inversion of rg waveforms recorded in southern spain abstract group velocity dispersion measurements of r g waves generated either by blasts or by local earthquakes are used to investigate the shallow crustal structure of almeria southern spain in principle the usable frequency range of 250 to 2000 mhz allows determination of structures to depths of about 4 km for this purpose the main operations are a detailed dispersion analysis of high frequency rayleigh waves propagating along very short paths and the inversion of r g wave group velocities a total of 21 seismic events were studied these events had small magnitudes 2 0 to 2 5 approximately and very shallow focal depths about 100 m and were taken from a set of 214 events that occurred in 1991 during a spanish italian seismic experiment the events were recorded at seven single component stations belonging to the regional seismic network of andalucia at approximate distances of between 15 and 57 km from the source these events were grouped into six seismic sources according to specific criteria we used digital filtering techniques providing a significant improvement in signal to noise ratio to determine ray path group velocities and we inverted dispersion data via generalized inversion in order to obtain refined dispersion data we have carried out a further regionalization of group velocities and thus six small subregions have been resolved in group velocities the highest group velocity values from 1 93 to 2 25 km sec 1 correspond to the filabres mountain range which is an area containing materials of the nevado filabride complex of paleozoic and triassic age on the other hand low velocity values between 1 39 and 1 56 km sec 1 correspond to the alhamilla mountain range which belongs to the alpujarride complex and contains conglomerates of the cambrian and tortonian periods the velocities obtained for the neogene quaternary basin of the andarax river with materials of the tortonian and pliocene periods are also very low between 1 35 and 1 68 km sec 1 we inverted the regionalized group velocities in order to obtain the shear velocity structure of the region for depths down to 4 km according to the regional earth models that we obtained we find clear variations in velocity both laterally and vertically for several zones with different composition the filabres mountain range shows high shear velocity values 2 14 to 2 83 km sec 1 in the opposite end we have the andarax basin that presents the lowest shear velocity values consistent with its sedimentary structure 1 56 to 2 55 km sec 1 intermediate shear wave velocities characterize the remaining regions the tabernas sorbas basin the gador mountain range and the volcanic region of nijar cape of gata although the relationship between lateral changes in r g dispersion and geologic structure may not be straightforward in this study we have observed a correlation between those changes and the sharply contrasting geology between adjacent geological formations

geoacoustic inversion with ships as sources estimation of geoacoustic parameters using acoustic data from a surface ship was performed for a shallow water region in the gulf of mexico the data were recorded from hydrophones in a bottom mounted horizontal line array hla the techniques developed to produce the geoacoustic inversion are described and an efficient method for geoacoustic inversion with broadband beam cross spectral data is demonstrated the performance of cost functions that involve coherent or incoherent sums over frequency and one or multiple time segments is discussed successful inversions for the first sediment layer sound speed and thickness and some of the parameters for the deeper layers were obtained with the surface ship at nominal ranges of 20 30 or 50 water depths the data for these inversions were beam cross spectra from four subapertures of the hla spanning a little more than two water depths the subaperture beams included ten frequencies equally spaced in the 120 200 hz band the values of the geoacoustic parameters from the inversions are validated by comparisons with geophysical observations and with the parameter values from previous inversions by other invesigators and by comparing transmission loss tl measured in the experiment with modeled tl based on the inverted geoacoustic parameters

the physical basis of mb ms and variable frequency magnitude methods for earthquake explosion discrimination we use recent results from deterministic numerical modeling of earthquakes and explosions to develop an improved physical basis for earthquake explosion discrimination we use six three dimensional finite difference simulations of earthquakes for the earthquake source models and four empirical and four numerical models for explosions in different materials four factors contribute to the success of the mb ms discriminant source spectra focal mechanism near source elastic properties and interference of p spectral differences contribute about 0 5 magnitude unit to the separation between earthquake and explosion populations for large events but this difference vanishes for events with mb 4 0 using a frequency band of 0 5 3 0 hz for small events the vfm discriminant is complementary to mb ms in several ways vfm is improved by p interference while mb ms is degraded vfm is more effective for hard rock explosions while mb ms will preferentially discriminate explosions in low velocity materials and vfm is insensitive to fault orientation vfm is more sensitive to noise and regional attenuation differences than mb ms simultaneous use of both methods should allow improved discrimination of small events

occurrence of low stress drop earthquakes in the garhwal himalaya region abstract the digital seismic data recorded by a telemetered seismic array has been used to compute seismic moments and stress drops of local earthquakes in the garhwal himalaya region based on brune s model of the earthquake source the seismic moments for 18 shallow focus earthquakes range from 7 10 18 dyn cm to 6 23 10 21 dyn cm these earthquakes are found to have very low stress drops less than 1 bar for seven events and between 1 bar and 10 bars for another seven events the maximum stress drop is found to be 38 bars for an earthquake of magnitude 4 2 with focal depth 15 km the shallow focus events having low stress drops in this region indicate that the crust has low strength to withstand the accumulated strain energy

hybrid geoacoustic inversion of broadband mediterranean sea data this paper describes an acoustic experiment prosim 97 carried out to investigate inversion for seabed properties at a site off the west coast of italy where previous acoustic and geophysical studies have been performed acoustic fields were measured at a vertical hydrophone array due to a swept frequency source towed over weakly range dependent bathymetry based on the known geology the seabed is modeled as a sediment layer overlying a semi infinite basement with unknown model parameters consisting of the sediment thickness sediment and basement sound speeds source range and depth water depth at the source and array and array tilt a hybrid inversion algorithm is applied to determine the model values that minimize the mismatch with the measured acoustic fields multiple data sets are analyzed to examine the consistency of the inversion results it is found that the low sound speed of the sediment layer together with a large uncertainty in bathymetry leads to strong correlations between the water

source investigation of a small event using empirical green s functions and simulated annealing we propose a two step inversion of three component seismograms that 1 recovers the far field source time function at each station and 2 estimates the distribution of co seismic slip on the fault plane for small earthquakes magnitude 3 to 4 the empirical green s function egf method consists of finding a small earthquake located near the one we wish to study and then performing a deconvolution to remove the path site and instrumental effects from the main event signal the deconvolution between the two earthquakes is an unstable procedure we have therefore developed a simulated annealing technique to recover a stable and positive source time function stf in the time domain at each station with an estimation of uncertainties given a good azimuthal coverage we can obtain information on the directivity effect as well as on the rupture process we propose an inversion method by simulated annealing using the stf to recover the distribution of slip on the fault plane with a constant rupture velocity model this method permits estimation of physical quantities on the fault plane as well as possible identification of the real fault plane we apply this two step procedure for an event of magnitude 3 recorded in the gulf of corinth in august 1991 a nearby event of magnitude 2 provides us with empirical green s functions for each station we estimate an active fault area of 0 02 to 0 15 km2 and deduce a stress drop value of 1 to 30 bar and an average slip of 0 1 to 1 6 cm the selected fault of the main event is in good agreement with the existence of a detachment surface inferred from the tectonics of this half graben

surface wave dispersion crustal structure and sediment thickness variations across the barents shelf summary rayleigh and love wave group velocities were determined for 21 paths across the barents shelf those group velocities exhibit regional variations of 1 0 km or more at short periods depending upon the location of the path within the shelf only two different crustal shear velocity models beneath sedimentary layers are required however to explain all of the group velocity data one model pertains to most of the shelf from a longitude near the eastern coast of svalbard to novaya zemlya the other pertains to a 200 or 300km wide region at the western edge of the shelf shear velocities in the upper crust of the western region are significantly higher and the crust is much thinner than they are for the rest of the shelf that region is known to have moved to its present prosition from a point several hundred kilometres to the north during the caledonian orogeny surface wave group velocities within each of the two regions are strongly influenced by sediments which have accumulated in basins within the barents shelf some of these basins in the southern portion of the shelf may be 10 km or more in thickness

an evaluation of seismic decoupling and underground nuclear test monitoring using high frequency seismic data an effective solution to the problem of the detection and identification of low yield coupled and fully decoupled underground nuclear explosions appears available via use of high frequency seismic data ranging up to 30 or 40 hz in order to evaluate detection identification capabilities when using such data it is necessary to estimate 1 spectral characteristics and relative amplitudes of both p and s waves from explosions and earthquakes over the frequency band from 5 to 40 hz 2 signal transmission characteristics over this band through pertinent types of earth structure and 3 recording system and ground noise characteristics over this frequency band in this study each of these topics is considered in turn as they relate to detection and discrimination of the signals from low yield coupled and decoupled explosions in the regional and teleseismic distance ranges estimates of the capabilities of specific hypothetical networks to detect and identify insofar as signal to noise ratio is an important factor in identification explosions within the ussr are then considered these estimates of signal detection capability provide the central focus for the study as they serve to translate diverse and rather complex sets of observational data and theory into concrete predictions of monitoring capability following the assessment of detection capabilities the problem of identification of small events is considered with particular emphasis on discrimination at regional distances where the network is calculated to provide signals of high signal to noise ratio the principal results and conclusions of this study are as follows 1 seismic system noise can be suppressed to levels well below ground noise at quiet sites up to frequencies at least as high as 30 40 hz when using presently available hardware 2 average amplitudes of high frequency noise in a variety of geological environments are very low and change little with time or season 3 transmission of high frequency p and s wave signals in the regional distance range in stable continental areas and shields is nearly as efficient as at 1 hz with effective q factors in shield areas being about 9000 and 4000 for high frequency pn and sn respectively while the effective q for pn waves in tectonic areas is about 1000 4 a properly designed and deployed network of 25 simple three component nonarray stations internal to the ussr and 15 similar stations surrounding the ussr is predicted to be capable of multistation detection at high signal to noise ratio of fully decoupled 1 kt explosions located at all potential decoupling sites within the ussr 5 by inference from the quantitative agreement of empirical observations and theoretical predictions when using lower frequency data over a great range of explosion yields we conclude that procedures based on the use of both detectable p and s waves will serve to identify explosion generated seismic signals at least as small as those expected from a fully decoupled 1 kt explosion

