strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

intrinsic and scattering q near 1 hz across the east african plateau abstract crustal attenuation across the east african plateau in tanzania an area of uplifted and rifted precambrian crust has been investigated using seismic data from regional earthquakes recorded by the 1994 1995 tanzania broadband seismic experiment we use 1 hz lg coda waves from the 17 events together with the energy flux model of frankel and wennerberg 1987 to obtain estimates of intrinsic q i and scattering q s attenuation for east africa q i values across the plateau are fairly uniform ranging from a low of 300 to a high of 600 q i values for the tanzania craton in the middle of the plateau are similar to those for the mobile belts which form the sides of the plateau q i of 300 to 600 is somewhat lower than the average crustal q for precambrian terrains elsewhere heat flow from the tanzania craton and surrounding mobile belts is not elevated therefore we attribute the lower than average q values not to elevated crustal temperatures but instead to rift faults in the crust that are interconnected and filled with fluids q s ranges from 1000 in the mobile belts and along the eastern margin of the tanzania craton to 2200 in the north central part of the plateau just south of lake victoria we attribute the variability in q s to scattering of lg by surface topography in particular rift basins along the eastern side of the plateau

seismic frequency laboratory measurements of shear mode viscoelasticity in crustal rocks ii thermally stressed quartzite and granite forced torsional oscillation techniques have been used to explore the seismic frequency shear mode viscoelasticity of specimens of two crustal rocks cape sorell quartzite and delegate aplite cycled between room temperature and 700 c under conditions of moderate confining pressure the anisotropy and intergranular inhomogeneity of thermal expansivity in these materials give rise to large deviatoric stresses resulting in thermal cracking at temperatures above a pressure dependent threshold temperature associated with the onset of very pronounced temperature sensitivity of the shear modulus in general accord with the predictions of fracture mechanics models for delegate aplite in particular the shear modulus behaves reproducibly during multiple thermal cycles at different confining pressures consistent with the notion that the thermal cracks are of low aspect ratio minimum maximum dimension and are therefore readily closed by the prevailing confining pressure once the thermal stresses are removed marked frequency dependent dissipation of shear strain energy is observed on heating each rock to temperatures 500 c although the attenuation varies significantly with prior thermal history probably as a result of progressive dehydration and relaxation of deviatoric stresses temperature and pressure dependent crack densities for delegate aplite have been estimated by comparison of the observed shear moduli with those expected for a crack free aggregate in parallel with the forced oscillation tests measurements have been made of the rate at which argon pore pressure equilibrium is re established following a perturbation combination of these results which provide a proxy for permeability with the inferred crack densities indicates that the variation of permeability with crack density is well described by a percolation model with a threshold crack density of 0 2

seismic frequency laboratory measurements of shear mode viscoelasticity in crustal rocks i competition between cracking and plastic flow in thermally cycled carrara marble in the first phase of a seismic frequency study of the viscoelasticity of cracked and fluid saturated crustal rocks forced torsional oscillation experiments have been conducted on specimens of carrara marble subjected to multiple thermal cycles up to 560 c under confining pressure of 50 mpa the experimental results showed distinctly different behaviour of both the shear modulus and internal friction between the first and subsequent thermal cycles a pronounced reduction in shear modulus g observed during the first series of thermal cycles at temperature t 400 c presumably due to dislocation relaxation contrasts with frequency independent q 1 at lower temperatures the relatively sudden onset of markedly frequency dependence seismic wave attenuation might prove to be a useful diagnostic of temperature under crustal conditions

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

azimuthal anisotropy of rayleigh waves in east asia 1 we present a surface wave study aimed to resolve the azimuth dependent propagation velocities of rayleigh waves 10 184s in east asia the resultant anisotropy patterns demonstrate that east asia consists of a number of tectonic domains each displays a characteristic and self consistent deformation mode the depth variation of the anisotropy pattern in each domain helps to understand the boundary actions on the lithosphere and the depth varying deformation styles owing to rheology change

crustal fabric in the tibetan plateau based on waveform inversions for seismic anisotropy parameters 1 the tibetan plateau has the thickest continental crust on earth and fabrics within the crust that are anisotropic to seismic waves may provide clues to how it reached such extreme proportions and how it is currently deforming waveform modeling using a global minimization inversion technique applied to receiver functions computed from 11 stations spanning the north south length of the eastern plateau has yielded a suite of crustal models that include anisotropy these models suggest that the tibetan crust contains 4 14 anisotropy at different depths that is likely a result of both fossil fabrics and more recent deformation all models contain anisotropy in the surface layer and for most stations the alignment of the slow symmetry axis suggests a relationship with crustal fabrics associated with e w trending thrust faults or sutures middle to lower crustal anisotropy is present at most stations with a fast axis trending n s to nw se in the south nearly e w in the central plateau and n s to ne sw in the northern plateau this pattern appears consistent with recent ductile deformation due to both topographically induced flow and to boundary forces from subducting lithosphere at the northern and southern margins of the plateau the orientations of crustal anisotropy determined for most stations in this study are significantly different from shear wave splitting fast polarization directions implying distinct deformation in the crust and mantle

evidence for upper crustal anisotropy in the songpan ganze northeastern tibet terrane 1 the radial and transverse receiver functions obtained at several stations located on the songpan ganze terrane tibetan plateau are characterized by strong p to s conversions in the first 4 s after the direct p wave we show that the amplitude variations of these phases with back azimuth can be well explained by a simple model with one or two anisotropic layers the anisotropy we obtained is quite strong about 15 and might be related to the presence of highly deformed flyschs and schists in the first 25 km of the crust this study shows that anisotropy in the crust could be locally high and has to be taken into account when analyzing teleseismic data

lg attenuation in northeast china using necessarray data nishath r ranasinghe 1 andrea c gallegos 1 andrea r trujillo 1 2 alexander r blanchette 1 eric a sandvol 3 james ni 1 thomas m hearn 1 youcai tang 4 5 stephen p grand 4 fenglin niu 5 6 yongshun j chen 7 jieyuan ning 7 hitoshi kawakatsu 8 satoru tanaka9 and masayuki obayashi9 1department of physics new mexico state university las cruces nm 88001 usa e mail nrana001 nmsu edu 2apache corporation 303 veterans airpark lane 600 midland tx 79705 usa 3department of geological sciences university of missouri 101 geology bldg columbia mo 65211 usa 4jackson school of geosciences the university of texas at austin austin tx 78712 usa 5state key laboratory of petroleum resource and prospecting and unconventional natural gas institute china university of petroleum beijing 102249 china 6department of earth science ms 126 rice university houston tx 77005 usa 7institute of theoretical and applied geophysics sess peking university beijing china 8earthquake research institute the university of tokyo tokyo japan 9institute for frontier research on earth and evolution japan agency for marine earth science and technology yokosuka japan

