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

propagation characteristics of lg across the tibetan plateau abstract we present the results of a study on the propagation of lg within the tibetan plateau we analyze over 1200 seismograms for lg propagation from 185 local and regional earthquakes recorded at distances from 150 to 2000 km 52 of which are located within the tibetan plateau although the propagation of lg has been observed across most of asia lg has not been observed for paths crossing the tibetan plateau we analyze paths that cross the plateau boundaries and paths contained entirely within the tibetan plateau lg is generated within the tibetan plateau and can propagate to epicentral distances of at least 600 km for events located outside of the tibetan plateau lg is either absent or severely diminished for paths that cross both the himalayan and kunlun ranges confirming that the margins of the plateau weaken lg transmission based on the observations from our qualitative analysis we invert lg amplitudes for event station paths that are confined to the tibetan plateau to estimate the quality factor q the inversion yields a frequency dependent q function q f 366 37 f 0 45 0 06 0 5 f 16 h z similar observations in other areas indicate that frequency dependent apparent q within the tibetan plateau is well below that expected for a typical continental interior instead it has a frequency dependence that is characteristic of a tectonically active region

site amplification scattering and intrinsic attenuation in the mississippi embayment from coda waves we estimate site amplification intrinsic q q i and scattering q q s values from three component coda of broadband data recorded by cooperative new madrid sesimic network stations for center frequencies 0 2 0 5 1 0 2 0 and 5 0 hz near 0 5 and 1 0 hz site amplification factors for the transverse component recorded by stations in the mississippi embayment are between 1 7 and 18 compared with stations outside the embayment the amplification factor becomes smaller at frequencies lower than 0 5 hz and higher at those greater than 1 0 hz the radial component shows a similar amplification however the vertical component shows relatively less amplification q i and q s are estimated from coda using an energy flux model for two time windows after the direct s wave and at 0 5 and 1 0 hz center frequencies the two time windows include coda up to and after twice the s wave travel time at each station coda parameters change with the time window and are attributed to a mixture of incoherent coda with coherent s wave reverberations directly after the s wave q i and q s inferred from stations outside of the embayment have higher values than those from stations within the embayment a plane wave model qualitatively explains the lack of coda amplification at high frequencies compared with a low frequency without recourse to anelastic attenuation arguments however the plane wave model cannot explain the very large observed coda amplifications at 1 and 0 5 hz seen in the data suggesting that other wave scattering effects must be occurring in the embayment velocity structure this is consistent with the relatively low q s values inferred from the energy flux model for stations within the embayment in addition to coda amplification we performed a synoptic noise study that revealed that pre event ambient background noise behaves in a similar manner to the coda for stations within and outside of the embayment the presence of large lateral velocity heterogeneities in the embayment may cause q s values to be low there

geophysical investigations of the lake lappajarvi impact structure western finland abstract a circular negative bouguer anomaly approximately 10 mgal in amplitude and 17 km in diameter is associated with the lappajarvi impact structure the related mass deficit is 440 10 11 kg there are irregularities in the anomaly pattern due to differentially faulted bedrock blocks and lateral density contrasts between bedrock shocked bedrock breccia suevite impact melt rock and overburden the minimum amount of impact melt rock was interpreted from the positive residual bouguer anomaly between the centre and the rim in the northwestern part of the crater as being 1 5 2 0 km 3 there are a few small aeromagnetic anomalies up to 200 nt at an altitude of 40 m within the crater which is mostly characterized by a lack of magnetized material strong magnetic anomalies a few thousand nt which are abundant in the surroundings disappear in the crater area the aero electromagnetic aem data show strong anomalies with inphase quadrature ratios close to 1 indicating moderately conducting material within the crater an electromagnetic wide band sampo em profile was measured across the annular depression between the centre and the rim of the crater three layers were interpreted the thickness of the uppermost layer mostly conductive overburden is about 100 200 m and the resistivity 60 170 m in the annular depression beneath the uppermost layer there is a 50 250 m thick layer of suevites and breccias with resistivity of about 17 50 m the resistive lowest layer follows the base of breccia and suevite some separate soundings were carried out near the centre of the crater according to one of these soundings there is a conductive layer of suevite and breccia with a resistivity of 30 40 m beginning at depth of 150 m beneath the exposed impact melt rock the results of geophysical interpretations such as the annular depression between the centre and the rim of the crater the small thickness diameter ratio of the geophysically anomalous part of the crater and the calculated amount of impact melt rock indicate that the lappajarvi structure belongs to the category of complex craters rhomboidal geophysical anomaly patterns in particular aem anomalies parallel to old bedrock structures suggest that the collapse of the transient crater and or subsequent crater modification were controlled by nw se and ne sw trending pre existing fracture and fault zones

