the location and source parameters of the lompoc california earthquake of 4 november 1927 in this paper we address the relocation magnitudes and the style of faulting of the lompoc earthquake from a sparse assortment of teleseismic and regional seismograms the highest quality teleseismic waveform data come from a station at de bilt netherlands that remains in operation thus recordings of numerous modern events in central coastal california i e the 1969 santa lucia banks 1983 coalinga 1978 santa barbara and 1989 loma prieta earthquakes have been used for comparison with the 1927 records location constraints for the lompoc event were established from the de bilt recording by comparing s p and sss s waveform matches against the above master events to avoid the effect of unknown clock errors on locations that use absolute times these same seismograms were modeled to estimate the depth faulting parameters and source strength a similar approach using observational comparisons and numerical modeling was applied to the regional waveform data obtained from the stations at berkeley tucson and pasadena our results indicate a north northwesterly striking reverse event located about 40 km west of point conception which is in excellent agreement with the recent tsunami modeling results by satake and somerville 1992 this location is 25 km south of that proposed by hanks 1979 and well within his error bars we obtain a body wave moment of 1 10 26 dyne cm a trapezoidal time history of 2 2 2 sec and a source depth of 10 km the weak beginning of the pnl wavetrain at berkeley indicates some source complexity which is characteristic of many large events the fault parameters are strike n20 w dip 66 ne and rake 95 most seismicity catalogs report a m s 7 3 for this event after gutenberg and richter 1956 but this was a long period body wave magnitude and not a surface wave result their original worksheets indicate a smaller ms 7 0 the body waves of the loma prieta event m s 7 1 appear distinctly larger than those of the lompoc event at de bilt in agreement with our lower estimate of source strength

exploration of the lower lithosphere northeastern united states abstract in this study we report on the modeling of broadband seismograms from moderate sized earthquakes at regional distances we demonstrate that the longer period motions world wide standardized seismograph network wwssn preceding the direct s arrival can be modeled reasonably well with a crustal model consisting of a layer over a half space although a generic model assumed to be the same everywhere can be used to model the pnl waveshape some adjustments are required to fit the pn and sn timing for specific paths inversion of three component seismograms assuming such simple models proves effective in source retrieval using a single station shorter periods can then be modeled by adding more detail to the velocity structure holding the source fixed and repeating the process application of this modeling procedure to earthquakes occurring in the new england region proves interesting in particular paths on opposite sides of the appalachian thrust belt produce distinct models paths along the western side of the belt can be modeled with an average crustal thickness of 35 km the compressional and shear velocities of the crust are 6 5 km s 1 and 3 6 3 7 km s 1 and those of the upper mantle 8 4 8 5 km s 1 and 4 7 4 8 km s 1 respectively the results are in agreement with the normal shield parameters paths along the eastern side of the belt indicate more variability but yield consistently lower lithospheric velocities by 3 4 for p waves these models predict p wave travel time delays across this zone of about 0 3 s in agreement with teleseismic delays

broadband modelling along a regional shield path harvard recording of the saguenay earthquake summary the saguenay earthquake 1988 november 25 is one of the first large shield type events recorded by a broadband high dynamic range instrument the streckeisen system installed at harvard station hrv the event is sufficiently large to be well recorded teleseismically and thus the source characteristics can be determined by independent means and considered known this allows a detailed study of the propagational effects along this path at an epicentral distance of 625 km where the strengths of the surface waves can be compared with the crustal body phases broadband modelling using standard analytical techniques and flat layered models works amazingly well over the period range of 0 5 20 s a detailed strategy for modelling broadband regional phases is given in terms of a decomposition of the vertical and radial seismograms into three segments pnl containing pn ppn spn pmp p coupled pl waves snl containing sn ssn sms etc and the fundamental rayleigh waves information about the upper crust is obtained from the fundamental rayleigh waves while crustal thickness and velocity gradients in the mantle are obtained from pnl and snl this particular crustal model has a thickness of 35 km with a sharp moho and a substantial gradient in the top 20 km of the mantle 0 01 km s 1 per km for both p and s velocities the mantle velocities 8 2 and 4 55 km s 1 are slower than expected for a shield environment attenuation is not required for waveform modelling or for absolute amplitude estimation

