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

engineering implications of source parameters and 3d wave propagation modeling for the 2004 parkfield california earthquake abstract the 2004 m w 6 0 parkfield california earthquake took place in a very well instrumented area producing a substantial quantity of high quality near field recordings taking advantage of the rare luxury of having a large number of near field ground motion recordings distributed around the fault zone and the availability of various slip models as well as an earth structure model of the region we study the effects of various kinematic rupture parameters to derive implications for strong ground motion simulation in engineering applications we model the 3d wave propagation resulting from this earthquake using the 3d staggered grid finite difference method using a grid spacing of 100 m in our fourth order explicit finite difference code we could properly resolve frequencies of up to 1 hz with a minimum of eight grids per wavelength for shear waves except in the immediate vicinity of the fault where fault trapped waves dominate the records we assess the effects of various simulation parameters such as slip model rise time constant or variable rupture velocity and the earth model 1d versus 3d on the resulting waveforms we also investigate the distribution of engineering parameters such as peak ground velocities peak ground displacements and spectral accelerations at specific periods on the earth s surface an outstanding feature is that at high frequencies fault normal components near the edge of fault segments dominate the ground motion field fault parallel components are dominated by lower frequencies the difference between fault parallel and fault normal components is clearly observed in such engineering parameters as peak ground velocity and peak ground displacement

lateritic paleoweathering profiles in french massif central paleomagnetic datings 1 geological records of the history of ancient land surfaces are scarce and often fragmented by unconformities determining the age of these surfaces has long been the limiting factor in correlating continental paleosurfaces with marine basin deposits age determination of the weathering product of continental paleosurfaces as well as weathered material within the crystalline basement is of prime importance for geodynamic reconstruction in general and more so for assessing differential dynamics between basin and basement areas erosion rates etc in this paper we examine layers of ferruginous kaolinitic paleoweathering products overlying the basement rocks of the french massif central these layers which are mainly formed in situ and are azoic devoid of fossils have been classically related to the siderolithic eocene oligocene period because of their geographic ties with tertiary deposits the paleomagnetic ages of these paleoweathered surfaces have been obtained by comparing their acquired remanent magnetization converted into paleomagnetic poles with the apwp apparent polar wander path of the eurasian plate recovered ages indicate late jurassic to early cretaceous ages 140 160 ma with a confidence limit of 10 ma which are much older than the previously assumed tertiary age the ages obtained in this study considerably alter the present knowledge of the geodynamic evolution of the french massif central our results suggest a lack of sedimentary cover at the jurassic cretaceous boundary this implies that at the time of deposition of the siderolithic formations the basement was cropping out and devoid of any mesozoic cover furthermore the profiles are not compacted indicating that they have never been buried beneath a significant sedimentary cover the obtained ages point to a great stability of french massif central throughout the mesozoic with very low ablation erosion while neighbouring basins aquitaine and paris were subsiding about 2000 m

simulation of multiple station ground motions using stochastic point source method with spatial coherency and correlation characteristics abstract existing algorithms including the stochastic point source method used to simulate synthetic ground motion records are aimed at sampling records at a single station or records at multiple stations the application of the algorithms may not adequately reproduce the observed coherency structure of the actual records and the intraevent spatial correlation characteristics of peak ground acceleration or spectral accelerations to improve these we suggest an extension to the stochastic point source method by introducing a target spatial coherency structure and the spatially correlated uncertainties in the fourier amplitude spectrum for each recording station the use of the extended model to simulate the multiple station records is illustrated and the spatial correlation of the ground motion measures for the simulated records are compared with the empirical spatial correlation model derived based on the 1999 chi chi taiwan earthquake

