crustal structure of a transform plate boundary san francisco bay and the central california continental margin wide angle seismic data collected during the bay area seismic imaging experiment provide new glimpses of the deep structure of the san francisco bay area block and across the offshore continental margin san francisco bay is underlain by a veneer 300 m of sediments beneath which p wave velocities increase rapidly from 5 2 km s to 6 0 km s at 7 km depth consistent with rocks of the franciscan subduction assemblage the base of the franciscan at 15 18 km depth is marked by a strong wide angle reflector beneath which lies an 8 to 10 km thick lower crust with an average velocity of 6 75 0 15 km s the lower crust of the bay area block may be oceanic in origin but its structure and reflectivity indicate that it has been modified by shearing and or magmatic intrusion wide angle reflections define two layers within the lower crust with velocities of 6 4 6 6 km s and 6 9 7 3 km s prominent subhorizontal reflectivity observed at near vertical incidence resides principally in the lowermost layer the top of which corresponds to the 6 s reflector of brocher et al 1994 rheological modeling suggests that the lower crust beneath the 6 s reflector is the weakest part of the lithosphere the horizontal shear zone suggested by furlong et al 1989 to link the san andreas and hayward calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust a transect across the continental margin from the paleotrench to the hayward fault shows a deep crustal structure that is more complex than previously realized strong lateral variability in seismic velocity and wide angle reflectivity suggests that crustal composition changes across major transcurrent fault systems pacific oceanic crust extends 40 50 km landward of the paleotrench but contrary to prior models probably does not continue beneath the salinian block a cretaceous arc complex that lies west of the san andreas fault in the bay area the thickness 10 km and high lower crustal velocity of pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby pioneer seamount the salinian block consists of a 15 km thick layer of velocity 6 0 6 2 km s overlying a 5 km thick high velocity 7 0 km s lower crust that may be oceanic crust cretaceous arc derived lower crust or a magmatically underplated layer the strong structural variability across the margin attests to the activity of strike slip faulting prior to and during development of the transcurrent pacific north american plate boundary around 29 ma

inverse kinematic and forward dynamic models of the 2002 denali fault earthquake alaska we perform inverse kinematic and forward dynamic models of the m 7 9 2002 denali fault alaska earthquake to shed light on the rupture process and dynamics of this event which took place on a geometrically complex fault system in central alaska we use a combination of local seismic and global positioning system gps data for our kinematic inversion and find that the slip distribution of this event is characterized by three major asperities on the denali fault the rupture nucleated on the susitna glacier thrust fault and after a pause propagated onto the strike slip denali fault approximately 216 km to the east the rupture abandoned the denali fault in favor of the more southwesterly directed totschunda fault three dimensional dynamic models of this event indicate that the abandonment of the denali fault for the totschunda fault can be explained by the totschunda fault s more favorable orientation with respect to the local stress field however a uniform tectonic stress field cannot explain the complex slip pattern in this event we also find that our dynamic models predict discontinuous rupture from the denali to totschunda fault segments such discontinuous rupture helps to qualitatively improve our kinematic inverse models two principal implications of our study are 1 a combination of inverse and forward modeling can bring insight into earthquake processes that are not possible with either technique alone and 2 the stress field on geometrically complex fault systems is most likely not due to a uniform tectonic stress field that is resolved onto fault segments of different orientations rather other forms of stress heterogeneity must be invoked to explain the observed slip patterns

site response in las vegas valley nevada from nts explosions and earthquake data we report site response in las vegas valley lvv from historical recordings of nevada test site nts nuclear explosions and earthquake recordings from permanent and temporary seismic stations our data set significantly improves the spatial coverage of lvv over previous studies especially in the northern deeper parts of the basin site response at stations in lvv was measured for frequencies in the range 0 2 5 0 hz using standard spectral ratios ssr and horizontal vertical spectral ratios hvr for the ssr measurements we used a reference site approximately nehrp b rock classification located on frenchman mountain outside the basin site response at sedimentary sites is variable in lvv with average amplifications approaching a factor of 10 at some frequencies we observed peaks in the site response curves at frequencies clustered near 0 6 1 2 and 2 0 hz with some sites showing additional lower amplitude peaks at higher frequencies the spatial pattern of site response is strongly correlated with the reported depth to basement for frequencies between 0 2 and 3 0 hz although the frequency of peak amplification does not show a similar correlation for a few sites where we have geotechnical shear velocities the amplification shows a correlation with the average upper 30 meter shear velocities v 30 we performed two dimensional finite difference simulations and reproduced the observed peak site amplifications at 0 6 and 1 2 hz with a low velocity near surface layer with shear velocities 600 750 m s and a thickness of 100 200 m these modeling results indicate that the amplitude and frequencies of site response peaks in lvv are strongly controlled by shallow velocity structure

the site response of two rock and soil station pairs to strong and weak ground motion abstract the site response to strong and weak ground motion depends largely on the subsurface conditions at the soil site for the two rock soil station pairs studied the first station pair consists of a soft soil site treasure island and a sandstone and shale site yerba buena island these stations recorded strong ground shaking from the loma prieta mainshock and weak ground motion from four aftershocks the range of peak ground acceleration is from approximately 0 00006 to 0 07 g at the rock site compared to the rock site the strong ground motion at the soft soil site is amplified by a factor of about 3 over a frequency range from 0 5 to 2 0 hz the amplification is much higher for weak motion and suggests a dependence on signal amplitude for example near 1 hz the site response shows an increasing amplification as magnitude and the peak velocity at the rock site decreases for events of local magnitude 7 0 4 3 4 1 3 5 and 3 3 the maximum soil site amplifications are 4 12 17 19 and 25 respectively a second station pair consisting of a stiff soil site gilroy 2 and a sandstone site gilroy 1 was also studied with contrasting results these two stations recorded strong ground shaking from the 1979 coyote lake 1984 morgan hill and 1989 loma prieta mainshocks weak ground motion was recorded at these stations after the loma prieta mainshock the range of peak ground acceleration is from 0 006 to 0 43 g at the rock site unlike the results for the soft soil study above the estimated stiff soil site responses are not significantly different for strong and weak motion from 0 5 to 2 0 hz near 0 7 hz the stiff soil site responses range from 2 5 to 4 5 for strong ground shaking from three mainshocks and from 1 5 to 4 0 for weak ground shaking from thirteen aftershocks

crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

modeling and validation of a 3d velocity structure for the santa clara valley california for seismic wave simulations a 3d seismic velocity and attenuation model is developed for santa clara valley california and its surrounding uplands to predict ground motions from scenario earthquakes the model is developed using a variety of geologic and geophysical data our starting point is a 3d geologic model developed primarily from geologic mapping and gravity and magnetic surveys an initial velocity model is constructed by using seismic velocities from boreholes reflection refraction lines and spatial autocorrelation microtremor surveys this model is further refined and the seismic attenuation is estimated through waveform modeling of weak motions from small local events and strong ground motion from the 1989 loma prieta earthquake waveforms are calculated to an upper frequency of 1 hz using a parallelized finite difference code that utilizes two regions with a factor of 3 difference in grid spacing to reduce memory requirements cenozoic basins trap and strongly amplify ground motions this effect is particularly strong in the evergreen basin on the northeastern side of the santa clara valley where the steeply dipping silver creek fault forms the southwestern boundary of the basin in comparison the cupertino basin on the southwestern side of the valley has a more moderate response which is attributed to a greater age and velocity of the cenozoic fill surface waves play a major role in the ground motion of sedimentary basins and they are seen to strongly develop along the western margins of the santa clara valley for our simulation of the loma prieta earthquake

simulated ground motion in santa clara valley california and vicinity from m 6 7 scenario earthquakes abstract models of the santa clara valley scv 3d velocity structure and 3d finite difference software are used to predict ground motions from scenario earthquakes on the san andreas saf monte vista shannon south hayward and calaveras faults twenty different scenario ruptures are considered that explore different source models with alternative hypocenters fault dimensions and rupture velocities and three different velocity models ground motion from the full wave field up to 1 hz is exhibited as maps of peak horizontal velocity and pseudospectral acceleration at periods of 1 3 and 5 sec basin edge effects and amplification in sedimentary basins of the scv are observed that exhibit effects from shallow sediments with relatively low shear wave velocity 330 m sec scenario earthquakes have been simulated for events with the following magnitudes 1 m 6 8 7 4 calaveras sources 2 m 6 7 6 9 south hayward sources 3 m 6 7 monte vista shannon sources and 4 m 7 1 7 2 peninsula segment of the saf sources ground motions are strongly influenced by source parameters such as rupture velocity rise time maximum depth of rupture hypocenter and source directivity cenozoic basins also exert a strong influence on ground motion for example the evergreen basin on the northeastern side of the scv is especially responsive to 3 5 sec energy from most scenario earthquakes the cupertino basin on the southwestern edge of the scv tends to be highly excited by many peninsula and monte vista fault scenarios sites over the interior of the evergreen basin can have long duration coda that reflect the trapping of seismic energy within this basin plausible scenarios produce predominantly 5 sec wave trains with greater than 30 cm sec sustained ground motion amplitude with greater than 30 sec duration within the evergreen basin

