the 23 19 aftershock of the 15 october 1979 imperial valley earthquake more evidence for an asperity the well recorded strong ground motion data for the 23 19 aftershock of the 15 october 1979 imperial valley earthquake provide a good opportunity to study the high frequency source characteristics and the path effects at near source distances the best fitting point source model has a strike slip mechanism n40 w which is nearly identical to the main event the estimated stress drop is extremely high roughly 500 bars with a triangular time history 0 1 0 1 sec but with a moment of 1 0 10 24 dyne cm a double source model found by inversion fits the high frequency data better but indicates complex faulting the first source with strike n319 e dip 42 ne and slip angle 165 has a moment of 0 7 10 24 dyne cm the second source with strike n324 e dip 82 sw and slip angle 181 lies about 0 5 km to the north and has a seismic moment twice that of the first source source dimensions appear very small for this amount of energy release many of the anomalous behaviors observed at certain stations for the main event are also present in the aftershock data these features are examined in terms of path effects

high resolution seismic reflection surveys and modeling across an area of high damage from the 1994 northridge earthquake sherman oaks california approximately 3 6 km of p wave seismic reflection data were acquired along two orthogonal profiles in sherman oaks california to determine whether shallow less than 1 km depth geologic structures contributed to the dramatic localized damage resulting from the 1994 northridge earthquake both lines one along matilija avenue and one along milbank street crossed areas of both high and low damage we believe these data reveal a geologic structure in the upper 600 m that contributed to the increased earthquake ground shaking in the high damage areas south of and along the los angeles river of interest in these data is a reflection interpreted to be from bedrock that can be traced to the north along the matilija avenue profile this reflecting interface dipping northward at 15 22 may be an important impedance boundary because it is the lower boundary of a wedge of overlying low velocity sediments the wedge thins and terminates in the area where we interpret down warped reflections as evidence of a shallow subbasin the low velocity subbasin sediments v s of 200 m sec v p of 500 m sec may be up to 150 m thick beneath the channelized los angeles river the area across the subbasin experienced greater earthquake damage from possible geometric focusing effects three dimensional basin effects may be responsible for the variable damage pattern but from these seismic profiles it is not possible to determine the regional structural trends two dimensional elastic and sh mode finite difference modeling of the imaged structural geometry along matilija avenue suggests that a peak horizontal velocity amplification factor of two and over can be explained in the high damage area above the shallow subbasin and sediment wedge amplification factors up to 5 were previously observed in aftershock data at frequencies of 2 to 6 hz amplification in the elastic simulation at the santa monica mountains range front on the southern end of the matilija profile with the geologic layering and geometry interpreted from the seismic data is also consistent with aftershock observations

microzonation of zeytinburnu region with respect to soil amplification a case study abstract a microzonation study is performed as a part of the zeytinburnu pilot project within the framework of the earthquake master plan for istanbul to determine the effects of local soil conditions on the earthquake forces that will act on structures for this purpose detailed geological and geotechnical studies are conducted at the site a geological map which demonstrates the local geological features of the site is prepared and the site is classified with respect to the dynamic behaviour based on the data gathered from the soil borings in order to investigate the effects of local soil conditions on the dynamic behaviour site response analyses are performed with the computer code eera by utilizing the findings of field and laboratory investigations the behaviour of the region during a probable earthquake is investigated through one dimensional response analyses and microzonation maps are prepared with respect to ground shaking intensity in accordance with the new microzonation manual ansal a laue j buchheister j erdik m springman s studer j and koksal d 2004 site characterization and site amplification for a seismic microzonation study in turkey 11th int conference on soil dynamics and earthquake engineering and 3rd earthquake geotechnical engineering san francisco studer j and ansal a 2004 belediyeler icin sismik mikrobolgeleme el kitabi arastirma raporu afet isleri genel mudurlugu bayindirlik ve iskan bakanligi afet risk yonetimi dunya enstitusu

integrated use of seismograph and strong motion data to determine soil amplification response of the fraser river delta to the duvall and georgia strait earthquakes we present a new method to estimate site amplification in which re gional seismographic data are used to constrain the input motions to the soil profile the empirical inferred amplification is determined as the ratio of the fourier spec trum of the recorded motion on soil to the spectrum of the inferred input motion at the underlying bedrock interface the method is demonstrated for sites on the fraser river delta british columbia b c that recorded the 1996 m 5 1 duvall wash ington and 1997 m 4 3 georgia strait b c earthquakes regional seismographic recordings on rock sites are used to determine the apparent source spectra of these events these spectra are attenuated to sites in the fraser river delta then input to a specified soil profile we compare the empirical inferred amplification to alternative estimates of the soil amplification based on 1 the quarter wavelength approach and 2 the assumption that the amplification is approximately equal to the observed h v ratio the empirical inferred amplification estimates are grossly consistent with the theoretical amplification estimates but have larger peaks at specific frequencies probably due to resonant effects not accounted for in the quarter wavelength ap proach the h v ratio appears to be a simple and useful first approximation to the site response the fraser river delta amplifies weak motions by factors of three to six over a broad frequency range from 0 2 to 4 hz high frequency motions f 10 hz are attenuated the long period amplifications pose a serious concern in the event of strong earthquake shaking from large earthquakes a useful by product of the analysis is the determination of apparent source spectra for the events the apparent source spectrum of the georgia strait earthquake is in good agreement with the brune point source model with a stress drop of approxi mately 45 bars the apparent source spectrum of the slightly larger duvall earthquake is in reasonable agreement with the brune model with a stress drop of about 70 bars however for the duvall earthquake there is evidence of a sag in spectral amplitudes relative to the brune model at intermediate frequencies 0 3 to 1 hz this inter mediate frequency sag is an expected result of source complexity it is well dem onstrated by the broadband seismic data

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

complex terrain experiments in the new european wind atlas the new european wind atlas project will create a freely accessible wind atlas covering europe and turkey develop the model chain to create the atlas and perform a series of experiments on flow in

the 1994 northridge earthquake sequence in california seismological and tectonic aspects the m w 6 7 northridge earthquake occurred on january 17 1994 beneath the san fernando valley two seismicity clusters located 25 km to the south and 35 km to the north northwest preceded the mainshock by 7 days and 16 hours respectively the mainshock hypocenter was relatively deep at 19 km depth in the lower crust it had a thrust faulting focal mechanism with a rake of 100 on a fault plane dipping 35 to the south southwest and striking n75 w because the mainshock did not rupture the surface its association with surficial geological features remains difficult to resolve nonetheless its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the transverse ranges the northridge earthquake is part of the temporal increase in earthquake activity in the los angeles area since 1970 the mainshock was followed by an energetic aftershock sequence eight aftershocks of m 5 0 and 48 aftershocks of 4 m 5 occurred between january 17 and september 30 1994 the aftershocks extend over most of the western san fernando valley and santa susana mountains they form a diffuse spatial distribution around the mainshock rupture plane illuminating a previously unmapped thrust ramp extending from 7 10 km depth into the lower crust to a depth of 23 km no flattening of the aftershock distribution is observed near its bottom at shallow depths above 7 10 km the thrust ramp is topped by a dense distribution of aftershock hypocenters bounded by some of the surficial faults the dip of the ramp increases from east to west the west side of the aftershock zoae is characterized by a dense steeply dipping and north northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting these events coincided with the gillibrand canyon lateral ramp along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms the focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks with some strike slip and normal faulting in the smaller aftershocks the 1971 san fernando and the 1994 northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge both earthquakes accommodated north south contractional deformation of the transverse ranges the two earthquakes differ primarily in the dip direction of the faults and the depth of faulting the 1971 north northeast trend of left lateral faulting chatsworth trend was not activated in 1994