the mw 8 0 antofagasta northern chile earthquake of 30 july 1995 a precursor to the end of the large 1877 gap abstract an m w 8 0 earthquake occurred on 30 july 1995 in the antofagasta region northern chile the main rupture corresponding to thrust faulting developed from 10 to 50 km in depth along the subduction interface between the nazca and the south american plates the 1995 earthquake took place just south of the large seismic gap where a great earthquake m 9 had occurred in 1877 most of the 1995 rupture was located within a local network consisting of nine short period stations that had been previously installed at the southern end of the 1877 gap and the aftershock sequence could be accurately monitored little destruction resulted from the 1995 earthquake in spite of its large size ground acceleration in antofagasta reached 29 of gravity a tsunami wave 2 to 2 5 m high was observed along the coast from mejillones to taltal one strong foreshock m w 6 2 occurred in the 1995 hypocentral region 6 months before the main event body wave modeling of broadband seismograms from the global network along with the analysis of the aftershock distribution allows us to propose a well constrained model for the whole rupture process some additional details of the rupture were obtained from an accelerometer record at antofagasta the main rupture started as a double even with thrust mechanism below the southern part of the mejillones peninsula and it propagated southward in a n200 e direction with an average velocity of 2 8 km sec it ended near the trench in normal faulting the total rupture area and seismic moment are 185 90 km 2 and 1 2 10 28 dyne cm respectively the aftershock distribution delineates a well defined rupture surface along the subduction interface the distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the mejillones peninsula hence the 1995 main rupture did not propagate north of the mejillones peninsula into the 1877 gap aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north the mechanisms of the strongest aftershocks are similar to that of the mainshock the down dip termination of the main rupture corresponds to the maximum depth 50 km of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event a well constrained dislocation model is proposed for the 1995 main rupture which produces surface displacements in good agreement with available observations of coastal uplift and gps measurements the dislocation model as well as global positioning system gps measurements indicate that the 1995 earthquake generated e w extension in the coastal region of antofagasta the atacama fault located 40 to 50 km above the 1995 main rupture showed small fresh surface ruptures near sierra remiendos 70 km to the sse of antofagasta with a maximum vertical offset of 20 cm this offset corresponds to normal faulting which is in agreement with the e w co seismic extension the mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap modeling of static stress changes induced by the antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface thus the chances for the reactivation of the 1877 gap after this event are greater now

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

on the nature of regional seismic phases ii on the influence of structural barriers summary the blockage of the lg wave by crustal barriers such as continental margins and graben structures has long been recognized as providing a very useful tool for mapping large scale lateral crustal variations along the propagation path numerical simulation of lg wave propagation in complex anelastic media using the pseudospectral method provides insight into the nature of the propagation process using both snapshots of the wavefield and synthetic seismograms a variety of 2 d structures have been investigated including the influence of sediments crustal thickness and attenuation thick sedimentary basins covering a graben structure can have a major influence since they remove lg energy by generating p conversion and scattering the principal mechanisms for strong lg attenuation across a graben the reduction of the lg energy is reinforced by anelastic attenuation in the sediments as well as the influence of the gradually thinning crustal waveguide associated with an elevated moho the extinction of lg in a sequence of explosions fired across the central graben of the north sea can be simulated by numerical calculations for the structure derived from refraction experiments

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

injection of tibetan crust beneath the south qaidam basin evidence from indepth iv wide angle seismic data 1 the international deep profiling of tibet and the himalaya phase iv indepth iv active source seismic profile in northeast tibet extends 270 km roughly north south across the songpan ganzi terrane the predominantly strike slip north kunlun fault along the kunlun suture the east kunlun mountains and the south qaidam basin refraction reflection and gravity modeling provide constraints on the velocity and density structure down to the moho the central qaidam basin resembles average continental crust whereas the songpan ganzi terrane and east kunlun mountains exhibit thickened lower velocity crust also characteristic of southern tibet the crustal thickness changes from 70 km beneath the songpan ganzi terrane and east kunlun mountains to 50 km beneath the qaidam basin this jump in crustal thickness is located 100 km north of the north kunlun fault and 45 km north of the southern kunlun qaidam boundary farther north than previously suggested ruling out a moho step caused by a crustal penetrating north kunlun fault the qaidam moho is underlain by crustal velocity material 6 8 7 1 km s for 45 km near the crustal thickness transition the southernmost 10 km of the qaidam moho are underlain by a 70 km reflector that continues to the south as the tibetan moho the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath stronger qaidam crust

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

moho undulations beneath tibet from grace integrated gravity data summary knowledge of the variation of crustal thickness is essential in many applications such as forward dynamic modelling numerical heat flow calculations seismologic applications and geohistory reconstructions we present a 3 d model of the moho undulations over the entire tibetan plateau derived from gravity inversion the gravity field has been obtained by using the gravity recovery and climate experiment grace potential field development which has been integrated with terrestrial data and is presently the best available in the studied area for the effective use of the global geopotential model that has no height information of observation stations upward continuation is applied the moho model is characterized by a sequence of troughs and ridges with a semi regular pattern which could reflect the continent continent collision between the indian and eurasian plates the three deep moho belts troughs and shallow moho belts ridges between them are clearly found to have an e w directional trend parallel to the border of the plateau and tectonic lines while variation of the directionality is observed in central to southeast tibet to describe the distinctive shape of the moho troughs beneath tibet we introduce the term moho ranges the most interesting aspects of the moho ranges are 1 that they run in parallel with the border and tectonic sutures of the plateau 2 that the distances between ranges are found at regular distances of about 330 km except in northeast tibet and 3 that the splitting of the ranges into two branches is found as the distance between them is increasing from our study we conclude that the distinctive undulations of the tibetan moho have been formed by buckling in a compressional environment superimposed on the regional increase in crustal thickness according to our analysis the grace satelliteonly data turns out to have good enough resolution for being used to determine the very deep moho beneath tibet our moho model is the first one that covers the entire plateau