source parameters for small events associated with the 1986 north palm springs california earthquake determined using empirical green functions using small events as empirical green functions which include path site and instrument effects on the p waveforms source parameters were estimated for 25 ml 3 4 to 4 4 events associated with the 1986 north palm springs earthquake the static stress drops ranged from 3 to 80 bars for moments of 0 7 to 11 1021 dyne cm there was a spatial pattern to the stress drops of the aftershocks which showed increasing values along the fault plane toward the northwest compared to relatively low values near the hypocenter of the mainshock the highest values were outside the main area of slip and are believed to reflect a loaded area of the fault that still has an higher level of stress which was not released during the main shock the relatively high stress drop of a preshock in 1983 located close to the hypocenter of the main shock is also interpreted as evidence that stress drops of magnitude 3 and 4 events can be used as indicators of the stress level in a particular area

source parameters from locally recorded aftershocks of the 9 january 1982 miramichi new brunswick earthquake abstract source parameters are derived from 111 locally recorded three component seismograms of 40 aftershocks magnitude 1 0 to 3 5 of the 9 january 1982 miramichi new brunswick earthquake this data set greatly expands the set of three component data recorded at small source to receiver distances in eastern north america preliminary analysis showed that some aftershock spectra were modified by strong site effects the need to understand and account for site effects when analyzing the aftershock data in terms of simple source models led to a two part data analysis first site effects were modeled as a seismic wave resonance in a thin layer 0 to 10 m of low seismic impedance glacial till over a high seismic impedance bedrock half space second we calculated source parameters from sh wave seismograms and amplitude spectra incorporating what had been learned about site resonances correcting for obvious site effects partially succeeded in reducing scatter in the source parameter determinations but some site dependent discrepancies remain seismic moments range from 1 10 18 to 5 10 20 dyne cm and corner frequencies from 5 to 45 hz brune model stress drops primarily range between 5 and 300 bars stress drop apparently increases with increasing seismic moment trends in apparent stress calculated from radiated energy and seismic moment parallel trends in stress drop corner frequencies from these locally recorded data are greater than those derived from regionally recorded l g waves for earthquakes of comparable moment this result contradicts a recently proposed m 0 4 constant source scaling law based primarily on l g data

regional propagation characteristics and source parameters of earthquakes in northeastern north america abstract the vertical components of the s wave trains recorded on the eastern canadian telemetered network ectn from 1980 through 1990 have been spectrally analyzed for source site and propagation characteristics the data set comprises some 1033 recordings of 97 earthquakes whose magnitudes range from m 3 to 6 the epicentral distances range from 15 to 1000 km with most of the data set recorded at distances from 200 to 800 km the recorded s wave trains contain the phases s s m s s n and l g and are sampled using windows that increase with distance the acceleration spectra were analyzed from 1 0 to 10 hz to separate the source site and propagation characteristics an inversion for the earthquake corner frequencies low frequency levels and average attenuation parameters is alternated with a regression of residuals onto the set of stations and a grid of 14 distances ranging from 25 to 1000 km the iteration between these two parts of the inversion converges in about 60 steps the average attenuation parameters obtained from the inversion were q 1997 10 and 0 998 0 003 the most pronounced variation from this average attenuation is a marked deamplification of more than a factor of 2 at 63 km and 2 hz which shallows with increasing frequency and increasing distance out to 200 km the site response spectra obtained for the ectn stations are generally flat the source spectral shape assumed in this inversion provides an adequate spectral model for the smaller events m o 21 dyne cm in the data set whose brune stress drops range from 5 to 150 bars for the five events in the data set with m o 10 23 dyne cm however the source spectra obtained by regressing the residuals suggest that an 2 spectrum is an inadequate model for the spectral shape in particular the corner frequencies for most of these large events appear to be split so that the spectra exhibit an intermediate behavior where u is roughly proportional to

source parameters of microearthquakes on an interplate asperity off kamaishi ne japan over two earthquake cycles summary we have estimated the source parameters of interplate earthquakes in an earthquake cluster off kamaishi ne japan over two cycles of m 4 9 repeating earthquakes the m 4 9 earthquake sequence is composed of nine events that occurred since 1957 which have a strong periodicity 5 5 0 7 yr and constant size m4 9 0 2 probably due to stable sliding around the source area asperity using p and s wave traveltime differentials estimated from waveform cross spectra three m 4 9 main shocks and 50 accompanying microearthquakes m1 5 3 6 from 1995 to 2008 were precisely relocated the source sizes stress drops and slip amounts for earthquakes of m2 4 or larger were also estimated from corner frequencies and seismic moments using simultaneous inversion of stacked spectral ratios relocation using the double difference method shows that the slip area of the 2008 m 4 9 main shock is co located with those of the 1995 and 2001 m 4 9 main shocks four groups of microearthquake clusters are located in and around the mainshock slip areas of these two clusters are located at the deeper and shallower edge of the slip areas and most of these microearthquakes occurred repeatedly in the interseismic period two other clusters located near the centre of the mainshock source areas are not as active as the clusters near the edge the occurrence of these earthquakes is limited to the latter half of the earthquake cycles of the m 4 9 main shock similar spatial and temporal features of microearthquake occurrence were seen for two other cycles before the 1995 m5 0 and 1990 m5 0 main shocks based on group identification by waveform similarities stress drops of microearthquakes are 3 11 mpa and are relatively constant within each group during the two earthquake cycles the 2001 and 2008 m 4 9 earthquakes have larger stress drops of 41 and 27 mpa respectively these results show that the stress drop is probably determined by the fault properties and does not change much for earthquakes rupturing in the same area the occurrence of microearthquakes in the interseismic period suggests the intrusion of aseismic slip causing a loading of these patches we also found that some earthquakes near the centre of the mainshock source area occurred just after the earthquakes at the deeper edge of the mainshock source area these seismic activities probably indicate episodic aseismic slip migrating from the deeper regions in the mainshock asperity to its centre during interseismic periods comparison of the source parameters for the 2001 and 2008 main shocks shows that the seismic moments 1 04 1016 nm and 1 12 1016 nm for the 2008 and 2001 earthquakes respectively and source sizes radius 570 m and 540 m for the 2008 and 2001 earthquakes respectively are comparable based on careful phase identification and hypocentre relocation by constraining the hypocentres of other small earthquakes to their precisely located centroids we found that the hypocentres of the 2001 and 2008 m 4 9 events are located in the southeastern part of the mainshock source area this location does not correspond to either episodic slip area or hypocentres of small earthquakes that occurred during the earthquake cycle

geoacoustic inversion of broadband data by matched beam processing this paper describes results of geoacoustic inversion using broadband signals from an experiment carried out at a site near the south florida ocean measurement centre in the florida straits m sequence coded pulse trains at different center carrier frequencies from 100to3200hz were recorded at a vertical line array at a distance around 10km geoacoustic inversion was carried out to determine the feasibility of inverting the environmental parameters from this long range propagation experiment the received signal at lower frequencies below 400hz consisted of a dominant water column signal and a secondary arrival delayed by 0 4s the secondary signal was spatially filtered by beamforming the array data and the beam data were inverted by matched beam processing in the time domain combined with an adaptive simplex simulated annealing algorithm the estimated values of compressional wave speed and density were in good agreement with ground truth values from sediment cores the inverted shear wave speed appea

preliminary results from vertical seismic profiling of oroville microearthquake s waves seismograms recorded along a 0 5 km deep borehole in oroville ca show that ground level s wave velocity spectra at this location are a product of site effects in the 6 25 to 50 hz range studied both the cleveland hill fault through which the borehole was drilled and the rock above it contribute to a substantial loss of s wave energy in first order approximation the average apparent s wave quality factor qsa along the well is 9 across the fault zone the qsa drops to a low of 3 downhole the seismograms can readily be separated into longitudinal and transverse components particle motion diagrams show that the s arrival is made up of two differently polarized waves this splitting is possibly an effect of wave propagation in the highly cracked and probably anisotropic rock of the region

a comparison of broadband and narrow band modal inversions for bottom geoacoustic properties at a site near corpus christi texas detailed comparison between narrow band and broadband techniques used in a series of acoustic experiments are presented with emphasis on a comparisons of the pressure field versus range predicted b comparisons of the bottom compressional wave speed model generated c the variance of the bottom model estimated and d the resolution of the bottom model generated the range dependence of the bottom medium and the seasonal dependence of the near surface sediment sound speed profile are observed and discussed

simultaneous determination of site responses and source parameters of small earthquakes along the atotsugawa fault zone central japan velocity seismograms recorded at nine local stations from 102 small earthquakes on the atotsugawa fault zone central japan with seismic moment from 3 10 17 to 3 10 21 dyne cm are analyzed to obtain the s wave spectra a genetic algorithm is applied to determine the earthquake source parameters and site responses simultaneously the stability of both site responses and source parameters are experimentally tested as well as the effects of different q models the resulting source parameters show that the stress drops appear to vary systematically with the location of earthquakes events located on the mozumi fault or the junction of the mozumi and the atotsugawa faults have higher stress drops than those on the creeping section and southernmost part of the atotsugawa fault the relationship between seismic moment and corner frequency constructed by combining our results and those of iio 1986 for other regions in japan shows that a single scaling relation does not apply in a moment range of 10 10 to 10 25 dyne cm a steplike change in scaling relationship occurs between 5 10 18 and 5 10 20 dyne cm corresponding to corner frequencies between 15 and 25 hz this observation suggests that earthquakes are most likely a multihierarchical system divided by characteristic sizes according to aki9s 1980 model a characteristic source dimension 100 200 meters may represent a critical size of the barriers distributed in the fault zone at which the barrier effects are balanced by the dynamic effect of the source process

