three dimensional tomographic velocity structure of upper crust coaxial segment juan de fuca ridge implications for on axis evolution and hydrothermal circulation three dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23 564 ocean bottom seismometer p wave arrivals define systematic lateral variations in seismic structure of the coaxial segment of the juan de fuca ridge jdfr over much of the segment the across axis structure is roughly axisymmetric characterized by a progressive increase in dike velocities moving away from the ridge axis this trend is most apparent in the basal dikes where on axis velocities are about 800 m s slower than those measured elsewhere within the rift valley the on axis sheeted dikes also exhibit ridge oriented azimuthal anisotropy with a peak to peak amplitude of about 600 m s outboard of the rift valley beneath ridge flanks with fault scarps velocities in the upper 1500 m of crust are reduced the maximum amplitude of this anomaly is about 700 m s located near the top of the sheeted dikes variations in the three dimensional velocity model are believed to reflect changes in crustal porosity from which we infer an axisymmetric porosity model for seismic layer 2 of the coaxial segment as the crust ages the evolution of layer 2 porosity could occur in the following way 1 the porosity of zero age on axis dikes is set at formation by the contraction of molten material 2 hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley and 3 normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off axis at the north end of the segment dike velocities are several hundred meters per second slower on average and the across axis structure is lost the transition from a coherent aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the cobb hotspot on the ridge axis the porosity structure we have derived for the coaxial segment suggests an alternative to the usual hydrothermal circulation model of cross axis convection cells a circulation model with along axis convection cells located entirely within the axial valley appears to be more compatible with our data

lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

double ramp on the main himalayan thrust revealed by broadband waveform modeling of the 2015 gorkha earthquake sequence abstract the 2015 m w 7 8 gorkha earthquake sequence that unzipped the lower edge of the main himalayan thrust mht in central nepal provides an exceptional opportunity to understand the fault geometry in this region however the limited number of focal mechanisms and the poor horizontal locations and depths of earthquakes in the global catalog impede us from clearly imaging the ruptured mht in this study we generalized the amplitude amplification factor aaf method to teleseismic distance that allows us to model the teleseismic p waves up to 1 5 hz we used well constrained medium sized earthquakes to establish aaf corrections for teleseismic stations that were later used to invert the high frequency waveforms of other nearby events this new approach enables us to invert the focal mechanisms of some early aftershocks which is challenging by using other long period methods with this method we obtained 12 focal mechanisms more than that in the gcmt catalog we also modeled the high frequency teleseismic p waves and the surface reflection phases pp and sp to precisely constrain the depths of the earthquakes our results indicate that the uncertainty of the depth estimation is as small as 1 2 km finally we refined the horizontal locations of these aftershocks using carefully hand picked arrivals the refined aftershock mechanisms and locations delineate a clear double ramp geometry of the mht with an almost flat decollement sandwiched in between the flat dip 7 degrees portion of the mht is consistent with the coseismic rupture of the mainshock which has a well constrained slip distribution the fault morphology suggests that the ramps both along the up dip and down dip directions play a significant role in stopping the rupture of the 2015 gorkha earthquake our method can be applied to general subduction zone earthquakes and fault geometry studies

attenuation of high frequency seismic waves in eastern iran we investigated the frequency dependent attenuation of the crust in eastern iran by analysis data from 132 local earthquakes having focal depths in the range of 5 25 km we estimated the quality factor of coda waves q c and body waves q p and q s in the frequency band of 1 5 24 hz by applying the single backscattering theory of s coda envelopes and the extended coda normalization method respectively considering records from recent earthquakes rigan m w 6 5 2010 12 20 goharan m w 6 2 2013 5 11 and sirch m w 5 5 2013 1 21 the estimated values of q c q p and q s vary from 151 49 63 6 and 93 14 at 1 5 hz to 1 994 124 945 84 and 1 520 123 at 24 hz respectively the average frequency dependent relationships q q o f n estimated for the region are q c 108 10 f 0 96 0 01 q p 50 5 f 1 01 0 04 and q s 75 6 f 1 03 0 06 these results evidenced a frequency dependence of the quality factors q c q p and q s as commonly observed in tectonically active zones characterized by a high degree of heterogeneity and the low value of q indicated an attenuative crust beneath the entire region

ground motion relations for eastern north america predictive relations are developed for ground motions from eastern north american earthquakes of 4 0 m 7 25 at distances of 10 r 500 km the predicted parameters are response spectra at frequencies of 0 5 to 20 hz and peak ground acceleration and velocity the predictions are derived from an empirically based stochastic ground motion model the relations differ from previous work in the improved empirical definition of input parameters and em pirical validation of results the relations are in demonstrable agreement with ground motions from earthquakes of m 4 to 5 there are insufficient data to adequately judge the relations at larger magnitudes although they are consistent with data from the saguenay m 5 8 and nahanni m 6 8 earthquakes the underlying model parameters are constrained by empirical data for events as large as m 6 8

frequency dependent attenuation of shear waves in the crust of the southern kanto area japan the attenuation characteristics of shear waves in the crust of the southern kanto area central japan were estimated using strong motion data including acceleration data recorded in the pre tertiary basement rocks by means of downhole observation the quality factor qs f was determined for a range of discrete frequencies from 0 5 to 16 hz from the analysis of data from 13 local earthquakes with focal depths of less than about 50 km that occurred in the philippine sea plate and in the boundary zone between the lower part of the eurasian plate and the upper part of the philippine sea plate the estimated 1 qs f shows a peaked structure in this frequency range on the assumption that the geometrical spreading exponent is 1 body waves the estimated peak 1 qs f is of the order of 10 2 at 0 8 hz

regional seismic wave propagation lg and sn and pn attenuation in the arabian plate and surrounding regions an edited version of this paper was published by blackwell publishing copyright 2004 blackwell publishing see also http www blackwell synergy com doi abs 10 1111 j 1365 246x 2004 02246 x http atlas geo cornell edu middleeastnorthafrica publications al damegh2004 htm

seismic amplitude tomography for crustal attenuation beneath china summary amplitude tomography reconstructs seismic attenuation directly from recorded wave amplitudes we have applied the tomography to amplitude data reported in the annual bulletin of chinese earthquakes and interpreted the regionally varying crustal attenuation in terms of tectonics the seismic amplitudes were originally recorded for determining the m l and m s magnitudes they generally correspond to the maximum amplitudes of the horizontal components of the short period s waves and intermediate period rayleigh waves both sets of measurements are sensitive to crustal structure the peak amplitudes from m l amplitudes spread spherically with significant dispersion and scattering m s amplitudes show cylindrical spreading with little dispersion average crustal q values for attenuation at 1 hz are 737 and 505 for m l and m s respectively with substantial regional variations frequency dependence in the attenuation is also indicated regions with the lowest attenuation high q values are beneath the south china block sichuan basin ordos platform the daxinganling and the korea craton these tend to be tectonically inactive regions which are generally dominated by intrusive and cratonic rocks in the upper crust regions with the highest attenuation low q values are beneath bohai basin yunnan eastern songpan ganzi terrain margins of the ordos platform and the qilian shan these are predominantly active basins grabens and fold belts the continental margin also highly attenuates both s and surface waves

