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

slow and go pulsing slip rates on the creeping section of the san andreas fault rising and falling slip rates on the creeping section of the san andreas fault have been inferred from variations of recurrence intervals of characteristically repeating microearthquakes but this observation has not previously been confirmed using modern geodetic data here we report on observations of this pulsing slip obtained from advanced multitemporal interferometric synthetic aperture radar insar data confirmed using continuous gps sites of the plate boundary observatory the surface deformation time series show a strong correlation to the previously documented slip rate variations derived from repeating earthquakes on the fault interface at various spatial and temporal scales time series and spectral analyses of repeating earthquake and insar data reveal a quasiperiodic pulsing with a roughly 2 year period along some sections of the fault with the earthquakes on the fault interface lagging behind the far field deformation by about 6 months this suggests a temporal delay between the pulsing crustal strain generated by deep seated shear and the time variable slip on the shallow fault interface and that at least in some places this process may be cyclical there exist potential impacts for time dependent seismic hazard forecasting in california and as it becomes better validated in the richly instrumented natural laboratory of the central san andreas fault the process used here will be even more helpful in characterizing hazard and fault zone rheology in areas without california s geodetic infrastructure

temporal changes in microseismicity and creep near parkfield california the 25 km long section of the san andreas fault near parkfield california ruptured in similar magnitude 6 earthquakes in 1881 1901 1922 1934 and 1966 on the basis of a number of lines of seismological evidence a section of the san andreas fault now termed the parkfield preparation zone has been identified as the locus of the next parkfield earthquake1 3 here we describe coincident changes in surface creep rates and deep seismicity near the parkfield preparation zone following the 2 may 1983 coalinga earthquake and suggest that both respond to the same stimuli these changes were concentrated near the point of initiation of the magnitude 6 characteristic parkfield earthquakes lending credence to the hypothesis that this section of the san andreas fault is characterized by a unique set of physical properties4 6 which make it ideal for earthquake prediction studies and which may also be useful for identifying hypocentral regions in other areas

seismic slip segmentation and the loma prieta earthquake we have plotted the cumulative seismic slip projected onto a vertical plane for earthquakes occurring during the last 20 years along 210 km of the san andreas fault that includes the section that moved in the loma prieta earthquake these plots illustrate the differences in depth and character of the seismicity between the locked and creeping portions of the fault or fault zone and define the segment upon which the loma prieta earthquake occurred working by analogy from the relation between pre main shock microseismicity and presumed main shock slip regions at parkfield and loma prieta we identify a segment on the san francisco peninsula where we believe the m 7 1838 earthquake occurred and which we believe may have accumulated sufficient strain that rupture should be expected in the coming decades

measurements of the strain field associated with episodic creep events on the san andreas fault at san juan bautista california over the past 6 years repeated strain steps recorded on a tensor strainmeter installed close to the san andreas fault trace at san juan bautista in california correlated closely with episodic creep events registered on a nearby creepmeter the strain events were remarkably similar in character were of about 1 hour duration and were followed within hours to days by creep events of 2 6 days duration the episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data we postulate that the additional stress transferred to this region by the loma prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region thus accounting for the increased frequency of the creep strain events following the loma prieta earthquake

geologic versus geodetic deformation adjacent to the san andreas fault central california we combine geologic and global positioning system gps data to characterize the style and magnitude of off fault deformation across the san andreas fault system in central california geologic structures record 12 km of both fault parallel and fault perpendicular displacements across creeping and locked portions of the san andreas fault analysis of 150 gps site velocities suggests that the borderlands record 4 6 mm yr of fault parallel and 3 5 mm yr of fault perpendicular motion alongside the creeping segment where elastic strain is minimized the distribution of both long term geologic and short term geodetic deformation is affected by basement type where more deformation is concentrated northeast of the san andreas fault on franciscan basement we suggest that at least half the fault parallel gps deformation measured by gps bordering the creeping segment must be accommodated by geologic structures this permanent deformation needs to be incorporated into dynamic models of the fault system elastic modeling of the san andreas fault in central california which incorporates its well known transition from locked to creeping behavior near parkfield predicts first order variations in the gps velocity field along the fault and corresponding variations in dilatational strain rates the strain rate pattern is dominated by a large contractional region northeast of the transition from locked to creeping behavior and a large extensional region southwest of the transition the former coincides with the coalinga and kettleman hills anticlines the growth and development of which seem to have occurred under at least two kinematic conditions we suggest that the onset of fault creep in central california promoted the growth of these folds by implication fault creep has been active over geologic time scales

retardations in fault creep rates before local moderate earthquakes along the san andreas fault system central california records of shallow aseismic slip fault creep obtained along parts of the san andreas and calaveras faults in central california demonstrate that significant changes in creep rates often have been associated with local moderate earthquakes an immediate postearthquake increase followed by gradual long term decay back to a previous background rate is generally the most obvious earthquake effect on fault creep this phenomenon identified as aseismic afterslip usually is characterized by above average creep rates for several months to a few years in several cases minor step like movements called coseismic slip events have occurred at or near the times of mainshocks one extreme case of coseismic slip recorded at cienega winery on the san andreas fault 17 5 km southeast of san juan bautista consisted of 11 mm of sudden displacement coincident with earthquakes ofm l 5 3 andm l 5 2 that occurred 2 5 minutes apart on 9 april 1961 at least one of these shocks originated on the main fault beneath the winery creep activity subsequently stopped at the winery for 19 months then gradually returned to a nearly steady rate slightly below the previous long term average