three dimensional finite difference modeling of the san andreas fault source parameterization and ground motion levels olsen et al 1995 recently simulated an mw 7 75 earthquake on the san andreas fault predicting long period t 2 5 sec ground velocities of 140 cm sec in the los angeles basin about 60 km from the fault these motions are much larger than estimates derived from empirical relations or other numerical simulations standard area magnitude relations predict that the 170 16 km fault used in the simulations would produce an mw 7 5 earthquake giving a moment of 2 0 1027 dyne cm which is 2 4 times smaller than the moment used by olsen et al 1995 further self similar scaling predicts a rise time of 3 sec for an mw 7 75 event and 2 2 sec for an mw 7 5 event the filtered impulse slip function used by olsen et al 1995 has an effective rise time of 1 6 sec yielding a response that is about 2 times larger than expected for periods less than 5 sec this combination of high seismic moment and short rise time along with the use of a uniform slip distribution leads to the extreme ground motion levels predicted by olsen et al 1995 to quantify the sensitivity of the long period ground motion response to source parameterization we have performed 3d finite difference fd simulations using various combinations of seismic moment source rise time and slip heterogeneity these calculations incorporate the same grid dimensions fault size and bandwidth employed by olsen et al 1995 with a moment of 2 0 1027 dyne cm a rise time of 2 sec and a smoothly heterogeneous slip distribution we simulate peak long period ground velocities of 155 cm sec in the near fault region and 40 cm sec in the los angeles basin these values are much closer to although still higher than empirical predictions a uniform slip distribution produces the largest peak motions both in the near fault region and in the los angeles basin whereas a rough asperity slip distribution noticeably reduces the maximum near fault ground velocities our results indicate that the accurate simulation of long period ground motions requires a realistic source parameterization including appropriate choices of seismic moment and rise time as well as the use of spatial and temporal variations in slip distribution

broadband waveform modeling of moderate earthquakes in the san francisco bay area and preliminary assessment of the usgs 3d seismic velocity model abstract recently the united states geologic survey earthquake hazards program developed a 3d seismic velocity model of northern california based on geology including a detailed model of the urbanized san francisco bay area in this study we report comparisons of observed three component broadband 0 03 0 25 hz waveforms with synthetic seismograms computed with the new 3d model using an elastic finite difference method we selected a set of 12 moderate earthquakes m w 4 0 5 1 that occurred within the greater san francisco bay area having well constrained source parameters and broadband recordings with high signal to noise ratios the objective of this study was to investigate how well the 3d velocity model predicts observed waveforms and to identify features of the model that may require revision to improve the waveform fits and predictions of ground motions for future events we show for the 3 september 2000 yountville earthquake that reported source parameters accurately predict waveforms at two close strong motion stations approximately 10 km from the epicenter by comparing synthetics for the average 1d model gil7 stidham et al 1999 and the 3d structures we show that the effects of seismic wave propagation in the 3d model become important for frequencies at and above 0 1 hz periods less than 10 sec comparison of observed and synthetic seismograms for the 3d model consistently shows that the model predicts energy arriving earlier than is observed particularly for the surface waves indicating that the shear velocities in the upper crust must be reduced we cross correlated the observed and synthetic waveforms and recorded the delay time and linear correlation for best alignment of the data and delayed synthetic the results indicate that generally the 3d model predicts the observed waveforms well mean linear correlations 0 41 and includes features that arise from the interaction of the wave field with 3d structure especially the major sedimentary basins in san pablo bay san francisco bay santa clara valley and livermore valley simple conversion of the observed delay times for optimal alignment suggests shear velocities should be reduced by 4 5 on average based on these findings we conclude that the model is an excellent first step suggesting that the overall structure of the model is accurate i e the basin and discontinuity geometry however the velocities must be reduced to improve the observed timing of surface wave arrivals

a fault model with variable slip duration for the 1989 loma prieta california earthquake determined from strong ground motion data abstract a finite fault model with variable slip duration is inferred from strong ground motion data for the loma prieta earthquake unlike previous models slip duration is found to be consistent with fault width scaling slip duration varies between 1 and 6 sec at points along the fault surface with values between 3 and 6 sec where slip amplitudes obtain or exceed the average slip of 98 cm modest high frequency modifications of the slip function shape greatly enhance the data fit without significantly changing the inferred static offset or rupture characteristics this model exhibits bilateral rupture with the propagation rate of the main energy release of 3 km sec or less the moment is 2 3 10 26 dyne cm and the largest slip amplitudes occur northwest of the hypocenter the rake varies with position along the fault from dominantly strike slip in the southeast to dominantly reverse slip in the northwest

regional wave propagation in new england and new york we validate and improve 1d velocity models of the two main crustal provinces in the northeastern united states neus using seismograms from the 20 april 2002 m 5 au sable forks earthquake which is the largest earthquake in the region to be recorded by multiple recently deployed good quality regional broad band stations to predict and mitigate the effects of future earthquakes in the north eastern united states more information is needed regarding both the local earthquake sources and how seismic waves travel through the region we investigate the source and regional wave propagation for the au sable forks earthquake the earthquake epicenter is located near the boundary of two distinct geological provinces the appalachian new england and grenville new york we use a forward modeling approach to study the waveforms recorded at 16 stations located within 400 km of the epicenter we generate synthetic seismograms using the frequency wavenumber method testing several published models for the two provinces several models per form well at low frequencies 0 1 hz we refine these models and generate two alternative 1d crustal models for intermediate frequencies 1 hz of engineering interest our new grenville model performs better than previously published models for all six source station paths modeled in that province according to goodness of fit statistics variance reduction and correlation coefficient our alternative appalachian model improves the fit of synthetics to data for five of the ten paths modeled in that province from the results we identify two specific sources of wave field complex ities that should be investigated in future studies of earthquake ground motions in neus 3 azimuthal anisotropy in the appalachian province and complex wave paths along the boundary between the two provinces

site response shallow shear wave velocity and wave propagation at the san jose california dense seismic array ground motion records from a 52 element dense seismic array near san jose california are analyzed to obtain site response shallow shear wave velocity and plane wave propagation characteristics the array located on the eastern side of the santa clara valley south of the san francisco bay is sited over the evergreen basin a 7 km deep depression with miocene and younger deposits site response values below 4 hz are up to a factor of 2 greater when larger regional records are included in the analysis due to strong surface wave development within the santa clara valley the pattern of site amplification is the same however with local or regional events site amplification increases away from the eastern edge of the santa clara valley reaching a maximum over the western edge of the evergreen basin where the pre cenozoic basement shallows rapidly amplification then decreases further to the west this pattern may be caused by lower shallow shear wave velocities and thicker quaternary deposits further from the edge of the santa clara valley and generation trapping of surface waves above the shallowing basement of the western evergreen basin shear wave velocities from the inversion of site response spectra based on smaller local earthquakes compare well with those obtained independently from our seismic reflection refraction measurements velocities from the inversion of site spectra that include larger regional records do not compare well with these measurements a mix of local and regional events however is appropriate for determination of site response to be used in seismic hazard evaluation since large damaging events would excite both body and surface waves with a wide range in ray parameters frequency wavenumber plane wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array conventional high resolution and multiple signal characterization f k power spectra and stacked slowness power spectra are compared these spectra show surface waves generated scattered at the edges of the santa clara valley and possibly within the valley at the western edge of the evergreen basin

moderate earthquake ground motion validation in the san francisco bay area abstract we performed 3d ground motion simulations for 10 recent small to moderate earthquakes m w 4 1 5 4 in the san francisco bay area to evaluate two versions of the usgs3d velocity model brocher 2005 jachens et al 2006 brocher 2008 comparisons were made in terms of modeling phase arrival timing peak ground motion amplitudes and the seismic waveforms in the simulations we assumed the source parameters reported in the berkeley seismological laboratory bsl moment tensor catalog broadband seismic data from the berkeley digital seismic network bdsn and strong motion data from the usgs and the california geological survey cgs strong motion arrays were used in the analysis the comparison of peak ground velocity pgv for both models reveals that both 3d models predict the observed pgv well over four orders of magnitude and p and s wave timing and pseudospectral acceleration psa are well modeled by the 3d structure while the revised model model 8 3 0 significantly improved the timing of the first arrival and the waveform fit is generally good there remain discrepancies in estimated amplitudes and durations that require improvements to the structure nevertheless from our low frequency 0 5 hz analysis we found that the 3d model is suitable for the simulation of pgv to assess the strong shaking hazard of future large earthquakes because earthquakes larger than m 6 have pgv carried by waves of 1 to several seconds period

the pacifica earthquake of 28 april 1979 abstract an example of how first motion polarities can precisely fix lateral velocity variations along active faults is given by an earthquake m l 4 4 and m o 3 4 10 21 dyne cm that occurred on 28 april 1979 at 0044 utc along the san andreas fault zone in the vicinity of pacifica 37 38 7 n 122 28 0 w near the western coastline of san francisco peninsula this moderate earthquake is of particular interest in that it is the largest and best recorded earthquake that has occurred in the region in the past 20 yr the well constrained fault plane solution indicates that there is approximately a 20 per cent velocity contrast across the fault zone with the velocity increasing toward the southwest and that the earthquake had a right lateral strike slip focal mechanism orientated parallel to the strike of the san andreas fault the distribution of the duration of the first half cycle of the p wave with azimuth indicated that the rupture proceeded unilaterally northwestward along the san andreas fault at a velocity of 1 6 0 3 km sec in contrast the fault plane solution for the m l 5 3 earthquake of 22 march 1957 centered about 4 km northwest of the 28 april 1979 earthquake had a reverse faulting mechanism with the continental northeast side rising with respect to the oceanic side unlike other moderate earthquakes which have occurred in the region in the past 22 yr this earthquake had no detectable foreshocks or aftershocks the depth of focus is estimated to be 12 9 1 0 km