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

source parameters of earthquakes in eastern and western north america based on finite fault modeling the ground motion from large earthquakes is often predicted based on finite fault modeling in which the fault plane is discretized into small independently rupturing subfaults the radiation from all subfaults is summed at the observation point despite the success of the method in matching observed ground motion char acteristics the physical interpretation of the subfaults has remained largely unclear and a rationale for the choice of the subfault attributes has been lacking two key parameters the subfault size and the maximum slip velocity on the fault govern the amplitude of the source spectrum at intermediate and high frequencies respec tively we determined these key source parameters on an event by event basis for all well recorded moderate to large earthquakes in western north america wna by fitting simulated to observed response spectra we compare the values of these source parameters with those obtained previously for eastern north america ena and the michoacan mexico earthquakes a total of 26 modeled events we find that the characteristic subevent size increases linearly with moment mag nitude in an apparently deterministic manner the subevent size relationship obtained for wna is not statistically different from that obtained for ena in both regions the subevent size follows the trend of log dl 2 0 4 m 4 m 8 where dl is the subfault size in km this trend agrees well with independent studies by somerville et al 1999 and aki 1992 in which the characteristic size of the patches asperities or barriers on earthquake faults was determined these results indi cate that large earthquakes should be viewed as a sequence of smaller events that comprise the large rupture interestingly the characteristic size of these constituent small events appears to be directly related to the size of the overall rupture the slip velocities determined for all 26 earthquakes vary in a narrow range from about 0 25 to 0 60 m sec with a mean of 0 40 m sec and standard deviation of 0 09 m sec the slip velocities for the ena events are distributed randomly over this range while those for the wna region appear to exhibit a decreasing trend with increasing magnitude our results indicate that a generic region independent earthquake source model for engineering prediction of strong ground motions can be developed

attenuation of high frequency body waves in the crust of western tibet abstract the attenuation characteristics of western tibet have been analyzed by estimating the seismic attenuation parameters q p 1 and q s 1 at five different central frequencies 1 5 3 6 12 and 18 hz the extended coda normalization method has been used on 469 three component local earthquakes which are recorded at y2 network of broadband stations deployed in western tibet during years 2007 2011 we have estimated the frequency dependent q p 1 and q s 1 values at each station the results indicate strong body wave attenuation in the medium below western tibet the highest attenuation is observed at wt19 and wt13 stations located near to the karakoram fault kkf and indus yarlung suture iys across the kkf we have subdivided the entire study area into two regions to explore the lateral variations of crustal attenuation properties the first part covers the northeastern of kkf referred as region 1 while the second part covers the southwestern of kkf referred as region 2 the frequency dependent relations are found as follows q p 1 19 4 4 4 10 3f 0 9 0 2 q s 1 13 3 2 1 10 3f 0 8 0 1 for region 1 and q p 1 18 4 1 8 10 3f 0 8 0 1 q s 1 13 2 2 2 10 3f 0 8 0 1 for region 2 both the regions show strong frequency dependent nature of q 1 values with no significant lateral variations it may suggest the presence of similar tectonic complexities and heterogeneities beneath both the regions we also find that all the estimated values of q p 1 q s 1 ratios are greater than unity at all the frequency range which are likely to be associated with both the intrinsic and scattering phenomenon in the crust of western tibet

p and s wave attenuation in the region of marche italy we have found empirical functions to describe the amplitude decay with distance of p and s waves in the marche region of italy these functions result from the analysis of spectral amplitudes of 161 three component digital records from 26 earthquakes recorded by the marchesan seismograph network rsm jointly run by the italian seismological survey of rome ssn and the geophysical observatory of macerata ogsm the events have magnitudes in the range 1 8 m 3 6 and were recorded at hypocentral distances between 9 and 125 km in order to study the possible variations of the quality factor q with the direction of wave propagation q anisotropy we made independent estimates of the attenuation for p sv and sh waves using the longitudinal vertically polarized and transversely polarized components respectively for the whole distance range and in the frequency band analyzed 1 f 25 hz the frequency dependence of q can be approximated as qp 108 4 f0 70 for the p waves qsv 105 6 f0 65 for sv waves and qsh 86 1 f0 70 for sh waves the small difference between the sh and sv spectral decay is within the statistical uncertainties of the estimates suggesting that q anisotropy is negligible however when only deeper wave paths are used in the analysis we observed that at low frequencies f 4 hz the values of qsv are significantly above the estimates of qsh this observation provides additional evidence of the seismic anisotropy reported by margheriti et al geophys res lett 23 2721 2724 1996 for the region the low q anisotropy is possibly related with the weakly deformed adriatic foreland the high attenuation observed for both p and s waves can be explained by the double low velocity layer reported by ponziani et al tectonophysics 252 391 418 1995 since the seismic energy can be trapped between these layers in addition the low values of q obtained for distance ranges between 5 and 50 km can be due to the presence of hot material in the lower crust beneath the northern apennines as suggested by mele et al j geophys res 102 11863 11875 1997

seismic attenuation in northeastern sonora mexico we studied the attenuation of local and regional earthquake body waves recorded by the seismic network of northeastern sonora mexico red sismica del noreste de sonora resnes the source station paths of the selected data set cover the epicentral region of the may 1887 m w 7 5 sonora earthquake we used the best located earthquakes recorded by resnes between 2003 and 2004 to analyze the spectral amplitude decay of body waves with hypocentral distance the database consists of 44 earthquakes with magnitudes between 1 8 and 3 2 and hypocentral distances between 25 and 150 km we calculated fourier spectral amplitudes from more than 400 records and defined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes at 23 individual frequencies between 0 4 and 63 1 hz we determined regional attenuation curves based on events located more than 100 km northwest of the center of the network and local curves based on earthquakes located near the fault zone that ruptured in the 1887 earthquake the obtained attenuation curves indicate that the spectral amplitudes decay faster in the area of the 1887 rupture than in the surrounding region to analyze the possible sources of attenuation we parameterized the attenuation functions considering three terms one that accounts for geometrical spreading another one for anelastic attenuation and a third for near surface attenuation we found the s wave quality factor q to be similar in both the general region and near the 1887 rupture however in the region of the 1887 rupture the near surface attenuation exceeds the regional average finally in the region of the 1887 rupture the nonparametric attenuation functions and the estimates of q indicate that at between 0 6 and 63 1 hz p waves attenuate more than s waves q p 20 8 f 1 1 whereas q s 83 8 f 0 9

