earthquake locations using single station deep borehole recordings implications for microseismicity on the san andreas fault in southern california we have located and estimated source parameters of 109 earthquakes 0 to 5 m using seismograms recorded at 2 5 km depth in the cajon pass borehole southern california the borehole is about 4 km from the san andreas fault at the boundary between the locked almost aseismic mojave and san bernardino segments where the san jacinto fault approaches the san andreas this area is of interest both on account of its tectonic complexity and its high potential seismic hazard the clear relatively unattenuated downhole recordings are rotated to determine the incoming azimuth of the p wave and the delay time between the p and s arrivals is used to estimate the hypocentral distance the difference between the hypocenters located this way and the southern california seismic network scsn locations of the same earthquakes increases with hypocentral distance from the borehole of the earthquakes within 20 km of the borehole 95 are within 1 2 km of the scsn epicenters and all are within 5 km of the scsn depths all but three of the 58 earthquakes located here which the scsn did not record were within 20 km of the borehole and so the location errors are similar to those of scsn a and b quality events most of the small earthquakes occurred within the relatively active san jacinto fault zone but at least eight are located close to or within the san andreas fault zone the stress drops for these events range from 0 1 to 18 mpa these earthquakes appear similar to the other events in this study suggesting that the san andreas fault is similar to other faults at the scale of these small earthquakes the variation in stress drop and slip orientation of these small earthquakes suggests that larger scale heterogeneity continues to these small scales also the location of these small earthquakes in major established fault zones implies that earthquake source dimensions are not geometrically controlled by fault zone width

the attenuation of seismic shear waves in quaternary alluvium in santa clara valley california we used shear waves generated by an air powered source at the ground surface and recorded in a borehole to estimate the shear wave quality factor at strong motion station gilroy no 2 we find similar values of q using both the decay of the spectra with depth and the slope of the spectral ratio at two depths we find no evidence of a frequency dependence of q the mean value of q over the depth range 10 to 115 m is close to 10 the use of this value over the depth of the borehole and the observed travel time of 0 358 sec gives a cumulative attenuation factor t of 0 036 sec for the upper 180 m of the quaternary alluvium this is comparable to the differential decay between gilroy no 2 and a rock site 1 9 km away gilroy no 1 as measured from the decay of the high frequency spectra of accelerograms from large earthquakes plotted on a log linear scale t 0 05 0 04 and 0 03 sec for the 1979 coyote lake 1984 morgan hill and 1989 loma prieta earthquakes respectively the similarity between the attenuations measured from the low strain surface source and those from the larger amplitude earthquake sources suggests that increases of damping due to nonlinear wave propagation effects are limited

the relation between site amplification factor and surficial geology in central california abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 132 stations of the usgs seismic network in central california from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method we found that the site amplification at a station is systematically related to the geology underlying that station the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz the rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies to quantify the relation between site amplification factor and site geology the surface geology of station sites was classified into five groups namely 1 quaternary sediments 2 pliocene sediments 3 miocene through cretaceous sediments 4 franciscan formation and mesozoic granitic rocks and 5 pre cretaceous metamorphic rocks the station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group a smooth power law relation was found between the mean site amplification and the median geologic age this relation provides a simple way of estimating site effect at a specific site with known surficial geology our site amplification factors were compared with magnitude station residuals determined by eaton 1990 for the same network a remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value confirming that the coda method provides an effective means of estimating site amplification factor comparison of our site amplification factor with strong motion results obtained from the loma prieta earthquake boore et al 1989 maley et al 1989 shakal et al 1989 chin and aki 1991 suggests that weak and strong motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km within the epicentral source region however the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion suggesting a nonlinear behavior at sediment sites these results together with studies on the correlation between weak motion site amplification and earthquake intensity borcherdt 1970 borcherdt and gibbs 1976 chavez garcia et al 1990 king et al 1990 demonstrate the importance of in situ determination of weak motion site amplification for seismic zonation

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

high frequency site effects at two parkfield downhole and surface stations abstract using stacked ratios of microearthquake source spectra recorded at the surface and 200 m downhole we have established the p and s wave site effect at two locations in parkfield california the frequency responses of both sites are bimodal being completely dominated by multiples and amplification below 10 hz and by attenuation above 30 hz the low frequency amplification factors show roughly three to five times more uphole motion compared to that downhole the multiples produce peaks with a maximum of nearly 10 times more uphole motion above 50 hz the downhole motion is always larger approaching a factor of 5 at the high frequency limits of our study when fit with a linear attenuation model the high frequency ratios yield compressional and shear q values between 6 and 19 the data were divided into subsets based on event location and time of year statistical comparison of these subsets shows no systematic variation with azimuth or season the results of our study accounts for the lack of high frequency surface reflections in the parkfield downhole data but not for the lack of source scaling of parkfield microearthquakes

lithologic column of the arkoma drillhole and its relation to the cajon pass deep drillhole the 1795 m arkoma federal 1 26 well 48 5 m from the cajon pass deep drillhole cpddh provides additional lithologic and structural data pertinent to that project basement surface was encountered 158 m deeper than in the cpddh rock units correlate well between the holes the offset persists to 1128 m below this depth both lithologic unit thicknesses and fault zones correlate between holes on subhorizontal projections a combination of previously unrecognized high angle and low angle faults of several ages are required to explain the structures blind low angle faults may be regionally important

stress orientation inferred from shear wave splitting in basement rock at cajon pass s waves generated by mode conversion of p waves from surface compressional sources and recorded by oriented geophones at depths between 1300 and 1820 meters in the cajon pass scientific drillhole arrive at higher apparent velocities when polarized along the direction n70 e than when polarized in the orthogonal direction the observed polarization dependent velocities are consistent with seismic anisotropy due to in situ fractures or cracks that are preferen tially aligned n70 e subnormal to the san andreas fault this result is consistent with the maximum principal horizontal stress direction at cajon pass also being subnormal to the san andreas fault

seismic wave attenuation from borehole and surface records in the top 2 5km beneath the city of basel switzerland summary we investigate attenuation q 1 of sediments of 2 5 3 5 km thickness underneath the city of basel switzerland we use recordings of 195 induced events that were obtained during and after the stimulation of a reservoir for a deep heat mining project in 2006 and 2007 the data set is ideally suited to estimate q as all events are confined to a small source volume and were recorded by a dense surface network as well as six borehole sensors at various depths the deepest borehole sensor is positioned at a depth of 2 7 km inside the crystalline basement at a mean hypocentral distance of 1 8 km this allows us to measure q for frequencies between 10 and 130 hz we apply two different methods to estimate q first we use a standard spectral ratio technique to obtain q and as a second measure we estimate q in the time domain by convolving signals recorded by the deepest sensor with a q operator and then comparing the convolved signals to recordings at the shallower stations both methods deliver comparable values for q we also observe similar attenuation for p and s waves qp qs as expected q increases with depth but with values around 30 50 it is low even for the consolidated permian and mesozoic sediments between 500 and 2700 m

changes in vp vs with depth implications for appropriate velocity models improved earthquake locations and material properties of the upper crust properties associated with the arrival times of shallow microearthquakes are determined for three tectonically different sites soviet central asia the central united states and southern california simple graphical presentations of the earthquake data including wadati and riznichenko diagrams are used to resolve the trade off in origin time and focal depth estimates of the average half space velocities for p and s waves and of the travel time ratio ts tp are also obtained that are relatively free of any model constraints used to initially locate the earthquake in all cases p and s arrivals exhibit a systematic decrease in ts tp with depth in the upper few kilometers this decrease is significant and matches similar variations observed in situ for the velocity ratio vp vs based on laboratory and theoretical studies such observations are consistent with the closing of saturated microcracks with increasing confining pressure independent determinations of velocity structure and rock properties from both refraction studies and borehole experiments show analogous results by specifying separate velocity models for both p and s waves that reflect this variation rather than assuming a constant value of vp vs we find improved stability in hypocenter determination and increased resolution of shallow seismicity patterns

