simulated ground motions for hypothesized mw 8 subduction earthquakes in washington and oregon the amplitude and duration of strong ground motions from hypothesized mw 8 subduction zone thrust earthquakes in the puget sound portland region were estimated using a semi empirical method the simulation procedure assumes the rupture surface may be represented by a grid of fault elements finite difference wave simulation in a detailed 2 d velocity structure identifies direct s and the moho postcritical reflection as the primary components of the s wave field at horizontal distances of 0 to 150 km green s functions containing these two arrivals are computed with generalized ray theory in an equivalent 1 d structure for each source element receiver propagation path scattering and attenuation structure are empirically modeled by the use of corrected accelerograms from mw 7 michoacan mexico and valparaiso chile aftershocks for the fault element source functions different site conditions are empirically included in the simulation procedure by using empirical source functions recorded on rock and soil sites spatial variations in slip on the fault asperities are introduced by weighting the fault elements the technique has been validated for large subduction zone earthquakes by modeling acceleration time histories and response spectra from the 1985 michoacan mw 8 0 and valparaiso mw 8 0 main shocks somerville et al 1991 fault models for the puget sound and portland regions and seismic velocity structure models have been adapted from published regional studies uncertainty in the location of the asperities on the fault surface results in a large degree of uncertainty in the simulated ground motions at a given site if distance is defined as the distance to the closest asperity then the variability in the ground motions is reduced indicating that this uncertainty can be lessened by constraining the depth of the asperity the ground motion estimates are relatively insensitive to the difference in fault dip between the puget sound and portland fault models the attenuation of peak acceleration with distance r from the fault asperity for a mw 8 subduction earthquake is given by in pga 1 5 3 33 in r 128 0 79 s where s is a site term equal to 0 for rock and 1 for soil when distance r is the distance to the fault plane the attenuation relation is in pga 2 8 1 26 in r 0 79 s formal estimates of uncertainty in the calculated ground motions have been obtained by estimating both parametric uncertainty from the range of source models of hypothesized cascadia subduction earthquakes and modeling and random uncertainty from the misfit between recorded and simulated ground motions of the 1985 michoacan and valparaiso earthquakes for periods less than 1 sec the estimated response spectral velocities on soil sites in the seattle olympia region are about twice those recorded during the 1949 olympia and 1965 seattle wadati benioff zone earthquakes and the durations of strong motion on rock sites are significantly longer 45 to 60 sec versus 10 to 15 sec

active tectonics of the seattle fault and central puget sound washington implications for earthquake hazards we use an extensive network of marine high resolution and conventional industry seismic reflection data to constrain the location shallow structure and displacement rates of the seattle fault zone and crosscutting high angle faults in the puget lowland of western washington analysis of seismic profiles extending 50 km across the puget lowland from lake washington to hood canal indicates that the west trending seattle fault comprises a broad 4 6 km zone of three or more south dipping reverse faults quaternary sediment has been folded and faulted along all faults in the zone but is clearly most pronounced along fault a the northernmost fault which forms the boundary between the seattle uplift and seattle basin analysis of growth strata deposited across fault a indicate minimum quaternary slip rates of about 0 6 mm yr slip rates across the entire zone are estimated to be 0 7 1 1 mm yr the seattle fault is cut into two main segments by an active north trending high angle strike slip fault zone with cumulative dextral displacement of about 2 4 km faults in this zone truncate and warp reflections in tertiary and quaternary strata and locally coincide with bathymetric lineaments cumulative slip rates on these faults may exceed 0 2 mm yr assuming no other crosscutting faults this north trending fault zone divides the seattle fault into 30 40 km long western and eastern segments although this geometry could limit the area ruptured in some seattle fault earthquakes a large event ca a d 900 appears to have involved both segments regional seismic hazard assessments must 1 incorporate new information on fault length geometry and displacement rates on the seattle fault and 2 consider the hazard presented by the previously unrecognized north trending fault zone

the 23 19 aftershock of the 15 october 1979 imperial valley earthquake more evidence for an asperity the well recorded strong ground motion data for the 23 19 aftershock of the 15 october 1979 imperial valley earthquake provide a good opportunity to study the high frequency source characteristics and the path effects at near source distances the best fitting point source model has a strike slip mechanism n40 w which is nearly identical to the main event the estimated stress drop is extremely high roughly 500 bars with a triangular time history 0 1 0 1 sec but with a moment of 1 0 10 24 dyne cm a double source model found by inversion fits the high frequency data better but indicates complex faulting the first source with strike n319 e dip 42 ne and slip angle 165 has a moment of 0 7 10 24 dyne cm the second source with strike n324 e dip 82 sw and slip angle 181 lies about 0 5 km to the north and has a seismic moment twice that of the first source source dimensions appear very small for this amount of energy release many of the anomalous behaviors observed at certain stations for the main event are also present in the aftershock data these features are examined in terms of path effects