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

lg attenuation characteristics across the indian shield abstract new measurements of lg q are obtained using a reliable two station method to understand the attenuation characteristics across the indian subcontinent we use data from 514 events with magnitude 3 5 and depth less than 40 km collected during 2006 2008 from 17 stations deployed across india we measure 1 hz lg q q 0 values between many pairs of stations finally 34 high quality interstation paths were selected from 336 possible pairs that are 1 aligned approximately with the source and 2 separated enough to permit the use of the standard two station method for lg q measurement we found some spatial variation in lg wave attenuation across india low q regions are present in western india we attribute this to the recent tectonic activities in this region the central indian platform shows a moderate to high q 0 value the southernmost part of india exhibits higher values which may indicate that the crust in this part is more stable the obtained values correlate with the regional crustal heterogeneities and tectonic trends as a whole the estimated interstation q 0 values compare well with the areas of moderate seismicity across the earth

seismogenesis of clustered seismicity beneath the kangra chamba sector of northwest himalaya constraints from 3d local earthquake tomography abstract to investigate subsurface structure and seismogenic layers 3d velocity inversion was carried out in the source zone of 1905 kangra earthquake m8 0 in the northwestern himalaya p wave and s wave phase data of 159 earthquakes recorded by a network of 21 stations were used for this purpose inverted velocity tomograms up to a depth range of 18 km show significant variations of 14 in vp and vs and 6 in the vp vs across the major tectonic zones in the region synthesis of seismicity pattern velocity structure distinctive focal mechanisms coupled with nature of stress distribution allows mapping of three different source regions that control regional seismotectonics accumulating strains are partly consumed by sliding of chamba nappe to the southwest through reverse fault movements along chamba panjal main boundary thrusts this coupled with normal fault type displacements along chenab normal fault in the north account for low magnitude widespread seismicity in upper 8 10 km of the crust at intermediate depths from 8 to 15 km adjusting to residual compressive stresses the detachment or lower end of the mbt slips to produce thrust dominated seismicity nucleation of secondary stresses in local ne sw oriented structure interacts in complex manner with regional stresses to generate normal type earthquakes below the plane of detachment and therefore three seismic regimes at different depths produce intense seismicity in a block of 30 30 km 2 centered ne to the epicenter of kangra earthquake

seismic evidence for the influence of fluids on the 2005 west off fukuoka prefecture earthquake in southwest japan abstract high resolution 3 d seismic velocity vp and vs and poisson s ratio structures in the epicentral area of the 2005 west off fukuoka prefecture earthquake were determined by inverting a large number of high quality p and s wave arrival times and sp depth phase data to better understand the genesis of the main shock and its aftershock sequence the hypocenters of the earthquakes used in this study were relocated with a combined method which enables to accurately relocate the earthquakes occurred outside the seismic network by using sp depth phase data together with double difference travel time data this new approach of earthquake location can be applied to other offshore regions of subduction zones where few seismic stations exist while numerous suboceanic earthquakes occur strong crustal velocity heterogeneities were revealed in the hypocentral area which might reflect the existence of fluids geometrical irregularities of the fault zone and geothermal anomalies in the region the main shock hypocenter was located near a distinct zone characterized by low velocity high poisson s ratio and high conductivity anomalies being considered to be associated with the existence of fluids in and around the source area such fluids might have reduced the seismic velocity of the crustal materials around the main shock hypocenter and weakened the mechanic strength of the fractured rock matrix in the source area and thus triggered the 2005 west off fukuoka prefecture earthquake

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

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

seismic attenuation characteristics along the hi climb profile in tibet from lg q inversionseismic attenuation characteristics along the hi climb profile in tibet from lg q inversion

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

stochastic finite fault modeling of ground motions from the 1994 northridge california earthquake i validation on rock sites the stochastic method of simulating ground motions from finite faults is validated against strong motion data from the m 6 7 1994 northridge california earthquake the finite fault plane is subdivided into elements each element is as signed a stochastic o92 spectrum and the delayed contributions from all subfaults are summed in the time domain simulated horizontal acceleration time histories and fourier spectra at 28 rock sites are compared with observations we first perform simulations using the slip distribution on the causative fault derived from strong motion teleseismic gps and leveling data wald et al 1996 we then test the performance of the method using quasi random distributions of slip and alternative hypocenter locations this is important because the rupture initiation point and slip distribution are in general not known for future earthquakes the model bias is calculated as the ratio of the simulated to the observed spectrum in the frequency band of 0 1 to 12 5 hz averaged over a suite of rock sites the mean bias is within the 95 confidence limits of unity showing that the model provides an accurate prediction of the spectral content of ground motions on average the maximum excursion of the model bias from the unity value when averaged over all 28 rock stations is a factor of approximately 1 6 at most frequencies it is below a factor of 1 4 interestingly the spectral bias and the standard deviation of the stochastic simu lations do not depend on whether the fault slip distribution and hypocenter location are based on data or are randomly generated this suggests that these parameters do not affect the accuracy of predicting the average characteristics of ground motion or they may have their predominant effect in the frequency range below about 0 1 hz below the range of this study the implication is that deterministic slip models are not necessary to produce reasonably accurate simulations of the spectral content of strong ground motions this is fortunate because such models are not available for forecasting motions from future earthquakes however the directivity effects controlled by the hypocenter location are important in determining peak ground ac celeration at individual sites although the method is unbiased when averaged over all rock sites the simulations at individual sites can have significant errors generally a factor of 2 to 3 which are also frequency dependent factors such as local geology site topography or basin propagation effects can profoundly affect the recordings at individual stations to generate more accurate site specific predictions empirical responses at each site could be established

electrical resistivity structure of the valu fa ridge lau basin from marine controlled source electromagnetic sounding summary in december 1995 we carried out a comprehensive controlled source electromagnetic survey of the valu fa ridge at 22u25k si n the lau basin the valu fa ridge is ab ack arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity seismic studies have imaged a melt lens at an average depth of 3 2 km below the seafloor surrounded by a zone of lowered seismic velocity interpreted as a region of partial melt in the crust the electromagnetic experiment was part of a multidisciplinary study which included wide angle and reflection seismics bathymetry and potential field measurements electromagnetic signals at frequencies between 0 25 and 40 hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea bottom receivers at ranges of up to 20 km from the source over 80 hr of data consisting of the magnitude of the horizontal electric field at the seafloor were collected these data have extremely low scatter compared to similar data from previous surveys the data were interpreted using a combination of 1 and 2 d forward modelling and inversion the vertical resistivity gradient in the upper crust at the valu fa ridge is abnormally low with resistivities of less than 10 v m observed throughout layer 2 of the crust to a depth of 3 km this is significantly more conductive at depth than the axis of the slow spreading reykjanes ridge at 57u45kn and the fastspreading east pacific rise at 13un where similar data sets have been collected in the past although the structure of layer 2 is well constrained by the electromagnetic data its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it and hence the data are not sensitive to the structure in layer 3 in particular the structure of the melt lens or surrounding low velocity zone the seismic velocity structure of the valu fa ridge determined from the coincident wide angle seismic study is similar to that observed at other mid ocean ridges with a steep seismic velocity gradient through layer 2 although overall velocities are slightly lower the seismic velocity anomaly calculated relative to an average off axis structure is also small this suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity however hot and or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity since the conductive region extends unbroken from 3 km depth to the seafloor it is probable that these fluids circulate to or close to the magma chamber itself

