crustal attenuation and site effects at parkfield california at parkfield california the san andreas fault separates strongly contrasting rock types high velocity salinian granite to the sw and lower velocity franciscan basement to the ne i use the recordings of nine earthquakes in 10 boreholes 1000 m deep and calculate p and s wave spectral ratios between borehole recordings at different depths to eliminate the source i then invert for q on each side of the san andreas fault i find that the attenuation on the ne side of the fault q ne 100 is approximately twice that on the sw side q sw 200 in the depth range 200 m to 5 km q p and q s are similar but their ratio is not well resolved i estimate q p 50 and q s 80 in the fault zone itself by combining recordings at a site within the fault zone with the inversion results and confirm that it is a low q zone q is also observed to increase with depth and the rate of this increase decreases as the depth increases in the upper 1 km at the varian well on the ne side of the san andreas fault q p 30 and q s 20 with q p increasing from 20 between 0 and 300 m to 55 between 600 and 900 m this study demonstrates that the borehole seismometers at parkfield are not deep enough for the effects of attenuation to be ignored in analysis of small earthquakes also the attenuation below 1 km on the ne side of the fault is actually larger than that below 200 m on the sw side the results of this study suggest that the attenuation at depths of a few hundred to a few thousand meters should be considered when estimating site effects in seismic hazard studies

a comparison and test of various site response estimation techniques including three that are not reference site dependent abstract we compare various site response estimation techniques using after shock data of the 1989 loma prieta earthquake collected in oakland california because of recent interest in comparing results from weak and strong motion to infer any nonlinearity and between direct s and coda waves we pay particular attention to the uncertainties first sediment to bedrock spectral ratio estimates between pairs of sites are compared with those obtained from various generalized inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously we find that the site amplification factors are very similar among these approaches but that the uncertainties can be significantly different depending on how the data are weighted we also examine and test three site response estimation techniques that do not rely on a reference site to estimate source and path effects the first involves a parameterized source and path effects inversion even when the bedrock data are excluded from consideration this approach is found to reveal the frequency dependence of site response at each of the sediment sites the second technique involves taking horizontal to vertical component spectral ratios receiver function type estimates of shear wave aftershock data these are also found to reveal the frequency dependence of site response at the sediment sites and the results for the bedrock site are relatively flat and near unity the third estimate is formed by taking horizontal to vertical component ratios of ambient seismic noise and these are shown to reveal the fundamental resonant frequency of the sediment sites unfortunately discrepancies exist among all of the site response estimates and with one dimensional predictions with respect to a frequency independent scaling factor nevertheless the highly frequency dependent character of site response is well constrained and the fact that non reference site dependent methods are capable of revealing this is promising for site specific hazard assessments in regions that lack adequate reference sites

seismic source modeling seismic source modeling is taken to mean the modeling of seismograms to investigate kinematic properties of the earthquake source current trends such as the use of other data types e g geodetic measurements of coseismic deformation and the explicit consideration of source dynamics when modeling seismograms are beginning to blur this definition but i will adhere to it in this review i have divided seismic source modeling into teleseismic regional and near source modeling teleseismic modeling has the advantage of global coverage for earthquakes of magnitude 5 5 and larger but does not provide much more than point source parameters regional distance modeling has the advantage of a more rapid assessment of earthquake source properties but is complicated by relatively complex wave propagation the coverage is also limited to areas of broadband regional networks typically continental areas of developed countries near source modeling has the potential for detailed imaging of source properties but very few earthquakes occur in regions with sufficient instrumentation to allow this i have also included a section on historical earthquakes because of the additional issues that seismologists face in working with pre wwssn data and because of the important issues such as earthquake recurrence that require historical data in addition to reviewing the progress in seismic source modeling during the last four years i have also briefly reviewed some of the progress in understanding earthquake physics and hazards that has come about because of basic research in earthquake source modeling

source scaling of microearthquakes at mt etna volcano and in the calabrian arc southern italy microearthquake data recorded at mt etna volcano and in the calabrian arc southern italy have been used to study the scaling of the main source parameters after correction of the p wave displacement spectra for both path attenuation and the site amplification effects an inverse method has been applied to compute spectral parameters low frequency spectral level corner frequency and high frequency roll off results show that seismic moments m0 range from ca 1016 to 1021 dyne cm at mt etna and from ca 1017 to 1020 dyne cm in southern calabria corner frequencies range from ca 3 to 15 hz in both areas we found that the stress drop decreases from ca 100 to few bars with decreasing seismic moment a simple brune source model is not able to explain the source scaling because the typical fall off observed at higher frequencies 3 as an average is larger than that of brune s theory furthermore the relation between the seismic moment and the corner frequency points to a scaling law behaving as m0 c 4 5 and m0 c 3 6 on average at mt etna and in southern calabria respectively and higher for smaller events in both areas therefore the cube law m0 3 generally accepted for large earthquakes does not hold true for the analysed microearthquakes neither for those which occur along the tectonic structures of southern calabria or along those hosting the roots of the feeding system of the largest european volcano