characteristics of observed peak amplitude for strong ground motion from the 1995 hyogoken nanbu kobe earthquake over 200 peak amplitudes of strong motion were observed at distances of less than 250 km from the fault during the 1995 hyogo ken nanbu kobe earth quake we analyzed the attenuation of the peak ground acceleration and velocity as a function of distance and geological site conditions the observed peak amplitudes agree well with those predicted by an empirical attenuation relation that was devel oped for japanese earthquakes this demonstrates that on average the peak amplitude of the ground motion generated by this damaging earthquake did not exceed the level predicted by the empirical attenuation relation we found a significant effect of the surface geology on the observed ground motion peak amplitude in particular for soft soil sites located near the fault the peak horizontal acceleration decreases rap idly with distance as a result of the nonlinear response of soils in order to take into account the effect of the site conditions we introduced correction factors to the ex isting attenuation relation this resulted in a significant reduction of the residuals between the predicted and observed peak amplitudes based on the attenuation re lation corrected for the site condition effect we generated a map of horizontal peak ground acceleration in the kobe and osaka area for the kobe earthquake the area of simulated large ground motion agrees well with the severe damage zone of inten sity vii jma scale

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

site response shallow shear wave velocity and wave propagation at the san jose california dense seismic array ground motion records from a 52 element dense seismic array near san jose california are analyzed to obtain site response shallow shear wave velocity and plane wave propagation characteristics the array located on the eastern side of the santa clara valley south of the san francisco bay is sited over the evergreen basin a 7 km deep depression with miocene and younger deposits site response values below 4 hz are up to a factor of 2 greater when larger regional records are included in the analysis due to strong surface wave development within the santa clara valley the pattern of site amplification is the same however with local or regional events site amplification increases away from the eastern edge of the santa clara valley reaching a maximum over the western edge of the evergreen basin where the pre cenozoic basement shallows rapidly amplification then decreases further to the west this pattern may be caused by lower shallow shear wave velocities and thicker quaternary deposits further from the edge of the santa clara valley and generation trapping of surface waves above the shallowing basement of the western evergreen basin shear wave velocities from the inversion of site response spectra based on smaller local earthquakes compare well with those obtained independently from our seismic reflection refraction measurements velocities from the inversion of site spectra that include larger regional records do not compare well with these measurements a mix of local and regional events however is appropriate for determination of site response to be used in seismic hazard evaluation since large damaging events would excite both body and surface waves with a wide range in ray parameters frequency wavenumber plane wave analysis is used to determine the backazimuth and apparent velocity of coherent phases at the array conventional high resolution and multiple signal characterization f k power spectra and stacked slowness power spectra are compared these spectra show surface waves generated scattered at the edges of the santa clara valley and possibly within the valley at the western edge of the evergreen basin

cave ice the imminent loss of untapped mid latitude cryospheric palaeoenvironmental archives abstract cave hosted ice bodies have been reported to suffer significant mass loss worldwide although some successful achievements were reported recently to confirm the potential of stable isotope pollen or trace element records in cave glaciers for climate and environmental reconstructions these archives are still weakly studied this brief paper is to be an invitation to the geoscience community to face this research need and devote more scientific attention to the complex research of cave ice deposits before accelerated melt will determine the complete loss of the unique palaeoenvironmental information stored in these deposits

nonlinear and linear site response and basin effects in seattle for the m 6 8 nisqually washington earthquake we used recordings of the m 6 8 nisqually earthquake and its m l 3 4 aftershock to study site response and basin effects for 35 locations in seattle washington we determined site amplification from fourier spectral ratios of the recorded horizontal ground motions referenced to a soft rock site soft soil sites generally national earthquake hazard reduction program nehrp class e on artificial fill and young alluvium have the largest 1 hz amplifications factors of 3 7 for both the mainshock and aftershock these amplifications are correlated with areas of higher damage from the mainshock to major buildings and liquefaction there are several indications of nonlinear response at the soft soil sites for the mainshock ground motions despite relatively modest peak accelerations in the s waves of 15 22 g first the mainshock spectral ratios do not show amplification at 2 8 hz as do the aftershock spectral ratios spectral peaks at frequencies below 2 hz generally occur at lower frequencies for the mainshock spectral ratios than for the aftershock ratios at one soft soil site there is a clear shift of the resonant frequency to a lower frequency for the mainshock compared with the aftershock the frequency of this resonance increases in the coda of the mainshock record indicating that the site response during the weaker motions of the coda is more linear than that of the initial s wave three of the soft soil sites display cusped one sided mainshock accelerograms after the s wave these soft soil sites also show amplification at 10 20 hz in the s wave relative to the rock site that is not observed for the aftershock the cusped waveforms and 10 20 hz amplification are symptomatic of nonlinear response at the soft soil sites these sites had nearby liquefaction the largest amplifications for 0 5 hz occur at soft soil sites on the southern portion of the seattle basin stiff soil sites nehrp classes d and c on pleistocence age glacial deposits display similar spectral amplification for the mainshock and aftershock indicating approximately linear response the stiff soil sites generally have moderate amplification factors of 1 1 2 4 at 0 5 and 1 hz amplifications at 1 and 5 hz for all sites generally increase with decreasing shear wave velocity measured in the top 30 m v s 30 however larger amplifications at 0 5 and 1 hz for sites with similar v s 30 values are observed for sites in the seattle basin illustrating the amplification from the deeper 30 m sediments and the contribution from basin surface waves record sections for the mainshock and aftershock show that basin surface waves produce the peak velocities for many of the sites in the seattle basin and often dominate the amplitude at 1 hz and lower frequencies

