peak ground motions and site response at anza and imperial valley california power spectra of shear waves for eighteen earthquakes from the anza imperial valley region were inverted for source mid path q site attenuation and site response the motivation was whether differences in site attenuation parameterized as t r cq where r is distance along ray path near the site c is shear velocity and q is the quality factor that parameterizes attenuation and site response could be correlated with residuals in peak values of velocity or acceleration after removing the affect of distance dependent attenuation we decomposed spectra of s waves from horizontal components of 18 earthquakes from 2010 to 2018 into a common source for each event with 2 spectral fall off at high frequencies and then projected the residuals onto path and site terms following the methodology of boatwright et al bull seismol soc am 81 1754 1782 1991 the site terms were constrained to have an amplification at a particular frequency governed by vs30 at two of the sites which had downhole shear wave logs the 18 events 3 m 4 had moments between approximately 1020 and 1022 dyne cm and stress drops between 1 and 100 bars average mid crust attenuation had a q of 844 reflecting the average path through the crystalline rock of the san jacinto mountains t for each station corresponded to the geologic environment such that stations on hard rock had low t e g stations knw pfo and rdm a station in the san jacinto fault zone station snd had a moderate t of 0 035 s and stations in the imperial valley usually had higher t s generally t correlated with average amplification suggesting that sites characterized by low surface velocities and higher attenuation also have more amplification in the 1 6 hz band residuals of peak values were determined by subtracting the prediction of boore and atkinson 2008 there is a correlation between average amplification and peak velocity but not peak acceleration interestingly there is less scatter at high values of amplification although there is also less data scatter in values of peak velocity and peak acceleration are higher at shorter compared to longer durations when using a frequency dependent form for q variances are higher sometimes much higher the dataset does not support frequency dependent q which is not similar to results from the imperial valley and northeastern north america

a comparison of spectral parameter kappa from small and moderate earthquakes using southern california anza seismic network data kappa is a one parameter estimator of the spectral amplitude decay with frequency of a seismogram low values 5 ms indicate limited attenuation of high frequency energy whereas higher values 40 ms indicate high frequency energy has been removed kappa is often assumed to be a site term and used in seismic designs we address two key questions about kappa 1 how to identify source path and site contributions to kappa and 2 can kappa estimates from smaller earthquakes and more readily accessible weak motion recordings be reasonably extrapolated to esti mate kappa of larger earthquakes the use of small earthquakes ml 3 5 earthquakes inside the network we find kappa from small earthquakes predicts the relative values of kappa for larger earthquakes e g measurements at stations pfo and knw are small compared with those at stations tro and snd for the snd and tro data however kappa values from small earth quakes overpredict those from moderate and large earthquakes site effects are the most important contributor to kappa estimates but the scatter within kappa measure ments at a given station is likely caused by a significant contribution from near the source perhaps related to near source scattering because of this source side varia bility care is recommended in using individual small events as green s functions to study source time effects of moderate and large events

dependence of sediment filled valley response on input amplitude and valley properties abstract for three sediment filled valleys average site responses were experimentally determined and found to apply to within 0 25 log units to earthquakes at any azimuth and to s waves and coda waves for one of these valleys ground accelerations were measured over the range 10 5 to 0 2 g and no significant differences in site responses were apparent in two valleys with sediment basement impedance contrasts of 5 1 and 6 1 average site amplification is as much as a factor of five in certain frequency bands in another valley with a sediment basement impedance contrast of 3 1 sediment amplification is about a factor of two the duration of motion on the valley sediments in narrow frequency bands is equal to that on the bordering rock implying that the observed amplification is not due to a high q resonance of the sediments

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

a holistic model for the origin of orogenic gold deposits and its implications for exploration the term orogenic gold deposits has been widely accepted but there has been continuing debate on their genesis early syn sedimentary or syn volcanic models and hydrothermal meteoric fluid models are now invalid magmatic hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships the most plausible models involve metamorphic fluids but the source of these fluids is equivocal intra basin sources within deeper segments of the hosting supracrustal successions the underlying continental crust subducted oceanic lithosphere with its overlying sediment wedge and metasomatized lithosphere are all potential sources several features of precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic fluid source these include the presence of hypozonal deposits in amphibolite facies domains their anomalous multiple sulfur isotopic compositions and problems of derivation of gold related elements from devolatilization of dominant basalts in the sequences the phanerozoic deposits are largely described as hosted in greenschist facies domains consistent with supracrustal devolatilization models a notable exception is the jiaodong gold deposits of china where ca 120 ma gold deposits are hosted in precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization other deposits in china are comparable to those in the massif central and elsewhere in france in that they are hosted in amphibolite facies domains or clearly post date regional metamorphic events imposed on hosting supracrustal sequences if all orogenic gold deposits have a common genesis the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold bearing sulfide rich sedimentary package or the associated metasomatized mantle wedge with co2 released during decarbonation and s and ore related elements released from transformation of pyrite to pyrrhotite at about 500 c although this model satisfies all geological geochronological isotopic and geochemical constraints and is consistent with limited computer based modeling of fluid release from subduction zones the precise mechanisms of fluid flux are model driven and remain uncertain from an exploration viewpoint the model re emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction related orogenic belts and importance of continental scale lithosphere tapping fault and shear zones to focus large volumes of auriferous fluid it confirms the importance of the consistent spacing between world class deposits broadly equivalent to the depth of the moho as derived from empirical observations