some key features of the strong motion data from the m 6 0 parkfield california earthquake of 28 september 2004 the 2004 parkfield california earthquake was recorded by an extensive set of strong motion instruments well positioned to record details of the motion in the near fault region where there has previously been very little recorded data the strong motion measurements obtained are highly varied with significant variations occurring over only a few kilometers the peak accelerations in the near fault region range from 0 13 g to over 1 8 g one of the highest acceleration recorded to date exceeding the capacity of the recording instrument the largest accelerations occurred near the northwest end of the inferred rupture zone these motions are consistent with directivity for a fault rupturing from the hypocenter near gold hill toward the northwest however accelerations up to 0 8 g were also observed in the opposite direction at the south end of the cholame valley near highway 41 consistent with bilateral rupture with rupture southeast of the hypocenter several stations near and over the rupturing fault recorded relatively weak motions consistent with seemingly paradoxical observations of low shaking damage near strike slip faults this event had more ground motion observations within 10 km of the fault than many other earthquakes combined at moderate distances peak horizontal ground acceleration pga values dropped off more rapidly with distance than standard relationships at close in distance the wide variation of pga suggests a distance dependent sigma may be important to consider the near fault ground motion variation is greater than that assumed in shakemap interpolations based on the existing set of observed data higher density of stations near faults may be the only means in the near future to reduce uncertainty in the interpolations outside of the near fault zone the variance is closer to that assumed this set of data provides the first case where near fault radiation has been observed at an adequate number of stations around the fault to allow detailed study of the fault normal and fault parallel motion and the near field s wave radiation the fault normal motions are significant but they are not large at the central part of the fault away from the ends the fault normal and fault parallel motions drop off quite rapidly with distance from the fault analysis of directivity indicates increased values of peak velocity in the rupture direction no such dependence is observed in the peak acceleration except for stations close to the strike of the fault near and beyond the ends of the faulting

assessment of coherency for bidirectional horizontal ground motions and its application for simulating records at multiple stations estimation of the coherency of ground motion records in two horizontal orthogonal directions at single or multiple stations is carried out using records from smart 1 array for several seismic events in taiwan empirical coherency function is suggested for the components of records in two horizontal orthogonal directions at single and multiple recording stations it was also found that the coherency for the records along the major and minor principal axes at a recording station is similar to that for two randomly oriented orthogonal directions the spatial coherency and the spatial correlation of the records are used to establish a procedure for simulating bidirectional horizontal ground motions at multiple stations considering that the stochastic point source method or the stochastic finite fault method can be used to define the reference fourier amplitude spectra for scenario events use of the proposed procedure to simulate records is illustrated for a scenario seismic event the adequacy of the simulated records in approximating the target spatial correlation and coherency is presented

estimated ground motion from the 1994 northridge california earthquake at the site of the interstate 10 and la cienega boulevard bridge collapse west los angeles california we have estimated ground motions at the site of a bridge collapse during the 1994 northridge california earthquake the estimated motions are based on correcting motions recorded during the mainshock 2 3 km from the collapse site for the relative site response of the two sites shear wave slownesses and damping based on analysis of borehole measurements at the two sites were used in the site response analysis we estimate that the motions at the collapse site were probably larger by factors ranging from 1 2 to 1 6 than at the site at which the ground motion was recorded for periods less than about 1 sec

modelling the spatial correlation of earthquake ground motion insights from the literature data from the 2016 2017 central italy earthquake sequence and ground motion simulations abstract over the past decades researchers have given increasing attention to the modelling of the spatial correlation of earthquake ground motion intensity measures ims particularly when the seismic risk of spatially distributed systems is being assessed the quantification of the seismic performance of these systems requires the estimation of simultaneous ims at multiple locations during the same earthquake for which the correlation between pairs of locations needs to be defined numerous spatial correlation models of common ims such as peak ground acceleration and spectral acceleration have been published although the functional forms of the models are generally similar significant discrepancies exist in terms of the rate of decay of the correlation with increasing inter site separation distance the main reasons for such differences lie with the selected databases the ground motion models used to derive the spatial correlation models estimation approaches and regional geological conditions in this study we aim to provide a comprehensive review of spatial correlation models analysing factors that most affect the spatial dependency of ims we use strong motion records from the 2016 2017 central italy earthquake sequence combined with ground motion simulations to examine the influence of various factors on spatial correlation models we investigate the dependency on 1 the estimation method and model fitting technique 2 the magnitude 3 the response spectral period and 4 local soil conditions our results suggest that the rate of decay is not only period dependent but also regionally dependent so that a single universal correlation model based on large datasets is not appropriate when describing the correlation behaviour of small geographical areas our outcomes could be used to guide the development of new spatial correlation models