regional earthquakes in northern tibetan plateau implications for lithospheric strength in tibet 1 a total of 400 regional earthquakes were located in northern tibetan plateau from data recorded by indepth iv and pku eastern kunlun arrays from may 2007 to june 2009 the distribution of these earthquakes is compatible with a continuously deforming tibetan lithosphere most earthquakes occur at a depth range of 0 15 km but no event is deeper than 30 km this observation strongly supports the existence of a hot and weak lower crust beneath the northern tibet the crustal seismogenic zone appears slightly thicker beneath the northern tibet than in the southern plateau possibly reflecting a difference in the rheological dry vs wet structure of the crust the absence of lower crustal and uppermost mantle earthquakes in northern tibet is consistent with a localized asthenospheric upwelling under the qiangtang and songpan ganze terranes finally the lack of mantle earthquakes should be fully addressed in any models of subduction in northern tibet

a novel technique for measuring lg attenuation results from eastern canada between 1 to 10 hz spectral amplitudes of regionally recorded lg waves are studied in detail between 0 6 and 10 hz using vertical component velocity seismograms of the eastern canada telemetered network stations and a supplementary seismic research observatory type station at glen almond gac quebec we find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest furthermore they are found to be strongly frequency dependent consequently it is essential that they be taken into consideration in studies of lg wave attenuation and lg source spectra of regionally recorded seismic events we present a new method of measuring interstation surface wave attenuation which is closely related to the conventional two station method while retaining all the desirable features of the conventional two station method the new technique which we will call the reversed two station method allows simple direct one parameter determination of the lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error the reversed two station method is successfully tested over weakly attenuating short 53 to 210 km interstation paths in eastern canada a difficult experimental condition by normal standards the lg attenuation coefficient 0 6 to 10 hz in eastern canada is found to be frequency dependent and of the form f 0 0008 f 0 81 km 1 at higher frequencies the lg attenuation appears to be essentially frequency independent this latter finding is preliminarily interpreted as evidence that regionally recorded lg waves in the canadian shield are as in the case of lg waves propagating through the structurally complex appalachian province contaminated by the high frequency coda of sn waves the lg contamination over the shield paths becomes severe starting at 14 hz twice the frequency above which the lg signal propagating over the appalachian province becomes completely dominated by non lg arrivals

lateral variations of crustal seismic attenuation along the indepth profiles in tibet from lg q inversion 1 fourier spectra of crustal guided lg waves are collected to study seismic attenuation or q along the indepth profiles the 1 hz lg q q0 values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying q0 models along the indepth iii profile in central tibet the q0 model consists of low and nearly constant values of 90 these low values are consistent with abnormally high temperature as well as partial melts in the crust along the indepth ii profile in southern tibet the q0 values increase southward they are extremely low 60 over the northernmost segment but increase by a factor of 2 over a distance of 100 km between the indus yalong suture and the kangmar dome farther south and into the high himalayas q0 values are higher than 300 regional rayleigh waves observed along the northern indepth ii profile are used to infer a low velocity low q layer at midcrustal depths between 15 and 30 km in southern tibet the aqueous fluid trapped in the upper crust and a midcrust partial melting zone associated with the underthrusting of the indian lithosphere are the likely causes of the high lg attenuation in southern tibet low q rather then a systematic lg conversion along a dipping moho is the main cause of the previously observed lg blockage over paths crossing the southern tibetan boundary

lithospheric and upper mantle stratifications beneath tibet new insights from sp conversions 1 we assess the upper mantle structure under the tibetan plateau from a s to p converted waves receiver functions study contrary to the ps receiver functions blurred by multiples from moho to 350 km depth the sp are better suited for imaging at these depths the moho is clearly recovered and often exhibits a complex and warped structure the upper crust is marked by a significant low velocity zone in the southern part of the plateau probably associated with partial melt which vanishes north of the bangong suture in the upper mantle between the moho and the 660 km discontinuity four stratification levels are identified the strongest converter at a depth ranging between 120 to 180 km corresponds to the bottom of a low shear wave velocity layer imaged by surface wave inversion

azimuthal anisotropy of lg attenuation in eastern tibetan plateau 1 the q of seismic phase lg is measured to approach the crustal q which can be used to infer crustal rheology a reverse two station event method is used to measure the interstationqlg which theoretically eliminates effects from the source and site response strong azimuthal anisotropy of 1 qlg in northern tibet is observed which approximately correlates with the seismic velocity anisotropy observed using both surface waves and teleseismic s waves both the intrinsic and scattering attenuation contribute the isotropic q whereas the anisotropic q may be associated with the fractures and faults in the crust strong 1 qlgis observed in the northwestern songpan ganzi fold belt where the high qdirections correspond with the orientations of major strike slip faults intrinsic and scatteringq values are estimated for different tectonic terranes in eastern tibetan plateau the intrinsic qvalues are approximately 500 in the northern tibetan plateau which suggests a hot 700 c crust consistent with low seismic velocity measurements our result suggests that the crust of northwestern songpan ganzi fold belt may be hotter than that of the qiangtang terrane which may be related to the strain heating along the major strike slip fault zones that dominate the northernmost tibet

rayleigh wave phase velocity maps of tibet and the surrounding regions from ambient seismic noise tomography ambient noise tomography is applied to the significant data resources now available across tibet and surrounding regions to produce rayleigh wave phase speed maps at periods between 6 and 50 s data resources include the permanent federation of digital seismographic networks five temporary u s program for array seismic studies of the continental lithosphere passcal experiments in and around tibet and chinese provincial networks surrounding tibet from 2003 to 2009 totaling 600 stations and 150 000 interstation paths with such a heterogeneous data set data quality control is of utmost importance we apply conservative data quality control criteria to accept between 5000 and 45 000 measurements as a function of period which produce a lateral resolution between 100 and 200 km across most of the tibetan plateau and adjacent regions to the east misfits to the accepted measurements among passcal stations and among chinese stations are similar with a standard deviation of 1 7 s which indicates that the final dispersion measurements from chinese and passcal stations are of similar quality phase velocities across the tibetan plateau are lower on average than those in the surrounding nonbasin regions phase velocities in northern tibet are lower than those in southern tibet perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained at short periods 20 s very high velocities are imaged in the tarim basin the ordos block and the sichuan basin these phase velocity dispersion maps provide information needed to construct a 3 d shear velocity model of the crust across the tibetan plateau and surrounding regions