the relation between site amplification factor and surficial geology in central california abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 132 stations of the usgs seismic network in central california from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method we found that the site amplification at a station is systematically related to the geology underlying that station the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz the rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies to quantify the relation between site amplification factor and site geology the surface geology of station sites was classified into five groups namely 1 quaternary sediments 2 pliocene sediments 3 miocene through cretaceous sediments 4 franciscan formation and mesozoic granitic rocks and 5 pre cretaceous metamorphic rocks the station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group a smooth power law relation was found between the mean site amplification and the median geologic age this relation provides a simple way of estimating site effect at a specific site with known surficial geology our site amplification factors were compared with magnitude station residuals determined by eaton 1990 for the same network a remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value confirming that the coda method provides an effective means of estimating site amplification factor comparison of our site amplification factor with strong motion results obtained from the loma prieta earthquake boore et al 1989 maley et al 1989 shakal et al 1989 chin and aki 1991 suggests that weak and strong motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km within the epicentral source region however the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion suggesting a nonlinear behavior at sediment sites these results together with studies on the correlation between weak motion site amplification and earthquake intensity borcherdt 1970 borcherdt and gibbs 1976 chavez garcia et al 1990 king et al 1990 demonstrate the importance of in situ determination of weak motion site amplification for seismic zonation

seismic attenuation tomography in southwestern china insight into the evolution of crustal flow in the tibetan plateau abstract crustal flow in southwestern china is commonly attributed to the continental collision between the indian and the eurasian plates therefore studying crustal flow in this area can provide valuable insight into the collisional process the quality factor q is an indicator of seismic wave attenuation that provides constraints on lithology rheology and geodynamic processes which are vital for interpreting the nature of the crustal flow in a given region in this study we used local seismic waveforms to obtain regional attenuation models for p qp and s qs waves in the sichuan yunnan region syr the emeishan large igneous province elip is characterized by high qp and qs values in the middle yunnan block myb at depths greater than 5 km the qp and qs values in most of the fault zones surrounding the myb were lower than 300 indicating that the material within crustal faults causes high waveform attenuation our results suggest that the crustal flow was diverted into two channels in the northern myb due to resistance from the elip to illuminate the tectonic dynamics of the syr we propose a simple model that explains both the evolution of the crustal flow and the variation in the seismic wave attenuation

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

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

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

p wave tomography and dynamics of the crust and upper mantle beneath western tibet abstract the continental collision between the indian and asian plates plays a key role in the geologic and tectonic evolution of the tibetan plateau in this article we present high resolution tomographic images of the crust and upper mantle derived from a large number of high quality seismic data from the antilope project in western tibet both local and distant earthquakes were used in this study and 35 115 p wave arrival times were manually picked from the original seismograms geological and geochemical results suggested that the subducting indian plate has reached northward to the lhasa terrane whereas our new tomography shows that the indian plate is currently sub horizontal and underthrusting to the jinsha river suture at depths of 100 to 250 km suggesting that the subduction process has evolved over time the asian plate is also imaged clearly from the surface to a depth of 100 km by our tomography and it is located under the tarim basin north of the altyn tagh fault there is no obvious evidence to show that the asian plate has subducted beneath western tibet the indian and asian plates are separated by a prominent low velocity zone under northern tibet we attribute the low velocity zone to mantle upwelling which may account for the warm crust and upper mantle beneath that region and thus explain the different features of magmatism between southern and northern tibet but the upwelling may not penetrate through the whole crust we propose a revised geodynamic model and suggest that the high velocity zones under lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost tibetan plateau