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

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

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

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

site amplification from s wave coda in the long valley caldera region california following the work of phillips 1985 we have computed site amplification factors for coda waves at many sites in the long valley region in the eastern sierra nevada we computed ratios of coda amplitudes measured at 15 stations in and around long valley caldera relative to a granitic site mmpm for six frequency bands centered at 1 5 3 0 6 0 9 0 12 0 and 15 0 hz all station sites located within the caldera experienced large ground motion amplification at 1 5 and 3 0 hz ranging between five and 17 times that of the reference site however at higher frequencies these same sites exhibited significantly less amplification than the reference granite site this is attributed to the competing effects of an impedance contrast between the basement rock and caldera fill and very high absorption in the caldera fill at high frequencies station mmlm located on top of a volcanic plug displayed the largest amplitudes of all the sites studied for frequencies between 9 0 and 15 0 hz a dike structure attached to the plug couples the basement rock to the surface at high frequencies the resulting large amplitudes at mmlm are not due to amplification resulting from a strong impedance contrast rather the absorption under this site is very low perhaps lower than at the reference site mmpm outside the caldera another hard rock site located at devil s postpile mdpm generally behaved like the reference site for all frequencies the lowest amplifications observed came from a site outside the caldera mdcm located on thin pyroclastic ash deposits overlying granitic basement this can be attributed to a dominance of absorption over the amplification caused by lower impedance of this layer variations among sites on similar surface geology may be due to small local variations in impedance and absorption under and adjacent to the site the range in the spectral decay parameter between caldera and rock sites are comparable to results of anderson and hough 1984 for sites on alluvium and rock in the san fernando region these surprisingly different amplifications support the need for additional site specific studies amplifications determined in this study for the frequency range 1 5 and 3 0 hz correlate remarkably well with eaton s 1990 residuals for duration magnitude fmag and amplitude magnitude xmag for the usgs northern california seismic array further supporting the use of coda waves in determining site specific amplification

constraints on the geometries of structures within the sedimentary rocks at the cajon pass scientific drillsite california an analysis of downhole geophysical logs and their relation to surface geological data constrain the structure in the first 700 m of the cajon pass scientific drillhole several angular folds mapped in a small canyon 150 m to the southeast of the drillsite are associated with a steep northeast dipping reverse fault at depth this fault is inferred to cut the scientific drillhole in basement at 700 m and to intercept a nearby oil exploration well arkoma federal 1 26 within the sediments at 350 m the folds mapped at the surface are thought to have developed in late miocene to early pliocene time during the squaw peak thrusting event the inferred reverse fault offsets the sediment basement contact 180 m between the two holes

ground motion and intraevent variability from 3d deterministic broadband 0 7 5 hz simulations along a nonplanar strike slip faultground motion and intraevent variability from 3d deterministic broadband simulations

geologic setting and lithologic column of the cajon pass deep drillhole the cajon pass deep drillhole penetrates a late tertiary basin developed on crystalline basement in the sw mojave desert 4 km from the san andreas fault cores continuous cuttings and geophysical logs from phase i show great diversity in rock types structure and alteration the hole encountered cajon formation arkoses granodiorite tonalite unusual megacrystic granite and augen gneiss granitic and pelitic gneisses with quartzite quartzofeldspathic orthogneiss cut by sheets of hornblende gabbro and interlayered mafic and quartzofeldspathic orthogneisses with rare calcsilicate intervals foliation and compositional layering have low dips throughout the column and layered gneisses contain 10 cm scale recumbent folds faults and alteration zones bound several rock units with low apparent dips basement cores are typically cut by steep fractures 1 mm wide that contain zeolites calcite or chlorite epidote fractures and faults decrease in abundance with depth

empirical distance attenuation and the local magnitude scale for the central united states seismic waveform data from the cooperative new madrid seismic network cnmsn the u s national seismograph network usnsn and stations operated by the u s geological survey and the incorporated research institutions for seismology iu are corrected to the theoretical wood anderson response and the peak amplitudes are used to determine a local magnitude scale for the central united states the distance correction function can be expressed as graphic 1 with amplitude a in millimeters and hypocentral distance r in kilometers showing a weak attenuation with distance synthetic seismograms are calculated based on a detailed crustal model obtained from local well log acoustic data in the mississippi embayment to examine the effect of structure on station factors the results show that the mississippi embayment sediment as a whole may amplify seismic waves at some specific frequencies such as 0 5 and 5 0 6 0 hz with a factor less than 4 0 and may not amplify seismic waves in the frequency band from 0 5 to 9 9 hz on average even though the amplification factors do increase with the source magnitude estimations for b values and magnitudes of earthquakes with a return period of 500 yr in the central united states are performed based on the original new madrid catalog m d or m blg and the newly created m l catalog results show that the b value is smaller for the m l catalog 0 790 yielding one m 7 5 event every 500 yr a result that converges with the paleoearthquake estimate for the region on average the relation between m l and m d or m blg for 1 5 m d 5 0 can be expressed as m l 1 008 m d 0 0714 1 embed inline graphic 1 gif

a comparison of direct s wave and coda wave site amplification determined from aftershocks of the little skull mountain earthquake abstract site amplifications of direct s waves and coda waves are studied and compared using high quality three component digital data from aftershocks of the little skull mountain neveda earthquake we use data from 12 stations installed on a variety of geological and topographic site conditions distributed widely in space with different azimuths and epicentral distances s wave site amplifications are obtained in the frequency range from about 0 5 to over 30 hz while coda wave site amplifications are obtained in a frequency range from about 1 5 to over 30 hz a thorough statistical analysis of these results was performed we find that 1 for s waves all three components at a station follow a similar frequency dependent trend the amplitudes of the two horizontal components match more closely while the vertical component shows consistently lower amplification than the horizontal components at low frequencies 2 for coda waves all three components share both similar frequency dependent trends and amplification level 3 s wave and coda wave site amplification are consistent for stations with epicentral distance greater than about 10 km which is about the average focal depth of the earthquakes we used within the epicentral distance of 10 km however some stations show discordant s wave and coda wave site amplifications possible factors are that the direct s waves are affected by particular wave propagation paths and that at short distance sv is partitioned with more energy on the horizontal components and less on the vertical components than at larger distances