seismic reflection images beneath puget sound western washington state the puget lowland thrust sheet hypothesis seismic reflection data show that the densely populated puget lowland of western washington state is underlain by subhorizontal paleogene and neogene sedimentary rocks deformed by west and northwest trending faults and folds from south to north beneath the lowland features seen on the seismic data include the horizontally stratified 3 5 km thick tacoma sedimentary basin the seattle uplift with south dipping 20 strata on its south flank and steeply 50 to 90 north dipping strata and the west trending seattle fault on its north flank the 7 5 km thick northward thinning seattle sedimentary basin the antiformal kingston arch and the northwest trending transpressional southern whidbey island fault zone swif interpreting the uplifts as fault bend and fault propagation folds leads to the hypothesis that the puget lowland lies on a north directed thrust sheet the base of the thrust sheet may lie at 14 to 20 km depth within or at the base of a thick block of basaltic crescent formation its edges may be right lateral strike slip faults along the base of the cascade range on the east and the olympic mountains on the west our model suggests that the seattle fault has a long term slip rate of about 0 25 mm year and is large enough to generate a m7 6 to 7 7 earthquake

variability in nonlinear sediment response during the 1994 northridge california earthquake abstract spectral ratios between sediment sites and rock sites for strong and weak ground motion are compared to infer the extent of nonlinear sediment response in the los angeles area during the 1994 northridge earthquake northridge mainshock records form the strong motion database and aftershock records form the weak motion database the degree of nonlinearity is evaluated at 25 locations in and around the san fernando and los angeles basins based on the ratio of weak to strong spectral amplification factors wsaf values of wsaf vary greatly from site to site and for a given site from one frequency band to another over the range from 0 75 to 20 hz this complexity is attributed to differences in several factors material properties under the sites and the amplitude and frequency content of the input ground motions given these observations it is not possible to assign an acceleration or velocity level at which nonlinearity initiates for all the stiff soil sites analyzed however no nonlinearity is seen below a peak acceleration of about 200 to 300 cm sec 2 or peak velocity of 20 to 30 cm sec nonlinearity is seen at all sites in or near the san fernando valley nonlinearity in the los angeles basin is limited to a few isolated sites for the northridge earthquake even sites with strong nonlinearity still show amplification levels greater than 1 0 relative to rock sites for frequencies below 10 hz

influence of a sedimentary basin infilling description on the 2 d p sv wave propagation using linear and non linear constitutive models numerical modelling is a useful tool to understand the role of different parameters site geometry impedance contrast material properties constitutive model input motion governing site effects we focus here on the 2 d p sv seismic wave propagation in a simple shaped asymmetric model we consider two ways of describing the sedimentary infilling properties the basin is either composed of distinct homogeneous geological layers or is described through properties progressively changing as a function of depth smooth model in order to study the wave propagation in the two basins with similar soil properties p and s velocities of the layered basin are deduced from the smooth basin ones making both models compatible in a kinematic point of view p and s velocities globally increase with depth following epri non linear properties are considered as constant in layers for the two basins we assess the influence of the model choice on the wave propagation while taking into account linear and non linear constitutive models we test the influence of the input motion by propagating two different input motions the first being a simple impulsive one and the second a real complex accelerogram we find that transfer functions computed in each basin are quite similar irrespective of the soil constitutive behaviour and the input motion moreover we show that the maximum shear strain spatial distribution depends on the input motion frequency content and maximum values depend on the input motion strength and complexity maximum shear strains are generally higher for superficial layers than for deeper ones we show that the highest shear strain values are located above discontinuities where velocities are lower and the soil is more non linear than in the underlying layer regardless of the input motion indeed in a layered basin the shear strain is higher above interfaces for all the soil constitutive models that were used in the smooth visco elastoplastic model a kind of layering appears absent in a viscoelastic model due to the different non linear properties in each layer moreover no matter what the model structure and the input motion used maximum shear strain levels are higher in the visco elastoplastic soil than in the viscoelastic one such a numerical study helps to understand the non linearity location and therefore the consequences of a model choice when performing a site specific study

sedimentary basin effects in seattle washington ground motion observations and 3d simulations seismograms of local earthquakes recorded in seattle exhibit surface waves in the seattle basin and basin edge focusing of s waves spectral ratios of s waves and later arrivals at 1 hz for stiff soil sites in the seattle basin show a dependence on the direction to the earthquake with earthquakes to the south and southwest producing higher average amplification earthquakes to the southwest typically produce larger basin surface waves relative to s waves than earthquakes to the north and northwest probably because of the velocity contrast across the seattle fault along the southern margin of the seattle basin s to p conversions are observed for some events and are likely converted at the bottom of the seattle basin we model five earthquakes including the m 6 8 nisqually earthquake using 3d finite difference simulations accurate up to 1 hz the simulations reproduce the observed dependence of amplification on the direction to the earthquake the simulations generally match the timing and character of basin surface waves observed for many events the 3d simulation for the nisqually earthquake produces focusing of s waves along the southern margin of the seattle basin near the area in west seattle that experienced increased chimney damage from the earthquake similar to the results of the higher frequency 2d simulation reported by stephenson et al 2006 waveforms from the 3d simulations show reasonable agreement with the data at low frequencies 0 2 0 4 hz for the nisqually earthquake and an m 4 8 deep earthquake west of seattle