aftershock slip behavior of the 1989 loma prieta california earthquake an analysis of 745 aftershocks of the m7 1 loma prieta earthquake of 17 october 1989 reveals a wide variety of focal mechanisms at the northwestern end of the aftershock zone earthquakes that apparently occurred off the main rupture plane exhibit mechanisms with predominantly reverse slip on planes nearly parallel to the san andreas fault at the southeastern end the mechanisms exhibit right lateral motion on near vertical planes suggesting that these aftershocks involve slip on the san andreas fault few of the aftershock mechanisms in the central zone resemble the main shock mechanism strike n130 degree e dip 70 degree sw rake 104 degree but instead exhibit reverse right lateral left lateral and normal motion on planes subparallel to the main shock rupture plane the dip of the aftershock zone is parallel to the main shock slip plane and includes the main shock hypocenter however the lack of agreement between the main shock and the aftershock mechanisms suggests that few of the aftershocks occurred on the main shock slip plane this behavior is consistent with observations of aftershock sequences for other dip slip events and also with studies indicating that main shock rupture zones are at all other times mostly aseismic if the stress drop for the main shock relievedmore most of the tectonic stress the mechanisms could reflect the heterogeneity of the near field stress redistribution alternatively the variety of the aftershock mechanisms may reflect deformation by block motion within a narrow zone adjacent to the main shock rupture plane less

reflected seismic waves and their effect on strong shaking during the 1989 loma prieta california earthquake our data indicate that critical and postcritical reflections from crustal layers and the moho produced increased shaking at discrete distances along the san francisco peninsula during the 1989 loma prieta earthquake these reflections may have produced an increase in amplitude that is as much as 10 times greater than that of the direct arrival peak amplitude distance patterns measured from explosive sources synthetic seismograms aftershocks and the mainshock of the 1989 loma prieta earthquake indicate that 1 point sources such as explosions produce similar peak amplitude distance relations as distributed double couple sources when the crustal structure is approximately a poisson solid 2 peak amplitudes from smaller point sources explosions may be scaled to those of larger sources earthquakes and 3 reflections caused a pattern of high amplitudes at specific distances along the san francisco peninsula that geographically correlates with areas of reported damage following from the loma prieta mainshock our study indicates that critical and postcritical reflections were stronger influences on the locations of strong shaking than local geology because a number of sites around the san francisco bay that are underlain by unconsolidated sediments experienced much less shaking than sites that were underlain by hard rock furthermore some severely shaken hard rock sites were farther from the epicenter than many of the less severely shaken soft sediment sites models of the regional crustal structure and seismic wave propagation paths may be useful in identifying sites around the san francisco bay region that are at risk due to reflected seismic energy and this type of analysis may be useful in other seismically active regions

simulations of strong ground motion in sw iberia for the 1969 february 28 ms 8 0 and the 1755 november 1 m 8 5 earthquakes i velocity model summary this is the first paper of a series of two concerning strong ground motion in sw iberia due to earthquakes originating from the adjacent atlantic area the aim of this paper is to build and calibrate a velocity model that will be used in the companion paper for seismic intensity modelling of the 1969 m s 8 0 and 1755 m 8 5 8 7 earthquakes taking into account the geological evolution of the region since the palaeozoic we build a 3 d velocity model down to the moho discontinuity substantially different from a simple 1 d layered model the velocity model presented in this paper is built a priori using information originating from a variety of geological and geophysical studies its resolution is sufficient to simulate realistically seismic wave propagation in the low frequency f 0 5 hz range which is the most significant for the study of the destructive effects of large earthquakes at a regional scale to validate the model we compute synthetic seismograms for three recent earthquakes of moderate magnitudes 4 6 m w 5 3 located offshore in the most seismically active area in the region synthetics are generated using a wave propagation code based on the finite difference method which was chosen for its simplicity and accuracy in the frequency range considered in this study 0 1 0 5 hz we compare simulated waveforms with threecomponent seismograms for 9 different stations traveltimes of the direct p waves and the amplitude of ground motion are accurately reproduced at all stations the frequency content of the seismograms fits the observations especially for the lowest frequencies investigated 0 1 0 3 hz for each earthquake the estimated seismic moment is in good agreement with values obtained by other authors using different methods we conclude that the velocity model provides encouraging results for the computation of low frequency seismograms in the region and can be used for the study of larger earthquakes for which the radiated wavefield has a predominant low frequency spectrum

modified frequency wavenumber algorithm for regional seismograms using filon s quadrature modelling of lg waves in eastern north america summary the main focus of this study is to develop an efficient frequency wavenumber code to synthesize high frequency say 10 hz regional seismograms up to a distance of more than 1000 km in a medium consisting of a large number of crustal layers and investigate effects of regional waveguides on lg waves in this newly developed code the frequency wavenumber response is evaluated by using the filon s quadrature at high frequency and the polynomial approximation of bessei s functions implementing trapezoidal integration at low frequency this has reduced the computation time by several folds compared to the time needed for the trapezoidal integration using this algorithm both broad band seismograms at harvard and short period ectn eastern canadian telemetered network seismograms from 1988 november 25 saguenay earthquake were successfully modelled a moment magnitude of 5 9 was used initially an average waveguide was calibrated to successfully excite the regional pnl and snl waves at hrv its near surface structure constrained by matching the recorded frequency content the crust mantle transition zone was constrained by modelling the strength of the broad band snl waves consisting of sn ssmsnsmssms and ssmssms phases relative to the initial pnl waves the model was modified to consist of thin layers with an alternating high and low velocity distribution to match the high frequency ectn lg waves observed from 300 to 600 km away in addition the effect of a near surface irregular receiver structure on the duration of lg waves was also investigated by convolving its response with the regional lg seismograms initial study suggests this method as viable for modelling the entire lg waves including the coda numerical studies suggest that the shapes and peak amplitudes on lg seismograms depend more strongly on the source depth anelasticity and crustal waveguide than do the pnl seismograms

site response at treasure and yerba buena islands california a variety of methods are utilized to reinvestigate the physical relationship between the seismic response of treasure island ti and yerba buena island ybi in california these islands are a soil ti and rock ybi site pair separated by 2 km the site pair has been used previously by researchers to identify soil response to earthquake shaking linear regime ground motions mw4 0 mw4 6 and pga 0 014 0 017 g recorded in the ti vertical array indicate a coherent wavefield in the sediments and an incoherence between the rock and sediments our analyses show that the greatest change in the wavefield occurred between the rock and soil layers corresponding to a significant impedance contrast the waveforms change very little as they propagate through the sediments indicating that the site response is a cumulative effect of the entire soil structure and not a result of wave propagation within individual soil layers in order to highlight the complexity of the site response correlation analysis was used to demonstrate that the rock and soil ground motions were not highly coherent between the two sites ybi was therefore shown to be an inappropriate reference site for ti one dimensional 1d vertical wave propagation and inverse techniques were used to differentiate between 1d site response and more complex site behavior both 1d methods vertical wave propagation and inverse transfer functions proved incapable of capturing the site response at ti beyond the initial four seconds of motion finite difference waveform modeling based on a two dimensional velocity structure of the northern san francisco bay was needed to explain the linear site response at ti as horizontally propagating surface waves trapped in the bay sediments a simplified velocity structure for the san francisco bay including a single 100 m basin layer constant shear wave velocity of 400 m s over a 1 5 km s layer of franciscan bedrock was able to trap energy in the basin and produce surface waveform ringing similar to that observed in the ti data due to surface waves propagating in the san francisco bay sediments any 1d model will not fully characterize site response at ti all 1d models will fail to produce the late arriving energy observed in the ground motions

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

three dimensional structure influences on the strong motion wavefield of the 1989 loma prieta earthquake abstract the accurate calculation of ground motions for future large earthquakes depends on detailed knowledge of three dimensional 3d geologic structure and the earthquake source process as well as sufficient computational resources in this article we describe the results of finite difference simulations for the 1989 loma prieta ca earthquake with a 3d seismic velocity model for the san francisco bay region and a heterogeneous slip model of the source additionally we explore the sensitivity of the synthetics to the major geologic structures in the velocity model the san francisco bay region particularly the loma prieta region is a unique area for the study of 3d wave propagation because of the pronounced lateral velocity contrasts across the strike slip faults of the region understanding the effects of such long wavelength structure is doubly important when considering a source located close to such a contrast our simulations show that the lateral velocity contrast across the san andreas fault saf would be expected to substantially affect the propagation of elastic waves radiated from a source in the loma prieta region indeed we find that the refraction of energy by the saf serves to reduce ground motions at stations located along the san francisco peninsula and the quaternary and tertiary alluvial basins of the san francisco bay region are found to amplify and extend the duration of ground motions in santa clara valley livermore valley and san pablo bay we find that the 3d model as currently defined accurately describes the spatial variation of peak ground velocity for frequencies less than 0 5 hz which suggests that this model may be used to estimate ground motions for future earthquake scenarios