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

the effects of attenuation and site response on the spectra of microearthquakes in the northeastern caribbean abstract the spectra of microearthquakes recorded by a seismic network in the northeastern caribbean were analyzed to determine the effects of attenuation and site response on these spectra the ratios of the spectral amplitudes at 5 and 20 hz were measured for several earthquakes at different distances from a fixed receiver to estimate q for p and s waves in the crust and upper mantle the average q p determined from three stations was 380 for epicentral distances between 40 and 200 km the average q s determined from two stations was estimated at 400 spectra of earthquakes closer than 40 km to one station indicate that a high q path q 800 exists for these short distances in the island arc platform the near equivalence of q p and q s observed in this study of the upper lithosphere contrasts with estimates of anderson et al 1965 that q p 9 4 q s in the asthenosphere this finding that q p is approximately equal to q s suggests that the attenuation mechanism in the upper lithosphere differs from that in the asthenosphere the spectra of nearby microearthquakes unaffected by crustal attenuation display corner frequencies which are uniform despite large differences in the seismic moments of the events the corner frequencies of these spectra appear to be characteristic of the receiver site and insensitive to the location and moment of the earthquakes at one station the s wave corner frequency of a nearby blast was identical to the corner frequency of a magnitude 4 8 earthquake derived from the record of a strong motion accelerograph at the same site

broadband sound propagation in shallow water and geoacoustic inversion part of an experiment to test a measurement package in a shallow water region in the gulf of mexico was designed to gather broadband acoustic data suitable for inversion to estimate seabed geoacoustic parameters continuous wave tow acoustic signals at multiple frequencies and broadband impulsive source signals were recorded on a horizontal line array in a high noise environment simulated annealing with a normal mode forward propagation model is utilized to invert for a geoacoustic representation of the seabed several inversions are made from different data samples of two light bulb implosions the measured sound speed profiles at the hla and at the positions of the light bulb deployments and for two different cost functions the different cost functions measured sound speed profiles and measured time series result in different inverted geoacoustic profiles from which transmission loss is generated for comparison with measurements on the basis of physical consistency and from the comparison of the transmission loss and time series a best estimate geoacoustic profile is selected and compared to those obtained from previously reported inversions uncertainties in the sound speed profile are shown to affect the uncertainties of the estimated seabed parameters

passive bottom reflection loss estimation using ship noise and a vertical line array an existing technique for passive bottom loss estimation from natural marine surface noise generated by waves and wind is adapted to use noise generated by ships the original approach based on beamforming of the noise field recorded by a vertical line array of hydrophones is retained however additional processing is needed in order for the field generated by a passing ship to show features that are similar to those of the natural surface noise field a necessary requisite is that the ship position relative to the array varies over as wide a range of steering angles as possible ideally passing directly over the array to ensure coverage of the steepest angles the methodology is illustrated through simulation and applied to data from a field experiment conducted offshore of san diego ca in 2009

broadband localization by matched fields in range and bearing in shallow water data from a moving source received by three horizontally separated bottomed sensors in a shallow water environment are analyzed using a broadband matched field approach to source localization the water depth was approximately 150 m and the low bottom loss in the area results in low spatial coherence of the broadband field the modeled acoustic fields are constructed via a ray model localizations are made in the x y plane in a large search area the time evolution of the ambiguity surfaces produced from the data agree fairly well with simulated surfaces ambiguity surfaces are generated successively at intervals of about 1 6 s in small search areas with a fine spatial grid to test the possibility of tracking a source results of the source tracking are presented the analysis also considers the effects of the sea floor on localization in the context of spatial coherence

comparison of dynamic source parameters for earthquakes in different tectonic regions of the northern cascadia subduction zone abstract based on characteristics of displacement amplitude spectra of near distant s waves from 52 earthquakes in the magnitude m l range 0 6 to 5 4 dynamic source parameters were computed and compared for three tectonic regions in the northern cascadia subduction zone a downgoing slab of the juan de fuca plate with earthquake depths of about 30 km the same slab further east with deeper events about 60 km and an overriding part of the north american plate no distance dependence of observed corner frequencies was obtained nor any significant regional difference in source parameters although there is an indication of smaller stress drop in the north american plate for larger events factors which may distort a regional comparison were examined but neither source radiation directivity nor propagation path and station site characteristics have significant influence the structural model has influence however and two different density velocity models were used for the deeper juan de fuca slab events seismic moments m 0 range from e 17 to 0 4 e 24 dyn cm source radii from 0 07 to 1 6 km stress drops from less than 0 1 to 40 bar and average dislocations from less than 0 1 to 7 cm in the range e 18 to e 21 dyn cm in which the vast majority of the events are confined both the stress drop and displacement increase rapidly with event size seismic moment or magnitude as the seismic moment increases rapidly with fault dimension the relation between seismic moment dyn cm and m l magnitude computed from data from all three regions is similar to relations presented for california and reads log m 0 17 65 1 02 m l there are however differences between events in the north american plate and the juan de fuca plate with larger m l for the former at equivalent seismic moment

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

inversion for p and s wave attenuation structure long valley caldera california p and s wave amplitude data from digital three component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three dimensional attenuation structure in the upper crust of long valley caldera california differential attenuation q 1 values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios the spectral ratios are inverted for q 1 using the lsqr algorithm zones of high positive p wave q 1 greater p wave attenuation are observed at a depth of 4 5 km beneath the east flank of mammoth mountain and at 6 8 km beneath the southwestern edge of the resurgent dome a region of positive s wave q 1 is seen at 7 8 km beneath the southern edge of the resurgent dome whereas at 6 7 km the s wave results are near average the region of positive s wave q 1 at 7 8 km beneath the resurgent dome may indicate the roof of a magmatic system the positive p wave q 1 at 4 5 km beneath mammoth mountain and 6 8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water these zones may represent some of the source regions for the long valley geothermal system

proof of principle for inversion of vector sensor array data acoustic data collected from a 22 m long horizontal line array with elements comprised of omni directional hydrophone sensors co located with acceleration vector sensor triplets are analyzed for seabed geo acoustic information broadband and continuous wave tone data were recorded from the passage of a surface vessel source the r v montague in an august 2006 experiment conducted in monterey bay the water depth at the array and along the source track was approximately 85 m simultaneous inversions for source track and geo acoustic parameters were performed with 18 tones from the 62 415 hz tone data for source ranges 0 5 2 0 km the efficacy of inversions with vector sensor data is demonstrated by the fact that data from vector sensor acceleration components parallel to the array line of bearing produced inversion solutions approximately identical to the solutions obtained from the inversion of data for the same tones from the hydrophone array components in addition the source spectra derived from these inversions are in nominal agreement with the source spectra obtained from an independent measurement

test of tectonic models by great circle rayleigh waves summary a large set of published great circle rayleigh wave phase velocities in the period range 125 350s is used to compare three recent tectonic models okal leveque jordan prior to any regionalization the symmetry property of the great circle integrals is used to obtain a lower limit of the signal noise ratio in the data it turns out that the signal is responsible for at least 30 per cent of the data variance in the period range 175 300s a standard regression method is applied for computing the pure path velocities and the model efficiency is derived from a variance analysis it is shown that even at great depth none of the three models explains more than 60 per cent of the energy due to the long wavelength lateral heterogeneities 6500 km the three models have nearly the same efficiency for explaining the short period data t 125s between 200 and 300s the higher performance of okal s model indicates that it is important to separate the subduction zones from the other orogenic zones by perturbing the lateral extension of the subduction zones it comes out that they constitute on both sides of the subducting slabs wider anomalies than often assumed suggesting large downgoing flows on the contrary the effect of surface features such as marginal seas are restricted to a close region in front of the trenches finally the anomalous ellipticity values deduced directly from great circle data are partly explained by a coupling between tectonics and ellipticity

source parameter analysis of microearthquakes recorded around the underground gas storage in the montello collalto area southeastern alps italy abstract the study of seismic source parameters is crucial for understanding the origin of seismicity and retrieving information on the energy balance and the stress involved in earthquake rupture processes in active tectonic areas where underground industrial activities are carried out such parameters may help to understand whether earthquakes are induced triggered or natural the montello collalto area southeastern alps is located in an active tectonic environment and hosts a depleted natural reservoir used to store gas since 2012 a high quality seismic network monitors the microseismicity occurring around the underground gas storage reservoir to understand if the storage activity might induce seismicity in this paper we estimate the source parameters of low magnitude events representative of the seismicity occurring in the area surrounding the reservoir the analysis includes a preliminary removal of the site effects specifically computed within this study from all the records then using a parametric multistep inversion scheme we estimate the seismic moment the corner frequency and the static stress drop that can be set as reference for the microseismicity occurring in the study area all the investigated earthquakes show low seismic efficiency compatible with overshoot processes which is typical of natural i e tectonic earthquakes our procedure can be implemented in other tectonic regions hosting underground industrial activities to support the decisional processes related to real time monitoring