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

tomographic regionalization of crustal lg q in eastern eurasia 1 we measure q of crustal lg waves in eastern eurasia using the reliable two station methods more than 5 000 spectral ratios are collected over 594 interstation paths and are used to determine values of lg q0 and q at 1 hz and its frequency dependence over these paths these values are used to derive tomographic models of laterally varying q0 and with resolutions between 4 and 10 the q0 model contains values that vary between 100 and 900 q0 are the lowest in the tibetan plateau increase to moderate levels towards the east and north and reach maxima in the siberian and eastern europe cratons q0 values correlate well with regional tectonics most values are between 0 14 and 0 50 with a mean of 0 18 these values are lower than those obtained previously using either an lg coda q method or a method of simultaneous inversion of source spectra and q

three dimensional crustal structure and magma chamber geometry at the intermediate spreading back arc valu fa ridge lau basin results of a wide angle seismic tomographic inversion in this paper the results of a tomographic analysis of a 3 d wide angle seismic refraction data set acquired at the valu fa ridge vfr in 1995 are presented the vfr is an intermediate spreading ridge located in the southern lau back arc basin in the southwest pacific the ridge comprises three morphological segments the southern central and northern valu fa ridges separated by overlapping spreading centres oscs previous seismic experiments have identified a robust axial magmatic system beneath the central segment cvfr and the osc with the northern segment nvfr the experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post rift crust a regularized inversion scheme that minimizes model roughness was applied to the first arrival traveltime picks made from the wide angle data a quantitative approach for determining data uncertainties is described based on the signal to noise ratio of the arrivals several initial model assumptions were tested including one with a thin melt lens representing a seismic reflector identified in previous studies explicitly included in the initial model the inversion results suggest that crustal layer 2 exhibits northward thickening which mirrors a similar northward thickening of the whole crust in addition local thinning of layer 2 is identified in the vicinity of the boundary between pre and post rift crust which is thought to represent thinning of the crust prior to the onset of rifting axial low velocity anomalies are identified in layer 2b c and layer 3 the models are consistent with a continuous 6 km wide negative velocity anomaly in layer 3 with an amplitude of 0 7 0 9 km s 1 relative to off axis post rift crust this anomaly is consistent with the presence of an axial mush zone comprising a small percentage 1 per cent of partial melt the negative velocity anomaly in layer 2b c is modelled with its largest amplitude 0 5 km s 1 beneath the northern osc possible origins for this anomaly include locally thicker crust or locally higher porosity near the osc or a high temperature anomaly associated with the axial magmatic system

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

the three dimensional seismic velocity structure of the east pacific rise near latitude 9 30 n three dimensional images of crustal seismic structure beneath the east pacific rise show pronounced axial heterogeneity over distances of a few kilometres a linear high velocity anomaly approximately 1 2 km in width and restricted to the uppermost 1 km of the crust is centred on the rise axis an axial low velocity anomaly at depths of 1 3 km varies in amplitude along the axis consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis this apparent thermal segmentation along axis is consistent with injection of mantle derived melt midway along a locally linear 12 km long segment of the rise

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

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

crustal and uppermost mantle structures in the frontal himalaya and indo gangetic basin using surface wave tectonic implications abstract characteristics of dispersion curves of rayleigh and love waves of 2005 kashmir earthquake and its aftershocks are utilized to investigate the sub surface structural variations within the northern margin of the indian continent i e indo gangetic plains and the frontal part of the northwestern himalaya surface waves of these earthquakes recorded in the broad band seismometers bbs installed at ajmer delhi and shimla were used for this investigation estimated group velocities along three paths from earthquake source region to these stations are in the range of 2 17 km s to 3 94 km s which are sufficient to investigate the crustal and uppermost mantle structures paths covering northern margin of the indian continent has very low group velocities for periods less than 10 s that indicate presence of thick sediments high differences are observed between love and rayleigh wave group velocities for the paths crossing himalayan wedge that can be ascribed to the effect of anisotropy due to india eurasia collision the inversion of dispersion curves was performed to investigate sub surface shear wave velocity structure and variation of anisotropy with depth for crust and uppermost mantle a 6km thick sediments divided into two layers unconsolidated and consolidated are seen in the indo gangetic plain deposited above underthrusting indian plate the conrad and moho depths increase from ajmer the southernmost station in the igp to shimla the station lying in the himalayan wedge this shows the dipping geometry of the indian plate the average moho depth along these paths from earthquake source to ajmer station is 42 5 km to delhi station is 44 km and to shimla station is at 46 km average values of conrad depth for these ray paths are 18 5 km 19 5 km and 26 5 km respectively the s wave velocity along ajmer path is higher than that along the other two paths this may be due to lateral heterogeneity of the crust and mantle for the study area frontal western himalaya is anisotropic in nature whereas the igp region is not anisotropy effect in the western himalaya is most pronounced in the lower crustal material of the underthrusted indian plate it is quite large in the uppermost mantle below the frontal himalayan region we infer that anisotropy in the lower crust and uppermost mantle could be due to development of preferably oriented folds faults and fractures and possible reorientation of crystals as a consequence of ongoing deformation caused by continent continent collision

electrical structure of the himalaya of central nepal high conductivity around the mid crustal ramp along the mht twelve broadband magnetotelluric mt soundings were performed across the himalaya of central nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics the mt impedance tensors were obtained for frequencies between 0 001 and 500 hz the 2 d section derived from joint inversion of te and tm mode after rri and groom bailey decomposition shows high conductivity in the foreland basin 30 m that contrasts with the resistive indian basement 300 m and lesser himalaya 1000 m in addition our mt sounding reveals a major conductive feature beneath the front of the higher himalaya also characterized by intense microseismic activity and the position of a mid crustal ramp along the major active thrust fault mht this high conductivity zone probably reflects metamorphic fluids released during underthrusting of the indian basement and pervading well connected microcracks induced by interseismic stress build up or distributed brittle deformation around the ramp

