crustal strains in the japanese islands as deduced from dense gps array data from the nationwide continuous gps network operated by the geographical survey institute of japan were used to estimate crustal strains in the japanese islands we used horizontal velocity vectors at 610 sites for the period april 1996 to march 1997 and employed the least squares prediction method to interpolate the velocities to a uniform 7km 7km grid over japan the gridded velocities were then differentiated in space to calculate principal components of strains results clearly portray tectonic strainings of the japanese islands in one year 1 dilatational strains show that the japanese islands are under a compressive strain regime induced by the subducting oceanic plates 2 distribution of high maximum shear strains coincides with the regions of crustal activities and 3 principal axes of the strains indicate influences of e w and nw se compression due to the converging oceanic plates north south extension in kyushu is also visible

s wave velocity structure of sediments in anchorage alaska estimated with array measurements of microtremors the array measurements of microtremors were carried out at nine sites to estimate the subsurface s wave velocity structures of the sedimentary deposits beneath the metropolitan area of anchorage alaska the data were recorded by ten three component accelerometers arranged in a triangular manner for three different array sizes the phase velocities c f were estimated at each site from the vertical component of the recorded microtremor data by using the frequency wavenumber technique the c f data from different arrays were combined after checking their consistency for a series of overlapped frequency bands from different arrays and were inverted using a stochastic least squares inversion technique to estimate the 1d structure underneath each site the inversion results show that the engineering basement 750 m sec lies at a relatively shallower depth 40 m in the eastern part of the basin along the foothills of the chugach mountains cm and at deeper depths toward the southcentral 100 m and western 150 m parts of the basin in accord with the general dip of the basin below the engineering basement a well developed low velocity zone lvz with values in the range of 900 1040 m sec is found to be present in the eastern as well as along the knik arm side in the western part of the basin at a depth of 200 m and 900 m respectively moreover the central part of the basin is associated with a weakly developed lvz below the engineering basement depth in the rest of the basin the value increases gradually with depth the spatial variations of in the basin at different depths from 20 to 500 m are represented by using 2d interpolation of the structures obtained from the inversion of c f data the depth to the crystalline basement of the basin however could not be ascertained and it seems to be much deeper than the maximum depth 2000 m resolved by the data gathered in this study to validate the results of inversion the spectral ratio between the horizontal and vertical components h v of the recorded microtremor data at each site has been compared with the computed h v of rayleigh waves based on the respective structure the results showed good agreement in the frequency range of about 0 4 6 0 hz in this frequency range the h v peaks are due to the overall effect of the velocity contrasts between layers representing the subsurface structure

inversion of gps velocity and seismicity data to yield changes in stress in the japanese islands summary to estimate stress changes within the crust from observed displacement rates we have devised an ew stress inversion method for a 2 d plate subjected to a planar stress which uses airy s stress function the merits of this stress inversion method are that it allows us to estimate the stress field without full knowledge of the elastic properties of the object and it improves precision because it is not necessary to take derivatives of observed quantities we applied this stress inversion method to the japanese islands where the geographical survey institute has been operating a nationwide gps array called geonet we make the assumption that the inelastic deformation in the japanese islands has no dilatational component we used velocity data derived from 3 yr of gps observations we estimated the change in boundary traction from the seismic parameters of large earthquakes stress drop slip direction and recurrence interval we compared the observed total strain and estimated the elastic strain the results suggest that the latter looks larger than the former indicating that it is feasible to estimate the distribution of rigidity from the stress inversion comparison with seismicity data suggests that inland shallow earthquakes occur where rigidity is lower