on the characteristics of local geology and their influence on ground motions generated by the loma prieta earthquake in the san francisco bay region california strong ground motions recorded at 34 sites in the san francisco bay region from the loma prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions peak horizontal acceleration and velocity inferred for sites underlain by rock generally occur on the transverse component of motion they are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong motion data theoretical amplitude distributions and synthetic seismograms calculated for 10 layer models suggest that bedrock motions were elevated due in part to the wide angle reflection of s energy from the base of a relatively thin 25 km continental crust in the region characteristics of geologic and geotechnical units as currently mapped for the san francisco bay region show that average ratios of peak horizontal acceleration velocity and displacement increase with decreasing mean shear wave velocity ratios of peak acceleration for sites on soil alluvium fill bay mud are statistically larger than those for sites on hard rock sandstone shale franciscan complex spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill bay mud average spectral amplifications inferred for vertical and the mean horizontal motion are respectively 1 1 for sites on the franciscan complex kjf 1 4 1 5 for sites on mesozoic and tertiary rocks tmzs 2 1 2 0 for sites on the santa clara formation qts 2 3 2 9 for sites on alluvium qal and 2 1 4 0 for sites on fill bay mud qaf qhbm these mean values are not statistically different at the 5 significance level from those inferred from previous low strain data analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 km in the cities of san francisco and oakland

ground motion amplification in the santa monica area effects of shallow basin edge structure abstract the 1994 northridge earthquake produced ground motions in the northwest portion of the los angeles basin that were significantly larger than rock site motions observed at locations just north of the basin the santa monica area was hit particularly hard with numerous structures being damaged or destroyed by the strong ground shaking in this region the basin edge geology is controlled by the active strand of the east west striking santa monica fault and virtually all of the structural damage occurred at or south of the fault location we have used 2d finite difference ground motion simulations to investigate the effect of the basin edge structure in amplifying ground response constraints on the basin edge structure come from geologic cross sections geophysical data and seismological observations our simulations indicate that the shallow basin edge structure 1 km deep formed by the active strand of the santa monica fault creates a large amplification in motions immediately south of the fault scarp in very good agreement with mainshock damage patterns recorded ground motions and locations of elevated site response this large amplification results from constructive interference of direct waves with the basin edge generated surface waves and is quite similar to the basin edge effect associated with the 1995 kobe earthquake in addition we find that focusing effects created by the deeper basin structure 3 to 4 km deep cannot explain the large motions observed immediately south of the fault scarp this strongly suggests that the deep basin focusing models proposed by gao et al 1996 and alex and olsen 1998 are not likely explanations of the observed pattern of ground motion amplification in the santa monica area

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

site response in southern california for probabilistic seismic hazard analysis this study determines site response factors that can be applied as cor rections to a rock attenuation relationship for use in probabilistic seismic hazard analysis the site response factors are amplitude and site class dependent these amplification factors are determined by averaging ratios between observed and pre dicted ground motions for peak ground acceleration pga and for 5 damped re sponse spectral acceleration at 0 3 1 0 and 3 0 sec oscillator periods the observa tions come from the strong motion database of the southern california earthquake center scec and the predictions are based on the sadigh 1993 rock attenuation relation when separated and averaged according to surface geology significantly different site response factors are found for quaternary and mesozoic units but a subclassification of quaternary is generally not justified by the data the low input motion amplification factors are consistent with those obtained from independent aftershock studies at the pga and 0 3 second period an observed trend of decreasing quaternary site amplification with higher input motion is consistent with nonlinear soil behavior however the trend exists for mesozoic sites as well implying that this may be an artifact of the sadigh relationship there is a correlation between larger site response factors and lower average shear wave velocity in the upper 30 m for low predicted pga input motions with an increase in the correlation with increasing period the 0 3 sec site response factors for quaternary data in southern california determined in this study are consistent with 0 3 sec nehrp site response correction factors however at 1 0 sec period some inconsistencies remain a trend is also seen with respect to sediment basin depth where deeper sites have higher average site response factors these results constitute a customized attenuation relationship for southern california the implication of these customized attenuation models with respect to probabilistic hazard analysis is examined in field and petersen 2000

neutral turbulent flow over forested hills numerical simulations of neutral turbulent flow over periodic forested hills are carried out using a first order turbulence closure scheme the forest is represented by the inclusion in the model of a canopy near the surface where additional drag is exerted on the flow as a result of the trees simulations with a more traditional roughness length parametrization of the surface which does not explicitly model the flow within the forest are also carried out for comparison comparison of simulations with existing wind tunnel data shows that the simple mixing length canopy closure scheme provides a good estimate of the mean flow over a forested hill and is a significant improvement on a traditional roughness length parametrization in this case the numerical results are seen to be in close agreement with existing linear analytical solutions for flow over a low and wide forested hill the model is used to investigate the breakdown of the analytical solution when the hill becomes steep and the perturbations to the flow are no longer linear even in the case of a gentle slope the analytical solution can break down when the hill is narrow or the canopy is deep and the vertical velocities near the top of the canopy become large this leads to a downwind shift in the surface pressure perturbation and an enhanced drag compared to a roughness length model model results also show enhanced flow separation in the presence of a canopy these results are important for a range of meteorological problems including the interpretation of flux measurements parametrization of orographic drag predicting wind damage to trees and calculating the yield from wind turbines royal meteorological society 2005 s b vosper s contribution is crown copyright

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

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

site classification for earthquake hazard and risk assessment in australia while site response phenomena are well recognized globally the influence of regolith properties on ground motion has not previously been investigated for much of australia in an effort to characterize the regolith in terms of its ability to modify earthquake ground shaking this validation study assesses the national site classification map of australia which has been developed for application in first order earthquake hazard and risk assessment in the absence of regolith thickness and fundamental period data site classes are assigned based on a method developed in california that uses the relationship between geological material and the shear wave velocity of the upper 30 m v s 30 the classification scheme is then adjusted to suit the australian geological environment including a correction for the presence of weathered in situ regolith commonly encountered in this stable tectonic setting the results are validated using geophysical and geotechnical data from a variety of quaternary sedimentary environments in the newcastle sydney and perth urban areas and from bedrock dominated environments at a range of sites across australia the results of these analyses demonstrate the utility but also highlight the limitations of applying a depth limited shear wave velocity method in site classification for seismic hazard assessment

seismic wave gradiometry using the wavelet transform application to the analysis of complex surface waves recorded at the glendora array sullivan indiana usa abstract this article describes a new development in seismic wave gradiometry the work follows previous work on time domain wave gradiometry but proposes applying a wavelet transform to the data prior to wave gradiometric analysis the result is a more complete picture of wave attributes which vary in time as well as frequency in this article the new data analysis approach is used to analyze two underwater shots that were recorded by a portion of the glendora array the glendora site is a former surface coal mine that has subsequently been filled by mine tailings and as such forms an artificial sedimentary basin in which the geometry is well known the array used for the analysis was located in a corner formed by the intersection of two vertically oriented boundaries between mine tailings and the surrounding bedrock the analysis of the data revealed the presence of typical body waves and surface waves in addition to several phases that are interpreted to be 1 reflections of surface waves off of the vertical tailings bedrock boundaries 2 surface waves generated by the interaction of direct arrival waterborne acoustic waves with the water tailing interface the lake shore and 3 the interaction of other waterborne acoustic waves in glendora lake with other water tailing interfaces the surface waves are highly time dependent and frequency dependent and highlight the extreme complexity of surface wave propagation in geologically complex areas