attenuation of multiphase surface waves in the basin and range province part i lg and lg coda summary broad band data recorded by the lawrence livermore laboratory network are used to investigate whether or not lg q and lg coda q have similar values in the basin and range province of north america in order to retrieve spectral estimates of the broad band lg and lg coda signals a hybrid method is used which retains high fidelity at lower frequencies while strongly smoothing the spectral estimates and minimizing the number of spectral points at high frequencies a stochastic approach is used to model the lg signal and ambient noise and to analyse results from two station determinations of lg q in the basin and range the effects of ambient noise frequency dependent station site response and focusing defocusing on lg q inversion are analysed it is shown that after averaging spectral ratios determined from repeated observations over four two station paths a modest site effect will not cause significant errors in the resulting lg q relation using this inversion scheme and spectral ratio estimates averaged over those paths at frequencies between 0 2 and 2 5 hz we obtain the relation qlg f 267 f 56 f0 37 0 06 for the basin and range province further constraints on lg q were obtained from lg coda which due to the natural averaging effect of scattering is insensitive to effects of focusing defocusing to investigate the validity of the single isotropic scattering model for lg coda in the basin and range province we calculated ischrons for scatterers corresponding to later lg coda signals as required by the single scattering model it is found that when significant portions of these isochrons are oceanic lg coda amplitude decay becomes irregular and abnormally slow suggesting a strong scattering reflection effect caused by the rapidly changing crustal structure along the coast applying the single isotropic scattering model to lg coda records with isochrons mostly confined to the continental crust results in q f 275 f 26 f 0 36 0 03 which is nearly identical to our qlg relation this indicates that even in a complicated geological province a joint interpretation of the dynamic properties of lg and lg coda can be successful and fruitful inconsistent lg q and coda q relations found in earlier studies in the basin and range province may be attributed to different settings of parameter trade offs different seismic phases or different dynamic properties being used

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

direct evidence for the undulation of the 660 km discontinuity beneath tonga comparison of japan and california array data short period seismograms of tonga deep earthquakes recorded by japanese and californian seismic networks are stacked to identify the s p converted wave associated with the 660 km discontinuity the travel time difference between this s p converted wave and the direct p wave is used to constrain the depth of the 660 km discontinuity analysis of a total of 29 events produced a detailed topographical map of the discontinuity beneath the tonga subduction zone two events which exhibit clear s p conversions in both japan and california data are selected to show directly the depth variations of the 660 km discontinuity adjacent to the subducting slab the s p conversion points on the ray paths to japan are observed to be approximately 10 to 30 km deeper than the conversion points on those to california which represents direct evidence for a slab induced depression of the 660 km discontinuity

upper mantle discontinuity structure in the region of the tonga subduction zone we study the mantle structure below the southwest pacific in order to examine the geometry of the tonga slab at depth and its interaction with the 410 and 660 km discontinuities hereafter called the 410 and the 660 we utilize data from stations of both the lau basin ocean bottom seismogram experiment and island stations of the southwest pacific seismic experiment the tectonic complexity of this region containing both the tonga subduction zone and the associated lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions we produce a high resolution image of the upper mantle near the tonga subduction zone to show the interaction between the discontinuities and the subducting slab our results show the 410 uplifted by 30 km near the tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

upper mantle structure of the southwest pacific from regional waveform inversion we determine the upper mantle seismological structure of the southwest pacific by inverting complete regional seismograms recorded by the southwest pacific seismic experiment a deployment of 12 broadband seismographs in the fiji tonga and niue islands we first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure partial derivatives are computed using an efficient reflectivity code for differential seismograms randall 1994 and a conjugate gradient method is used to perform the nonlinear inversion band pass filtered 10 55 mhz vertical and radial component seismograms extending from the p arrival to the surface waves are used to constrain the structure waveforms are selected to provide regional distance 400 1500 km propagation paths predominantly within one of the tectonic regions of the southwest pacific source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths results show extremely large lateral variability in the upper mantle structure of the southwest pacific the inactive south fiji basin shows upper mantle velocities much lower than those previously observed for the pacific lithosphere of a similar age the north fiji and lau basins containing active back arc spreading systems show upper mantle shear velocities as slow as 3 8 km s 1 the exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world the velocity difference between the active back arc basins and the old pacific lithosphere immediately east of the tonga trench is maximum at 30 90 km depth with a variation of 18 indicating partial melting in this depth range velocity heterogeneity of 4 extends to a depth of 170 km velocity structure below 220 km depth is poorly resolved but there is some evidence of a 2 difference extending to depths of 350 km