subsurface fault damage zone of the 2014 mw 6 0 south napa california earthquake viewed from fault zone trapped waves the aftershocks of the 24 august 2014 m w 6 0 south napa earthquake generated prominent fault zone trapped waves fztws that were recorded on two 1 9 km long seismic arrays deployed across the northern projection array 1 a1 and the southern part a2 of the surface rupture of the west napa fault zone wnfz we also observed fztws on an array a3 deployed across the intersection of the franklin and southampton faults which appear to be the southward continuations of the wnfz a1 a2 and a3 consisted of 20 20 and 10 l28 4 5 hz three component seismographs we analyzed waveforms of fztws from 55 aftershocks in both time and frequency to characterize the fault damage zone associated with this m w 6 0 earthquake post s coda durations of fztws increase with epicentral distances and focal depths from the recording arrays suggesting a low velocity waveguide along the wnfz to depths in excess of 5 7 km locations of the aftershocks showing fztws combined with 3d finite difference simulations suggest the subsurface rupture zone having an s wave speed reduction of 40 50 between a1 and a2 coincident with the 14 km long mapped surface rupture zone and at least an 500 m wide deformation zone the low velocity waveguide along the wnfz extends further southward to at least a3 but with a more moderate velocity reduction of 30 35 at ray depth this last fztw observation suggests continuity between the wnfz and franklin fault the waveguide effect may have localized and amplified ground shaking along the wnfz and the faults farther to the south see a companion paper by catchings et al 2016

ground motion modeling of the 1906 san francisco earthquake part i validation using the 1989 loma prieta earthquake we compute ground motions for the beroza 1991 and wald et al 1991 source models of the 1989 magnitude 6 9 loma prieta earthquake using four different wave propagation codes and recently developed 3d geologic and seismic velocity models in preparation for modeling the 1906 san francisco earthquake we use this well recorded earthquake to characterize how well our ground motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the san francisco bay area all of the simulations generate ground motions consistent with the large scale spatial variations in shaking associated with rupture directivity and the geologic structure we attribute the small variations among the synthetics to the minimum shear wave speed permitted in the simulations and how they accommodate topography our long period simulations on average under predict shaking intensities by about one half modified mercalli inten sity mmi units 25 35 in peak velocity while our broadband simulations on average under predict the shaking intensities by one fourth mmi units 16 in peak velocity discrepancies with observations arise due to errors in the source models and geologic structure the consistency in the synthetic waveforms across the wave propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure in agree ment with earlier studies we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the beroza and wald source models although the new 3d seismic velocity model improves upon previous velocity models we identify two areas needing improvement nevertheless we find that the seismic velocity model and the wave propagation codes are suitable for modeling the 1906 earthquake and scenario events in the san francisco bay area online material modified mercalli intensities and velocity waveforms and a movie of simulated wave propagation

near source modeling of the loma prieta earthquake evidence for heterogeneous slip and implications for earthquake hazard i have analyzed records from 20 strong motion instruments that recorded the 1989 loma prieta earthquake ml 6 9 to determine the spatial and temporal distribution of slip using a tomographic back projection technique i find that the earthquake ruptured bilaterally at approximately 80 of the local shear wave velocity over a distance of slightly more than 30 km from 13 km northwest to 20 km southeast of the hypocenter slip within the rupture zone was highly variable both in magnitude and direction slip at the hypocenter and immediately up dip was relatively low about 1 m there were two main areas of concentrated slip one centered 7 km northwest of the hypocenter at a depth of 14 km and the other centered 6 km southeast of the hypocenter at a depth of 12 km peak slip amplitude on these high slip patches exceeded 4 5 m a surprising aspect of the rupture model is that rake varies from being predominately strike slip to the southeast of the hypocenter to being predominately reverse slip to the northwest despite this variation in rake the average rake determined from the strong motion data is in agreement with that determined teleseismically there is a correlation between areas of high slip and areas of low aftershock activity a simple model to explain this observation is that the areas of high slip are also areas of high strength which slip only during infrequent large mainshocks other areas of the fault are weaker and may slip both seismically and aseismically most of the slip in the loma prieta earthquake occurred between 9 and 16 km depth on a structure that dips to the southwest and runs underneath the surface trace of the san andreas fault at a depth of 8 to 10 km this observation complicates the assessment of seismic hazard on the southern santa cruz mountains segment of the san andreas fault

a structural vp model of the salton trough california and its implications for seismic hazard we present a high resolution three dimensional p wave seismic veloc ity model of the sedimentary basin in the salton trough southern california and use the model for spectral element method sem wave propagation and ground motion simulations to quantitatively assess seismic hazard in the region the basin geometry is defined by a surface representing the top of crystalline basement which was constrained by seismic refraction profiles and free air gravity data sonic logs from petroleum wells in the imperial valley and isovelocity surfaces defined by seismic refraction studies were used to define p wave velocity within the sedimentary basin as a function of two variables 1 absolute depth and 2 depth of the underlying crystalline basement surface cbs this velocity function was used to populate cells of a three dimensional spatial array voxet defining the p wave velocity structure in the basin the new model was then resampled in a computational mesh used for earthquake wave propagation and strong ground motion simulations based upon the sem komatitsch et al 2004 simulation of the 3 november 2002 mw 4 2 yorba linda earthquake demonstrates that the new model provides accurate simulation of strong ground motion amplification effects in the salton trough sedimentary basin offering substantial improvements over previous models a hypothetical mw 7 9 earthquake on the southern san andreas fault was then simulated in an effort to better understand the seismic hazard associated with the basin structure these sim ulations indicate that great amplification will occur during large earthquakes in the region due to the low seismic velocity of the sediments and the basin shape and depth

azimuthal dependence of the ground motion variability from scenario modeling of the 2014 mw6 0 south napa california earthquake using an advanced kinematic source model strong ground motion simulations require physically plausible earthquake source model here i present the application of such a kinematic model introduced originally by ruiz et al geophys j int 186 226 244 2011 the model is constructed to inherently provide synthetics with the desired omega squared spectral decay in the full frequency range the source is composed of randomly distributed overlapping subsources with fractal number size distribution the position of the subsources can be constrained by prior knowledge of major asperities stemming e g from slip inversions or can be completely random from earthquake physics point of view the model includes positive correlation between slip and rise time as found in dynamic source simulations rupture velocity and rise time follows local s wave velocity profile so that the rupture slows down and rise times increase close to the surface avoiding unrealistically strong ground motions rupture velocity can also have random variations which result in irregular rupture front while satisfying the causality principle this advanced kinematic broadband source model is freely available and can be easily incorporated into any numerical wave propagation code as the source is described by spatially distributed slip rate functions not requiring any stochastic green s functions the source model has been previously validated against the observed data due to the very shallow unilateral 2014 mw6 south napa california earthquake the model reproduces well the observed data including the near fault directivity seism res lett 87 2 14 2016 the performance of the source model is shown here on the scenario simulations for the same event in particular synthetics are compared with existing ground motion prediction equations gmpes emphasizing the azimuthal dependence of the between event ground motion variability i propose a simple model reproducing the azimuthal variations of the between event ground motion variability providing an insight into possible refinement of gmpes functional forms

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

rupture history of the 1989 loma prieta california earthquake strong motion records of the 1989 loma prieta earthquake are inverted to determine a model of the rupture history uncorrected horizontal and vertical accelerograms are integrated to particle velocity time histories for 38 stations within an epicentral range of 75 km the time histories are bandpassed filtered with corners at 0 05 and 1 0 hz these bandpassed time histories are inverted using a nonlinear method to solve for the distribution of slip amplitudes and rupture times at specified locations on the fault plane the fault plane is specified a priori 38 km long and 17 km wide extending from 3 to 19 km depth at a constant dip of 70 starting models have rupture times based on constant rupture velocities of 2 5 2 8 and 3 0 km sec and uniform slip with rise times of 0 5 1 0 1 5 and 3 0 sec the waveform inversion results show the strike slip displacement is concentrated at the southern end of the rupture rake 156 and the dip slip displacement is concentrated at the northern end of the rupture rake 115 the average total slip is partitioned almost equally between strike slip and dip slip rake 137 the hypocentral area has an unusually small amount of slip with almost no slip in a region just to the north and up dip from the hypocenter the rupture front is complex propagating up dip to the south faster than it propagates to the north the region of maximum strike slip to the southeast radiates simultaneously with the region of maximum dip slip to the northwest the average rupture velocity is 3 0 km sec approximately 0 83 times the local shear wave speed the calculated seismic moment is 3 5 0 5 1026 dyne cm

the loma prieta earthquake ground motion and damage in oakland treasure island and san francisco abstract the basis of this study is the acceleration velocity and displacement wave forms of the loma prieta earthquake 18 october 1989 m 7 0 at two rock sites in san francisco a rock site on yerba buena island an artificial fill site on treasure island and three sites in oakland underlain by thick sections of poorly consolidated pleistocene sediments the waveforms at the three rock sites display a strong coherence as do the three sedimentary sites in oakland the duration of strong motion at the rock sites is very brief suggestive of an unusually short source duration for an earthquake of this size while the records in oakland show strong amplification effects due to site geology the s wave group at treasure island is phase coherent with the oakland records but at somewhat diminished amplitudes until the steps in acceleration at approximately 15 sec apparently signaling the onset of liquefaction all seven records clearly show shear wave first motion opposite to that expected for the mainshock radiation pattern and peak amplitudes greater than expected for sites at these distances 95 3 km from an earthquake of this magnitude while the association between these ground motion records and related damage patterns in nearby areas has been easily and eagerly accepted by seismological and engineering observers of them we have had some difficulty in making such relationships quantitative or even just clear the three oakland records from sites that form a nearly equilateral triangle about the cypress street viaduct collapse are dominated by a long period resonance 1 1 2 sec period far removed from the natural frequency of the structure to transverse motion 2 5 hz or from high frequency amplification bands observed in aftershock studies a spectral ratio arbiter of this discrepancy confuses it further the failure of the east bay crossing of the san francisco oakland bay bridge cannot be attributed to relative displacements of the abutments in oakland and yerba buena island but the motions of the bay bridge causing failure remain unknown the steps in acceleration at treasure island present unusual strong motion accelerogram processing problems and modeling suggests that the velocity and displacement waveforms are contaminated by a spurious response of the filtering operations to the acceleration steps a variety of coincidences suggests that the treasure island accelerogram is the most likely strong motion surrogate for the filled areas of the marina district for which no mainshock records are available but the relative contributions of bad ground poor construction and truly strong ground motion to damage in the marina district will never by known in any quantitative way the principal lesson of all of this is that until a concerted effort is mounted to instrument ground and structures that are likely to fail during earthquakes our understanding of the very complex relationships between strong ground motion and earthquake damage will in general remain rudimentary imprecise and vague