stresses and strains in a twisted subduction zone fiordland new zealand summary in the fiordland region of the south island of new zealand the subducted australian plate steepens from south to north becoming near vertical below 75 km depth between doubtful and milford sounds this steepening is accompanied by a sharp change in strike of the subduction zone of about 17 here we investigate stress and strain in this twisted subduction zone using earthquakes recorded during a three month deployment of 24 portable digital seismographs we determine focal mechanisms of 115 earthquakes of ml 2 5 4 5 using both first motion polarity data and amplitudes of seismogram envelopes we also invert for the stress tensor orientation for regions within both the subducted and overlying plates using a method which invokes the coulomb failure criterion and considers all p wave first motions together regardless of whether or not they are sufficient to define single event focal mechanisms focal mechanisms and stress inversions for earthquakes in the overlying plate indicate normal faulting shallower than 16 km and thrust faulting at greater depths this stress and strain regime is related to the 17 change in strike of the subduction zone which leads to an arching up of the subducted plate at the bend such arching is consistent with the exposure of lower crustal granulite facies rocks near the bend earthquakes in the subducted plate show predominantly normal faulting relative to the plate arrival time inversions for 3 d seismic velocity structure indicate that a zone of high vp 8 5 km s 1 in the uppermost mantle of the overlying plate abuts the vertical portion of the subduction zone stresses and strains in the subducted plate are consistent with a model in which the sharp change in strike of the subduction zone is a consequence of the obliquely converging subducted plate having to bend around this zone the steep dip of the subducted plate to the north appears to be a natural consequence of the minimization of membrane stress in the plate arising from the sharp bend along strike this mechanism for steepening the plate is quite different to that of trench retreat usually used to explain steep subduction

evaluation of methods for estimating linear site response amplifications in the los angeles region we have evaluated two methods of estimating linear site response amplifications and the standard index for classifying sites by comparing the results of each technique to observed ground motions at 33 sites in the los angeles region using velocity and density profiles from 33 boreholes we evaluated the use of the average 30 m shear wave velocity and associated site classifications the quarter wavelength method and the haskell propagator matrix method we correlated the average 30 m shear wave velocity and nehrp site classification at the borehole sites colocated within 290 m of a site with observed ground motion the observed data follow the expected trend of higher ground motion amplifications for lower average shear wave velocities but there is a significant degree of variability also there is a great deal of scatter in the observed amplifications within each national earthquake hazards reduction program nehrp class we used the velocity and density information in the database to calculate the average frequency dependent site response in the frequency ranges 1 3 hz 3 5 hz and 5 7 hz for a one dimensional flat layered structure using the haskell propagator matrix method and the quarter wavelength method there is a general correlation with the ground motion data however once again there is a great degree of scatter although slightly less for the quarter wavelength method we note that both the observed and predicted amplifications can change dramatically over a distance as small as 1 km or less even though all techniques considered give results that follow the expected trend of higher amplifications for softer sediments the predicted site response at any particular site with the current level of information may not be representative of the shaking that will actually occur during an earthquake the disparity between observed and predicted amplifications appears to be a result of oversimplification inherent in the amplificationestimation methods such as the use of average or assumed values for the site conditions in the absence of measured values the smoothing effect of using an average velocity limiting the properties considered to the uppermost 30 m of material and complexities in the wave propagation that are not addressed by these methods

analysis of borehole broadband recordings test site in the po basin northern italy we analyze the broadband recordings from a borehole station installed in the bedrock underneath the po basin northern italy and from a second station temporarily deployed on the surface at the same site to determine the surface sedimentary layer response and test the robustness of the different empirical techniques used to estimate local site effects we investigate the site amplification by evaluating the surface to downhole spectral ratios and the horizontal to vertical spectral ratios on both weak motion events and microtremors all the different empirical methods except the horizontal to vertical spectral ratios on weak motions reveal the main amplification peak at about 0 8 hz although the amplification factors are not always in agreement the 0 8 hz peak is the fundamental mode of the site due to the upper 130 m quaternary alluvium column v s 300 m sec we compare these experimental spectral functions with the theoretical 1d site response including and not including the borehole site response first we evaluate the theoretical transfer function at the surface then at depth and take the smoothed ratio the shallow velocity and attenuation structure at this site is assumed from previous studies we attempt to account for the effect of the reflected wave field in the borehole recordings in two different ways first we include the reflected wave field in the theoretical response function taking the theoretical transfer function9s ratio hst hbt and comparing it to the experimental earthquake ratios of horizontal components recorded at surface and at depth hs hb alternatively we multiply the hs hb earthquake ratio by the coherence and compare it to the surface response function this second approach does not provide a good fit to the amplitude of model response even if the shape of the ratio is in agreement with the theoretic response suggesting that multiplying by the coherence is not a good way to correct for the effect of reflected phases to further understand the goodness of our theoretical 1d model hst hbt theoretical evaluations we simulate the propagation through the shallower soil structure of the seismic waveforms recorded by the borehole sensor for a couple of events obtaining reasonably good agreement with the surface time histories both in phase and amplitude this test site and the results of this study are useful for understanding the influence of sedimentary layers on the ground shaking in the po basin the largest and most populated alluvial basin in italy

an inversion of lg wave attenuation and site response from seismic spectral ratios in the eastern china region abstract both lg wave attenuation and site responses are determined in the eastern china region the region ranges from 26 n to 33 n and 117 e to 124 e we analyze over 1360 three component seismograms from 92 local and regional earthquakes the seismograms are recorded in the distance range of 100 600 km the earthquakes occurred between 2002 and 2012 in the region and varied in magnitude between 2 0 and 4 7 m l a combined inversion method which can simultaneously estimate attenuation and site response from seismic spectral ratios is implemented in the study modeling errors are determined using a delete j jackknife resampling technique fourier spectral amplitudes are measured from lg phases windowed between the group velocity limits 2 6 and 3 6 km s the inversion are performed in a frequency range of 1 6 hz with an interval of 0 2 hz the frequency dependent quality factor is modeled in the form of q q 0 f the eastern china region displays high q 0 and a weaker frequency dependence characterized by the functions q 335 16 f 0 45 0 02 q 411 21 f 0 26 0 01 and q 432 26 f 0 24 0 01 for vertical north south and east west components respectively the values are similar to the average for other stable regions of the globe site responses from 32 stations are estimated in the region most stations do not show obvious amplification the site responses are mainly smooth flat and less than 5 in the frequency range of 1 6 hz

local earthquake wave propagation through mississippi embayment sediments part i body wave phases and local site responses p and s body waves from microearthquakes in the new madrid seismic zone nmsz are investigated at selected sites in an effort to understand wave propagation from future large earthquakes earthquake body waveforms display distinctive features that constrain the nature of p and s wave local site responses and wave propagation within the unconsolidated mississippi embayment sediments modeling of the waveforms demonstrates that a near surface low velocity zone is characteristic of structure within the upper 60 m of the sedimentary column and produces large p and s wave resonance effects that can be used to infer near site conditions site resonance effects change because of velocity heterogeneity between individual receivers but imply that embayment sediments will substantially amplify ground motions at high frequencies site resonance affects p and s wave amplitude spectra and can bias estimates of source and anelastic attenuation parameters travel times of observed body wave phases such as p ppphp the first p wave reverberation within the entire sedimentary column ps sp s and ssphp can be used to estimate the average wave slownesses and poisson s ratio within the embayment sediments an average poisson s ratio of 0 44 is obtained for the central nmsz under station pebm detailed s wave velocities are derived for a nafe drake sediment model using acoustic well logs and the travel time constraints of observed seismic phases v p v s ratios vary from 5 5 near the surface to approximately 2 4 at the base of the sediments use of the well log data in wave calculations also explains much of the nature of p and s wave coda within the waveforms and shows that 1d heterogeneity is a first order influence on seismic wave propagation within the mississippi embayment manuscript received 11 march 2003