moho offset across the northern margin of the tibetan plateau anomalous double pulse teleseismic p wave arrivals were observed at one station near the northern margin of the tibetan plateau the azimuthal dependence of the waveform distortion and its absence at nearby stations indicated that the distortion was produced by receiver side crustal heterogeneity modeling of the three component data revealed a 15 to 20 kilometer moho offset that occurs over a narrow lateral range of less than 5 kilometers this east west striking offset separates the thick tibetan plateau crust from the qaidam basin crust such a sharp crustal thickness change implies a weak tibetan plateau crust that thickens vertically in response to penetration by india from the south and to blockage caused by a strong qaidam basin crust to the north

lateral variation of pn and lg attenuation at the cdsn station lsa abstract the propagation efficiencies of the regional phases lg and pn are indicative of how active and recent tectonics influence crust and uppermost mantle properties respectively in this study regional scale lateral heterogeneity of lg and pn attenuation for the region in and around the southern tibetan plateau is investigated the data set is comprised of seismograms recorded at the chinese digital seismogram network cdsn station lsa from regional events with epicentral distances within 11 attenuation of lg and pn is calculated using spectral methods and assuming constant q lg and q pn models for the frequency bands 0 3 to 3 0 hz and 0 5 to 4 0 hz respectively lateral variation in attenuation is estimated by analyzing data on an event by event basis significant lateral variation is observed with q lg decreasing from about 520 for events south of lsa to about 340 for events north of lsa and q pn 670 for southern backazimuths while q pn 240 for northern events for pn this north south variation is consistent with other observations indicating partially melted upper mantle beneath north central tibet for lg the azimuthal variation in attenuation indicates that lg propagation as observed at lsa is efficient for paths crossing the southern boundary of the tibetan plateau relative to paths within the plateau itself

seismic evidence for stepwise thickening of the crust across the ne tibetan plateau we performed a receiver function analysis on teleseismic data recorded along two 550 km long profiles crossing the northeastern tibetan plateau results from time to depth migration grid search vp vs determination and simulated annealing inversion of waveforms reveal that the crust thickens from 50 km near the northern edge of the plateau to 80 km south of the jinsha suture in the qiang tang block crustal thickening occurs in staircase fashion with steps located beneath the main reactivated sutures the vp vs ratio close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern tibetan crust

deep structures along the boundary of a collisional belt attenuation tomography of p and s waves in the greater caucasus summary seismic attenuation q 1 o fp and s waves shows a major discontinuity from the russian platform to the tectonically active greater caucasus broadband records show this boundary over paths 4 long as revealed by the decay of amplitude spectra from a digital seismic network flanking the greater caucasus we measure attenuation from individual spectra using a non linear least squares procedure to determine an attenuation parameter t simultaneously with source parameters at frequencies between 1 and 15 hz the t measurements are then inverted for spatial variations of q 1 with parametrizations of varying complexity model variance for heterogeneous structures improves by more than 30 per cent compared with homogeneous parametrizations site corrections also significantly improve the fit in these inversion results mountainous regions exhibit q values 2 3 times lower q s 775 75 than do the adjacent shields q s 2060 315 showing that the discontinuity is large for both regions q p is roughly equal to q s comparison of body wave to coda spectra indicates that intrinsic absorption rather than scattering dominates the q 1 measurements at least beneath the mountains hence q 1 variations may give a reasonable proxy for temperature if so then temperature beneath the mountains exceeds that beneath the shield by 70 400 c these temperature increases may not be high enough to generate widespread partial melting beneath the mountains but could produce regional metamorphism and could contribute substantially to isostatic compensation of the mountains whatever their origin the boundary in seismic attenuation is abrupt and large between stable craton and an adjacent mountain belt demonstrating that q 1 is a sensitive indicator of tectonic process

a back projection method for imaging large scale lateral variations of lg coda q with application to continental africa summary a new method is developed which uses back projection tomography to regionalize large scale lateral variations of coda q for lg waves which have traversed long continental paths successful use of this method requires precise and stable single trace measurements of lg coda q xie nuttli 1988 the method converges rapidly and requires minimal computer storage it also allows quantitative estimation of resolution and error in imaging lateral variations of coda q the spatial resolution of this method is limited by the uneven spatial coverage of the data base by our limited knowledge of lg coda generation and by the trade off between the stability and the spatial resolution of the coda q inversion the method is applied to a large set of digital lg coda data from africa where large scale lateral variations of lg coda q are found to correlate well with major tectonic features most of africa is stable and like other stable regions has relatively high coda q values the lowest values of coda q are associated with the african rift system other regions of low q values include the atlas mountains and cape fold belt regions of mesozoic and younger deformation the lateral variation of frequency dependence of lg coda q correlates in most regions with that of q at 1 hz our analysis of the spatial resolution of this method indicates that the resolving power of lg coda q imaging is comparable to that of velocity tomography using long period surface waves using empirical approaches we estimate that uncertainties in lg coda q and its power law frequency dependence parameter are less than about 60 and about 0 2 respectively for most of africa

receiver function imaging of the lithosphere at the kunlun qaidam boundary northeast tibet abstract we studied the crustal structure and tectonics in the north tibetan plateau from the songpan ganzi terrane to the qaidam basin using teleseismic receiver function imaging across a major lithospheric boundary the kunlun qaidam boundary where previous studies suggest a 15 20 km change in crustal thickness from thicker crust in the kunlun mountains to thinner crust in the qaidam basin we report p receiver functions for 70 stations largely the international deep profiling of tibet and the himalaya indepth phase iv experiment our most dense station coverage is located along the roughly north south indepth iv active source seismic profile at approximately 95 e longitude azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and vp vs from across the study area receiver functions show strong converters that we interpret as the moho at 70 km depth beneath the qiangtang songpan ganzi terranes and kunlun mountains and at 50 km depth beneath the central qaidam basin this large change in crustal thickness occurs 50 km north of the north kunlun strike slip fault on which the 2001 m8 1 kunlun earthquake occurred receiver functions for some of the stations north of the thickness change at the kunlun qaidam boundary also show a deeper 70 km bright converter in addition to the 50 km converter the two converters appear to overlap by up to 30 km in some locations along the south qaidam basin we combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north tibetan plateau including crustal flow and crustal injection at depths imaged here shallower than 100 km we see no evidence of southward subduction of eurasian lithosphere

high resolution regional seismic attenuation tomography in eastern tibetan plateau and adjacent regions 1 the q of regional seismic phases lg and pg within the crust is assumed as a proxy for crustal q and q which is used as a constraint of crustal rheology we measure regional phase q of the eastern tibetan plateau and adjacent areas this method eliminates contributions from source and site responses and is an improvement on the two station method tsm we have generated tomographic images of crustal attenuation anomalies with resolution as high as 1 in general we observe low q in the northernmost portions of the tibetan plateau and high q in the more tectonically stable regions such as the interior of the qaidam basin the calculated site responses appear to correlate with topography or sediment thickness furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the rtm method effectively removes the source response and may be used as an alternative to source magnitude