three dimensional upper crustal velocity structure beneath san francisco peninsula california this paper presents new seismic data from and crustal models of the san francisco peninsula in much of central california the san andreas fault juxtaposes the cretaceous granitic salinian terrane on its west and the late mesozoic early tertiary franciscan complex on its east on san francisco peninsula however the present day san andreas fault is completely within a franciscan terrane and the pilarcitos fault located southwest of the san andreas marks the salinian franciscan boundary this circumstance has evoked two different explanations either the pilarcitos is a thrust fault that has pushed franciscan rocks over salinian rocks or the pilarcitos is a transform fault that has accommodated significant right lateral slip in an effort to better resolve the subsurface structure of the peninsula faults we established a temporary network of 31 seismographs arrayed across the san andreas fault and the subparallel pilarcitos fault at 1 2 km spacings these instruments were deployed during the first 6 months of 1995 and recorded local earthquakes air gun sources set off in san francisco bay and explosive sources travel times from these sources were used to augment earthquake arrival times recorded by the northern california seismic network and were inverted for three dimensional velocity structure results show lateral velocity changes at depth 0 5 7 km that correlate with downward vertical projections of the surface traces of the san andreas and pilarcitos faults we thus interpret the faults as high angle to vertical features constrained to a 70 110 dip range from this we conclude that the pilarcitos fault is probably an important strike slip fault that accommodated much of the right lateral plate boundary strain on the peninsula prior to the initiation of the modern day san andreas fault in this region sometime after about 3 0 m y ago

evidence for the collision of the salinian block with the franciscan subduction zone california recent detailed field mapping near pilarcitos lake san francisco peninsula california indicates that a subduction related suture between the salinian block continental fragment and the franciscan subduction complex may be preserved in the area in this region a ne dipping mylonite zone separates salinian rocks and structurally overlying franciscan rocks asymmetric augen pressure shadows and and s surfaces in the mylonite suggest down dip movement and a ne over sw sense of shear indicating the thrusting of franciscan over salinian rocks the mylonite is part of a 1 km wide zone of ductile thrust faulting these structures may have formed as a result of a collision between the salinian block and the franciscan subduction zone franciscan rocks being thrust westward over the salinian block because paleocene rocks are involved in the thrusting and clasts of the mylonitized rocks are present in upper paleocene and eocene in the area the age of thrusting is bracketed at approximately 50 60 ma th

simulations of strong ground motion in sw iberia for the 1969 february 28 ms 8 0 and the 1755 november 1 m 8 5 earthquakes ii strong ground motion simulations summary this is the second paper of a series of two concerning strong ground motion in sw iberia due to earthquakes originating from the adjacent atlantic area the aim of this paper is to use the velocity model that was proposed and validated in the companion paper for seismic intensity modelling of the 1969 m s 8 0 and 1755 m 8 5 8 7 earthquakes first we propose a regression to convert simulated values of peak ground velocity pgv into modified mercalli intensity mmi in sw iberia and using this regression we build synthetic isoseismal maps for a large m s 8 0 earthquake that occurred in 1969 based on information on the seismic source provided by various authors we show that the velocity model effectively reproduces macroseismic observations in the whole region we also confirm that seismic intensity distribution is very sensitive to a small number of source parameters rupture directivity fault strike and fault dimensions then we extrapolate the method to the case of the great m 8 5 8 7 1755 earthquake for a series of hypotheses recently proposed by three authors about the location of the epicentral region the model involving a subduction

short baseline variations in site response and wave propagation effects and their structural causes four examples in and around the santa clara valley california abstract ground motion records of local and regional events from a portable array are used to investigate the structural causes of variations in ground motion over distances of a few hundred meters to a few kilometers in the sedimentary basin environment of the santa clara valley california and its margins arrays of portable seismic stations are used to target four study areas with different ground motion patterns 1 an edge of the alluvial basin extending up onto a marginal ridge blossom hill 2 a cenozoic basin with a nearly flat bottom cupertino basin 3 a long narrow cenozoic basin with a steep v profile evergreen basin and 4 a line perpendicular to the trace of the hayward fault average peak velocities on blossom hill from local earthquakes are a factor of 2 5 times higher than nearby valley sites three dimensional 3d modeling is used to conclude that the majority of the amplification is due to lower shear wave velocities along a local fault zone shannon berrocal site amplification over the cupertino basin in the frequency band 0 5 4 hz is generally low less than 2 0 relative to a mesozoic rock site and spatially uniform this response is attributed to the shallow flat bottomed shape of the basin and the uniform flat laying sedimentary fill in contrast site amplification in the evergreen basin generally exceeds 3 0 and is attributed to the deep v shaped geometry of the basin and younger sedimentary fill 3d waveform modeling shows the elongated shape of the evergreen basin causes more efficient trapping of long period waves for sources along the long axis of the basin a low velocity zone is postulated along the hayward fault with a width between 100 and 200 m based on elevated site response along the fault trace and 4 5 hz fault zone guided waves on the horizontal components of stations near the fault

a new nonlinear finite fault inversion with three dimensional green s functions application to the 1989 loma prieta california earthquake 1 we present a new procedure to invert for kinematic source parameters on a finite fault on the basis of the reciprocity relation of the green s functions we use a newly developed fourth order viscoelastic finite difference algorithm to calculate three dimensional 3 d green s functions actually the tractions on the fault we invert the data for the unknown source parameters at the nodes or corners of the subfaults the source parameters within a subfault area are allowed to vary this variation is calculated through bilinear interpolation of the four nodal quantities we have developed a global nonlinear inversion algorithm that is based on simulated annealing methods to solve efficiently for the nodal parameters we apply this method to the 1989 loma prieta california m 6 9 earthquake for both a 1 d and 3 d velocity structure we show 1 the bilinear interpolation technique reduces the dependence of inversion results on the subfault size by naturally including the effects of nearby subfaults 2 while the number of synthetic seismograms that must be computed is greatly increased by the bilinear interpolation the structure of the inversion method minimizes the actual numbers of computations 3 as expected complexity in the velocity structure is mapped into the source parameters that describe the rupture process there are significant differences between faulting models derived from 1 d and 3 d structural models

p velocity models and earthquake locations in the livermore valley region california p wave travel times from 223 well recorded earthquakes and five timed explosions recorded by the us geological survey lawrence livermore national laboratory seismic network are inverted simultaneously for one and three dimensional velocity structures and hypocenter locations the stability of earthquake locations between the one and three dimensional inversions demonstrates the adequacy of a layered earth model combined with station corrections for routine hypocenter locations in the livermore valley initial one dimensional velocity inversions indicate a fairly strong positive velocity gradient in the upper crust of about 6 km three dimensional models show that the strongest velocity contrasts occur along the eastern and southern edges of the valley where lower velocity down dropped tertiary sediments and basin full alluvium are found in structural contact with higher velocity late mesozoic and tertiary sediments of the altamont anticline and the diablo range the low velocity basin structure is truncated along the western margin of the livermore valley by the calaveras fault across which higher velocity aftershocks from the livermore are exposed some of the relocated cretaceous and tertiary formations earthquake sequence show a good correlation with surface trace of the northwest trending greenville fault in the southeast corner of the livermore valley a number of short 5 km or less linear andmore discontinuous epicentral zones are observed these seismic lineations appear to define a conjugate set of right stepping faults in a right lateral shear zone focal mechanisms for these aftershocks show many complexities but are generally consistent with this interpretation less

the october 17 1989 loma prieta california earthquake and its aftershocks geometry of the sequence from high resolution locations hypocenters of the loma prieta sequence form a dipping zone that rises from the mainshock hypocenter and is parallel to the mainshock nodal plane most aftershocks cluster around the perimeter of the zone surrounding a relatively aseismic center which approximates the region of mainshock rupture at its southeastern end the dipping aftershock zone warps into a vertical surface that corresponds to the san andreas fault in the central and northwestern parts of the zone at depths above 10 km the aftershocks define numerous disjoint fault structures the large component of reverse slip observed in this event agrees with a simple model for slip on a dipping plane within a compressional fault bend we do not believe that the loma prieta earthquake occurred on the sargent fault however we are unable to conclude whether it ruptured the principal plate boundary fault or a less frequently active fault