advances and limitations of the environmental seismic intensity scale esi 2007 regarding near field and far field effects from recent earthquakes in greece implications for the seismic hazard assessment the new environmental seismic intensity scale esi 2007 introduced by inqua incorporates the advances and achievements of palaeoseismology and earthquake geology and evaluates earthquake size and epicentre solely from the earthquake environmental effects eee this scale is tested and compared with traditional existing scales for the 1981 alkyonides earthquake sequence in the corinth gulf ms 6 7 ms 6 4 ms 6 3 the 1993 pyrgos event ms 5 5 and the 2006 kythira event mw 6 7 these earthquakes were of different magni tudes focal mechanisms and focal depths and produced well documented environmental effects the esi 2007 intensity values and the isoseismal pattern for the 1993 pyrgos and the 2006 kythira events are similar to those resulting from the traditional scales demonstrating that for moderate intensity levels vii and viii the esi 2007 and the traditional scales comply well in contrast the 1981 alkyonides earthquake sequence shows that there is an inconsistency between the esi 2007 and the traditional scales both in the epicentral area where higher esi 2007 intensity values have been assigned and for the far field effects the esi 2007 scale offers higher objectivity in the process of assessing macroseismic intensities particularly in the epicentral area than traditional intensity scales that are influenced by human parameters the esi 2007 scale follows the same criteria environmental effects for all events and can compare not only events from different settings but also contemporary and future earthquakes with histori cal events a reappraisal of historical earthquakes so as to constrain the esi 2007 scale may prove beneficial for seismic hazard assessment by reducing the uncertainty implied in the attenuation laws which constitute one of the most important seismic hazard parameters

large eddy simulation of turbulent flow over a forested hill validation and coherent structure identification modelling turbulent flow at a very fine scale within and above vegetation canopies over complex terrain is of great interest for many environmental applications over the last decade it has been demonstrated that the large eddy simulation les technique is able to reproduce in detail many observed features of turbulent flow over homogeneous and heterogeneous vegetation canopies on flat terrain for the first time a nested les of turbulent flow within and above a forested canopy on an isolated two dimensional hill is analysed and validated against pressure and velocity data from a wind tunnel experiment the model is shown to be able to reproduce accurately the main features observed over a forested hill the results also confirm recent observations on the intermittency of the recirculation region behind the hill and on the domination of sweep motions in momentum transfer at the canopy top all along the hill the main characteristics of turbulent structures have also been investigated from the analysis of vorticity fields and two point velocity correlations across the hill it is shown that the streamwise wind velocity upwind from the summit is not correlated with the flow within the wake region but only with the flow above it while in the wake region the streamwise wind velocity at the canopy top is only correlated with the flow within the wake region this result suggests that turbulence within the wake region results from the superposition of various turbulent structures large turbulent structures induced by the elevated shear layer that may result from the rolling over of kelvin helmotz instabilities structures induced at the lee side foot of the hill by an adverse pressure gradient and structures induced by the presence of the canopy implications of hilly terrain on canopy atmosphere exchanges and tree vulnerability to windload are briefly discussed copyright 2008 royal meteorological society

did two massive earthquakes in the holocene induce widespread landsliding and near surface deformation in part of the ottawa valley canada recent evidence based on ages of large landslides and the existence of severely disturbed terrain in champlain sea sediments to the east of ottawa suggests that this area may have been the site of two of the most geologically destructive earthquakes in eastern canada the walls of several paleochannels downcut into sensitive marine clayey silts are scarred by numerous large earthflows of sizes unequaled in historical time while radiocarbon ages of 15 landslides as much as 35 km apart range from ca 1870 to 5130 yr b p the majority of events cluster at ca 4550 yr b p the coincidence of numerous large failures in different paleovalleys occurring concurrently long after channel abandonment and at a time of drier climate suggests that the widespread landsliding was triggered by a strong earthquake close to the landslide area areas of very disturbed terrain in a flat erosional plain show severely deformed bedding and irregular ground subsidence and are attributed to a large earthquake ca 7060 yr b p that strongly shook a thick sequence of sensitive material filling deep steep sided bedrock basins

analysis of the ground accelerations radiated by the 1980 livermore valley earthquakes for directivity and dynamic source characteristics the strong motion accelerograph recordings of the 24 january 1980 main shock and the 27 january 1980 aftershock of the livermore valley earthquake sequence are analyzed for systematic variations with azimuth or station location the variation of the peak accelerations with epicentral azimuth is apparently reversed for the two events the main shock accelerations are larger to the south and the aftershock accelerations are larger to the northwest we eliminate the site effects by forming the ratio of the peak accelerations recorded at the same station after correcting for the epicentral distance this analysis indicates that source directivity caused a total variation of a factor of 10 in the peak accelerations comparison of this variation with the spatial extent of the aftershock sequences suggests that the strong directivity in the radiated accelerations is the result of unilateral ruptures in both events the accelerograms recorded at 10 stations within 35 km of the events were digitized to analyze the azimuthal variation of the rms acceleration the peak velocity and the radiated energy flux the variation of rms acceleration correlates almost exactly with the variation of the peak accelerations this correlation is analyzed using both deterministic and stochastic models for the acceleration waveforms the peak velocities corrected for epicentral distance vary with azimuth by a factor of 5 for both events while the radiated energy flux varies by a factor of 30 for the main shock and 15 for the aftershock the peak velocities are strongly correlated with the radiated energy flux the radiated seismic energies are estimated to be 2 6 0 9 1020 dyne cm for the main shock and 1 5 0 3 1020 dyne cm for the aftershock

accounting for site effects in probabilistic seismic hazard analyses of southern california overview of the scec phase iii report this article presents an overview of the southern california earthquake center scec phase iii effort to determine the extent to which probabilistic seismic hazard analysis psha can be improved by accounting for site effects the contri butions made in this endeavor are represented in the various articles that compose this special issue of bssa given the somewhat arbitrary nature of the site effect distinction it must be care fully defined in any given context with respect to psha we define the site effect as the response relative to an attenuation relationship averaged over all damaging earthquakes in the region a diligent effort has been made to identify any attributes that predispose a site to greater or lower levels of shaking the most detailed maps of quaternary geology are not found to be helpful either they are overly detailed in terms of distinguishing different amplification factors or present southern california strong motion observations are inadequate to reveal their superiority a map based on the average shear wave velocity in the upper 30 m however is found to delineate significantly different amplification factors a correlation of amplification with basin depth is also found to be significant implying up to a factor of two difference between the shallowest and deepest parts of the los angeles basin in fact for peak accel eration the basin depth correction is more influential than the 30 m shear wave ve locity questions remain however as to whether basin depth is a proxy for some other site attribute in spite of these significant and important site effects the standard deviation of an attenuation relationship the prediction error is not significantly reduced by making such corrections that is given the influence of basin edge induced waves subsur face focusing and scattering in general any model that attempts to predict ground motion with only a few parameters will have a substantial intrinsic variability our best hope for reducing such uncertainties is via waveform modeling based on first principals of physics finally questions remain with respect to the overall reliability of attenuation re lationships at large magnitudes and short distances current discrepancies between viable models produce up to a factor of 3 difference among predicted 10 in 50 yr exceedance levels part of which results from the uncertain influence of sediment nonlinearity

