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

the complete synthesis of seismic crustal phases at regional distances using a new method to evaluate green s functions for elastic layered media we compute complete synthetic seismograms at regional distances for an earthquake in the earth crust we choose as source parameters the ones inferred for an earthquake in central france and compare the computed seismograms to the ones recorded at several short period stations of the french seismic network the major features of the records including complexity and duration are well reproduced by the synthetics lg waves which are the prominant phase are shown to be guided waves made up of sh and sv waves incident on the moho at angles of incidence more grazing than the critical angle and multiply reflected within the crust the length of the computed lg wave coda extends between group velocities of 3 5 and 2 8 km s and is in good agreement with the data

modeling the effect of san andreas fault structure on receiver functions using elastic 3d finite difference abstract teleseismic p wave data collected at the san andreas observatory sao seismic station show strong frequency dependent nonrectilinear particle motions consisting of off azimuth horizontal displacements that are sometimes larger than the vertical displacements the azimuthal dependence of the horizontal components amplitude is directly related to the symmetry of the fault zone a 3d finite difference program frankel and vidale 1992 is used to examine the response of the san andreas fault zone velocity structure to incident teleseismic p waves synthetic seismograms are formed using a 3d representation of an assumed 2d fault zone structure the fault zone velocity model produces large amplitude phases that contain combinations of refracted and internally reflected p to s converted phases diffractions and possible p to rayleigh converted phases the largest amplitude phases observed in the data are nondispersive and have complicated particle motions confined to the vertical plane perpendicular to the fault zone the data are fit best by synthetic receiver functions for field points similar to the location of sao relative to the san andreas fault zone since the velocity model used in the computations is nonunique sensitivity studies were performed by modifying the structure synthetics match the timing and nature of the observed phases at sao showing that assumption of plane layered structure is invalid in the area of this station

use of empirical green s functions spectral ratios and kinematic source models for simulating strong ground motion abstract ground motions for large and moderately large earthquakes at short and moderate distances are particularly important for seismic hazard estimation in eastern north america ena very few direct observations of such ground motions have been obtained however because of the sparsity of recording sites and the relatively low rates of occurrence of large earthquakes inside the region estimation of strong ground motion must therefore rely heavily on theoretical models to extend empirical results obtained from small earthquakes and from the few larger ones for which reliable data are available because of the generally large distances between recording stations the main source of useful data comes from lg wave trains observed at relatively large distances for the two largest earthquakes to have occurred near populated regions of southeastern canada during the past decade spectral ratios of the lg wave trains of the mainshocks with respect to those of their aftershocks are found to depend almost entirely upon the source radiation characteristics of the sources alone this result is utilized to derive elastodynamically based kinematic rupture models that are consistent with the empirical spectral ratio data such models provide a firm physical basis from which to infer the most probable spectral characteristics for future large earthquakes in the region in converse application it is shown that spectral ratios obtained from such models along with empirical seismograms from small earthquakes can be used to accurately simulate strong ground motions at short and moderate as well as large distances as such small event seismograms are relatively plentiful the problem of reliable strong ground motion estimation is therefore reduced to that of obtaining reliable representative source spectral models the solution of this latter problem must continue to depend upon whatever empirical data are available and upon appropriately detailed theoretical modeling

estimates of q in central asia as a function of frequency and depth using the coda of locally recorded earthquakes digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the hindu kush region of afghanistan were used to determine the attenuation factors of the s wave coda qc and primary s waves q an anomalously rapid decay of the coda shortly after the s wave arrival observed also in a study of coda in central asia by rautian and khalturin 1978 seems to be due primarily to depth dependent variations in qc in particular we deduce the average qc in the crust and uppermost mantle 100 km depth is approximately four times lower than the deeper mantle 400 km depth over a wide frequency range 0 4 to 24 hz further while qc generally increases with frequency at any depth the degree of frequency dependence of qc depends on depth except at the highest frequency studied here 48 hz the magnitude of qc at a particular frequency increases with depth while its frequency dependence decreases for similar depths determinations of q and qc agree suggesting a common wave composition and attenuation mechanism for s waves and codas comparison of these determinations of qc in afghanistan with those in other parts of the world shows that the degree of frequency dependence of qc correlates with the expected regional heterogeneity such a correlation supports the prejudice that qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