climate scientists shine light on cave ice a small but growing number of researchers are investigating whether persistent ice that drapes the inside of some caves can reveal what the climate outside was once like at a recent workshop in austria about 50 such scientists discussed this formidable challenge and reported tantalizing progress by dating layers of cave ice that form from ponds of trapped rainwater and then analyzing the isotopes and other substances that make up those layers these scientists aim to chart past changes in temperature rainfall and other climate indicators such as carbon dioxide levels it9s a strategy paleoclimatologists have long pursued with ice cores taken from glaciers and polar caps or with layers of lake sediment but cave ice has proven much more difficult to study and interpret every cave is unique in how water trickles in and freezes and researchers have to go to great lengths to establish whether the ice layers they see represent annual seasonal or other time scales of deposition but it9s worth pursuing researchers said at the austrian workshop as polar ice cores don9t offer insight into the climate histories of temperate regions where most people live

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 1994 northridge california earthquake investigation of rupture velocity risetime and high frequency radiation a hybrid global search algorithm is used to solve the nonlinear problem of calculating slip amplitude rake risetime and rupture time on a finite fault thirty five strong motion velocity records are inverted by this method over the frequency band from 0 1 to 1 0 hz for the northridge earthquake four regions of larger amplitude slip are identified one near the hypocenter at a depth of 17 km a second west of the hypocenter at about the same depth a third updip from the hypocenter at a depth of 10 km and a fourth updip from the hypocenter and to the northwest the results further show an initial fast rupture with a velocity of 2 8 to 3 0 km s followed by a slow termination of the rupture with velocities of 2 0 to 2 5 km s the initial energetic rupture phase lasts for 3 s extending out 10 km from the hypocenter slip near the hypocenter has a short risetime of 0 5 s which increases to 1 5 s for the major slip areas removed from the hypocentral region the energetic rupture phase is also shown to be the primary source of high frequency radiation 1 15 hz by an inversion of acceleration envelopes the same global search algorithm is used in the envelope inversion to calculate high frequency radiation intensity on the fault and rupture time the rupture timing from the low and high frequency inversions is similar indicating that the high frequencies are produced primarily at the mainshock rupture front two major sources of high frequency radiation are identified within the energetic rupture phase one at the hypocenter and another deep source to the west of the hypocenter the source at the hypocenter is associated with the initiation of rupture and the breaking of a high stress drop asperity and the second is associated with stopping of the rupture in a westerly direction

site response for seattle and source parameters of earthquakes in the puget sound region we analyzed seismograms from 21 earthquakes ml 2 0 4 9 recorded by digital seismographs we deployed in urban seattle to determine site response and earthquake stress drops the seismometers were situated on a wide variety of geologic units including artificial fill e g kingdome harbor island pleistocene age soils glacial till and outwash deposits of seattle s hills modified land downtown seattle space needle and tertiary sedimentary rock two mainshock aftershock sequences were recorded the june 1997 bremerton sequence mainshock ml 4 9 and the february 1997 south seattle sequence mainshock ml 3 5 along with other events in the puget sound region we developed a new inversion procedure to estimate site response source corner frequencies and seismic moments from the s wave spectra this inversion uses corner frequencies determined from spectral ratios of mainshock aftershock pairs as constraints the site responses found from the inversion are not relative to the rock site but are relative to an idealized site with a flat frequency response the response of the rock site is also found from the inversion the inversion results show high response for the sites on artificial fill more moderate amplification for most sites on stiff pleistocene soils or modified land and low response for the rock site some sites display resonances such as a strong 2 hz resonance at our site near the kingdome which is caused by the surficial layers of fill and younger alluvium the sites in west seattle exhibit high amplification even though they are on relatively stiff soils of glacial outwash this may be partly caused by basin surface waves produced by conversion of incident s waves this high response in west seattle is consistent with damage reports from the 1949 mb 7 1 and 1965 mb 6 5 earthquakes stress drop estimates for the events we recorded were generally low between 0 4 and 25 bars although some of the events may have had higher stress drops that could not be resolved because of the limited passband we calculated a stress drop of 24 bars for the bremerton mainshock and 10 bars for the south seattle mainshock

amplification of seismic waves by the seattle basin washington state recordings of the 1999 m w 7 6 chi chi taiwan earthquake two local earthquakes and five blasts show seismic wave amplification over a large sedimentary basin in the u s pacific northwest for weak ground motions from the chi chi earthquake the seattle basin amplified 0 2 to 0 8 hz waves by factors of 8 to 16 relative to bedrock sites west of the basin the amplification and peak frequency change during the chi chi coda the initial s wave arrivals 0 30 sec had maximum amplifications of 12 at 0 5 0 8 hz whereas later arrivals 35 65 sec reached amplifications of 16 at 0 3 0 5 hz analysis of local events in the 1 0 to 10 0 hz frequency range show fourfold amplifications for 1 0 hz weak ground motion over the seattle basin amplifications decrease as frequencies increase above 1 0 hz with frequencies above 7 hz showing lower amplitudes over the basin than at bedrock sites modeling shows that resonance in low impedance deposits forming the upper 550 m of the basin beneath our profile could cause most of the observed amplification and the larger amplification at later arrival times suggests surface waves also play a substantial role these results emphasize the importance of shallow deposits in determining ground motions over large basins