stress drop distribution of the aftershocks of the 1993 hokkaido nansei oki earthquake the hokkaido nansei oki earthquake m 7 8 occurred off the south west coast of hokkaido japan at 22 17 jst on july 12 1993 we observed its aftershocks by broad band seismometers at five stations for about four months our analysis of the broad band data indicates that the stress drops of aftershocks have a regional dependency the stress drops in the central part of the aftershock zone are larger than those in the northern and the southern parts this result suggests that the stress field in this region is closely related to some previous seismic events that occurred in the vicinity

a comparison of coda and s wave spectral ratios as estimates of site response in the southern san francisco bay area abstract we calculate spectral ratios for s waves and codas to evaluate the amplification of 14 sites in the southern san francisco bay area relative to a nearby bedrock site in the coyote hills our data are seismograms written by loma prieta aftershocks the epicentral distances and azimuths to the stations are effectively the same all the sites in the study are amplified with respect to the reference site at frequencies from 0 5 to 7 hz the shapes of the s wave and coda spectral ratios are similar but the coda ratios are greater than the s wave ratios by as much as a factor of 4 the difference is larger for sites on alluvial and bay mud deposits particularly at frequencies around 1 hz suggesting the presence of waves trapped in the alluvial basin in general the length of the analysis window affects the s wave spectral ratios for alluvial sites longer windows give ratios similar to coda ratios apparently because these windows include more of the phases that contribute to the coda we classify the sites according to their geological characteristics and surficial shear wave velocities for the s wave ratios the differences between the classes showed no systematic trend the softest and hardest soil classes we consider have practically identical s wave amplifications the average coda ratios for the site classes clearly increase as the soil classes include slower and softer materials after correction for differences in reference sites the coda amplifications are very similar to the relative amplifications for these site classes estimated by borcherdt and glassmoyer 1992 from the strong motion recordings of the 1989 loma prieta earthquake

nonlinear inversion for ocean bottom properties high resolution methods based on simulated annealing and full wave sound propagation models are developed for nonlinear inversion for ocean bottom properties simulated annealing is used to search the high dimensional parameter space of ocean bottoms for the parameter set corresponding to the best replica field the parabolic equation method is used to solve range dependent inversion problems for data taken by lynch et al from a range dependent region of the gulf of mexico j acoust soc am 89 648 665 1991 this approach achieves excellent agreement between the theoretical and measured acoustic pressures the recovered sediment parameters suggest that a sound speed boundary layer exists in the upper part of the sediment and that the depth of an interface in the sediment is range dependent for locally range independent problems inversion is performed in wave number space large efficiency gains are possible with this approach because the number of wave number samples required for inversion is mu

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

empirical green s function analysis of loma prieta aftershocks abstract following the 18 october 1989 loma prieta earthquake an array of 22 three component iris passcal instruments was deployed in the santa cruz mountains we use an empirical green9s function method to study source properties of aftershocks as well as attenuation in the epicentral area using pairs of closely spaced events we show that the largest aftershocks are well modelled by an 2 source spectrum and we obtain estimates of the p wave corner frequency for a total of 15 events brune stress drops are observed to have no systematic variation over the range of moments spanned by the data set down to 10 19 dyne cm there is a suggestion that aftershocks that occurred outside the mainshock rupture have higher stress drops than those that occurred within the mainshock rupture a m l 3 4 foreshock in the subsequence associated with the 18 april 1990 m l 5 4 watsonville earthquake yields a stress drop estimate of 820 bars this value is well outside the stress drops obtained for all other events 6 to 266 bars site specific attenuation parameters suggest large lateral and vertical heterogeneity in p wave q structure station holy in a valley near the surface trace of the san andreas fault is characterized by low q values while stations to the north of the aftershock zone are characterized by the highest q values

matched field geoacoustic inversion with a horizontal array and low level source this paper applies geoacoustic inversion to acoustic field data collected on a bottom moored horizontal line array due to a continuous wave towed source at a shallow water site in the barents sea the source transmitted tones in the frequency band of 30 160hz at levels comparable to those of a merchant ship with resulting signal to noise ratios of 9 15db bayesian inversion is applied to cross spectral density matrices formed by averaging spectra from a sequence of time series segments snapshots quantifying data errors including measurement and theory errors is an important component of bayesian inversion to date data error estimation for snapshot averaged data has assumed either that averaging reduces errors as if they were fully independent between snapshots or that averaging does not reduce errors at all this paper quantifies data errors assuming that averaging reduces measurement error dominated by ambient noise but does not reduce theory modeling error providing a physically reasonable

imaging of the near surface shear wave velocity structure of the granada basin southern spain the seismic velocity structure of the granada basin southern spain at depths of 1 4 km is investigated by dispersion analysis of 0 5 to 5 9 sec rayleigh waves we have used records from 12 quarry blasts detonated between 1990 and 2000 and 16 local earthquakes with magnitudes between 2 6 and 3 8 that occurred between 1990 and 1997 all events were recorded on the regional seismic network of andalucia spain and on the national seismic network p s and rg phases were recorded at 13 stations at distances between 10 and 72 km from the source the analysis and inversion of rg waveforms is performed using multiple filtering techniques and a generalized inversion approach we measured ray path group velocities from 0 83 to 2 15 km sec in the 0 6 to 5 8 sec period interval and obtained shear wave velocity profiles for 29 source station paths the shear velocities lie between 0 8 and 3 34 km sec within a standard deviation band of 0 05 0 14 km sec from these models it is possible to infer the most conspicuous features in the upper 4 km of the region and the existence of some degree of lateral variation in velocity after applying an imaging method aimed at volumetric modeling and data visualization we present the first velocity images obtained for the granada basin our results characterize the basin and are correlated with the surface geology manuscript received 7 september 2001

earthquake source parameters and the frequency dependence of attenuation at coalinga mammoth lakes and the santa cruz mountains california least squares best fits were calculated to the fourier amplitude spectra of p and s waves of locally recorded earthquakes attenuation amplification and earthquake source parameters were calculated from parameters of the fits approximately 8200 spectra were fit at 39 sites at coalinga mammoth lakes and the santa cruz mountains california typically the spectra were fit from 2 to 45 hz for p waves and 2 to 20 hz for s waves the source spectral falloff of p waves was limited to be no greater than f 2 5 f is frequency by low attenuation sites in the sierra nevada attenuation was measured through t the travel time divided by q attenuation was considered to be frequency dependent according to t to f which is equivalent to q qof negative values of were found to minimize the sum of squared residuals at most of the sites studied attenuation and amplification varied from site to site systematically with site geology source parameters were determined for individual earthquakes by averaging fit parameters from between four and 18 spectra depending on the earthquake seismic moments ranged from roughly 5 1012 to 1015 nm at coalinga and mammoth lakes moments ranged from 2 1011 to 3 1014 nm in the santa cruz mountains when the fits were determined using negative values of a the seismic moments trended approximately as fc 3 fc is the corner frequency for 0 5 stress drops varied from roughly 0 2 to 50 mpa at coalinga and mammoth lakes stress drops were lower in the santa cruz mountains ranging from approximately 0 05 to 8 mpa

azimuthal variation of short period rayleigh waves from cast blasts in northern arizona we have examined the generation and propagation of short period sp rayleigh waves rg from two cast blasts of similar yield 726 000 kg of explosives delay sequence and near source geology at a coal mine in northern arizona the blasts were oriented approximately perpendicular to each other allowing for radiation pattern studies at regional distances near source seismic p and lg phases the observation of these rg radiation patterns at regional distances could act as a cast blast discriminant in regions of monitoring concern

site response for seattle and source parameters of earthquakes in the puget sound region we analyzed seismograms from 21 earthquakes ml 2 0 4 9 recorded by digital seismographs we deployed in urban seattle to determine site response and earthquake stress drops the seismometers were situated on a wide variety of geologic units including artificial fill e g kingdome harbor island pleistocene age soils glacial till and outwash deposits of seattle s hills modified land downtown seattle space needle and tertiary sedimentary rock two mainshock aftershock sequences were recorded the june 1997 bremerton sequence mainshock ml 4 9 and the february 1997 south seattle sequence mainshock ml 3 5 along with other events in the puget sound region we developed a new inversion procedure to estimate site response source corner frequencies and seismic moments from the s wave spectra this inversion uses corner frequencies determined from spectral ratios of mainshock aftershock pairs as constraints the site responses found from the inversion are not relative to the rock site but are relative to an idealized site with a flat frequency response the response of the rock site is also found from the inversion the inversion results show high response for the sites on artificial fill more moderate amplification for most sites on stiff pleistocene soils or modified land and low response for the rock site some sites display resonances such as a strong 2 hz resonance at our site near the kingdome which is caused by the surficial layers of fill and younger alluvium the sites in west seattle exhibit high amplification even though they are on relatively stiff soils of glacial outwash this may be partly caused by basin surface waves produced by conversion of incident s waves this high response in west seattle is consistent with damage reports from the 1949 mb 7 1 and 1965 mb 6 5 earthquakes stress drop estimates for the events we recorded were generally low between 0 4 and 25 bars although some of the events may have had higher stress drops that could not be resolved because of the limited passband we calculated a stress drop of 24 bars for the bremerton mainshock and 10 bars for the south seattle mainshock

source pulse enhancement by deconvolution of an empirical green s function observations of the earthquake source time function are enhanced if path recording site and instrument complexities can be removed from seismograms assuming that a small earthquake has a simple source its seismogram can be treated as an empirical green s function and deconvolved from the seismogram of a larger and or more complex earthquake by spectral division when the deconvolution is well posed the quotient spectrum represents the apparent source time function of the larger event this study shows that with high quality locally recorded earthquake data it is feasible to fourier transform the quotient and obtain a useful result in the time domain in practice the deconvolution can be stabilized by one of several simple techniques in this paper the method is implemented and tested on high quality digital recordings of aftershocks of the jan 9 1982 miramichi new brunswick earthquake in particular seismograms from a jan 17 aftershock 017 13 33 gmt local mag 3 5 exhibit path or site effects which complicate the determination of source parameters after deconvolution the apparent far field source of this event is a simple pulse in displacement with duration 0 07 second for both p and s

source parameters of some significant earthquakes near koyna dam india seismic moment stress drop source radius and fault dislocation have been determined for nineteen significant earthquakes in the konya area india using displacement spectra of shear waves computed from strong motion accelerograph records though the stress drop shows a definite increasing trend with the seismic moment its correlations with source radius and corner frequency indicate that a constant stress drop of 170 bars represents a good mean value to describe the source mechanism of the koyna dam earthquakes an empirical relationship also has been established between magnitude and seismic moment