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

s wave attenuation and site effects in the region of friuli italy we used strong motion records from the 1976 friuli earthquake m 6 4 and 10 of the biggest aftershocks recorded by the national accelerograph network of the electrical power company of italy to estimate the quality factor q of s waves in this region the wide distance range of the recordings 10 r 190 km permits us to analyze the spectral amplitude decay of the records using a nonparametric approach e g anderson and quaas 1988 castro et al 1990 anderson 1991 we obtained attenuation functions for a set of 18 frequencies ranging between 0 4 and 25 0 hz the values of q retrieved from the attenuation functions obtained follow the frequency dependent relation q 20 4f a test of the method was made using a second data set consisting of digital seismograms from the friuli venezia giulia seismograph network in spite of the different size of the volume sampled by these data 10 r 131 km the frequency dependence of q obtained q 16 1f0 92 is similar to that obtained with the strong motion data set the near surface attenuation was also estimated using the model proposed by anderson and hough 1984 and anderson 1991 we found that 0 is smaller for the strong motion stations located on rock compared to stations located on either shallow or soft sediments to estimate the site response of the strong motion stations we corrected the spectral records for the attenuation effect and then inverted the corrected records to separate source and site effects using the inversion scheme proposed by andrews 1986 to verify the site amplification estimates obtained we also calculated the transfer function of each site using nakamura s 1989 method for s wave e g lermo and chavez garcia 1993 in general the shapes of the site functions obtained with the inversion are consistent with the transfer functions obtained calculating the horizontal to vertical component ratio

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

ubiquitous weakening of faults due to thermal pressurization faults weaken during earthquakes analysis of the amount of energy released during earthquakes globally suggests that heat induced pressurization of pore fluids can weaken faults during earthquakes of all sizes laboratory simulations of earthquakes show that at high slip rates faults can weaken significantly aiding rupture1 2 3 various mechanisms such as thermal pressurization and flash heating have been proposed to cause this weakening during laboratory experiments1 4 5 6 yet the processes that aid fault slip in nature remain unknown measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening known as an event s fracture energy7 8 9 here we compile new and existing8 9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes we observe a distinct transition in how fracture energy scales with event size which implies that faults weaken differently during small and large earthquakes and earthquakes are not self similar we use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms we find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes over seven orders of fault slip magnitude we conclude that thermal pressurization is a widespread and prominent process for fault weakening

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

constraints on explosive sources with spall from near source waveforms spall the tensile failure of a material due to high stress loading has been observed in a number of contained and surface explosions the phenomenon results in a repartition of the initial spherical explosion energy source yielding a second energy source which is cylindrical and delayed in time recent spall models by day et al 1983 demanding conservation of momentum have shown the phenomenon to have little contribution to 20 sec surface waves these models are extended to include the effect of the process on near source seismograms the spall model is constrained by observations within the nonlinear regime of the source which bound the mass momentum and timing of the process comparison of these forward models with the inverse vertical point force source inferred from seismic recordings of a bermed surface explosion yields excellent agreement the spall model developed from the contained explosion cheat is used to create synthetic seismograms comparisons of these waveforms with those from a mueller murphy contained explosion indicate that the waveform contribution from spall is similar in size to the spherical explosion waveform the complete synthetic composed of the spall and explosion contribution compares favorably with observational data from the cheat experiment in both amplitude and energy distribution

site response shallow shear wave velocity and damage in los gatos california from the 1989 loma prieta earthquake aftershock records of the 1989 loma prieta earthquake are used to calculate site response in the frequency band of 0 5 10 hz at 24 locations in los gatos california on the edge of the santa clara valley two different methods are used spectral ratios relative to a reference site on rock and a source site spectral inversion method these two methods complement each other and give consistent results site amplification factors are compared with surficial geology thickness of alluvium shallow shear wave velocity measurements and ground deformation and structural damage resulting from the loma prieta earthquake higher values of site amplification are seen on quaternary alluvium compared with older miocene and cretaceous units of monterey and franciscan formation however other more detailed correlations with surficial geology are not evident a complex pattern of alluvial sediment thickness caused by crosscutting thrust faults is interpreted as contributing to the variability in site response and the presence of spectral resonance peaks between 2 and 7 hz at some sites within the range of our field measurements there is a correlation between lower average shear wave velocity of the top 30 m and 50 higher values of site amplification an area of residential homes thrown from their foundations correlates with high site response this damage may also have been aggravated by local ground deformation severe damage to commercial buildings in the business district however is attributed to poor masonry construction