ground motion in the presence of complex topography ii earthquake sources and 3d simulations abstract eight seismic stations were placed in a linear array with a topographic relief of 222 m over mission peak in the east san francisco bay region for a period of one year to study topographic effects seventy two well recorded local earthquakes are used to calculate spectral amplitude ratios relative to a reference site a well defined fundamental resonance peak is observed with individual station amplitudes following the theoretically predicted progression of larger amplitudes in the upslope direction favored directions of vibration are also seen that are related to the trapping of shear waves within the primary ridge dimensions spectral peaks above the fundamental one are also related to topographic effects but follow a more complex pattern theoretical predictions using a 3d velocity model and accurate topography reproduce many of the general frequency and time domain features of the data shifts in spectral frequencies and amplitude differences however are related to deficiencies of the model and point out the importance of contributing factors including the shear wave velocity under the topographic feature near surface velocity gradients and source parameters electronic supplement figures of recorded and simulated acceleration response spectra and earthquake ground motion directional response

intermediate term pre earthquake phenomena in california 1975 1986 and preliminary forecast of seismicity for the next decade intermediate term observations preceding earthquakes of magnitude 5 7 or greater in california from 1975 through 1986 suggest that 1 the sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years 2 these larger earthquakes tend to occur towards the ends of creeping fault segments 3 one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area and 4 marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area a common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep and suggests that surface fault creep is a sensitive indicator of changes in stress the preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade

long term creep rate changes and their causes 1 we discuss measurements of the long term creep rate from the san juan bautista section of the san andreas fault these observations of aseismic slip span a period of approximately half a century making it the longest creep record ever measured along an active fault zone the records show systematic changes in creep rate over time scales of tens of years and we address potential causes of these changes in principle they may be caused either by changes in resistive forces within the fault zone or by variations in plate tectonic driving forces such driving force variations have recently been suggested to have occurred in the same area western north america and within the same time window these should give rise to observable effects in the creep data but we find no evidence for temporal changes in plate tectonic forcing at these time scales a natural explanation for the observed long term changes is elastic reloading of adjacent locked portions of the san andreas fault after large earthquakes and possibly aseismic strain release that occurs before the earthquakes

modeling 3d surface topography by finite difference method kobe jma station site japan case study we propose a simple approach for treating non planar free surface boundary conditions that can be easily implemented into an existing finite difference code the approach is based on the three dimensional staggered grid finite difference method fdm hereafter the proposed scheme gives very good results when compared with other numerical methods its accuracy depends on how well the topography is matched by the 3d fd grid based on the proposed technique we made a qualitative evaluation of the local topography effect on ground motion at kob jma site kobe japan the local topography at jma station amplifies the horizontal ground motion in the frequency range 1 3 hz by a factor of less than two and deamplifies it at frequencies around 5 hz by a factor of five

variability of fault slip behavior along the san andreas fault in the san juan bautista region an improved understanding of the time history of fault slip at depth is an essential step toward understanding the underlying mechanics of the faulting process using a waveform cross correlation approach we document spatially and temporally varying fault slip along the northernmost creeping section of the san andreas fault near san juan bautista sjb california by systematically examining spatiotemporal behaviors of characteristically repeating earthquakes cres the spatial distribution of pre 1998 sjb earthquake 1984 1998 fault slip rate inferred from the cres reveals a 15 km long low creep or partially locked section located near the 1998 mw 5 1 sjb earthquake rupture a finite fault slip inversion reveals that the rupture of the 1998 sjb earthquake is characterized by the failure of a compact 4 km2 asperity with a maximum slip of about 90 cm and corresponding peak stress drop of up to 50 mpa whereas the mean stress drop is about 15 mpa following the 1998 earthquake the cre activity was significantly increased in a 5 10 km deep zone extending 2 7 km northwest of the main shock which indicates triggering of substantial aseismic slip the postseismic slip inferred from the cre activity primarily propagated to the northwest and released a maximum slip of 9 cm in this 5 10 km depth range the estimated postseismic moment release is 8 6 1016 n m which is equivalent to mw 5 22 the aseismic slip distribution following the 1998 earthquake is not consistent with coseismic stress driven afterslip but represents a triggered long lasting slow earthquake

fault interaction near hollister california a numerical model is used to study fault stress slip near hollister california the geometrically complex system of interacting faults including the san andreas calaveras sargent and busch faults is approximated with a two dimensional distribution of short planar fault segments in an elastic medium the steady stress and slip rate are simulated by specifying frictional strength and stepping the remote stress ahead in time the resulting computed fault stress is roughly proportional to the observed spatial density of small earthquakes suggesting that the distinction between segments characterized by earthquakes and those with aseismic creep results in part from geometry a nonsteady simulation is made by introducing in addition stress drops for individual moderate earthquakes a close fit of observed creep with calculated slip on the calaveras and san andreas faults suggests that many changes in creep rate averaged over several months are caused by local moderate earthquakes in particular a 3 year creep lag preceding the august 6 1979 coyote lake earthquake on the calaveras fault seems to have been a direct result of the november 28 1974 thanksgiving day earthquake on the busch fault computed lags in slip rate preceding some other moderate earthquakes in the area are also duemore to earlier earthquakes although the response of the upper 1 km of the fault zone may cause some individual creep events and introduce delays in others the long term rate appears to reflect deep slip less