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

near surface velocities and attenuation at two boreholes near anza california from logging data to investigate near surface site effects in granite rock we drilled 300 m deep boreholes at two sites which are collocated with stations from the digital array at anza california the first borehole was sited at station knw keenwild fire station which is located along a ridge line about 8 7 km east of the san jacinto fault zone station pfo pinon flat observatory chosen for the second site is another 6 km further to the east of station knw and is located on a gently sloping hillside we logged each borehole for p and s wave velocities as well as for crack density and orientation p waves were generated by striking a plate with a hammer at the surface a tool consisting of weighted anvils driven by compressed air against end plates along a 3 5 m beam was used to generate shear waves signals were recorded downhole with a three component sensor package at 2 5 m intervals from the surface to 50 m depth and at 5 m intervals from 50 m depth to the bottom of the hole velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth travel times were computed for the first breaks at shallow depths however below about 100 m depth times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable the knw site yielded a shear velocity of 1 9 km sec by only 30 m in depth and reached close to 2 6 km sec at the bottom of the hole p wave velocities at knw were also high at 5 4 km sec starting at 120 m depth the pfo site had similar but slightly higher shear wave velocities the bottom hole shear wave velocity reached 3 0 km sec and the p wave velocity was 5 4 km sec shear wave attenuation was computed using both the pulse rise time and spectral ratio methods at station knw attenuation was significant only in an interval between 17 5 and approximately 40 m in depth over the top 50 m attenuation corresponding to a q of about 8 was obtained a total t of 0 004 sec was measured for this interval pulse rise times also increased rapidly in this zone the spectral ratio data for station pfo yields two peaks in attenuation above 50 m similar to the attenuation found for station knw the peak in attenuation corresponds to a q of about 11 averaged over the top 50 m spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 hz significant motion perpendicular to the polarizations of the first shear wave arrival was recorded within a few meters of the surface apparently the rock structure is sufficiently complicated that body waves are being converted sh to sv at oblique incidence very close to the surface the presence of these elliptical particle motions within a mere few m of the pure shear wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not by itself a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth s crust crack densities and orientations were determined from televiewer records these records showed cracks with a preferred direction at station knw and of a greater density than at station pfo at station pfo crack densities were smaller and more diffuse in orientation

determination of causative fault parameters for some recent iranian earthquakes using near field sh wave data abstract a method based on a point source representation and non linear least square formulation which estimates the strike dip and rake of the causative fault has been used to infer the causative fault plane parameters using strong ground motion data for the some recent earthquakes in iran this study indicates that the analysis of sh waves of near field accelerographs can distinguish the fault plane from the auxiliary plane this method is especially helpful when lack of information such as surface rupture aftershock data and isoseismal maps makes it difficult to identify causative fault plane in the fault plane solution the estimated fault plane parameters from inversion of the near field data based on the point source model for these earthquakes are in fair agreement with solutions given by hrv and neic

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

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

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

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

the anza array a high dynamic range broadband digitally radiotelemetered seismic array a small aperture seismic array has been deployed along a 30 km stretch of the san jacinto fault in the vicinity of the town of anza california the array was installed to study specifically the scaling laws of body wave spectra the character of high frequency ground motion the physical interpretation of seismic stress drops and the interaction of earthquakes this region was chosen for these studies due to its high rate of seismic activity in the 2 m 4 5 range the likelihood of a m 6 event in the next decade and the existence of the southern california batholith on either side of the fault reducing problems associated with attenuation and large scale anisotropy these studies employ an instrument package with a frequency response of 100 hz a dynamic range of 138 db a sampling rate of 250 times per second per component and a 16 bit a d converter the array consists of 10 three component stations telemetered via digital vhf radio to a nearby mountain peak and thence via a microwave link to la jolla california a minicomputer system monitors the array s performance detects events and records data upon demand initial results demonstrate the feasibility of digital transmission with an inherent increase in data quality over analog systems