delineation of spatial variation of shear wave velocity with high frequency rayleigh waves in anchorage alaska high frequency rayleigh waves were generated in the frequency range of about 1 100 hz by an electromagnetic vibrator in co operation with the vibration instrument company japan and the ensol corporation north carolina at 36 sites in anchorage from phase delay times between two sensors the phase velocity of the fundamental mode at each site was computed these data were inverted in terms of shear wave velocity structure by a stochastic inversion scheme of the 36 sites values of shear wave velocity as a function of depth were available at seven sites from downhole measurements at these sites the comparison of the results obtained by others between surface and downhole measurements showed reasonable agreement these results were therefore combined with those of the remaining 29 sites obtained in this study the structure of the 36 sites could be subdivided into four groups on the basis of nehrp provision and from the time averaged structures for the uppermost 30 m corresponding to the four groups it has been possible to identify the lateral extent of soil classes c and d in the anchorage area from statistical analysis of the time averaged data it is shown that the lithology of the area in soil class c is distinct from that in d the same is true for the subunits namely glaciofluvial deposits in areas of c and d moreover the areas in soil class d along the knik arm in west anchorage with relatively low values coincide with the areas of high ground failure susceptibility identified by others from observations following the prince william sound earthquake mw 9 2 of 1964

array estimators and the use of microseisms for reconnaissance of sedimentary basins a natural field seismic technique is possible to attain by observing microseisms with a suitably designed array and by digitally processing the data to obtain estimates of the phase velocities of rayleigh waves wavelengths of interest in detecting depth to the basement of sedimentary basins are in the range 2 to 20 km and correspond to wave periods from 1 to 7 s an array of five or seven seismometers deployed as an expanding cross configuration simplifies field procedures and is adequate for phase velocity measurements of rayleigh waves in the required wavelength range provided high resolution frequency wavenumber spectral analysis is used this analysis can be implemented on a minicomputer in the field results obtained from observation in a sedimentary basin of known structure show predominantly fundamental mode rayleigh wave propagation the scatter of velocity estimates is small enough to allow inversion by curve matching and depth to the basement can be computed to an accuracy of 30 percent w

transient and permanent deformation of central japan estimated by gps 1 interseismic loading and subduction kinematics abstract we analyze continuous global positioning system gps data to estimate the kinematics of the philippine sea plate subduction below southern central japan using a back slip dislocation model we invert the strain rate tensors derived from the gps velocity field to obtain the velocity of the downgoing slab with respect to the tokai and kanto forearcs along the eastern nankai suruga and sagami troughs respectively in both cases these velocities are smaller and slightly rotated with respect to the philippine sea plate subduction velocity in central nankai this result implies strain partitioning between the subduction thrusts and on land and or offshore deformation zones in both the eastern nankai suruga and the sagami area in southern kanto we predict 20 mm yr 1 of dextral shear in agreement with motion observed along the boso strike slip fault in the tokai region the slip deficit accumulated since the last earthquake 1854 ranges between 6 and 3 m considering the extent of the locked zone it would correspond to a magnitude m w 8 15 earthquake if the slip deficit were to be released now during a single event

the use of earthquake rate changes as a stress meter at kilauea volcano stress changes in the earth s crust are generally estimated from model calculations that use near surface deformation as an observational constraint but the widespread correlation of changes of earthquake activity with stress has led to suggestions that stress changes might be calculated from earthquake occurrence rates obtained from seismicity catalogues although this possibility has considerable appeal because seismicity data are routinely collected and have good spatial and temporal resolution the method has not yet proven successful owing to the non linearity of earthquake rate changes with respect to both stress and time here however we present two methods for inverting earthquake rate data to infer stress changes using a formulation for the stress and time dependence of earthquake rates application of these methods at kilauea volcano in hawaii yields good agreement with independent estimates indicating that earthquake rates can provide a practical remote sensing stress meter

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

near surface shear wave velocity structure of the chiayi area taiwan this paper uses microtremor array measurements to obtain an overall view of s wave velocity structure in the chiayi area taiwan the rayleigh wave dispersion curves are calculated using the frequency wavenumber graphic 1 spectrum method and then the s wave velocity structure of the area is estimated by surface wave inversion according to the inversion results the s wave velocities decrease from east to west if the s wave velocity of the bedrock is assumed to be 1500 m s the depths of the alluvium are between 560 and 1400 m gradually increasing from east to west in order to understand the variations of the shallow s wave velocity in the area we sketch 2d and 3d maps using the imaging techniques based on interpolation algorithms from the 3d results the thickness of the sediments having an s wave velocity in the range 270 1500 m s increases from east to west whereas the thickness of the layers having an s wave velocity in the range 1500 2370 m s decreases from east to west the results are also in good agreement with the geological and geophysical information of the chiayi area therefore microtremor array measurement provides a good alternate to estimate shallow s wave velocity structure online material figures showing observed and calculated phase velocities f k spectra dispersion curves inverted velocity models sensitivity kernels and v s contour maps and electric resistivity maps 1 embed inline graphic 1 gif

