earthquake ground motion estimation using strong motion records a review of equations for the estimation of peak ground acceleration and response spectral ordinates abstract engineering seismology is the link between earth sciences and engineering the main input of engineering seismology in engineering design are loading conditions which must satisfy certain conditions regarding their level and frequency of occurrence during the lifetime of a structure one method for estimating these loading conditions is through equations based on strong ground motion recorded during previous earthquakes these equations have a handful of independent parameters such as magnitude and source to site distance and a dependent parameter such as peak ground acceleration pga or spectral acceleration and the coefficients in the equation are usually found by regression analysis this review examines such equations in terms of data selection accelerogram processing techniques of the strong motion records used to construct the equations the characterisation of earthquake source travel path and local site used and regression techniques employed to find the final equations it is found that little agreement has been reached in the past 30 years of ground motion estimation relation studies workers have chosen their techniques based on the available data which varies greatly with geographical region also it is noted that there is a need to include more independent parameters into ground motion estimation equations if the large uncertainties associated with such equations are to be significantly reduced the data required to do this is unfortunately scarce

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

a comparison and test of various site response estimation techniques including three that are not reference site dependent abstract we compare various site response estimation techniques using after shock data of the 1989 loma prieta earthquake collected in oakland california because of recent interest in comparing results from weak and strong motion to infer any nonlinearity and between direct s and coda waves we pay particular attention to the uncertainties first sediment to bedrock spectral ratio estimates between pairs of sites are compared with those obtained from various generalized inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously we find that the site amplification factors are very similar among these approaches but that the uncertainties can be significantly different depending on how the data are weighted we also examine and test three site response estimation techniques that do not rely on a reference site to estimate source and path effects the first involves a parameterized source and path effects inversion even when the bedrock data are excluded from consideration this approach is found to reveal the frequency dependence of site response at each of the sediment sites the second technique involves taking horizontal to vertical component spectral ratios receiver function type estimates of shear wave aftershock data these are also found to reveal the frequency dependence of site response at the sediment sites and the results for the bedrock site are relatively flat and near unity the third estimate is formed by taking horizontal to vertical component ratios of ambient seismic noise and these are shown to reveal the fundamental resonant frequency of the sediment sites unfortunately discrepancies exist among all of the site response estimates and with one dimensional predictions with respect to a frequency independent scaling factor nevertheless the highly frequency dependent character of site response is well constrained and the fact that non reference site dependent methods are capable of revealing this is promising for site specific hazard assessments in regions that lack adequate reference sites

integrated model of the crustal structure in the gulf of california extensional province abstract a method of constructing 3d earth models from a suite of 1d profiles is demonstrated using delaunay triangulation accurate crustal models are required to correct for crustal contamination in seismic tomography of the mantle and can be used as starting points for higher resolution tomography of the lithosphere and asthenosphere using a simple input data set 1d vertical profiles provides for rapid prototyping model building and modification any structural length scale in the final 3d model is possible using this method through precise placement of 1d profiles and triangulation collections of layers within the 1d profiles are grouped and identified by keys called superlayers allowing for 3d discontinuities and variations along these important interfaces the method was applied to the gulf extensional province and baja california regions to account for the laterally discontinuous crustal variations during a tomographic inversion for mantle structure use of the 3d model and appropriate regional crustal wave speeds minimizes wave speed variations in the tomographic model and assists interpretation of the final model

source spectral characteristics of two large intra slab earthquakes along the southern kurile hokkaido arc abstract in 1993 and 1994 two large earthquakes occurred along the southern kurile hokkaido arc the 1993 kushiro oki earthquake 15 january mw 7 6 and the 1994 shikotan earthquake 4 october mw 8 2 these were intra slab earthquakes that ruptured through a substantial part of the subducting oceanic lithosphere we study source spectral characteristics of these large intra slab earthquakes based on strong motion records for the kushiro oki earthquake the s wave spectra on three rock site stations are directly compared with the theoretical spectra based on the omega squared source model and the source parameters estimated from long period seismic waves the observed spectra are roughly consistent with the theoretical ones at low frequencies

nonlinearity liquefaction and velocity variation of soft soil layers in port island kobe during the hyogo ken nanbu earthquake clear nonlinear behavior is analyzed from the acceleration records of the 1995 hyogo ken nanbu earthquake at port island kobe from four triaxial in struments placed at four different depths the surficial effects during strong ground motions were compared with those during weak motions before and after the main shock we used a spectral ratio technique and a nonlinear inversion for velocity structure to analyze the data from the spectral analysis we observed a large variation of the spectral ratios between the surface and different depths during the strong ground motions and during the liquefied state the spectral ratios after the mainshock i e after the liquefied state are different from those before the mainshock the peak frequencies in the spectral ratios after the mainshock are shifted to lower fre quencies with respect to those in the spectral ratios before the mainshock we in verted the s wave velocities using a genetic algorithm technique to determine the velocity structure before during and after the mainshock the s wave velocity struc ture before and after the mainshock was found to be different specifically the s wave velocity of the second layer 5 m to 16 m depth after the mainshock was 20 lower than before our analysis shows that the liquefied state remains at least 3 hr after the mainshock but no more than 24 hr the rigidity of the soil decreased close to zero when liquefaction happened and later increases gradually following a trend that resembles a consolidation curve the strong influence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions so that the observed horizontal peak acceleration was only about 25 of the peak acceler ation expected from the linear theory however the nonlinear effects in the vertical peak acceleration were not significant

nonlinear behavior of soil sediments identified by using borehole records observed at the ashigra valley japan abstract we evaluate the nonlinear behavior of soil sediments during strong ground shaking based on the identification of their s wave velocities and damping factors for both the weak and strong motions observed on the surface and in a borehole at kuno in the ashigara valley japan first we calculate spectral ratios between the surface station ks2 and the borehole station kd2 at 97 6 m below the surface for the main part of weak and strong motions the predominant period for the strong motion is apparently longer than those for the weak motions this fact suggests the nonlinearity of soil during the strong ground shaking to quantify the nonlinear behavior of soil sediments we identify their s wave velocities and damping factors by minimizing the residual between the observed spectral ratio and the theoretical amplification factor calculated from the one dimensional wave propagation theory the s wave velocity and the damping factor h 2 q 1 of the surface alluvial layer identified from the main part of the strong motion are about 10 smaller and 50 greater respectively than those identified from weak motions the relationships between the effective shear strain 65 of the maximum shear strain calculated from the one dimensional wave propagation theory and the shear modulus reduction ratios or the damping factors estimated by the identification method agree well with the laboratory test results we also confirm that the soil model identified from a weak motion overestimates the observed strong motion at ks2 while that identified from the strong motion reproduces the observed thus we conclude that the main part of the strong motion whose maximum acceleration at ks2 is 220 cm sec 2 and whose duration is 3 sec has the potential of making the surface soil nonlinear at an effective shear strain on the order of 0 1 the s wave velocity in the surface alluvial layer identified from the part just after the main part of the strong motion is close to that identified from weak motions this result suggests that the shear modulus recovers quickly as the shear strain level decreases

velocity change in the zone of a moderate mw 5 0 earthquake revealed by autocorrelations of ambient noise and by event spectra a moderate mw 5 0 earthquake occurred near mogul nevada just west of reno nevada on 26 april 2008 this mainshock was surrounded by notable foreshock and aftershock sequences due to the long foreshock sequence the area was well instrumented at the time of the mainshock we investigated the foreshock and aftershock sequences for evidence of velocity changes in the structure immediate to the hypocenter and above it in the area of observed strong ground motion using autocorrelations of the time periods with nearly continuous foreshocks and aftershocks we detected a nearly 1 negative change in velocity on recordings of station mogl which was approximately over the hypocenter of the mainshock we also observed from these recordings a shift in the spectral peaks to lower frequency following the mainshock again indicative of a velocity decrease due to the different spatial sampling of the two methods the effects could not be attributed to the same subsurface volume however both results are strong evidence for coseismic velocity changes accompanying an earthquake which is very much smaller than those for which previous velocity changes have been reported we hypothesize that these changes can be observed for even smaller earthquakes given a fortuitous placement of observing stations