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

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

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

qp qs relations in the sedimentary basin of the upper mississippi embayment using converted phases abstract three component digital seismograms recorded by the 40 panda portable array for numerical data acquisition stations in the new madrid seismic zone were analyzed to study seismic wave attenuation in the sedimentary basin using the spectral ratio method a prominent s to p s p converted phase was generated at the boundary between the uppermost sedimentary basin and the underlying paleozoic rocks the direct s wave on the horizontal component is characterized by a lower frequency content than that of the converted s p wave on the vertical component the differences in frequency content between the direct s and the s p converted waves can be attributed to the different attenuation effects between p and s waves in the unconsolidated sediments the spectral ratio between the low frequency s wave and the high frequency converted s p wave from the bottom of the sediments can be used to yield a relationship between q p and q s in the sediments results from panda stations with well constrained spectral ratios in the frequency range from 2 to 25 hz give the q p value ranging from 25 to 60 and q s from 25 to 30 for the sedimentary basin in the upper mississippi embayment

site amplification and attenuation of strong ground motion abstract at the mcgee creek california site three component strong motion accelerometers are located at depths of 166 m 35 m and 0 m the surface material is glacial moraine to a depth of 30 5 m overlying hornfels accelerations were recorded from two california earthquakes round valley m l 5 8 23 november 1984 18 08 utc and chalfant valley m l 6 4 21 july 1986 14 42 utc anti plane shear strains for the round valley and chalfant valley events are less than 1 8 10 5 and 1 2 10 5 respectively by separating out the sh components of acceleration we were able to determine the orientations of the downhole instruments peak sh accelerations at the surface were 5 7 times greater than those at 166 m in both earthquakes a constant phase velocity haskell thomson model was applied to generate synthetic sh seismograms at the surface using the accelerations recorded at 166 m in the frequency band 0 10 to 10 0 hz we compared the filtered synthetic records to the filtered surface data the onset of the sh pulse and the reflections from the interface at 30 5 m are clearly seen in both the synthetics and the recorded data the synthetic records closely match the data in both amplitude and phase the fit between the synthetic accelerograms and the data indicates that the seismic amplification at the surface is a result of the resonance of the surface layers and the contrast of the impedances shear stiffness of the near surface materials the impedance contrast of the materials does not account for all of the amplification of the peak accelerations at the surface the resonances of the layer system below 10 hz contribute the most to this amplification we find that a linear model predicts the soil response for frequencies up to 10 hz material damping is added to the soil layers of the model in the form of a quality factor q the damping moderates the strong resonance effects by allowing energy to radiate into the half space the quality factor q 10 is constant in this frequency range but does not accurately model the attenuation at frequencies above 10 hz

petrophysical analysis of the cajon pass scientific well implications for fluid flow and seismic studies in the continental crust in the framework of the cajon pass scientific drilling project in the vicinity of the san andreas fault system petrophysical analysis of the retrieved core was focused on elastic properties density porosity and permeability simultaneous study of rock mineral composition texture degree of metamorphism and fracture filling yielded a petrophysical classification of the studied rock that is convenient for the interpretation of the downhole logging data given the small percentage of the hole drilled with coring the geophysical data were used to construct a lithologic petrophysical column of the well most of the petrophysical variability seen in this column is the result of laumontite alteration of granitoids during retrograde metamorphism in the zeolite facies the intensity peaks of the alteration process are spatially confined to the oblique reverse faults 25 40 dip angles as observed in the core and borehole televiewer images accompanied by brittle deformation of the adjacent granitoids the marked drop in density velocity and elastic moduli and the noticeable increase in porosity and permeability of these altered fault zone rocks are enhanced owing to in situ stress shielding the low velocity and density and higher permeability associated with these faults makes them detectable seismic reflectors and hydrologic conduits in the upper crust by contrast based on our data analysis the matrix of the intact crystalline rock in situ is inferred to be essentially free of microcracks and interconnected channels with free water a detailed analysis of the various factors affecting the microcrack introduction into the cores retrieved from deep and superdeep holes is presented to show that a unique interpretation of in situ stresses from analysis of microcracks in cores is not possible without a better understanding of the mechanisms of crack formation

two major modes of variability of the east asian summer monsoon we study the two primary modes of variability associated with the east asian summer monsoon as identified using a multivariate empirical orthogonal function eof analysis the second mode is shown to be related to changes in intensity of the south asian high at 100 hpa while consistent with previous work the first mode is associated with an index for the shear vorticity of the 850 hpa zonal wind over the monsoon region we show that a linear dry dynamical model when driven by the diabatic heating anomalies associated with each mode can reproduce many of the anomalous circulation features especially for the first eof and in the lower troposphere the model results indicate the importance of diabatic heating anomalies over the tropical indian ocean in the dynamics of both modes especially eof 1 and illustrate the role of local diabatic feedback for intensifying the circulation anomalies in particular the subtropical anticyclonic anomalies that are found in the positive phase of both modes and the circulation anomaly associated with the meiyu changma baiu rain band a running cross correlation analysis shows that the second eof is consistently linked to both the decaying and the onset phase of el nino southern oscillation enso events throughout the study period 1958 2001 we attribute the connection in the onset phase to zonal wind anomalies along the equator in the west pacific associated with this mode on the other hand a link between the first eof and enso is found only in the post 1979 period we note also the role of sea surface temperature anomalies in the tropical indian ocean in the dynamics of eof 1 and a link to the variability of the indian summer monsoon in the case of eof 2 copyright 2010 royal meteorological society