ground motion observations at hotel montana during the m 7 0 2010 haiti earthquake topography or soil amplification unusually severe structural damage was reported during the 2010 m 7 0 haiti earthquake in the vicinity of hotel montana located on top of a ridge in the district of petionville prompted by the observations u s geological survey seismic stations were deployed and aftershock recordings indicated ground motion amplification on the top of the hill compared to adjacent stations on reference site conditions the presence of topographic relief has been shown to significantly aggravate the consequences of strong ground motion during past events and topographic effects were brought forward to explain the observations in this paper we test the hypothesis of topographic amplification as the dominant factor that contributed to the damage concentration in the vicinity of hotel montana we initially conduct numerical simulations of the ridge seismic response assuming elastic homogeneous site conditions and show that numerical predictions of topographic amplification disagree with the field data both in amplitude and in frequency conversely while 1d ground response analyses for the site conditions at the hilltop predict amplification in the same frequency range as the field data they significantly underestimate the recorded amplitude we then conduct numerical simulations of the foothill ridge response to seismic motion while accounting for soil layering and qualitatively demonstrate that the recorded amplification is most likely attributed to coupled site topographic amplification effects namely to seismic waves trapped in the soft soil layers of the near surface amplified as a consequence of reverberations and further modified due to diffraction and scattering upon incidence on the irregular ground surface parametric investigations of the topography soil amplification coupling effects are then conducted and our results show that when accounting for a hypothetical soil bedrock interface at 100 m depth predictions are in excellent agreement with the observed motion

topographic site effects and hvsr a comparison between observations and theory we present a comparison between observed and predicted amplification on a topographic feature our data come from an ad hoc experiment carried out during the summer of 1989 in epire northern greece ten digital autonomous three component stations recorded a total of 68 small earthquakes as these data were inadequate to compute empirical transfer functions using the standard spectral ratio ssr technique we explored the possible use of horizontal to vertical spectral ratios hvsr first we compare hvsr computed from theoretical transfer functions for incident p sv and rayleigh waves with the horizontal transfer function for each incidence type second we compute hvsr for our weak motion data set and show that the results are stable with a scatter not larger than that usually accepted in ssr studies we observe significant differences between n s and e w motion when we compare hvsr with theoretical transfer functions we obtain a better agreement for sv motion than for sh motion amplification levels are similar between hvsr and predicted amplification even for the sh case this suggests that discrepancies that have been signaled for the topographic amplification between observations and predictions could be due to site effects being present in the reference station used to computed ssr our results support the use of hvsr to determine topographic site effects thus hvsr may have a large potential to evaluate site effects in a wider variety of geological conditions than previously thought without the shortcomings of ssr finally we show that hvsr computed from noise recordings gives a reasonable estimate of hvsr computed from weak motion data although this conclusion should be better substantiated our preliminary results are encouraging

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

effects of topography on seismic wave propagation an example from northern taiwan topography influences ground motion and in general increases the amplitude of shaking at mountain tops and ridges whereas valleys have reduced ground motions as is observed from data recorded during and after real earthquakes and from numerical simulations however recent publications have focused mainly on the implications for ground motion in the mountainous regions themselves whereas the impact on surrounding low lying areas has received less attention here we develop a new spectral element mesh implementation to accommodate realistic topography as well as the complex shape of the taipei sedimentary basin which is located close to the central mountain range in northern taiwan spectral element numerical simulations indicate that high resolution topography can change peak ground velocity pgv values in mountainous areas by 50 compared to a half space response we further demonstrate that large scale topography can affect the propagation of seismic waves in nearby areas for example if a shallow earthquake occurs in the i lan region of taiwan the central mountain range will significantly scatter the surface waves and will in turn reduce the amplitude of ground motion in the taipei basin however as the hypocenter moves deeper topography scatters body waves which subsequently propagate as surface waves into the basin these waves continue to interact with the basin and the surrounding mountains finally resulting in complex amplification patterns in taipei city with an overall pgv increase of more than 50 for realistic subduction zone earthquake scenarios off the northeast coast of taiwan the effects of topography on ground motion in both the mountains and the taipei basin vary and depend on the rupture process the complex interactions that can occur between mountains and surrounding areas especially sedimentary basins illustrate the fact that topography should be taken into account when assessing seismic hazard

interrelation between fault zone structures and earthquake processes in order to develop capabilities for predicting earthquake processes on the basis of known fault zone structures and stress conditions we need to find relations between seismogenic structures and processes in the present paper we search for the scale dependence in various earthquake phenomena with the hope to find some structures in the earth that may control the earthquake processes among these phenomena we shall focus on 1 geologic structures which play some role in nucleation and stopping of earthquake fault rupture 2 depth ranges of the brittle seismogenic zone 3 asperities and barriers distributed over a fault plane 4 source controlled f max effect 5 nonfractal behavior of creep events and 6 temporal correlation between coda q 1 and seismicity of earthquakes with magnitude characteristic to a given area our review of various scale dependent phenomena leads us to propose a working hypothesis that the temporal change in coda q 1 may reflect the activity of creep fractures near the brittle ductile transition zone

the shakeout earthquake scenario verification of three simulation sets summary this paper presents a verification of three simulations of the shakeout scenario an mw 7 8 earthquake on a portion of the san andreas fault in southern california conducted by three different groups at the southern california earthquake center using the scec community velocity model for this region we conducted two simulations using the finite difference method and one by the finite element method and performed qualitative and quantitative comparisons between the corresponding results the results are in good agreement with each other only small differences occur both in amplitude and phase between the various synthetics at ten observation points located near and away from the fault as far as 150 km away from the fault using an available goodness of fit criterion all the comparisons scored above 8 with most above 9 2 this score would be regarded as excellent if the measurements were between recorded and synthetic seismograms we also report results of comparisons based on time frequency misfit criteria results from these two criteria can be used for calibrating the two methods for comparing seismograms in those cases in which noticeable discrepancies occurred between the seismograms generated by the three groups we found that they were the product of inherent characteristics of the various numerical methods used and their implementations in particular we found that the major source of discrepancy lies in the difference between mesh and grid representations of the same material model overall however even the largest differences in the synthetic seismograms are small thus given the complexity of the simulations used in this verification it appears that the three schemes are consistent reliable and sufficiently accurate and robust for use in future large scale simulations