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

convective upwelling in the mantle beneath the gulf of california in the past six million years baja california has rifted obliquely apart from north america opening up the gulf of california between transform faults seafloor spreading and rifting is well established in several basins other than hotspot dominated iceland the gulf of california is the only part of the world s seafloor spreading system that has been surrounded by enough seismometers to provide horizontal resolution of upper mantle structure at a scale of 100 kilometres over a distance great enough to include several spreading segments such resolution is needed to address the long standing debate about the relative importance of dynamic and passive upwelling in the shallow mantle beneath spreading centres here we use rayleigh wave tomography to image the shear velocity in the upper 200 kilometres or so of the mantle low shear velocities similar to those beneath the east pacific rise oceanic spreading centre underlie the entire length of the gulf but there are three concentrated locations of anomalously low velocities spaced about 250 kilometres apart these anomalies are 40 to 90 kilometres beneath the surface at which depths petrological studies indicate that extensive melting of passively upwelling mantle should begin we interpret these seismic velocity anomalies as indicating that partial melting triggers dynamic upwelling driven by either the buoyancy of retained melt or by the reduced density of depleted mantle

regional mapping of the crustal structure in southern california from receiver functions lateral variations of the crustal structure in southern california are determined from receiver function rf studies using data from the southern california seismic network broadband stations and los angeles regional seismic experiment surveys the results include crustal thickness estimates at the stations themselves and where possible cross sections are drawn the large scale moho depth variation pattern generally correlates well with the current status of the mesozoic batholith deep moho of 35 39 km is observed beneath the western peninsula ranges sierra nevada and san bernardino mountains where the batholith is relatively intact and shallow moho of 26 32 km is observed in the mojave desert where the batholith is highly deformed and disrupted high resolution lateral variations of the crustal structure for individual geographic provinces are investigated and distinctive features are identified the crustal structure is strongly heterogeneous beneath the central transverse ranges and deep moho of 36 39 km is locally observed beneath several station groups in the western san gabriel mountains moho is relatively flat and smooth beneath the western mojave desert but gets shallower and complicated to the east anomalous rfs are observed at two stations in the eastern mojave desert where a moho step of 8 10 km is found between the nw and se back azimuthal groups of station dan in the fenner valley asymmetric extension of the salton trough is inferred from the moho geometry depth extension of several major faults such as the san andreas fault and san gabriel fault to the moho is inferred

site amplifications for generic rock sites seismic shear wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities and these velocities have been used to compute frequency dependent amplifications for zero attenuation for use in simulations of strong ground motion we define a genetic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data most of the boreholes are in populated areas for that reason the rock sites sampled are of particular engineering significance we consider two genetic rock sites rock corresponding to the bulk of the borehole data and very hard rock such as is found in glaciated regions in large areas of eastern north america or in portions of western north america the amplifications on rock sites can be in excess of 3 5 at high frequencies in contrast to the amplifications of less than 1 2 on very hard rock sites the consideration of unattenuated amplification alone is computationally convenient but what matters for ground motion estimation is the combined effect of amplification and attenuation for reasonable values of the attenuation parameter k 0 the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 hz with a maximum level of less than 1 8 the combined effect is about a factor of 1 5 at 1 hz and is less than unity for frequencies in the range of 10 to 20 hz depending on t using these amplifications we find provisional values of about ao 70 bars and k 0 0 035 sec for rock sites in western north america by fitting our empirically determined response spectra for an m 6 5 event to simulated values the borehole data yield shear velocities 033o of 618 and 306 rrdsec for rock and soil sites respectively when averaged over the upper 30 m from this we recommend that v30 equals 620 and 310 m sec for applications requiring the average velocity for rock and soil sites in western north america by combining the amplifications for rock sites and the site factors obtained from our analysis of strong motion data we derive amplifications for sites with 17 3o 520 m sec nehrp class c corresponding to a mix of rock and soil sites and v3o 310 and 255 m sec average soil and nehrp class d sites respectively for the average soil site the combined effect of amplification and attenuation exceeds a factor of 2 0 for frequencies between 0 4 and about 4 hz with a peak site effect of 2 4 the peak of the nehrp class d site effect is 2 7

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

imaging lateral heterogeneity in the northern apennines from time reversal of reflected surface waves summary prominent arrivals in the coda of seismograms from the wider alpine area can be associated with lateral reflections of love waves at the northern apennines mountain chain italy where structural heterogeneity causes an abrupt contrast in phase velocity we discuss an approach to image lateral heterogeneity from reflected surface waves using intermediate period threecomponent coda waveforms as sources for an adjoint wavefield that propagates the reflections backward in time we numerically compute three dimensional sensitivity kernels for the dependence of coda waveforms on p velocity s velocity and density based upon correlations between the adjoint and the regular forward wavefields we consider synthetic coda waveforms for a simplified model of the northern apennines as well as real coda observations from five moderate magnitude earthquakes m w 4 6 5 6 in the southern alps wave propagation is simulated using the spectral element method for which a 3 d regional earth model is used in the case of real data single and combined event sensitivity kernels provide clear images of the reflectivity associated with the northern apennines in kernels for density and s wave speed the kernels show that surface wave reflections occur near the axial zone of the mountain chain apart from the apennines the approach is able to image other smaller reflectivity patches from the coda waveforms like the ivrea zone in the southern alps our coda misfit kernels can be integrated in a gradient based waveform tomography where they could enhance the sharpness of the model at lateral discontinuities

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

regional crustal thickness variations of the peninsular ranges southern california we used the teleseismic receiver function technique to obtain a profile of the crustal thickness of the northern peninsular ranges california depth to the moho varies from 37 km beneath the western peninsular ranges batholith to 27 km at the western edge of the salton trough an average apparent dip of 10 to the west over a lateral distance of 60 km we previously obtained a similar result for a profile 100 km to the south a moho dip of 20 over 30 km lateral distance in both cases the moho depth variations do not correlate with topography of the eastern batholith but rather appear to parallel the trend of a boundary that separates compositionally distinct eastern and western terranes these observations suggest that a steeply dipping moho is a regional feature beneath the eastern peninsular ranges and that compensation is through lateral variations in crustal or upper mantle density rather than through an airy root

a seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

ground motion and intraevent variability from 3d deterministic broadband 0 7 5 hz simulations along a nonplanar strike slip faultground motion and intraevent variability from 3d deterministic broadband simulations

detailed evaluation of site response estimation methods across and along the sedimentary valley of volvi euro seistest using records from the euro seistest area near thessaloniki greece we studied empirically the amplification effects of a sedimentary valley one of the main advantages of this site is the very detailed knowledge of the near subsurface structure due to exhaustive geophysical and geotechnical measurements and data analyses jongmans et al 1998 the uniqueness of the seismological data set is the very dense network 31 stations were installed perpendicular and parallel to the valley axis with minimum interstation distances of 250 m the mean ampli fications along both axes estimated from 13 local events were imaged using the traditional spectral ratio technique variations of the mean amplifications due to the choice of different data subsets were tested by the use of up to 89 local events supplemented by four teleseismic events offering good low frequency resolution results obtained using the traditional spectral ratio technique were compared with results from a generalized inversion technique the h v ratio technique a coda wave technique and nakamura s technique advantages and disadvantages of each method are outlined especially from a practical point of view

a modified ground motion attenuation relationship for southern california that accounts for detailed site classification and a basin depth effect the attenuation relationship presented by boore et al 1997 has been evaluated and customized with respect to southern california strong motion data for peak ground acceleration pga and 0 3 1 0 and 3 0 sec period spectral acceleration this study was motivated by the recent availability of a new site classification map by wills et al 2000 which distinguishes seven different site categories for california based on the 1994 nehrp classification with few exceptions each of the five site types represented in the southern california strong motion database exhibit distinct amplification factors supporting use of the wills et al 2000 map for microzonation purposes following other studies a basin depth term was also found to be significant and therefore added to the relationship sites near the center of the la basin exhibit shaking levels up to a factor of 2 greater on average than otherwise equivalent sites near the edge relative to boore et al 1997 the other primary difference here is that pga exhibits less variation among the wills et al 2000 site types in fact the pga amplification implied by the basin depth effect is greater than that implied by site classification the model does not explicitly account for nonlinear sediment effects which if important will most likely influence rock site pga predictions the most evidence for a magnitude dependent variability or prediction uncertainty is also found and included as an option

a comparison of direct s wave and coda wave site amplification determined from aftershocks of the little skull mountain earthquake abstract site amplifications of direct s waves and coda waves are studied and compared using high quality three component digital data from aftershocks of the little skull mountain neveda earthquake we use data from 12 stations installed on a variety of geological and topographic site conditions distributed widely in space with different azimuths and epicentral distances s wave site amplifications are obtained in the frequency range from about 0 5 to over 30 hz while coda wave site amplifications are obtained in a frequency range from about 1 5 to over 30 hz a thorough statistical analysis of these results was performed we find that 1 for s waves all three components at a station follow a similar frequency dependent trend the amplitudes of the two horizontal components match more closely while the vertical component shows consistently lower amplification than the horizontal components at low frequencies 2 for coda waves all three components share both similar frequency dependent trends and amplification level 3 s wave and coda wave site amplification are consistent for stations with epicentral distance greater than about 10 km which is about the average focal depth of the earthquakes we used within the epicentral distance of 10 km however some stations show discordant s wave and coda wave site amplifications possible factors are that the direct s waves are affected by particular wave propagation paths and that at short distance sv is partitioned with more energy on the horizontal components and less on the vertical components than at larger distances