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

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

seismic attenuation above 10 hz in southern california from coda waves recorded in the cajon pass borehole we estimate the attenuation around cajon pass southern california for frequencies above 10 hz and we find total q to exhibit only weak frequency dependence ranging from 800 at 10 hz to 1500 at 100 hz the intrinsic attenuation is approximately twice the level of the scattering attenuation measurements are made using earthquake seismograms recorded at 0 3 km depth using the multiple lapse time window method the results are not dependent on receiver depth and are consistent with previous estimates of q made from direct waves recorded at 2 5 km depth our q values are therefore thought to be representative of the seismogenic crust and the technique used is uncontaminated by the highly attenuating near surface at least above 10 hz we also calculate q from surface data between 1 and 10 hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at 10 hz q exhibits strong frequency dependence below 10 hz 1 8 consistent with previous studies in active tectonic regions and only weak frequency dependence at higher frequencies 0 34 this change in behavior renders it unwise to extrapolate q measurements outside the frequency range from which they were derived for example in earthquake source studies possible factors responsible for the apparent change in frequency dependence of q are considered further work is required however to resolve the causes of this change

source parameters of small earthquakes recorded at 2 5 km depth cajon pass southern california implications for earthquake scaling a 2 5 km deep triaxial seismometer at cajon pass in southern california has recorded several hundred earthquakes ml4 0 occurring within the san andreas fault system at 2 5 km seismic background noise is below amplifier sensitivity and the 2 250 hz spectral range of recorded seismic motion is wider and higher than that of most natural event catalogs compared with downhole recorded motion seismic amplitudes at the surface are amplified below 10 hz and severely attenuated above 30 hz we estimate that qs is at least 1000 for wave motion at 2 5 km and below and qp is over 2000 the range of source dimensions in the downhole recorded catalog is 10 m to 70 m ml 2 0 mo 108 nm to ml 2 7 mo 1013 nm the plot of log source radius vs log moment has a straight line trend compatible with earthquake scaling at constant stress drop inferred stress drops are scattered between 1 and 500 bars there is no evidence in the catalog for the proposed minimum source dimension at 100 m when the cajon pass borehole catalog containing some of the smallest recorded natural earthquakes is combined with 800 larger events from previous studies the moment radius trend suggests that natural earthquakes are self similar over a magnitude range m 2 to 8 we suggest that inferences of minimum source dimension are more likely due to bias in bandlimited individual catalogs than to properties of the seismic crust

regional and local factors in attenuation modelling hong kong case study seismic attenuation behaviour is controlled by a large number of wave modification mechanisms the characteristics of some of these mechanisms are specific to a local area whilst the remainder can be generalised to the entire seismic region factors representing these mechanisms are often not resolved a new attenuation modelling approach is demonstrated in this paper using hong kong as a case study to evaluate individual regional and local wave modification factors shear wave velocity swv information for the four prevalent geological formations found in hong kong was first obtained a at shallow depths from instrumented boreholes b at depths of up to 100 200 m from measurements using the microtremor spatial auto correlation spac technique c at depths of up to 1 5 km from the monitoring of quarry blasts and d at depths from 1 5 to 8 km in the hard basement rock layers from results of seismological refraction surveys the upper crust amplification factor calculated from the four modelled rock swv profiles was then combined with predicted attenuation parameters to determine the upper crust modification factor filter function incorporating the local wave modification characteristics associated with hong kong geological formations such functions may then be combined with the regional attenuation characteristics in that part of the south china region a seismic attenuation model was developed by combining the upper crust modification factor with the regional source function of intra plate earthquakes based on stochastic simulations the ground shaking model developed from the presented methodology is supported by the comparison with macro seismic data of seven historical earthquake events affecting hong kong

local magnitude discrepancies for near event receivers implications for the u k traffic light scheme local seismic magnitudes provide a practical and efficient scale for the implementation of regulation designed to manage the risk of induced seismicity such as traffic light schemes tls we demonstrate that significant magnitude discrepancies up to a unit higher occur between seismic events recorded on nearby stations 5 km compared with those at greater distances this is due to the influence of sedimentary layers which are generally lower in velocity and more attenuating than the underlying crystalline basement rocks and requires a change in the attenuation term of the m l scale this has a significant impact on the united kingdom s u k hydraulic fracturing tls whose red light is set at m l 0 5 because the nominal detectability of the u k network is m l 2 this scheme will require the deployment of monitoring stations in close proximity to well sites using data collected from mining events near new ollerton nottinghamshire we illustrate the effects that proximity has on travel path velocities and attenuation then perform a damped least squares inversion to determine appropriate constants within the m l scale we show that the attenuation term needs to increase from 0 00183 to 0 0514 and demonstrate that this higher value is representative of a ray path within a slower more attenuating sedimentary layer compared with the continental crust we therefore recommend that the magnitude scale m l log a 1 17log r 0 0514 r 3 0 should be used when local monitoring networks are within 5 km of the event epicenters

q for p waves in the sediments of the virginia coastal plain the seismic quality factor q for p waves in atlantic coastal plain sediments is estimated using data from the 2004 u s geological survey seismic survey in eastern virginia the estimates are based on spectral ratios derived from reflections and sediment guided p waves in late cretaceous and tertiary sediments within the annular trough of the late eocene chesapeake bay impact structure the estimates of q for the frequency range of 10 150 hz are from 75 to 100 with the best estimate of 80 based on multichannel stacking of spectral ratios from receivers in the offset range of 200 2000 m this result is approximately a factor of 2 larger than the results previously reported for the charleston south carolina area and it is approximately one half of that recently reported for the mississippi embayment

attenuation characteristics of coda waves in mainland gujarat india abstract the attenuation characteristics based on coda waves of mainland gujarat india have been investigated in the present study the broadband waveforms of 53 local earthquakes mw 1 1 3 3 having focal depths in the 6 0 33 6 km range recorded at five stations of mainland gujarat region has been used for the analysis the frequency dependent relationships q q0fn for coda q qc and dependency of coda q on lapse time windows have been determined for the said region the average lapse time dependent coda q relations estimated for the region are qc 87 13 f 1 01 0 06 lapse time 30 s qc 112 20 f 0 94 0 08 lapse time 40 s and qc 120 22 f 0 76 0 07 lapse time 50 s the increase in qc values with lapse time shows the depth dependence of qc as longer lapse time windows will sample larger area the observed quality factor is strongly dependent on frequency and lapse time which indicates that the upper lithosphere is more heterogeneous and seismotectonically active while the lower lithosphere is homogeneous and relatively less active a comparison of the coda q estimated for mainland gujarat region with those of nearby kachchh and saurashtra regions shows that mainland gujarat region is more heterogeneous the rate of decay of attenuation q 1 with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ the obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the mainland gujarat region where no such relations were available earlier these relations are also important for the simulation of earthquake strong ground motions in the region

vertical seismic profiling of oroville microearthquakes velocity spectra and particle motion as a function of depth direct evidence of site distortion of p and s wave microearthquake source spectra at oroville california is presented the data were gathered by placing vertical and three component seismometers at 90 m intervals in a 500 m borehole through the cleveland hill normal fault on which the 1975 ml 5 7 oroville earthquake took place high pressure hydraulic locking mechanisms were used to firmly lock the seismometer packages against the borehole wall digital event triggering geos recorders were used to receive the data some 30 seismic events including 12 microearthquakes were recorded during the 4 months the seismometers were deployed by comparing the velocity spectra of microearthquake waves at the different depths it can be seen that scattering and attenuation in the shallow crust around the borehole dominantly affect high frequency s waves above 15 hz the value of the apparent s wave quality factor qas for the upper 500 m of crust at this site is 9 one feature of this low qas is a gross difference between s wave corner frequencies observed at ground level and 500 m downhole for example the uphole and downhole s wave corner frequencies for the mcoda 0 4 microearthquake of julian day 259 differ by a factor of 2 or more low quality factors and depth dependent corner frequencies were also observed for p waves but these data are less definitive due to lower signal to noise ratios based on their three component particle motions and polarizations the direct s waves of the microearthquakes appear to be composed of two phases which have similar amplitudes but different apparent velocities it is possible that the velocity differences are a result of anisotropy in the underlying rock