characteristics of long period microtremors and their applicability in exploration of deep sedimentary layers abstract applicability of long period microtremors in inferring subsurface structure is examined using measurements of microtremors in the northwestern part of the kanto plain in japan short term continuous measurements of long period microtremors at both sediment and basement sites were taken a spectral peak at a period of 4 to 5 sec is stable with time while peaks at periods less than 2 sec are time variant suggesting a variation of microtremor sources however it was found that the spectral ratio between vertical and horizontal microtremors ellipticity at each site is stable with time good agreement was found between ellipticities of microtremors at the sediment site and those computed for rayleigh waves in which the structure of the sediments beneath the site was taken into account we also found that the ellipticities of rayleigh waves in earthquake ground motions were consistent with those of the microtremors these comparisons provide strong evidence that long period microtremors in the area studied consist mainly of rayleigh waves the ellipticity of microtremors was investigated by observing microtremors at temporary observation sites in the kanto plain where the sediment thickness varied from 0 to 1 km the subsurface structures were deduced by trial and error fitting of observed ellipticities with theoretical ellipticities that were calculated assuming rayleigh waves these results show that ellipticity of long period microtremors is effective for deducing structure from microtremor data at a single site

site specific issues for strong ground motions during the kocaeli turkey earthquake of 17 august 1999 as inferred from array observations of microtremors and aftershocks array observations of microtremors and aftershocks were carried out near the permanent strong motion observation sites and the damaged areas after the kocaeli turkey earthquake of 17 august 1999 the major objectives were to determine s wave velocity structures at the sites and to understand the site effects on strong motion or damage array observation of microtremors is a useful method for determining the s wave velocity structures in a sedimentary basin with less practical restriction than the other geotechnical methods the spatial autocorrelation method spac was applied to array data of microtremors for determining s wave velocity structures the spac method generally provides an equivalent result with that of the frequency wavenumber method using fewer array sites and a smaller array size most strong motion sites near the fault are classified into stiff and or very hard soils the records cannot directly be used for interpreting damage to buildings in the sedimentary basin records of long durations of strong motion at ats near avcilar west of istanbul are closely related to the low velocity v s 200 m sec of the surface layers the s wave velocity structure at avcilar where there was severe damage during the mainshock is similar to that of the lowland ats and it differs significantly from that of cna located 4 km northeast of avcilar where the strong motion record was obtained from the mainshock the strong ground motion at avcilar during the mainshock is estimated to be similar to that at ats of the strong motion sites sakarya skr is located on very hard soil whereas thick and soft sediments cover downtown adapazari it is plausible that strong ground motions during the mainshock in the damaged area adc were significantly different from those of skr a large difference between the strong motions of a hillside and the lzmit bay area in and around golcuk is also indicated by a comparison of aftershock records

s wave velocity structure of the taiwan chelungpu fault drilling project tcdp site using microtremor array measurements the taiwan chelungpu fault drilling project tcdp drilled a 2 km deep hole 2 4 km east of the surface rupture of the 1999 chi chi earthquake mw 7 6 near the town of dakeng geophysical well logs at the tcdp site were run over depths ranging from 500 to 1 900 m to obtain the physical properties of the fault zones and adjacent damage zones these data provide good reference material for examining the validity of velocity structures using microtremor array measurement therefore we conduct array measurements for a total of four arrays at two sites near the tcdp drilling sites the phase velocities at frequencies of 0 2 5 hz are calculated using the frequency wavenumber f k spectrum method then the s wave velocity structures are estimated by employing surface wave inversion techniques the s wave velocity from the differential inversion technique gradually increases from 1 52 to 2 22 km s at depths between 585 and 1 710 m this result is similar to those from the velocity logs which range from 1 4 km s at a depth of 597 m to 2 98 km s at a depth of 1 705 m the stochastic inversion results are similar to those from the seismic reflection methods and the lithostratigraphy of tcdp a borehole comparatively these results show that microtremor array measurement provides a good tool for estimating deep s wave velocity structure