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

observations of loma prieta aftershocks from a dense array in sunnyvale california a small aperture 300 m four station array was deployed in sunnyvale california for 5 days to record aftershocks of the loma prieta earthquake of october 1989 the purpose of the array was to study the seismic response of the alluvium filled santa clara valley and the role of surface waves in the seismic shaking of sedimentary basins strong motion records of the loma prieta mainshock indicate that surface waves produced the peak velocities and displacements at some sites in the santa clara valley we use the recordings from the dense array to determine the apparent velocity and azimuth of propagation for various arrivals in the seismograms of four aftershocks with magnitudes between 3 6 and 4 4 apparent velocities are generally observed to decrease with increasing time after the s wave in the seismograms phases arriving less than about 8 sec after the s wave have apparent velocities comparable to the s wave and appear to be body waves multiply reflected under the receiver site or reflected by crustal interfaces for times 10 to 30 sec after the direct s wave we observe long period 1 to 6 sec arrivals with apparent velocities decreasing from 2 5 to 0 8 km sec we interpret these arrivals to be surface waves and conclude that these surface waves produce the long duration of shaking observed on the aftershock records much of the energy in the 40 sec after the s wave is coming approximately from the direction of the source although some arrivals have backazimuths as much as 60 different from the backazimuths to the epicenters two of the aftershocks show arrivals coming from 30 to 40 more easterly than the epicenters this energy may have been scattered from outcrops along the southeastern edge of the basin in contrast the deepest aftershock studied d 17 km displays later arrivals with backazimuths 30 to 40 more westerly than the epicenter a distinct arrival for one of the aftershocks propagates from the southwest possibly scattered from the western edge of the basin synthetic seismograms derived from a plane layered crustal model do not produce the long period love waves observed in the waveforms of the ml 4 4 aftershock these love waves may be generated by the conversion of incident s waves or rayleigh waves near the edge of the basin

late neogene recent uplift of the cabo de gata volcanic province almeria se spain abstract cabo de gata is a miocene volcanic province in the betic cordillera in southeastern spain the distribution of coastal deposits in the successive marine sedimentary units overlying the last volcanic episode about 7 5 ma old has been used to reconstruct the post volcanic palaeogeographic evolution of the region during the late neogene the current elevation of well dated shoreline marker rocks has been used to estimate uplift amount and rates since the late tortonian a n45 e aligned the strike of the carboneras fault system topographic relief was emergent in the cabo de gata region the extension and height of this island increased throughout the late neogene smaller independent islands were emergent and finally became connected to the main island during the messinian the carboneras and agua amarga pliocene sub basins were the last two marine basins prior to the final emergence of the region since the last volcanic eruption ca 7 5 ma maximum uplift of sedimentary rocks in cabo de gata has taken place on the western margin of the n45 e aligned palaeorelief the altitude of the shoreline marker rocks in the successive sedimentary units decreases eastwards to the present day coast and northwards of the rambla del plomo uplift rates since deposition remain nearly constant for the successive messinian rocks and decrease slightly for the lower pliocene outcrops most of the uplift took place before the pliocene while the main island enlarged uplift amounts and rates since deposition of the upper neogene sedimentary units in the cabo de gata area are similar to the ones estimated for laterally equivalent units in the eastern betic basins approximately 50 m ma despite its volcanic nature and the occurrence of the carboneras fault system the cabo de gata probably became elevated as a consequence of regional uplift in connection with the rest of the betic cordillera

fine structure of p nl waves from explosions the long period character of p nl waves from many tectonic regimes has been modeled in the past using a simple homogeneous layer over a half space crust model scant attention has been paid to the short period wave form character in this study the p nl waves from nevada test site explosions as recorded on a regional digital seismic array have been modeled in the period range 1 10 s the fine structure of the p nl waveform has been carefully analyzed the effects of a realistic velocity gradient at the free surface or a velocity transition zone at the crust mantle boundary have been calculated a free surface gradient of even a moderate nature has a major effect on the waveforms whereas a finite crust mantle transition apparently does not we have developed an efficient scheme to implement the wavenumber integration theory for computing short period p nl waves at regional distances we utilized both the wavenumber integration and a generalized ray theory synthetic seismogram code for use in our analysis the former was used to compute exact though band limited synthetics the latter was used to determine which generalized ray paths contributed the primary energy to the major features of the waveforms for the simple one layer crust model the dominant energy to the critically reflected p wave portion of the wave comes from the primary ray reflected from the mantle we refer to this as pmp similar to the notation of pcp for the primary core reflection the next high amplitude energy is associated with a group of ray paths with at least one converted mode i e p converting to s and vice versa if the wave velocities are gradientlike within the top few kilometers of the model 6 km in this study the contribution from 2pmp a ray reflecting twice from the crust mantle interface ray path becomes significant the converted group of rays still contributes but not strongly free surface reflections become significant with the surface gradient present whereas they are not important for the simple one layer model besides the direct pn four major ray groups have been specifically identified in the observed waveforms and shown to dominate the wave shape these are the pmp and 2pmp groups the converted group and the 2pn head wave group the 2pmp and 2pn groups are portions of the same generalized rays since the converted group of rays loses significance in the presence of a realistic free surface gradient the onset of the critically reflected portion of the pnl waveform can be modeled as primarily an interaction between pmp and 2pmp the increased importance of depth phases in this part of the waves implies that it may be useful in improving source discrimination capabilities low q surface layers have been explored as a possible physical mechanism for the resolving power of regional spectral discriminants such a mechanism appears plausible if this is the dominant mechanism in differentiating earthquake and explosion spectra then earthquakes should have a much more energetic pmp phase with later arrivals being comparable in frequency content for the two types of sources

site response estimation for the 2003 miyagi oki earthquake sequence considering nonlinear site response the m w 7 0 miyagi oki earthquake which occurred on 26 may 2003 was well recorded by the kik net and k net networks a large number of stations recorded very high peak ground accelerations above 0 5g and large peak ground velocities above 0 5 m sec these high ground motion values are thought to come from a combination of the effect of shallow sediment layers of the upper couple of meters and the enhanced high frequency ground motion content associated with this intraslab earthquake the objective of this study is to examine the effect of sediment amplification at network stations with peak ground acceleration 0 3g linear site response is first estimated from observed weak motion aftershock records in this case we use a spectral inversion method without reference stations to separate the source path and site response effects the resulting weak motion analysis for the source path and site response agree with other previous studies the mainshock site response is obtained separately using the same spectral inversion technique with the addition of a frequency dependent radiation pattern the comparison of the site am plification from aftershocks with the mainshock indicates the possibility of nonlinear site response at many stations during the mw 7 0 event the results also suggest a correlation between lownear surface materialvelocity and thedegreeofnonlinearity

the information content of high frequency seismograms and the near surface geologic structure of hard rock recording sites due to hardware developments in the last decade the high frequency end of the frequency band of seismic waves analyzed for source mechanisms has been extended into the audio frequency range 20 hz in principle the short wavelengths corresponding to these frequencies can provide information about the details of seismic sources but in fact much of the signal is the site response of the nearsurface several examples of waveform data recorded at hard rock sites which are generally assumed to have a flat transfer function are presented to demonstrate the severe signal distortions includingfmax produced by near surface structures analysis of the geology of a number of sites indicates that the overall attenuation of high frequency 1 hz seismic waves is controlled by the whole path q between source and receiver but the presence of distinctfmax site resonance peaks is controlled by the nature of the surface layer and the underlying near surface structure models of vertical decoupling of the surface and nearsurface and horizontal decoupling of adjacent sites on hard rock outcrops are proposed and their behaviour is compared to the observations of hard rock site response the upper bound to the frequency band of the seismic waves that contain significant source information which can be deconvolved from a site response or an array response is discussed in terms offmax and the correlation of waveform distortion with the outcrop scale geologic structure of hard rock sites it is concluded that although the velocity structures of hard rock sites unlike those of alluvium sites allow some audio frequency seismic energy to propagate to the surface the resulting signals are a highly distorted limited subset of the source spectra

directional site resonances observed from aftershocks of the 18 october 1989 loma prieta earthquake abstract two weeks after the 18 october 1989 loma prieta california earthquake 18 three component digital seismometers were deployed in the epicentral area to form three six station subarrays the subarray configuration allowed us to investigate the presence of direction and frequency dependent site resonances we measured the shear wave polarization from the recordings of 10 aftershocks from the loma prieta earthquake our observations show that the site response has a strong azimuthal dependence and that both the shear wave polarization and the spectral amplitude of the ground motions are affected by site characteristics in the frequency range from 1 to 18 hz the majority of stations examined showed preferred azimuths of ground motion for the scattered waves that did not depend either on the earthquake location or on the polarization of the shear waves expected from the known focal mechanism the measurements were made from 5 sec windows that included direct and scattered shear waves which contain the largest amplitude motions in the near source region and are therefore of most interest to earthquake engineers however in the 0 to 2 hz frequency range the first pulse of shear waves shows a polarization that is well predicted by the mechanism and location of the earthquake the rapid spatial variation of the preferred directions and their corresponding frequencies indicate that geologic structures within a distance of the order of 50 m probably control these site effects we suggest that the site amplified the motion of scattered waves in one preferred direction altering the resulting polarization and modulating the spectral amplitude