nonlinear ground motion amplification by sediments during the 1994 northridge earthquake it has been known since at least 1898 ref 1 that sediments can amplify earthquake ground motion relative to bedrock for the weak ground motion accompanying small earthquakes the amplification due to sediments is well understood in terms of linear elasticity hooke s law 2 but there has been a long standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes the view of geotechnical engineers based largely on laboratory studies is that hooke s law breaks down at larger strains causing a reduced nonlinear amplification seismologists on the other hand have tended to remain sceptical of this nonlinear effect mainly because the relatively few strong motion observations seemed to be consistent with linear elasticity although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances3 4 the significance of nonlinearity for the type of stiff soil sites found in the greater los angeles region remains unresolved5 here we report that ground motion amplification due to sediments for the main shock of the 1994 northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks these observations imply significant nonlinearity in such amplification and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites

discrimination between pressure and fluid saturation changes from time lapse seismic data explicit expressions for computing saturation and pressure related changes from time lapse seismic data have been derived and tested on a real time lapse seismic data set necessary input is near and far offset stacks for the baseline seismic survey and the repeat survey the method has been tested successfully in a segment where pressure measurements in two wells verify a pore pressure increase of 5 to 6 mpa between the baseline survey and the monitor survey estimated pressure changes using the proposed relationships fit very well with observations between the baseline and monitor seismic surveys 27 of the estimated recoverable hydrocarbon reserves were produced from this segment the estimated saturation changes also agree well with observed changes apart from some areas in the water zone that are mapped as being exposed to saturation changes which is unlikely saturation changes in other segments close to the original oil water contact and the top reservoir interface are also estimated and conf

observation of local site effects at a downhole and surface station in the marina district of san francisco abstract the marina district of san francisco california with its artificial fill and a thick section of sand and clay covering a northwest trending valley in the bedrock suffered extensive damage during the 18 october 1989 loma prieta earthquake following the earthquake the usgs drilled a hole at winfield scott school at beach and divisadero streets the borehole intersects bedrock surface at a 79 5 m depth two three component seismometers one in bedrock at a 88 m depth and one located at the surface have been installed at the site each seismometer consists of one vertical and two orthogonally oriented horizontal geophones having a natural period of 0 5 sec between august 1990 and january 1991 more than 50 earthquakes have been recorded digitally eight among these ranging in magnitude between 2 8 and 3 6 and originating on the calaveras franklin greenville and hayward faults and on faults parallel and close to the san andreas fault generated seismograms with high signal to noise ratio horizontal ground motion amplification expressed as spectral ratio between ground motions at the surface and those in the bedrock has been calculated for motions in two orthogonal directions along divisadero and beach street each ground motion spectrum has been calculated using an entire seismogram consisting of body waves surface waves multiply reflected and scattered coda waves and a short section 2 sec of pre event ambient noise before calculating spectral ratio each spectrum has been smoothed using a truncated gaussian window 0 61 hz wide except for the lowest frequency spectral ratio peak at 1 hz frequency of other peaks depends on earthquake location amplitude of spectral ratio peaks also show variation depending on ground motion direction and earthquake location for example amplitude of the 1 hz spectral ratio peak varies from 7 2 to 12 7 the surface downhole spectral ratio therefore provides only partial information on how ground motions are amplified by sediment deposits if we choose to use this ratio for earthquake engineering applications the ratios from the eight earthquakes give an indication of the variation in spectral ratio to be expected from earthquakes with similar magnitudes and epicentral distances on various bay area faults also noteworthy are the observations that the two horizontal component seismograms recorded by each seismometer have similar coda amplitude and duration regardless of earthquake location and that particle motion polarization becomes complex shortly after the p wave and s wave onset the complex particle motion polarization indicates that wave fields in the bedrock and at the surface are three dimensional the bedrock topography underlying the site has been delineated previously to be three dimensional from drill hole information we suggest from these observations that three dimensional effects need to be considered when modeling site amplification in the marina district finally the eight earthquakes are divided into two groups comprising those whose epicenters are located east of san francisco bay and those whose epicenters are located south of san francisco bay within each group spectral ratio peaks from different earthquakes line up with each other thus showing consistency in spectral ratio peaks as a function of earthquake location