analysis of earthquake body wave spectra for potency and magnitude values implications for magnitude scaling relations we develop a simple methodology for reliable automated estimation of the low frequency asymptote in seismic body wave spectra of small to moderate local earthquakes the procedure corrects individual p and s wave spectra for propagation and site effects and estimates the seismic potency from a stacked spectrum the method is applied to 11 000 earthquakes with local magnitudes 0 m l 4 that occurred in the southern california plate boundary region around the san jacinto fault zone during 2013 moment magnitude m w values derived from the spectra and the scaling relation of hanks kanamori follow a gutenberg richter distribution with a larger b value 1 22 from that associated with the m l values 0 93 for the same earthquakes the completeness magnitude for the m w values is 1 6 while for m l it is 1 0 the quantity m w m l linearly increases in the analysed magnitude range as m l decreases an average earthquake with m l 0 in the study area has an m w of about 0 9 the developed methodology and results have important implications for earthquake source studies and statistical seismology

three dimensional compressional wave attenuation tomography for the crust and uppermost mantle of northern and central california i present a frequency independent three dimensional 3 d compressional wave attenuation model indicated by quality factor qp for the crust and uppermost mantle of northern and central california the tomographic inversion used t values measured from amplitude spectra of 80 988 p wave arrivals of 3247 events recorded by 463 network stations through a 3 d seismic velocity model the model has a uniform horizontal grid spacing of 15 km and the vertical node intervals range between 2 and 10 km down to 45 km depth in general the resulting qp values increase with depth and agree with the surface geology at shallow depth layers the most significant features observed in the qp model are the high qp values in the sierra nevada mountains and low qp anomalies in the western fault zones low qp values are also imaged in owens valley and long valley at shallow depths and the cape mendocino region in the lower crust 25 km depth an overall contrast of qp values across the fault is observed in the creeping parkfield and cholame carrizo sections of the san andreas fault the new 3 d qp model provides an important complement to the existing regional scale velocity models for interpreting structural heterogeneity and fluid saturation of rocks in the study area

numerical simulation of the atmospheric signature of artificial and natural seismic events the mechanical coupling between solid planets and their atmospheres enables seismically induced acoustic waves to propagate in the atmosphere we numerically simulate this coupled system for two application cases active seismic experiments ases and passive seismic experiments a recent ase krishnamoorthy et al 2018 https doi org 10 1002 2018gl077481 observed the infrasonic signals produced by a seismic hammer to measure the sensitivity of observations to seismic parameters we attempt to reproduce the results from this experiment at short range by considering a realistic unconsolidated subsurface and an idealized rock solid subsurface at long range we investigate the influence of the source by using two focal mechanisms we found surface waves generate an infrasonic plane head wave in the ase case of the rock solid material for the passive seismic experiments the amplitude of atmospheric infrasound generated by seismic surface waves is investigated in detail despite some limitations the simulations suggest that balloon measurement of seismically induced infrasound might help to constrain ground properties

attenuation and velocity structure for site response analyses via downhole seismogram inversion a seismic waveform inversion algorithm is proposed for the estimation of elastic soil properties using low amplitude downhole array recordings based on a global optimization scheme in the wavelet domain complemented by a local least square s fit operator in the frequency domain the hybrid scheme can efficiently identify the optimal solution vicinity in the stochastic search space whereas the best fit model detection is substantially accelerated through the local deterministic inversion results presented for selected aftershocks of the mw 7 0 sanriku minami earthquake in japan recorded by the kik net strong motion network illustrate robustness of the impedance structure estimation by contrast the attenuation structure is shown to be sensitive to the frequency content of seismic input data attributed to the deterministic description of the continuum in the forward model that cannot simulate late arrivals of multiple scattered energy sensitivity analyses illustrate that for the same forward model results can be substantially different based on the definition of the objective function it is concluded that even for engineering purposes inversion should aim to decouple intrinsic and scattering attenuation mechanisms

the matter of size on the moment magnitude of microseismic events abstractwe investigated the method of estimating seismic moment and moment magnitude for microseismic events we determined that the m0t defined by bowers and hudson is the proper scalar moment to be used in microseismic studies for characterizing the size of an event and calculating its moment magnitude for non double couple sources the proportional relationship between body wave amplitude and seismic moment in the brune model breaks down so under such situations the brune model is not an appropriate way to estimate the seismic moment and magnitude moreover the s wave alone is not sufficient for determining the total seismic moment instead the p wave must be analyzed an example barnett shale data set was studied and the results concluded that the magnitudes estimated with the brune model could be off by as much as 1 92 with an absolute average of 0 35 the moment magnitudes based on the scalar moment m0t also gave a significantly different event size distribution and b value estimation finall

a summary of attenuation measurements from borehole recordings of earthquakes the 10 hz transition problem earthquake seismograms recorded by instruments in deep boreholes have low levels of background noise and wide signal bandwidth they have been used to extend our knowledge of crustal attenuation both in the near surface and at seismogenic depths site effects are of major importance to seismic hazard estimation and the comparison of surface shallow and deep recordings allows direct determination of the attenuation in the near surface all studies to date have found that q is very low in the near surface 10 in the upper 100 m and increases rapidly with depth unlike site amplification attenuation at shallow depths exhibits little dependence on rock type these observations are consistent with the opening of fractures under decreasing lithostatic pressure being the principal cause of the severe near surface attenuation seismograms recorded in deep boreholes are relatively unaffected by near surface effects and thus can be used to measure crustal attenuation to higher frequencies 100 hz than surface recordings studies using both direct and coda waves recorded at over 2 km depth find q to be high 1000 at seismogenic depths in california increasing only weakly with frequency between 10 and 100 hz intrinsic attenuation appears to be the dominant mechanism these observations contrast with those of the rapidly increasing q with frequency determined from surface studies in the frequency range 1 to 10 hz further work is necessary to constrain the factors responsible for this apparent change in the frequency dependence of q but it is clearly unwise to extrapolate q estimates made below about 10 hz to higher frequencies

microearthquake s wave observations from 0 to 1 km in the varian well at parkfield california abstract high gain three component seismometers from 0 to 1 km deep along the varian a 1 well at parkfield california were used to record the waveforms of nearby microearthquakes despite being in the thick tertiary sediments of the parkfield syncline the s wave amplification at this site is only about a factor of 3 the spectral content and spectral ratios of s waves along the well show that the average qs in the top 1 km at this site is 37 with the qs in different subintervals varying between 8 and 65 based on initial s wave polarizations a complex s wave velocity structure must exist at and below the varian site this structure appears to include position dependent anisotropy as well as steep lateral velocity gradients at a depth of 1 km s wave splitting parallel and normal to the san andreas fault zone is consistently observed this splitting scales at roughly 0 01 sec km subsequent to the split s waves the particle motion seems to be controlled by event focal mechanism above 1 km the upgoing s waves attenuate and change directions of polarization with a new splitting rate of 0 1 sec km uniquely for some events on the san andreas fault immediately below the varian site large post s wave signals with normal dispersion are present we propose that these phases are fault zone guided waves channeled from the san andreas fault to the varian site along the gold hill fault