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

on the nature of regional seismic phases i phase representations for pn pg sn lg summary an operator development of the seismic wavefield is used to generate descriptions of the propagation processes contributing to the main regional seismic phases pn pg sn lg these operator forms are valid for laterally heterogeneous crust and mantle models and include the major processes of interconversion between wavetypes these representations of the regional phases are used to examine the theoretical basis for discriminants between earthquakes and underground explosions based on the relative amplitudes of p and s phases the ratio of sn to pn amplitude looks promising as a high frequency discriminant however the ratio of lg to pn amplitudes is not as useful because of the complex nature of the propagation characteristics of lg

detailed structure and sharpness of upper mantle discontinuities in the tonga subduction zone from regional broadband arrays 1 recordings of deep tonga earthquakes from two arrays of 12 broadband seismographs each in the fiji and tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the tonga subduction zone the arrays operated as part of the seismic arrays in fiji and tonga saft experiment from july 2001 to august 2002 in comparison with the commonly used teleseismic approaches the short path lengths for the local data provide smaller fresnel zones and high frequency content for precise mapping of discontinuity topography and sharpness to enhance the low amplitude discontinuity phases s410p p660p and s660p deconvolved seismograms from each event array pair are aligned on the maximum amplitude of the direct p wave and subsequently slant stacked for the 410 km discontinuity the results show no systematic variations in depth with distance to the cold slab the 660 km discontinuity varies between 656 and 714 km in depth for the southern and central parts of the subduction zone the largest depths occur in the core of the tonga slab for the northern part two separate depressions of the 660 km discontinuity are observed these anomalies are interpreted as being induced by the active steeply subducting tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil vityaz trench respectively interpreting the deflections of the 660 km discontinuity in terms of local temperatures implies a thermal anomaly of 800 k to 1200 k at 660 km depth except for the southern region where it may thicken the width of the depressed 660 km discontinuity region implies that the tonga slab seems to penetrate the 660 km discontinuity with little deformation waveform modeling suggests that both the 410 and 660 km discontinuities are sharp the 660 km discontinuity is at most 2 km thick in many parts of the region and a first order discontinuity cannot be precluded the 410 km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab this suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities

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

strong lg wave attenuation in the northern and eastern tibetan plateau measured by a two station two event stacking method 1 the regional seismic phase lg which involves shear wave reverberations in the crust is strongly attenuated along propagation paths in northern and eastern tibet robust estimates of the attenuation quality factor qlg in these regions are obtained using a two station two event stacking method for broadband vertical component lg data in the passband 0 2 1 0 hz we find average 1 hz qlg values qo of 79 4 in northern central tibet and 119 17 in eastern tibet for the 0 2 0 5 hz passband localized regions of eastern tibet have qo estimates ranging from 66 to 121 these results confirm and augment the spatial coverage of recent estimates of low qo values in north central and south central tibet and support the notion of widespread partial melting of the crust throughout the plateau

seismic evidence for a thinner mantle transition zone beneath the south pacific superswell broadband seismograms recorded by the seismic stations deployed on oceanic islands in the south pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the south pacific where a large scale hot plume might ascend from the core mantle boundary cmb by stacking s waves reflected at the mantle transition zone discontinuities we find clear signals associated with the 410 km and 660 km discontinuities the transition zone thickness which is constrained from the travel time difference between the reflected waves from the 410 km and 660 km discontinuities is observed to be approximately 15 km thinner beneath the south pacific superswell than that beneath other regions the result suggests that the transition zone temperature beneath french polynesia is approximately 100 200 k higher than the surrounding mantle

estimation of site effects and q factor using a reference event a method to determine site effects and the q factor using a reference event is described spectral ratios between pairs of earthquakes and a reference one were written as logarithmic summations in order to obtain individual solutions for the numerators and denominators given the seismic moment and corner frequency for one of the events following the 2 model a source model was assumed to constrain the amplitude of the solution the validity and limitations of this technique were tested on synthetically generated data we analyzed the variances of the results depending on the different number of stations used the amount of noise present in the records and the change in the seismic moment corner frequency and source model of the reference event aftershocks of the 2000 tottori japan earthquake were used to estimate the site effects and q factor in that region two strong motion networks k net and kik net recorded the aftershocks the latter network contained both surface and borehole receivers at every station data were inverted first using only surface records and then using the borehole records from kik net stations in order to study amplitude differences our results suggest that the study area presents a low q value and that there is also amplification at borehole sites manuscript received 18 january 2002