a one step newmark displacement model for probabilistic seismic slope displacement hazard analysis abstract estimating the earthquake induced sliding displacement is important in assessing the stability of slopes during earthquakes current newmark displacement models generally use ground motion intensity measures ims as predictors and the uncertainties of predicting im values need to be accounted for in probabilistic seismic slope displacement hazard analysis this paper aims at developing a simple one step predictive model for the newmark displacement based on only four seismological parameters moment magnitude rupture distance fault categories and shear wave velocity at top 30 m vs30 rather than any ims the predictive model is suitable for critical acceleration from 0 02 g to 0 25 g covering most of the susceptible earthquake induced landslide cases first the proposed one step model is compared with some recently developed im based newmark displacement models it is found that both the median predictions and the variabilities of the proposed one step model are in reasonable agreement with those obtained by the im based models second several hypothetical slopes are used to compare the slope displacement hazard curves between the one step model and the im based models the new model can be used as an alternative method for a fully probabilistic analysis of the earthquake induced slope displacements in addition spatial correlations of the newmark displacement residuals are also investigated using strong motion data from the northridge and chi chi earthquakes

long offset time lapse seismic tested on the valhall lofs data conventional time lapse seismic has been less successful for stiff rock reservoir monitoring such as carbonates this is mainly because of the negligible time lapse changes in the seismic properties therefore we propose to use long offset time lapse seismic as an alternative method to estimate small velocity changes more specifically we monitor the maximum amplitude offset that is beyond critical offset the properties of the maximum amplitude offset are similar to critical offset except that they appear for longer offsets and are frequency dependent increased frequency reduces the gap between this offset and the critical offset we find that the maximum amplitude offset is a function of overburden and reservoir velocity and practically independent of density this method requires a velocity increase across the interface that is to be analyzed this criterion usually is satisfied for stiff rock reservoirs also by long offset acquisition we mean typically 1 to 2 km beyond the critical offset for

nonlinear sediment response during the 1994 northridge earthquake observations and finite source simulations we have addressed the long standing question regarding nonlinear sediment response in the los angeles region by testing whether sediment amplification was similar between the northridge earthquake and its aftershocks comparing the weak and strong motion site response at 15 sediment sites we find that amplification factors were significantly less for the main shock implying systematic nonlinearity the difference is largest between 2 and 4 hz a factor of 2 and is significant at the 99 confidence level between 0 8 and 5 5 hz the inference of nonlinearity is robust with respect to the removal of possibly anomalous sediment sites and how the reference site motion is defined furthermore theoretical ground motion simulations show no evidence of any bias from finite source effects during the main shock nonlinearity is also suggested by the fact that the four sediment sites that contain a clear fundamental resonance for the weak motion exhibit a conspicuous absence of the peak in the strong motion although we have taken the first step of establishing the presence of nonlinearity it remains to define the physics of nonlinear response and to test the methodologies presently applied routinely in engineering practice the inference of nonlinearity implies that care must be exercised in using sediment site data to study large earthquakes or predict strong ground motion