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

asperities barriers characteristic earthquakes and strong motion prediction recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip stable asperities and barriers which survive many earthquakes can explain these results the activity of volcano usu japan since 1977 which produced a dacite dome with a total upheaval of 180 m offered a great opportunity to study the recurrence behavior of a fault seismologists at hokkaido university revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall along which the dome moved upward this family can be explained by the existence of a stable asperity on the crater wall and is called the asperity type family they found however another type of family of earthquakes which share the same fault plane but show a great range of variation in the amount of slip the behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers and this family is referred to as the barrier type family this type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from california and the results reveal the stability of local stress drop among different earthquakes the agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault

observations of hard rock site effects abstract differences were measured in seismograms recorded at eight proximate hardrock sites five in tunnels and three on outcrops on hills tunnel sites affected the amplitude of incident signals by as much as a factor of three at frequencies inversely proportional to the depth of the site into the tunnel for example the maximum effect of a site 12 m deep was at 25 hz while that of a site 85 m deep was at 11 hz most of the observed tunnel effects could be explained in terms of interference between the incident and surface reflected waves rock outcrop sites on hills affected the amplitude of incident signals by as much as a factor of eight at frequencies inversely proportional to the dimension of the hill and more for horizontally than for vertically polarized signals hard rock site effects typically varied by no more than a factor of three for input signals from different earthquakes having widely different azimuth and incident angles

dispersion of rg waves in southeastern maine evidence for lateral anisotropy in the shallow crust abstract group velocities of short period rayleigh waves rg are sensitive to variations in the velocity structure of the upper few kilometers of the crust in this study we analyzed dispersive properties of rg waves recorded from blasts detonated as part of the maine seismic refraction experiment conducted in 1984 by the u s geological survey the period range of the rg waves in this study is 0 4 to 1 6 sec group velocities in this period range are sensitive to the velocity structure at depths ranging from very near the surface to about 2 km group velocities were found to vary significantly from one path to the next a simple explanation of the observed pattern of group velocities is that higher velocities were observed for paths parallel to the structural grain of the appalachians than for paths transverse to the grain for example 1 sec rg waves have group velocities of about 3 0 to 3 2 km sec for paths parallel to the grain and about 2 6 to 2 7 km sec for paths transverse to the grain thus our results suggest that the shallow crust beneath the appalachians of southeastern maine is laterally anisotropic rg group velocities from this study were inverted to obtain shear wave velocity models of the upper 1 5 to 2 0 km of the crust underlying southeastern maine to compare our results with refraction and laboratory results v p was estimated from v s using a v p v s ratio of 1 78 inverting the average group velocities for all paths in this study resulted in a v p model with velocities ranging from 4 9 km sec in the upper 0 3 km to 6 2 km sec at 2 km depth the range in observed rg group velocities from one path to the next is evidence of a variation in shallow crustal structure depending on geographical location and or orientation of the paths in the upper 0 3 km v p appears to range from about 4 8 to 5 3 km sec and at 2 km depth v p appears to range from about 5 6 to 6 6 km sec

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

inversion for geometric and geoacoustic parameters in shallow water experimental results experimental results on the estimation of both geometric and geoacoustic parameters in shallow water are presented genetic algorithms are used for estimation of the forward model parameters the estimated parameters are then used by a standard bartlett processor for source localization a stationary source at a range of 5 6 km and a moving source at ranges from 5 8 7 7 km were successfully localized in range and depth using a single frequency bartlett processor the results indicate that global estimation of the forward model parameters significantly improves source localization performance

ground motions underground compared to those on the surface a case study from sudbury ontario we compare signals recorded on hard rock seismographic stations in sudbury ontario to those recorded in caverns below the surface stations in the sudbury neutrino observatory signals from shallow local events show strong apparent amplification of ground motion observed on the surface relative to those recorded at depths of 1 4 2 1 km the amplification peaks near 2 1 hz where it is on average a factor of 3 to 5 the peak narrows decreases in amplitude and shifts toward lower frequencies 0 8 1 hz for regional and teleseismic events the peak in apparent amplification for the local events is attributed to a strong rg phase from shallow events at close distances the frequency shift and decay of the amplitude of this peak for more distant events reflects the rapidly diminishing influence of the higher frequency components of this phase with distance coupled with the increasing importance of shear wave phases

attenuation of short period p waves at mount st helens an increase of seismicity occurred at mount st helens between may and july 1998 with more than 900 events md 2 2 recorded by the pacific north west seismograph network this article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before january 1995 april 1998 during and after august december 1998 the period of higher seismicity and recorded on three different one component seismic stations epicentral distances of the studied events ranged between 0 5 and 15 km we performed the analysis distinguishing the deep depth 5 5 km from shallow depth 5 5 km events we used the frequency decay method to estimate the quality factor q and station site correction s for p waves in the frequency bands 2 7 and 18 30 hz the results show that the attenuation varies from site to site systematically and decreases with depth lower qp values are obtained for focal depths less than 5 5 km moreover the spatial variations of qp show minimum values 30 in the crater area that are interpreted as due to a low density mass distribution under the crater we find that qp was 30 higher before the period of high seismicity than after this change may be attributed to an increase of pressure in the magma chamber producing new cracks and or reopening of pre existing cracks which are the most viable mechanisms for increasing attenuation

shallow velocity structure at the shagan river test site in kazakhstan during 1997 and 1998 twelve chemical explosions were detonated in boreholes at the former soviet nuclear test site near the shagan river sts in kazakhstan the depths of these explosions ranged from 2 5 to 550 m while the explosive yield varied from 2 to 25 tons the purpose of these explosions was for closure of the unused boreholes at sts and each explosion was recorded at local distances by a network of seismometers operated by los alamos national laboratory and the institute of geophysics for the national nuclear center nnc short period fundamental mode rayleigh waves rg were generated by these explosions and recorded at the local stations resultingly the waves exhibited normal dispersion between 0 2 and 3 seconds dispersion curves were generated for each propagation path using the multiple filter analysis and phase match filtering techniques tomographic maps of rg group velocity were constructed and show a zone of relatively high velocities for the southwestern sw region of the test site and slow propagation for the northeastern ne region for 0 5 sec rg the regions are separated by the 2 1 km sec contour as propagation in the sw is greater than 2 1 km sec and less in the ne region at 1 0 sec period the 2 3 km sec contour separates the two regions finally for 1 5 and 2 0 sec the separation between the two regions is less distinct as velocities in the ne section begin to approach the sw except for a low velocity region 2 1 km sec near the center of the test site local geologic structure may explain the different regions as the sw region is composed predominantly of crystalline intrusive rocks while the ne region consists of alluvium tuff deposits and paleozoic sedimentary rocks low velocities are also observed along the shagan river as it passes through the sw region of the test site for shorter period rg 0 5 1 0 sec iterative least squares inversions of the rg group velocity dispersion curves show shear wave velocities for the southwestern section that are on average 0 4 km sec higher than the ne region at depths greater than 1 5 km the statistical difference between the models is no longer significant the observed group velocities and different velocity structures correlate with p wave complexity and with spatial patterns of magnitude residuals observed from nuclear explosions at sts and may help to evaluate the mechanisms behind those observations

the magnitude frequency distribution of earthquakes recorded with deep seismometers at cajon pass southern california abstract the cumulative b value the slope of the gutenberg richter relationship between earthquake occurrence rate and magnitude is commonly found to be constant 1 network catalogues however reveal a decrease at small magnitudes b value is not just an artifact of catalogue incompleteness but that small earthquakes are really not as numerous as a constant b value extrapolated from larger events would predict in the cajon pass area southern california the b value of seismicity recorded by the local network scsn appears to decrease below about m l 1 6 in order to investigate whether this decrease is real or simply represents the network detection threshold we use seismicity recorded by the deep 1 5 and 2 5 km seismometers deployed in the cajon pass scientific drillhole between april 1992 and october 1994 the maximum amplitudes recorded downhole are compared to scsn magnitudes for events recorded by the network to determine the relationship between amplitude and m l as a function of hypocentral distance from the borehole magnitudes are then calculated for 1300 earthquakes which occurred within 40 km of the borehole magnitude frequency curves are calculated for those events within 18 km of the borehole and a constant b value is observed to m l 0 5