estimation of earthquake source parameters and site response in anchorage alaska from strong motion network data using generalized inversion method abstract generalized inversion of the s wave amplitude spectra from the strong motion network data in anchorage alaska has been analyzed to estimate simultaneously source parameters site response sr and the s wave attenuation qs 1 the data represented 60 earthquakes of magnitudes ml from 3 0 to 6 3 in the hypocentral distance range from 30 to 300 km the source term obtained from inversion was analyzed to estimate various source parameters namely seismic moment mo seismic energy es corner frequency fc apparent stress a and brune s stress drop for each event it is shown that mo and es varies from 1014 to 2 1 1018 n m and form 5 8 108 to 0 8 1014 j respectively while varies from 0 2 to 24 mpa the regressional analysis of the source parameters shows the following 1 es mo is approximately equal to 1 2 10 5 and 2 mo is proportional to fc 3 it is also shown that a and approximately satisfies the relation a 0 23 the sr values obtained for the anchorage basin from the above inversion were averaged for two frequency bands 0 5 2 5 and 3 0 6 3 hz with geometric center frequencies of about 1 0 and 5 0 hz respectively the spatial variation of sr at 1 0 hz shows a gradual increase from 1 0 along the foothills of chugach mountains to 3 5 in the western part of the basin at 5 0 hz the trend of the variation of sr values is from north to south moreover at 1 0 hz the above trend of sr values is in agreement with that of shear wave velocity of the uppermost 30 m of the sedimentary formations of the basin the variation of qs 1 in the region is found to be 0 0066f 1 2 in the frequency range from 0 25 to 20 hz

induced dynamic nonlinear ground response at garner valley california we present results from a prototype experiment in which we actively induce observe and quantify in situ nonlinear sediment response in the near surface this experiment was part of a suite of experiments conducted during august 2004 in garner valley california using a large mobile shaker truck from the network for earthquake engineering simulation nees facility we deployed a dense accel erometer array within meters of the mobile shaker truck to replicate a controlled laboratory style soil dynamics experiment in order to observe wave amplitude dependent sediment properties ground motion exceeding 1g acceleration was pro duced near the shaker truck thewave field was dominated by rayleigh surface waves and ground motions were strong enough to produce observable nonlinear changes in wave velocity we found that as the force load of the shaker increased the rayleigh wave phase velocity decreased by as much as 30 at the highest frequencies used up to 30 hz phase velocity dispersion curves were inverted for s wave velocity as a function of depth using a simple isotropic elastic model to estimate the depth depen dence of changes to the velocity structure the greatest change in velocity occurred nearest the surface within the upper 4 m these estimated s wave velocity values were used with estimates of surface strain to compare with laboratory based shear modulus reduction measurements from the same site our results suggest that it may be possible to characterize nonlinear soil properties in situ using a noninvasive field technique

s wave velocity structures of the taipei basin taiwan using microtremor array measurements abstract the s wave velocity structures of the taipei basin in taiwan are investigated using the array records of microtremors at 15 sites dispersion curves at these sites are calculated using the frequency wavenumber f k spectrum method the s wave velocity structures in the taipei basin are then estimated by employing surface wave inversion technique harder strata sites have higher phase velocities than softer sites if the s wave velocity of the tertiary basement is assumed to be 1000 m s then the quaternary alluvial thicknesses in the taipei basin are between about 100 m and 650 m the thickness of the alluvium gradually increases from the southeast to the northwest the inversion results are also in good agreement with well logging data and seismic reflection studies of the taipei basin the study concludes that microtremor array measurement is a useful tool for estimating s wave velocity structure