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

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

temporal and spatial variation on coda q 1 associated with the north palm springs earthquake of july 8 1986 a temporal and spatial change of codaq 1 associated with the occurrence of the north palm springs earthquake of july 8 1986 was studied by using 242 small local earthquakes in the vicinity of the mainshock we found that the codaq 1 of earthquakes which occurred before the mainshock was significantly higher than that of the aftershocks in the mainshock area while the codaq 1 for the surrounding area remained almost constant throughout 1986 codaq 1 was determined separately for the lapse time windows of 10 to 20 sec and 15 to 40 sec for the period from 1981 to 1987 the result for the time window 10 to 20 sec showed a peak in codaq 1 before the time of mainshock at all frequencies the peak appeared earlier at lower frequencies there was no significant change in codaq 1 for the time window 15 to 40 sec probably because the change was restricted to a small area

an organic geochemical profile of the toarcian anoxic event in northern italy abstract a suite of black shales calcareous shales and limestones representing the toarcian anoxic event jurassic 192 ma was collected from a 14 m length of core taken from the belluno trough northern italy and examined in detail to assess variations in depositional environment during this interval geochemical and optical data indicate organic matter of algal and bacterial origin with only trace contribution from terrestrial sources detailed biological marker data interpreted with the aid of multivariate statistics demonstrate fluctuations in primary producers and bacterial populations during black shale deposition the onset of the anoxic event was not abrupt but characterized by an apparent gradual development of anoxia and consequent enhanced organic matter preservation different hopane biomarkers proved especially useful in recognizing preservation potential in the section and correlate well with other preservation indicators increased oxygenation at the end of the event may have been associated with changes in primary productivity

inversion of strain dependent nonlinear characteristics of soils using weak and strong motions observed by borehole sites in japan during the 1995 hyogo ken nanbu earthquake strong motions with horizontal peak ground accelerations of 507 cm sec 2 or more 187 cm sec 2 and 126 cm sec 2 were observed at surface stations of borehole sites at amagasaki takasago and nanko in japan respectively we showed qualitative evidence of nonlinear soil response during strong motions at amgasaki and takasago but not at nanko using cross correlation and spectral ratio analyses of weak and strong motions we evaluated soil nonlinearity by inversion for s wave velocities and frequency dependent damping factors the strain dependence of shear modulus ratios inverted for gravel is larger than that for clay which is consistent with laboratory test results in the shear strain range from about 10 4 to 10 3 the strain dependence of shear modulus ratios inverted for clay and gravel at amagasaki and takasago is consistent with empirical relations based on laboratory test results for clay and gravel respectively these results support using laboratory estimates of shear modulus ratios to simulate strong motions at clay and gravel sites the damping factors for gravel inverted from the mainshock at amagasaki are the largest among the others but the strain dependence of damping factors are not as significant as that of shear modulus ratios the reason why the strain dependence of damping factors is not so significant may be because scattering attenuation is relatively dominated in the low strain range

complex rayleigh waves resulting from deep sedimentary basins abstract two earthquakes in northern mexico recorded by trinet stations in southern california show complex surface wave propagation through the salton trough stations contained in a narrow corridor extending from the salton trough north towards the central valley california show recorded waveforms with two rayleigh wave arrivals the time separation between the rayleigh wave arrivals is nearly constant beyond the northern edge of the salton trough we are able to reproduce both arrivals with a simple 1 d model of a low velocity deep basin in the salton trough the slower rayleigh wave is propagating within the sediments of the basin while the faster rayleigh wave propagates mainly in faster basement rock further we reproduce the constant time separation through the implementation of a simple 2 d model this 2 d model has a low velocity deep basin near the source and a crust without a basin at distances greater than 200 km

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

a comparison of coda and s wave spectral ratios as estimates of site response in the southern san francisco bay area abstract we calculate spectral ratios for s waves and codas to evaluate the amplification of 14 sites in the southern san francisco bay area relative to a nearby bedrock site in the coyote hills our data are seismograms written by loma prieta aftershocks the epicentral distances and azimuths to the stations are effectively the same all the sites in the study are amplified with respect to the reference site at frequencies from 0 5 to 7 hz the shapes of the s wave and coda spectral ratios are similar but the coda ratios are greater than the s wave ratios by as much as a factor of 4 the difference is larger for sites on alluvial and bay mud deposits particularly at frequencies around 1 hz suggesting the presence of waves trapped in the alluvial basin in general the length of the analysis window affects the s wave spectral ratios for alluvial sites longer windows give ratios similar to coda ratios apparently because these windows include more of the phases that contribute to the coda we classify the sites according to their geological characteristics and surficial shear wave velocities for the s wave ratios the differences between the classes showed no systematic trend the softest and hardest soil classes we consider have practically identical s wave amplifications the average coda ratios for the site classes clearly increase as the soil classes include slower and softer materials after correction for differences in reference sites the coda amplifications are very similar to the relative amplifications for these site classes estimated by borcherdt and glassmoyer 1992 from the strong motion recordings of the 1989 loma prieta earthquake

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

simultaneous study of the source path and site effects on strong ground motion during the 1989 loma prieta earthquake a preliminary result on pervasive nonlinear site effects we considered the effects of source propagation path and local site conditions on strong ground motion from the loma prieta earthquake in the following steps first the local site effect was removed from the strong motion records applying the weak motion amplification factors estimated from nearby stations of usgs regional network using the coda method then we confirmed the approximate validity of the 1 r law of geometrical spreading factor for the crustal model applicable to the region and we determined the attenuation parameter q f and source spectrum s f empirically from the site effect corrected spectral amplitude both q f and s f are consistent with the results obtained earlier for major california earthquakes we then synthesized the time history of ground acceleration by applying the stochastic simulation technique the agreement between the observed and predicted records was good for duration and spectral content we found however a strong systematic discrepancy with regard to peak acceleration namely for distances less than 50 km the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the franciscan formation sites since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter it is unlikely that the effect is due to the radiation pattern near source structure or topography it appears that the strong difference in the amplification factor between the franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0 1 to 0 3 g if this is due to the nonlinear response of the sediment sites it is in the range expected from geotechnical engineering studies

site amplification of coda waves from local earthquakes in central california the goals of this study are two fold 1 to examine the site effect of coda waves believed to be the average site effect of shear waves in order to understand its spatial and frequency dependent behavior and 2 to learn what we can about the processes that generate the coda itself we use digital data from over 90 earthquakes each recorded by a subset of some 150 stations in the coast ranges of central california between san francisco and san luis obispo results from the band 1 5 to 24 hz indicate that site amplification depends strongly on surface geology and frequency at low frequencies 1 5 to 3 0 hz changes in amplification up to a factor of 20 are observed amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and franciscan basement sites at high frequencies 6 to 24 hz the pattern changes many granite sites the majority in the gabilan range exhibit increasing amplification with frequency relative to the average station this behavior differs strikingly from that observed at sediment and franciscan sites which roll off at an intermediate rate and many sediment sites adjacent to the san andreas fault zone which decay away even more rapidly with frequency these results can be explained qualitatively by appealing to variable near site impedance and attenuation however complicating phenomena such as mode excitation in valley sediments must exist at low frequencies peaked behavior at a few hard rock sites at high frequency 12 hz currently lacks explanation at a few sediment sites the low frequency excitation is so strong that the coda envelopes begin to take on different shapes this contradicts the assumption based on previous observations that the coda shape is independent of source receiver location thus some site measurements may be artificially high the working model of coda waves as body wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near site resonance excited by incident direct and coda wave energy