earthquake ground motion and 3d georgia basin amplification in southwest british columbia shallow blind thrust scenario earthquakes abstract finite difference modeling of 3d long period 2 s ground motions for large m w 6 8 scenario earthquakes is conducted to investigate the effects of the georgia basin structure on ground shaking in greater vancouver british columbia canada scenario earthquakes include shallow blind thrust north america na plate earthquakes simulated in locations congruent with linear clusters of shallow seismicity that is potential active faults a slip distribution model of the m w 6 7 northridge california blind thrust earthquake with the hypocenter modified to 5 km depth is used to characterize the source rupture process two sets of simulations are performed for a given scenario earthquake using models with and without georgia basin sediments the ratio of predicted peak ground velocity pgv for the two simulations is applied here as a quantitative measure of amplification due to 3d basin structure a total of eight shallow blind thrust na plate scenario earthquakes are simulated within 100 km of greater vancouver overall predicted ground motions are higher in the down dip direction of each epicenter due to the source radiation pattern hence scenario earthquakes located south of vancouver produce the highest motions in the city the average maximum pgv at stiff soil sites across greater vancouver considering all eight scenario earthquakes is 17 8 cm s modified mercalli intensity vii the average increase in peak motion due to the presence of georgia basin sediments is a factor of 4 1 the effective duration of moderate level 3 4 cm s shaking within greater vancouver is an average of 22 s longer when georgia basin sediments are included in the 3d structure model online material snapshots and videos of wave propagation and peak ground velocity maps

ground motion simulation in the lower tagus valley basin throughout history the lower tagus valley ltv region has been shaken by several earthquakes including some with moderate to large magnitudes and with sources located inside the basin for example the 1344 m6 0 and 1909 m6 0 earthquakes previous simulations bezzeghoudet al natural hazard 69 1229 1245 2011 have revealed strong amplification of the amplitude waves in the region because of the effect of the low velocity sediments that fill the basin the model used in those simulations was updated in this work by including new high resolution geophysical and geotechnical data available for the area seismic reflection aeromagnetic gravimetric deep wells standard penetration tests and geological data to contribute to improved assessment of seismic hazard in the ltv we simulated propagation of seismic waves produced by moderate earthquakes in a 3d heterogeneous medium by using elastic finite difference wave propagation code the method successfully used by grandinet al geophys j int 171 1144 1161 2007 involves evaluation of the seismic potential of known faults in the area studied and three dimensional seismic ground motion modelling by use of finite difference methods on the basis of this methodology we calculated the ground motion for the april 23 1909 benavente portugal earthquake mw 6 0 in dense grid points then computed the synthetic isoseismic map of the area by use of appropriate relationships between seismic intensity mmi and peak ground velocity pgv the synthetic results in contrast with available macroseismic and instrumental data enable validation of the source models proposed for the area identification of the sources of historical earthquakes and could also indicate which areas are more exposed to seismic ground motion

variability of site response in seattle washington ground motion from local earthquakes and the ships seismic hazards investigation in puget sound experiment is used to estimate site amplification factors in seattle earthquake and ships records are analyzed by two methods 1 spectral ratios relative to a nearby site on tertiary sandstone and 2 a source site spectral inversion technique our results show site amplifications between 3 and 4 below 5 hz for west seattle relative to tertiary rock these values are approximately 30 lower than amplification in the duwamish valley on artificial fill but significantly higher than the calculated range of 2 to 2 5 below 5 hz for the till covered hills east of downtown seattle although spectral amplitudes are only 30 higher in the duwamish valley compared to west seattle the duration of long period ground motion is significantly greater on the artificial fill sites using a three dimensional displacement response spectrum measure that includes the effects of ground motion duration values in the duwamish valley are 2 to 3 times greater than west seattle these calculations and estimates of site response as a function of receiver azimuth point out the importance of trapped surface wave energy within the shallow low velocity sedimentary layers of the duwamish valley one dimensional velocity models yield spectral amplification factors close to the observations for till sites east of downtown seattle and the duwamish valley but underpredict amplifications by a factor of 2 in west seattle a two dimensional finite difference model does equally well for the till sites and the duwamish valley and also yields duration estimates consistent with the observations for the duwamish valley the two dimensional model however still underpredicts amplification in west seattle by up to a factor of 2 this discrepancy is attributed to 3d effects including basin edge induced surface waves and basin geometry focusing effects caused by the proximity of the seattle thrust fault and the sediment filled seattle basin