time lapse seismic surveys in the north sea and their business impact in a time lapse 3 d seismic tl3d project a monitor seismic survey is compared to a base survey ideally the base survey is a 3 d data set acquired before production from the field started normally the monitor survey is acquired after several years of production the observed differences between the base and the monitor survey can be interpreted in terms of changes in fluid saturation and pressure when embarking on a seismic monitoring project it is important to keep the following questions in mind in this article we will answer these questions and illustrate our solutions with two examples what field information might technically be obtained fluid drainage information from tl3d may be used directly to locate fluid fronts in each major flow unit it may also be used to derive lateral variations in permeability of the reservoir and sealing capacity of faults these variations can cause areas of bypassed oil and blocks that are not being drained in general it will help to understand the observed production from the field it is a very powerful method to map production related changes in reservoir fluid saturation and pressure away from the wells tl3d very often reveals some flow behavior that had not been predicted even by history matched flow models this information can be critical to future reservoir development what impact will it have on reservoir management time lapse seismic may be used to optimize the number and location of infill wells to access untapped reserves or to accelerate production ideally this will minimize infill costs and maximize recovery it may also indicate the need to shut off thief zones to avoid premature water or gas breakthrough it will

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

spatial correlation of ground motions from vrancea romania intermediate depth earthquakes the evaluation of the spatial correlation of ground motions is the first step toward an assessment of the seismic risk on a regional scale this short study focuses on the evaluation of the spatial correlation of ground motions recorded during intermediate depth earthquakes produced in the vrancea subcrustal seismic source in romania for eight spectral periods in the 0 0 3 0 s range a recently developed ground motion model is used as reference and the database consists of around 300 ground motions recorded during eight intermediate depth vrancea earthquakes an exponential model is fitted on the experimental semivariograms and the resulting correlation ranges are compared with the values obtained in previous studies

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

probabilistic characterization of spatially correlated response spectra for earthquakes in japan seismic hazard and risk assessments of spatially distributed infrastructural systems require seismic demand models that capture random but correlated simultaneous seismic effects at multiple sites this study characterizes spatially correlated ground motion parameters probabilistically using comprehensive databases of the k net and kik net strong motion networks in japan by developing a ground motion prediction equation and then investigating the correlation structure of regression residuals from the prediction equation analysis results indicate that 1 interevent residuals of ground motion parameters at different vibration periods are more strongly correlated than intraevent and total residuals with zero separation distance and 2 intraevent spatial correlation coefficients can be described as a simple exponential decay function that is independent of the way the event based intraevent standard deviation is calculated of the earthquake type and of the vibration period the developed overall correlation model of spatially correlated ground motion parameters may be used for seismic hazard and risk assessments in a subduction environment

monitoring a shallow subsurface gas flow by time lapse refraction analysis studying time lapse time shifts for refracted events is a complementary method to conventional 4d analysis especially for detecting changes in relatively shallow subsurface layers we find that this method has potential by studying data associated with an old underground blowout we find consistent time shifts of up to 4 ms when comparing a base survey that was acquired before the underground blowout with a monitor survey acquired 20 months after the blowout these time shifts are interpreted as caused by shallow gas migration the 4d refraction time shifts are estimated by a standard windowed crosscorrelation technique observed time shifts are compared with a simple synthetic modeling to find the possible property and location of the anomaly the synthetic result was qualitatively matched with real data we believe that more quantitative matches can obtained when the method is combined with full waveform inversions time lapse refraction analysis has some clear limitations for instance the presence of refracted events which requires a positive velocity contrast for the interface to be studied another limitation is that for deeper events the signal from shallower refractions and noise associated with the water layer will distort the 4d time shifts however we found that these results demonstrate the potential of refraction time shifts as a complementary 4d analysis technique we believe that the method has a significant potential for quantitative 4d interpretation