empirical corrections for basin effects in stochastic ground motion prediction based on the los angeles basin analysis amplification corrections are presented for the finite fault stochastic ground motion simulation model these corrections represent the total effect of the los angeles basin on the ground motion spectra spectral amplification ratios were calculated by dividing the observed spectra for the 1994 northridge and 1987 whittier narrows earthquakes including shear and basin generated waves by the simulated spectra created assuming an average rock site condition smoothed amplification data were plotted above 3d images of the basin revealing a general correlation between the estimated basin depth and total basin amplification for both earthquakes over three frequency ranges low 0 2 2 hz intermediate 2 8 hz and high 8 12 5 hz the depth dependent corrections are derived from the regression of the combined data from both earthquakes in order to reduce an uncertainty caused by the azimuth of incoming waves ground motion duration is defined as the time for 95 of the acceleration spectral energy to pass after the s wave arrival due to ambiguity in defining a basin parameter controlling duration it was impossible to develop a generic equation that would relate the duration ratio observed synthetic to some characteristic of the basin users are cautioned though that the durations within the basin may be as much as four times longer than the simulated ones the procedure is outlined for potential users who wish to use the results of this study in synthesizing more accurate earthquake ground motions taking into account complicated basin geometry and near surface effects the results are directly applicable to engineering simulation of strong ground motions in a sedimentary basin environment manuscript received 21 may 2002

relations among fault behavior subsurface geology and three dimensional velocity models the development of three dimensional p wave velocity models for the regions surrounding five large earthquakes in california has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities these relations point toward a physical understanding of why faults are divided into segments that can fail independently an understanding that could improve our ability to predict earthquakes and strong ground motion

three dimensional simulation of earthquakes on the los angeles fault system we have used a 3d finite difference method to simulate ground motion from elastodynamic propagating ruptures with constant slip on faults in the metropolitan area of los angeles california simulations are carried out for hypothetical m 6 75 earthquakes on the palos verdes and elysian park faults and for comparison an approximation to the 17 january 1994 m 6 7 northridge earthquake the dominant subsurface features of this area are the deep sedimentary los angeles basin and the smaller and shallower san fernando basin simulated ground motions are restricted to the frequency range 0 0 to 0 4 hz ground motion time histories show that in general sites associated with the largest particle velocities and cumulative kinetic energies are located 1 in the epicentral area 2 above the deepest parts of the basins and 3 near the steepest edges of the los angeles basin we find maximum particle velocities for the palos verdes elysian park and northridge simulations of 0 44 0 67 and 0 58 m sec respectively in each case both the directivity of the rupture and the lower impedance of the basins significantly amplify the ground motion although the gross radiation pattern from these ruptures is observable the 3d basin structure distorts the wave field and becomes a source for edge generated waves signal durations at some basin sites last beyond 90 sec due to love waves and refracted s waves that propagate into the sediments from the basin edges compared with the palos verdes event the durations are generally smaller for the elysian park earthquake due to a smaller amount of love waves generated at the basin edges a simple approximation to the northridge earthquake reproduces the overall spatial pattern of the long period particle velocities successfully predicts the timing of late arriving waves and matches the peak velocities with discrepancies generally less than a factor of 2 however for localized areas immediately north and south of the santa monica mountains the computed ground motion underpredicts the observed horizontal peak velocities but matches the vertical ones the pattern of simulated total cumulative kinetic energies is similar to that for the damage intensities observed near the epicenter of the northridge event while the northridge earthquake caused damage in the los angeles area the 3d simulations show that earthquakes with the same magnitude on the palos verdes or elysian park faults produce more severe ground shaking in the los angeles basin

seismic evidence for a lower crustal detachment beneath san francisco bay california results from the san francisco bay area seismic imaging experiment basix reveal the presence of a prominent lower crustal reflector at a depth of 15 kilometers beneath san francisco and san pablo bays velocity analyses indicate that this reflector marks the base of franciscan assemblage rocks and the top of a mafic lower crust because this compositional contrast would imply a strong rheological contrast this interface may correspond to a lower crustal detachment surface if so it may represent a subhorizontal segment of the north america and pacific plate boundary proposed by earlier thermo mechanical and geological models

a regional view of urban sedimentary basins in northern california based on oil industry compressional wave velocity and density logs compressional wave sonic and density logs from 119 oil test wells provide knowledge of the physical properties and impedance contrasts within urban sedimentary basins in northern california which is needed to better understand basin amplification these wire line logs provide estimates of sonic velocities and densities for primarily upper cretaceous to pliocene clastic rocks between 0 1 and 5 6 km depth to an average depth of 1 8 km regional differences in the sonic velocities and densities in these basins largely 1reflect variations in the lithology depth of burial porosity and grain size of the strata but not necessarily formation age for example miocene basin filling strata west of the calaveras fault exhibit higher sonic velocities and densities than older but finer grained and or higher porosity rocks of the upper cretaceous great valley sequence as another example hard eocene sandstones west of the san andreas fault have much higher impedances than eocene strata mainly higher porosity sandstones and shales located to the east of this fault and approach those expected for franciscan complex basement rocks basement penetrations define large impedence contrasts at the sediment basement contact along the margins of several basins where quaternary pliocene and even miocene deposits directly overlie franciscan or salinian basement rocks at depths as much as 1 7 km in contrast in the deepest geographic centers of the basins such logs exhibit only a modest impedance contrast at the sediment basement contact at depths exceeding 2 km prominent up to 1 km sec and thick up to several hundred meters velocity and density reversals in the logs refute the common assumption that velocities and densities increase monotonically with depth online material borehole log regression parameters and metadata

regional extent of great valley basement west of the great valley california implications for extensive tectonic wedging in the california coast ranges interpretation and modeling of the magnetic field of central california indicate that the magnetic basement of the forearc deposits of the great valley sequence extends westward beneath the coeval subduction related rocks of the franciscan complex the basement surface slopes gently to the west reaching midcrustal depths 15 19 km at distances of 50 100 km west of the great valley this magnetic basement is disrupted by the hayward rodgers creek fault system and is cut by the san andreas fault at the south end of the great valley and possibly throughout much of central california the widespread presence of the great valley basement beneath rocks of the franciscan complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the franciscan complex great valley sequence contact this result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones the eastern boundary fault of the franciscan complex coast range fault is not and never was a subduction zone thrust fault but rather was originally a roof thrust wedge roof fault formed above the eastward wedging mass of franciscan complex intruded along the top of the basement beneath the great valley deposits this tectonic interpretation offers a solution for the question of how high pressure metamorphic rocks of the franciscan complex were juxtaposed at the coast range fault against low pressure metamorphic rocks of the great valley sequence this interpretation also implies an older flat lying thrust fault wedge floor fault that forms the top of magnetic basement between the active san andreas and hayward faults at depths of 15 17 km this older thrust fault may today transfer strain between the two young strike slip faults possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults the former east dipping subduction zone along which the rocks of the franciscan complex accumulated must lie west of the western limit of the great valley magnetic basement

a three dimensional simulation of seismic waves in the santa clara valley california from a loma prieta aftershock abstract the finite difference method is used to propagate elastic waves through a 3 d model of the santa clara valley an alluvium filled basin that underlies the city of san jose california the model was based on depth to bedrock information from water wells in the area the simulation corresponded to a region 30 east west by 22 north south by 6 depth km and contained about 4 million grid points synthetic seismograms from the simulation are accurate at frequencies up to 1 hz motions from a magnitude 4 4 aftershock of the loma prieta earthquake were modeled snapshots of ground motion and synthetic seismograms from the simulation are presented the simulation illustrates s to surface wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics because of the slow group velocity of the love waves sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges sites near the center of the basin also show larger peak amplitudes on the transverse component array analysis of the synthetic seismograms indicates that love waves tend to propagate parallel to the eastern and western edges of the valley rayleigh waves are produced along the southern margin of the basin from incident s waves large radial motions occur where a rayleigh wave impinges on the northeast margin of the valley some rayleigh waves travel westward across the basin after being scattered from the eastern edge of the valley synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the sunnyvale dense array for this aftershock although the duration of the tranverse component is not matched by the synthetic seismogram using this basin model the simulation indicates that the love waves observed on the actual seismograms were produced by conversion of incident s waves at the southern margin of the santa clara valley 2 d simulations show how the s to love wave conversion is affected by the angle of incidence of the s wave and the sharpness of the velocity transition between the alluvium and bedrock as more accurate basin models are developed 3 d simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes

waveform tomography of crustal structure in the south san francisco bay region 1 we utilize a scattering based seismic tomography technique to constrain crustal structure around the southern san francisco bay region sfbr this technique is based on coupled traveling wave scattering theory which has usually been applied to the interpretation of surface waves in large regional scale studies using fully three dimensional kernels this technique is here applied to observed p s and surface waves of intermediate period 3 4 s dominant period observed following eight selected regional events we use a total of 73 seismograms recorded by a u s geological survey short period seismic array in the western santa clara valley the berkeley digital seismic network and the northern california seismic network modifications of observed waveforms due to scattering from crustal structure include positive or negative amplification delay and generation of coda waves the derived crustal structure explains many of the observed signals which cannot be explained with a simple layered structure there is sufficient sensitivity to both deep and shallow crustal structure that even with the few sources employed in the present study we obtain shallow velocity structure which is reasonably consistent with previous p wave tomography results we find a depth dependent lateral velocity contrast across the san andreas fault saf with higher velocities southwest of the saf in the shallow crust and higher velocities northeast of the saf in the midcrust the method does not have the resolution to identify very slow sediment velocities in the upper approximately 3 km since the tomographic models are smooth at a vertical scale of about 5 km