chromophoric and fluorescent dissolved organic matter in and above the oxygen minimum zone off peru key points two amino acid like and three humic like fdom components were found in and above the oxygen minimum zone off the coast of peru the distribution of cdom and amino acid like fdom covaried with chl a suggesting phytoplankton as their major source presence of dom microbial reworking and dom release by anoxic sediment was illustrated by the distribution of humic like fdom as a result of nutrient upwelling the peruvian coastal system is one of the most productive regions in the ocean sluggish ventilation of intermediate waters characteristic for the eastern tropical south pacific etsp and microbial degradation of a high organic matter load promotes deoxygenation at depth dissolved organic matter dom plays a key role in microbial respiration and carbon cycling but little is known on dom distribution and cycling in the etsp dom optical properties give important insights on dom sources structure and biogeochemical reactivity here we present data and a conceptual view on distribution and cycling of chromophoric cdom and fluorescent fdom dom in and above the oxygen minimum zone omz off peru five fluorescent components were identified during parafac analysis highest intensities of cdom and of the amino acid like fluorescent component c3 occurred above the omz and coincided with maximum chl a concentrations suggesting phytoplankton productivity as major source high intensities of a marine humic like fluorescent component c1 observed in subsurface waters indicated in situ microbial reworking of dom fdom release from inner shelf sediment was determined by seawater analysis and continuous glider sensor measurement and included a humic like component c2 with a signature typical for terrestrially derived humic acids upwelling supplied humic like substances to the euphotic zone photo reactions were likely involved in the production of a humic like fluorescent component c5 our data show that variable biological and physical processes need to be considered for understanding dom cycling in a highly dynamic coastal upwelling system like the etsp off peru

effect of soil conditions on predicted ground motion case study from western anatolia turkey abstract we present a site effect study for the city of izmir western anatolia turkey local amplification was evaluated using state of practice tools ten earthquakes recorded at 16 sites were analysed using spectral ratios relative to a reference site horizontal to vertical spectral ratios and an inversion scheme of the fourier amplitude spectra of the recorded s waves seismic noise records were also used to estimate site effects the different estimates are in good agreement among them although a basic uncertainty of a factor of 2 seems difficult to decrease we used our site effect estimates to predict ground motion in izmir for a possible m6 5 earthquake close to the city using stochastic modelling site effects have a large impact on psv pseudospectral velocity where local amplification increases amplitudes by almost a factor of 9 at 1 hz relative to the firm ground condition our results allow identifying the neighbourhoods of izmir where hazard mitigation measurements are a priority task and will also be useful for planning urban development

source parameters of loma prieta aftershocks and wave propagation characteristics along the san francisco peninsula from a joint inversion of digital seismograms abstract aftershock recordings from 10 digital seismographs deployed along the axis of the san francisco peninsula from the epicenter of the 18 october 1989 loma prieta earthquake to san francisco provide the data for a joint inversion for source wave propagation and site effects the inversion procedure is a least squares method using the singular value decomposition algorithm and is done in two parts the first solves only for the source and attenuation terms for all events and the second projects the residuals from the first inversion onto the site terms the coefficient of geometrical spreading is solved for as part of the model of attenuation residuals plotted versus distance and frequency provide a check on the appropriateness of the attenuation model the earthquakes are all located near the san andreas fault mostly in the aftershock zone of the loma prieta mainshock but some area near daly city on the peninsula the data set provided by geos recorders consists of 148 digital seismograms for 33 events from 10 sites that form a linear profile from the epicentral region to the city of san francisco a distance of approximately 100 km strong motion accelerograms are also available for the larger events body wave spectra are fit surprisingly well by our model especially at distances beyond about 20 km where differences due to radiation pattern effects and changes in angle of incidence become less important results from the inversion show that estimates of moment are only 1 higher for the p waves compared to the s waves apparently there are no major biases in the estimates as each is determined independently of the other p wave corner frequencies are 1 5 times higher than s wave corner frequencies on average stress drops with an average of 176 bars for events that range in magnitude from 1 7 to 4 2 are higher than those found in previous studies where site and path were not explicitly taken into account the exponents of geometrical spreading are very close to one 1 019 for p waves and 1 017 for s waves for this data set which extends to about 100 km the whole path q assumed to be frequency independent was about 800 for both p and s waves specifically 790 for p waves and 830 for s waves the average p wave attenuation at all sites is quantified by a t of 0 020 sec which compares to 0 03 sec for s waves t determined for each site range from a low of 0 008 sec at san bruno mountain a hard rock site to a high of 0 065 sec at corralitos in the epicentral area residuals of the model fits which are distance dependent show larger amplitudes near the epicenter but fall off with a minimum at about 30 km where amplitudes are about 30 to 60 lower than average at distances farther than 30 km amplitudes rise out to the limit of the data at about 105 km where amplitudes are about average at frequencies less than 10 hz but are slightly above average for higher frequencies the location of this low in amplitude can be explained as the boundary between the amplitude decay of direct arrivals and the emergence of wide angle reflections from the lower crust and moho which increase in amplitude beyond 30 km these amplitudes correlate with damage patterns from the mainshock as less damage was observed along the peninsula but appeared to increase towards san francisco even when damage in the marina and china basin districts of san francisco are not included in the damage census