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

crustal motion north and south of the arica deflection comparing recent geodetic results from the central andes 1 an intercomparison of recent velocity solutions for the global positioning system gps networks constructed by the south american nazca plate project snapp based in bolivia and peru and our central andes gps project cap based in chile and argentina indicates a velocity discontinuity of order 10 mm yr near the boundary between these networks we suggest that this velocity jump manifests measurement bias in the snapp velocity field our results indicate that no major slip partioning occurs within the forearc of northern chile in response to the obliquity of subduction but that nazca south america plate convergence is partitioned between the forearc and the backarc regions the present rate of shortening across the southern part of the sub andean belt in argentina is 8 9 1 6 mm yr

strong crustal heterogeneity in the bolivian altiplano as suggested by attenuation of lg waves using the seismological data of four temporary networks installed across the andes of bolivia and chile between 1990 and 1995 we study the amplitudes of the lg phase for 45 regional events located at crustal depths this analysis reveals that lg is strongly attenuated for most of the ray paths crossing the altiplano independent of the azimuth of propagation to map the anomalies and evaluate their frequency dependence we conduct a regionalization of the crustal average qs factor using a damped least squares inversion technique in the frequency range 0 6 5 hz the resulting maps delineate regions with contrasting values of apparent attenuation the altiplano is characterized by strong attenuation with an average qs of 100 at 1 hz whereas the cordilleras are regions of weaker attenuation the frequency dependence of qs is investigated in an attempt to separate the source of the apparent attenuation into scattering and anelastic absorption within the hypothesis of a weak frequency dependence of the intrinsic q we show that at 1 hz scattering plays a major part in the attenuation of lg waves whereas at 4 hz both mechanisms are of equal importance these results indicate that the attenuation of lg waves in the altiplano is due more to scattering by small scale heterogeneities than to a high percentage 1 of partial melt involving a large part of the crust

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

complex multilayered azimuthal anisotropy beneath tibet evidence for co existing channel flow and pure shear crustal thickening of the two debated end member models for the late cenozoic thickening of tibetan crust one invokes channel flow rapid viscous flow of the mid lower crust driven by topography induced pressure gradients and transporting crustal rocks eastward and the other pure shear faulting and folding in the upper crust with viscous shortening in the mid lower crust deep crustal deformation implied by each model is different and would produce different anisotropic rock fabric observations of seismic anisotropy can thus offer a discriminant we use broadband phase velocity curves each a robust average of tens to hundreds of measurements to determine azimuthal anisotropy in the entire lithosphere asthenosphere depth range and constrain its amplitude inversions of the differential dispersion from path pairs region average inversions and phase velocity tomography yield mutually consistent results defining two highly anisotropic layers with different fast propagation directions within each the middle crust and the asthenosphere in the asthenosphere beneath central and eastern tibet anisotropy is 2 4 per cent and has a nne ssw fast propagation azimuth indicating flow probably driven by the nne ward shallow angle subduction of india the distribution and complexity of published shear wave splitting measurements can be accounted for by the different anisotropy in the mid lower crust and asthenosphere the estimated splitting times that would be accumulated in the crust alone are 0 25 0 8 s in the upper mantle 0 5 1 2 s depending on location in the middle crust 20 45 km depth beneath southern and central tibet azimuthal anisotropy is 3 5 and 4 6 per cent respectively and its e w fast propagation directions are parallel to the current extension at the surface the rate of the extension is relatively low however whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals implying large finite strain and consistent with high rate horizontal flow together radial and azimuthal anisotropy suggest eastward mid crustal channel flow in central tibet along the regional topography gradient in ne high tibet mid crustal azimuthal anisotropy is 4 8 per cent and has wnw ese and nw se fast propagation directions parallel to the net extension at the surface these fast directions are inconsistent with channel flow following the sw ne regional topography gradient instead they suggest similar net deformation in the decoupled shallow and deep crust in the brittle upper crust it is accommodated by strike slip faulting in the ductile mid lower crust by shear oriented at 45 to the faults although mid crustal flow beneath ne tibet may transport some material towards the plateau periphery at a low region average rate the dominant mid crust deformation pattern is shear parallel to the plateau boundary this implies that channel flow from central tibet is not the main cause of the on going crustal thickening farther northeast

frequency dependent attenuation in the crust the shear wave attenuation q l in the crust beneath the northeastern us and scandinavia is determined as a function of depth and frequency using theoretical modeling of complete waveforms in comparison with observed data to determine attenuation in the upper crust of the northeastern us short period 0 25 t 2 sec rg waves recorded by linear arrays of seismometers are used for scandinavia rg and lg waves are combined to extend the investigation into the deeper crust the combination of rg waves and short period t 1 0 sec lg waves makes it possible to determine attenuation as a function of depth and frequency in the crust parameterized as q qo f to match the observations theoretical models for three mechanisms that contribute to attenuation in the crust frequency independent anelastic qa 1 and frequency dependent fluid flow qf 1 and scattering qs 1 are considered the best fitting models require low q q0 100 in the shallow crust that increases with increasing frequency as f0 5 above 1 hz in the middle crust between about 2 to 10 km depth q is moderate 100 q0 500 with a frequency dependence of f1 in the lower crust the attenuation is very low q 500 and its frequency dependence is difficult to establish the low intrinsic q and the frequency dependence in the uppermost crust can be explained by fluid flow in fractures in the middle crust the frequency dependence suggests scattering the increase of q in the lower crust is most likely due to annealing of microcracks under pressure and temperature as observed in the laboratory