long period regional wave moment tensor inversion for earthquakes in the western united states source parameters of moderate to large size mw 4 5 earthquakes in the western united states from 1992 to 1994 are determined by point source moment tensor inversion of complete long period t 35 50 s ground motions recorded at regional distances 1 12 stable long period signals are obtained by low pass filtering records from the very broadband seismometers recently deployed in several networks in the western united states these signals are dominated by fundamental mode rayleigh and love waves which have very simple waveforms due to the limited dispersion on the short paths to regional stations since long period motions are relatively insensitive to the attenuation model and crustal structure used in the inversion they provide robust constraints on the seismic moment and faulting geometry as long as adequate azimuthal coverage is available comparisons of solutions for 21 events with results of other regional and teleseismic wave inversions are made to assess the model dependence and uncertainties of our regional centroid moment tensor rcmt solutions rcmt inversion has limited source depth resolution for shallow crustal events but the focal mechanism and seismic moment determinations prove quite stable over a range of source depths in the crust as well as over a range of crustal propagation models simultaneous waveform inversion of shorter period body wave signals can improve the source depth resolution by applying path corrections for heterogeneous crustal structure shorter period surface wave energy can also be inverted allowing the methodology to be extended to lower magnitude regional events as well the rcmt procedure requires minimal signal processing only a sparse broadband network and a simple laterally homogeneous propagation model thus it can readily be automated and applied in near real time to events in the magnitude range from 4 5 to 7 5 distributed over an area as large as the western united states

polarization anisotropy and fine scale structure of the eurasian upper mantle we have observed shear wave splitting with tsh tsv tsv up to 1 5 on long period records of multiply reflected s waves bottoming in the upper mantle beneath the russian and siberian platforms the dispersion of love and rayleigh waves over these paths shows discrepancies of comparable or larger magnitude with respect to smooth isotropic si structures consistent with a model of the uppermost mantle having significant apparent vertical anisotropy although the splitting and dispersion data can be fit by smooth anisotropic sa models we have investigated the apparent anisotropy associated with fine scale rough structure beneath stable eurasia we fit the data with a rough isotropic ri model having an rms shear velocity fluctuation that varies from 14 in the uppermost mantle to zero at 400 km depth these fluctuations are larger than the variation in isotropically averaged parameters expected for even a diverse assemblage of upper mantle ultrabasic rocks which we take to be evidence for some sort of intrinsic local anisotropy

the siquisique ophiolites northern lara state venezuela a discussion on their middle jurassic ammonites and tectonic implications ammonites from the pillow basalt associations of the siquisique ophiolites located in west central venezuela indicate a minimum bajocian to early bathonian age for emplacement of the complex the ammonites have been identified as stephanoceras skirroceras cf macrum quenstedt emileia ex gr multiformis gottsche et quenstedti westermann and parkinsonia sp this association of cosmopolitan pacific and tethyan elements aids in the location of the western end of the middle jurassic seaway set between the tethys and the pacific oceans hispanic corridor the apparent age of the ammonites indicates that some of the siquisique sediments pillow basalts and serpentines are the oceanic remnants of an arrested early phase of rifting between north and south america

residual flows in cook strait a large tidally dominated strait abstractflows at subtidal frequencies are resolved from 20 months of current observations at multiple sites across the narrows of cook strait new zealand the substantial tidal flows spring tides 3 m s 1 are challenging to measure with moored adcps along strait residual flows are on the order of 0 05 0 1 m s 1 but there were differences across the strait and also through the annual cycle the calculated net residual transport is 0 25 sverdrups sv 1 sv 106 m3 s 1 i e negative sign implies to the south from tasman sea to pacific ocean this is smaller by a factor of 2 3 than that estimated from previous sparse current measurements crucially however these new data also suggest that over the austral summer autumn period there is persistent residual flow northward on the western side of the strait so that the summer autumn net 0 15 sv flow can be separated into 0 065 sv westside and 0 22 sv eastside flow this western northward flow had a seasonal bias appearing only in the summer

comments on neogene block tectonics of eastern turkey and northern south america continental applications of the finite difference method by j f dewey and j l pindell