ground motion amplification geology and damage from the 1989 loma prieta earthquake in the city of san francisco main shock and aftershock accelerograms from the loma prieta earthquake sequence are analyzed to study relative ground amplification at sites in the city of san francisco main shock spectra are estimated for those stations that recorded only aftershocks by comparing their records to those from stations that recorded both the ratios of the spectra with respect to a stable bedrock station located at fort mason show distinct spectral shapes that are strongly correlated with the rock type underlying the site earthquake damage is quantified using estimates of the modified mercalli intensity at the recording sites and safety tags issued by the city of san francisco building department these intensities are compared with ground amplification in three frequency bands and with the intensities at the same sites from the 1906 san francisco earthquake there is sufficient correlation to show that site amplification is one of the controlling factors in damage distribution

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

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

comparison of seismic body wave and coda wave measures of q measurements of seismic attenuation q 1 can vary considerably when made from different parts of seismograms or using different techniques particularly at high frequencies these discrepancies may be methodological or may reflect earth processes to investigate this problem we compare body wave with coda q 1 results utilizing three common techniques i parametric fit to spectral decay ii coda normalization of s waves and iii coda amplitude decay with lapse time q 1 is measured from both body and coda waves beneath two mountain ranges and one platform from recordings made at seismic arrays in the caucasus and kopet dagh over paths 4 long if q is assumed frequency independent spectral decay fits show q s and q coda near 700 800 for both mountain paths and near 2100 2200 for platform paths similar values are determined with the coda normalization technique however frequency dependent parameterizations fit the data significantly better with q s 1 hz and q coda l hz near 200 300 for mountain paths and near 500 600 for platform paths lapse decay measurements are close to the frequency dependent values showing that both spectral and lapse decay methods can give similar results when q has comparable parameterizations above 6 hz coda measurements suggest some enrichment relative to body waves perhaps due to scattering but intrinsic absorption appears to dominate at lower frequencies all approaches show sharp path differences between the eurasian platform and adjacent mountains and all are capable of resolving spatial variations in q

topographic site effects a comparison of two nonreference methods abstract we present an experimental study of topographic site effects the data we use come from an experiment carried out during the summer of 1989 in epire northern greece ten digital stations recorded a total of 68 small earthquakes a recent article chavez garcia et al 1996 presented a comparison between site effects determined using horizontal to vertical spectral ratios hvsr for this data set and theoretical modeling in this note we compare the topographic site effects determined using hvsr with another independent experimental estimate a generalized inversion scheme gis neither hvsr nor gis depend on the availability of a reference site we obtain a very good agreement between both estimates of topographic site effects for both horizontal components our results support the use of hvsr to determine topographic site effects

comment on simultaneous study of the source path and site effects on strong ground motion during the 1989 loma prieta earthquake a preliminary result on pervasive nonlinear site effects by byau heng chin and keiiti aki

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

determination of site amplification in the los angeles urban area from inversion of strong motion records abstract the amplification of strong ground motion at sites in the greater los angeles california region is determined using the generalized inverse method of andrews 1986 site amplification estimates are determined at 281 strong motion sites that provided horizontal component accelerograms from the 1971 san fernando 1987 whittier narrows 1991 sierra madre or 1994 northridge mainshocks the estimates are determined relative to the spectral level recorded at a single reference site in a second inversion a source site interaction term is added to andrews9s 1986 model to quantify the effect selected mainshock records have on site amplification estimates the source site interaction term is applied to the san fernando valley sites9 records of the northridge earthquake and to three los angeles basin sites9 records of the whittier narrows mainshock site amplification spectra are averaged within two frequency bands the intermediate frequency band ifb from 0 5 to 1 5 hz and the high frequency band hfb from 2 to 6 hz results are displayed on maps of surficial geology average spectral levels are correlated with average shear wave velocity in the uppermost 30 m a geotechnical parameter used by the national earthquake hazards reduction program nehrp to characterize site amplification statistically significant correlation is found in both frequency bands average spectral amplification levels for nehrp class b c and d sites in the la urban area are determined these averages display an expected increase in spectral amplification with category with a more pronounced variation in the ifb than in the hfb considerable overlap in the one standard deviation range of the c and d site levels is found in both ifb and hfb suggesting that site specific spectral amplification is influenced by more than just near surface shear wave velocity average site amplification levels are compared with those obtained from northridge aftershock records at 28 collocated sites and are found to be in reasonably good agreement in both frequency bands some interesting outliers that is sites for which the mainshock and aftershock determined amplifications differ significantly are identified as sites that experienced ground failure during the northridge mainshock