energy flux model of seismic coda separation of scattering and intrinsic attenuation a new model of seismic coda is presented based on the balance between the energy scattered from the direct wave and the energy in the seismic coda this energy flux model results in a simple formula for the amplitude and time decay of the seismic coda that explicitly differentiates between the scattering and intrinsic anelastic attenuation of the medium this formula is valid for both weak and strong scattering and implicitly includes multiple scattering the model is tested using synthetic seismograms produced in finite difference simulations of wave propagation through media with random spatial variations in seismic velocity some of the simulations also included intrinsic dissipation the energy flux model explains the coda decay and amplitude observed in the synthetics for random media with a wide range of scattering q in contrast the single scattering model commonly used in the analysis of microearthquake coda does not account for the gradual coda decay observed in the simulations for media with moderate or strong scattering attenuation scattering q less than or equal to 150 the simulations demonstrate that large differences in scattering attenuation cause only small changes in the coda decay rate as predicted by the energy flux model the coda decay rate is sensitive however more to the intrinsic q of the medium the ratio of the coda amplitude to the energy in the direct arrival is a measure of the scattering attenuation thus analysis of the decay rate and amplitude of the coda can in principle produce separate estimates for the scattering and intrinsic q values of the crust we analyze the coda from two earthquakes near anza california intrinsic q values determined from these seismograms using the energy flux model are comparable to coda q values found from the single scattering theory the results demonstrate that coda q values are at best measures of the intrinsic attenuation of the lithosphere and are unrelated to the scatteing q less

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

regional spectral analysis of three moderate earthquakes in northeastern north america abstract we analyze fourier spectra obtained from the horizontal components of broadband and accelerogram data from the 1997 cap rouge the 2002 ausable forks and the 2005 riviere du loup earthquakes recorded by canadian and american stations sited on rock at hypocentral distances from 23 to 602 km we check the recorded spectra closely for anomalies that might result from site resonance or source effects we use beresnev and atkinson s 1997 near surface velocity structures and boore and joyner s 1997 quarter wave method to estimate site response at hard and soft rock sites we revise the street et al 1975 model for geometrical spreading adopting a crossover distance of r o 50 km instead of 100 km we obtain an average attenuation of q 410 25 f 0 50 0 03 for s l g surface waves with ray paths in the appalachian and southeastern grenville provinces we correct the recorded spectra for attenuation and site response to estimate source spectral shape and radiated energy for these three earthquakes and the 1988 m 5 8 saguenay earthquake the brune stress drops range from 130 to 419 bars and the apparent stresses range from 39 to 63 bars the corrected source spectral shapes of these earthquakes are somewhat variable for frequencies from 0 2 to 2 hz falling slightly below the fitted brune spectra

effects of volcano topography on seismic broad band waveforms volcano seismology often deals with rather shallow seismic sources and seismic stations deployed in their near field the complex stratigraphy on volcanoes and near field source effects have a strong impact on the seismic wavefield complicating the interpretation techniques that are usually employed in earthquake seismology in addition as most volcanoes have a pronounced topography the interference of the seismic wavefield with the stress free surface results in severe waveform perturbations that affect seismic interpretation methods in this study we deal predominantly with the surface effects but take into account the impact of a typical volcano stratigraphy as well as near field source effects we derive a correction term for plane seismic waves and a plane free surface such that for smooth topographies the effect of the free surface can be totally removed seismo volcanic sources radiate energy in a broad frequency range with a correspondingly wide range of different fresnel zones a 2 d boundary element method is employed to study how the size of the fresnel zone is dependent on source depth dominant wavelength and topography in order to estimate the limits of the plane wave approximation this approximation remains valid if the dominant wavelength does not exceed twice the source depth further aspects of this study concern particle motion analysis to locate point sources and the influence of the stratigraphy on particle motions furthermore the deployment strategy of seismic instruments on volcanoes as well as the direct interpretation of the broad band waveforms in terms of pressure fluctuations in the volcanic plumbing system are discussed

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

development and testing of a 3d seismic velocity model of the po plain sedimentary basin italy abstract we built a 3d seismic model of the po plain and neighboring regions of northern italy covering altogether an area about 600 km by 300 km with an approximately 1 km spaced grid we started by collecting an extensive and diverse set of geological and geophysical data including seismic reflection and refraction profiles borehole logs and available geological information major geological boundaries and discontinuities have thus been identified and mapped into the model we used kriging to interpolate the geographically sparse information into continuous surfaces delimiting geological bodies with laterally varying thickness seismic wave properties have been assigned to each unit using a rule based system and v p v s and derived from other studies sedimentary strata although with varying levels of compaction and hence material properties may locally reach a thickness of 15 km and give rise to significant effects in seismic wave propagation we have used our new model to compute the seismic response for two recent earthquakes to test its performance results show that the 3d model reproduces the large amplitude and the long duration of shaking seen in the observed waveforms recorded on sediments whereas paths outside the basin may be well fit by more homogeneous 1d hard rock structure we conclude that the new model is suited for simulation of wave propagation mostly for t 3 s and may serve well as a constraint for earthquake location and further improvements via body or surface wave inversion