combining double difference relocation with regional depth phase modelling to improve hypocentre accuracy summary precise and accurate earthquake hypocentres are critical for various fields such as the study of tectonic process and seismic hazard assessment double difference relocation methods are widely used and can dramatically improve the precision of event relative locations in areas of sparse seismic network coverage however a significant trade off exists between focal depth epicentral location and the origin time regional depth phase modelling rdpm is suitable for sparse networks and can provide focal depth information that is relatively insensitive to uncertainties in epicentral location and independent of errors in the origin time here we propose a hybrid method in which focal depth is determined using rdpm and then treated as a fixed parameter in subsequent double difference calculations thus reducing the size of the system of equations and increasing the precision of the hypocentral solutions based on examples using small earthquakes from eastern canada and southwestern usa we show that the application of this technique yields solutions that appear to be more robust and accurate than those obtained by standard double difference relocation method alone

regional wave propagation in new england and new york we validate and improve 1d velocity models of the two main crustal provinces in the northeastern united states neus using seismograms from the 20 april 2002 m 5 au sable forks earthquake which is the largest earthquake in the region to be recorded by multiple recently deployed good quality regional broad band stations to predict and mitigate the effects of future earthquakes in the north eastern united states more information is needed regarding both the local earthquake sources and how seismic waves travel through the region we investigate the source and regional wave propagation for the au sable forks earthquake the earthquake epicenter is located near the boundary of two distinct geological provinces the appalachian new england and grenville new york we use a forward modeling approach to study the waveforms recorded at 16 stations located within 400 km of the epicenter we generate synthetic seismograms using the frequency wavenumber method testing several published models for the two provinces several models per form well at low frequencies 0 1 hz we refine these models and generate two alternative 1d crustal models for intermediate frequencies 1 hz of engineering interest our new grenville model performs better than previously published models for all six source station paths modeled in that province according to goodness of fit statistics variance reduction and correlation coefficient our alternative appalachian model improves the fit of synthetics to data for five of the ten paths modeled in that province from the results we identify two specific sources of wave field complex ities that should be investigated in future studies of earthquake ground motions in neus 3 azimuthal anisotropy in the appalachian province and complex wave paths along the boundary between the two provinces

measurements of tidal flow around a headland with a shipboard acoustic doppler current profiler a shipboard acoustic doppler current profiler adcp and moored current meters were used to obtain detailed measurements of the spatial structure of the tidal flow around a headland in vineyard sound massachusetts where tidal currents typically range from 50 to 70 cm s 1 eight shipboard surveys were conducted each of which followed one of five trapezoidal tracks in the vicinity of the headland completing 11 12 circuits in the course of the tidal cycle 12 4 hours the measurements from the adcp compare favorably with moored velocity measurements at two locations rms deviations of 3 cm s 1 but the comparison showed more scatter at two other sites rms deviations of 7 cm s 1 the adcp measurements from separate cruises were merged to form a composite spatial representation of the tidal and residual currents providing resolution of the spatial structure of the flow around the headland at scales from several hundred meters to 10 km the semidiurnal tidal flow is relatively uniform in space roughly following the bathymetry around the headland while a residual current field consists of counterrotating eddies on either side of the headland with spatial scales of 5 8 km and velocities as high as 25 cm s 1 the instantaneous current field indicates the formation of transient eddies on the downstream side of the headland during flood and ebb with clear evidence of flow separation near the tip of the headland

depth and age in the cayman trough abstract caribbean plate boundaries are almost everywhere either strike slip or convergent so oceanic crust accreted at the spreading axis within the cayman trough provides information critical to determining the relative motions and tectonic interactions among the caribbean and surrounding plates the tectonic evolution of the trough as inferred from magnetic anomaly identifications is not entirely compatible with either its crustal subsidence or the geological evidence for time of opening and rate of offset movement along the north american caribbean plate boundary a close examination of basement topography and depth within the trough suggests that the total opening of the cayman trough is about 1100 km and that this opening occurred in two stages crust generated by the present ongoing stage of very slow spreading has extensive relief and shows a distinct increase of crustal depth with increasing distance from the spreading center crust generated by an earlier episode of faster spreading is characterized by reduced topographic relief and no appreciable differential subsidence and is separated from the younger crust by distinct topographic breaks the rate of spreading of the current slow spreading episode is15 5mm yr based upon a new crustal subsidence curve which accommodates lateral as well as vertical cooling of the accreting slab the trough has opened at this rate for about 30 ma the rate of opening prior to 30 ma cannot be determined with any reliability from crustal subsidence a reexamination of magnetic anomalies in the central part of the trough shows that these anomalies can be generated at a much slower spreading rate than previously thought this revised interpretation of cayman trough evolution brings magnetic subsidence and geological evidence into much better agreement