quantifying the spatial distribution of site response by use of the yokohama high density strong motion network we have estimated the spatial variation of site response using three component accelerograms from the yokohama high density strong motion net work 150 surface and 9 borehole accelerometers located in a 20 30 km 2 area the site response is the average from 28 earthquakes 16 were recorded at all borehole stations we inverted data recorded at the boreholes to determine 1 source param eters seismic moment m0 and corner frequency fc for each event 2 q f for the path and 3 frequency dependent site factors for the boreholes the inversion scheme is independent of a reference station that is there are no a priori constraints about site response for events with depths greater than 30 km we find q f almost independent of frequency q f 285f 0 06 for the four shallow events that were analyzed q f 70 f 0 28 having determined m0 fc and q f using the borehole data we computed the site response for each of the 150 surface stations by dividing the observed surface spec trum by the path modified source spectrum we then averaged this ratio for all 24 events to quantify the variability of the site response between sites we introduced a site response ratio that depends on station separation and frequency from this we computed cumulative probability functions for different frequencies for two cases 1 the average vs30 ratio is most similar for two sites with the same separation distance and 2 the average vs30 ratio is least similar when two sites have the most similar vs30 ratio these functions indicate that 95 of the time a frequency dependent site response f 1 hz can be predicted within a factor of two for a site up to 5 km from a site with a known site response and 90 of the time the unknown response is within a factor of three for frequencies 1 10 hz when two sites have the least similar average vs30 ratio the cumulative probability functions have similar shapes for all frequencies above 0 5 hz for frequencies up to 10 hz the site ampli fication can be predicted within a factor of three for 80 of the station pairs separated by as much as 5 km

urban seismology northridge aftershocks recorded by multi scale arrays of portable digital seismographs we deployed portable digital seismographs in the san fernando valley sfv the los angeles basin lab and surrounding hills to record aftershocks of the 17 january 1994 northridge california earthquake the purpose of the deployment was to investigate factors relevant to seismic zonation in urban areas such as site amplification sedimentary basin effects and the variability of ground motion over short baselines we placed seismographs at 47 sites not all concurrently and recorded about 290 earthquakes with magnitudes up to 5 1 at five stations or more we deployed widely spaced stations for profiles across the san fernando valley as well as five dense arrays apertures of 200 to 500 m in areas of high damage such as the collapsed interstate 10 overpass sherman oaks and the collapsed parking garage at calstate northridge aftershock data analysis indicates a correlation of site amplification with mainshock damage we found several cases where the site amplification depended on the azimuth of the aftershock possibly indicating focusing from basin structures for the parking garage array we found large ground motion variabilities a factor of 2 over 200 m distances for sites on the same mapped soil unit array analysis of the aftershock seismograms demonstrates that sizable arrivals after the direct s waves consist of surface waves traveling from the same azimuth as that of the epicenter these surface waves increase the duration of motions and can have frequencies as high as about 4 hz for the events studied here we do not observe large arrivals reflected from the southern edge of the san fernando valley

site response for urban los angeles using aftershocks of the northridge earthquake ground motion records from aftershocks of the 1994 northridge earthquake are used to estimate site response in the urban los angeles area over 1300 shear wave records from 61 sources and 90 sites are used in a linear inversion for source and site response spectra the methodology makes no assumptions about the shape of the source spectrum to obtain a stable unique inverse a q model and geometrical spreading factor are assumed in addition the site response at a hard rock site is constrained to be approximately 1 0 with a kappa of 0 02 the site response spectra compare favorably with the results of previous and on going investigations in los angeles a couple of first order effects are lower site response in the surrounding mountains dominated by mesozoic and tertiary rocks and higher values in the san fernando and los angeles basins containing surficial pleistocene and holocene alluvial deposits results show good correlation of high site response spectral values with localized areas of severe damage interstate 10 collapse sherman oaks northridge interstate 5 14 collapse however widespread trends in site response across the sedimentary basins are not obvious the data suggest that site responses are lower near the southern margin of the san fernando valley for sources to the north due to north dipping sedimentary structures but the general pattern of site response is characterized by high variability on length scales less than a kilometer variations of a factor of 2 in site response are observed over the length scale of 200 m and for the same surficial geologic unit for some of the alluvial basin sites surface wave generation is a significant contributor to elevated site response at lower frequencies below 2 hz the total damage pattern for the northridge earthquake is influenced by strong source directivity to the north and strong local site effects the correlation of weak motion site response estimates with areas of significant damage demonstrates the value of these field measurements in future urban planning and in the reduction of seismic risk in urban areas

preliminary analysis of long period basin response in the los angeles region from the 1994 northridge earthquake long period 1 10 sec ground motions recorded in the los angeles basin region during the northridge earthquake show complex waveforms extended durations and multiple sets of arrivals which cannot be attributed solely to source processes or wave propagation within a plane layered medium these features suggest a strong interaction between the propagating seismic wave field and the laterally varying subsurface geologic structure of this region recorded motions at a hard rock site in the santa monica mountains scrs are smaller by nearly a factor of three in peak velocity compared to recordings at more distant sites in the adjacent northwest portion of the los angeles basin in addition although the rock site recording has a relatively simple wave shape the basin site recordings are dominated by late arriving large amplitude pulses of energy we interpret these arrivals to be surface waves which are generated by body waves interacting with the thickening margin of the basin a preliminary modeling analysis of these data indicates that a combination of both large scale deep basin and small scale shallow micro basin structures are needed to explain the observed responses

variability of site response in seattle washington ground motion from local earthquakes and the ships seismic hazards investigation in puget sound experiment is used to estimate site amplification factors in seattle earthquake and ships records are analyzed by two methods 1 spectral ratios relative to a nearby site on tertiary sandstone and 2 a source site spectral inversion technique our results show site amplifications between 3 and 4 below 5 hz for west seattle relative to tertiary rock these values are approximately 30 lower than amplification in the duwamish valley on artificial fill but significantly higher than the calculated range of 2 to 2 5 below 5 hz for the till covered hills east of downtown seattle although spectral amplitudes are only 30 higher in the duwamish valley compared to west seattle the duration of long period ground motion is significantly greater on the artificial fill sites using a three dimensional displacement response spectrum measure that includes the effects of ground motion duration values in the duwamish valley are 2 to 3 times greater than west seattle these calculations and estimates of site response as a function of receiver azimuth point out the importance of trapped surface wave energy within the shallow low velocity sedimentary layers of the duwamish valley one dimensional velocity models yield spectral amplification factors close to the observations for till sites east of downtown seattle and the duwamish valley but underpredict amplifications by a factor of 2 in west seattle a two dimensional finite difference model does equally well for the till sites and the duwamish valley and also yields duration estimates consistent with the observations for the duwamish valley the two dimensional model however still underpredicts amplification in west seattle by up to a factor of 2 this discrepancy is attributed to 3d effects including basin edge induced surface waves and basin geometry focusing effects caused by the proximity of the seattle thrust fault and the sediment filled seattle basin