structural explanation for low creep rates on the san andreas fault near monarch peak central california comparison of fault slip throughout most of the central creeping section of the san andreas fault zone indicates that near fault about 100 m aperture and intermediate scale about 1 to 2 km aperture geodetic measurements of right lateral slip are in good agreement an exception is present near monarch peak near the center of the creeping section here near fault measuring techniques yield no more than two thirds of the slip rate determined by intermediate scale measurements anomalously low near fault creep at this site has continued during 10 yr of measurement detailed geologic mapping shows the presence of geologically youthful subsidiary faults along an area about 7 km long and parallel with the main trace centered on monarch peak we infer that the subsidiary faults are active taking up the difference in horizontal offset if the subsidiary faults are in fact active it makes the structural setting near monarch peak anomalous in the creeping section of the fault zone

effects of realistic surface topography on seismic ground motion in the yangminshan region of taiwan based upon the spectral element method and lidar dtm we combine light detection and ranging lidar digital terrain model dtm data and an improved mesh implementation to investigate the effects of high resolution surface topography on seismic ground motion based upon the spectral element method in general topography increases the amplitude of shaking at mountain tops and ridges whereas valleys usually have reduced ground motion as has been observed in both records from past earthquakes and numerical simulations however the effects of realistic topography on ground motion have not often been clearly characterized in numerical simulations especially the seismic response of the true ground surface here we use lidar dtm data which provide two meter reso lution at the free surface and a spectral element method to simulate three dimensional 3d seismic wave propagation in the yangminshan region in taiwan incorporating the effects of realistic topography a smoothed topographic map is employed beneath the model surface in order to decrease mesh distortions due to steep ground surfaces numerical simulations show that seismic shaking in mountainous areas is strongly affected by topography and source frequency content the amplification of ground motion mainly occurs at the tops of hills and ridges whilst the valleys and flat topped hills experience lower levels of ground shaking interaction between small scale to pographic features and high frequency surface waves can produce unusually strong shaking we demonstrate that topographic variations can change peak ground accel eration pga values by 50 in mountainous areas and the relative change in pga between a valley and a ridge can be as high as a factor of 2 compared to a flat surface response this suggests that high resolution realistic topographic features should be taken into account in seismic hazard analysis especially for densely populated moun tainous areas

ground motion amplitude across ridges abstract local amplification and wave diffraction on an elongated ridge near sourpi in central greece were studied by the analysis of seismic records of local and regional earthquakes data were obtained during field work especially designed for this purpose these data were analyzed in the frequency and time domains in the frequency domain spectral ratios show amplifications of 1 5 to 3 at the ridge top relative to the base of the ridge the horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations theoretical spectral ratios calculated by the indirect boundary element method are dependent on earthquake location but are in general agreement with the observed spectral ratios another dataset from mont st eynard in the french alps showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank these relative amplifications are within the range predicted by numerical simulations the numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge the use of a seven station array on the ridge at sourpi made it possible to identify such waves the analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography induced amplification on ridge tops our results also show that such amplification is moderate for the ridges under study

an experimental approach for estimating seismic amplification effects at the top of a ridge and the implication for ground motion predictions the case of narni central italy abstract from march to september 2009 a velocimetric network was installed in narni central italy a village on the top of a limestone ridge the aim was to investigate local site effects due to the 220 m high ridge which is characterized by slopes ranging from 22 to 35 to investigate amplification without and with a reference site three stations were installed at the base of the hill and seven at the crest the network recorded 702 earthquakes many of them from the 2009 l aquila sequence to determine the dependence of amplification on the morphological features the spectra were computed for horizontal components rotated into a range of azimuths both the ratio of the horizontal to vertical component spectra and the ratio of the spectra at the ridge crest with respect to a reference station at the base of the ridge showed amplification by a factor of circa 4 5 for frequencies between 4 hz and 5 hz the highest amplifications were seen for the directions of the ground motion perpendicular to the main elongation of the ridge finally considering events with an epicentral distance less than 30 km empirical ground motion models were calibrated for maximum horizontal peak ground acceleration pga velocity and acceleration response spectra 5 damping up to 1 s to estimate the site corrective coefficients for topographic amplification the data show corrective coefficients between 0 35 and 0 48 log 10 scale amplification 2 2 3 0 for the spectral ordinates between 0 2 s and 0 3 s

spatial and temporal correlation between coda q 1 and seismicity and its physical mechanism from the analysis of local earthquake coda waves recorded by the benioff short period vertical seismograph at riverside california we found a surprisingly large and systematic temporal variation in coda q 1 in the southern california during the 55 year period from 1933 to 1987 comparison of the coda q 1 variation with the b value obtained for earthquakes with m 3 occurred in the area within 180 km from riverside corresponding to the area for which q 1 is estimated by the coda method led to an extraordinary result that coda q 1 correlates positively with b value with the correlation coefficient of 0 79 the significance of correlation cannot be rejected at the confidence level of 0 974 since the data used for measuring q 1 namely the coda part of seismograms and the data used for measuring b namely the seismicity catalog are totally independent the observed correlation supports strongly the physical reality of temporal change in both coda q 1 and b value the correlation between coda q 1 and b value has been reported for several other cases but the correlation was positive in some cases and negative in others our current preferred model for explaining the observed correlation is the following aseismic creep is going on from time to time over various sizes of cracks in the ductile part and in some cases in the brittle part of lithosphere at and near the plate boundary creep activities tend to increase the crack density and coda q 1 observed in the region if the creek occurs over distributed cracks with a certain predominant scale length spatial stress concentration may enhance seismicity for earthquakes with a similar size if this characteristic size is in the lower part of the magnitude range for which the b value is estimated the enhanced seismicity will increase b on the other hand if it is in the upper part of the magnitude range it will decrease b thus we can explain the observed strong correlation between coda q 1 and b value for both positive and negative cases the existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling

frequency scaled curvature as a proxy for topographic site effect amplification and ground motion variability we introduce a new methodology to predict the topographic site effect amplification ground motions obtained from a large database of 3d earthquake simulations show that the curvature of the earth s surface defined as the second spatial derivative of the elevation map is correlated with the topographic site amplification the highest correlation between the frequency dependent topographic amplification and the topographic curvature is reached when the curvature is smoothed over a characteristic length equal to the s wavelength divided by two i e frequency scaled curvature fsc this implies the amplification is caused by topographic features for which horizontal dimensions are similar to half of the s wavelength the largest ground motion variabilities are found at sites located on slopes and on the largest summits whereas intermediate variabilities occur over narrow ridges and a stable behavior in the bottom valleys the fsc proxy allows the identification of topographic features with similar characteristic dimensions and probabilistic estimates of amplification values accounting on the variability of ground motions due to source site interactions amplification estimates using the fsc proxy are robust and easily computed from digital elevation maps provided that reasonable values of s wave velocities are available in the area of interest

long term fault creep observations in central california the u s geological survey usgs has been monitoring aseismic fault slip in central california for more than 10 years as part of an earthquake prediction experiment since 1968 the usgs creep network has grown from one creep meter at the cienega winery south of hollister to a 44 station network that stretches from hayward east of san francisco bay to palmdale in southern california in general the long term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period e g calaveras fault near hollister and san andreas fault between san juan bautista and bear valley these fault sections are the most difficult to characterize with a single long term slip rate in contrast sections of the faults that are seismically relatively quiet e g san andreas fault between bear valley and parkfield produce the steadiest creep records and are easiest to fit with a single long term slip rate appendix is available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j82 004 1 00 payment must accompany order

localized damage caused by topographic amplification during the 2010 m 7 0 haiti earthquake microzonation maps use local geological conditions to characterize seismic hazard but do not generally consider topography ground motions during the haiti earthquake are found to have been significantly amplified along a high topographic ridge which caused substantial structural damage indicating that topography can play an important role in seismic hazard local geological conditions including both near surface sedimentary layers1 2 3 4 and topographic features5 6 7 8 9 are known to significantly influence ground motions caused by earthquakes microzonation maps use local geological conditions to characterize seismic hazard but commonly incorporate the effect of only sedimentary layers10 11 12 microzonation does not take into account local topography because significant topographic amplification is assumed to be rare here we show that although the extent of structural damage in the 2010 haiti earthquake was primarily due to poor construction topographic amplification contributed significantly to damage in the district of petionville south of central port au prince a large number of substantial relatively well built structures situated along a foothill ridge in this district sustained serious damage or collapse using recordings of aftershocks we calculate the ground motion response at two seismic stations along the topographic ridge and at two stations in the adjacent valley ground motions on the ridge are amplified relative to both sites in the valley and a hard rock reference site and thus cannot be explained by sediment induced amplification instead the amplitude and predominant frequencies of ground motion indicate the amplification of seismic waves by a narrow steep ridge we suggest that microzonation maps can potentially be significantly improved by incorporation of topographic effects

the response of creeping parts of the san andreas fault to earthquakes on nearby faults two examples rates of shallow slip on creeping sections of the san andreas fault have been perturbed on a number of occasions by earthquakes occurring on nearby faults one example of such perturbations occurred during the 26 january 1986 magnitude 5 3 tres pinos earthquake located about 10 km southeast of hollister california seven creepmeters on the san andreas fault showed creep steps either during or soon after the shock both left lateral ll and right lateral rl steps were observed a rectangular dislocation in an elastic half space was used to model the coseismic fault offset at the hypocenter for a model based on the preliminary focal mechanism the predicted changes in static shear stress on the plane of the san andreas fault agreed in sense ll or rl with the observed slip directions at all seven meters for a model based on a refined focal mechanism six of the seven meters showed the correct sense of motion two possible explanations for such coseismic and postseismic steps are 1 that slip was triggered by the earthquake shaking or 2 that slip occurred in response to the changes in static stress fields accompanying the earthquake in the tres pinos example the observed steps may have been of both the triggered and responsive kinds a second example is provided by the 2 may 1983 magnitude 6 7 coalinga earthquake which profoundly altered slip rates at five creepmeters on the san andreas fault for a period of months to years the xmm1 meter 9 km northwest of parkfield california recorded ll creep for more than a year after the event to simulate the temporal behavior of the xmm1 meter and to view the stress perturbation provided by the coalinga earthquake in the context of steady state deformation on the san andreas fault a simple time evolving dislocation model was constructed the model was driven by a single long vertical dislocation below 15 km in depth that was forced to slip at 35 mm yr in a rl sense a dislocation element placed in the seismogenic layer under xmm1 was given a finite breaking strength of sufficient magnitude to produce a parkfield like earthquake every 22 years when stress changes equivalent to a coalinga earthquake were superposed on the model running in a steady state mode the effect was to make a segment under xmm1 that could slip in a linear viscous fashion creep ll and to delay the onset of the next parkfield like earthquake by a year or more if static stress changes imposed by earthquakes off the san andreas can indeed advance or delay earthquakes on the san andreas by months or years then such changes must be considered in intermediate term prediction efforts