attenuation and source parameters of earthquakes in the cascadia region digital ground motion data recorded by the western canada telemetered network wctn are used to examine the attenuation and source parameters of earthquakes in the cascadia region of southwestern british columbia and northwestern washington the data base is comprised of over 1000 vertical component fourier spectra from earthquakes of magnitude 3 to 7 at distances from 10 to 500 km regression analyses determine the shape and coefficients of the regional attenuation curve and source spectra for 68 earthquakes seismic moment and stress drop are inferred from the amplitudes of the source spectra shallow h 10 km earthquakes have an attenuation curve with a complex shape exhibiting significant flattening no apparent geometric spreading in the distance range from 75 to 230 km this shape is probably a result of strong reflected phases from mid crustal discontinuities and the subducting slab events within the subducting slab and the lower crust exhibit a simple r 1 attenuation curve the anelastic attenuation coefficient for the region as a whole is given by q 380 f 0 39 the duration of motion for each wctn record is determined as the value that yields the observed relationship between time domain and spectral domain amplitudes according to random process theory these durations are approximately constant within 50 km of the source then increase with distance as 0 07 r the high frequency ground motions from cascadia earthquakes are relatively weak cascadia source spectra are characterized by an average brune stress drop of about 30 bars this is significantly lower than the average california stress drop of 70 to 100 bars and dramatically lower than the average eastern stress drop of 150 to 200 bars it is concluded that there are significant regional variations in source parameters hazard estimates for the cascadia region based on california groundmotion relations may be overly conservative

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

response of high rise and base isolated buildings to a hypothetical mw 7 0 blind thrust earthquake high rise flexible frame buildings are commonly considered to be resistant to shaking from the largest earthquakes in addition base isolation has become increasingly popular for critical buildings that should still function after an earthquake how will these two types of buildings perform if a large earthquake occurs beneath a metropolitan area to answer this question we simulated the near source ground motions of a m w 7 0 thrust earthquake and then mathematically modeled the response of a 20 story steel frame building and a 3 story base isolated building the synthesized ground motions were characterized by large displacement pulses up to 2 meters and large ground velocities these ground motions caused large deformation and possible collapse of the frame building and they required exceptional measures in the design of the base isolated building if it was to remain functional

abrupt uplift within the past 1700 years at southern puget sound washington shorelines rose as much as 7 meters along southern puget sound and hood canal between 500 and 1700 years ago evidence for this uplift consists of elevated wave cut shore platforms near seattle and emerged peat covered tidal flats as much as 60 kilometers to the southwest the uplift was too rapid for waves to leave intermediate shorelines on even the best preserved platform the tidal flats also emerged abruptly they changed into freshwater swamps and meadows without first becoming tidal marshes where uplift was greatest it adjoined an inferred fault that crosses puget sound at seattle and it probably accompanied reverse slip on that fault 1000 to 1100 years ago the uplift and probable fault slip show that the crust of the north america plate contains potential sources of damaging earthquakes in the puget sound region

katabatic wind fields in coastal areas studied by ers 1 synthetic aperture radar imagery and numerical modeling synthetic aperture radar sar images acquired over mediterranean coastal waters from the first european remote sensing satellite ers 1 showing sea surface manifestations of katabatic wind fields are presented in particular sea surface roughness patterns generated by katabatic winds blowing from 1800 in high mountains through a broad valley at the western coast of calabria southern italy into the gulf of gioia is investigated the ers 1 sar images show that their areal extent and shape vary strongly depending on the meteorological conditions the roughness pattern sometimes has the form of a mushroom an elongated tongue a broad blob or a narrow truncated band for one event september 8 1992 we have simulated the wind field at the sea surface by using a nonhydrostatic mesoscale atmospheric model and then compared it with the wind field derived from the ers 1 sar image by using the c band wind scatterometer model cmod4 the comparison shows that the atmospheric model reproduces quite well the mushroom like form of the wind field pattern while the wind speed is obtained somewhat lower than the one inferred from the sar image this study demonstrates that sar images acquired over coastal waters are well suited to study local wind fields in coastal areas and to test numerical models that describe local wind fields which extend from the coast onto the sea

a three dimensional simulation of seismic waves in the santa clara valley california from a loma prieta aftershock abstract the finite difference method is used to propagate elastic waves through a 3 d model of the santa clara valley an alluvium filled basin that underlies the city of san jose california the model was based on depth to bedrock information from water wells in the area the simulation corresponded to a region 30 east west by 22 north south by 6 depth km and contained about 4 million grid points synthetic seismograms from the simulation are accurate at frequencies up to 1 hz motions from a magnitude 4 4 aftershock of the loma prieta earthquake were modeled snapshots of ground motion and synthetic seismograms from the simulation are presented the simulation illustrates s to surface wave conversion at the edges of the basin and the large amplitude and long duration of ground motion in the basin compared to the surrounding rock love waves produced at the edge of the basin are the largest arrivals in the transverse synthetics because of the slow group velocity of the love waves sites near the center of the basin have longer durations of significant motions than basin sites near the valley edges sites near the center of the basin also show larger peak amplitudes on the transverse component array analysis of the synthetic seismograms indicates that love waves tend to propagate parallel to the eastern and western edges of the valley rayleigh waves are produced along the southern margin of the basin from incident s waves large radial motions occur where a rayleigh wave impinges on the northeast margin of the valley some rayleigh waves travel westward across the basin after being scattered from the eastern edge of the valley synthetic seismograms from the simulation have similar peak amplitudes as seismograms recorded by the sunnyvale dense array for this aftershock although the duration of the tranverse component is not matched by the synthetic seismogram using this basin model the simulation indicates that the love waves observed on the actual seismograms were produced by conversion of incident s waves at the southern margin of the santa clara valley 2 d simulations show how the s to love wave conversion is affected by the angle of incidence of the s wave and the sharpness of the velocity transition between the alluvium and bedrock as more accurate basin models are developed 3 d simulations should become valuable for predicting ground motions in sedimentary basins for future large earthquakes