testing long period ground motion simulations of scenario earthquakes using the mw 7 2 el mayor cucapah mainshock evaluation of finite fault rupture characterization and 3d seismic velocity models using a suite of five hypothetical finite fault rupture models we test the ability of long period t 2 0 s ground motion simulations of scenario earthquakes to produce waveforms throughout southern california consistent with those recorded during the 4 april 2010 m w 7 2 el mayor cucapah earthquake the hypothetical ruptures are generated using the methodology proposed by graves and pitarka 2010 and require as inputs only a general description of the fault location and geometry event magnitude and hypocenter as would be done for a scenario event for each rupture model two southern california earthquake center three dimensional community seismic velocity models cvm 4m and cvm h62 are used resulting in a total of 10 ground motion simulations which we compare with recorded ground motions while the details of the motions vary across the simulations the median levels match the observed peak ground velocities reasonably well with the standard deviation of the residuals generally within 50 of the median simulations with the cvm 4m model yield somewhat lower variance than those with the cvm h62 model both models tend to overpredict motions in the san diego region and underpredict motions in the mojave desert within the greater los angeles basin the cvm 4m model generally matches the level of observed motions whereas the cvm h62 model tends to overpredict the motions particularly in the southern portion of the basin the variance in the peak velocity residuals is lowest for a rupture that has significant shallow slip 5 km depth whereas the variance in the residuals is greatest for ruptures with large asperities below 10 km depth overall these results are encouraging and provide confidence in the predictive capabilities of the simulation methodology while also suggesting some regions in which the seismic velocity models may need improvement

determination and analysis of optimal kinetic parameters for hydrocarbon generation from source rock sequences of cambay basin india abstract pyrolysis studies have been carried out on 16 rock samples containing type iii organic matter of paleogene age from the cambay basin kinetic parameters for hydrocarbon generation were obtained by mathematical optimization of pyrolysis curves assuming a discrete distribution of activation energies with a constant preexponential factor the values of the initial potentials corresponding to the first few activation energies are used to throw light on the thermal stress undergone by the samples while the values corresponding to the last few activation energies are used to infer the depositional environment the study indicates that tmax is mainly determined by the set of activation energies associated with the dominant initial generation potentials

the influence of critical moho reflections on strong ground motions recorded in san francisco and oakland during the 1989 loma prieta earthquake the amplitudes of strong ground motions from the loma prieta earthquake recorded in the san francisco and oakland areas exceeded the levels predicted by standard empirical attenuation relations preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust these reflections which are identified on the basis of their arrival times and phase velocity and by comparison with simulated accelerograms were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust these motions were further amplified presumably by impedance contrast effects at soft soil sites in san francisco and oakland the effect of the critical reflections in amplifying peak accelerations of the loma prieta earthquake in the san francisco and oakland regions was as large as the effect of soft soil site conditions focal depth has an important influence on strong motion attenuation at distances beyond about 40 km and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in san francisco and oakland and the conclusions drawn from that explanation

the seismic response of the los angeles basin california using strong motion data recorded in the los angeles region from the 1992 mw 7 3 landers earthquake we have tested the accuracy of existing three dimensional 3d velocity models on the simulation of long period 2 sec ground motions in the los angeles basin and surrounding san fernando and san gabriel valleys first the overall pattern and degree of long period excitation of the basins were identified in the observations within the los angeles basin the recorded amplitudes are about three to four times larger than at sites outside the basins amplitudes within the san fernando and san gabriel valleys are nearly a factor of 3 greater than surrounding bedrock sites then using a 3d finite difference numerical modeling approach we analyzed how variations in 3d earth structure affect simulated waveforms amplitudes and the fit to the observed patterns of amplification significant differences exist in the 3d velocity models of southern california that we tested magistrale et al 1996 graves 1996a hauksson and haase 1997 major differences in the models include the velocity of the assumed background models the depth of the los angeles basin and the depth location and geometry of smaller basins the largest disparities in the response of the models are seen for the san fernando valley and the deepest portion of the los angeles basin these arise in large part from variations in the structure of the basins particularly the effective depth extent which is mainly due to alternative assumptions about the nature of the basin sediment fill the general ground motion characteristics are matched by the 3d model simulations validating the use of 3d modeling with geologically based velocity structure models however significant shortcomings exist in the overall patterns of amplification and the duration of the long period response the successes and limitations of the models for reproducing the recorded ground motions as discussed provide the basis and direction for necessary improvements to earth structure models whether geologically or tomographically derived the differences in the response of the earth models tested also translate to variable success in the ability to successfully model the data and add uncertainty to estimates of the basin response given input scenario earthquake source models

an empirical analysis of peak horizontal acceleration for the loma prieta california earthquake of 18 october 1989 abstract a total of 137 ground level accelerograms recorded during the 18 october 1989 m s 7 1 loma prieta california earthquake were used to study the dependence of peak horizontal acceleration on distance azimuth and site geology according to this analysis 1 peak accelerations recorded on alluvium at distances of 50 to 79 km from the inferred seismogenic rupture zone exhibited little or no attenuation with distance 2 peak accelerations recorded on alluvium at seismogenic distances exceeding 50 km were significantly higher than those predicted from attenuation relationships available from the literature 3 both the amplitude and rate of attenuation of peak accelerations recorded on alluvium exhibited a strong dependence on azimuth with accelerations recorded at azimuths corresponding to san francisco and oakland having 68 higher amplitudes and a lower rate of attenuation than those recorded at more northerly azimuths and 4 peak accelerations recorded at all azimuths exhibited a strong dependence on site geology with bay mud sites having 88 higher accelerations soft rock sites having 15 lower accelerations and hard rock sites having 32 per cent lower accelerations than alluvial sites the dependence of peak acceleration on azimuth is consistent with the geographic distribution of modified mercalli intensity although it is possible to explain both the low rate of attenuation and the constant level of acceleration at distances of 50 to 79 km by the arrival of critical reflections from the base of the crust moho it is also possible that source directivity and radiation pattern path dependent attenuation and systematic differences in local site conditions could have been partly responsible for these observations

rupture history of 2014 mw 6 0 south napa earthquake inferred from near fault strong motion data and its impact to the practice of ground strong motion prediction the spatiotemporal rupture history of the 2014 south napa earthquake is constrained using near fault strong motion records an aggressive source parameterization with 1372 subfaults is adopted to match the signals in the transverse components up to 4 hz the result reveals that the rupture of the napa earthquake initiated at a depth of 9 84 km and propagated mainly to north northwest nnw and updip on a 13 km long fault patch a gradual increase in average rise time when the rupture propagates to shallower depth is observed however it is the rupture of a small mw 4 9 isolated and high stress drop fault patch that excited the largest ground acceleration at stations south of the epicenter such fine scale rupture heterogeneity shall be considered during seismic hazard analysis

2d analysis of earthquake ground motion in haifa bay israel we study the earthquake response of the zevulun valley basin underlying northern israel s largest urban area with 2d viscoelastic seismic modeling of a detailed geological section we found that amplification of the horizontal vibrations the ratio of basin to no basin response spectra correlates with basin depth in the deepest portion of the basin qishon graben long periods 2 5 sec are amplified by 400 in the shallowest portion of the basin afeq horst shorter periods 0 5 sec are amplified by 300 400 these resonances in the vertical direction through the basin are strong enough that their amplitude overwhelms the amplitude of a previously recognized basin edge effect the horizontal vertical h v fourier spectral ratios based on 124 ambient noise measurements do not fully coincide with the basin to no basin fourier spectral ratios of the simulation but the resonance frequencies found in both methods are alike moreover the relation between the resonance frequency and the depth of the corresponding seismic reflector in the simulation is almost identical to the empirical frequency depth relations obtained from measurements this indicates that the average shear wave velocity of the sedimentary column in the model is consistent with measurements to evaluate the necessity of 2d analysis we performed additional 1d simulations at two locations along the section for the qishon graben 1d analysis underestimates the amplification factor relative to 2d by 25 whereas for the afeq horst 1d and 2d simulations are similar for a hard layer within the soft qishon graben fill we found that when the hard layer is thinner than 50 m its influence on ground motion is small

effects of three dimensional crustal structure on the estimated slip history and ground motion of the loma prieta earthquake abstract the strong ground motion of the loma prieta earthquake is synthesized using a known three dimensional 3d crustal model of the region a rupture model determined under the assumption of laterally homogeneous structure and green9s functions computed by superposition of gaussian beams compared to results obtained assuming a laterally homogeneous crust stations within 30 km of the hypocenter and 5 km of the fault trace are predicted to be strongly focused by factors up to 10 or more while stations lying at the northeastern edge of the santa clara valley are predicted to be weakly defocused by factors of 0 5 the focusing is caused by low velocities lying between the zayante and san andreas faults and the defocusing is caused by a wedge of high velocity material between the san andreas and sargent faults the magnitude of amplitude variations resulting from known 3d structure is similar to that incorporated in site effects resulting from variations in surface impedance and shallow structure beneath receivers the existence of these effects add to the uncertainty in estimates of future strong ground motion the strong path dependence of the effects makes it possible to include them in estimates of future ground motions only by knowing both the 3d structure and the sites of localized high slip in future earthquakes to match observed waveforms with the structure model of eberhart phillips et al 1990 the slip model of beroza 1991 is modified by reducing the magnitude of the northwest maximum in slip by a factor of 0 7 and increasing the magnitude of the southeast maximum slip by a factor of 2 5 these modifications are in the range of variations in the estimated slip history published by investigators using different data sets and inversion procedures