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

qvoa analysis p wave attenuation anisotropy for fracture characterization this paper investigates q anisotropy for characterizing fractured reservoirs specifically the variation of the seismic quality factor q versus offset and azimuth qvoa w e derive an analytical expression for p wave attenuation in a transversely isotropic medium with horizontal symmetry axis hti and provide a method qvoa for estimating fracture direction from azimuthally varying q in pp wave reflection data the qvoa formula is similar to ruger s approximation for pp wave reflection coefficients the theoretical basis for amplitude variation with angle offset avoa analysis the technique for qvoa analysis is similar to azimuthal avo analysis we introduce two new seismic attributes q versus offset qvo gradient and intercept qvo gradient inversion not only indicates fracture orientation but also characterizes q anisotropy we relate the q anisotropy parameter q to fractured medium parameters and invert the qvo gradient to estimate q the attenuation parameter q and thomsen style anisotropy parameter v are found to be interdependent the attenuation anisotropy magnitude strongly depends on the host rock s vs vp parameter whereas the dependence on fracture parameters is weak this complicates the qvo gradient inversion for the fracture parameters this result is independent of the attenuation mechanism to illustrate the qvoa method in synthetic data we use hudson s first order effective medium model of a dissipative fractured reservoir with fluid flow between aligned cracks and random pores as a possible mechanism for p wave attenuation

crustal thickening and variations in architecture from the qaidam basin to the qang tang north central tibetan plateau from wide angle reflection seismology abstract wide angle reflection refraction profiles at crustal scale have been obtained along a 700 km transect of northeastern tibet from six shotpoints a combination of sampling by broadside fan recordings and in line profiling allows us to image increases in moho depth towards the centre of the plateau these occur with different styles across the two block boundaries between north kun lun bayan har and qang tang as shown also by the changes resolved in the internal architecture and layer velocities in the crust clear s waves are observed together with p waves they bring rare constraints on the composition of crustal units which reveals an abnormally felsic average composition for the whole crust and its lower half along the whole length of the transect across the north kun lun qaidam block variations of crustal thickness are detected with moho depth between 55 and 65 km and an inverse correlation with basement topography in the north of the bayan har block a strong heterogeneity in the middle of the 70 km thick crust is shown as suggested from synthetic seismograms modelling it may result from thin layer inclusion marking the northward tectonic superposition of the bayan har crust on the north kun lun crust as imaged by a fan profile the corresponding upper part could have been transported northwards to thicken also the upper crust of the present north kun lun block in the north of qang tang a model is obtained with a 75 km thick crust and a lower half of the crust with average velocities characteristic of felsic composition from bayan har to qang tang variations of deep architecture are documented across the jinsha suture and xianshuihe fault from recording in line and broadside to several shotpoints together with the felsic composition of the lower part of the crust a moho offset suggests the imbrication of the bayan har crust in the qang tang lithosphere between a south dipping moho above and a north dipping moho below a model of indentation of the qang tang by an originally thinner bayan har crust and lithosphere with part of the qang tang brought to greater depth would also be consistent also with the moho topography across the transition from the qang tang to bayan har as imaged by fan profile

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

tibetan basement rocks near amdo reveal missing mesozoic tectonism along the bangong suture central tibet the u pb and 4 0 ar 3 9 ar studies of a unique exposure of crystalline basement along the jurassic early cretaceous bangong suture of central tibet reveal previously unrecognized records of mesozoic metamorphism magmatism and exhumation the basement includes cambrian and older orthogneisses that underwent amphibolite facies metamorphism coeval with extensive granitoid emplacement at 185 170 ma the basement cooled to 300 c by 165 ma and was exhumed to upper crustal levels in the hanging wall of a south directed thrust system during early cretaceous time we attribute jurassic metamorphism and magmatism to the development of a continental arc during bangong ocean subduction and early cretaceous exhumation to northward continental underthrusting of the lhasa terrane beneath the qiangtang terrane we speculate that a jurassic arc extended regionally along the length of the bangong suture but in all other places in tibet has been buried either depositionally or structurally beneath supracrustal assemblages

the effect of 3 d structure on lg propagation patterns the method of ray diagrams based on the constructive interference of multiple s reflections gives a visual measure of the interaction of guided wavetrains with variations in crustal structure the technique is extended to 3 d heterogeneity and by representing the s ray systems in plan view through their basement reflection points a semi quantitative measure of the disruption of lg at different phase velocities can be made in addition the tilt of the ray systems can be used as an indication of the degree of interconversion between rayleigh and love wave energy two simple crustal models are examined to investigate the effects of oblique incidence and transverse topographic gradients on multiple s reflections using ray diagrams the method is then applied to several real earth models of the crust in central asia using topographic data and assuming isostasy interpretations of the ray diagrams for paths across the major tectonic features compare well with many of the observations from previous studies ruzaikan et al 1977 ni barazangi 1983 and suggest that the gross characteristics of lg propagation can be attributed to the shape of the crustal waveguide

frequency dependent attenuation in the crust beneath central france from lg waves data analysis and numerical modeling abstract the quality factor of shear waves in the crust beneath central france is evaluated by the combined use of data analysis and theoretical modeling the measurement is made from lg wave records of 18 earthquakes at 22 short period vertical seismic stations of the french seismic network the geometrical attenuation of lg waves has been calculated through a numerical simulation in a perfectly elastic model the station responses including site effects have been evaluated from lg waves and from records of the seismic noise q has been calculated for a set of 11 frequencies between 0 5 and 10 hz and has been found to be in the form q 290 f 0 52 where f is the frequency the numerical simulation leads to the interpretation of q of lg waves as the mean value of q within the crust theoretical and experimental decay of energy with distance are in very good agreement the lg waves present a different sensitivity to a low q zone whether this zone is associated with the sediments or with scattering in the lower crust the lower crust beneath central france appears from our experimental results to be associated with high values of q