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

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

apparent break in earthquake scaling due to path and site effects on deep borehole recordings 1 we reexamine the scaling of stress drop and apparent stress rigidity times the ratio between seismically radiated energy to seismic moment with earthquake size for a set of microearthquakes recorded in a deep borehole in long valley california in the first set of calculations we assume a constant q and solve for the corner frequency and seismic moment in the second set of calculations we model the spectral ratio of nearby events to determine the same quantities we find that the spectral ratio technique which can account for path and site effects or nonconstant q yields higher stress drops particularly for the smaller events in the data set the measurements determined from spectral ratios indicate no departure from constant stress drop scaling down to the smallest events in our data set mw 0 8 our results indicate that propagation effects can contaminate measurements of source parameters even in the relatively clean recording environment of a deep borehole just as they do at the earth s surface the scaling of source properties of microearthquakes made from deep borehole recordings may need to be reevaluated

source spectra and site response from p and s waves of local earthquakes in the oaxaca mexico subduction zone we compare p and s wave source functions to determine the level of p s radiation at high frequencies for this purpose spectral records from local earthquakes with magnitudes ranging between 2 8 and 4 8 are inverted to separate source and path effects we used digital records from 16 local events recorded at short hypocentral distances r 91 km in the region of oaxaca mexico since most of the seismograms used were recorded on scale and the three components of velocity are available this data set offers the opportunity of comparing source spectra and site response of p and s waves the digital seismograms analyzed were recorded by a temporary network of six seismic stations five of them located on hard rock sites and one on sediments the spectral amplitudes of the records used were corrected for attenuation with a 1 r geometrical spreading function r being the hypocentral distance and using estimates of q of p and s waves reported by castro and munguia 1993 for the region the corrected amplitudes were then inverted to separate source and site responses for 15 different frequencies selected between 1 and 25 hz the site responses for p and s waves have similar trends however the maximum p wave amplification shifts toward higher frequencies relative to the maximum s wave amplification in consistency with the expected vp vs ratio while the stations located on metamorphic rocks show small amplifications in narrow frequency bands the station sited on sediments shows amplification factors ranging from 2 to 10 in a broader frequency band in general the p wave source spectra show corner frequencies greater than those of the s wave although the s wave source acceleration spectra exceed the spectral amplitudes of p at low frequencies f 10 hz the p s source function ratios tend to increase at frequencies greater than 5 hz beyond 10 hz p wave source spectra are frequently above the s wave source amplitudes these results indicate that the high p wave radiation observed in the spectral records is a source effect and also support the hypothesis proposed by brune et al 1989 and brune and anooshehpoor 1992 that normal vibrations are an important characteristic of the rupture process

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

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

a general inversion scheme for source site and propagation characteristics using multiply recorded sets of moderate sized earthquakes abstract we derive an inversion scheme that fits the logarithms of seismic body wave spectra to 2 source shapes conditioned by anelastic attenuation with a frequency independent q the residuals from this fit are then projected onto the set of sources and sites with the source terms damped to estimate residual source spectra and site response spectra this two part inversion process is iterated until the square of the residuals summed over frequency for all recordings is minimized the absolute amplitudes of the source and site spectra are determined by fitting the site response spectra to estimates of the site amplification derived from geotechnical data we apply this inversion process to three sets of recordings of loma prieta aftershocks the first data set contains s waves recorded both in the marina district and at nearby hard rock stations we constrain the site response using the s wave velocity measured in a nearby borehole the absolute site amplifications for the marina stations are 15 to 20 at 1 hz and 10 at 2 5 hz the other two data sets comprise the p and s wave recordings from 28 aftershocks ranging in size from m d 2 1 to 4 6 obtained at accelerograph sites near the epicentral area the p and s wave attenuations determined from the inversions are q 414 and 380 while the average near site attenuations are t 0 012 and 0 026 sec respectively the average ratio of the s to p wave seismic moments is 1 1 0 1 the ratio of the p to s wave corner frequencies is 1 24 0 11 and the ratio of s to p wave radiated energy is 13 9 3 2 the brune stress drops for these aftershocks range from 8 to 800 bars the largest earthquakes exhibit the largest stress drops but there is no apparent dependence of stress drop on seismic moment for earthquakes with 10 19 m 0 21 dyne cm