transient and permanent deformation of central japan estimated by gps 2 strain partitioning and arc arc collision abstract deformation of central japan can be divided into two main components elastic deformation due to the interseismic locking of subduction thrusts and deformation associated with intra arc tectonics in a companion paper henry et al earth planet sci lett 2000 this issue we estimated an elastic deformation field based on the modeling of global positioning system gps strain rates in this study we show that the corrected field gps minus predicted elastic deformation is consistent with geological and seismological observations and should represent long term plastic deformation we use this corrected gps deformation field to describe the kinematics of partitioning of the philippine sea plate amurian plate amu convergence in relation with the izu bonin arc collision about one third of the relative philippine sea amur motion is transferred to the central japan crust north of the collision front this transfer induces high strain rates in southern kanto and in the northern japanese alps the latter region corresponds to the eastern boundary of the amu we propose that the independent motion of central japan results from the delamination and the underthrusting of the izu bonin arc lower crust and the upward bent oceanic slab

discrimination of fault planes from auxiliary planes based on simultaneous determination of stress tensor and a large number of fault plane solutions it is well known that there is a large difference in focal mechanism solutions of events even if they occur within a certain small area considering earthquake generating stress to be uniform in a studied area we developed a new method for simultaneously determining the stress tensor and the orientation of fault planes for many events by the use of polarity data of p waves first the number of inconsistent stations is calculated for various values of three parameters in the focal mechanism solution with intervals of 10 this calculation is made for all the events used then four parameters defining the stress tensor are determined by a grid search using the data calculated above with the assumption that the slip direction in the fault plane should be parallel to the direction where the shear stress on the plane becomes maximum a numerical test is made by applying the present method to an artificial data set composed of 25 events with 25 25 readings it is found that a stable solution of the stress tensor is determined by the inversion and about 60 of fault planes are distinguished from auxiliary planes there are no events whose auxiliary planes are incorrectly determined as fault planes this method is used to estimate the earthquake generating stress in the aftershock area of the 1984 western nagano prefecture earthquake central japan the results obtained show that the maximum principal stress is horizontal in the direction of n80 w s80 e there are strike slip events having fault planes almost perpendicular to the fault plane of the main shock

investigation of shallow sedimentary structure of the anchorage basin alaska using simulated annealing inversion of site response abstract this study deals with shallow sedimentary structure of the anchorage basin in alaska for this purpose inversion of site response sr f data in the frequency range 0 5 11 0 hz from various sites of the basin has been performed using the simulated annealing method to compute subsurface layer thickness shear wave velocity density and shear wave quality factor the one dimensional 1d models for the aforementioned parameters were obtained with preset bounds on the basis of available geological information such that the l 2 norm error between the observed and computed site response attained a global minimum next the spatial distribution of the important parameter was obtained by interpolating values yielded by the 1d models the results indicate the presence of three distinct velocity zones as the source of spatial variation of sr f in the anchorage basin in the uppermost part of the basin the values of fine grain quaternary sediments mainly lie in the range of 180 500 m sec with thickness varying from 15 to 50 m this formation overlies relatively thick 80 200 m coarse grain quaternary sediments with values in the range of 600 900 m sec these two quaternary units are in turn overlain on tertiary sediments with 1000 m sec located at depths of 100 and 250 m respectively in the central and western side along the knik arm parts of the basin the important implication of the result is that the sources of spatial variation of sr f in the anchorage basin for the frequency band 0 5 11 hz besides in the uppermost 30 m are found to be deeper than this depth thus use of commonly considered geological formations in the depth intervals from 0 to 30 m for the ground motion interpretation will likely yield erroneous results in the anchorage basin

continuous gps array and present day crustal deformation of japan a gps array with about 1 000 permanent stations is under operation in japan the gps array revealed coseismic deformations associated with large earthquakes and ongoing secular deformation in the japanese islands based on daily coordinate data of the gps stations strain rate distribution is estimated most regions with a large strain rate are related to plate boundaries and active volcanoes in addition the niigata kobe tectonic zone nktz is recognized as a region of large strain rate along the japan sea coast and in the northern chubu and kinki districts this newly found tectonic zone may be related to a hypothetical boundary between the eurasian or amurian and the okhotsk or north america plates precise observation of crustal deformation provides important boundary conditions on numerical modeling of earthquakes and other crustal activities appropriate computation methods of continuous deformation field are directly applicable to data assimilation for such numerical simulations