pervasive seismic wave reflectivity and metasomatism of the tonga mantle wedge subduction zones play critical roles in the recycling of oceanic lithosphere and the generation of continental crust seismic imaging can reveal structures associated with key dynamic processes occurring in the upper mantle wedge above the sinking oceanic slab three dimensional images of reflecting interfaces throughout the upper mantle wedge above the subducting tonga slab were obtained by migration of teleseismic recordings of underside p and s wave reflections laterally continuous weak reflectors with tens of kilometers of topography were detected at depths near 90 125 200 250 300 330 390 410 and 450 kilometers p and s wave impedances decreased at the 330 kilometer and 450 kilometer reflectors and s wave impedance decreased near 200 kilometers in the vicinity of the slab and near 390 kilometers just above the global 410 kilometer increase the pervasive seismic reflectivity results from phase transitions and compositional zonation associated with extensive metasomatism involving slab derived fluids rising through the wedge

depression of the 660 km discontinuity beneath the tonga slab determined from near vertical scs reverberations the occurrence of the march 9 1994 mw 7 6 deep tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab near vertical scs reverberations from the 410 520 and 660 discontinuities are visible at 10 stations in the south pacific at frequencies between 0 003 0 033 hz without stacking we use a regional velocity model and ray theory to model the interval from scs to scs3 a grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab and 410 km 500 km and 640 km to the east of the slab analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the tonga slab in a region of possible near horizontal slab these results are consistent with a large cool region just to the west of the intersection of the tonga slab with the 660

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

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

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

crustal structure of western tibet revealed by lg attenuation tomography abstract the lateral lg wave attenuation in western tibet and surrounding regions is studied a total of 706 two station method tsm and 533 reverse two station method rtsm pairs are generated from 282 regional earthquakes the earthquakes are recorded at 48 seismic stations operated across western kunlun and western tibet regions the lg phases are extracted from the vertical component of seismograms with magnitude greater than 4 6 mb and focal depth less than 50 km to measure the tsm and rtsm lg q at 1 hz q0 the lgq0 tomography map is generated with grid size 0 5 0 5 using the least squares orthogonal factor decomposition lsqr algorithm lateral variation of lg attenuation is found to exist across the region the results provide new insights into the attenuation properties of the crust and its linkage with regional geodynamics the low lgq0 200 is revealed in the central part of lhasa terrane in western tibet from both tsm and rtsm images the high lgq0 values complemented with the high pn velocities beneath the central part of lhasa terrane reveal the trace of underthrusting indian lithosphere beneath the western tibet

pg attenuation tomography within the northern middle east abstract a seismic attenuation map of regional phase pg is constructed for the northern middle east using two station frequency dependent q measurements and seismic attenuation tomography pg is widely used in source discrimination but its attenuation behavior is not well understood we investigate the functional form of the geometrical spreading of pg for both uniform and more complex seismic velocity structures we find that there is significant trade off between attenuation and geometrical spreading of pg and we quantify this trade off to interpret the observed pg attenuation in the middle east the northern middle east attenuation tomographic images show significant lateral q pg variations which may be caused by lateral variations of crustal rheology if intrinsic attenuation dominates the seismic attenuation mechanism of this region we observe very large variations in the q pg model such as low q pg within the anatolian plate and high q pg within much of the stable arabian plate the low q pg anomalies generally coincide with active fault systems or quaternary volcanisms in order to perform a systematic comparison with lg propagation we have also updated the q lg map of zor et al 2007 these two new maps provide estimates for crustal q pg and q lg for the northern middle east

convergence of the indian and eurasian plates under eastern tibet revealed by seismic tomography 1 a three dimensional p wave velocity model of the crust and upper mantle down to 400 km depth beneath eastern tibet is obtained using many temporary seismic stations of the ascent project and the namche barwa broadband seismic network we collected 16 508 arrival times of p pn and pg phases from 573 local and regional earthquakes and 7 450 p wave arrivals from seismograms of 435 teleseismic events our high quality data set enables us to reconstruct the 3 d velocity structure under eastern tibet in more detail than the previous studies in the shallow depth our results show that the low velocity zones are not interconnected well no wide spread low velocity zones which may reflect the complex pattern of material flowing in the study region below the moho we find that the indian lithospheric mantle underthrusts sub horizontally under eastern tibet and the extent of the northward advancing indian lithosphere decreases from west to east in the north the asian lithospheric mantle is detected under the vicinity of the qaidam basin between the indian and asian lithospheric mantles there is an obvious low velocity anomaly which may reflect an upwelling mantle diapir