shallow seismic attenuation and shear wave splitting in the short period range of deception island volcano antarctica abstract the occurrence of a seismic series in deception island volcano antarctica composed of hundreds of local volcano tectonic earthquakes has permitted us to study the seismic attenuation of such a volcanic environment in the short distance and high frequency range this study has been performed using p waves s waves and coda waves and applying different frequency dependent and independent techniques the methods used for this analysis have been spectral and broadening of the pulse for direct p and s waves coda normalization for s waves and single back scattering model for coda waves the results show that in general q values are significantly smaller for the entire frequency range used 6 30 hz than those found in other volcanic and tectonic areas the attenuation for p waves is greater than for s waves in the frequency independent methods with a q q p ratio that ranges between 1 9 and 3 2 comparing the q factor obtained for s waves we have observed clear differences as a function of the method used the coda normalization method has supplied significantly higher q values q d than the other two methods q we have interpreted this discrepancy as an effect of the methods coda normalization and single back scattering methods eliminate the contribution of the near surface attenuation in their q values comparing both q and q d we have estimated the near surface attenuation under the recording site named q on the other hand we have observed that q d has anomalous frequency dependence with a minimum value at 21 hz this pattern is interpreted as an effect of strong scattering of the seismic waves in the source area of the earthquakes q c values depend clearly with frequency and lapse time and the lapse time dependence could be interpreted as a depth dependence of the seismic attenuation in deception island volcano antarctica the obtained q values have allowed us to separate the contribution of intrinsic and scattering attenuation deriving that the scattering attenuation is predominant over the intrinsic effects finally in order to investigate how the heterogeneous medium of the volcanic island could produce other effects we have checked whether it produces polarization of the shear waves the preliminary results of the polarization direction indicate a main e w strain direction all these evidences reveal the strongly heterogeneous structure of deception island volcano

reliable and economical high temperature deep borehole seismic recording abstract recording earthquakes with borehole seismometers has become increasingly popular in recent years as the advantages in noise reduction and also the distorting effects of near surface rocks especially sediments have become well known borehole recording can be extremely complex involving active sensors special cables and downhole electronics such installations however are often not very reliable at the high temperatures reached in active tectonic areas at depths of 1 km and greater here we describe a simple and reliable system for 3 component recording of local earthquakes at single and multiple depths greater than 1 5 km and temperatures up to 120 c our system was designed and developed for experiments in the cajon pass scientific drillhole of southern california the borehole packages are made of titanium with spring loaded clamping allowing easy retrieval standard seven conductor oil well logging cables are used together with a specially designed cablehead the data recorded have been used for investigating earthquake source scaling attenuation in the mid crust and also near surface site effects

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

garner valley downhole array of accelerometers instrumentation and preliminary data analysis the garner valley downhole array gvda in southern california 33 41 60 n 116 40 20 w is a five element array of three component dual gain force balance accelerometers that are capable of measuring accelerations from 3 10 6 to 2 0 g over a frequency range from 0 0 to 100 hz the accelerometers are placed at depths of 0 6 15 22 and 220 m the lithostratigraphy is 19 m of soil overlying a 24 m layer of weathered granite below which is granite tonalite the array only 7 km from the san jacinto fault is located at the northern end of the anza seismic gap on the san jacinto fault where an m 6 5 or greater earthquake can be expected it is only 35 km from the indio segment of the san andreas fault which last ruptured with an m 8 earthquake around 1700 a d from its installation in july 1989 through july 1991 gvda has recorded 280 earthquakes with magnitudes ranging from 1 2 to 4 7 and epicentral distances ranging from from 0 to 110 km the maximum acceleration recorded was 89 cm sec 2 from an m l 4 2 earthquake 15 8 km hypocentral distance from gvda the average amplification of the apparent seismic moment between 220 m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes the spectral ratio 0 220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2 0 to 30 hz for 17 events resonance peaks exist at about 1 7 3 0 and 12 0 hz where the spectral ratio 0 220 is nearly 40 analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes m 4 2 and 2 5 shows that the weathered granite zone beneath the soil strongly attenuates the spectral amplitudes for frequencies greater than 40 hz the impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3 leading to spectral levels that are greater at the surface than at 22 m depth near the top of the weathered granite the acceleration spectra at each depth show resonant peaks at the same frequencies the spectra at each depth are well correlated for both earthquakes the acceleration spectrum at 220 m is nearly constant for frequencies less than 60 hz which implies that there is little or no attenuation

source parameters of earthquakes recorded near the itoiz dam northern spain we calculate the source parameters and attenuation from earthquakes recorded near the itoiz dam from 2004 to 2009 with magnitudes ranging between 1 2 and 5 2 we use a genetic algorithm in order to fit the three component p wave spectra with the spectral level corner frequency and attenuation factor as searching parameters the obtained moments range from 1 72 1011 to 2 65 1015 nm the radii span from 0 09 to 1 00 km and the stress drops vary from 0 006 to 29 462 mpa the maximum value for the q attenuation factor is 794 and the minimum value is 53 we find a good agreement between empirical and theoretical relationships between moment and magnitude there seems to be a breakdown of self similarity but it could be due to the method used we group the data by means of a self organizing map and the clusters found are related by their magnitude and not by other considerations

depth dependence of constitutive law parameters for shear failure of rock at local strong areas on faults in the seismogenic crust 1 in the framework of slip dependent constitutive formulation which leads to a unifying law that governs not only frictional slip failure at precut interface areas on faults but also shear fracture of intact rock at local strong areas on the faults a thorough investigation has been made about how the constitutive law of dimensionless form for rock failure at local strong areas on faults is affected by seismogenic crustal conditions of effective normal stress pore pressure and temperature on the basis of laboratory data on the shear fracture of granite rock obtained by kato et al 2003a under crustal conditions simulated in the laboratory it has been revealed how dimensionless constitutive law parameters distinctively vary with depth depending on the crustal conditions simulated consequently the constitutive behavior including the slip weakening phase also distinctively varies with depth this behavior in the regime of suprahydrostatic pore pressure clearly differs from that in the regime of hydrostatic pore pressure the results of this study are suited for implementation in a numerical code and the constitutive properties of rock failure at local strong areas called asperites on faults will be easily assimilated into numerical models of earthquake ruptures for realistic computer simulation

forward and inverse three dimensional p wave velocity models of the southern california crust we construct a three dimensional p wave velocity model of the southern california crust by combining existing one dimensional models each describing a region defined by surface geology and calibrate the model with travel times from three explosions the model is expressed as blocks each of a given slowness the variance of the p wave travel time residuals of 1000 earthquakes relocated in and near the los angeles basin where the model is most detailed is half that of the catalog locations in the standard one dimensional model for southern california starting from the forward model we invert 21 000 p wave arrivals from earthquakes for hypocenters and block slownesses using the technique of roecker 1981 the variance of these p wave travel time residuals decreases 47 during the inversion many of the blocks representing the upper crust and midcrust are well sampled and well resolved the resulting model is useful both for locating earthquakes and for comparing the geologies of the different regions for example the velocity structure of the los angeles basin represents seismically slow sediments on top of basement rocks having velocities similar to the granitic rocks under the peninsular ranges moho is between 26 and 32 km depth in contrast the ventura basin has mostly slower sediments above a deeper higher velocity basement compared to catalog locations relocations in the final three dimensional model of 98 ml 4 earthquakes throughout southern california tend to deepen below sediment filled valleys and basins shallow in regions without sedimentary cover and have a 44 lower p wave travel time residual variance