crustal thickness variations beneath the peninsular ranges southern california we investigate the crustal thickness under the peninsular ranges using p to s converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method ps minus p times at sites west of a compositional boundary that separates the peninsular ranges batholith into east and west zones indicate a relatively flat deep moho ps minus p times at sites east of the compositional boundary decrease eastward moho depth estimates made from the ps delays and crustal velocities from seismic tomography indicate a relatively constant 36 to 41 km thick crust in the western zone in the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the salton trough a lateral distance of 30 km the lack of correlation between topography and moho depths suggests compensation via lateral density variations in the lower crust or upper mantle we propose that the compositional boundary decouples the eastern and western portions of the batholith and that the eastern portion has thinned in response to regional miocene extension or salton trough rifting or both

seismic structure beneath the gulf of california a contribution from group velocity measurements rayleigh wave group velocity dispersion measurements from local and regional earthquakes are used to interpret the lithospheric structure in the gulf of california region we compute group velocity maps for rayleigh waves from 10 to 150 s using earthquakes recorded by broad band stations of the network of autonomously recording seismographs in baja california and mexico mainland unm in mexico bor dpp and gor in southern california and tuc in arizona the study area is gridded in 120 longitude cells by 180 latitude cells with an equal spacing of 10 10 km assuming that each gridpoint is laterally homogeneous for each period the tomographic maps are inverted to produce a 3 d lithospheric shear wave velocity model for the region near the gulf of california rift axis we found three prominent low shear wave velocity regions which are associated with mantle upwelling near the cerro prieto volcanic field the ballenas transform fault and the east pacific rise upwelling of the mantle at lithospheric and asthenospheric depths characterizes most of the gulf this more detailed finding is new when compared to previous surface wave studies in the region a low velocity zone in northcentral baja at 28on which extends east south eastwards is interpreted as an asthenospheric window in addition we also identify a well defined high velocity zone in the upper mantle beneath central western baja california which correlates with the previously interpreted location of the stalled guadalupe and magdalena microplates we interpret locations of the fossil slab and slab window in light of the distribution of unique post subduction volcanic rocks in the gulf of california and baja california we also observe a high velocity anomaly at 50 km depth extending down to 130 km near the southwestern baja coastline and beneath baja which may represent another remnant of the farallon slab

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

site amplification and attenuation via downhole array seismogram inversion a comparative study of the 2003 miyagi oki aftershock sequence weak motion geotechnical array recordings at 38 stations of the japanese strong motion network kik net from the 2003 m w 7 0 miyagi oki aftershock sequence are used here to quantify the amplification and attenuation effects of near surface formations to incident seismic motion initially a seismic waveform optimization algorithm is implemented for the evaluation of high resolution low strain velocity v s attenuation q s and density profiles at the sites of interest based on the inversion results v s versus q s correlations are developed and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation surface to downhole traditional spectral ratios ssr cross spectral ratios c ssr and horizontal to vertical h v site response estimates are next evaluated and compared while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average vs of the upper 30 m v s30 of the formations single and reference station site response estimates are successively compared to surface to rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results comparison of the observed ssr and estimated surface to rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class site amplification factors are successively computed in reference to the national earthquake hazards reduction program nehrp b c boundary site conditions v s30 760 m sec and results are compared to published values developed on the basis of strong motion data and site response analyses finally weak motion ssr estimates are compared to the mainshock spectra and conclusions are drawn for the implications of soil nonlinearity in the near surface results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near surficial soil formations

the atakoy vertical array turkey insights into seismic wave propagation in the shallow most crustal layers by waveform deconvolution summary a vertical array of accelerometers was installed in atak oy western istanbul with the longterm aim of improving our understanding of in situ soil behaviour to assess the modelling and parametric uncertainties associated with the employed methodologies for strong motion site response analysis and for shallow geological investigations geotechnical and geophysical investigations were carried out to define the subsoil structure at the selected site data associated with 10 earthquakes 2 7 m 4 3 collected during the first months of operation of the array were used to image the upgoing and downgoing waves by deconvolution of waveforms recorded at different depths results have shown that the velocity of propagation of the imaged upgoing and downgoing waves in the borehole is consistent with that of s or p waves depending on the component of ground acceleration analysed but independent of the chosen signal window in particular an excellent agreement was found between the observed upgoing and downgoing wave traveltimes and the ones calculated using a model derived by seismic noise analysis of array data the presence of a smaller pulse on the waveforms obtained by deconvolution of the horizontal components suggests both internal s wave reflection and s to p mode conversion as well as a not normal incidence of the wavefield the presence of a pulse propagating with s wave velocity in the uppermost 25 m in the waveforms obtained by the deconvolution of the vertical components suggests p to s mode conversion these evidences imply that even when site amplification is mainly related to 1 d effects the standard practice in engineering seismology of deconvolving the surface recording down to the bedrock using an approximate s wave transfer function generally valid for vertical incidence of sh waves might lead to errors in the estimation of the input ground motion required in engineering calculations finally downgoing waves with significant amplitudes were found down to 70 m and even to 140 m depth this result provides a warning about the use of shallow borehole recordings as input for the numerical simulation of ground motion and for the derivation of ground motion prediction relationships

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

joint inversion of surface wave velocity and gravity observations and its application to central asian basins shear velocity structure 1 we implement and apply a method to the jointly inverted of surface wave group velocities and gravity anomalies observations surface wave dispersion measurements are sensitive to seismic shear wave velocities and the gravity measurements supply constraints on rock density variations our goal is to obtain a self consistent three dimensional shear velocity density model with increased resolution of shallow geologic structures we apply the method to investigate the structure of the crust and upper mantle beneath two large central asian sedimentary basins the tarim and junggar the basins have thick sediment sections that produce substantial regional gravity variations up to several hundred milligals we used gravity observations extracted from the global gravity model derived from the gravity recovery and climate experiment grace satellite mission we combine the gravity anomalies with high resolution surface wave slowness tomographic maps that provide group velocity dispersion values in the period range between 8 and 100 s for a grid of locations across central asia to integrate these data we use a relationship between seismic velocity and density constructed through the combination of two empirical relations one determined by nafe and drake most appropriate for sedimentary rocks and a linear birch s law more applicable to denser rocks the basement an iterative damped least squares inversion including smoothing is used to jointly model both data sets using shear velocity variations as the primary model parameters results show high upper mantle shear velocities beneath the tarim basin and suggest differences in lower crust and upper mantle shear velocities between the eastern and western tarim

mode of extension and rifting history of upper tiburon and upper delfin basins northern gulf of california 1 the crustal structure of the northern gulf of california transtensional margin has been investigated by a 280 km long nw se profile including deep multichannel seismic reflection and densely sampled refraction wide angle reflection seismic information combined with gravity modeling the seismic and gravity modeling constrains two thinned crustal areas corresponding to the upper delfin and the upper tiburon basins on both sides of the profile toward the baja california peninsula and the mexico mainland a progressive thickening of the continental crust is observed our results indicate that the crustal thickness is 19 km below the coastline and it decreases to 14 and 17 km below the upper delfin and upper tiburon basins respectively in the area between both basins the crust thickens to 19 5 km there are significant lateral thickness variations for the different levels of the crust the interpreted structure is consistent with the existence of an aborted rift below the upper tiburon basin prominent dipping reflections in the multichannel data under upper tiburon basin and the ridge between upper tiburon and upper delfin basins can be explained as a mylonite like zone related to a detachment fault this interpretation suggests that the structural evolution of upper tiburon basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust the mode and locus of extension have evolved from a core complex in upper tiburon to a narrow rift mode in upper delfin basin