site response models for charleston south carolina and vicinity developed from shallow geotechnical investigations the study models the response of near surface materials in charleston south carolina and the adjacent area geotechnical investigations at 281 locations were made available by local engineering firms the data used for dynamic site response analysis were derived from shear wave velocity measurements at 52 loca tions site response was quantified as the ratio of surface motion to hypothetical hard rock basement outcrop motion scenario earthquake motions were developed with the stochastic model acceleration response ratios for 5 critical oscillator damping were computed for 12 frequencies ranging from 0 1 to 30 hz and for peak ground acceleration two features determine the general nature of site response in the study area the impedance contrast between mesozoic basement and cretaceous sediments and the shallow impedance contrast between quaternary and tertiary sediments average s wave velocities in the quaternary are relatively uniform and range from 150 to 250 m sec they are not strongly correlated with surface geology the velocities of the immediately underlying tertiary sediments range from 300 to 500 m sec because of the uniformity of velocity in the quaternary depth to the quaternary tertiary contact appears to be the most important variable leading to differences in calculated site response this surface is irregular and varies in depth from near surface at inland sites to approximately 30 m at sites near the coast as a consequence estimated site response in the frequency band 1 10 hz varies by as much as a factor of 3 site response at frequencies less than 1 hz is dominated by the first few resonant har monics of the entire sedimentary section with fundamental frequency near 0 2 hz

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

sensitivity analysis and application of time lapse full waveform inversion synthetic testing and field data example from the north sea norway time lapse refraction can provide complementary seismic solutions for monitoring subtle subsurface changes that are challenging for conventional p wave reflection methods the utilization of refraction time lapse has lagged behind in the past partly due to the lack of robust techniques that allow extracting easy to interpret reservoir information however with the recent emergence of the full waveform inversion technique as a more standard tool we find it to be a promising platform for incorporating head waves and diving waves into the time lapse framework here we investigate the sensitivity of 2d acoustic time domain full waveform inversion for monitoring a shallow weak velocity change 30 m s or 1 6 the sensitivity tests are designed to address questions related to the feasibility and accuracy of full waveform inversion results for monitoring the field case of an underground gas blowout that occurred in the north sea the blowout caused the gas to migrate both vertically and horizontally into several shallow sand layers some of the shallow gas anomalies were not clearly detected by conventional 4d reflection methods i e time shifts and amplitude difference due to low 4d signal to noise ratio and weak velocity change on the other hand full waveform inversion sensitivity analysis showed that it is possible to detect the weak velocity change with the non optimal seismic input detectability was qualitative with variable degrees of accuracy depending on different inversion parameters we inverted the real 2d seismic data from the north sea with a greater emphasis on refracted and diving waves energy i e most of the reflected energy was removed for the shallow zone of interest after removing traces with offset less than 300 m the full waveform inversion results provided more superior detectability compared with the conventional 4d stacked reflection difference method for a weak shallow gas anomaly 320 m deep

comparisons of ground motions from colocated and closely spaced one sample per second global positioning system and accelerograph recordings of the 2003 m 6 5 san simeon california earthquake in the parkfield region the 2003 san simeon california earthquake m 6 5 generated a set of colocated and closely spaced high rate 1 sample per second global positioning system gps positions and ground motions from digital accelerographs in the park field region at epicentral distances of 50 to 70 km the waveforms of displacements derived from the 13 gps receivers in the region have dominant periods between about 7 and 18 sec the waveforms are similar in shape with a systematic change in waveform as a function of distance from the source the gps motions are smaller than the accelerograph motions for periods less than about 2 sec from this we conclude that the 1 sample per sec gps receivers provide a good representation of ground motion at periods longer than about 2 sec perhaps more important for earth quake engineering is that the accelerograph data are similar to the gps data for periods as long as 30 sec if not longer this means that data from digital accelero graphs can provide reliable relative displacement response spectra at the periods needed in the design of large structures at least for earthquakes with magnitudes of 6 5 or above at distances within 70 km we combine the colocated or very closely spaced gps and accelerograph data sets in the frequency domain to obtain a single broadband time series of the ground motion at each accelerograph station these broadband ground motions may be useful to seismologists in unraveling the dynamic process of fault rupture and to engineers for designing large structures with very long period response