localized amplification of seismic waves and correlation with damage due to the northridge earthquake evidence for focusing in santa monica abstract the analysis of seismograms from 32 aftershocks recorded by 98 seismic stations installed after the northridge earthquake in the san fernando valley the santa monica mountains and santa monica california indicates that the enhanced damage in santa monica is explained in the main by focusing due to a lens structure at a depth of several kilometers beneath the surface and having a finite lateral extent the diagnosis was made from the observation of late arriving s phases with large amplitudes localized in the zones of large damage the azimuths and angles of incidence of the seismic rays that give rise to the greatest focusing effects correspond to radiation that would have emerged from the lower part of the rupture surface of the mainshock thus the focusing and hence the large damage in santa monica were highly dependent on the location of the northridge event and an earthquake of similar size located as little as one source dimension away would not be likely to repeat this pattern we show from coda wave analysis that the influence of surface geology as well as site effects on damage in santa monica is significantly smaller than are the focusing effects

nonlinear site response at kik net kmmh16 mashiki and heavily damaged sites during the 2016 mw 7 1 kumamoto earthquake japan abstract damage from severe ground motion occurred in the downtown area of mashiki in the kumamoto prefecture during the 2016 kumamoto earthquake japan such damage was heavy in the center of the downtown area nonlinear site responses for the first shock and mainshock which occurred on 14 and 16 april 2016 respectively are important factors that explain why the area was heavily damaged we analyzed soil nonlinearity using surface and borehole records obtained from the kik net kmmh16 mashiki station from our analysis we found that s wave velocity models clearly depended on the amplitude of input ground motion we estimated the strain dependent shear stiffness and damping ratio to explain this s wave velocity dependence we conducted equivalent linear analyses at the kmmh16 site based on a nonlinear model from these analyses we concluded that our synthetic surface ground motions agreed well with the observed ones especially for the s wave amplitudes and phases of the first shock and mainshock noted above in addition we performed the same analyses at the tmp3 site which was actually located within one of the heavily damaged zones the synthetic motions here also agreed with the observed ones with differences in spectral accelerations being well explained by our analyses our results indicated that soil nonlinearity played a major role in causing the difference of ground motions thus leading to the heavily damaged zone in the downtown area of mashiki

initial investigation of site and topographic effects at robinwood ridge california abstract following the 1989 loma prieta earthquake a dense array of seven digitally recorded three component seismograph stations was deployed on robinwood ridge 7 3 km northwest of the epicenter the purpose of this array was to investigate the cause of high levels of structural damage and ground cracking observed on the ridge crest aftershocks recorded by the array allow a comparison of ground motion up the slope of the ridge from the base to the crest the data present an extremely complicated pattern of ground motion that demonstrates the importance of the three dimensionality of the problem slowness analysis of p wave trains show initial arrivals propagating away from the source with small angles of incidence and large apparent velocities consistent with direct arrivals after 0 5 sec propagation azimuths become more random and apparent velocities drop indicating nearly horizontal wave propagation and multiply reflected and diffracted phases within the ridge slowness analysis and particle motion diagrams of horizontal components of motion show dramatic variations in ground motion with changes in azimuth of the source and a complicated interaction between body waves and rayleigh and love waves results suggest that the larger amplitude more coherent arrivals at the array stations favor a propagation direction parallel to the ridge axis an amplification factor of from 1 5 to 4 5 is seen for frequencies from 1 0 to 3 0 hz with wavelengths comparable to the base of the ridge part of which may be caused by local site effects and part by topographic amplification in addition amplifications of up to a factor of 5 are seen at higher frequencies and are attributed to local site effects these effects are most notable from 4 to 8 hz on the vertical components and from 6 to 9 hz on the horizontal components the entire robinwood ridge area may also have been situated in a region of heightened mainshock ground motion due to source directivity and radiation pattern effects

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

analytical models for the mean flow inside dense canopies on gentle hilly terrain simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments laser doppler anemometry lda measurements are conducted in a neutrally stratified boundary layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy the hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy h l 1 and lc l 1 where h is the hill height l the half length and lc the canopy adjustment length scale the lda data suggest that the often criticized linearizations of the advective terms turbulent shear stress gradients and drag force appear reasonable except in the deep layers of the canopy as predicted by a previous analytical model the lda data reveal a recirculation region within the lower canopy on the lee slope adjusting the outer layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model s performance for the velocity perturbations in the deeper layers of the canopy a new analytical model which retains a balance between mean horizontal advection mean pressure gradient and mean drag force but neglects the turbulent shear stress gradient is developed the proposed model reproduces the lda measurements better than the earlier analytical model which neglected advection but retained the turbulent shear stress gradient in the lower layers of the canopy and near the hill top this finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small in essence the newly proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean momentum equation we briefly discuss how to integrate all these findings with recent advances in canopy lidar remote sensing measurements of general topography and canopy height copyright 2008 royal meteorological society

late quaternary fold deformation along the northridge hills fault northridge california deformation coincident with past northridge blind thrust earthquakes and other nearby structures paleoseismic investigation of the northridge hills fault in the northern san fernando valley california helps assess the timing and style of near surface late quaternary deformation in the epicentral area of the 1994 northridge earthquake the northridge hills fault a 15 km long north dipping reverse fault exhibits geomorphic evidence of late quaternary surface deformation including topographic scarps across late quaternary fluvial terraces and aligned alluvial fan apices on the footwall block we excavated one 40 m long trench and six test pits and drilled nine boreholes across a 2 m high scarp developed on a probable holocene fluvial terrace adjacent to aliso canyon wash a continuous clayey gravel identified in the trench test pits and boreholes defines a south facing monocline with 6 1 m of vertical separation across the northridge hills fault based on pedochronology the clayey gravel ranges in age from 6 to 30 ka the borehole data also suggest that an unconformity developed on the plio pleistocene saugus formation is warped into a monocline that has 13 2 m of vertical separation across the fault these preliminary data yield a dip slip rate of 1 0 0 7 mm yr for the northridge hills fault the absence of distinct scarp derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small incremental uplifts rather than large uplifts that generate distinct scarp relief we postulate that such uplift could be produced via moderate magnitude earthquakes m w 6 on the northridge hills fault or secondary deformation induced by earthquakes on other faults for instance evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the northridge hills fault produced by movement on the underlying northridge blind reverse fault or other nearby large structures based on our geologic investigations the distribution of aftershocks following the 1994 earthquake and pre and post 1994 leveling and geodetic surveys we interpret that the northridge hills fault underwent triggered slip during the 1994 earthquake

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

3d ground motion simulations of mw 7 earthquakes on the salt lake city segment of the wasatch fault zone variability of long period t 1 s ground motions and sensitivity to kinematic rupture parameters abstract we examine the variability of long period t 1 s earthquake ground motions from 3d simulations of m w 7 earthquakes on the salt lake city segment of the wasatch fault zone utah from a set of 96 rupture models with varying slip distributions rupture speeds slip velocities and hypocenter locations earthquake ruptures were prescribed on a 3d fault representation that satisfies geologic constraints and maintained distinct strands for the warm springs and for the east bench and cottonwood faults response spectral accelerations sa 1 5 10 s 5 damping were measured and average distance scaling was well fit by a simple functional form that depends on the near source intensity level sa 0 t and a corner distance r c sa r t sa 0 t 1 r r c 1 period dependent hanging wall effects manifested and increased the ground motions by factors of about 2 3 though the effects appeared partially attributable to differences in shallow site response for sites on the hanging wall and footwall of the fault comparisons with modern ground motion prediction equations gmpes found that the simulated ground motions were generally consistent except within deep sedimentary basins where simulated ground motions were greatly underpredicted ground motion variability exhibited strong lateral variations and at some sites exceeded the ground motion variability indicated by gmpes the effects on the ground motions of changing the values of the five kinematic rupture parameters can largely be explained by three predominant factors distance to high slip subevents dynamic stress drop and changes in the contributions from directivity these results emphasize the need for further characterization of the underlying distributions and covariances of the kinematic rupture parameters used in 3d ground motion simulations employed in probabilistic seismic hazard analyses electronic supplement description of seismic velocity model and wave propagation code and interpretation of directivity effects and figures of fault geometry z1 and z2 5 depths effects of rupture speed variations on directivity effects and amplifications