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

unexpected values of qs in the unconsolidated sediments of the mississippi embayment we studied the attenuation of shear waves at three sites in the mississippi embayment using data recorded in boreholes drilled to depths of up to 60 m the source was a highly repeatable compressed air driven hammer to estimate attenuation we used a spectral ratio technique for fixed depth and variable frequency the best fit line for each depth z gives a measure of the cumulative attenuation indicated by z then we fit a straight line to z for a range of values of z the slope of this line gives an estimate of the average attenuation per distance and was used to determine an average q s for one of the sites newport northeastern arkansas q s ranges between 34 1 5 m z 44 2 m and 44 1 5 m z 51 8 m these values are significantly higher than the more typical value of about 10 determined for unconsolidated sediments by other authors in addition these high values correspond to sediments with low average shear wave velocity about 300 m sec in contrast average q s and velocity for sediments in shelby forest near memphis tennessee are 22 and 348 m sec 22 6 m z 60 1 m respectively therefore these results go against the conventional wisdom that low velocity implies low q for the third site marked tree northeastern arkansas average q s and velocity are 18 and 251 m sec 9 8 m z 33 6 m respectively this site is about 75 km from newport and the differences in attenuation appear related to differences in lithology manuscript received 5 april 2001

refined thresholds for non linear ground motion and temporal changes of site response associated with medium size earthquakes summary we systematically analyse non linear effects and temporal changes of site response associated with medium size earthquakes using seismic data recorded by the japanese strong motion network kik net we apply a sliding window spectral ratio technique to surface and borehole strong motion records at six sites and stack results associated with different earthquakes that produce similar peak ground acceleration pga in some cases we observe a weak coseismic drop in the peak frequency when the pga is as small as 20 30 gal and near instantaneous recovery after the passage of the direct s waves the percentage of drop in the peak frequency starts to increase with increasing pga values we also observe a coseismic drop in the peak spectral ratio for two sites when the pga is larger than 60 gal to more than 100 gal we observe considerably stronger drops of the peak frequencies followed by logarithmic recovery with time the observed weak reductions of peak frequencies with near instantaneous recovery likely reflect non linear response with essentially fixed level of damage while the larger drops followed by logarithmic recovery reflect the generation and then recovery of additional rock damage the results indicate clearly that non linear site response may occur during medium size earthquakes and that the pga threshold for in situ non linear behaviour is lower than the previously thought value of 100 200 gal

the cenomanian turonian oceanic anoxic event ii palaeoceanographic controls on organic matter production and preservation correlation of the 13c spike with the well dated occurrences of strata rich in organic carbon detailed in schlanger et al this volume indicates that a global episode of intense organic carbon orgc burial took place during the latest cenomanian earliest turonian oceanic anoxic event oae a plenus through i labiatus macrofossil zones and upper r cushmani trz through w archecretacea prz foraminiferal zones over a period of no more than 1 million years m y the shape of the 13c curve indicates that rates of orgc burial gradually increased in the early part of the late cenomanian increased more rapidly in the later cenomanian and levelled off at peak values in latest cenomanian early turonian time during the maximum rate of orgc burial the 13c values decreased nearly to pre late cenomanian levels in the early to middle turonian the decrease in 13c reflects decreasing rates of orgc burial following the cenomanian turonian oceanic anoxic event as well as the probable oxidation and return of significant amounts of orgc to the oceans following regression and re oxygenation of much of the deeper water masses in contact with the seafloor

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

simulation of broadband ground motion including nonlinear soil effects for a magnitude 6 5 earthquake on the seattle fault seattle washington the seattle fault poses a significant seismic hazard to the city of seattle washington a hybrid low frequency high frequency method is used to calculate broadband 0 20 hz ground motion time histories for a m 6 5 earthquake on the seattle fault low frequencies 1 hz are calculated by a stochastic method that uses a fractal subevent size distribution to give an 2 displacement spectrum time histories are calculated for a grid of stations and then corrected for the local site response using a classification scheme based on the surficial geology average shear wave velocity profiles are developed for six surficial geologic units artificial fill modified land esperance sand lawton clay till and tertiary sandstone these profiles together with other soil parameters are used to compare linear equivalent linear and nonlinear predictions of ground motion in the frequency band 0 15 hz linear site response corrections are found to yield unreasonably large ground motions equivalent linear and nonlinear calculations give peak values similar to the 1994 northridge california earthquake and those predicted by regression relationships ground motion variance is estimated for 1 randomization of the velocity profiles 2 variation in source parameters and 3 choice of nonlinear model within the limits of the models tested the results are found to be most sensitive to the nonlinear model and soil parameters notably the overconsolidation ratio

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

site amplification from coda waves validation and application to s wave site response abstract the subject of this article is to see if the response to s waves at a specific site can be determined from coda waves we used aftershock records of the 1992 landers and big bear earthquakes observed at four terrascope stations pas pfo svd and gsc in southern california the wide dynamic range of the terrascope instrument provided a broadband seismogram for s waves and at the same time coda waves over a long lapse time window we analyzed a total of 708 three component waveforms from 65 events whose magnitudes ranged from 3 3 to 5 4 the station svd is located on quaternary alluvium and the remaining stations are on rocks of cretaceous age we first evaluated the coda site amplification factor using a spectral ratio method may eda et al 1991 koyanagi et al 1992 for frequency bands centered at 0 75 1 0 1 5 2 0 3 0 4 0 and 6 0 hz for lapse time windows after 70 sec from the origin time we found that the site amplification factor for the radial component is similar to that for the transverse component this similarity was expected because codas consist of backscattered waves coming from all directions into the station resulting in a directionally averaged site response we then evaluated the s wave site amplification factor using an inversion method andrews 1986 on the direct s wave portion of the same data set we found that the coda site response agrees with the s wave site response within a factor of about 1 5 supporting the earlier findings by tsujiura 1978 and justifying the use of coda waves in determining site specific amplification for s waves this agreement between s wave site response and coda site response is understandable if we assume that coda waves are dominated by s waves as theoretically predicted by aki 1992 and zeng 1993

predicting nonlinear site response using spectral acceleration vs pgv vs30 a case history using the volvi test site in this study we analyze the efficiency of the ratio between particle velocity and shear wave velocity as a strain proxy for evaluating the nonlinear seismic response of sediments the in situ stress strain relationships are derived from accelerometric vertical array recordings at the tst site in volvi thessaloniki greece first the shear wave velocity between two successive sensors was computed by seismic interferometry and strain was computed as the velocity ratio or the relative displacement between sensors the shear wave velocity profile and in situ shear modulus degradation curve with strain were compared with previous studies performed at the tst site finally the stress strain relationships were derived from data recorded at the surface by extending the strain proxy and stress values to the ratio between peak ground velocity and the vs30 parameter used for site classification i e without requiring the accelerometric vertical array our model captures the in situ nonlinear response of the site without consideration of azimuth or distance of the earthquakes in conclusion the acceleration stress values based on the accelerometric response spectra instead of peak ground acceleration compared with the deformation strain proxy provide an effective model of the in situ nonlinear response providing information that can be integrated into ground motion prediction equations

attenuation and velocity structure for site response analyses via downhole seismogram inversion a seismic waveform inversion algorithm is proposed for the estimation of elastic soil properties using low amplitude downhole array recordings based on a global optimization scheme in the wavelet domain complemented by a local least square s fit operator in the frequency domain the hybrid scheme can efficiently identify the optimal solution vicinity in the stochastic search space whereas the best fit model detection is substantially accelerated through the local deterministic inversion results presented for selected aftershocks of the mw 7 0 sanriku minami earthquake in japan recorded by the kik net strong motion network illustrate robustness of the impedance structure estimation by contrast the attenuation structure is shown to be sensitive to the frequency content of seismic input data attributed to the deterministic description of the continuum in the forward model that cannot simulate late arrivals of multiple scattered energy sensitivity analyses illustrate that for the same forward model results can be substantially different based on the definition of the objective function it is concluded that even for engineering purposes inversion should aim to decouple intrinsic and scattering attenuation mechanisms

spectral analysis of k net and kik net data in japan part ii on attenuation characteristics source spectra and site response of borehole and surface stations abstract in this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the k net and kik net networks in japan in order to derive attenuation characteristics source spectra and site response for this purpose we use a total of more than 67 000 s wave records from 2178 earthquakes m jma 2 7 8 obtained at 1555 stations at the earth s surface and more than 29 000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m attenuation characteristics are investigated in five separate regions showing that crustal q depicts lower values in central compared to southern japan and a significant frequency dependence is observed in every region the source spectra follow the 2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones while strong amplification effects dominate the site contributions for the surface sensors those for the borehole sensors are characterized by smaller variability nevertheless consistent with observations from deconvolution of borehole surface recording pairs downgoing wave effects are visible in the site contributions for many borehole stations finally the site amplification functions obtained at the surface are compared with surface to borehole s b and horizontal to vertical h v spectral ratios showing that the s b ratios generally provide better estimates of the horizontal amplification than the h v ratios due to amplification of the vertical component of ground motion