source parameters of the 23 april 1992 m 6 1 joshua tree california earthquake and its aftershocks empirical green s function analysis of geos and terrascope data source parameters of the m 6 1 23 april 1992 joshua tree mainshock and 86 m 1 8 to 4 9 aftershocks are determined using an empirical green s function methodology for the aftershocks deconvolved p and s wave spectra are calculated for 126 pairs of closely spaced events recorded on portable geos stations s wave spectra from the two horizontal components are averaged the deconvolved spectra are fit by a ratio of omega square source models yielding an optimal least squares corner frequency for both the large and the small event in each pair we find no resolved difference between the inferred p and s wave corner frequencies using the standard brune model for stress drop we also find no resolved nonconstant scaling of stress drop with moment although we also conclude that detailed scaling systematics would be difficult to resolve in particular a weak increase of stress drop with moment over a limited moment magnitude cannot be ruled out for magnitudes smaller than m 3 to 3 5 the inferred stress drop values will be limited by the maximum observable corner frequency value of 60 hz for the mainshock source time functions are obtained from mainshock recordings at three terrascope stations pfo pas and gsc using an m 4 3 foreshock as an empirical green s function the results indicate a fairly simple single pulse source time function with clear south to north directivity and an inferred rupture radius of 5 to 6 km the deconvolved source time functions are inverted to obtain a finite rupture model that gives a robust estimate of rupture dimension early aftershocks are found to lie along the perimeters of regions with high mainshock slip the inferred mainshock stress drop value 56 bars is within the range determined for the aftershocks our derived mainshock source spectra do not show resolvable deviation from the omega square model

constraints on lateral variation in upper crustal structure beneath southern new england from dispersion of rg waves group velocity dispersion was determined for rayleigh waves between periods of 0 5 and 2 0 sec rg recorded from quarry blasts in southern new england the paths of these surface waves traverse various geologic structures and distinct dispersion regions have been identified normal dispersion was consistently observed in the period range studied indicating a superficial layer of relatively low velocity 4 1 vp 5 5 at least 1 km thick in the upper crust throughout this region seismic velocity in this upper layer varies laterally and correlates to some extent with geologic features the observed dispersion also indicates that seismic velocity at depths of 5 to 10 km beneath southern new england varies laterally

high frequency spectral falloff of earthquakes fractal dimension of complex rupture b value and the scaling of strength on faults the high frequency falloff omega sup minus gamma of earthquake displacement spectra and the b value of aftershock sequences are attributed to the character of spatially varying strength along fault zones the author assumes that the high frequency energy of a main shock is produced by a self similar distribution of subevents where the number of subevents with radii greated than r is proportional to r sup minus d d being the fractal dimension in this model an earthquake is composed of a hierarchical set of smaller earthquakes the static stress drop is parameterized to be proportional to r sup eta and strength is assumed to be proportional to static stress drop he finds that a distribution of subevents with d 2 and stress drop independent of seismic moment eta 0 produces a main shock with an omega sup minus 2 falloff if the subevent areas fill the rupture area of the main shock by equating subevents to islands of high stress of a random self similar stress field on a fault he relates d to the scaling of strength on a fault such that d 2 minus eta thus d 2 corresponds to constant stress drop scaling eta 0 and scale invariant fault strength amore self similar model of aftershock rupture zones on a fault is used to determine the relationship between the b value the size distribution of aftershock rupture zones and the scaling of strength on a fault the b value for aftershock sequences on a fault is found to equal 3 minus 1 5 eta 3 eta therefore this model indicates that the typically observed spectral falloffs of omega sup minus 2 and b values of 1 can be entirely caused by scale invariant strength eta 0 along fault zones less

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

source parameters and rupture velocities of microearthquakes in western nagano japan determined using stopping phases we use an inversion method based on stopping phases imanishi and takeo 2002 to estimate the source dimension ellipticity and rupture velocity of microearthquakes and investigate the scaling relationships between source parameters we studied 25 earthquakes ranging in size from m 1 3 to m 2 7 that occurred between may and august 1999 at the western nagano prefecture japan which is characterized by a high rate of shallow earthquakes the data consist of seismograms recorded in an 800 m borehole and at 46 surface and 2 shallow borehole seismic stations whose spacing is a few kilometers these data were recorded with a sampling frequency of 10 khz in particular the 800 m borehole data provide a wide frequency bandwidth with greatly reduced ground noise and coda wave amplitudes compared with surface recordings high frequency stopping phases appear in the body waves in hilbert transform pairs and are readily detected on seismograms recorded in the 800 m borehole after correcting both borehole and surface data for attenuation we also measure the rise time which is defined as the interval from the arrival time of the direct wave to the timing of the maximum amplitude in the displacement pulse the differential time of the stopping phases and the rise times were used to obtain source parameters we found that several microearthquakes propagated unilaterally suggesting that all microearthquakes cannot be modeled as a simple circular crack model static stress drops range from approximately 0 1 to 2 mpa and do not vary with seismic moment it seems that the breakdown in stress drop scaling seen in previous studies using surface data is simply an artifact of attenuation in the crust the average value of rupture velocity does not depend on earthquake size and is similar to those reported for moderate and large earthquakes it is likely that earthquakes are self similar over a wide range of earthquake size and that the dynamics of small and large earthquakes are similar

rupture characteristics and tomographic source imaging of ml 3 earthquakes near anza southern california the rupture characteristics of several ml 3 earthquakes near anza california are determined from time domain analysis of path corrected displacement pulses these pulses are corrected for propagation effects apparent attenuation scattering site resonance using the waveforms of adjacent small events ml 2 1 as empirical green s functions the earthquakes studied occurred during two swarm sequences near cahuilla in an area characterized by swarm activity at shallow depth 5 km the data consist of seismograms recorded by the anza seismic network a high dynamic range local array with digital telemetry the waveforms of adjacent small events ml 2 1 are deconvolved from those of the ml 3 events to yield the source time functions of the larger earthquakes the ml 3 earthquakes display a variety of rupture modes similar to those reported for large shocks but over much shorter time scales and distances events with single and multiple ruptures are observed relative locations of the subevents are obtained from azimuthal differences in the time separation of pulses in the deconvolved displacement waveforms some events as small as ml 2 4 display unilateral rupture propagation as manifested by azimuthal variations in the pulse widths of the deconvolved displacement pulses a tomographic back projection technique is applied to the deconvolved displacement pulses to image the slip velocity on the fault as a function of time after nucleation and distance along the fault using a one dimensional fault model the tomographic inversion for one of the events ml 2 4 reveals a unilateral rupture with rupture velocity about equal to the shear wave velocity vs and fault length of 300 m a similar inversion for an ml 3 7 event resolves two subevents separated by about 200 m and yields an average rupture velocity of 0 8vs these rupture velocities are similar to those reported for large earthquakes indicating that rupture velocity is independent of seismic moment down to 3 1 1019 dyn cm the smallest event studied there are clear variations in pulse width for neighboring events with similar seismic moments implying significant differences in static stress drops factor of five variations in stress drop 11 61 bars are calculated for events within 200 m of each other thus the stress and or strength along this fault zone varies considerably over distances of hundreds of meters

an intraslab seismic sequence activated by the 2011 tohoku oki earthquake evidence for fluid related embrittlement 1 using quantitative analysis of waveforms we investigate a seismic cluster that occurred at a depth of 67 km in the philippine sea slab 8 months after the 11 march 2011 megathrust tohoku oki earthquake mw 9 0 the sequence started with an m 4 1 normal fault event on 14 november in the deepest part of the cluster and subsequent earthquakes migrated upward by 6 km along a narrow conduit like zone the earthquakes have stress drops of 0 5 40 mpa and groups of earthquakes with coherent waveforms are observed we explain these observations in terms of fluid related embrittlement and the migration of overpressured fluids the low permeability plate interface of the pacific plate may have been broken by the coseismic or postseismic slips of the tohoku oki earthquake with the fluids subsequently being liberated from the underlying crust of the pacific slab and migrating into the philippine sea slab due to the pore pressure gradient the tensional stresses generated by the coseismic slip promoted the efficient upward migration of fluids and also acted to enhance the deviatoric stress around the source area the heightened pore pressures and the resulting reduced effective normal stress lowered the strength of the faults sufficiently to bring the system into the brittle regime under the enhanced deviatoric stress the lag of 8 months may represent the time needed for the unsealing of the plate interface the upward migration of fluids and the increase in pore pressure to become sufficient to overcome the lithostatic pressure

source parameters of a m4 8 and its accompanying repeating earthquakes off kamaishi ne japan implications for the hierarchical structure of asperities and earthquake cycle 1 we determine the source parameters of a m4 9 0 1 characteristic earthquake sequence and its accompanying microearthquakes at 50 km depth on the subduction plate boundary offshore of kamaishi ne japan the microearthquakes tend to occur more frequently in the latter half of the recurrence intervals of the m4 9 0 1 events our results show that the microearthquakes are repeating events and they are located not only around but also within the slip area for the 2001 m4 8 event from the hierarchical structure of slip areas and smaller stress drops for the microearthquakes compared to the m4 8 event we infer the small repeating earthquakes rupture relatively weak patches in and around the slip area for the m4 8 event and their activity reflects a stress concentration process and or change in frictional property healing at the area we also infer the patches for the m4 9 0 1 and other repeating earthquakes undergo aseismic slip during their interseismic period

a seismotectonic analysis of the anza seismic gap san jacinto fault zone southern california small earthquake epicenters near the anza seismic gap define a 20 km quiescent segment of fault bounded to the northwest and southeast by areas of relatively high seismicity recent moderate earthquakes on and near the san jacinto fault in the gap and their relatively depressed aftershock activity indicate that the fault is seismogenic and highly stressed but locked by some mechanism the locked nature of the fault may be due to relatively high compressive stress normal to the fault resulting from the convergent geometries of the local active discontinuous faults and the oblique orientation of the regional maximum compressive stress strain is not being relieved by aseismic fault creep a swarm of small earthquakes in the crustal block 13 km southwest of the anza gap beneath the cahuilla valley recently released stress in an area which was previously highly active before the 1918 m 6 8 and 1937 m l 6 0 earthquakes the occurrence of these periods of increased seismicity near cahuilla in the years immediately before the nearby closer than 35 km large earthquakes and the recent swarm suggest that the ground beneath cahuilla may be acting as a stress meter signaling the presence of high stresses before large local earthquakes the length of the quiescent fault segment suggests potential for about an m 6 5 earthquake if the entire segment ruptures at once