estimation of s wave velocity structures in and around the sendai basin japan using array records of microtremors we conducted array measurements of microtremors at six sites in and around the sendai basin to estimate deep s wave velocity structures above the seismological bedrock pre tertiary bedrock after estimating phase velocities of microtremors in the frequency range from about 0 5 to 3 hz we succeeded to estimate the deep structures above the pre tertiary bedrock at four soil sites kata okin arah and myg15 and only the upper part just beneath the pliocene layer at the other soil site trma using a rayleigh wave inversion technique at trma and a rock site tama we roughly estimate the deep s wave velocity structures using empirical site amplification factors derived from strong motion records the deepest bedrock depth is 1 km and the shallowest depth is 200 m among these six sites the difference of the bedrock depth between the eastern side and the western side of an active thrust fault the nagamachi rifu line is only 120 m the difference of the bedrock depth between two sites in the east of the basin reaches about 500 m although any geological boundaries or buried faults have not been mapped since the deep s wave velocity structure of the sendai basin had been basically unknown we delineate it for the first time in this article using array measurement of microtremors

relocation of earthquakes in the cook inlet area south central alaska using the joint hypocenter determination method abstract the joint hypocenter determination jhd method was used to relocate 1604 earthquakes that occurred during 1988 to 1993 in the wadati benioff zone wbz beneath the cook inlet in south central alaska the study utilizes both p and s wave arrivals recorded by a recently improved regional seismic network and it provides an improvement over previous studies the jhd locations of the events allowed improved delineation of the geometry of the subducted slab and the detection of a double seismic zone within the wbz in the 40 to 100 km depth range the jhd station corrections obtained correlate with the major surface geological features in the study area slow velocity sedimentary basins as well as with crustal and mantle structure high velocity subducting slab and low velocity anomalies beneath the active volcanoes to investigate the dependence of the jhd relocation procedure on factors such as the hypocentral depths of the events used the velocity model the p s ratio and random and systematic errors in travel time readings synthetic travel times were calculated using an exact raytracing technique a three dimensional tomographic p wave velocity model the present network configuration and the actual seismicity distribution then the synthetic events were located with the jhd algorithm the synthetic experiments showed that the jhd procedure is rather insensitive to the choice of plane layered velocity model and p s ratio preserves relative hypocenter locations well and can handle systematic and large random reading errors effectively

in situ measurements of nonlinear and nonequilibrium dynamics in shallow unconsolidated sediments abstract we present results and in situ measurements from a field experiment in which a large seismic shaker truck is used to induce nonlinear and nonequilibrium dynamics in shallow unconsolidated sediments an array of accelerometers was deployed adjacent to the shaker truck to record strong ground motions exceeding 1 g we determined high strain rayleigh wave dispersion across the array in the band from 5 to 60 hz rayleigh wave phase velocities at frequencies above 10 hz are wave amplitude dependent and a function of the driving force amplitude as driving force was increased phase velocity decreased consistent with nonlinear dynamics we demonstrate the existence of a temporary nonequilibrium state occurring in the near surface soils during and after the induced nonlinear behavior nonlinear conditioning is demonstrated by measuring changes in rayleigh wave phase velocity for input signals with the same applied driving force amplitude a logarithmic slow dynamic recovery process is observed by analyzing the temporal variation in velocity of the noise field produced by the shaker truck when sitting idle measurements from weak motion seismic surveys taken before and after shaking show compelling evidence that induced nonlinear behavior in the shallow sediments is reversible we demonstrate that an active source field based approach has the potential to expand our knowledge of how sediments respond to strong ground motions provide additional insight into the poorly understood slow dynamic recovery process and possibly even lead to a new site specific and noninvasive technique for characterizing the nonlinear properties of sedimentary deposits