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

what is a reference site many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a reference motion the critical assumption in these methods is that the surface rock site record reference is equivalent to the input motion at the base of the soil layers data collected in this study show that surface rock sites can have a site response of their own which could lead to an underestimation of the seismic hazard when these sites are used as reference sites data were collected from local and regional earthquakes on digital recorders both at the surface and in boreholes at two rock sites and one basin site in the san jacinto mountains southern california the two rock sites keenwild and pinon flat are located on granitic bedrock of the southern california peninsular ranges batholith the basin site garner valley is an ancestral lake bed with watersaturated sediments on top of a section of decomposed granite which overlies the competent bedrock ground motion is recorded simultaneously at the surface and in the bedrock at all three sites when the surface rock sites are used as the reference site i e the surface rock motion is used as the input to the basin the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 hz this underestimation by a factor of 2 to 4 depending on frequency and site results from the rock sites having a site response of their own above the 2 to 5 hz frequencies the near surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest even at sites that appear to be located on competent crystalline rock the bedrock borehole ground motion can be used as the reference motion but the effect of the downgoing wave field and the resulting destructive interference must be considered this destructive interference may produce pseudo resonances in the spectral amplification estimates if one is careful the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site

site effects and microzonation in the city of thessaloniki greece comparison of different approaches ten seismological stations were installed for 3 months in the city of thessaloniki greece to record earthquakes and ambient noise over areas of different geotechnical characteristics about 40 local earthquakes were recorded with a good signal to noise ratio in more than two stations we analyze the records with three different techniques a the classical spectral ratios to a reference station b the receiver functions and c the horizontal to vertical h v spectral ratios on noise recordings the results are compared in terms of predominant frequencies and amplification levels the three methods are found equally able to reveal the fundamental frequency regarding local site amplification they provide significantly different results with a general trend of the two h v ratio methods b and c to underestimate the amplification levels compared to that inferred from the classical spectral ratio technique a the results are then used in relation with the microzonation of the city of thessaloniki a good correlation is found between the local site amplification and the type and age of the local geological formation as well as with the depth of bedrock finally we compare our observations with the damage distribution in the city which was hit by a magnitude 6 5 earthquake in 1978 the amplification levels from spectral ratios obtained at each site exhibit a good agreement with the map of damage intensity

a mobile multi depth borehole sensor set up to study the surface to base seismic transfer functions abstract the best method to evaluate the seismic site response is by means of borehole vertical arrays that use earthquake records from different depths in this paper we introduce the implementation of a single borehole sensor system synchronized to a sensor on the surface that is fixed at variable depths within a single well this system is used for recording small amplitude earthquake signals at variable stiffness conditions in depth to compute empirical borehole transfer functions the computed average empirical borehole transfer functions allow the estimation of an s wave velocity model that is constrained using the frequency peak observed in the h v ratio curve pairs of surface and borehole earthquake records were obtained with the borehole sensor placed at 10 20 50 and 100 m depth in a test site in managua nicaragua the average velocity of the final model down to 100 m appeared to be in good agreement with the average velocity computed via cross correlation using the surface and borehole signals likewise an inverted masw profile and h v ratio at the same site agree with the s wave velocity model obtained

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

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

the mechanisms of earthquakes and faulting in the southern gulf of california abstract accurate earthquake locations and their focal mechanisms can illuminate the distribution and mode of deformation at rifted continental margins the pacific north america pa na plate boundary within the gulf of california goc provides an excellent opportunity to explore the evolution and kinematics of rifting as continental extension in the north transitions to seafloor spreading in the south from october 2005 to october 2006 an array of eight four component ocean bottom seismographs deployed in the goc recorded seismicity as part of the sea of cortez ocean bottom array scooba experiment by combining the data with those from the onshore network of autonomously recording seismographs nars baja array we detect and locate 700 earthquakes m w 2 2 6 6 mainly along the northwest southeast striking transform faults that delineate the plate boundary for 36 events m w 3 5 6 6 with high signal to noise ratio in a long period 10 20 s band we determine deviatoric moment tensors and associated double couple focal mechanisms by regional waveform inversion many focal mechanisms are consistent with right lateral strike slip faulting along the pa na transform fault system which suggests that the transform faults primarily accommodate seismic deformation within the gulf in addition we capture a swarm of events on baja california along the right lateral northwest southeast striking las viboras el azufre fault which may be related to ongoing geothermal activity and volcanic deformation within the peninsula the combination of high resolution earthquake locations and focal mechanisms improves our understanding of the distribution of seismic deformation within the greater extensional zone in the southern goc online material earthquake catalog of the 695 events detected and located during our combined on offshore deployment of seismometers

spectral amplification in a sediment filled valley exhibiting clear basin edge induced waves various site response estimates are presented for a linear array deployed in the coachella valley california during the 1992 landers big bear aftershock sequence this systematic comparison is unique in that the response of the site is clearly dominated by basin edge induced waves average sediment to bedrock spectral ratios for long s wave windows which include the basin edge induced waves exhibit amplification factors as high as 18 below 7 hz the deep basin structure which gives rise to the obvious multi dimensional effects produces a fundamental resonant peak that shifts between basin sites from 0 23 to 0 48 hz as the depth to bedrock changes above 0 6 hz where the largest amplifications occur the response is remarkably similar between sites and appears to be dominated by a near surface layer that is relatively uniform across the valley the apparent fundamental resonant frequency of this layer is between 0 8 and 1 hz sediment to bedrock spectral ratios computed using shorter windows that exclude the basin edge induced waves imply that the multi dimensional effects are significant only below 4 hz where they increase amplifications by an approximate factor of 2 spectral ratios computed using coda windows taken at twice the s wave travel time exhibit amplifications that are an average factor of 1 7 greater between 1 and 4 hz than those of the s wave estimates this discrepancy does not improve by taking coda windows later at four times the s wave travel time horizontal to vertical component s wave spectral ratios do not agree with the sediment to bedrock ratios however they do exhibit a clear peak at the fundamental resonant frequency of the deep basin structure sediment to bedrock spectral ratios of ambient seismic noise are also inconsistent with the s wave estimates however horizontal to vertical noise ratios exhibit clear peaks near the fundamental resonant frequencies of both the deep basin structure below 0 6 hz and the suspected near surface layer between 0 8 and 1 hz therefore ambient noise data appear to provide valuable constraints on the basin structure ongoing efforts involve multi dimensional modeling of the observed basin edge induced phases and resonant frequencies

garner valley downhole array of accelerometers instrumentation and preliminary data analysis the garner valley downhole array gvda in southern california 33 41 60 n 116 40 20 w is a five element array of three component dual gain force balance accelerometers that are capable of measuring accelerations from 3 10 6 to 2 0 g over a frequency range from 0 0 to 100 hz the accelerometers are placed at depths of 0 6 15 22 and 220 m the lithostratigraphy is 19 m of soil overlying a 24 m layer of weathered granite below which is granite tonalite the array only 7 km from the san jacinto fault is located at the northern end of the anza seismic gap on the san jacinto fault where an m 6 5 or greater earthquake can be expected it is only 35 km from the indio segment of the san andreas fault which last ruptured with an m 8 earthquake around 1700 a d from its installation in july 1989 through july 1991 gvda has recorded 280 earthquakes with magnitudes ranging from 1 2 to 4 7 and epicentral distances ranging from from 0 to 110 km the maximum acceleration recorded was 89 cm sec 2 from an m l 4 2 earthquake 15 8 km hypocentral distance from gvda the average amplification of the apparent seismic moment between 220 m depth and the surface is about 13 for over five orders of magnitude in the size of the recorded earthquakes the spectral ratio 0 220 of horizontal acceleration amplitude spectrum shows a mean amplification of about 10 for the frequency range of 2 0 to 30 hz for 17 events resonance peaks exist at about 1 7 3 0 and 12 0 hz where the spectral ratio 0 220 is nearly 40 analysis of the acceleration spectra of two specific earthquakes that have nearly the same hypocenter but with different magnitudes m 4 2 and 2 5 shows that the weathered granite zone beneath the soil strongly attenuates the spectral amplitudes for frequencies greater than 40 hz the impedance of the soil relative to the weathered granite amplifies all frequencies by about a factor of 3 leading to spectral levels that are greater at the surface than at 22 m depth near the top of the weathered granite the acceleration spectra at each depth show resonant peaks at the same frequencies the spectra at each depth are well correlated for both earthquakes the acceleration spectrum at 220 m is nearly constant for frequencies less than 60 hz which implies that there is little or no attenuation