the effects of site response on source parameters deduced for the 1980 long valley california earthquake sequence data taken from the university of nevada unr array of surface located wideband digital displacement seismographs placed over the 1980 long valley earthquake sequence show very strong site effects which control spectral parameters and hence inferred source parameters up to ml 5 0 observations from 4 000 seismograms show that differences in site effects can lead to inferred seismic moments at least a factor of two lower than previously reported observed s wave spectral corner frequencies show upper bound cutoffs which are widely disparate at different sites e g 15 hz at mg1 and 6 hz at wt1 thus fmax is quite variable by site this high frequency cutoff clearly a site effect at wt1 could be misinterpreted as an apparent departure from source similarity for seismic moments below 1021 dyn cm estimates of seismic moment and source duration from the unr rock sites may be more accurate than those previously published both because the unr instrumentation has greater resolution at longer periods 0 1 to 1 0 hz and because of less near site signal distortion at high frequencies

high frequency observations and source parameters of microearthquakes recorded at hard rock sites we have estimated the source parameters of 53 microearthquakes recorded in july 1983 which were aftershocks of the miramichi new brunswick earthquake that occurred on 9 january 1982 these events were recorded by local three component digital seismographs at 400 sps component from 2 hz velocity transducers sited directly on glacially scoured crystalline basement outcrop hypocentral distances are typically less than 5 km and the hypocenters and the seven digital seismograph stations established all lie essentially within the boundaries of a granitic pluton that encompasses the faults that ruptured during the main shock and major aftershocks the p wave velocity is typically 5 km sec at the surface and at least 6 km sec at depths greater than about 1 km the events have s wave corner frequencies in the band 10 to 40 hz and the calculated brune model seismic moments range from 1015 to 1018 dyne cm the corresponding stress drops are generally less than 1 0 bars but there is considerable evidence that the seismic source signals have been modified by propagation and or site effects the data indicate a there is a velocity discontinuity at 0 5 km depth b the top layer has strong scattering attenuating properties c some source receiver paths differentiate the propagated signal d there is a hard rock site p wave f max between 50 and 100 hz and e some hard rock sites are characterized by p wave resonance frequencies in the range 50 to 100 hz comparison of this dataset with the january 1982 new brunswick digital seismograms which were recorded at sites underlain by several meters of low velocity surface sediments suggests that some of the hard rock site phenomena listed above can be explained in terms of a layer over a half space model for microearthquakes this result implies that spectrally determined source dimension scales with site dimension thickness of the layer more generally it emphasizes that it is very difficult to accurately observe the details of seismogenic processes from the earth s surface

tomographic inversion of rg wave group velocities for regional near surface velocity structure the tomographic inversion of group velocities of rg waves is a viable technique for mapping lateral variations in the lithology of sedimentary basins in order to determine a velocity structure we first use the moving window maximum entropy spectral analysis to measure the average rg wave group travel times second the group velocities within blocks subregions covering the area sampled by the rg wave propagation paths are determined by tomographic inversion third the vertical shear wave velocity profile within each block is determined from the inversion of group velocities for that block finally a structural cross section is obtained by connecting blocks located along a profile rg waves recorded from mining explosions in alabama were used in a test of the technique by modeling the paleozoic sedimentary rocks in the black warrior basin of northern alabama the high resolution of moving window maximum entropy spectral analysis allows estimation of the rg wave group velocities with a precision of better than 3 the usable frequency range of 0 5 to 1 5 hz allows determination of structures to depths of about 2 4 km with layers of 0 2 km thickness blocks of widths of 20 km or less could be resolved with appropriate data shear wave velocities in the black warrior basin range from 1 6 to 4 0 km s the layering in the velocity structure corresponds to major geologic units within units an increase in velocity to the northeast corresponds to observed facies changes

tomographic inversion of geoacoustic properties in a range dependent shallow water environment this paper presents a matched field tomographic method to estimate the geoacoustic properties of the ocean bottom for a range dependent medium in shallow water the inversion method has been developed in order to interpret experimental data from the haro strait primer sea trial this experiment was carried out in june 96 and used low frequency broadband signals that were received on three vertical line arrays inversion of the data is particularly difficult because of the complex bathymetry of the haro strait experimental site for this inversion a range dependent ray code was developed to solve the forward problem allowing an arbitrarily layered bottom environment the inversion scheme is based on modeling the propagation time and the amplitude of the recorded data and a simple new cost function is proposed the signal ray paths are identified automatically using a simple process that compares calculated and measured travel times data from multiple source positions are used to invert the range dependence of the geoacoustic model the environment is separated into segments and within each segment the inversion is carried out layer by layer for a multilayer geoacoustic model starting with the topmost layer the range dependent thickness and sound speed are estimated via a monte carlo method inversion results are presented for synthetic and experimental data from the haro strait sea trial

seismic imaging of the site response using microearthquake recordings part ii application to the swabian jura southwest germany seismic network the subsurface site structure of the swabian jura southwest germany seismic network has been modeled from the inversion of locally recorded sh wave microearthquake seismograms the observed records have been deconvolved from the effects of apparent source pulse broadening due to the combined influence of frequency bandlimited source signals absorption peg leg multiple reflections and the recording system following the procedure of scherbaum and stoll 1985 corresponding pseudo reflection seismograms were calculated based on the kunetz claerbout relation for sh waves and inverted using a levison recursion algorithm the final structural models have been optimized in terms of numbers of layers and depth resolution using ferber s 1985 noise stabilization technique the resulting impedance models for the swabian jura southwest germany seismic network show some strongly correlated reflectors in the uppermost layers which are well understood in terms of the local stratigraphy the comparison of the inverted impedance model hsn with the p wave velocity log from a nearby exploration drilling trochtelfingen beb shows a high degree of similarity of the dominating reflectors down to a depth of several hundred meters as a consequence the coda generating process for the early part of the coda of the observed swabian jura microearthquake seismograms wood anderson magnitude mwa 3 0 can be completely interpreted as an effect of the local subsurface structure

geoacoustic inversion of short range source data using a plane wave reflection coefficient approach acoustic time series data were collected in a shallow hard bottom lake environment located in central texas using both short range 2 m implosive data obtained with the source and a single hydrophone located near mid depth in the waveguide along with longer range implosive and explosive data from a near surface source to a bottom mounted hydrophone matched field inversions using simulated annealing were performed with a ray trace plus complex plane wave reflection coefficient forward propagation model that was validated in previous work isolating bottom interacting paths to perform the inversions is shown to be essential to reduce parameter uncertainties in the hard bottom environment and enables a systematic approach to the inversions which establishes the number of layers needed to represent the lake environment measured transmission loss data from a towed source were compared through a rms error analysis to modeled transmission loss constructed with the parameters from inversions of data from several source types to further establish the validity of the inversion approach for this environment geoacoustic parameters obtained by inversions of short range low frequency impulsive data are used to predict transmission loss at longer ranges and higher frequencies the range dependence of the global minimum is discussed

scholte wave tomography for shallow water marine sediments summary we determine the 3 d in situ shear wave velocities of shallow water marine sediments by extending the method of surface wave tomography to scholte wave records acquired in shallow waters scholte waves are excited by air gun shots in the water column and recorded at the seafloor by ocean bottom seismometers as well as buried geophones our new method comprises three steps 1 we determine local phase slowness values from slowness frequency spectra calculated by a local wavefield transformation of common receiver gathers areal phase slowness maps for each frequency used as reference in the following step are obtained by interpolating the values derived from the local spectra 2 we infer slowness residuals to those reference slowness maps by a tomographic inversion of the phase traveltimes of fundamental scholte wave mode 3 the phase slowness maps together with the residuals at different frequencies define a local dispersion curve at every location of the investigation area from those dispersion curves we determine a model of the depth dependency of shear wave velocities for every location we apply this method to a 1 km2 investigation area in the baltic sea northern germany the phase slowness maps obtained in step 2 show lateral variation of up to 150 per cent the shear wave velocity models derived in the third step typically have very low values 60 80 m s 1 in the top four meters where fine muddy sands can be observed and values exceeding 170 m s 1 for the silts and sands below that level the upper edge of glacial till with shear wave velocities of 300 400 m s 1 is situated approximately 20 m below sea bottom a sensitivity analysis reveals a maximum penetration depth of about 40 m below sea bottom and that density may be an important parameter best resolvable with multimode inversion

seabed acoustics of a sand ridge on the new jersey continental shelf acoustic measurements were made on a sand ridge on the new jersey continental shelf data collected on two l arrays separated by 20km from a single multi frequency tow suggest small horizontal environmental variability values for the sound speed structure of the seabed are extracted by first applying a geo acoustic inversion method to broadband and narrowband acoustic data from short range sources then a parabolic equation algorithm is used to properly include the bathymetry and sub bottom layering finally the frequency dependence of the seabed attenuation is inferred by optimizing the model fit to long range transmission loss data in the 50 3000hz band