nonlinear attenuation and rock damage during strong seismic ground motions strong seismic waves cause nonlinear behavior in the shallow subsurface in fractured rocks seismologists use low amplitude signals from small repeating earthquakes to measure s wave velocity decrease after strong motion the 2004 parkfield california earthquake provides examples of such velocity changes in fractured sandstone with an s wave velocity of 300 m s 1 this nonlinear behavior occurred around the wave number depth of the incident waves 30 m for the 10 s 1 dominant angular frequency on a velocity seismogram the low amplitude s wave velocity gradually recovered with the logarithm of time the attenuation of strong waves in general depends nonlinearly on their amplitude high dynamic stress triggered small very shallow earthquakes at sites including parkfield the theoretical frictional behavior of a fractured medium with heterogeneous prestress relates these phenomena failure occurs in the highly prestressed domains causing small earthquakes and opening mode cracks the energy to dilate the cracks dissipates a significant fraction of the incoming seismic energy the local high porosity domains close with the logarithm of time as expected from the aging law of rate and state friction increasing the s wave velocity the domain model indicates that nonlinear effects increase gradually over a range of dynamic coulomb stresses as observed and as included in the widely used masing rules the linker and dieterich 1992 relationship provides the maximum sustainable dynamic coefficient of friction needed to utilize the masing rules this parameter is the coefficient of friction at a laboratory normal traction plus a constant 0 15 times the logarithm of the ratio of field normal traction to the laboratory normal traction it is helpful to relate s wave velocity to starting frictional strength as coefficient of friction near the quarter wavelength depth determines nonlinear behavior then the dynamic coefficient of friction and equivalently the maximum sustainable acceleration at the dominant frequency depend weakly on s wave velocity for example the coefficient of friction at an angular frequency of 10 s 1 is less than 1 5 for rocks ranging from parkfield sandstone to intact granite

the relationship between the microwave radar cross section and both wind speed and stress model function studies using frontal air sea interaction experiment data the frontal air sea interaction experiment fasinex provided a unique data set with coincident airborne scatterometer measurements of the ocean surface radar cross section rcs at ku band and near surface wind and wind stress these data have been analyzed to study new model functions which relate wind speed and surface friction velocity square root of the kinematic wind stress to the radar cross section and to better understand the processes in the boundary layer that have a strong influence on the radar backscatter studies of data from fasinex indicate that the rcs has a different relation to the friction velocity than to the wind speed the difference between the rcs models using these two variables depends on the polarization and the incidence angle the radar data have been acquired from the jet propulsion laboratory airborne scatterometer these data span 10 different flight days stress measurements were inferred from shipboard instruments and from aircraft flying at low altitudes closely following the scatterometer wide ranges of radar incidence angles and environmental conditions needed to fully develop algorithms are available from this experiment

the general circulation responses of high resolution north atlantic ocean models to synthetic scatterometer winds abstract high resolution 1 5 1 6 quasigeostrophic models of the north atlantic ocean are forced by daily wind stress curl fields of controlled wavenumber content in the low wavenumber case the wind stress curl is derived from a low pass filtering of ecmwf wind fields such that the retained wavenumber band is observed to obey a k 2 power law in the spectrum for each day where k is the wavenumber vector in a second case the wavenumber content of the wind stress curl fields is comparable to that derivable from an ideal scatterometer wind dataset decadal average streamfunction fields are compared with a climatology of dynamic topography and compared between the model calculations driven by these synthetic wind stress curl datasets with the goal of testing the sensitivity of the general circulation to high wavenumber forcing the largest signal in decadal average streamfunction response to high wavenumber forcing occurs in the eastern basin of the north atlantic fields of mean kinetic and eddy kine

three dimensional simulations of ground motions in the seattle region for earthquakes in the seattle fault zone we used the 3d finite difference method to model observed seismograms of two earthquakes m l 4 9 and 3 5 in the seattle region and to simulate ground motions for hypothetical m 6 5 and m 5 0 earthquakes on the seattle fault for periods greater than 2 sec a 3d velocity model of the seattle basin was constructed from studies that analyzed seismic reflection surveys borehole logs and gravity and aeromagnetic data the observations and the simulations highlight the importance of the seattle basin on long period ground motions for earthquakes occurring just south of the basin the edge of the basin and the variation of the thickness of the quaternary deposits in the basin produce much larger surface waves than expected from flat layered models the data consist of seismograms recorded by instruments deployed in seattle by the usgs and the university of washington uw the 3d simulation reproduces the peak amplitude and duration of most of the seismograms of the june 1997 bremerton event m l 4 9 recorded in seattle we found the focal mechanism for this event that best fits the observed seismograms in seattle by combining green9s functions determined from the 3d simulations for the six fundamental moment couples the february 1997 event m l 3 5 to the south of the seattle basin exhibits a large surface wave arrival at uw whose amplitude is matched by the synthetics in our 3d velocity model for a source depth of 9 km the m 6 5 simulations incorporated a fractal slip distribution on the fault plane these simulations produced the largest ground motions in an area that includes downtown seattle this is mainly caused by rupture directed up dip toward downtown radiation pattern of the source and the turning of s waves by the velocity gradient in the seattle basin another area of high ground motion is located about 13 km north of the fault and is caused by an increase in the amplitude of higher mode rayleigh waves caused by the thinning of the quaternary deposits

