seismic coupling at balapan and demonstrate the utility of combined seismic and satellite image analyses for seismic verification studies

spatial orientation and distribution of reservoir fractures from scattered seismic energy wepresentthedetailsofanewmethodfordeterminingthereflection and scattering characteristics of seismic energy from subsurface fractured formations the method is based upon observations we have made from 3d finite difference modeling of the reflected and scattered seismic energy over discrete systems of vertical fractures regularly spaced discrete vertical fracture corridors impart a coda signature which is a ringing tail of scatteredenergy toanyseismicwaveswhicharetransmittedthrough or reflected off of them this signature varies in amplitude and coherence as a function of several parameters including 1 the difference in angle between the orientation of the fractures and the acquisition direction 2 the fracture spacing 3 the wavelength of the illuminating seismic energy and 4 the compliance or stiffness of the fractures this coda energy is most coherent when the acquisition direction is parallel to the strike of thefractures ithasthelargestamplitudewhentheseismicwavelengths are tuned to the fracture spacing and when the fractures have low stiffness our method uses surface seismic reflection tracestoderiveatransferfunctionthatquantifiesthechangeinan apparent source wavelet before and after propagating through a fracturedinterval thetransferfunctionforanintervalwithnoor low amounts of scattering will be more spikelike and temporally compact the transfer function for an interval with high scattering will ring and be less temporally compact when a 3d survey is acquired with a full range of azimuths the variation in the derived transfer functions allows us to identify subsurface areas with high fracturing and to determine the strike of those fractures wecalibratedthemethodwithmodeldataandthenapplied ittotheemiliofieldwithafracturedreservoir themethodyielded results which agree with known field measurements and previously published fracture orientations derived from ps anisotropy

improving regional seismic event location in china in an effort to improve our ability to locate seismic events in china using only regional data we have developed empirical propagation path corrections and applied such corrections using traditional location routines thus far we have concentrated on corrections to observed p arrival times for crustal events using travel time observations available from the usgs earthquake data reports the international seismic centre bulletin the preliminary international data center reviewed event bulletin and our own travel time picks from regional data location ground truth for events used in this study ranges from 25 km for well located teleseimic events down to 2 km for nuclear explosions located using satellite imagery we also use eight events for which depth is constrained using several waveform methods we relocate events using the evloc algorithm from a region encompassing much of china latitude 20 55 n longitude 65 115 e we observe that travel time residuals exhibit a distance dependent bias using iaspei91 as our base model to remedy this bias we have developed a new 1 d model for china which removes a significant portion of the distance bias for individual stations having sufficient p wave residual data we produce a map of the regional travel time residuals from all well located teleseismic events residuals are used only if they are smaller than 10 s in absolute value and if the seismic event is located with accuracy better than 25 km from the residual data correction surfaces are constructed using modified bayesian kriging modified bayesian kriging offers us the advantage of providing well behaved interpolants and their errors but requires that we have adequate error estimates associated with the travel time residuals from which they are constructed for our p wave residual error estimate we use the sum of measurement and modeling errors where measurement error is based on signal to noise ratios when available and on the published catalog estimate otherwise our modeling error originates from the variance of travel time residuals for our 1 d china model we calculate propagation path correction surfaces for 74 stations in and around china including six stations from the international monitoring system the statistical significance of each correction surface is evaluated using a cross validation technique we show relocation results for nuclear tests from the balapan and lop nor test sites and for earthquakes located using interferometric synthetic aperture radar these examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations

a preliminary study of regional seismic discrimination in central asia with emphasis on western china abstract in support of a comprehensive test ban treaty we have started to evaluate regional seismic event discrimination capabilities for central asia emphasizing western china we have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband digital station wmq in western china and over 120 earthquakes and five nuclear explosions at station aak in kyrgyzstan the explosions are from the kazakh test site kts and lop nor china the earthquakes are mostly from northwest china event magnitudes measured as either m b or m l range between 2 5 and 6 5 and maximum event station distance is about 1700 km we formed phase spectral cross spectral short period long period and long period ratios to test many possible event discriminants all ratios were corrected for distance effects before forming ratio versus magnitude discrimination plots the most consistent discriminants for separating earthquakes from explosions are p s phase ratios for frequencies above 3 to 4 hz p wave f 1 hz to rayleigh wave 0 03 hz f p s ratios separation between earthquakes and explosions increases with frequency and for the p r ratios separation increases with increasing p frequency the p 3 to 6 hz s 0 75 to 1 5 hz cross spectral ratios also separate earthquakes and explosions these cross spectral ratios may prove quite useful because due to weak high frequency l g signals from explosions the high frequency p l g ratio sometimes cannot be evaluated but the p 3 to 6 hz l g 0 75 to 1 5 hz can be evaluated successfully p spectral ratios 3 to 6 hz 0 75 to 1 5 hz also separate earthquakes and explosions but not quite as consistently as the p s ratios however p spectral ratios may prove useful in cases where path effects block or highly attenuate s n and l g waves rendering the p s ratios unavailable in contrast to nevada test site nts events the l g 3 to 6 hz 0 75 to 1 5 hz spectral ratio does not separate earthquakes and nuclear explosions in central asia this observation may be a source effect as asian explosions are thought to be detonated in highly lithified rocks below the water table while most of the smaller m b for station aak the short period spectral and cross spectral ratios identify a few earthquakes not recorded at wmq from the north pamir region that plot with the lop nor explosions the waveforms from these earthquakes lack surface waves and a distinct l g phase these events which we interpret as having occurred in a subducted slab do separate from the explosions on high frequency f 4 hz p s discrimination plots for station wmq long period r l rayleigh wave love wave ratios separate kts explosions and asian earthquakes however r l ratios fail to identify the large lop nor explosions recorded at station aak because the love waves are as large as the rayleigh waves suggesting that strong tectonic release occurs with these explosions because we have examined only one path at a single station for these explosions source radiation pattern may be influencing this observation

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

a method for the joint inversion of geodetic and seismic waveform data using abic application to the 1997 manyi tibet earthquake geodetic imaging data and seismicwaveformdata have complementary strengthswhen considering the modelling of earthquakes the former particularly modern space geodetic techniques such as interferometric synthetic aperture radar insar permit high spatial density of observation and thus fine resolution of the spatial pattern of fault slip the latter provide precise and accurate timing information and thus the ability to resolve how that fault slip varies over time in order to harness these complementary strengths we propose a method through which the two data types can be combined in a joint inverse model for the evolution of slip on a specified fault geometry we present here a derivation of akaike s bayesian information criterion abic for the joint inversion of multiple data sets that explicitly deals with the problem of objectively estimating the relative weighting between data sets as well as the optimal influence of model smoothness constraints in space and time we demonstrate our abic inversion scheme by inverting insar displacements and teleseismic waveform data for the 1997 manyi tibet earthquake we test using a simplified fault geometry three cases insar data inverted alone vertical component teleseismic broad band waveform data inverted alone and a joint inversion of both data sets the insar only model and seismic only model differ significantly in the distribution of slip on the fault plane that they predict the joint inversion model however has not only a similar distribution of slip and fit to the insar data in the insar only model suggesting that those data provide the stronger control on the pattern of slip but is also able to fit the seismic data at a minimal degradation of fit when compared with the seismic only model the rupture history of the preferred joint inversion model indicates bilateral rupture for the first 20 s of the earthquake followed by a further 25 s of westward unilateral rupture afterwards with slip peaking at 7m in the upper 6 km of the fault this joint inversion approach is thus shown to be a viable method for the study of large shallow continental earthquakes and may be of particular benefit in cases where near field seismic observations are not available the authors 2014 published by oxford university press on behalf of the royal astronomical society

receiver functions for the tien shan analog broadband network contrasts in the evolution of structures across the talasso fergana fault we determined the structure of the crust beneath the central and western tien shan by analyzing broadband 1 20 s analog seismograms of converted p sv phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations the one dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the talasso fergana fault specifically the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault also velocities at depths between 10 and 35 km are about 10 lower in the east although the depth range of these lower velocities is not well resolved the talasso fergana fault is an important boundary for other observables in addition to the structural discontinuities observed at the surface the areas east of the talasso fergana fault are associated with abnormally low mantle velocities outcrops of basalt low q and short wavelength variations in anisotropy integrating these observations we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust likewise the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids the gross contrast in structure east and west of the talasso fergana fault could reflect a contrast in the dynamics of mountain building in these two regions we postulate that while crustal shortening is the dominant mechanism controlling topography west of the talasso fergana fault vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault