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

acceleration source spectra anticipated for large earthquakes in northeastern north america a set of 16 large intraplate earthquakes are considered on the basis of their tectonic settings compressive focal mechanisms and shallow focal depths as possible analogs for large earthquakes that may be anticipated to occur in northeastern north america the earthquakes range in seismic moment from 5 1023 to 3 1026 dyne cm and include the mainshocks and largest aftershocks of the 1982 miramichi canada the 1985 to 1988 nahanni canada and the 1988 tennant creek australia earthquake sequences the teleseismic recordings of these earthquakes are spectrally analyzed by correcting the logarithmically averaged acceleration spectra for the frequency dependent effects of attenuation and interference of the depth and direct phases the resulting acceleration source spectra are generally flat at high frequencies the mainshock spectra exhibit intermediate spectral falloffs that is u where 1 over frequencies that characteristically extend from 0 1 to 0 5 hz in contrast the largest aftershocks of the three sequences named above exhibit 2 spectral shapes the high frequency source spectral levels increase with seismic moment approximately as ru m 1 3 the 1988 saguenay canada earthquake which occurred significantly deeper than the other earthquakes exhibits an anomalous spectral shape combining the teleseismic data with strong motion recordings indicates that the intermediate spectral falloff for this earthquake extends from 0 2 to 3 hz the spectral amplitude at 3 hz is a factor of 3 higher than the amplitude anticipated for an earthquake of this size from the other 15 earthquakes

implications of felt area magnitude relations for earthquake scaling and the average frequency of perceptible ground motion abstract observed relations between felt area and moment magnitude m for stable continental regions scr and california earthquakes can be explained by a simple model with geometrical spreading attenuation q 1 and earthquake stress drops that remain constant from m 4 to 8 differences in the slopes of the observed felt area m relations between scr and california earthquakes are consistent with differences in q between these tectonic settings fitting the felt area m data to the model yields an estimate of f q where f is the average frequency of minimum perceptible ground motion using fits to the data and instrumentally derived values of q i find that f is between about 2 and 4 hz i present an estimate of the spectral acceleration level associated with the lower limit of perceptibility

ground motion relations for eastern north america predictive relations are developed for ground motions from eastern north american earthquakes of 4 0 m 7 25 at distances of 10 r 500 km the predicted parameters are response spectra at frequencies of 0 5 to 20 hz and peak ground acceleration and velocity the predictions are derived from an empirically based stochastic ground motion model the relations differ from previous work in the improved empirical definition of input parameters and em pirical validation of results the relations are in demonstrable agreement with ground motions from earthquakes of m 4 to 5 there are insufficient data to adequately judge the relations at larger magnitudes although they are consistent with data from the saguenay m 5 8 and nahanni m 6 8 earthquakes the underlying model parameters are constrained by empirical data for events as large as m 6 8

regional propagation characteristics and source parameters of earthquakes in northeastern north america abstract the vertical components of the s wave trains recorded on the eastern canadian telemetered network ectn from 1980 through 1990 have been spectrally analyzed for source site and propagation characteristics the data set comprises some 1033 recordings of 97 earthquakes whose magnitudes range from m 3 to 6 the epicentral distances range from 15 to 1000 km with most of the data set recorded at distances from 200 to 800 km the recorded s wave trains contain the phases s s m s s n and l g and are sampled using windows that increase with distance the acceleration spectra were analyzed from 1 0 to 10 hz to separate the source site and propagation characteristics an inversion for the earthquake corner frequencies low frequency levels and average attenuation parameters is alternated with a regression of residuals onto the set of stations and a grid of 14 distances ranging from 25 to 1000 km the iteration between these two parts of the inversion converges in about 60 steps the average attenuation parameters obtained from the inversion were q 1997 10 and 0 998 0 003 the most pronounced variation from this average attenuation is a marked deamplification of more than a factor of 2 at 63 km and 2 hz which shallows with increasing frequency and increasing distance out to 200 km the site response spectra obtained for the ectn stations are generally flat the source spectral shape assumed in this inversion provides an adequate spectral model for the smaller events m o 21 dyne cm in the data set whose brune stress drops range from 5 to 150 bars for the five events in the data set with m o 10 23 dyne cm however the source spectra obtained by regressing the residuals suggest that an 2 spectrum is an inadequate model for the spectral shape in particular the corner frequencies for most of these large events appear to be split so that the spectra exhibit an intermediate behavior where u is roughly proportional to

characteristics of the aftershock sequence of the borah peak idaho earthquake determined from digital recordings of the events abstract the u s geological survey menlo park deployed and maintained a network of twelve digital instruments over the 2 weeks following the 28 october 1983 borah peak idaho earthquake the network recorded 45 events with m 3 0 and 6 events with m 4 0 the epicenters are located in a narrow band which parallels the trace of the surface fauiting up to the willow creek summit the depths of the events range from 5 to 16 km in the south the distribution of hypocenters delineate a plane which dips to the southwest at 50 to the north the hypocenters dip steeply to the east composite focal mechanisms for three groups of events show normal faulting mechanisms the mechanism of the aftershocks in the north appear rotated in both strike and dip from the aftershocks in the south the seismic moments of the aftershocks increase with increasing hypocentral depth below 12 km the dynamic stress drops of the events do not show any systematic variation with depth however most of the events with large stress drops are clustered in the northwest limb of the aftershock distribution the average stress drop of the southern events is 31 16 bars while the average stress drop of the events in the northwest limb is 77 52 bars this clustering of events with large stress drops marks an apparent stress concentration possibly associated with the arrest of the main shock rupture propagation by a fracture barrier at depth

source scaling and moment estimation for the pacific northwest seismograph network using s coda amplitudes earthquake coda waves can be used for investigating both source and medium parameters in this investigation we introduce a method for estimating the scalar seismic moment of local earthquakes using s coda amplitudes recorded by regional network stations assuming a single scattering coda model more than 8000 coda amplitude measurements from 78 local western washington earthquakes with coda duration magnitudes from 2 4 to 5 0 are used to estimate source spectra in the narrow frequency band 2 to 12 hz using a transmission model based on previous studies we find that our source modeling best supports an omega square constant stress drop far field scaling model an average stress drop of 32 bars is inferred for the earthquakes studied to calibrate the moment estimation procedure a reference event with known moment and corner frequency is required using a suitable reference event we estimate moments for the remaining 77 events in the data set the moment values range from 5 1 1019 to 4 4 1023 dyne cm

calibration of time history simulation methods abstract ground motion time histories for use in engineering analyses of structures in eastern north america are often simulated from seismological models owing to the paucity of real recordings in the magnitude and distance ranges of interest two simulation methods have been widely used in recent years the stochastic method and the ray theory method in the stochastic method as im plemented in this study ground motion is treated as filtered gaussian noise whose underlying spectrum is determined from an empirical region specific seismological model of the source and propagation processes in the ray theory method as implemented in this study the ground motions are simulated by convolving an empirical source function with theoretical green s functions for a specified crustal structure model this article compares results of the two simulation methods for four well recorded calibration events and assesses the applicability of the methods the assessment is based on comparisons of ground motion parameters from the simulated data with those of the actual recordings ground motion parameters in the frequency range from 1 to 10 hz are sat isfactorily predicted by both methods averaged over the four events studied the stochastic method underpredicts 1 hz response spectra by 20 to 40 but accurately predicts response spectra for frequencies of greater than 2 hz it also accurately predicts peak ground acceleration and velocity the wave propaga tion method underpredicts 1 hz response spectra by 10 to 40 but accurately predicts response spectra for higher frequencies it overpredicts peak ground acceleration and velocity by 10 to 40 both methods are imprecise the stan dard error of an estimate is a factor of about 2 2 the bias and standard error of an estimate for the wave propagation method are generally slightly lower than for the stochastic method if the focal depth of the event can be specified i e as for a past earthquake if the focal depth of the event is not known i e as for a future earthquake then the accuracy and precision of thetwo methods are about the same the chief advantage of the wave propagation method is its predictive power since its attenuation func tion is derived from the focal depth and crustal structure it does not require knowledge of the empirical attenuation function the chief advantage of the stochastic model is its economy and simplicity introduction seismic hazard studies are often used to identify design earthquakes in terms of magnitude and dis tance from a site of interest ground motion time his tories for the desired magnitude s and distance s are then sought to be used as input for engineering analyses of a proposed structure for regions such as california where strong ground motion records are relatively abun dant a suite of recordings from past earthquakes is usu ally available in eastern north america by contrast relevant records for the desired magnitudes and distances are seldom available it is therefore necessary to simu late the required records based on seismological models of earthquake ground motion generation processes two simulation methods have been widely used in recent years the stochastic method and the ray theory method in the stochastic method ground motion is treated as bandlimited finite duration gaussian noise whose underlying spectrum is determined from a seismological model of source and propagation processes in the ray theory method the ground motions are simulated by convolving an empirical source function usually derived from a past event with theoretical green s functions for 400

seismic hazard maps for puget sound washington abstract a set of maps have been constructed which estimate the hazard of seismic ground shaking in the puget sound washington area as measured by the peak ground acceleration pga the average pga in this region for which there is a 5 per cent chance of exceedence in 50 yr is approximately 30 per cent of g pgas on the map vary from a low of 20 per cent of g to a maximum of near 60 per cent of g the effects of local seismicity patterns type of soil and subsurface geology are all included and all contribute significantly to the variation of expected pga across puget sound the predicted pgas are slightly larger than or comparable to the pgas experienced in magnitude 6 5 and 7 2 earthquakes in this century and the average pga is consistent with work by previous investigators the hazard estimates are controlled by earthquakes in the magnitude 4 5 to 7 0 range and are largely independent of the rate of occurrence of earthquakes outside of that range

the effect of crustal structure on strong ground motion attenuation relations in eastern north america strong ground motion attenuation relations are usually described by smoothly decreasing functions of distance however consideration of wave propagation in the crust suggests that attenuation relations should be more complex such complexity may be present in strong ground motion data for eastern north american earthquakes which show amplitudes in the distance range of 60 to 150 km that lie above the trends at smaller and greater distances using a wavenumber integration method to compute green s functions and close in recordings of several earthquakes as empirical source functions we have generated synthetic seismograms that are in good agreement with regional and strong motion recordings of eastern north american earthquakes from these synthetic seismograms we have shown that the observed interval of relatively high amplitudes may be attributable to postcritically reflected s waves from the moho the presence and location of the interval of relatively high amplitudes is highly dependent on the crustal velocity structure and may therefore be expected to show regional variation however for any realistic structure model there will be a transition in the attenuation relation from an interval at shorter distances less than about 100 km that is dominated by direct waves to an interval at greater distances that is dominated by postcritically reflected waves the synthetic seismograms have response spectral velocities that match those of the recorded data and their m blg values are in good agreement with observed values

seismic potential associated with subduction in the northwestern united states despite good evidence of present day convergence of the juan de fuca and north american plates there has been remarkably little historical seismic activity along the shallow part of the juan de fuca subduction zone although we cannot completely rule out the possibility that the plate motion is being accommodated by aseismic creep we find that the juan de fuca subduction zone shares many features with other subduction zones that have experienced great earthquakes

the shape of ground motion attenuation curves in southeastern canada the shape of the attenuation curve of the fourier spectrum of acceleration is established for southeastern canada from empirical data the dataset is comprised of 1200 digital vertical component seismograms from 100 earthquakes recorded on 30 stations of the eastern canada telemetered network the distance range of the observations is 10 to 1700 km focal depths are from 5 to 30 km results indicate that the attenuation curve has three distinct sections at distances less than 70 km corresponding to attenuation of the direct wave amplitudes decay slightly more rapidly than 1 r r 1 1 between 70 and 130 km where the direct wave is joined by sms spectral amplitudes are approximately constant beyond 130 km corresponding to the lg phase amplitudes decay at a rate that is consistent with r 0 5 and q 670 f 0 33 the findings support the hypothesis of burger et al 1987 that postcritical reflections from the moho discontinuity play an important role in determining the shape of the attenuation curve however the influence of sms is subtle allowing the shape to be approximated by simple functional forms for example the commonly assumed bilinear form r 1 r 0 5 with frequency dependent q fits well if the transition from direct wave to lg spreading is placed at approximately 60 km a linear form r 1 over all distances with constant q 2000 also provides a satisfactory fit overall we find little evidence for the dependence of attenuation on focal depth or tectonic province

earthquake source spectra in eastern north america source spectra for earthquakes of 3 m 7 in eastern north america ena are determined from regional seismograph recordings strong motion records teleseismic data and modified mercalli intensity data the inclusion of mmi data is made possible by the finding that the felt area of an earthquake is an accurate measure of its high frequency spectral level at the source current ground motion relations for ena are based on a brune source model with a stress parameter of 100 bars this model is a poor fit to ena source spectra for events of m 4 at high frequencies 5 to 10 hz observed spectral amplitudes are more nearly matched by a brune stress parameter of 200 bars intermediate frequency amplitudes near 1 hz are matched by a stress parameter of approximately 50 bars an empirical two corner source model is proposed to describe ena source spectra for events of 4 m 7