broadband modeling of local earthquakes three component broadband waveforms of two small earthquakes near upland california recorded on the pasadena broadband high dynamic range instrument were modeled to obtain useful green s functions for this path and to examine the sensitivity of the synthetic seismograms to perturbations of the crustal model we assumed that the source of each event was both simple and known as determined from the caltech usgs array first motions a trapezoidal time function was chosen to fit the width of the direct s wave generalized rays reflectivity and finite difference techniques were used to compute the synthetic seismograms we found that a simple layer over a half space model is an adequate approximation of the upper crust along this profile in particular the waveforms are controlled by a relatively slow 4 km thick surficial layer 4 5km s 1 2 6 km s 1 over a faster layer 5 9 km s 1 3 5 km s 1 the relative amplitudes of direct and multiple s indicate that the main shock occurred at a depth of 6 km while the aftershock occurred at a depth of 8 to 9 km sensitivity analyses indicate that for distances less than 50 km and for periods longer than 1 sec the synthetic seismograms are not very sensitive to perturbations of the deep crustal structure analysis of upper crustal model perturbations revealed that the surficial layer is between 3 to 5 km thick in addition the contact between this layer and the underlying material can be smoothed with a 2 km wide velocity gradient without adversely affecting the fit to the data two dimensional finite difference calculations show that a ridge structure beneath the recorder acts as a lowpass filter the lower frequency phases are largely unaffected other two dimensional models with ridges between the source and receiver clearly did not fit the data synthetic seismograms computed for the best fitting model were used to estimate a long period moment of 6 2 1022 dyne cm ml 4 6 and 1 1022 dyne cm ml 3 7 with identical triangular source time durations of 0 3 sec assuming the same fault dimension of 0 4 km from standard scaling laws stress drop estimates of 410 and 70 bars are obtained for the two events respectively generally we found that it is possible to reproduce local waveforms at frequencies up to 1 hz without a complete knowledge of fine structural detail resulting green s functions can be useful in studying historic events and in simulations of large events from a given source region

modified frequency wavenumber algorithm for regional seismograms using filon s quadrature modelling of lg waves in eastern north america summary the main focus of this study is to develop an efficient frequency wavenumber code to synthesize high frequency say 10 hz regional seismograms up to a distance of more than 1000 km in a medium consisting of a large number of crustal layers and investigate effects of regional waveguides on lg waves in this newly developed code the frequency wavenumber response is evaluated by using the filon s quadrature at high frequency and the polynomial approximation of bessei s functions implementing trapezoidal integration at low frequency this has reduced the computation time by several folds compared to the time needed for the trapezoidal integration using this algorithm both broad band seismograms at harvard and short period ectn eastern canadian telemetered network seismograms from 1988 november 25 saguenay earthquake were successfully modelled a moment magnitude of 5 9 was used initially an average waveguide was calibrated to successfully excite the regional pnl and snl waves at hrv its near surface structure constrained by matching the recorded frequency content the crust mantle transition zone was constrained by modelling the strength of the broad band snl waves consisting of sn ssmsnsmssms and ssmssms phases relative to the initial pnl waves the model was modified to consist of thin layers with an alternating high and low velocity distribution to match the high frequency ectn lg waves observed from 300 to 600 km away in addition the effect of a near surface irregular receiver structure on the duration of lg waves was also investigated by convolving its response with the regional lg seismograms initial study suggests this method as viable for modelling the entire lg waves including the coda numerical studies suggest that the shapes and peak amplitudes on lg seismograms depend more strongly on the source depth anelasticity and crustal waveguide than do the pnl seismograms

upper mantle structure of the southwest pacific from regional waveform inversion we determine the upper mantle seismological structure of the southwest pacific by inverting complete regional seismograms recorded by the southwest pacific seismic experiment a deployment of 12 broadband seismographs in the fiji tonga and niue islands we first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure partial derivatives are computed using an efficient reflectivity code for differential seismograms randall 1994 and a conjugate gradient method is used to perform the nonlinear inversion band pass filtered 10 55 mhz vertical and radial component seismograms extending from the p arrival to the surface waves are used to constrain the structure waveforms are selected to provide regional distance 400 1500 km propagation paths predominantly within one of the tectonic regions of the southwest pacific source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths results show extremely large lateral variability in the upper mantle structure of the southwest pacific the inactive south fiji basin shows upper mantle velocities much lower than those previously observed for the pacific lithosphere of a similar age the north fiji and lau basins containing active back arc spreading systems show upper mantle shear velocities as slow as 3 8 km s 1 the exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world the velocity difference between the active back arc basins and the old pacific lithosphere immediately east of the tonga trench is maximum at 30 90 km depth with a variation of 18 indicating partial melting in this depth range velocity heterogeneity of 4 extends to a depth of 170 km velocity structure below 220 km depth is poorly resolved but there is some evidence of a 2 difference extending to depths of 350 km

determining source parameters of moderate size earthquakes from regional waveforms abstract waveform modeling of the teleseismic long period body wave phases of shallow earthquakes has proven quite effective in determining source parameters for events of magnitude larger than six unfortunately these teleseismic phases become too weak for smaller events and regional data must be used which are generally much more complicated this is because the crust mantle system acts as a leaky waveguide and a large number of rays are required to model the observations in this report we review a procedure for the systematic determination of source parameters from regional body waves a least squares inversion technique is used which is based on a cross correlation of the data and a synthetic seismogram the synthetic waveforms are a linear combination of those for the three fundamental types of fault a set of profiles of the synthetic waveforms is presented although the synthetic waveforms are generated for a model developed for the western united states this model seems to be adequate for most continental regions the inversion is parameterized in terms of fault strike dip and rake these parameters are relatively insensitive to small changes in crustal structure several examples are presented

seismodynamics of sweden deduced from earthquake focal mechanisms summary focal mechanisms from 18 major swedish earthquakes ml upp 3 reveal that 1 several of the studied earthquakes have focal mechanisms with significant extensional stresses which fit well to the idea of postglacial rebound as an important stress generator 2 in areas like south western sweden the north westerly trending compressional axes indicate that ridge push from the north atlantic ridge is also a considerable stress contributor 3 some local tectonic features like the skalderviken depression in kattegat are momentous for seismotectonic interpretations in addition to first p polarities full waveform modelling for frequencies up to 3 hz and epicentral distances up to 225 km provides rather robust focal mechanism determinations focal depths of seven events ranging from 11 to 37 km are well resolved within a few km by waveform modelling

calibration of the regional crustal waveguide and the retrieval of source parameters using waveform modeling regional crustal waveguide calibration is essential to the retrieval of source parameters and the location of smaller m 4 8 seismic events this path calibration of regional seismic phases is strongly dependent on the accuracy of hypocentral locations of calibration or master events this information can be difficult to obtain especially for smaller events generally explosion or quarry blast generated travel time data with known locations and origin times are useful for developing the path calibration parameters but in many regions such data sets are scanty or do not exist we present a method which is useful for regional path calibration independent of such data i e with earthquakes which is applicable for events down to m w 4 and which has successfully been applied in india central asia western mediterranean north africa tibet and the former soviet union these studies suggest that reliably determining depth is essential to establishing accurate epicentral location and origin time for events we find that the error in source depth does not necessarily trade off only with the origin time for events with poor azimuthal coverage but with the horizontal location as well thus resulting in poor epicentral locations for example hypocenters for some events in central asia were found to move from their fixed depth locations by about 20 km such errors in location and depth will propagate into path calibration parameters particularly with respect to travel times the modeling of teleseismic depth phases pp sp yields accurate depths for earthquakes down to magnitude m w 4 7 this m w threshold can be lowered to four if regional seismograms are used in conjunction with a calibrated velocity structure model to determine depth with the relative amplitude of the p nl waves to the surface waves and the interaction of regional spmp and ppmp phases being good indicators of event depths we also found that for deep events a seismic phase which follows an s wave path to the surface and becomes critical developing a head wave by s to p conversion is also indicative of depth the detailed characteristic of this phase is controlled by the crustal waveguide the key to calibrating regionalized crustal velocity structure is to determine depths for a set of master events by applying the above methods and then by modeling characteristic features that are recorded on the regional waveforms the regionalization scheme can also incorporate mixed path crustal waveguide models for cases in which seismic waves traverse two or more distinctly different crustal structures we also demonstrate that once depths are established we need only two stations travel time data to obtain reliable epicentral locations using a new adaptive grid search technique which yields locations similar to those determined using travel time data from local seismic networks with better azimuthal coverage

upper mantle shear structure beneath the northwest atlantic ocean we have modeled the sh motion from earthquakes in the northwest atlantic ocean to derive an oceanic upper mantle shear velocity model the signals were recorded on long period wwssn and canadian network stations on the east coast of north america the travel times and waveforms of seismograms in the distance range 11 16 were used to constrain the lid structure these data indicate a fast 4 75 km s lid of about 100 km thickness in the older western atlantic from 16 to 28 the s waves pass through triplications due to velocity jumps near 400 and 660 km the branches from both discontinuities are visible in the s wave data these arrivals were modeled using synthetic seismograms to obtain accurate travel times the two triplications are also apparent in ss data from 30 to 50 the triplication curves derived from the s waves agree with the ss data given the lid structure the waveforms and travel times from the more distant data put tight constraints on the shear velocities at greater depths we find shear velocities from 200 to 400 km to be slow in relation to a shear model of the canadian shield derived by a similar technique indicating continent ocean heterogeneity to about 400 km

source spectra for the 1988 saguenay quebec earthquakes abstract source spectra are obtained for the 1988 saguenay quebec earthquake m n 6 5 its foreshock m n 4 8 and largest aftershock m n 4 1 using recordings from analog and digital seismographs on rock sites at epicentral distances from 40 to 700 km the source spectra of the foreshock and the aftershock are well matched by a single corner frequency 2 model with seismic moments of 2 2 10 22 and 2 2 10 21 dyne cm respectively and a stress parameter of 65 bars for both events by contrast the spectrum of the mainshock differs dramatically from this simple shape a stress parameter of 500 bars is required to match the observed high frequency amplitudes using the seismic moment determined from teleseismic observations as a constraint this single corner frequency model however overestimates amplitudes at frequencies near 1 hz by a factor of about 2 the relative radiation of short and long period energy for the mainshock is inconsistent with that from other intraplate earthquakes the fore and aftershocks appear typical in this regard the extraction of source spectra from the regional data required a careful examination of attenuation azimuthal variations were observed with the attenuation being most severe to the southeast normal to the st lawrence valley and the dominant structural grain of the region the st lawrence river coincides in part with the boundary between the grenville and appalachian tectonic provinces and the difference in attenuation may be related to the difference in tectonic province the distribution of data however makes it difficult to obtain good estimates of the attenuation in each province lumping the data from both provinces and the three earthquakes together results in an anelastic attenuation factor given by q 755 f 0 5 for frequencies f from 0 6 to 18 hz using data at distances r beyond 100 km from the sources and assuming 1 r geometrical spreading this attenuation is similar to that obtained in a number of other studies in the general area three component recordings were available at a few stations and from these the following frequency dependent ratio of horizontal to vertical motion was obtained hiv 1 14 0 118 f 0 00638 f 2 for frequencies from 0 2 to 15 hz different methods of correcting observations for attenuation in order to derive source spectra were examined source spectra derived by simple regression analyses of observed data assuming a uniform medium are very similar to those based on a more complicated synthetic seismogram correction which accounts for wave propagation in a layered earth

single station location of seismic events a review and a plea for more research abstract traveltime based location methods are ineffective for locating regional seismic events which are recorded by few stations however the comprehensive test ban treaty ctbt makes it imperative that we monitor sparsely observed events this paper reviews the current status of single station location ssl methods the location of seismic events using records from one three component broadband digital station we argue here that ssl methods may ultimately provide the most effective and economical means for monitoring small magnitude m 3 seismic events over most of the earth potentially station event distance and focal depth may be determined accurately by utilizing broadband three component data and matching observed and synthetic waveforms station event azimuth is determinable from the polarization of the p wave arrival and p coda however more research is required before ssl methods become a practical alternative to traveltime based methods currently the most significant problem is that regional seismograms are strongly dependent on regional crustal structure thus future research efforts should focus both on determining appropriate crustal models in specific regions and also on improved methods for dealing with regional variations in crustal structure a second important problem concerns location errors with ssl they may involve 180 errors in station event azimuth or other highly non gaussian blunders