an improved method to extract very broadband empirical green s functions from ambient seismic noise the extraction of green s functions by cross correlation of continuous seismic records at station pairs is best achieved in a diffuse wave field where energies radiated by random sources have equal power to partially satisfy the diffuse wave field condition original seismic records must be normalized as they are highly nonstationary and may have amplitude variations of orders of magnitude within the cross correlation time window we adapt a frequency time normalization method to obtain seismograms with an even spectrum at all times within the data processing unit compared with the commonly used one bit normalization the new method improves the signal to noise ratio of empirical green s functions by a factor of 2 and increases the effective data recording duration by a factor of 4 assuming random local and instrument noise it yields useful rayleigh waves at periods up to 300 s for program for array seismic studies of the continental lithosphere passcal type deployments and 600 s for permanent stations with very broadband sensors thus the new method makes it possible to extend surface wave signals to frequencies beyond those in traditional earthquake based surface wave tomography at both the high and low frequency ends

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

1 hz lg q tomography in central asia we have applied tomographic techniques to a data set of over 1700 regional distance lg amplitudes from 12 stations for paths inside a 30 by 40 region of central asia our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination as well as to study lg propagation characteristics in one of the most heterogeneous portions of the earth for 1 0 hz data tomography reduced variance 33 relative to a best fit uniform q model lgq varied with geologic region with low values in tibet the tien shan and the pamir range q 200 to 400 intermediate values in basins such as the tarim q 500 and high values for platforms and older crust q 800 the relatively efficient propagation across the tarim differs from the poor propagation that is generally observed across sedimentary basins

distinct lateral contrast of the crustal and upper mantle structure beneath northeast tibetan plateau from receiver function analysis abstract we investigate the crustal thickness poisson s ratio and the thickness variation of the mantle transition zone using the p receiver function technique on data recorded by the 68 seismic station of the ascent experiment beneath the northeast tibetan plateau our results reveal a distinct crustal and upper mantle structure contrast between the western and eastern part of the northeast tibetan plateau along two migrated profiles in the eastern region the moho is relatively smooth and continuous and varies in depth between 80 and 51 km whereas the depth of the moho beneath the western region varies between 77 and 52 km and is offset 20 km beneath the boundary between the kunlun fault and qaidam basin the delay times for the ps phases from both the 410 km and 660 km discontinuities in the western region are 1 2 s later than that in the eastern region indicating that the average velocities of the upper mantle in the western region are lower than that in the eastern region crustal rocks with low to moderate poisson s ratios are interpreted to be dominated by felsic to intermediate compositions implying that large scale middle lower crustal flow does not occur easily the nearly constant thickness of the mantle transition zone with an average value of 255 km 24 4 s from south to north implies that no lithospheric fragments correlating to the india asia collision have entered into the mantle transition zone

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

pgq model for nepal himalaya abstract we investigate the spatial variations of pg attenuation structure in nepal himalaya we implement the two station method on 2325 waveforms obtained from 435 events recorded at 151 stations deployed across nepal himalaya to measure the pg wave quality factor q these station station measurements are inverted further for investigating the spatial variations of pg attenuation using a back projection algorithm the model contains variations across the region with much of the region being dominated by high attenuation it is observed that the areas around the existing faults and lineaments exhibit very low q values a few exceptional high q pockets are observed to be limited predominantly in the north western side of our study area we hypothesize that intrinsic attenuation plays a major role in causing the high apparent pg attenuation in the crust of nepal himalaya this may be mainly caused by highly pressurized fluids trapped within a thin low velocity layer lvl at shallow depths moreover it is interesting to observe that all the past earthquakes of m w 7 since 1934 fall into the regions of low q values but not the lowest it may be anticipated that the very low q areas are too weak to sustain the accumulation of high energy release during a large magnitude earthquake therefore the observed low q structures across the study area may imply a weak crust and subsequently a favorable zone for future rupture

seismic evidence against a mantle chemical discontinuity near 660 km depth beneath izu bonin we analyze p wave codas from 12 deep izu bonin earthquakes recorded by hundreds of western united states seismograph stations for evidence of small amplitude phases caused by near source mantle discontinuities for nearly every event the dominant phase in the coda is the result of an s to p conversion from a nearly horizontal discontinuity ranging in depth from 650 to 690 km this is interpreted as a thermally depressed spinel to perovskite and magnesiowustite phase transition if the 660 km seismic discontinuity is also associated with a change in chemistry it would be dynamically depressed by a subducting slab we consistently see that there is no nearly horizontal discontinuity between 700 and 1000 km with shear wave velocity contrast exceeding 1 this observation places constraints on the properties of a postulated chemical discontinuity separating the upper and lower mantles