lens effect in santa monica we used finite difference simulations of 10 hz p sv and sh waves to estimate the contribution from the deep basin structure to the large ground motion amplification in santa monica california observed for seismic waves incident from the north our simulations of a 17 km deep northridge aftershock with epicenter 30 km north of santa monica show that focusing at the lens shaped boundary of the sediments can only account for less than half the amplification observed in the area that was heavily damaged during the 1994 m 6 7 northridge earthquake the focusing in the simulations caused amplification of up to 1 6 times in a zone 0 65 2 4 km south of the santa monica fault and de amplification at sites just south of the fault where some of the largest amplification is observed in the data

northridge earthquake damage caused by geologic focusing of seismic waves despite being located 21 kilometers from the epicenter of the 1994 northridge earthquake magnitude 6 7 the city of santa monica experienced anomalously concentrated damage with mercalli intensity ix an intensity as large as that experienced in the vicinity of the epicenter seismic records from aftershocks suggest that the damage resulted from the focusing of seismic waves by several underground acoustic lenses at depths of about 3 kilometers formed by the faults that bound the northwestern edge of the los angeles basin the amplification was greatest for high frequency waves and was less powerful at lower frequencies which is consistent with focusing theory and finite difference simulations

variability of site response in the los angeles urban area this article addresses the variability of site response in the los angeles area and possible structural causes for the observations aftershock records from 231 sites in the san fernando and los angeles basins and the surrounding mountains are used in this study spectral ratios taken with respect to a low amplitude reference site are used to document the variation in site amplification in the frequency range 2 to 6 hz both spatially and with backazimuth to the source at higher frequencies 6 to 10 hz spectral ratios are shown to have greater spatial variability interstation spectral ratios are used to measure the standard deviation among source as a function of station separation an increase in the variation in ground motion is shown to take place at a station separation of 1 km relative site response estimates between nearby stations are used to demonstrate that preferred directions of motion can exist even in areas with no surface topographic effects significant variations in site response exist over short baselines up to a factor of 2 over 200 m that are not explained by differences in surficial geology or shallow shear wave velocity a variety of investigative approaches is used including spectral ratios arrival time variations 1d and 2d waveform modeling and comparison with seismic reflection lines to determine the most likely causes of these observations a correlation is demonstrated between late arrival times of p and s waves and larger site amplification in sherman oaks and northridge this observation in conjunction with waveform modeling and seismic reflection profiles is used to infer that sedimentary structures in the upper 1 to 2 km and topography on the sediment basement interface play an important role in determining site amplification these structures in the form of folds and buried basins focus energy in spatially restricted areas at the surface comparison of displacement waveforms at nearby stations having disparate site amplifications complemented by known shallow shear wave velocities at selected sites is used to support the argument that these structures in some cases can be the dominant factor in the modification of local ground motions

separation of site effects and structural focusing in santa monica california a study of high frequency weak motions from earthquakes and blasts recorded during the los angeles region seismic experiment near surface site factors and the effects of deep structural focusing were estimated in the santa monica mountains and santa monica california from a portable array of 75 seismic stations deployed during the los angeles region seismic experiment phase ii larse ii the objective was to examine further the origin of seismic wave amplification in the region of intense damage south of the santa monica fault from the northridge earthquake the analysis used normalized spectral amplitudes in the 4 to 8 and 8 to 12 hz range in direct and coda waves from local earthquakes in santa paula northridge redlands and hector mine coda waves indicated that site factor amplifications are larger south of the santa monica fault relative to the north spectral ratios of direct s waves corrected for site effects show additional amplification south of and adjacent to the santa monica fault attributable to focusing by a deeper structure gao et al 1996 concluded that localized focusing effects contributed to anomalous p and s wave amplification in the santa monica damage zone for northridge aftershocks within a specified range of azimuths in an attempt to reproduce the hypothesized focusing from the northridge earthquake two shots 4000 and 3750 lb were detonated one at pyramid lake a distance of about 69 km to the north northwest of central santa monica and the other near fort tejon a distance of 91 km the azimuth of the shots was chosen to be that expected to give anomalous amplification at these distances steeply incident seismic energy from pg pmp waves are expected to pass through the underground focusing structure and be selectively amplified after the local site factors are removed the waveforms from the fort tejon shot exhibited localized amplification adjacent to and south of the fault 2 3 times larger than that of the surrounding area the effect is less for waves from the pyramid lake shot which could be due to their higher angle of incidence the observations lend support to the argument that deep structural focusing is an important factor in determining azimuth dependent amplification of seismic waves along a basin edge manuscript received 21 may 2001

first generation site response maps for the los angeles region based on earthquake ground motions abstract ground motion records from aftershocks of the 1994 northridge earthquake and mainshock records from the 1971 san fernando 1987 whittier narrows 1991 sierra madre and 1994 northridge earthquakes are used to estimate site response relative to a rock site for the urban los angeles area site response is estimated at 232 mainshock and 201 aftershock sites relative to a low amplitude site in the santa monica mountains average amplification values are calculated for the frequency bands 1 to 3 3 to 5 and 5 to 7 hz these bands are chosen based on limitations in aftershock recording equipment at lower frequencies and reduced significance to the building inventory at higher frequencies site amplification factors determined at the instrumented locations are grouped by the surficial geology and contoured to produce a continuous spatial estimation of amplification the maps in this article represent the first attempt to produce estimates of site amplification based on observations of ground motion for such a large areal extent of the los angeles region these maps are expected to evolve as more data become available and more analysis is done

site amplification in the san fernando valley california variability of site effect estimation using the s wave coda and h v methods during the months that followed the 17 january 1994 m 6 7 northridge california earthquake portable digital seismic stations were deployed in the san fernando basin to record aftershock data and estimate site amplification factors this study analyzes data recorded on 31 three component stations from 38 aftershocks ranging from m 3 0 to m 5 1 and depths from 0 2 to 19 km site responses from the 31 stations are estimated from coda waves s waves and ratios of horizontal to vertical h v recordings for the coda and the s waves site response is estimated using both direct spectral ratios and a generalized inversion scheme results from the inversions indicate that the effect of qs can be significant especially at high frequencies site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other depending on the choice of the reference site the difference is reduced when an average of six rock sites is used as a reference site in addition when using this multi reference site the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the s waves however for nonrock sites the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the s waves the h v method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods the amplifications determined from the hiv method are however different from the amplifications determined from the other methods finally the stations were grouped into classes based on two different classifications general geology and a more detailed classification using a quaternary geology map for the los angeles and san fernando areas average site response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification

ground motion in the presence of complex topography earthquake and ambient noise sources to study the influence of topography on ground motion eight seismic recorders were deployed for a period of one year over poverty ridge on the east side of the san francisco bay area california this location is desirable because of its proximity to local earthquake sources and the significant topographic relief of the array 439 m topographic amplification is evaluated as a function offrequency using a variety of methods including reference site based spectral ratios and single station horizontal to vertical spectral ratios using both shear waves from earthquakes and ambient noise field observations are compared with the predicted ground motion from an accurate digital model of the topography and a 3d local velocity model amplification factors from the theoretical calculations are consistent with observa tions the fundamental resonance of the ridge is prominently observed in the spectra of data and synthetics however higher frequency peaks are also seen primarily for sources in line with the major axis of the ridge perhaps indicating higher resonant modes excitations of lateral ribs off of the main ridge are also seen at frequencies consistent with their dimensions the favored directions of resonance are shown to be transverse to the major axes of the topographic features