neogene transpressive evolution of the california coast ranges the san andreas fault has long been viewed as a vertical structure that extends from the ground surface to the base of the lithosphere and separates the pacific plate from the north american plate along a transform plate boundary analysis of neogene structures in the central coast ranges indicates however that crustal blocks defined by strands of the san andreas fault system have undergone pervasive compressive deformation that has shifted these blocks and their boundary faults east relative to north america this suggests that the blocks and their boundary structures are bounded below by an active decollement that appears to coincide with the midcrustal brittle to ductile transition at the base of the seismogenic zone these compressive structures may be related to a currently seismically active fold and thrust belt that characterizes the eastern front of the coast ranges seismic profiles from the central valley east of the range front show that tectonic wedges blind thrusts developed within stratified rocks are present throughout this zone of deformation and that crystalline basement that underlies these stratified rocks is not involved in the deformation crystalline basement may underlie most of the coast ranges as indicated by the presence of metamorphic rocks of possible sierran affinity preserved in neogene thrust sheets as far west as loma prieta and by seismic velocities in the lower crust appropriate for mafic to intermediate granitoid rocks on the basis of these relations we suggest that the san andreas fault system is confined to the brittle crust above the decollement and does not penetrate to the base of the lithosphere thus the san andreas fault should not be considered as a plate boundary instead the functional plate boundary within the coast ranges appears to be the inferred subhorizontal midcrustal decollement this decollement corresponds approximately to the 350 c isotherm the temperature at which quartz becomes ductile if our hypothesis is proven to be correct by geologic and geophysical investigations currently underway then extensive revisions in popular plate tectonic models applied to the coast ranges will be required in addition assessment of seismic hazards will be rendered more difficult but more realistic owing to realization of the greater likelihood for occurrence of thrust faulting on blind or buried faults or on faults currently deemed inactive

wave propagation and site response in the santa clara valley forty two portable digital instruments were deployed across the santa clara valley from june until early november 1998 this array recorded 14 small and moderate local events and 7 large teleseismic events we analyze the ground motion from these events to determine station delays and relative site amplification within the valley p waves from an event at the southern edge of the valley are early t 0 35 sec at stations over an axial ridge in the basement interface in the middle of the valley but late t 0 20 sec for stations over the cupertino and evergreen basins to either side the s wave delays are approximately twice as large teleseismic p waves from an m 7 0 event beneath the bonin islands show a similar pattern in travel time delays the p waves are amplified by factors of 1 5 3 for frequencies below 2 hz at stations within either basin compared with stations on the axial ridge the p wave coda appear enhanced at 2 3 sec but coda q estimates at frequencies from 0 2 to 1 1 hz are not markedly different at stations over the basin compared with stations on the ridge with the possible exceptions of consistently high values over the northern end of the evergreen basin we invert the s wave spectra for site specific attenuation and amplification from the 14 local events by assuming a common source spectra for each event 1 r geometrical spreading and constraining the inversion using the 30 m velocity profile at four stations in the array the largest amplifications occurred in the 1 to 6 hz band at stations near the northwest edge of the evergreen basin while the highest amplifications occur at stations with the lowest s wave velocities the scatter obscures the correlation between velocity and amplification the stations in the basins are characterized by higher attenuation than the stations on the basement ridge manuscript received 2 july 2001

basin related effects on ground motion for earthquake scenarios in the lower rhine embayment summary the deterministic calculation of earthquake scenarios using complete waveform modelling plays an increasingly important role in estimating shaking hazard in seismically active regions here we apply 3 d numerical modelling of seismic wave propagation to m 6 earthquake scenarios in the area of the lower rhine embayment one of the seismically most active regions in central europe using a 3 d basin model derived from geology borehole information and seismic experiments we aim at demonstrating the strong dependence of ground shaking on hypocentre location and basin structure the simulations are carried out up to frequencies of ca 1 hz as expected the basin structure leads to strong lateral variations in peak ground motion amplification and shaking duration depending on source basin receiver geometry the effects correlate with basin depth and the slope of the basin flanks yet the basin also affects peak ground motion and estimated shaking hazard thereof outside the basin comparison with measured seismograms for one of the earthquakes shows that some of the main characteristics of the wave motion are reproduced cumulating the derived seismic intensities from the three modelled earthquake scenarios leads to a predominantly basin correlated intensity distribution for our study area

microseisms observations in the santa clara valley california we have investigated ground motion amplification in the santa clara valley scv using microseisms observed during the 1998 deployment of 41 short period seismometers the santa clara valley seismic experiment scvse lindh et al 1999 fletcher et al 2003 recorded many local regional and teleseismic events in our previous work we investigated the 3d velocity structure of the scv by modeling the teleseismic p waves recorded during the scvse dolenc et al 2005 to complement these results we now focus on the microseisms that were recorded during the same period and relate these observations to local earthquake wave amplification it is found that the seismic noise is related to the ocean wave heights measured on the weather buoy west of half moon bay california the spectral ratio of the horizontal to vertical h v microseisms at each scvse site is stable with time and the period of the dominant peak in the h v ratio is related to the basin depth the results of this study show that seismic noise can be used in the assessment of the effects of deep sediments on long period earthquake ground motions

liquefaction limit during earthquakes and underground explosions implications on ground motion attenuation liquefaction of saturated soils and sediments documented during earthquakes shows an empirical relation log r max 2 05 0 10 0 45 m where r max is the liquefaction limit in meters i e the maximum distance from liquefaction site to the hypocenter and m is the earthquake magnitude combining this with an empirical relation between m and the seismic energy of an earthquake we obtain a relation between the liquefaction limit and the seismic energy e a r max the prefactor corresponds to a threshold energy for liquefaction ranging from 0 004 to 0 1 j m3 the exponent ranging from 3 2 to 3 3 implies that the energy density of ground motion attenuates with distance according to 1 r 3 2 3 3 where r is the distance from the hypocenter the value of the threshold energy suggests a preliquefaction degradation of the shear modulus of soils by more than 3 orders of magnitude liquefaction documented during underground explosions is characterized by a threshold energy several orders of magnitude greater than that for liquefaction during earthquakes but shows a similar functional relation between e and r max as that for liquefaction during earthquakes and implies a similar attenuation relation between ground motion energy density and distance

the effect of shallow san francisco bay sediments on waveforms recorded during the mw 4 6 bolinas california earthquake to investigate the effect of the shallow low velocity sediments on the seismic wave field in the northern san francisco bay we modeled tangential component displacement seismograms recorded during the 18 august 1999 m w 4 6 bolinas california earthquake the modeling indicates that the velocity structure of pleistocene horizons in the san francisco bay is important for simulations of weak ground motions for bay area earthquakes models including the pleistocene sediments generate the 1 sec period surface waves observed at several stations modeling of treasure and yerba buena island records requires structures approximately an order of magnitude higher in spatial resolution than the current 3d velocity models for the region this pair of sites located only 2 km apart in the bay records a sixfold difference in peak ground acceleration during the bolinas earthquake three transects are forward modeled using 1d frequency wavenumber integration and 2d finite difference methods generally the ground motions are characterized by a direct shear wave s a midcrustal reflection s 1 a near receiver multiple s 2 and surface waves the direct s arrival at all six stations requires a faster model than gil7 the model routinely used to estimate earthquake source parameters using the berkeley digital seismic network in addition the timing of s 1 indicates the possibility of a dipping midcrustal interface s 2 can be matched with a single strong impedance contrast at 3 km depth a thin 200 m surface layer of weathered rock and sediments simulates the surface waves that follow s 2 at the richmond field station site however the surface waves at treasure island and the berkeley sites are longer in duration and higher amplitude than at richmond and require 2d structure a simple shallow uniform basin model for the san francisco bay consisting of stiff sediments shear wave velocity 400 m sec thickness 100 m over weathered rock 1 5 km sec of the franciscan assemblage produces surface waves in the 0 02 2 hz passband at treasure island and the berkeley sites manuscript received 27 july 2001

three dimensional seismic velocity structure of the san francisco bay area seismic travel times from the northern california earthquake catalogue and from the 1991 bay area seismic imaging experiment basix refraction survey were used to obtain a three dimensional model of the seismic velocity structure of the san francisco bay area nonlinear tomography was used to simultaneously invert for both velocity and hypocenters the new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm numerous inversions were performed with different parameters to test the reliability of the resulting velocity model most hypocenters were relocated 12 km under the sacramento river delta 6 km beneath livermore valley 5 km beneath the santa clara valley and 4 km beneath eastern san pablo bay the great valley sequence east of san francisco bay is 4 6 km thick a relatively high velocity body exists in the upper 10 km beneath the sonoma volcanic field but no evidence for a large intrusion or magma chamber exists in the crust under the geysers or the clear lake volcanic center lateral velocity contrasts indicate that the major strike slip faults extend sub vertically beneath their surface locations through most of the crust strong lateral velocity contrasts of 0 3 0 6 km s are observed across the san andreas fault in the middle crust and across the hayward rogers creek calaveras and greenville faults at shallow depth weaker velocity contrasts 0 1 0 3 km s exist across the san andreas hayward and rogers creek faults at all other depths low spatial resolution evidence in the lower crust suggests that the top of high velocity mafic rocks gets deeper from west to east and may be offset under the major faults the data suggest that the major strike slip faults extend sub vertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike slip motion the extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity strong ground motion and regional stress