mechanism of slab thickening near 670 km under indonesia we examine a new data set of reliably relocated earthquakes deeper than 400 km under indonesia developed by schoffel and das 1999 the accuracy of this data allows us to demonstrate that the slab thickens shortens and weakens before penetrating below 670 km by shearing along conjugate fault planes on the upper and lower portions of the seismic zone we also show that the earthquakes in the lower portion of the wadati benioff zone width are generally fewer and smaller than those in the central portions with only 8 of the total moment released in the period 1976 1996 being due to earthquakes on the lower side

lg attenuation tomographic models of himalaya and southern tibet abstract we investigate the lg attenuation structure of the crust beneath the tectonically complex himalaya and southern tibet regions adopting a tomographic regionalization method a total of 1671 earthquake waveforms registered at 38 seismic stations operated in the region are selected for the initial lgq measurements using the standard two station method q0 1 hz lgq values of 76 high quality interstation paths are finally considered as input for the tomographic inversion the estimates of q0 exhibit distinct variations in the crustal attenuation from north to south across the whole region the zones of lowest q0 values

strong lg wave attenuation in the middle east continental collision orogenic belt abstract using lg wave q tomography we construct a broadband crustal attenuation model for the middle east the q lg images reveal a relationship between attenuation and geological structures strong attenuation is found in the continental collision orogenic belt that extends from the turkish and iranian plateau to the pamir plateau we investigate the frequency dependence of q lg in different geologic formations the results illustrate that q lg values generally increase with increasing frequency but exhibit complex relationships both with frequency and between regions an average q lg value between 0 2 and 2 0 hz q lg 0 2 2 0 hz may be a critical index for crustal attenuation and is used to infer the regional geology low q lg anomalies are present in the eastern turkish plateau and correlate well with low pn velocities and cenozoic volcanic activity thus indicating possible partial melting within the crust in this region very strong attenuation is also observed in central iran the afghanistan block and the southern caspian sea this in line with the previously observed high crustal temperature high conductivity layers and thick marine sediments in these areas suggests the high lg attenuation is caused by abnormally high tectonic and thermal activities

transient and disruption cavity dimensions of complex terrestrial impact structures derived from magnetic data 1 accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact transient and disruption cavity size can in principle be inferred from morphometric relationships based on crater diameter however locating the crater rim can be difficult for eroded terrestrial craters and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin fortunately magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character a lower bound on this parameter can be defined by the outer limit of short wavelength intense magnetic anomalies produced by impact melt and or suevite deposits an upper bound is given by the inner limit of magnetic anomaly trends associated with the pre impact target rock configuration using published values of crater diameters d and values of collapsed disruption cavity diameters dcdc derived from magnetic data for 19 complex terrestrial impact structures we derive the relationship dcdc 0 49d these data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation

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

gravity and magnetic field modeling and structure of the chicxulub crater mexico the chicxulub crater is an 180 km diameter peak ring crater based on drill hole logs and samples potential fields seismic reflection profiles and surface fracture patterns a structural cross section produced based on these constraints has the features expected for a large complex impact crater the bouguer gravity anomaly consists of a broad 90 km radius 30 mgal low with a central 20 km radius 20 mgal high and two 5 mgal concentric lows at 35 and 60 km radius the gravity anomaly is disrupted by large scale basement anomalies and possibly by large scale slumping and backwash erosion effects the magnetic field anomaly over the crater consists of three zones an outer zone from 45 to 90 km radius of low amplitude short wavelength anomalies with an irregular perimeter a middle zone from 20 to 45 km radius of high amplitude short wavelength anomalies slightly elongated nnw sse and an inner 20 km radius single large amplitude anomaly magnetic field modeling indicates that the melt pool averages 90 km in diameter and the melt volume in the crater is estimated at 20 000 km3 the melt pool size constrains the collapsed transient cavity diameter to 90 km gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but 40 km in diameter and underlies or protrudes into the melt pool the preliminary structural cross section indicates that the inferred peak ring is decoupled from the structural uplift the geometry and bouguer gravity signature of the crater indicate that no significant uplift of the moho or relaxation of the crater has occurred