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

a shear strain anomaly following the loma prieta earthquake borehole tensor strain instruments deployed along the san andreas fault for the past ten years have provided sufficient resolution and stability to sample regional tectonic processes potentially enhancing earthquake prediction capability data obtained from the instrument at san juan bautista in the near field region of the 17 october 1989 loma prieta earthquake ms 7 1 provide the first opportunity to observe shear strain processes associated with a large earthquake we previously reported1 a prominent shear strain anomaly in those data for more than a year before the earthquake this anomaly ceased immediately after the earthquake but as we report here a new and higher rate of fault parallel shear accumulation 2 microstrain per year was established about four months later and has continued to the present associated changes in creep rate are apparent at a number of sites on the surface trace of the fault within 30 km of the strain meter we propose that the observed strain accumulation results from increased slip around a nearby locked section of the fault this slip arising from loading by the failed loma prieta source region to the north this model is consistent with suggestions of an increased probability of a moderate earthquake near san juan bautista9 10 and with evidence12 that interactions between fault regions are important in earthquake processes

carbonate platforms in the dolomites area of the southern alps historic perspectives on progress in sedimentology the area of the dolomites in the southern alps exhibits some of the best outcrops of carbonate platforms and platform to basin transitions in the world the region has attracted geologists since the early 19th century and has been a centre of stratigraphic and sedimentological research ever since the interpretation of the platforms in the dolomites as coral reefs dates back to the 1850s and was inspired by the work on modern reefs in the pacific and indian oceans at that time very soon however studies in the dolomites triggered developments of their own fieldwork on these triassic atolls led to a first understanding of the dramatic facies changes at the periphery of reefs and atolls including the recognition of slope facies and basinward transport of platform material by gravity in addition the platforms were used to prove long term syndepositional subsidence on the scale of kilometres as early as 1860 after world war ii the study of modern reefs and platforms led to process based facies models and this in turn guided the sedimentological analysis of the alpine mediterranean belt repeatedly however the roles became reversed and the alpine mediterranean platforms contributed to the fundamental underpinnings of sedimentology in the 1980s mountain size outcrops in the alps and the apennines were recognized as models for the interpretation of shoal water carbonates in seismic data seismic models of outcrops in the dolomites for instance gave rise to the concept of pseudo unconformities i e rapid lateral facies changes that appear as unconformities in seismic images in the 1990s the alpine mediterranean mesozoic along with the north american and european palaeozoic revealed the significance of microbially induced carbonate precipitation in constructing large limestone bodies studies of ancient rocks led this development because this particular carbonate factory is far less prominent now than it was at certain times in the past 2009 the authors journal compilation 2009 international association of sedimentologists

the 1984 morgan hill california earthquake the morgan hill california earthquake magnitude 6 1 of 24 april 1984 ruptured a 30 kilometer long segment of the calaveras fault zone to the east of san jose although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected no short term precursors were noted and so the time of the 1984 earthquake was not predicted unilateral rupture propagation toward the south southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion including an exceptionally large horizontal acceleration of 1 29g at a site on a dam abutment near the southeast end of the rupture zone

creep events preceding small to moderate earthquakes on the san andreas fault the san andreas fault in central california is unusual in that much of the fault displacement occurs as aseismic slip also known as creep changes in creep have been suggested as a possible earthquake precursor1 6 but clear associations between creep changes and subsequent earthquakes are uncommon here i report a test of the hypothesis that episodes of rapid creep creep events 1 3 precede small to moderate earthquakes on the creeping portion of the san andreas the test consists of a comparison of archival creep data to the earthquake catalogue for this area and an earthquake prediction experiment between 1970 and 1994 creep events occurred in the five days preceding half or more of the earthquakes of magnitude 3 3 in three 1 yr periods of above average seismic activity of five predictions based on the occurrence of creep events between october 1995 and mid january 1996 four were fulfilled there were also two misses earthquakes not preceded by a creep event if the relationship reported here holds for the entire creeping segment of the san andreas and applies to larger earthquakes these results suggest that a prediction based on precursory creep may be possible at parkfield california

dynamics of fault creep observations along the surface traces of active faults in california and elsewhere indicate that tectonic displacement can occur as seismic slip or as aseismic fault creep fault creep occurs as secular displacement and as displacement episodes called creep events which last a few hours to days and include from a millimeter or less to a few tens of millimeters of displacement instrumental measurements of displacement versus time during creep events show that many events have a characteristic simple form including a steep beginning followed by a gradual decay in the rate of displacement some creep events display small amounts of precursory slip and many seem to be composed of several discrete simple events propagation of creep events along the san andreas calaveras and imperial faults in california has been inferred from the nearly simultaneous observation of creep events at nearby creepmeters and from kinematic modeling of continuous measurements of strain during creep events fault creep can be viewed as arising from the interplay of three factors stress applied to the fault from external sources stress caused by the geometry and distribution of displacement on the fault arising from the elastic response of the surrounding medium to the displacements within the anelastic fault zone itself and the constitutive relations characterizing the resistance to slip on the fault analytic solutions are presented for a simple rectangular dislocation quasi static model of nonpropagating fault creep assuming viscous and quasi plastic power law creep fault zone rheology and for a multi element composed of several rectangular or striplike dislocation surfaces quasi static model assuming a viscous rheology viscous rheology implies solutions composed of exponentials that decay with time the simple one element quasi plastic model is in agreement with the form of creep events observed at melendy ranch california a matrix formulation is presented to calculate dislocation stresses on a fault zone in a half space or plate the formulation involves the division of the dislocation surface into strips or rectangles and the use of published solutions to calculate the stress resulting from displacement on each individual element results from the matrix method agree with analytic results for equilibrium displacements on stress free cracks the matrix formulation is then used as the basis for a numerical method to simulate one and two dimensional propagating creep assuming a quasi plastic rheology on the fault zone results from this method agree reasonably well with observations of afterslip from the 1966 parkfield 1975 oroville and 1979 imperial valley california earthquakes and of a propagating creep event observed along the southern calaveras fault central california during july 1977 this method of analysis and the assumption that creep at each point on the fault zone follows a power law rheology after the applied stress reaches a spatially varying yield stress can explain the secular and episodic nature of fault creep afterslip the simple shape of some individual events and the multiple composite shape of others and the propagation of creep events on the southern calaveras fault observations of propagating creep indicate a yield stress near zero at the surface and throughout much of the fault zone however they also indicate the presence of a zone at a depth of about 0 5 km and about 5 km long with a yield stress of about 15 bars 1 5 mpa the methods of solution presented here should be applicable to many problems in the dynamics of faulting