seismic recordings of the carlsbad new mexico pipeline explosion of 19 august 2000 on 19 august 2000 two seismometer networks in southeastern new mexico recorded signals from a natural gas pipeline explosion analysis of the particle motion arrival times and durations of the seismic signals indicates that three impulsive events occurred with origin times of 11 26 18 8 1 9 11 26 43 6 2 1 and 11 27 01 7 2 0 uct the first event was caused by the explosive blowout of the buried high pressure pipeline and the second event was caused by the ignition of the vented natural gas the nature of the third event is unclear however it was likely created by a secondary ignition there were also two extended seismic events that originated at the same time as the first two impulsive events the first resulted from the preignition venting of the gas and lasted for about 24 sec while the second resulted from the postignition roaring of the flames and lasted for about 1 hr many of the source constraints provided by the seismic data were not available from any other investigative technique and thus were valuable to a diverse range of parties including the new mexico state police law firms involved in litigation related to the accident the national transportation and safety board and the general public manuscript received 13 september 2002

bayesian geoacoustic inversion of ship noise on a horizontal array this paper applies geoacoustic inversion to low frequency narrow band acoustic data from a quiet surface ship recorded on a bottom moored horizontal line array in shallow water a bayesian matched field inversion method is employed which quantifies geoacoustic uncertainties and allows for meaningful comparison of inversion results from different data sets geoacoustic inversion results for ship noise data are compared with inversion results for multitone data from a towed controlled source collected in the same experiment and with independent geophysical measurements to increase the information content of low level ship noise data the effect of including multiple independent data segments in the inversion is investigated and shown to significantly reduce geoacoustic parameter uncertainties geoacoustic uncertainties are also shown to depend on ship range and orientation with increased uncertainties for long ranges and for the ship stern oriented away from the array

source parameters of microearthquakes at mount st helens usa summary we estimate the source parameters for a selection of microearthquakes that occurred at mount st helens in the period 1995 1998 excluding the activity of 2004 september this time period includes the most intense episode of earthquake activity since the last dome building eruption in 1986 october 200 seismograms were processed to obtain seismic moments source radii stress drops and average fault slip the source parameters were determined from the spectral analysis of p waves after correction for attenuation and site effects in particular p wave quality qp and site s factors have been previously calculated in the frequency ranges 2 7 hz and 18 30 hz because it was impossible to perform corrections for qp and s over the whole spectrum we applied a new approach based on the notion of holed spectrum to estimate spectral parameters the term holed spectrum indicates a spectrum lacking corrected spectral amplitude values at certain frequencies we carried out a statistical study to verify that dealing with the holed spectrum does not lead to significant differences in the estimates of spectral parameters we also investigated the dependence of spectral parameters low frequency level corner frequency and high frequency decay on the bandwidth of spectral hole and defined the threshold values for three different spectral models displacement holed spectra corrected by attenuation and site response are then used to determine spectral parameters in order to calculate seismic source parameters seismic moments range from 1017 to 1019 dyne cm source dimensions from 100 to 350 m and average fault slip from 0 003 to 0 1 cm self similarity seems to break down in that stress drops are very low 0 1 1 bars we postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime

mapping shallow heterogeneity with teleseismic p to rg scattered waves over 9000 records of teleseismic p wave coda from the southern california seismic network are used to map lateral variations in p to rg scattering strength and the group velocity of scattered rg building on the results of revenaugh 1995a we use the correlation of scattering strength and topographic roughness as an indicator of model fit to estimate group velocity dispersion over five frequency bands between 0 and 3 hz rg group velocity displays normal dispersion decreasing from 2 7 0 3 km sec for frequencies between 0 and 0 6 hz to a low of 2 2 0 4 km sec for the 2 0 to 3 0 hz band lateral heterogeneity is introduced by assuming rg velocity is a function of local topographic roughness grid search migration analysis yields group velocities of 2 2 and 2 85 km sec for smooth and rough terrain respectively and a 150 m threshold for the local standard deviation of topography distinguishing smooth from rough the map of scattering strength approximates topographic roughness throughout southern california implying that teleseismic p wave coda duration and relative amplitude level depend strongly on near surface 3 km geology

microearthquake spectra from the anza california seismic network site response and source scaling abstract we analyzed spectra of local microearthquakes recorded by the anza california seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes spectra of microearthquakes m m o 19 dyne cm at anza have shapes characteristic of the receiver sites and are generally independent of the source region thus the site response is a major conditioner of the observed spectral shape to remove the effects of site response from the spectra of a m 3 event and isolate its source spectrum we divided by the spectra of an adjacent aftershock used as an empirical green9s function event the spectral ratios indicate that the apparent corner frequencies of small earthquakes m o 19 dyne cm observed at even the high f max stations on hard rock are much lower than the source corner frequencies the spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment for events with moments as small as 10 18 dyne cm the spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra indicating that the spectral division effectively removed the site response the source spectrum of the m 3 event has a high frequency spectral fall off of about 2 an apparent dependence of high frequency fall off with seismic moment in the original spectra can also be explained by the effects of site response the difference between the p and s wave corner frequencies and high frequency roll offs in the observed spectra for these events is the result of the site response and is not a source property the shapes of the spectra of microearthquakes at anza can largely be explained by attenuation at shallow depth with a frequency independent q for some sites near surface resonances are also apparent in the spectra of microearthquakes it is indicated by t values determined for each site that q p q s for the shallow low q layer further evidence of low near surface q is observed in the anomalous spectra of an unusually shallow earthquake source depth 1 km in the network the spectra from this shallow event are depleted in high frequency energy at most stations relative to those of deeper events this observation can be explained by a low q surficial zone for stations of the network situated on alluvium this low q layer has a maximum thickness of about 3 km and maximum p and s wave q values of 30 to 50

determination of rupture duration and stress drop for earthquakes in southern california a simple technique is developed for determining the rupture duration and stress drop of earthquakes between magnitudes 3 5 and 4 0 using the time between the p wave onset and the first zero crossing 1 2 on seismograms from local seismic networks this method is applied to 10 main shocks in southern california to investigate regional variations in stress drop the initial pulse widths of 65 foreshocks or aftershocks of these events were measured values of 1 2 for small earthquakes below about magnitude 2 2 are generally observed to remain constant with decreasing magnitude in four sequences studied the relative pulse width of a particular main shock m 3 5 at a given station is found to be correlated with the relative pulse width of its aftershocks recorded at that station these observations are interpreted to signify that the waveforms of these small events m 2 2 are essentially the impulse response of the path between the source and receiver values of 1 2 determined from small foreshocks and aftershocks are therefore subtracted in effect deconvolved from those of each main shock to obtain an estimate of the rupture duration of the main shock which is corrected for path effects significant variations in rupture duration and stress drop are observed for the main shocks studied aftershock locations and azimuthal variations in 1 2 both indicate that the rupture zone of one earthquake expanded unilaterally a factor of 10 variation in stress drop is calculated for two adjacent events of similar seismic moments occurring 1 hr apart on the san jacinto fault system the first event in this pair had the highest stress drop of the events studied 860 bars and was followed within 8 months by a magnitude 5 5 earthquake 2 km away

differential analysis of coda q using similar microearthquakes in seismic gaps part 2 application to seismograms recorded by the parkfield high resolution seismic network abstract nearly two thirds of the microseismicity occurring at parkfield california consists of repeating clusters of closely located events exhibiting waveform cross correlation values exceeding 0 95 this observation coupled with a high probability for an m 6 earthquake within the next decade working group on california earthquake probabilities 1988 make this segment of the san andreas fault system a prime location to search for temporal variations in coda q associated with crustal changes that may occur during the earthquake cycle the coda comparison analysis method described in part 1 of this study aster et al 1996 is applied to data from four borehole stations of the parkfield high resolution seismic network hrsn for the period january 1987 through february 1994 we examine 21 of the nearly 300 repeating clusters observed to date the restriction of the data set to only the most highly similar sources is observed to be critical small differences in the source process even for event pairs with centroid locations within approximately 20 m significantly increase the spurious noise level in the parameter estimates absolute 68 confidence bound estimates obtained on the repeatability of coda q for all microearthquake pairs are 10 for frequencies below 20 hz tighter constraints are obtainable by reconstructing the temporal history of coda q from the first difference measurements between microearthquake pairs these functional reconstructions show that the mean value of coda q has not varied by more than approximately 5 from 3 to 30 hz at the 1 level during the study period however other observables such as an increased level of microseismicity suggest that a preparatory stage for the next parkfield earthquake has begun indicating that coda q is so far showing no sign of sensitivity to preparatory processes at parkfield

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

determination of near surface attenuation with κ parameter to obtain the seismic moment stress drop source dimension and seismic energy for microearthquakes in the granada basin southern spain abstract a set of 43 microearthquakes of the granada basin with magnitude duration m d from 1 4 to 3 5 have been spectrally analysed the digital data used in this study was recorded by two short period digital seismic network five permanent stations using andalusian seismic network red sismica de andalucia rsa and three stations of a portable digital seismic network red sismica portatil rsp the displacement spectra for p and s waves were analysed with brune s j geophys res 75 1970 4997 j geophys res 76 1971 5002 source model and the spectra parameters were determined by snoke s model to correct spectra from path attenuation coda q was used for the spectra correction from near surface attenuation parameter was calculated obtaining it from acceleration spectra the values of parameter range from 0 01 to 0 04 s for p waves and from 0 006 to 0 04 s to s waves once the spectra of these attenuation effects was corrected we obtained the seismic moments that range from 5 45 10 17 dyne cm to 1 53 10 20 dyne cm the source radii are between 0 13 and 0 39 km the seismic energy ranges from 7 17 10 3 j to 1 13 10 8 j the stress drop values were below 4 bars the scaling relations between seismic moment and stress drop indicated decreasing stress drop with decreasing seismic moment