strong lg attenuation in the tibetan plateau the widespread existence of strong lg attenuation in the tibetan plateau is further demonstrated by analysis of lg spectra on many paths within the plateau and quantitative estimates of the lg attenuation from low frequency lg signals for new localized path geometries strong path length dependent shifts of the lg spectra to lower corner frequencies with increasing distance are observed across the plateau consistent with a low regional average 1 hz lg attenuation value q 0 of about 125 there are clearly lateral variations within the plateau found in this and other recent studies with localized areas having q 0 values of 60 90 low enough to eliminate high frequency lg energy over path lengths of just several hundred kilometers while some localized areas may have higher q 0 values of up to 147 or higher a q 0 value of 103 is found in south central tibet compatible with recent work by others for higher frequencies on very localized scales and values from 83 to 147 are found in eastern tibet the lowest q 0 estimates found in tibet tend to be in areas for which there is evidence of volcanism and or partial melting within the crust however the strong regional attenuation may have a contribution from scattering by small scale crustal heterogeneity the strong lg attenuation in tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests

attenuation characteristics in eastern himalaya and southern tibetan plateau an understanding of the physical state of the medium abstract attenuation characteristics of the crust in the eastern himalaya and the southern tibetan plateau are investigated using high quality data recorded by himalayan nepal tibet seismic experiment himnt during 2001 2003 the present study aims to provide an attenuation model that can address the physical mechanism governing the attenuation characteristics in the underlying medium we have studied the coda wave attenuation q c in the single isotropic scattering model hypothesis s wave attenuation q s by using the coda normalization method and intrinsic q i 1 and scattering q sc 1 quality factors by the multiple lapse time window analysis mltwa method under the assumption of multiple isotropic scattering in a 3 d half space within the frequency range 2 12 hz all the values of q exhibit frequency dependent nature for a seismically active area at all the frequencies intrinsic absorption is predominant compared to scattering attenuation and seismic albedo b 0 are found to be lower than 0 5 the observed discrepancies between the observed and theoretical models can be corroborated by the depth dependent velocity and attenuation structure as well as the assumption of a uniform distribution of scatterers our results correlate well with the existing geo tectonic model of the area which may suggest the possible existence of trapped fluids in the crust or its thermal nature surprisingly the underlying cause of high attenuation in the crust of eastern himalaya and southern tibet makes this region distinct from its adjacent western himalayan segment the results are comparable with the other regions reported globally

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

seismic imaging of oceanic layer 2a between 9 30 n and 10 n on the east pacific rise from two ship wide aperture profiles we analyze two ship multichannel seismic wide aperture profiles wap acquired on the east pacific rise epr between 9 30 c and 10 n in may and june 1985 for offsets between 2 and 5 km most of the wap common depth point cdp gathers exhibit a strong retrograde reflection like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2a stacking of this postcritical arrival for hundreds of cdp gathers along the wap lines yields remarkably clear and virtually continuous images of the bottom of layer 2a for several tens of kilometers along and across the epr an innovative combination of this image with others of the seafloor sf and of the axial magma chamber amc provides an unusually clear composite picture of young upper crust we combine these images with detailed seismic velocity information also obtained from the cdp gathers to achieve an accurate determination of the layer 2a thickness in the area along the epr axis between 9 30 n and 10 n layer 2a shows a relatively constant thickness between 100 and 150 m 80 120 ms two way travel time across the axis however a line near 9 30 n indicates that layer 2a thickness rapidly increases by approximately a factor of 2 within 2 4 km of the axis and remains nearly constant afterwards thus the final thickness of layer 2a is attained before the development of large scale tectonic faulting strongly suggesting that the thickening of layer 2a away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2a downwards as the crust moves away from the axis

tomographic imaging of lg and sn propagation in the middle east observations based on relatively limited data recorded by sparsely distributed stations have indicated that regional seismic phase propagation lg and sn is very complex in the middle east accurate characterization of regional seismic wave propagation in this region necessitates the use of a large number of seismic stations we have compiled a large data set of regional and local seismograms recorded in the middle east this data set comprises approximately four years of data from national short period networks in turkey and syria data from temporary broadband arrays in saudi arabia and the caspian sea region and data from gsn mednet and geofon stations in the middle east we have used this data set to decipher the character and pattern of regional seismic wave propagation we have mapped zones of blockage as well as inefficient and efficient propagation for lg pg and sn throughout the middle east two tomographic techniques have been developed in order to objectively determine regions of lithospheric attenuation in the middle east we observe evidence of major increase in lg attenuation relative to pg across the bitlis suture and the zagros fold and thrust belt corresponding to the boundary between the arabian and eurasian plates we also observe a zone of inefficient sn propagation along the dead sea fault system which coincides with low pn velocities along most of the dead sea fault system and with previous observations of poor sn propagation in western jordan our observations indicate that in the northern portion of the arabian plate south of the bitlis suture there is also a zone of inefficient sn propagation that would not have been predicted from prior measurements of relatively low pn velocities mapped high attenuation of sn correlates well with regions of cenozoic and holocene basaltic volcanism these regions of uppermost mantle shear wave attenuation most probably have anomously hot and possibly thin lithosphere

ocean bottom seismograph tomographic experiments a consideration of acquisition geometries vs resources summary over the last 20 30 yr numerous seismic images of the earth s crust have revealed details of its gross structure including intra crustal layering the geometry of that layering and its composition as more and higher quality studies are undertaken it is becoming apparent that identified structures have a greater degree of 3 d variability than first anticipated thus the methodology of crustal imaging by seismic means has also developed into the third dimension with a tomographic approach now being widely adopted particularly so in the marine environment such surveys not only focus on mapping the finer scale 3 d structural variability they also aim to achieve sufficient density of azimuthal coverage and resolution to address preferential orientation patterns of features such as porosity fracturing and faulting recent developments in technology and consequently cheaper construction and deployment costs of instruments have resulted in an expansion in the number of instruments available in ocean bottom seismometer pools consequently individual experiments are being designed to accommodate the maximum number of instruments available and this coupled with dense grids of shot profiles significantly impacts on survey cost in this paper we consider a variety of approaches to achieving the best resolution of detail for minimal associated cost of acquisition and for instrument pools of various sizes a number of different geometries are compared including example grid designs in current use comparison of resolution tests and relative costings for a range of acquisition geometries suggest that if instrument numbers and or funds are limited the most cost effective ways of achieving the desired target resolution may be by 1 shooting additional shot profiles at the expense of deploying more instruments and 2 multiple overlapping deployments of a small geometry tailored in shape to the target structure and depth

seismic structure of the crust and the upper mantle beneath the himalayas evidence for eclogitization of lower crustal rocks in the indian plate 1 variations in the seismic velocity structure of the himalayan collision zone include significant differences between its north and south portions with transitions in physical properties across the greater himalaya we combined p and s wave traveltimes from a temporary broadband seismic network in eastern nepal and southern tibet with arrival times at the permanent station network of the department of mines and geology of nepal to determine the seismic velocity structure across the himalaya using local earthquake tomography and traveltimes of regional earthquakes the p to s velocity ratio vp vs structure marks the difference between the indian plate and the overlying materials with the vp vs ratios being high for the former and low for the latter we also found a significant increase in the uppermost mantle seismic velocities from south to north reaching p wave velocities vp over 8 4 km s north of the greater himalaya these high vp values do not seem to be the result of biases due to anisotropy in the upper mantle beneath the greater and tethyan himalayas instead we suggest that rocks in the lower crust of the underthrusting indian plate undergo metamorphism to eclogite as they plunge to greater depth beneath the mountain range explaining the high seismic velocities