is stress accumulating on the creeping section of the san andreas fault 1 the creeping section of the san andreas fault csaf in central california is a proposed barrier to propagation of large earthquakes yet recent studies show that the creeping section is not entirely uncoupled but is accumulating slip deficit at a rate equivalent to a mw 7 2 7 4 earthquake every 150years a critical piece to understanding earthquake potential on the csaf is determining whether slip deficit is occurring with stress accumulation on stick slip regions or without stress accumulation on stable sliding regions shadowed by surrounding locked areas we use a physical model to estimate the spatial distribution of locked stress accumulating areas of the fault constrained by surface creep rate measurements and gps derived velocities we find that the area of the fault accumulating stress if ruptured every 150years would release slip equivalent to at most a mw 6 75 earthquake significantly less than the mw 7 2 7 4 150year equivalent total slip deficit rate

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

effects of large scale surface topography on ground motions as demonstrated by a study of the san gabriel mountains los angeles california we investigate the effects of large scale surface topography on ground motions generated by nearby faulting we show a specific example studying the effect of the san gabriel mountains which are bounded by the mojave segment of the san andreas fault on the north and by the los angeles basin on the south by si mulating a mw 7 5 earthquake on the mojave segment of the san andreas fault we show that the san gabriel mountains act as a natural seismic insulator for metropo litan los angeles the topography of the mountains scatters the surface waves gen erated by the rupture on the san andreas fault leading to less efficient excitation of basin edge generated waves and natural resonances within the los angeles basin the effect of the mountains reduces the peak amplitude of ground velocity for some regions in the basin by as much as 50 in the frequency band up to 0 5 hz these results suggest that depending on the relative location of faulting and the nearby large scale topography the topography can shield some areas from ground shaking

high resolution interseismic velocity data along the san andreas fault from gps and insar 1 we compared four interseismic velocity models of the san andreas fault based on gps observations the standard deviations of the predicted secular velocity from the four models are larger north of the san francisco bay area near the creeping segment in central california and along the san jacinto fault and the east california shear zone in southern california a coherence spectrum analysis of the secular velocity fields indicates relatively high correlation among the four models at longer wavelengths 15 40 km with lower correlation at shorter wavelengths to improve the short wavelength accuracy of the interseismic velocity model we integrated interferometric synthetic aperture radar insar observations initially from advanced land observing satellite alos ascending data spanning from the middle of 2006 to the end of 2010 totaling more than 1100 interferograms with gps observations using a sum remove filter restore approach the final insar line of sight data match the point gps observations with a mean absolute deviation of 1 5 mm yr we systematically evaluated the fault creep rates along major faults of the san andreas fault and compared them with creepmeters and alignment array data compiled in uniform california earthquake rupture forecast version 2 ucerf2 moreover this insar line of sight dataset can constrain rapid velocity gradients near the faults which are critical for understanding the along strike variations in stress accumulation rate and associated earthquake hazard

the effect of topography on earthquake ground motion a review and new results a brief review of experimental and theoretical results about the effect of topography on seismic motion shows that they are consistent only on a qualitative basis amplification at mountain tops for wavelengths comparable with mountain widths is predicted and observed but the numerical simulations often underestimate the actual observed effects we propose that this disagreement is because current models are not complex enough we therefore computed the response to incident sh waves of a set of different complex configurations that include two dimensional surface topography with or without periodic ridges and subsurface layering the results show that 1 the topographic effect in itself is difficult to isolate from other effects like surface layering and therefore the amplification on top of geomorphologically complex sites cannot be predicted by a priori estimations based solely on topography and 2 the high crest base amplification ratio observed in the field cannot usually be matched even with complex two dimensional structures with incident plane sh waves which suggests that more complex models are needed to incorporate more complex wave fields e g sv surface and three dimensional geologic configurations

occurrence of a predicted earthquake on the san andreas fault in may 1985 we predicted1 that an earthquake would occur on the san andreas fault near stone canyon california within a year the prediction was based on the observation of seismic quiescence defined as a significant decrease in the average occurrence rate of earthquakes within the source volume of the future mainshock a mainshock of magnitude ml 4 6 occurred on 31 may 1986 rupturing exactly the specified segment of the fault this is the first successful prediction of an earthquake along the san andreas fault and the probability to have come true by chance is 5 although the prediction addressed only a small main shock its success was significant because the location size and occurrence time were correctly specified for an earthquake in a populated area larger earthquakes will undoubtedly be successfully predicted by the same method in the future but the major segments of the san andreas fault near san francisco and los angeles have such a low background seismicity rate that the method will probably not be applicable there