site amplification from coda waves validation and application to s wave site response abstract the subject of this article is to see if the response to s waves at a specific site can be determined from coda waves we used aftershock records of the 1992 landers and big bear earthquakes observed at four terrascope stations pas pfo svd and gsc in southern california the wide dynamic range of the terrascope instrument provided a broadband seismogram for s waves and at the same time coda waves over a long lapse time window we analyzed a total of 708 three component waveforms from 65 events whose magnitudes ranged from 3 3 to 5 4 the station svd is located on quaternary alluvium and the remaining stations are on rocks of cretaceous age we first evaluated the coda site amplification factor using a spectral ratio method may eda et al 1991 koyanagi et al 1992 for frequency bands centered at 0 75 1 0 1 5 2 0 3 0 4 0 and 6 0 hz for lapse time windows after 70 sec from the origin time we found that the site amplification factor for the radial component is similar to that for the transverse component this similarity was expected because codas consist of backscattered waves coming from all directions into the station resulting in a directionally averaged site response we then evaluated the s wave site amplification factor using an inversion method andrews 1986 on the direct s wave portion of the same data set we found that the coda site response agrees with the s wave site response within a factor of about 1 5 supporting the earlier findings by tsujiura 1978 and justifying the use of coda waves in determining site specific amplification for s waves this agreement between s wave site response and coda site response is understandable if we assume that coda waves are dominated by s waves as theoretically predicted by aki 1992 and zeng 1993

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

an empirical study of earthquake source spectra for california earthquakes we generalize source path and site effects for california earthquakes as functions of magnitude and distance based on regression analysis of 1000 fourier acceleration spectra from 43 california earthquakes in the moment magnitude range from 4 4 to 7 4 recorded at rupture distances from 1 to 200 km empirically derived source spectra for california earthquakes are generally inconsistent with the spectral shape implied by a brune omega squared point source model this is manifested by magnitude and frequency dependence of the brune model parameters for ex ample the brune stress parameter that best matches the data at high frequencies decreases from a value of about 120 bars at m5 5 to a value near 50 bars at m7 5 at frequencies below 1 hz though source spectra have much lower spectral ampli tudes than predicted by the brune point source model for these values of stress this discrepancy grows with increasing magnitude finite fault simulations indicate that this is a consequence of the breakdown of the validity of the point source approxi mation for large ruptures a stochastic finite fault model in which the fault is dis cretized as a number of subfaults each of which is represented by a brune omega squared point source correctly matches the observed spectral shapes and amplitudes the spectral decay parameter kappa representing average near surface attenuation of high frequency motion at rock sites increases with increasing magnitude from values near 0 035 sec at m5 5 to 0 050 sec at m7 5 magnitude dependence of kappa may be interpreted as evidence of nonlinearity for typical california sites subjected to strong ground motion comparisons of our empirical source spectra for california to corresponding spec tra for eastern north america suggest that the spectral amplitudes are similar in the two regions for low frequency motions f 2 hz for m5 5 f 0 5 hz for m7 5 for equivalent crustal conditions the eastern events appear to have enhanced high frequency near source amplitudes relative to the california events this is particularly pronounced for large magnitude earthquakes

seismic source parameters for microearthquakes of the granada basin southern spain abstract ninety five microearthquakes of the granada basin southern spain with duration magnitudes ranging from 1 3 to 3 5 moment magnitude from 0 9 to 2 5 have been spectrally analyzed with digital recordings from ten stations of the andalusian seismic network the coda q quality factor qc assumed to be frequency dependent was used instead of q for the path correction and the q adopted was equal to 9 4 qc the k parameter for site attenuation correction had values in the range of 0 02 0 05 fourier displacement spectra were calculated for p and s waves and analyzed with respect to the source model of brune 1970 1971 spectral parameters were automatically determined by the method of snoke 1987 the seismic moment ranged from 1017 to 1020 dyne cm with average m0 p to m0 s ratio of 1 02 and the source radii spanned from 0 14 to 0 40 km with average r p to r s ratio of 1 3 the seismic energy values ranged from 1 103 to 9 107 j with p to s energy ratio from 1 to 10 for about 85 of the events whereas the static stress drop varied from 0 02 to 2 6 bar the scaling relation between the seismic moment and the stress drop indicates a decrease in stress drop with decreasing seismic moment apparent stress is linearly correlated with stress drop and have on average similar values and this points out to a partial stress drop