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

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

relative role of intrinsic and scattering attenuation beneath the andaman islands india and tectonic implications abstract the attenuation characteristics of seismic waves traversing the andaman nicobar subduction zone ansz are investigated using high quality data from a network of broadband stations operational since 2009 we initially studied the coda wave attenuation q c 1 under the assumption of a single isotropic scattering model subsequently following the multiple isotropic scattering hypothesis we isolated the relative contributions of intrinsic q i 1 and scattering q sc 1 attenuation employing the multiple lapse time window analysis mltwa method within a frequency range 1 5 18 hz results reveal a highly attenuative nature of the crust with the values of q c being frequency dependent the intrinsic absorption is mostly found to be predominant compared to scattering attenuation the dominance of q i 1 in the crust may be attributed to the presence of fluids associated with the subducted slab our results are consistent with the low velocity zone reported for the region a comparison of our results with those from other regions of the globe shows that the ansz falls under the category of high intrinsic attenuation zone interestingly the character of ansz is identical to that of eastern himalaya and southern tibet but entirely different from the garhwal kumaun himalaya and the source zone of chamoli earthquake due to the underlying mechanisms causing high attenuation

attenuation of shear waves in the back arc region of the hellenic arc for frequencies from 0 6 to 16 hz abstract q for shear waves is determined for the inner part of the hellenic arc the back arc area as a function of frequency in the range 0 6 16 hz we used 314 digital records from 32 earthquakes with magnitudes m w ranging from 3 9 to 5 1 epicentral distances ranged from 65 to 515 km the data were obtained in 1997 during a 6 month operation of a digital portable network in greece the q estimates were made for five frequency bands centred at 0 8 1 5 3 0 6 0 and 12 0 hz and the q values obtained were 47 79 143 271 and 553 respectively the results show that q for s waves increases with frequency taking the form q 55 f 0 91 or q 1 0 018 f 0 91 the high attenuation and the strong frequency dependence found which is close to the frequency dependence of coda q for greece are characteristic of an area with high seismicity rapid extension and in agreement with other similar studies in greece

the seismic attenuation structure of a fast spreading mid ocean ridge the two dimensional p wave attenuation structure of the axial crust of the east pacific rise was obtained from an inversion of waveform spectra collected during an active source seismic tomography experiment the structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis the near axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation high attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers less than 1 kilometer beneath the axial magma lens to the base of the crust the levels of attenuation in this deeper region require at most only a small fraction of partial melt

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

the 2010 mw 6 5 rigan iran earthquake aftershock sequence abstract the 20 december 2010 m w 6 5 rigan earthquake that occurred in southeastern iran was followed by an extensive aftershock sequence a six station portable seismographic network was deployed by the institute of geophysics at the university of tehran around rigan in kerman province to understand the aftershock activity we have used the data recorded by this temporary seismic network distribution of the aftershocks shows that the earthquake sequence occurred along a right lateral strike slip fault with a northeast southwest trend the general pattern of the aftershocks distribution delineates a source fault with a length of approximately 20 km along strike and 5 km across the surface this source fault dips steeply southeastward and strikes along a nearly northeast southwest line the depth range of the aftershocks is 1 12 km the aftershock activity has its highest concentration in the northern part of the active fault and tapers off toward the southwestern part of the active fault segment indicating mainly a unilateral rupture on 27 january 2011 another moderate sized earthquake m w 6 2 occurred a few kilometers away from the southwestern end of the active zone which probably represents a slow diffusion of stress away from the main rupture online material aftershock catalog

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 attenuation tomography of the source zone of the 2016 kumamoto earthquake m 7 3 we study the three dimensional seismic attenuation qp and qs structure in the source area of the 2016 kumamoto earthquake m 7 3 by using 18 296 tp and 29 668 ts data from 742 local earthquakes recorded by a dense seismic network consisting of 112 stations deployed in kyushu island our results show that significant low q high attenuation anomalies exist in the crust and mantle wedge beneath the volcanic front and back arc area which reflect hot and wet zones caused by convective circulation in the mantle wedge and dehydration of the young philippine sea phs slab the 2016 kumamoto earthquake occurred in a high q and high velocity high v zone in the upper crust which is surrounded and underlain by low q and low v anomalies in the lower crust and upper mantle these results suggest that the 2016 kumamoto earthquake took place in a brittle seismogenic layer in the upper crust but its rupture nucleation was affected by fluids and arc magma ascending from the mantle wedge in addition an obvious low q zone is revealed in the fore arc mantle wedge which reflects serpentinization of the fore arc mantle due to abundant fluids from the phs slab dehydration

blockage of regional seismic waves by the teisseyre tornquist zone summary during the group of scientific experts technical test gsett second experiment 22 april 2 june 1991 several hundred seismic events were located in europe associating these events with the detecting stations altogether 28 european stations including seven arrays participated in the gsett 2 experiment clearly shows that the teisseyre tornquist zone ttz influences the propagation of regional seismic phases large explosions in the bay of gdansk for example were observed by the wellestablished scandinavian arrays norsar a 830 km and arcess a z 1650 km but not by the polish station ksp a 470km nor by the new highly sensitive geress array a 750 km both situated south west of the ttz for events in central europe with comparable magnitudes we observe a similar increase of the detection threshold at stations located north east of the ttz in scandinavia to explain these observations the wave propagation of pn and pg perpendicular to the ttz was modelled for a profile from the estonian russian border region to geress with gaussian beam seismograms published crustal and uppermost mantle models for poland and for europe were used as a starting point for developing a model of the ttz the observations cannot be explained only by a graben like crustal structure with a jump in moho depth from 30 km to 50 km to defocus the seismic energy the ttz as a structural anomaly between eastern and western europe must reach down into the upper mantle to a depth of at least about 200 km the proposed model has such a deep reaching root of the ttz