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

intra event spatial correlations for cumulative absolute velocity arias intensity and spectral accelerations based on regional site conditions abstract spatial correlations of ground motion intensity measures ims are essential for seismic analysis of spatially distributed systems in this paper geostatistical analysis is conducted to calculate the spatial correlations for cumulative absolute velocity cav arias intensity ia and spectral accelerations sa using a total number of more than 1500 earthquake records from nine recent earthquakes occurred in taiwan california and japan the results indicate that the spatial correlations for these ims are closely related to the regional site conditions and they can be predicted based on the spatial correlations of shear wave velocity in the top 30 m v s 30 in general an im recorded from a relatively homogeneous regional site condition tends to have a larger spatial correlation range than that from a heterogeneous site condition due to their intrinsic similarity to represent the integration of acceleration time histories cav and ia have similar spatial correlation coefficients besides the range of spatial correlation of sa generally increases as the spectral period increases simple predictive equations are proposed in this study to quantify the spatial correlations of cav ia and sa based on regional site conditions methods for data correction are also proposed to eliminate artificial correlations due to biased distance scaling and v s 30 estimation in the database finally monte carlo method is used to generate spatially distributed ims the results demonstrate that the annual frequency of exceedance curves for spatially distributed ims differ significantly if different ranges of spatial correlations are used

nonlinear soil response in the vicinity of the van normal complex following the 1994 northridge california earthquake ground motion recordings obtained at the van norman complex from the 1994 northridge california mainshock and its aftershocks constitute an excellent data set for the analysis of soil response as a function of ground motion amplitude we searched for nonlinear response by comparing the fourier spectral ratios of two pairs of sites for ground motions of different levels using data from permanent strong motion recorders and from specially deployed portable instruments we also compared the amplitude dependence of the observed ratios with the amplitude de pendence of the theoretical ratios obtained from 1 d linear and 1 d equivalent linear transfer functions using recently published borehole velocity profiles at the sites to provide the low strain material properties one pair of sites was at the jensen filtra tion plant jfp the other pair was the rinaldi receiving station r n and the los angeles dam lad most of the analysis was concentrated on the motions at the jensen sites portable seismometers were installed at the jfp to see if the motions inside the structures housing the strong motion recorders differed from nearby flee field motions we recorded seven small earthquakes and found that the high fre quency low amplitude motions in the administration building were about 0 3 of those outside the building this means that the lack of high frequencies on the strong motion recordings in the administration building relative to the generator building is not due solely to nonlinear soil effects after taking into account the effects of the buildings however analysis of the suite of strong and weak motion recordings indicates that nonlinearity occurred at the jfp as predicted by equivalent linear analysis the largest events the mainshock and the 20 march 1994 aftershock show a significant deamplification of the high frequency motion relative to the weak mo tions from aftershocks occurring many months after the mainshock the weak mo tion aftershocks recorded within 12 hours of the mainshock however show a relative deamplification similar to that in the mainshock the soil behavior may be a con sequence of a pore pressure buildup during large amplitude motion that was not dissipated until sometime later the motions at r n and lad are from free field sites the comparison among spectral ratios of the mainshock weak motion coda waves of the mainshock and an aftershock within ten minutes of the mainshock indicate that some nonlinearity occurred presumably at rin because it is the softer site the spectral ratio for the mainshock is between that calculated for pure linear response and that calculated from the equivalent linear method using commonly used modulus reduction and damping ratio curves in contrast to the jensen sites the ratio of motions soon after the high amplitude portion of the mainshock differs from the ratio of the mainshock motions indicating the mechanical properties of the soil returned to the low strain values as the high amplitude motion ended this may indicate a type of nonlinear soil response different from that affecting motion at the jensen administration building