compressional and shear wave velocity versus depth relations for common rock types in northern california abstract this article presents new empirical compressional and shear wave velocity vp and vs versus depth relationships for the most common rock types in northern california vp versus depth relations were developed from borehole laboratory seismic refraction and tomography and density measurements and were converted to vs versus depth relations using new empirical relations between vp and vs the relations proposed here account for increasing overburden pressure but not for variations in other factors that can influence velocity over short distance scales such as lithology consolidation induration porosity and stratigraphic age standard deviations of the misfits predicted by these relations thus provide a measure of the importance of the variability in vp and vs caused by these other factors because gabbros greenstones basalts and other mafic rocks have a different vp and vs relationship than sedimentary and granitic rocks the differences in vs between these rock types at depths below 6 or 7 km are generally small the new relations were used to derive the 2005 u s geological survey seismic velocity model for northern california employed in the broadband strong motion simulations of the 1989 loma prieta and 1906 san francisco earthquakes initial tests of the model indicate that the vp model generally compares favorably to regional seismic tomography models but that the vp and vs values proposed for the franciscan complex may be about 5 too high

effects of local soil conditions on the topographic aggravation of seismic motion parametric investigation and recorded field evidence from the 1999 athens earthquake during the 1999 athens earthquake the town of adames located on the eastern side of the kifissos river canyon experienced unexpectedly heavy damage despite the particular geometry of the slope that caused significant motion amplification topography effects alone cannot explain the uneven damage distribution within a 300 m zone parallel to the canyon s crest which is characterized by a rather uniform structural quality in this article we illustrate the important role of soil stratigraphy and material heterogeneity on the topographic aggravation of surface ground motion for this purpose we first conduct an extensive time domain parametric study using idealized stratified profiles and gaussian stochastic fields to characterize the spatial distribution of soil properties and using ricker wavelets to describe the seismic input motion the results show that both topography and local soil conditions significantly affect the spatial variability of seismic motion we next perform elastic two dimensional wave propagation analyses based on available local geotechnical and seismological data and validate our results by comparison with aftershock recordings

fault coupling and potential for earthquakes on the creeping section of the central san andreas fault the 150 km long central section of the san andreas fault csaf in central california creeps at the surface and has not produced a large earthquake historically however sections of the san andreas fault to the north and south are known to have ruptured repeatedly in m 7 8 earthquakes it is currently unclear whether the creeping csaf could rupture in large earthquakes either individually or along with earthquakes on the locked sections to the north and south we invert global positioning system and interferometric synthetic aperture radar data with elastic block models to estimate the degree of locking on the csaf and place bounds on the moment accumulation rate on the fault we find that the inferred moment accumulation rate is highly dependent on the long term fault slip rate which is poorly constrained along the csaf the inferred moment accumulation rate normalized by shear modulus ranges from 3 28 104 to 5 85 107 m3 yr which is equivalent to a mw 5 5 7 2 earthquake every 150 years for a long term slip rate of 26 mm yr and mw 7 3 7 65 for a long term slip rate of 34 mm yr the comparisons of slip distributions with microseismicity and repeating earthquakes indicate a possible locked patch between 10 and 20 km depth on the csaf that could potentially rupture with mw 6 5

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

correlation of 1 to 10 hz earthquake resonances with surface measurements of s wave reflections and refractions in the upper 50 m resonances observed in earthquake seismograms recorded in seattle washington the central united states and sherman oaks california are correlated with each site s respective near surface seismic velocity profile and reflectivity determined from shallow seismic reflection refraction surveys in all of these cases the resonance accounts for the highest amplitude shaking at the site above 1 hz these results show that imaging near surface reflections from the ground surface can locate impedance structures that are important contributors to earthquake ground shaking a high amplitude s wave reflection recorded 250 m northeast and 300 m east of the seattle kingdome earthquake recording station with a two way travel time of about 0 23 to 0 27 sec about 18 to 22 m depth marks the boundary between overlying alluvium v s 180 m sec and a higher velocity material v s about 400 m sec this reflector probably causes a strong 2 hz resonance that is observed in the earthquake data for the site near the kingdome in the central united states s wave reflections from a high impedance boundary an s wave velocity increase from about 200 m sec to 2000 m sec at about 40 m depth corresponds to a strong fundamental resonance at about 1 5 hz in sherman oaks strong resonances at about 1 0 and 4 hz are consistently observed on earthquake seismograms a strong s wave reflector at about 40 m depth may cause the 1 0 hz resonance the 4 0 hz resonance is possibly explained by constructive interference between the first overtone of the 1 0 hz resonance and a 3 25 to 3 9 hz resonance calculated from an areally consistent impedance boundary at about 10 m depth as determined by s wave refraction data

decrease in deformation rate as a possible precursor to the next parkfield earthquake crustal deformations along fault zones are monitored because they show the accumulation and possible premonitory release of strain energy before major earthquakes on the parkfield segment of the san andreas fault the next mainshock local magnitude ml 5 7 is expected to occur between 1983 and 1993 based on the relatively regular recurrence of characteristic earthquakes in 1857 1881 1901 1922 1934 and 19661 a concerted monitoring effort is therefore underway at parkfield with the aim of detecting precursory changes in observable quantities that can be measured reliably2 here we report data from two geodetic lines crossing the san andreas fault near parkfield which show that the rate of shortening decreased by 20 starting in august 1986 this change coincided with a rate decrease of 45 for earthquakes of ml 2 1 in the area3 4 both phenomena may have been caused by temporary locking of the fault surface or a decrease in strain accumulation rate and they are interpreted as intermediate term precursors to the next parkfield earthquake