coefficients for sea surface wind stress heat flux and wind profiles as a function of wind speed and temperature surface layer coefficients for wind profiles wind stress and heat flux in typical open sea conditions are briefly reviewed businger dyer flux gradient relationships and a charnock wind stress formula fit the empirical data and are dimensionally consistent these have been solved by an iterative method and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data

ocean model validation of the nasa scatterometer winds the suitability of basin scale satellite based scatterometer winds for forcing of numerical ocean models is examined using a reduced gravity primitive equation model of the tropical pacific ocean three surface forcing fields are validated in a comparison of upper layer thickness ult from the ocean model with observed sea level data the forcing fields are the florida state university observed winds winds derived from the nasa scatterometer nscat and stresses derived directly from nscat the sea level data sets are the world ocean circulation experiment fast sea level data set from island measurements and sea level anomalies from topex poseidon results of this comparison demonstrate that while the three model results are qualitatively similar the results are quantitatively better when forcing with the nscat derived stresses this is particularly true in the eastern tropical pacific and in convergent zones where forcing with the nscat stresses can lead to large differences in ult 40 m compared with results from the other two wind products

a comparison of two surface wind stress analyses over the tropical atlantic during 1980 1987 abstract two sets of monthly sea surface wind stress over the tropical atlantic ocean are compared the datasets are based on the ecmwf analyses during 1980 87 the e winds and the monthly pseudo wind stress from ship observations for the same period the s winds our examination shows that both datasets give qualitatively similar mean fields and annual cycles quantitatively the zonal component of the e winds is larger than that of the s winds especially in the winter hemisphere the strongest southeast trades of the e winds are also shifted to the east of the strongest southeast trades of the s winds in the vicinity of the itcz the e winds are more zonally oriented so that the convergence zone is not as clearly defined interannually both datasets show that the northeast trades were gradually strengthening from 1980 to 1986 the southeast trade winds on the other hand were anomalously strong during 1981 83 but weak during 1984 86 with the e winds the southeast trades decreased gradually duri

nonlinear behavior of strong surface waves trapped in sedimentary basins abstract numerical calculations and observations indicate that surface waves reverberate within sedimentary basins a site response approach is inapplicable to the nonlinear attenuation of these waves as the energy passes repeatedly through the shallow subsurface computed dynamic stresses obtained by using published shakeout calculations graves et al 2008 indicate that nonlinear attenuation is modest but not negligible in the los angeles california basin for large earthquakes on the san andreas fault the dominant surface waves have velocity amplitudes of 1 5 m sec 1 and periods 3 5 sec around 20 of the total elastic strain energy resides above dynamic lithostatic stress ratios of 0 2 for both rayleigh and love waves where prestressed rock is highly likely to fail somewhat in friction

predictions of shear wave velocities in southern california using surface geology a new model of the average shear wave velocity in the uppermost 30 m has been generated by extrapolation of discrete velocity profiles using surface geology at several scales statistical methods have been applied to create a map that is no more complicated than is supported by the velocity data several geologic units with similar responses are grouped together the resulting map is simpler than previous ones and yet fits the observed velocity profiles better than earlier more complicated maps analysis within a geographic information system will permit updates and modification of the map as new velocity data are added

estimation of strong ground motions from hypothetical earthquakes on the cascadia subduction zone pacific northwest strong ground motions are estimated for the pacific northwest assuming that large shallow earthquakes similar to those experienced in southern chile southwestern japan and colombia may also occur on the cascadia subduction zone fifty six strong motion recordings for twenty five subduction earthquakes ofms 7 0 are used to estimate the response spectra that may result from earthquakesmw 81 4 large variations in observed ground motion levels are noted for a given site distance and earthquake magnitude when compared with motions that have been observed in the western united states large subduction zone earthquakes produce relatively large ground motions at surprisingly large distances an earthquake similar to the 22 may 1960 chilean earthquake mw 9 5 is the largest event that is considered to be plausible for the cascadia subduction zone this event has a moment which is two orders of magnitude larger than the largest earthquake for which we have strong motion records the empirical green s function technique is used to synthesize strong ground motions for such giant earthquakes observed teleseismicp waveforms from giant earthquakes are also modeled using the empirical green s function technique in order to constrain model parameters the teleseismic modeling in the period range of 1 0 to 50 sec strongly suggests that fewer green s functions should be randomly summed than is required to match the long period moments of giant earthquakes it appears that a large portion of the moment associated with giant earthquakes occurs at very long periods that are outside the frequency band of interest for strong ground motions nevertheless the occurrence of a giant earthquake in the pacific northwest may produce quite strong shaking over a very large region