the digital array at anza california processing and initial interpretation of source parameters seismicity along the san jacinto fault suggests that a 40 km long section near the town of anza may constitute a gap in the occurrence of ml 6 7 earthquakes the potential for a gap filling shock and the high rate of seismicity at the southern end of the gap five events of 4 0 ml 5 5 since 1970 provided the impetus for deploying a digital seismic array to collect high quality ground motion recordings of all events 2 ml 4 5 on scale recording for shocks with magnitudes above ml 4 can be obtained from an existing array of eight strong motion accelerographs the anza site also had the advantage of being in the southern california batholith which appears to be relatively homogeneous compared to the franciscan gablian contrast of the central san andreas we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies the field instrumentation is specifically designed for broadband recording up to 70 hz and high dynamic range 96 db in the on site digitizer alone since both are necessary for determining the rupture history of earthquakes both local vhf and microwave digital telemetry transmit the data from the anza region to san diego for computer data logging in the first 30 months of the array s operation approximately 292 events have been recorded located and processed for source parameters most events occur in one of five clusters or in a diffuse zone near the buck ridge fault two of these clusters are located at right stepping en echelon offsets coyote creek san jacinto and san jacinto hot springs two others are directly below and about 8 km west of anza respectively the fifth cluster is just to the northeast of the hot springs fault although event depths are generally between 11 and 14 km at the southern end of the hot springs fault depths extend to 18 km these are some of the deepest strike slip earthquakes on the san andreas system we calculate source parameters such as the scalar moment and stress drop for the analysis of high frequency waves scaling relations and earthquake interaction the largest event recorded thus far had a moment of 4 4 1021 dyn cm m 3 8 and a stress drop of 55 bars both arms and brune stress drops increase with moment source radius increases only slowly with moment the maximum values of both the brune and arms stress drops increase with depth down to 10 km remain approximately constant to 14 km and may decrease below 14 km the data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi plastic flow laws although individual events may not be related stress drops on the san jacinto fault are high compared to those on the central creeping section of the san andreas where stress drops are about 10 bars or less this observation is consistent with the relative rupture area of events of equal ml 5 5 and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress

site amplification of coda waves from local earthquakes in central california the goals of this study are two fold 1 to examine the site effect of coda waves believed to be the average site effect of shear waves in order to understand its spatial and frequency dependent behavior and 2 to learn what we can about the processes that generate the coda itself we use digital data from over 90 earthquakes each recorded by a subset of some 150 stations in the coast ranges of central california between san francisco and san luis obispo results from the band 1 5 to 24 hz indicate that site amplification depends strongly on surface geology and frequency at low frequencies 1 5 to 3 0 hz changes in amplification up to a factor of 20 are observed amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and franciscan basement sites at high frequencies 6 to 24 hz the pattern changes many granite sites the majority in the gabilan range exhibit increasing amplification with frequency relative to the average station this behavior differs strikingly from that observed at sediment and franciscan sites which roll off at an intermediate rate and many sediment sites adjacent to the san andreas fault zone which decay away even more rapidly with frequency these results can be explained qualitatively by appealing to variable near site impedance and attenuation however complicating phenomena such as mode excitation in valley sediments must exist at low frequencies peaked behavior at a few hard rock sites at high frequency 12 hz currently lacks explanation at a few sediment sites the low frequency excitation is so strong that the coda envelopes begin to take on different shapes this contradicts the assumption based on previous observations that the coda shape is independent of source receiver location thus some site measurements may be artificially high the working model of coda waves as body wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near site resonance excited by incident direct and coda wave energy

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

lapse time and frequency dependent attenuation characteristics of kumaun himalaya abstract we have analyzed local earthquake data set consists of 84 well located events recorded by a digital seismic network in the kumaun himalaya region during 2004 2007 to study seismic attenuation characteristics of the region single back scattering assumption is used to estimate coda q q c values of the region at frequency range of 1 5 3 6 8 12 and 18 hz for different lapse time window length the values of q c show a ubiquitous observation of frequency dependence and follow a substantially similar trend as observed in other tectonically active parts of the himalaya the lapse time dependencies of coda waves are investigated which reveal that by increasing lapse time window from 20 to 50 s q 0 q c at 1 hz increases from 64 2 to 230 19 while frequency dependent parameter n decreases from 1 08 to 0 81 increase in q c with lapse time indicates decrease in heterogeneity with depth the observed values of q 0 and n infer that the medium beneath the study area is highly heterogeneous and tectonically very active the variation of q c with lapse time at 1 5 hz follow a similar pattern with the theoretically predicted values of gusev 1995 although the frequency parameter n does not match with the model it may suggest that the turbidity decays rapidly with depth for the kumaun himalaya our results are comparable with those obtained from tectonically active regions in the world