amplification factors for spectral acceleration in tectonically active regions empirical relationships are developed to predict amplification factors for 5 damped response spectral acceleration period range t 0 01 5 sec as a function of site category we evaluate amplification factors by normalizing response spectral accelerations computed from recordings by reference spectral accelerations derived from a modified attenuation relationship for active regions strong motion sites are categorized according to several schemes which are based on surface geology age only age depositional environment and age material texture shallow 30 m shear wave velocity and geotechnical data criteria for selection of the optimal classification scheme are that the amplification models for categories within the scheme 1 minimize the global dispersion of prediction residuals and 2 are significantly distinct across a broad period range the results of the regressions indicate that the greatest levels of distinction in amplification levels across categories occur for the shallow shear wave velocity scheme but that the dispersion of residuals for this scheme are relatively high for soil sites although not for rock sites conversely schemes based on detailed surface geology generally have the smallest dispersion for soil sites but amplification levels across quaternary categories are only significantly distinct at small periods these findings suggest that detailed surface geology provides an effective means of soil site categorization at small periods whereas shallow shear wave velocity provides an effective means for rock site categorization at small periods at longer periods none of the schemes are optimized relative to both the dispersion and distinction criteria the principal application of the amplification factors is as a modifier of attenuation relations much like a site term the present results significantly reduce the bias associated with the site terms in a widely used attenuation relationship for rock and soft soil site conditions manuscript received 25 february 2002

crustal thickness variations in the margins of the gulf of california from receiver functions receiver functions rfs from teleseismic events recorded by the nars baja array were used to map crustal thickness in the continental margins of the gulf of california a newly forming ocean basin although the upper crust is known to have split apart simultaneously along the entire length of the gulf little is known about the behaviour of the lower crust in this region the rfs show clear p to s wave conversions from the moho beneath the stations the delay times between the direct p and p to s waves indicate thinner crust closer to the gulf along the entire baja california peninsula the thinner crust is associated with the eastern peninsular ranges batholith prb crustal thickness is uncorrelated with topography in the prb and the moho is not flat suggesting mantle compensation by a weaker than normal mantle based on seismological evidence the approximately w e shallowing in moho depths is significant with extremes in crustal thickness of 21 and 37 km similar results have been obtained at the northern end of the gulf by lewis et al who proposed a mechanism of lower crustal flow associated with rifting in the gulf extensional province for thinning of the crust based on the amount of pre pliocene extension possible in the continental margins if the lower crust did thin in concert with the upper crust it is possible that the crust was thinned during the early stages of rifting before the opening of the ocean basin in this case we suggest that when breakup occurred the lower crust in the margins of the gulf was still behaving ductilely alternatively the lower crust may have thinned after the gulf opened the implications of these mechanisms are discussed

the scec southern california reference three dimensional seismic velocity model version 2 we describe version 2 of the three dimensional 3d seismic velocity model of southern california developed by the southern california earthquake center and designed to serve as a reference model for multidisciplinary research activities in the area the model consists of detailed rule based representations of the major southern california basins los angeles basin ventura basin san gabriel valley san fernando valley chino basin san bernardino valley and the salton trough embedded in a 3d crust over a variable depth moho outside of the basins the model crust is based on regional tomographic results the model moho is represented by a surface with the depths determined by the receiver function technique shallow basin sediment velocities are constrained by geotechnical data the model is implemented in a computer code that generates any specified 3d mesh of seismic velocity and density values this parameterization is convenient to store transfer and update as new information and verification results become available

the cenomanian turonian oceanic anoxic event i stratigraphy and distribution of organic carbon rich beds and the marine δ13c excursion marine strata deposited during late cenomanian and early turonian time display lithological faunal and geochemical characteristics which indicate that significant parts of the world ocean were periodically oxygen deficient at or very close to the cenomanian turonian boundary between 90 5 and 91 5ma ago oxygen deficiencies were particularly marked over a period of 1ma this short lived episode of oceanic oxygen deficiency has been termed the cenomanian turonian oceanic anoxic event oae the widespread distribution of anoxic sediments deposited synchronously during such a short lived event indicates that such sediments are not simply the product of coincidental local climatic or basinal water mass characteristics but are the result of a global expansion and intensification of the cenomanian turonian oxygen minimum zone related to feedback between sea level rise and regional palaeoceanography the palaeoceanography of the cenomanian turonian oae is discussed in detail in a companion paper by arthur et al 1987 from authors

location and mechanism of the little skull mountain earthquake as constrained by satellite radar interferometry and seismic waveform modeling we use interferometric synthetic aperture radar insar and broadband seismic waveform data to estimate source parameters of the 29 june 1992 m s 5 4 little skull mountain lsm earthquake this event occurred within a geodetic network designed to measure the strain rate across the region around yucca mountain the lsm earthquake complicates interpretation of the existing gps and trilateration data as the earthquake magnitude is sufficiently small that seismic data do not tightly constrain the epicenter but large enough to potentially affect the geodetic observations we model the insar data using a finite dislocation in a layered elastic space we also invert regional seismic waveforms both alone and jointly with the insar data because of limitations in the existing data set insar data alone cannot determine the area of the fault plane independent of magnitude of slip nor the location of the fault plane independent of the earthquake mechanism our seismic waveform data tightly constrain the mechanism of the earthquake but not the location together the two complementary data types can be used to determine the mechanism and location but cannot distinguish between the two potential conjugate fault planes our preferred model has a moment of 3 2 10 17 n m m w 5 6 and predicts a line length change between the wahomie and mile geodetic benchmarks of 5 mm

crustal structure of the borderland continent transition zone of southern california adjacent to los angeles we use data from the onshore offshore component of los angeles region seismic experiment larse to model the broad scale features of the midcrust to upper mantle beneath a north south transect that spans the continental borderland in the los angeles california region we have developed an analysis method for wide angle seismic data that consists primarily of refractions lacks near offset recordings and contains wide gaps in coverage although the data restrict the analysis to the modeling of broad scale structure the technique allows one to explore the limits of the data and determine the resolving power of the data set the resulting composite velocity model constrains the crustal thickness and location and width of the continent borderland transition zone we find that the mid to lower crust layer velocities of the inner borderland are slightly lower than the corresponding layers in the average southern california crust model while the upper mantle velocity is significantly higher the data require the moho to deepen significantly to the north we constrain the transition zone to initiate between the offshore slope and the southwest los angeles basin if the inner borderland crust is 22 km thick then the transition zone is constrained to initiate within a 2 km wide region beneath the southwest los angeles basin and have a width of 20 25 km the strong coherent and continuous pn phase suggests the moho is coherent and laterally continuous beneath the inner borderland and transition zone the inner california borderland seems to be modified and thickened oceanic crust with the oceanic upper mantle intact beneath it

surface and near surface effects on seismic waves theory and borehole seismometer results a simple plane wave model is adequate to explain many surface versus borehole seismometer data sets using such a model we present a series of examples which demonstrate the effects of the free surface near surface velocity gradients and low impedance surface layers on the amplitudes of upcoming body waves in some cases these amplitudes are predictable from simple free surface and impedance contrast expressions however in other cases these expressions are an unreliable guide to the complete response and the full plane wave calculation must be performed large surface amplifications are possible even without focusing due to lateral heterogeneities or nonlinear effects both surface and borehole seismometer site responses are almost always frequency dependent ocean bottom versus borehole seismic data from the 1983 ngendei seismic experiment in the southwest pacific are consistent with both a simple plane wave model and a more complete synthetic seismogram calculation the borehole seismic response to upcoming p waves is reduced at high frequencies because of interference between the upgoing p wave and downgoing p and sv waves reflected from the sediment basement interface however because of much lower borehole noise levels the borehole seismometer enjoys a p wave signal to noise advantage of 3 to 7 db over nearby ocean bottom instruments