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

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

medium term gully headcut retreat rates in southeast spain determined from aerial photographs and ground measurements this paper deals with gully retreat rates at different time scales whereby the short term time scale may span a time interval of 1 5 years the medium term time scale a time interval of 5 50 years and the long term time scale a time interval of more than 50 years an analysis of high altitude aerial photographs in combination with ground measurements allowed us to quantify volumetric gully head retreat rates for 12 permanent gullies in southeast spain guadalentin and guadix study areas over a 40 43 year time interval medium term time scale this resulted in an average retreat rate ve of 17 4 m3 year 1 a power relationship between drainage basin area a and medium term volumetric gully head retreat rate ve 0 069a0 380 r2 0 51 n 21 was found by combining the gully head retreat rates obtained in this study with those obtained by a dendrochronological method the exponent b and the coefficient of determination r2 of the power relationships ve aab increases from the short term to the long term time scale expressing the increasing importance of drainage basin area in gully development with time considerable differences between gully head retreat rates measured at the short and medium term time scales at individual gullies showed the importance of land use changes and unsuccessful management practices on gully head retreat and the episodic nature of gully head retreat when piping and tension cracking are involved higher gully head retreat rates are obtained at the medium term time scale compared to the short term time scale but the differences are not significantly different at 10 the medium term methods tend to measure proportionally more high gully retreat rates but less extreme values compared to the short term method this is explained by a more equal distribution of extreme rainfall events both in space and time at a longer term time scale and hence a higher probability of measuring the average effect of both small and extreme rainfall events at each gully

the influence of critical moho reflections on strong ground motions recorded in san francisco and oakland during the 1989 loma prieta earthquake the amplitudes of strong ground motions from the loma prieta earthquake recorded in the san francisco and oakland areas exceeded the levels predicted by standard empirical attenuation relations preliminary analysis of accelerograms having known trigger times strongly suggests that the elevation of ground motion amplitudes in the distance range of approximately 40 to 100 km was due to critical reflections from the base of the crust these reflections which are identified on the basis of their arrival times and phase velocity and by comparison with simulated accelerograms were large and occurred at relatively close range because of the deep focal depth of the earthquake and the strong velocity gradient at the base of the crust these motions were further amplified presumably by impedance contrast effects at soft soil sites in san francisco and oakland the effect of the critical reflections in amplifying peak accelerations of the loma prieta earthquake in the san francisco and oakland regions was as large as the effect of soft soil site conditions focal depth has an important influence on strong motion attenuation at distances beyond about 40 km and empirical attenuation relations derived from shallow crustal earthquakes may underpredict the ground motions of deeper crustal events in this distance range further analyses using an expanded data base that includes recordings of aftershocks are required to rigorously test the proposed explanation of the ground motions recorded in san francisco and oakland and the conclusions drawn from that explanation

seismic estimates of stress release five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 oroville california earthquake the dynamic stress drop the arms stress drop and the apparent stress provide stable estimates which are strongly correlated across the data set the uncertainties of these estimates the standard error of the mean divided by the mean are approximately 15 estimates of the brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks determined from the signal durations of the recorded s waves the brune stress drops are most strongly correlated with the arms stress drops implying that the brune stress drop provides an estimate of the average dynamic stress drop as brune originally proposed

prediction of strong ground motion in the puget sound region the 1965 seattle earthquake spatial variations in strong ground motion in the 1965 seattle washington earthquake ms 6 5 can be accurately reproduced using three dimensional ray tracing techniques synthetic accelerograms calculated at nearly 1600 receivers spread over the puget sound region indicate that peak ground accelerations are significantly affected by subsurface structure as well as by near receiver soil properties the large synthetic accelerations computed for the harbor island downtown seattle area appear to be due to subsurface focusing of rays within a deep sedimentary basin and are consistent with the observed isoseismal distribution in extreme circumstances such as a bedrock site adjacent to a site situated upon a soft sedimentary lens peak accelerations may vary by nearly an order of magnitude predicted values of peak ground acceleration as a function of distance reproduce both the trend and scatter in data from subduction zone earthquakes of comparable depth in japan

suspended sediment yield and transfer processes in a small high arctic glacier basin svalbard observations of suspended sediment concentration and discharge at two sites on the proglacial river network draining from a predominantly cold based high arctic glacier austre broggerbreen are described analysis of these observations illustrates i the relatively low suspended sediment yield from this basin in comparison with many other glacier basins reported in the open literature ii sustained and possibly increasing availability of suspended sediment to the fluvial system as the ablation season progresses and iii the role of the proglacial sandur as both a sediment source and sink field observations coupled with the results of the data analysis are used to make inferences concerning the changing nature and relative importance of sediment sources within the basin 1998 john wiley sons ltd

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