contrasts in regional seismic wave propagation to station wmq in central asia summary the varying character of regional seismic phase propagation in central asia is well illustrated by the character of the seismograms at the urumqi station wmq in xinjiang china three different classes of behaviour are seen for propagation across the relatively stable platforms to the north along the tien shan chain and across the tarim basin from tibet these paths demonstrate the influences of the variation in crustal structure on the regional phases particularly the tibetan plateau and its north and south boundaries but also the influence of the pamirs on propagation along the tien shan as an interference phase lg is sensitive to changes in crustal structure and can be used to study crustal heterogeneity lg can propagate along the tien shan chain trapped by the gradients in structure on either side but with a progressive loss in amplitude and frequency however once the northern boundary of tibet is crossed the high frequencies in the lg train are lost and there is little recognizable arrival the amplitude ratios of the different crustal phases pn sn and lg can be used to characterize the behaviour and the different classes of propagation can be further separated when account is made of the frequency content in the lg window the influence of the transitions in crustal structure at the northern edge of the tibetan plateau on propagation to wmq is investigated using numerical modelling with the pseudospectral method for models of crustal variations this demonstrates how the crustal pinch at the northern boundary operates to modify the fairly efficient propagation inside tibet with substantial transfer of energy into the mantle

shallow seismic structure of kunlun fault zone in northern tibetan plateau china implications for the 2001 ms8 1 kunlun earthquake summary the shallow seismic velocity structure of the kunlun fault zone klfz was jointly deduced from seismic refraction profiling and the records of trapped waves that were excited by five explosions the data were collected after the 2001 kunlun ms8 1 earthquake in the northern tibetan plateau seismic phases for the in line record sections 26 records up to a distance of 15 km along the fault zone were analysed and 1 d p and s wave velocity models of shallow crust within the fault zone were determined by using the seismic refraction method sixteen seismic stations were deployed along the off line profile perpendicular to the fault zone fault zone trapped waves appear clearly on the record sections which were simulated with a 3 d finite difference algorithm quantitative analysis of the correlation coefficients of the synthetic and observed trapped waveforms indicates that the kunlun fault zone width is 300 m and s wave quality factor q within the fault zone is 15 significantly s wave velocities within the fault zone are reduced by 30 45 per cent from surrounding rocks to a depth of at least 1 2 km while p wave velocities are reduced by 7 20 per cent a fault zone with such pand s low velocities is an indication of high fluid pressure because vs is affected more than vp the low velocity and low q zone in the klfz model is the effect of multiple ruptures along the fault trace of the 2001 ms8 1 kunlun earthquake

precise temperature estimation in the tibetan crust from seismic detection of the α β quartz transition in the deep crust temperature which is among the key pa rameters controlling lithospheric dynamics is inferred by extrap olation from the surface using several assumptions that may fail in regions of active tectonics and fluid migration in the rare case that temperatures of 700 8c or higher are exceeded in the upper and middle continental crust composed of quartz rich felsic rocks the a b quartz transition abqt will occur generating a mea surable seismic signature and offering the possibility for precisely estimating temperature from the known abqt phase diagram here it is shown that all expected seismic features of the abqt are met by the boundary between the upper and middle crust be low the indepth iii profile in central tibet this finding implies that a temperature of 700 8c is achieved at a depth of 18 km under the southern qiangtang block which agrees with the depth to the top of a high electrical conductivity anomaly likely representing partially melted crust to the south in the northern lhasa block the abqt is at 32 km depth corresponding to a temperature of 800 8c it thus appears that this seismic boundary representing the abqt is the result of recent geologic processes rather than being a lithologic boundary

high temperature experiments on the elastic and anelastic behaviour of magmatic rocks measurements of the complex shear modulus have been performed on various ultrabasic and basic rocks dunite peridotite gabbro and basalts and a synthetic forsterite at temperatures between 600 and 1500 c depending on the sample material and in the frequency range from 0 003 to 30 hz under forced torsional oscillations the maximum shear stress was 0 3 mpa and the strains were of the order 5 10 6 to 5 10 5 measurements of the electrical resistivity as a function of temperature have been carried out together with the mechanical experiments to indicate the beginning of partial melting the absolute values of the shear modulus below tm are comparable with the results of other authors the onset of melting shows up in a rapid decrease of both shear modulus and electrical resistivity at high temperatures t 0 8 tm the absorption factor q 1 follows a power law q 1 exp a rt with 0 15 0 30 and a between 100 and 200 kj mol 1 towards lower temperatures this high temperature background is superimposed by a flat absorption peak which shifts from high to low frequencies with falling temperature this peak can often be described by a lognormal distribution of simple relaxation processes with a standard deviation of about three and a mean relaxation time exp a rt with a 400 600 kj mol 1 above tm a third contribution to q 1 is observed that can be regarded as independent of frequency between 1 and 30 hz this last component which contributes 50 to the total absorption is attributed to the melt phase in the samples an explanation for the high temperature background and the peak is proposed which is based on the relaxation of pinned dislocation lines by some coupling mechanism via the reorganisation of the stress field with increasing deformation this process may be responsible for both features at very high temperatures and low frequencies a small influence of steady state flow on q 1 is observed and below t 0 6 tm thermal cracking or thermoelastic losses may contribute to q 1 the influence of lithostatic pressure on q 1 is estimated and the extrapolation of the experimental data to the p t conditions of the uppermost mantle are in good agreement with published earth models an attempt is made to establish a general q t relationship from the experimental data

l g coda q variation across the myanmar arc and its neighboring regions regional seismograms were collected to image the lateral variations of lg coda q at 1 hz q0 and its frequency dependence across burma and its neighboring regions the data include 660 vertical component traces recorded at 39 stations the resulting image indicates that lg coda q at a frequency of 1 hz varies between 100 and 500 lowest q values 250 are found in the eastern indian block from the lg coda q tomography we found that 1 the sagain fault acts as a rough boundary between the eastern indian plate and the three rivers area of the eurasia plate 2 near the eastern himalayan syntaxis higher q value appears in the background of relatively low q which may be the consequence of the northward intrusion of the assam block of the indian plate into the southern qinghai tibet plateau