multimethod characterization of the french pyrenean valley of bagneres de bigorre for seismic hazard evaluation observations and models a narrow rectilinear valley in the french pyrenees affected in the past by damaging earthquakes has been chosen as a test site for soil response characteriza tion the main purpose of this initiative was to compare experimental and numerical approaches a temporary network of 10 stations has been deployed along and across the valley during two years parallel various experiments have been conducted in particular ambient noise recording and seismic profiles with active sources for struc ture determination at the 10 sites classical observables have been measured for site amplification evaluation such as spectral ratios of horizontal or vertical motions between site and reference stations using direct s waves and s coda and spectral ratios between horizontal and vertical h v motions at single stations using noise and s coda records vertical shear velocity profiles at the stations have first been obtained from a joint inversion of rayleigh wave dispersion curves and ellipticity they have subsequently been used to model the h v spectral ratios of noise data from synthetic seismograms the h v ratio of s coda waves based on equipartition theory and the 3d seismic response of the basin using the spectral element method general good agreement is found between simulations and observations the 3d simulation reveals that topography has a much lower contribution to site effects than sedimentary filling except at the narrow ridge crests we find clear evidence of a basin edge effect with an increase of the amplitude of ground motion at some distance from the edge inside the basin and a decrease immediately at the slope foot

tau p seismic data for viscoelastic media part 2 linearized inversion1 an approach to extraction of viscoelastic parameters from seismic data is implemented and succesfully tested viscoelastic inversion is performed using adaptive damping factors to control the sensitivity of the viscoelastic parameters in relation to the p seismic data a priori information is incorporated through the damping factors as standard deviations of the data and of the viscoelastic model parameters the stability of the inversion process is controlled by the variation of the damping factors as a function of the residual errors and parameter updates at each iteration tests on synthetic and real data show that p and s wave quality factors qp and qs in addition to p and s wave velocities and density cp cs and p can be extracted from p seismic information singular value decomposition analysis demonstrates that estimated qp and qs values are more affected by the presence of data inaccuracies and noise than are those of cp and p cs and qs are not uniquely recovered due to the limited contribution of p s converted waves knowledge of the viscoelastic parameters is of particular importance in accurately describing petrophysical properties of rocks and pore fluids existing in the subsurface this is demonstrated with real data from the gulf of mexico

ground motion amplification in the marina district abstract following the 18 october 1989 loma prieta earthquake triggered seismographs were deployed in and around the marina district to investigate site amplification in the 3 weeks after the mainshock 16 aftershocks were recorded by two or more of these stations two additional stations deployed in march 1990 recorded three earthquakes that occurred in contra costa county by recasting the method of spectral ratios into a generalized inverse problem we combine the shear wave spectra from these 19 aftershocks to estimate the relative site amplification as a function of frequency all five stations located in the marina district are amplified by factors of 6 to 10 at 1 hz and 2 to 4 at 3 hz relative to a station located on franciscan sandstone at fort mason irrespective of whether they are sited on artificial fill or beach sand two stations located on dune sand just outside the marina district are amplified by factors of 2 to 4 for frequencies above 1 hz four stations located on franciscan sandstone in pacific heights nob hill rincon hill and diamond heights show little relative amplification conditional estimates of the mainshock spectra can be determined at stations that only recorded aftershocks if they can be linked to stations that recorded the mainshock these estimates are predicated on the assumption that the ground behaved linearly in the mainshock an assumption that is clearly violated for those sites where there was ground failure using this method of extrapolation for a site in the marina district that showed no evidence of ground failure yields mainshock spectra that are slightly greater than the mainshock spectra from the outer harbor wharf in oakland and near the pacific park plaza in emeryville and that significantly exceed the mainshock spectra from all the free field accelerograms located in san francisco

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

seismological structure of the upper mantle a regional comparison of seismic layering abstract we investigate seismic layering i e discontinuities regions of anomalous velocity gradients and anisotropy and its lateral variability in the upper mantle by comparing seismic models from three tectonic regions old 100 ma pacific plate younger 40 ma philippine sea plate and precambrian western australia these models were constructed by combining two data sets scs reflectivity profiles which provide travel times and impedance contrasts across mantle discontinuities and observations of frequency dependent travel times of three component turning s ss ss sss sss sa and surface r1 g1 waves which constrain the anisotropic velocity structure between discontinuities the models provide a better fit to observed seismograms from these regions than the current generation of global tomographic models the australian model is characterized by high shear velocities throughout the upper 350 km of the mantle with no low velocity zone lvz in the isotropically averaged shear velocities in contrast the oceanic models are characterized by a thin high velocity seismic lid underlain by a distinct lvz with a sharp boundary the g discontinuity separating them the g is significantly deeper beneath the western philippine sea plate than beneath the older pacific 89 and 68 km respectively implying that thermal cooling alone does not control the thickness of the lid we interpret this discontinuity as a compositional boundary marking the fossilized base of the melt separation zone msz active during sea floor spreading no discontinuity is detected at the base of the lvz in the oceanic models the s velocity gradient between 200 and 410 km depth is much steeper in the oceans than beneath australia this high oceanic gradient is probably controlled by a decrease in the homologous temperature over this depth interval the relative depths of the transition zone tz discontinuities are consistent with clapeyron slopes expected for an olivine dominated mineralogy the 660 km discontinuity displays variability in its amplitude that appears to correlate with its depth shallow and bright beneath the pacific deep and dim beneath australia and the philippine sea such behavior is possibly caused by the juxtaposition of the olivine and garnet components of the phase transition radial anisotropy extends through the upper 250 km of the mantle in the australia model and through the upper 160 km of the two oceanic models the magnitude of anisotropy is consistent with that expected for models of horizontally oriented olivine and the localization of anisotropy in the shallowest upper mantle implies that it reflects strain associated with past or present tectonic events

site response estimation from earthquake data abstract aftershocks of the 1989 loma prieta california earthquake are used to estimate site response along the san francisco peninsula a total of 215 shear wave records from 24 sources and 21 sites are used in a linear inversion for source and site response spectra the methodology makes no assumptions about the shape of the source spectrum however to obtain a stable unique inverse a q model and geometrical spreading factor are assumed as well as a constraint on site response that sets the site response averaged over two specific stations to 1 0 site responses calculated by this formulation of the problem are compared with other studies in the same region that use different methodologies and or data the shear wave site responses compare favorably with estimates based on an 2 constrained source model comparison with coda amplification factors is not as close but still favorable considering that the coda values were determined for nearby locations with similar geology and not the same sites the degree of agreement between the three methods is encouraging considering the very different assumptions and data used

determination of shallow s wave attenuation by down hole waveform deconvolution a case study in istanbul turkey summary the estimation of shear wave velocity and attenuation in near surface geology is of primary importance in engineering seismology in fact their knowledge is essential for site response studies when preparing improved seismic hazard scenarios in this study we propose two approaches for estimating the average shear wave quality factor qs by using recordings of a vertical array of accelerometers the methods are mainly based on the deconvolution of the wavefield recorded in a borehole with that recorded at the surface the first method requires the fourier transform of the deconvolved wavefield to be fitted with a theoretical transfer function valid for the vertical or nearly vertical in the case at hand up to 30 incidence angle propagation of s waves the second method is based on the spectral fitting of the fourier transform of only the acausal part of the deconvolved wavefield with a theoretical transfer function both methods can be applied without any prior knowledge of the subsoil structure since they are based on empirical data analysis and do not require a precise knowledge of the azimuthal orientation of the sensors in the boreholes which is seldom available first we describe the theoretical framework of the proposed methodologies for qs estimation which are based on the assumption that the structure in the borehole is weakly heterogeneous in the vertical direction i e no large impedance contrast exists between the borehole sensor and the surface second by using synthetic accelerograms we verify that in a realistic subsoil structure the assumption of vertical homogeneity can hold and we investigate the robustness and the suitability of the proposed methods finally only the method that was shown to provide the more stable results based on fitting the borehole to surface spectral ratio with a theoretical function is applied to earthquakes signals recorded by a vertical array of accelerometers installed in atak oy western istanbul results show that using borehole data provides a fair and robust estimate of an average qs of about 30 46 and 99 for the 0 50 0 70 0 140 m depth ranges respectively that can be used for numerical simulations of ground motion