attenuation of coda waves in northern greece the single scattering model has been applied for the estimation of codaq values for local earthquakes that occurred in northern greece during the period 1983 1989 and recorded by the telemetered network of the geophysical laboratory of the university of thessaloniki codaq estimations were made for four frequency bands centered at 1 5 hz 3 0 hz 6 0 hz and 12 0 hz and for the lapse time windows 10 20 sec 15 30 sec 20 45 sec 30 60 sec and 50 100 sec the codaq values obtained show a clear frequency dependence of the formq c q 0 f n whileq 0 andn depend on the lapse time window q 0 was found equal to 33 andn equal to 1 01 for the time window of 10 to 20 sec while for the other windowsq 0 increased from 60 to 129 withn being stable close to 0 75 this lapse time dependence is interpreted as due to a depth dependent attenuation the high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity in agreement with studies in other seismic regions

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

the anza array a high dynamic range broadband digitally radiotelemetered seismic array a small aperture seismic array has been deployed along a 30 km stretch of the san jacinto fault in the vicinity of the town of anza california the array was installed to study specifically the scaling laws of body wave spectra the character of high frequency ground motion the physical interpretation of seismic stress drops and the interaction of earthquakes this region was chosen for these studies due to its high rate of seismic activity in the 2 m 4 5 range the likelihood of a m 6 event in the next decade and the existence of the southern california batholith on either side of the fault reducing problems associated with attenuation and large scale anisotropy these studies employ an instrument package with a frequency response of 100 hz a dynamic range of 138 db a sampling rate of 250 times per second per component and a 16 bit a d converter the array consists of 10 three component stations telemetered via digital vhf radio to a nearby mountain peak and thence via a microwave link to la jolla california a minicomputer system monitors the array s performance detects events and records data upon demand initial results demonstrate the feasibility of digital transmission with an inherent increase in data quality over analog systems

deep crustal earthquakes and active faults along the rukwa trough eastern africa summary since june 1992 a seismic network of five digital stations has been operating in southwestern tanzania with the aim of investigating the seismic activity and its partitioning with regard to depth in the area between lake rukwa and lake malawi during the period 1992 june 15 to 1994 december 31 a reliable location was obtained for 199 local earthquakes 13 were recorded by all five seismic stations 71 by four stations and 115 by three seismic stations with both p and s wave arrival time measurements many earthquakes arc located in the south eastern part of the rukwa basin along its eastern border in a 20 km narrow zone centred on the lupa fault but the more active area is the songwe valley characterized by a narrowing of the basin in the proximity of the important uplift of the mbeya range seismic activity is also present inside intrabasinal faulted blocks the earthquake depth distribution presents an upper seismicity cut offal a depth of 10 to 12 km this upper stability zone is followed by the peak of seismic activity in the depth range 12 to 14 km where 20 per cent of the earthquakes occurred at greater depths the level of activity continuously decreases up to a depth of 34 km a crustal thickness of 42 4 5 km is estimated using arrivals identified as the pmp waves reflected at the base of the crust indicating that the deeper earthquakes are really occurring in the lower crust and not in the upper mantle this analysis confirms at a local scale what is suggested by geophysical and geological observations at a global scale in the western branch of the east african rift system border faults penetrate to lower crustal levels in a lithosphere that is cold and mechanically strong

three dimensional attenuation model of the shallow hikurangi subduction zone in the raukumara peninsula new zealand 1 the material properties of the shallow subduction zone are imaged with data from a passive seismic deployment in the raukumara peninsula attenuation images are obtained using t values measured from 2848 spectra of p wave arrivals the t values are inverted for frequency independent three dimensional 3 d qp using a 3 d velocity model linked nodes in a preliminary 2 d inversion are used to obtain the most reliable initial model for 3 d inversions the results show the benefits of obtaining 3 d q in addition to 3 d vp and vp vs q is related to localized permeability and heterogeneity and thus 3 d q is an important tool for interpreting active tectonic regions qp in the shallow overlying plate generally corresponds to the mapped geology two high qp features 600 represent subducted seamounts or cretaceous volcanics within the overlying plate the gradient above the subducting plate is the strongest feature in the qp model with the slab having high qp 600 900 below 20 km depth and above the plate interface there is a zone of 50 decrease in qp and high vp vs about 10 km thick which represents actively subducting sediment and has distributed microearthquake activity associated with active underplating