attenuation of high frequency shear waves in the crust measurements from new york state south africa and southern california we compare the attenuation of high frequency 3 30 hz shear waves for crustal paths in new york state south africa and southern california over source receiver distances of about 10 400 km the data consist of digital recordings of s waves 5 100 km and lg waves 100 400 km produced by earthquakes we use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the s and lg waves over the entire distance range studied 10 400 km the amplitude decay of 3 hz shear wave energy is considerably less for the tectonicaily stable areas of new york and south africa than for the tectonicaily active region of southern california high frequency 30 hz s wave attenuation is significantly less for new york and south africa than for southern california for distances between 15 and 90 km we parameterize the decay with distance r of coda normalized shear wave amplitudes with a frequency independent q and geometrical spreading exponent where geometrical spreading is proportional to r for new york state the s wave amplitude decay 3 30 hz is well described by a frequency independent q of 2100 330 490 and of 1 3 0 1 the decay of lg wave amplitudes from 3 to 15 hz in the new york state region is fit with a frequency independent q of 1600 280 330 of 0 70 0 2 the s wave amplitudes 3 30hz in south africa yield a q of 1500 190 380 and of 1 3 0 1 fixing the geometrical spreading at r 0 5 produces an lg wave q estimate at 3 hz in south africa of 360 50 80 this lg wave q is low considering that south africa is a cratonic tectonicaily stable area the s wave amplitudes from southern california are described with a frequency independent q of 800 150 240 and a large geometrical spreading exponent of of 1 9 0 2 we find an lg wave q at 3 hz of 260 30 for southern california after constraining the geometrical spreading at r 0 5

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

deep crustal structure and genesis from contrasting reflection patterns an integrated approach summary an integrated approach including technique development into data acquisition processing and modeling is used to aid the final interpretation of crustal reflection data by minimizing the many unknowns thus a project may start out with a series of experiments involving data acquisition designed to attack a particular problem and conclude with modeling as a test to the interpretation of a problem

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

the effect of quaternary alluvium on strong ground motion in the coyote lake california earthquake of 1979 abstract the effect of alluvium on strong ground motion can be seen by comparing two strong motion records of the coyote lake california earthquake of 6 august 1979 m l 5 9 one record at a site on franciscan bedrock had a peak horizontal acceleration of 0 13 g and a peak horizontal velocity of 10 cm sec the other at a site 2 km distant on 180 meters of quaternary alluvium overlying franciscan had values of 0 26 g and 32 cm sec amplifications by factors of 2 and 3 horizontal motions computed at the alluvial site for a linear plane layered model based on measured p and s velocities show reasonably good agreement in shape with the observed motions but the observed peak amplitudes are greater by a factor of about 1 25 in acceleration and 1 8 in velocity about 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance the remainder may represent focusing by refraction at a bedrock surface concave upward there is no clear evidence of nonlinear soil response fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model in particular the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value indicating that no significant nonlinearity occurs in the secant shear modulus computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0 5 to 1 0 or greater the data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections and more the simple effect of the low near surface velocity application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear wave velocity

the chronology of the last deglaciation implications to the cause of the younger dryas event it has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions this extraordinary event referred to by european palynologists as the younger dryas was centered in the northern atlantic basin evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern atlantic ocean further it appears that these mode changes may have been triggered by diversions of glacial meltwater between the mississippi river and the st lawrence river drainage systems we report here accelerator mass spectrometry ams radiocarbon results on two strategically located deep sea cores one provides a chronology for surface water temperatures in the northern atlantic and the other for the meltwater discharge from the mississippi river our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern atlantic basin with the meltwater history for the laurentian ice sheet

site response estimation from earthquake data abstract aftershocks of the 1989 loma prieta california earthquake are used to estimate site response along the san francisco peninsula a total of 215 shear wave records from 24 sources and 21 sites are used in a linear inversion for source and site response spectra the methodology makes no assumptions about the shape of the source spectrum however to obtain a stable unique inverse a q model and geometrical spreading factor are assumed as well as a constraint on site response that sets the site response averaged over two specific stations to 1 0 site responses calculated by this formulation of the problem are compared with other studies in the same region that use different methodologies and or data the shear wave site responses compare favorably with estimates based on an 2 constrained source model comparison with coda amplification factors is not as close but still favorable considering that the coda values were determined for nearby locations with similar geology and not the same sites the degree of agreement between the three methods is encouraging considering the very different assumptions and data used