stochastic finite fault modeling of ground motion for the 2010 rigan earthquake southeastern iran abstract we studied the 20 december 2010 rigan earthquake that occurred in southeastern iran using the ground motion simulation based on the stochastic finite fault model this earthquake was recorded by 23 stations the method requires region specific attenuation and generic site parameters we first found an empirical function to describe the amplitude decay with distance of shear waves in the rigan region of iran this function results from the analysis of spectral amplitudes of horizontal components from digital records from the rigan earthquakes the value of q s shows a dependence on frequency in the range of 1 24 hz for the rigan region the average frequency dependent relationship estimated for the region is q s 99 19 f 0 58 0 12 we then obtained a spectral decay parameter from the slope of smoothed amplitude of the fourier spectra of acceleration at higher frequencies the estimated zero distance kappa value is 0 06 in the next step we used these approximate values and the stochastic finite fault model in an iterative manner to optimally model the strong ground motion and rupture characteristics of the main event in terms of peak ground acceleration peak ground velocity peak ground displacement arias intensity the amplitude fourier spectra and the response spectra of ground shaking we compared our simulated results with recorded ones in both the frequency and time domain our near field estimates for the main event is in good conformity with the corresponding near field estimates reported by other studies

microearthquakes on and near the east pacific rise 9 10 n records from a seismic network deployed as part of an active tomography experiment at 9 30 n on the east pacific rise epr provide an opportunity to characterize local microearthquake activity over an 8 day period with the exception of the region around the 9 03 overlapping spreading center osc no events were located on the rise axis one microearthquake located from p and s wave arrival times lies adjacent to the network 18 km to the west of the rise axis five more events located from p wave and t phase data are to the south of the network three cluster around the western arm of the 9 03 osc while two are located 25 30 km to the east of the osc the seismic moments of the microearthquakes range from 5 1019 to 4 1020 dyn cm several orders of magnitude larger than those reported in other studies of shallow events along the axis of the epr these results suggest that overlapping spreading centers may be the loci of substantial microearthquake activity and that at this location the normal faults that form on the young flanks of the epr are active off axis to distances of at least 20 km

regional phases in continental and oceanic environments summary the very different crustal thicknesses between continental and oceanic crust lead to the establishment of distinct styles of regional seismic wavefield observations and numerical simulations of events near subduction zones in japan allow a direct comparison of seismic wave propagation in oceanic and continental structures the thick zone of lower seismic velocities in the continental crust acts to trap s wave energy in particular and generates crustally guided phases such as pg lg in the oceanic environment there is no efficient s wave trapping and crustal energy progressively leaks into the mantle reverberations in the crust and the water above help to sustain the amplitude of the mantle arrivals pn sn to large distances

regional body wave attenuation using a coda source normalization method application to mednet records of earthquakes in italy we develop a new methodology to determine apparent attenuation for the regional seismic phases pn pg sn and lg using coda derived source spectra the local to regional coda methodology mayeda 1993 mayeda and walter 1996 mayeda et al 2003 is a very stable way to obtain source spectra from sparse networks using as few as one station even if direct waves are clipped we develop a two step process to isolate the frequency dependent q first we correct the observed direct wave amplitudes for an assumed geometrical spreading next an apparent q combining path and site attenuation is determined from the difference between the spreading corrected amplitude and the independently determined source spectra derived from the coda methodology we apply the technique to 50 earthquakes with magnitudes greater than 4 0 in central italy as recorded by mednet broadband stations around the mediterranean at local to regional distances this is an ideal test region due to its high attenuation complex propagation and availability of many moderate sized earthquakes we find that a power law attenuation of the form q f q sub 0 f sup y fit all the phases quite well over the 0 5 to 8 hz band at most stations the measured apparent q values are quite repeatable frommore event to event finding the attenuation function in this manner guarantees a close match between inferred source spectra from direct waves and coda techniques this is important if coda and direct wave amplitudes are to produce consistent seismic results less

off axis volcanism at the east pacific rise detected by uranium series dating of basalts recent detailed surveys of the east pacific rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast spreading ocean ridge crests1 7 in parallel with geological and geochemical investigations it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium series disequilibria methods8 11 here we use 238u 230th and235u 231pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9 31 n on the east pacific rise most of the ages are younger than would be expected based on off axis distance and spreading rate we infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0 5 2 km outside the axial summit caldera with some volcanism occurring as far as 4 km off axis melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas

effect of baseline corrections on displacements and response spectra for several recordings of the 1999 chi chi taiwan earthquake displacements derived from many of the accelerogram recordings of the 1999 chi chi taiwan earthquake show drifts when only a simple baseline de rived from the pre event portion of the record is removed from the records the appearance of the velocity and displacement records suggests that changes in the zero level of the acceleration are responsible for these drifts the source of the shifts in zero level are unknown but in at least one case it is almost certainly due to tilting of the ground this article illustrates the effect on the ground velocity ground dis placement and response spectra of several schemes for accounting for these baseline shifts a wide range of final displacements can be obtained for various choices of baseline correction and comparison with nearby gps stations none of which are colocated with the accelerograph stations do not help in choosing the appropriate baseline correction the results suggest that final displacements estimated from the records should be used with caution the most important conclusion for earthquake engineering purposes however is that the response spectra for periods less than about 20 sec are usually unaffected by the baseline correction although limited to the analysis of only a small number of recordings the results may have more general significance both for the many other recordings of this earthquake and for data that will be obtained in the future from similar high quality accelerograph networks now being installed or soon to be installed in many parts of the world

lower edge of locked main himalayan thrust unzipped by the 2015 gorkha earthquake faults are unlocked by earthquakes analysis of seismic data from the 2015 nepal earthquake shows that only part of the main himalayan thrust fault was unzipped by the quake leaving much of the fault locked and ready to slip in a future event

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

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

crustal attenuation within the turkish plateau and surrounding regions the lateral variations of seismic attenuation in the crust of turkey and surrounding regions have been imaged from the inversion of interstation lg q measurements this study develops the first tomographic model for lg q 0 in this region that model is consistent with previous more qualitative lg attenuation models that showed inefficient or blocked lg across the eurasian arabian plate boundary in general the northern arabian platform has low to normal lg q 0 values 250 350 whereas high q 0 values 670 800 occur in the southern arabian plate additionally we have found a dramatic decrease in lg q 0 across the bitlis suture the plate boundary between the arabian and eurasian plates beneath the turkish plateau our high to moderate lg attenuation values q 0 100 200 probably originates from both scattering and intrinsic attenuation due to the tectonic complexity and the widespread young volcanics in the region we conclude that the lowest q 0 values for the east anatolian plateau 70 100 and the portion of western turkey around the menderes massif 60 150 are probably caused by intrinsic attenuation we find low to normal lg q 0 values 150 300 beneath the part of the taurus mountains in western anatolia these higher values may be related to the nature of the crust in the tauride mountain belt we also mapped the lateral variations in lg and observed a fairly consistent negative correlation between and lg q 0 in the northern middle east