dynamics of double subduction numerical modeling abstract double subduction is a geodynamic process in which two plates following each other are synchronously subducted double subductions are known for both modern izu bonin marianas and ryukyu arcs and ancient west himalaya collision zone plate tectonics however our knowledge about this process is limited to conceptual schemes and some restricted analogue experiments in order to fill this gap we performed 2d numerical experiments using a coupled petrological thermomechanical approach based on finite differences and marker in cell techniques combined with thermodynamic database for the mantle we investigated the influence of convergence rate intermediate plate length activation volume of the mantle dislocation creep and age of the lithosphere based on these experiments we conclude that a subduction rates at two zones running in parallel differ and vary in time even when the total convergence rate remains constant supremacy of either subduction zone depends on physical parameters such as i relative rates of the plates ii slab ages and iii length of the middle plate b subduction dynamics of the double subduction system involves several processes unknown in simple subduction systems such as i eduction i e un subduction ii subduction re initiation iii subduction flip triggered by shallow slab breakoff and iv turn over of detached slabs to up side down attitudes c simulated tomographic structures related to slab propagation account for both penetration and non penetration of the 660 km discontinuity non penetration is favored by i low convergence rate ii faster relative movement of the overriding plate iii young age of the subducting slab and iv up side down turn over of detached slab

thickness of the mantle transition zone beneath the south pacific as inferred from analyses of scs reverberated and ps converted waves abstract we study the thickness of the mantle transition zone beneath the south pacific in order to determine temperature anomalies associated with presumed hot plumes in the south pacific superswell the scs reverberation method is used to determine the average thickness between an event station pair and the velocity spectrum stacking vss method is used to determine the thickness near each station the thickness obtained by the scs reverberation method shows that the average thickness beneath the superswell is less than the globally averaged thickness by 6 km while the average thickness beneath the tonga fiji subduction is greater than the global average by 10 km the vss method shows nearly normal thickness beneath stations used in the superswell region we conclude that substantial thinning of the transition zone associated with presumed hot plumes occurs locally beneath the society hot spot as revealed by our previous ss precursor study and is not widespread beneath the whole area of the superswell

whole mantle shear structure beneath the east pacific rise we model broadband seismograms containing triplicated s s 2 and s 3 along with scs to produce a pure path one dimensional model extending from the crust to the core mantle boundary beneath the east pacific rise we simultaneously model all body wave shapes and amplitudes thereby eliminating depth velocity ambiguities the data consist of western north american broadband recordings of east pacific rise epr affiliate transform events that form a continuous record section out to 82 and sample nearly the entire east pacific rise the best fitting synthetics contain attenuation and small changes in lithospheric thickness needed to correct for variation in bounce point ages the 660 km discontinuity is particularly well resolved and requires a steep gradient 4 extending down to 745 km we find no discernible variation in apparent depths of the 405 and 660 km discontinuities over ridge orthogonal distances on the order of 1000 km or 20 ma lithosphere body waveform comparisons indicate that we can resolve discontinuity depths to less than 10 km providing an upper limit to transition zone topography these depth estimates in conjunction with the fan shot nature of the ray paths lower the detection limit from s 2 precursor analysis of the lateral length scale over which short wavelength topographic variation could occur and indicate the sub epr transition zone and upper mantle are remarkably homogeneous the lower mantle beneath the east pacific rise is well modeled by prem with the greatest variation occurring in scs reflecting strong heterogeneity along the core mantle boundary together these observations require that the east pacific rise spreading ridge cannot be actively supplied from the local lower mantle and that tomographically imaged lateral variation beneath the ridge likely reflects lateral smearing of outlying velocity gradients dynamically the transition zone therefore appears vertically decoupled from the overlying east pacific rise spreading system

crustal flow pattern beneath the tibetan plateau constrained by regional lg wave q tomography abstract as a prominent geophysical anomaly unusually high seismic wave attenuation is observed in the crust and upper mantle of the tibetan plateau particularly along its northern area theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting which may decrease the viscosity of the material and cause it to flow thus seismic attenuation distribution may provide useful constraints to the crust flows if they exist using lg wave q tomography we construct a 0 05 10 0 hz broadband high resolution crust attenuation model for the tibetan plateau and its surrounding regions the maximum spatial resolution is approximately 1 0 1 0 in well covered areas and for frequencies between 0 05 and 1 5 hz this attenuation model reveals an apparent low qlg belt stretching along the northern and eastern tibetan plateau combining the lg wave q model with other geophysical data two possible crust flow channels are found in the tibetan plateau the main flow channel is from north to east and then turns to southeastern tibet along the western edge of the rigid sichuan basin while a second channel starts from southern tibet and crosses the eastern himalayan syntaxis