high frequency attenuation in the crust and upper mantle of the northern tien shan abstract we analyze 243 three component broadband digital seismograms of aftershocks from the m s 7 3 suusamyr kyrgyzstan earthquake to determine seismic attenuation in the northern tien shan p g s g and s m s body waves and s coda waves were used to estimate the frequency dependent f 1 2 30 hz attenuation and scattering parameters of the crust and upper mantle as a function of depth using the equation q f q 0 f n for multiple layers we find that q 0 increases with depth from 76 upper crust to 1072 upper mantle and the value of n decreases from 0 99 to 0 29 over the same range the q coda results also demonstrated an azimuthal dependence q 0 736 in the north south direction and q 0 494 in the east west direction there is a strong 2 dependence on azimuth for high frequencies 1 2 hz the depth and azimuthal dependence of the quality factor show that the q is complicated and three dimensional estimates of the inhomogeneity scale a show two types of multiple scatterers wu and aki 1985 a velocity perturbation with a 0 5 3 0 km and an impedance perturbation with a 0 052 0 413 km it appears that a increases with depth the coefficient of scattering decreases from g 0 f 0 0055 km 1 at h 0 6 km to 0 0020 km 1 at h 6 11 km and 0 0048 km 1 at h 11 15 for forward scattering and g f 0 0006 km 1 at depth of low crust and upper mantle for backscattering estimates of seismic albedo b 0 show that the primary source of attenuation is intrinisc anelasticity and the variation of q 0 and n indicates that the level of heterogeneity varies with depth possibly due to pressure the lateral variation of q 0 can be connected with azimuthal anisotropy in the upper mantle related to current deformation under the tien shan

lg attenuation in the central and eastern united states as revealed by the earthscope transportable array abstract lg waveforms recorded by earthscope s transportable array ta are used to estimate lg q 0 in the central and eastern united states ceus attenuation is calculated based on lg spectral amplitudes filtered at a narrow band from 0 5 to 1 5 hz two station and reverse two station techniques are used to calculate q 0 values 39 events occurring from 2010 to 2012 and ranging from magnitude 3 to magnitude 6 are used in this study 39 258 and 4 627 interstation q 0 values are determined using the two station and reverse two station techniques respectively the results show northeast trending high q 0 regions low attenuation ranging from eastern new mexico to wisconsin regions of low q 0 high attenuation are seen along the minnesota wisconsin border the gulf coastal plain gcp along the south dakota nebraska border and along the oklahoma texas border a positive correlation between heat flow sediment thickness recent tectonic activity fluids and low q 0 was observed areas with low heat flow thin sediment cover no recent tectonic activity and a lack of fluids were observed to have consistently high q 0 these new models use a greater amount of data attain more coverage than previous studies and better constrain attenuation this increase in detail can improve high frequency ground motion predictions of future large earthquakes for more accurate far field hazard assessment and improve the overall understanding of the structure and assemblage of the ceus

modeling the effect of multiple sets of mesoscale fractures in porous rock on frequency dependent anisotropy this study models the seismic response of porous rock containing two fracture sets with different orientations sizes and connectivities modeling demonstrates frequency dependent anisotropy controlled by two characteristic frequencies that are defined by fluid mobility and the length scales of the fractures fracture related dispersion and attenuation typically occur over a wider frequency band than is seen in the case of a single fracture set when one set of fractures is modeled as being sealed the azimuthal variation of velocity and attenuation can be different with the azimuth of minimum attenuation coinciding with the strike direction of the open fracture set the results should support attempts to differentiate between open and closed fractures on the basis of seismic measurements

paleoseismology of the mejillones fault northern chile insights from cosmogenic 10be and optically stimulated luminescence determinations 1 we have undertaken the first paleoseismological study on an upper plate fault in chile the selected structure was the mejillones fault which is marked by a conspicuous fault scarp using cosmogenic 10be and osl dating and detailed sedimentary logging of trenches we have constrained the abandonment of two alluvial surfaces by fault activity at ca 35 ka and ca 14 ka based on stratigraphic observation we characterized the fault evolution in four intervals over the last ca 35 ka during the first three intervals the fault had a steady slip rate of 0 61 0 26 m ka the fourth interval is delineated by the last vertical fault slip and the accumulation of un deformed hillslope deposits after ca 3 3 ka and has a slip rate of 0 22 0 06 m ka the younger surface abandonment was caused by two mw 7 paleoearthquakes with a recurrence interval of 5 0 3 5 ka the third interval is characterized by the interaction of hillslope deposits and aseismic slip and or centimeter scale seismic slip events at ca 3 5 ka a last large mw 6 6 earthquake took place the recurrence intervals of large mw 8 5 subduction earthquakes do not appear to be the same as the recurrence intervals of the mw 7 events on the upper plate mejillones fault

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

explosion seismic p and s velocity and attenuation constraints on the lower crust of the north central tibetan plateau and comparison with the tethyan himalayas implications on composition mineralogy temperature and tectonic evolution p and s velocity and attenuation estimates in the lower crust are obtained from a set of wide angle reflection refraction profiles in the region of active tectonics at the ne edge of the tibetan plateau and discussed together with respect to similar data at its himalaya south tibet edge the quality factor is estimated in the lower half of the crust by accounting for the differential effect on amplitude frequency observed between waves of different penetrations and both in p and s modes attenuation values allow to exclude a significant proportion of partial melt and to estimate the homologous temperature ratio of in situ to solidus absolute temperatures the latter depend on the physical conditions being of dry wet or dehydration melting which are found different among the regions of the northern bayan har and northern qang tang boundaries between blocks as well as the tethyan himalayas south of the indus tsangpo suture their in situ temperatures differ also as estimated from their different vp for a similar felsic composition joint measurement of several parameters vp vs qp and qs reveals the composition mineralogy temperature and hydration conditions of the lower half of the thickened crust of tibet that may be discussed in terms of evolution the material presently in the thickened crust even its lower part has a felsic composition upper to middle crustal lithology and the temperature conditions estimated suggest that basic material that could have underlain it could be eclogitized and not appear anymore above the seismic moho under northern qang tang the felsic material in the lower half of the crust appears as hot and dry its burial may have occurred earlier or may have been moderate in the postcollisional phase this is consistent with a model of indentation of the qang tang crust by an originally thinner bayan har crust to bring part of its crust to greater depth suggested from imaging the crustal architecture under northern bayan har the material in the lower half of the crust appears as felsic at low temperature and not dry conditions this is evidence that it has been transported from a shallower depth and this recently enough not to be yet dehydrated and temperature equilibrated in a conductive geotherm it supports a model of recent overriding of the middle crust of the north kun lun block to the north independently suggested from the image of crustal architecture the tethyan himalayas case appears bracketed by these two cases in northern tibet for vp and temperature conditions but shows highest attenuation in the lower crust that is colder but less dry than under northern qang tang