variability in ground motions root mean square acceleration and peak acceleration for the 1971 san fernando california earthquake data from the 1971 san fernando california earthquake provided the opportunity to study the variation of ground motions on a local scale the uncertainty in ground motion was analyzed by studying the residuals about a regression with distance and by utilizing the network of strong motion instruments in three local geographic regions in the los angeles area our objectives were to compare the uncertainty in the peak ground acceleration pga and root mean square acceleration rmsa about regressions on distance and to isolate components of the variance we find that the rmsa has only a slightly lower logarithmic standard deviation than the pga and conclude that the rmsa does not provide a more stable measure of ground motion than does the pga as is commonly assumed by conducting an analysis of the residuals we have estimated contributions to the scatter in high frequency ground motion due to phenomena local to the recording station building effects defined by the depth of instrument embedment and propagation path effects we observe a systematic decrease in both pga and rmsa with increasing embedment depth after removing this effect we still find a significant variation a standard deviation equivalent to a factor of up to 1 3 in the ground motions within small regions circles of 0 5 km radius we conclude that detailed studies which account for local site effects including building effects could reduce the uncertainty in ground motion predictions as much as a factor of 1 3 attributable to these components however an irreducible component of the scatter in attenuation remains due to the randomness of stress release along faults during earthquakes in a recent paper joyner and boore 1981 estimate that the standard deviation associated with intra earthquake variability corresponds to a factor of 1 35

on the coherence of ground motion in the san fernando valley we present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the san fernando valley california using aftershocks of the 17 january 1994 m w6 7 earthquake recorded at a quasi dense array of portable stations we analyze the coherence of three well recorded magnitude 3 7 to 4 0 events over the frequency range 0 5 to 15 hz and a distance range of 0 5 to 5 3 km all stations are located at sites with broadly similar near site geology characterized by medium to fine grain quaternary alluvial sediments on average relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 hz coherence drops sharply at frequencies near and above 3 hz although average coherence functions are described reasonably well by a log linear relationship with frequency the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites the distance decay of coherence is also markedly less strong with high coherence values observed over station separations corresponding to multiple wavelengths this may reflect fundamental differences in shallow wave propagation in the two environments with high frequency scattering relatively more dominant in regions of hard rock near surface geology within a sedimentary basin or valley the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions however previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater los angeles region our results undoubtedly do not quantify the full range of ground motion variability at all sites but rather represent the level of that variability that can be expected and quantified for typical source receiver paths

time lapse full waveform inversion with ocean bottom cable data application on valhall field abstractknowledge of changes in reservoir properties resulting from extracting hydrocarbons or injecting fluid is critical to future production planning full waveform inversion fwi of time lapse seismic data provides a quantitative approach to characterize the changes by taking the difference of the inverted baseline and monitor models the baseline and monitor data sets can be inverted either independently or jointly time lapse seismic data collected by ocean bottom cables obcs in the valhall field in the north sea are suitable for such time lapse fwi practice because the acquisitions are of a long offset and the surveys are well repeated we have applied independent and joint fwi schemes to two time lapse valhall obc data sets which were acquired 28 months apart the joint fwi scheme is double difference waveform inversion ddwi which inverts differenced data the monitor survey subtracted by the baseline survey for model changes we have found that ddwi gave a cleaner and more easily interpr

incidents of ground failure from the 1994 northridge earthquake abstract numerous incidents of ground failure from the 1994 northridge earthquake caused thousands of pipe breaks and damaged many structures in this article significant occurrences of ground failure in the san fernando valley simi valley santa clara river area central los angeles and a number of coastal areas are described and the likely culprit mechanisms are identified ground failure in many areas could be readily associated with liquefaction and or seismically induced compaction of young alluvial soils or fill materials previously known to have a high ground failure susceptibility however ground failure also occurred in areas not previously thought to be susceptible to such hazards taken together these incidents provide insight into a variety of ground failure phenomena that can affect urban areas during earthquakes