separation of intrinsic and scattering q for volcanic areas a comparison between etna and campi flegrei abstract we studied the seismic attenuation in the short period range 1 24 hz at the campi flegrei volcanic area using the method of frankel 1991 to separate intrinsic and scattering attenuation parameters the results compared to those obtained at etna volcano show that scattering phenomena play a main role in the attenuation mechanism of the seismic waves for these two volcanic areas the comparison of the present results with those obtained in non volcanic areas allow us to re consider the importance of the molten materials magma as a main cause of the seismic attenuation in the active volcanic areas

array analysis of the large aperture array of the 1988 89 passcal basin and range passive source seismic experiment summary the 1988 89 passcal basin and range passive source seismic experiment consisted of two array deployments the large and small aperture arrays the large aperture array consisted of seven stations with three component mid period seismometers and independently triggered prototype passcal data recorders with goes clocks the array spatial distribution and the synchronized data have permitted us to employ simple beamforming operations to improve receiver function estimation from single teleseismic events the beamforming reinforces the coherent teleseismic p wave and the deterministic portions of the p wave coda primarily generated by interactions at major lithospheric interfaces microseismic background noise is incoherent across the array and is attenuated by beamforming beamforming also diminishes those portions of the coda due to scattering by inhomogeneities that appear random on the scale of the array aperture the enhanced signal to noise ratio of the teleseismic recording dramatically improves the single event receiver function deconvolution results

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

intrinsic and scattering attenuation of the mt fuji region japan summary mount fuji is the focus of intense study because of its potential hazard signaled by seismic geologic and historical activity based on extensive seismic data recorded in the vicinity of mt fuji coda quality factor q 1c using a single scattering model hypothesis and intrinsic and scattering quality factor q 1i and q 1s using the multiple lapse time window mltw method were measured to focus the study on the magmatic structure below mt fuji the data were separated into two groups a near fuji region of rays traversing an area with radius 5 km around the summit r 20 km this classification shows the largest discrepancy of q 1c at a range of sampling volumes corresponding to overlapped sampling depth of about 80 km further the spatial division shows significant difference of q 1i and q 1s at hypocentral distance of 80 km the large difference of q 1s in bandwidths 2 4 and 4 8 hz indicates lithospheric heterogeneity beneath mt fuji with a characteristic heterogeneity scale length of about 1 km the results have a small error range due to the large data sample showing that all q 1 values in the near fuji area are greater than those of the far fuji area and q 1i for both the near and far fuji areas is higher than q 1s at high frequencies the q 1i and q 1s values for far fuji are in the range of values for typical non volcanic areas the q 1i values of the near fuji area are lower than those of other volcanic areas considered where as values of q 1s are not the low q 1i for the volcanic region of near fuji suggests that the magmatic activity indicated by percent partial melt in the vicinity of mt fuji is not as active as hot spot volcanoes such as kilauea hawaii

coda q in the eastern caribbean west indies summary coda q q has been estimated from seismograms of local earthquakes recorded on short period seismographs on some of the eastern caribbean islands of the lesser antilles arc west indies using the s s single scattering model the region was subdivided into four distinct subregions in order to investigate lateral variations in q the qc values exhibit a strong frequency dependence in the frequency band 1 5 to 16 hz with calculated coda q at 1 hz qo ranging from a minimum of 97 in the dominica area to a maximum of 145 in the leewards the degree of frequency dependence is also largest in the dominica area n 1 09 and smallest in the leewards n 0 82 although there appears to be some lateral and depth variation of both q and n in the region this variation does not seem to be very significant

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a finite difference simulation of wave propagation in two dimensional random media a finite difference algorithm is used to generate synthetic seismograms for waves propagating through two dimensional random media the media have a significant component of their material properties varying randomly over length scales smaller than the seismic wavelength and are meant to approximate the heterogeneity of the crust and upper mantle the finite difference technique retains all multiply scattered and diffracted waves and also accounts for transmission losses the synthetic seismograms clearly exhibit coda and apparent attenuation caused by scattering for a medium with a white wavenumber spectrum of velocity fluctuations the coda is higher frequency than the initial pulse the apparent attenuation is greatest when the scatterer size is comparable to the seismic wavelength the spectra of the coda generally increase in frequency as the scatterers decrease in size examples demonstrate how scattering can produce spectra with broad peaks and sharp fall offs that can make the determination of the source spectra and corner frequencies of small earthquakes extremely difficult