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

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

can a simple dynamical system describe the interplay between drag and buoyancy in terrain induced canopy flows abstractunder nonneutral stratification and in the presence of topography the dynamics of turbulent flow within a canopy is not yet completely understood this has among other consequences serio

observations of basin ground motions from a dense seismic array in san jose california we installed a dense array of 41 digital seismographs in san jose california to evaluate in detail the effects of a deep sedimentary basin and shallow sedimentary deposits on earthquake ground motions this urban array is located near the eastern edge of the santa clara valley and spans the evergreen sedimentary basin identified by gravity data average station spacing is 1 km with three stations initially spaced 110 m apart despite the high noise urban environment the stations of the array successfully triggered on and recorded small local earthquakes m 2 5 2 8 at 10 25 km distance and larger regional events such as the m 5 0 bolinas earthquake 90 km distance m 4 6 5 6 earthquakes near mammoth lakes 270 km distance m 4 9 5 6 events in western nevada 420 km distance and the m 7 1 hector mine earthquake 590 km distance maps of spectral ratios across the array show that the highest amplitudes in all frequency bands studied 0 125 8 hz are generally observed at stations farther from the eastern edge of the santa clara valley larger spectral amplitudes are often observed above the western edge of the evergreen basin snapshots of the recorded wavefield crossing the array for regional events to the east reveal that large low frequency 0 125 0 5 hz arrivals after the s wave travel from south to north across the array a moving window cross correlation analysis finds that these later arrivals are surface waves traveling from the south the timing and propagation direction of these arrivals indicates that they were likely produced by scattering of incident s waves at the border of the santa clara valley to the south of the array it is remarkable that the largest low frequency phases at many of the valley sites for regional events to the east are basin surface waves coming from a direction about 70 degrees different from that of the epicenters basin surface waves emanating from the eastern edge of the valley are also identified by the cross correlation analysis

three dimensional simulations of ground motions in the seattle region for earthquakes in the seattle fault zone we used the 3d finite difference method to model observed seismograms of two earthquakes m l 4 9 and 3 5 in the seattle region and to simulate ground motions for hypothetical m 6 5 and m 5 0 earthquakes on the seattle fault for periods greater than 2 sec a 3d velocity model of the seattle basin was constructed from studies that analyzed seismic reflection surveys borehole logs and gravity and aeromagnetic data the observations and the simulations highlight the importance of the seattle basin on long period ground motions for earthquakes occurring just south of the basin the edge of the basin and the variation of the thickness of the quaternary deposits in the basin produce much larger surface waves than expected from flat layered models the data consist of seismograms recorded by instruments deployed in seattle by the usgs and the university of washington uw the 3d simulation reproduces the peak amplitude and duration of most of the seismograms of the june 1997 bremerton event m l 4 9 recorded in seattle we found the focal mechanism for this event that best fits the observed seismograms in seattle by combining green9s functions determined from the 3d simulations for the six fundamental moment couples the february 1997 event m l 3 5 to the south of the seattle basin exhibits a large surface wave arrival at uw whose amplitude is matched by the synthetics in our 3d velocity model for a source depth of 9 km the m 6 5 simulations incorporated a fractal slip distribution on the fault plane these simulations produced the largest ground motions in an area that includes downtown seattle this is mainly caused by rupture directed up dip toward downtown radiation pattern of the source and the turning of s waves by the velocity gradient in the seattle basin another area of high ground motion is located about 13 km north of the fault and is caused by an increase in the amplitude of higher mode rayleigh waves caused by the thinning of the quaternary deposits

a first order closure for diturbed plant canopy flows and its application to winds in a canopy on a ridge to calculate disturbed wind flows in plant canopies we studied a variant of the k k1 2 first order closure where k is the eddy viscosity k is the turbulent kinetic energy calculated from its simplified governing differential equation and is an algebraic length scale we compare numerical solutions using this closure which admits only three empirical constants with earlier obervations of mean wind shear stress and turbulent kinetic energy in three quite different uniform canopy flows and in disturbed canopy flow over a ridge the closure performs as well as or better than the solutions of others based on modifications of the k closure where is the rate of dissipation of k

flow over a hill covered with a plant canopy we develop an analytical model for atmospheric boundary layer flow over a hill that is covered with a vegetation canopy the slope of the hill is assumed to be small enough that the flow above the canopy can be treated within the linear framework of hunt perturbations to the flow within the canopy are driven by the pressure gradient associated with the flow over the hill in the upper canopy this pressure gradient is balanced by downwards turbulent transport of momentum and the canopy drag the flow there can be calculated from linearized dynamics which show that the maximum streamwise winds are where the perturbation pressure is at a minimum i e near the crest of the hill deep within the canopy the pressure gradient associated with the flow over the hill is balanced by the canopy drag here the nonlinear canopy drag this nonlinear balance shows how the streamwise winds are largest where the perturbation pressure gradient is largest i e on the upwind slope of the hill in the lee of the hill this nonlinear solution shows how the pressure gradient decelerates the wind deep within the canopy leading to separation with a region of reversed flow when the canopy is sufficiently deep coupling between the out of phase flows within and above the canopy means that the maximum velocity is further upwind of the hill crest than in flow over a rough hill while the extra turbulent mixing caused by the canopy significantly reduces the magnitude of the velocity speed up over the hill finally we find that there is no formal limit process where the solutions with a canopy yield the well known solutions for flow over a rough hill this finding calls into question the very use of a roughness length in accelerating or decelerating turbulent boundary layers copyright 2004 royal meteorological society

seismic wave observations with the global positioning system we describe the direct measurement of ground displacement caused by the hector mine earthquake in southern california m w 7 1 october 16 1999 we use a new method of instantaneous positioning which estimates site coordinates from only a single epoch of global positioning system gps data to measure dynamic as well as static displacements at 24 stations of the southern california integrated gps network scign with epicentral distances from 50 to 200 km for sites outside the los angeles basin the observed displacements are well predicted by an elastic half space model with a point shear dislocation within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3 4 min since we resolve the gps phase ambiguities and determine site coordinates independently at each epoch the gps solution rate is the same as the receiver sampling rate for the scign data this is 0 033 hz once per 30 s though sample rates up to 2 hz are possible with the scign receivers since the gps phase data are largely uncorrelated at 1 s a higher sampling rate would offer improved temporal resolution of ground displacement so that in combination with inertial seismic data instantaneous gps positioning would in many cases significantly increase the observable frequency band for strong ground motions

directional topographic site response at tarzana observed in aftershocks of the 1994 northridge california earthquake implications for mainshock motions abstract the northridge earthquake caused 1 78 g acceleration in the east west direction at a site in tarzana california located about 6 km south of the mainshock epicenter the accelerograph was located atop a hill about 15 m high 500 m long and 130 m wide striking about n78 e during the aftershock sequence a temporary array of 21 three component geophones was deployed in six radial lines centered on the accelerograph with an average sensor spacing of 35 m station c00 was located about 2 m from the accelerograph we inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion we identified a 3 2 hz resonance that is a transverse oscillation of the hill a directional topographic effect the top base amplification ratio at 3 2 hz is about 4 5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill this resonance is seen most strongly within 50 m of c00 other resonant frequencies were also observed a strong lateral variation in attenuation probably associated with a fault caused substantially lower motion at frequencies above 6 hz at the east end of the hill there may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill causing particle motions not spectral ratios at the top of the hill to be rotated about 20 away from the direction transverse to the hill the resonant frequency but not the amplitude of our observed topographic resonance agrees well with theory even for such a low hill comparisons of our observations with theoretical results indicate that the 3d shape of the hill and its internal structure are important factors affecting its response the strong transverse resonance of the hill does not account for the large east west mainshock motions assuming linear soil response mainshock east west motions at the tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill probable variations in surficial shear wave velocity do not account for the observed differences among mainshock acceleration observed at tarzana and at two different sites within 2 km of tarzana