attenuation and apparent source characteristics in the northern indian oceanic crust surrounding sri lanka a data analysis of 171 broadband vertical component records of 62 shallow crustal distant events in the northern indian ocean region was undertaken to investigate the far source geometric attenuation rate high frequency cutoff filter parameter kappa and apparent source characteristics event magnitude varied from 4 0 to 5 9 in the short period body wave magnitude m b scale and hypocentral distance ranged between 2 3 260 km and 14 1 1578 km for the dataset the far source geometric attenuation rate was estimated by fitting data to a predefined attenuation equation at a frequency range of 0 5 8 5 hz using the multiple linear regression method a secondary regression in the frequency domain on constants resulting from the main first regression was performed to find kappa and the second hinge point of the trilinear geometric attenuation function source characteristics stress drop corner frequency and moment magnitude of selected events were determined using the brune 1970 1971 model the average far source geometric attenuation rate was determined to be r 0 5 r is hypocentral distance for the selected frequency range but at low frequencies below about 2 hz slightly lower rates than the average were observed kappa was found to be 0 041 0 009 s the second hinge point of the geometric attenuation function was estimated to be 120 30 km and the complete form of the function after considering compliancy with actual records was collated as graphic 1 average static stress drops were found to be 9 5 and 16 0 mpa for the m b 4 5 and 5 6 magnitude bands respectively corner frequencies approximately fell within 1 0 7 0 hz for the dataset derived attenuation parameters were also validated using a ground motion comparison between recorded and stochastically simulated events online material tables of regression parameters estimated source parameters and stochastic simulation parameters and a figure comparing the ground motion for simulated and recorded events 1 embed inline graphic 1 gif

earthquake site response estimation a weak motion case study using weak motion recordings of aftershocks of the 1989 loma prieta earthquake recorded in oakland california near the failed nimitz freeway two methods have been applied to estimate the site response of an alluvium site and three mud over alluvium sites the first estimate is the traditional spectral ratio and the second utilizes the cross spectrum recordings obtained at a nearby bedrock site are used as estimates of the sediment site input motions while the two site response estimates produce similar peaks and troughs there is an approximate factor of 2 difference in amplitudes this discrepancy is evidence that there is a much greater level of noise than would be expected from the pre event ambient noise we interpret this as signal generated noise produced by scattering from heterogeneities which causes the true sediment site input to differ significantly from the bedrock site recording given this level of noise the cross spectrum estimate suffers a severe downward bias by a factor of 2 in this study and should probably not be used when the input motion is estimated from a bedrock site recording the spectral ratio estimates are relatively unbiased but the level of noise introduces a large degree of uncertainty therefore inferences about site response from individual spectral ratios should probably be avoided on the other hand ensemble averages of the estimates significantly reduce the scatter to reveal resonances that agree quite well in frequency and overall shape with those of one dimensional models whose parameters were determined independently a discrepancy of higher observed amplitudes than predicted by theory remains unexplained but most likely results from the effects of boundary layer topography which are not accounted for by the simple one dimensional models

damping values derived from surface source downhole receiver measurements at 22 sites in the san francisco bay area of central california and the san fernando valley of southern california

surface and near surface effects on seismic waves theory and borehole seismometer results a simple plane wave model is adequate to explain many surface versus borehole seismometer data sets using such a model we present a series of examples which demonstrate the effects of the free surface near surface velocity gradients and low impedance surface layers on the amplitudes of upcoming body waves in some cases these amplitudes are predictable from simple free surface and impedance contrast expressions however in other cases these expressions are an unreliable guide to the complete response and the full plane wave calculation must be performed large surface amplifications are possible even without focusing due to lateral heterogeneities or nonlinear effects both surface and borehole seismometer site responses are almost always frequency dependent ocean bottom versus borehole seismic data from the 1983 ngendei seismic experiment in the southwest pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation the borehole seismic response to upcoming p waves is reduced at high frequencies because of interference between the upgoing p wave and downgoing p and sv waves reflected from the sediment basement interface however because of much lower borehole noise levels the borehole seismometer enjoys a p wave signal to noise advantage of 3 to 7 db over nearby ocean bottom instruments

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

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

a composite source model of the 1994 northridge earthquake using genetic algorithms abstract the 17 january 1994 northridge earthquake m w 6 7 occurred on a buried thrust fault in the northwest los angeles metropolitan area we investigate the source process of this earthquake using the csmip strong motion records and a composite source model developed by zeng et al 1994a for realistic earthquake strong ground motion prediction our previous studies demonstrated the realism of the synthetic strong motions generated from the composite source model by comparing them with observed records from earthquakes in many areas of the world this article addresses an inverse study of the problem to find a specific composite source model for the northridge earthquake this is done by adjusting the location of a suite of composite subevents using genetic algorithms holland 1975 to best match the observed waveforms a test run of the genetic algorithm on synthetic data sets finds a very good convergence of the approach we reduce largely the intensive computation time by identifying subevents with major contribution to the waveform fit our result for the 1994 northridge earthquake indicates a complex earthquake rupture process with three large slip zones one at the hypocenter and the other two to the west of the hypocenter we then use this model to compute the high frequency strong motion velocity and acceleration the results show that the composite source model provides a very realistic broadband source description for the northridge earthquake

high frequency borehole seismograms recorded in the san jcinto fault zone southern california part 2 attenuation and site effects we examine surface and downhole p and s wave spectra from local earthquakes recorded at two borehole seismometer arrays knw bh and pfo bh installed in the southern california batholith region of the san jacinto fault zone by the u s geological survey to assess the influence of the weathered layer on the spectral content of high frequency 2 to 200 hz seismic signals earthquake signals recorded downhole at both sites show significantly improved seismic bandwidth due to both a reduction in ambient noise levels and especially to dramatically increased levels of high frequency signal significant seismic signal is observed up to approximately 190 hz for p waves at knw bh stacked spectral ratios from these signals indicate that the highly weathered near surface between 0 and 150 m at knw bh and pfo bh exerts a much larger influence on seismic signals than deeper between 150 and 300 m material modeling of uphole downhole spectral ratio data suggests q 6 5 and q 9 between 0 and 150 m increasing to q 27 and q 26 between 150 and 300 m an outcrop mounted anza network station deployed approximately 0 4 km from knw bh displays roughly similar high frequency content to the knw bh downhole sensors but it exhibits spectra that are significantly colored by directional resonances low q and low velocity near surface material forms a lossy boundary layer at these borehole sites that is advantageous to the high frequency downhole environment not only are noise levels reduced but reflections from the surface and near surface are greatly attenuated as a result high frequency recordings from below the weathered zone more nearly resemble those recorded in a whole space than would otherwise be expected