determination of shallow shear wave velocity profiles in the cologne germany area using ambient vibrations summary we have used both single station and array methods to determine shallow shear velocity site profiles in the vicinity of the city of cologne germany from ambient vibration records based on fk analysis we assume that fundamental mode rayleigh waves dominate the analysed wavefield in the frequency range of 0 7 2 2 hz according to this view a close relation exists between h v spectral ratios and the ellipticity of the contributing rayleigh waves the inversion of the shape of h v spectral ratios then provides quantitative information concerning the local shear wave velocity structure however based on tests with synthetic data believed to represent a typical situation in the lower rhine embayment dispersion curves were found to provide stronger constraints on the absolute values of the velocity depth model than the ellipticities the shape of the ellipticities was found to be subject to a strong trade off between the layer thickness and the average layer velocity we have made use of this observation by combining the inversion schemes for dispersion curves and ellipticities such that the velocity depth dependence is essentially constrained by the dispersion curves while the layer thickness is constrained by the ellipticities in order to test this method in practice we have used array recordings of ambient vibrations from three sites where the subsurface geology is fairly well known and geotechnical information is at least partially available in order to keep the parameter space as simple as possible we attempted to fit only a single layer over a half space model however owing to earlier studies from the region we assume a power law depth dependence for sediment velocities for all three sites investigated the inversion resulted in models for which the shear wave velocity within the sediment layer both in absolute value at the surface and in depth dependence are found to be remarkably similar to the results obtained by budny from downhole measurements this is strong support for the interpretation of h v spectral ratios as rayleigh wave ellipticities for all three sites the predicted sh wave site amplification factors at the fundamental frequency are of the order of 5 6 with a slightly smaller value south of cologne

estimation of s wave site response in anchorage alaska from weak motion data using generalized inversion method the generalized data inversion method has been used to estimate the spatial variation of site response sr in the anchorage basin in south central alaska the data analyzed represents weak motion of the horizontal components of s waves for local earthquakes they were recorded by a temporary portable 22 station network iris passcal equipment that was operated for about six months setting the path effect from independent sources the inversion of the data was carried out to determine sr the sr values thus obtained were averaged logarithmically for two frequency bands namely a low frequency band lfb and a high frequency band hfb from 0 5 to 2 5 hz and 3 0 to 7 0 hz respectively with center frequencies of about 1 0 hz and 5 0 hz in lfb sr increases from 1 0 along the foothills of chugach mountains in the east to about 3 5 in the west in and around campbell lake and government hill the areas with sr 3 0 are associated with extensive ground failure during the prince william sound earthquake m w 9 2 of 1964 in hfb there are two small areas adjoining tumagain arm in the southern side of anchorage with sr greater than 3 0 a comparison of the sr values obtained from the inversion with those reported by others for standard spectral ratio ssr and horizontal to vertical spectral ratio hvr showed 1 1 correspondence with the values of ssr but larger hvr values by a factor of about 2 areas of soil class d in the study area are characterized by sr 2 0 while those in c are characterized by sr 2 0 in lfb but soil class and sr lack correlation in hfb

near fault strong ground motions recorded during the morelia normal fault earthquakes of may 2006 in mexicali valley b c mexico by the end of may 2006 a series of earthquakes had occurred in the mexicali valley baja california mexico this activity which included an m w 5 4 event developed in the vicinity of the cerro prieto geothermal field the earthquakes occurred on the morelia fault one of the east dipping normal faults in the mexicali valley cracks and fissures visible on the ground after the stronger earthquake defined two parallel fault segments of 4 and 2 km length the shorter segment being 1 km to the east of the other one down dip displacements of up to 25 30 cm were measured at some places along this surface break zone the ten larger events of the series were recorded by strong motion stations that operate in the epicenter area so that accurate hypocenter locations were obtained on the basis of p wave arrival times from the strong motion recordings the estimated shallow depths combined with the earthquake magnitude explain the strong shaking felt by residents and some damage observed on the cerro prieto geothermal plant the m w 5 4 event produced peak ground accelerations that go from 0 002 g at cuc on rock to 0 5 g at geo on sediments at 13 and 1 7 km from the epicenter respectively the station geo recorded closer to the epicenters and on the hanging wall side of the fault static ground displacements and a predominance of the strike normal over the strike parallel velocity components were determined from the acceleration records of this station these and other ground motion characteristics are also seen on pseudovelocity and absolute acceleration response spectra calculated from data of the larger event altogether the observed ground motion characteristics provide useful insights into the levels of ground shaking that near fault structures in the mexicali valley should be designed to withstand