the attenuation of seismic intensity in italy a bilinear shape indicates the dominance of deep phases at epicentral distances longer than 45 km the attenuation of seismic intensity with distance in italy is analyzed by using felt intensity report data obtained from two comprehensive historical databases recently made available the observed attenuation pattern that in the past was interpreted as a logarithmic or root square or cubic attenuation law shows quite clearly two different linear trends in the near and in the far field at distances shorter than 45 km the decrease of the intensity with distance is about one degree per 20 km while at longer distances the slope is about one degree per 50 km this is in agreement with some recent findings of realistic modeling of seismic ground motion that has been explained as the transition from upper crust direct sg phases to waves reflected at the moho controlling the energy main release the slope of the curve in the far field shows a regional dependence in agreement with recent works on the attenuation of pn and sn phases in italy if effective this correlation might allow us to discriminate the contribution of crustal and subcrustal paths in seismic intensity attenuation studies manuscript received 31 may 2000

estimation of source parameters and scaling relations for moderate size earthquakes in north west himalaya abstract the scaling relation and self similarity of earthquake process have been investigated by estimating the source parameters of 34 moderate size earthquakes m b 3 4 5 8 occurred in the nw himalaya the spectral analysis of body waves of 217 accelerograms recorded at 48 sites have been carried out using in the present analysis the brune s 2 model has been adopted for this purpose the average ratio of the p wave corner frequency fc p to the s wave corner frequency fc s has been found to be 1 39 with fc p fc s for 90 of the events analyzed here this implies the shift in the corner frequency in agreement with many other similar studies done for different regions the static stress drop values for all the events analyzed here lie in the range 10 100 bars average stress drop value of the order of 43 19 bars for the region this suggests the likely estimate of the dynamic stress drop which is 2 3 times the static stress drop is in the range of about 80 120 bars this suggests the relatively high seismic hazard in the nw himalaya as high frequency strong ground motions are governed by the stress drop the estimated values of stress drop do not show significant variation with seismic moment for the range 5 10 14 2 10 17 n m this observation along with the cube root scaling of corner frequencies suggests the self similarity of the moderate size earthquakes in the region the scaling relation between seismic moment and corner frequency m o f c 3 3 47 10 16 n m s 3 estimated in the present study can be utilized to estimate the source dimension given the seismic moment of the earthquake for the hazard assessment the present study puts the constrains on the important parameters stress drop and source dimension required for the synthesis of strong ground motion from the future expected earthquakes in the region therefore the present study is useful for the seismic hazard and risk related studies for nw himalaya

ground motion at the san francisco international airport from the loma prieta earthquake sequence 1989 abstract following the loma prieta earthquake the u s geological survey installed four portable digital seismic recorders at the san francisco international airport sfo for one week to study aftershock ground motion at this important bay area lifeline this study was motivated largely by the need to anticipate strong ground motion from future major earthquakes affecting the bay area and to a lesser extent by the fact that sfo was shut down for 13 hours owing to damage from the loma prieta shock accordingly the recording sites were chosen so as to elucidate the effects of varying thicknesses of low velocity surficial alluvium on the ground motion three large aftershocks with magnitudes ranging from 4 2 to 4 5 each produced ground motion that was recorded at all four sfo stations one of our stations was collocated with a permanent ground motion recorder that indicated a peak horizontal velocity of 29 cm sec and a peak horizontal acceleration of 0 33 g during the 18 october mainshock from the aftershock data and one mainshock record it is possible to extrapolate approximately the mainshock ground motion to other locations at sfo and more generally to assess the effects of low velocity sedimentary cover including artificial fill material on the character of the ground motion the main shock ground motion recorded at the permanent station was apparently typical for most of sfo where the near surface alluvium resulted in peak horizontal ground velocity in the frequency band 0 1 to 3 hz amplified by a factor of about 2 5 relative to that recorded at bedrock sites observations in the epicentral distance range 59 to 95 km including sfo of the moho reflected phases pmp and sms from the aftershocks support the hypothesis presented elsewhere that the phase sms accounted for much of the peak ground motion throughout most of the san francisco bay area

physical properties and structure of the lower crust revealed by one and two dimensional modelling summary data from a refraction and a reflection seismic survey in the black forest southwest germany are used for extensive one and two dimensional modelling the data are available along approximately the same line and therefore the same piece of crust is probed by two seismic methods we utilize this favorable circumstance for detailed model calculations concerning both data sets lower crustal properties vary on the scale of a wavelength and thus full solutions of the elastic equations are required the reflectivity method for the evaluation of refraction seismograms and numerical solutions of the acoustic wave equation for the reflection response details of the geometry and physical properties of the lamination are derived vertical layering on a scale of 100 m is found horizontal extent of reflecting elements is in the range of a few hundreds of meters rocks with velocities between 5 6 and 7 2 kds constitute the lower crust

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