source characteristics of hypothetical subduction earthquakes in the northwestern united states historic earthquake sequences on subduction zones that are similar to the cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern united states based on systematic comparisons of several physical characteristics including physiography and seismicity subduction zones that are deemed most similar to the cascadia subduction zone are those in southern chile southwestern japan and colombia these zones have all experienced very large earthquake sequences and if the cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length then earthquakes of very large size m w 8 1 2 must be considered circumstantial evidence is presented that suggests but does not prove that large subduction earthquakes along the cascadia subduction zone may have an average repeat time of 400 to 500 yr

seismic imaging of the axial region of the valu fa ridge lau basin the accretionary processes of an intermediate back arc spreading ridge in 1995 a multidisciplinary geophysical experiment targetted the intermediate spreading valu fa ridge full rate 60 mm yr 1 which is centred on 22 20 s 176 40 w in the lau basin as part of this experiment wide angle and normal incidence seismic profiles were collected both along and across axis to determine the crustal structure of the central valu fa ridge cvfr and its overlap with the northern valu fa ridge nvfr controlled source electromagnetic profiles and underway gravity magnetic and bathymetry data were also collected in this paper we describe the results of forward modelling of the along and across axis wide angle and normal incidence seismic data an axial low velocity block and its underlying slightly broader zone of depressed seismic velocities low velocity zone have been identified and these features are interpreted as corresponding to a melt lens and underlying magma chamber the low velocity block is 1 2 km wide and has a first order upper boundary from which large amplitude reflections are observed amplitude analysis of these indicates an interconnected melt fraction the nature of the lower boundary is more poorly constrained as no reflection event corresponding to the base of the low velocity block is observed modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region 5 5 km s 1 are achieved by 250 m below the upper boundary possibly suggesting a gradational lower boundary with high velocity gradient the low velocity zone lvz is interpreted as an 4 km wide magma chamber delineated by a seismic velocity anomaly of 0 2 km s 1 extending down through layer 3 to within 1 5 2 km of the moho the velocity anomaly and dimensions of the lvz are generally smaller than those observed at the east pacific rise epr and mid atlantic ridge mar the observed along axis continuity of the low velocity block is remarkable extending from the southern tip of the cvfr to the overlapping spreading centre osc with the nvfr a low velocity block is modelled beneath the inside flanks i e the slopes that dip into the overlap basin of both ridges at the osc although the existence of a single low velocity block beneath the overlap basin itself cannot be ruled out the identification of a single lvz centred on the overlap region rather than two merged lvzs beneath each segment implies that the material in each low velocity block originates from the same crustal magma source a reflection event from the moho is observed from directly beneath the axis on both across axis profiles which indicates that a distinct crust mantle boundary may be formed within the axial region many of the observations at the valu fa ridge are consistent with those at the epr and the reykjanes ridge mar which implies that regardless of spreading rate crustal accretionary processes at mid ocean ridges with similar magmatic budgets are also broadly similar

seismic attenuation characteristics along the hi climb profile in tibet from lg q inversion we investigate the q of crustal lg waves along the himalayan tibetan continental lithosphere during mountain building hi climb profile in tibet using a reliable two station method four earthquakes of m 5 5 recorded along a 800 km long densely spaced seismic profile are used to extract lg wave spectra one hundred seven high quality interstation paths that allow us to use the standard two station method for lg q measurement are selected from 2369 possible pairs by using those 107 interstation q0 1 hz lg q values as input lateral variations in attenuation char acteristics are derived through inversion the region has q0 estimates ranging from 88 5 to 165 15 with the lowest value in the qiangtang terrane and the highest across some part of the lower himalaya the lowest value is attributed to the high poisson s ratio higher temperatures as well as partial melts in the crust the results do not contain any detectable variation in q0 either across the himalaya tibet collision zone i e the indus tsangpo suture zone or along the major bangong nujiang suture zone bnsz consistent low q0 values along the hi climb profile manifest the presence of omnipresent partial melting of the crust throughout the plateau

structure of young oceanic crust at 13 n on the east pacific rise from expanding spread profiles we present the results of the analysis of expanding spread profiles esps collected on and near the axis of the east pacific rise at 13 n these profiles were collected at 0 1 1 2 1 3 6 and 9 5 km from the rise axis and all but the most distant profile show a distinct low velocity zone lvz located within layer 3 of the oceanic crust at the ridge crest the top of the magma chamber is at the base of layer 2 while 3 6 km off axis the roof of the lvz is 1 1 km below the top of layer 3 the profile farthest from the ridge could possibly have a residual lvz confined to the lower 1 5 km of the crust the total width of the lvz as determined from the esp data is at least 6 km and possibly much greater this wide lvz apparently contradicts multichannel seismic data which show cross axis reflections from the magma chamber with a width of 5 km we suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt the esp data are sensitive to this larger region while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast we identify the presence of a layer at the top of the oceanic crust with initial p wave velocities between 2 35 and 2 6 km s while the s wave velocity is estimated as being 0 8 km s the layer thickness lies between 100 and 200 m these velocities are consistent with previous estimates for the pacific and recent results for the atlantic the thickness of this layer is consistent with that of layer 2a determined from geophysical measurements at deep sea drilling project hole 504b

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

surface expression of thrust faulting in eastern iran source parameters and surface deformation of the 1978 tabas and 1968 ferdows earthquake sequences summary previously unrecognised thrust faults in eastern iran were responsible for a destructive earthquake at tabas 1978 september 16 which produced over 80 km of distributed and discontinuous surface ruptures above a series of low anticlinal hills to the west of a major range front analysis of long period body wave seismograms shows a simple rupture on a gently dipping 16 thrust with a slight right lateral component this is compatible with the locally recorded aftershock distribution body wave analysis of two later smaller events show similar source orientations several indicators of long term active folding at tabas can be recognised in the geomorphology and surface ruptures from 1978 are consistent with co seismic fold growth drainage incision also indicates uplift at depth on thrust faults dipping eastwards beneath the folds body wave seismograms for two earthquakes near ferdows 150 km east of tabas in september 1968 also show thrust faulting at depths of 10 km again the surface geomorphology indicates a region of folding above an eastward dipping thrust fault which is 10 km west of a fault bounded range front in both tabas and ferdows the active faulting appears to show quaternary migration away from the range front possibly in response to stresses produced by the elevated topography the identification of zones of active fault related folding is important for earthquake hazard assessment and also for an understanding of the local tectonics we conclude that the structures which gave rise to the 1968 and 1978 earthquakes could have been recognised beforehand from the clear signals in the geomorphology had we known what to look for at that time