rapid computation of multioffset vertical seismic profile synthetic seismograms for layered media by rearranging the formulas for the responses of buried sources and receivers in the kennett reflectivity algorithm we have obtained a new algorithm that is very efficient for computing multioffset vsp synthetic seismograms the rearrangement of the response formulas is quite general inasmuch as it applies to both isotropic and anisotropic kennett codes our new algorithm and the original single receiver algorithm can both be made to run much faster on vector computers by taking the layer loop out of the p loop but leaving it inside the frequency loop the resulting vector codes can still compute the response of media with frequency dependent velocities

shear velocity structure of the crust and upper mantle beneath the tibetan plateau and southeastern china this paper addresses the velocity structure of the crust and upper mantle beneath southern china with special emphasis on the tibet region waveform data from 48 earthquakes as recorded on the wwssn and gdsn are used in this detailed forward modelling study constraints on the upper crustal section are derived from modelling local love waves in the time domain applying the mode sum modelling technique lower crustal constraints are derived by modelling the p nl wavetrain with the reflectivity method an average crustal thickness of 70 km is obtained beneath the tibetan plateau with a modest increase of velocity with depth the lithospheric and upper mantle structure is deduced from modelling s and ss triplication waveform data and relative traveltimes by applying a combination of wkbj and generalized ray methods s ss seismograms chosen with bounce points directly under tibet allow remote sensing of this inaccessible region the resulting model is an averaged 1 d model where corrections for lateral variation have been applied we conclude that the upper mantle structure in the entire region is basically shield like below 200 km sna however the velocity of the lithosphere is abnormally slow roughly 5 per cent beneath tibet the model for tibet derived does not have a distinct lid and has a positive velocity gradient in the crust suggesting crustal shortening a preliminary velocity model for southeastern china is also suggested

simulated hard rock motions in saint louis missouri from large new madrid earthquakes mw 6 5 abstract we simulated broadband hard rock ground motion time histories for the saint louis missouri metropolitan area due to three large earthquakes 6 5 m w 7 5 each occurring on the largest fault segment of the new madrid fault zone at a distance of about 200 km a deterministic procedure was implemented to simulate ground motion for frequencies less than 1 hz for which the propagation effects were calculated using full wave theory for signals above 1 hz a semi empirical simulation approach was used the fault dimensions were defined in conformity with a constant stress drop model developed for eastern north america the fault width was constrained to a maximum depth of 16 km following the occurrence of the natural seismicity for each event we simulated broadband time histories for ten randomly generated slip models at eight sites distributed in various locations in the city of saint louis this study shows that the level of peak ground motions at hard rock sites in saint louis is small the largest average peak horizontal ground motion acceleration simulated is only 26 45 cm sec 2 1 96 cm sec for the velocity even for the largest event to validate the simulation results we examined regional accelerograms and their peak values and response spectra recorded during the 25 november 1988 saguenay earthquake m w 5 8 at distances further than 230 km from its epicenter to be consistent with the distance of the city of saint louis from the new madrid earthquake zone the saguenay earthquake observations seem consistent with simulation values obtained for the m w 6 5 new madrid earthquake a preliminary equivalent linear analysis by applying these simulated motions at the base of a soil profile suggests that the level of ground motions is too weak to produce damaging effects in saint louis even for the largest event on the longest segment

observations of the phase of tidal currents along a strait abstract the phase of the m2 surface elevation tide changes by 100 along a 40 km length of cook strait new zealand acoustic doppler current profiler measurements are presented that show a change of only 12 in the phase of depth averaged semidiurnal tidal currents along this same length this phase change supports the general result developed by vennell which states that the phase of the cross sectional average velocity is approximately constant along a short strait of variable cross sectional area vennell also developed a relationship between the constant phase of tidal currents along the strait and the amplitude and phase of the surface elevation tide at the ends of a short strait the velocity phase given by this relationship is shown to agree well with the observed tidal velocity phase in cook strait and one other strait

source estimation from broadband regional seismograms recently developed source inversion techniques do not take full advantage of the broadband nature of regional seismograms the reason is that green s functions generated from flat layered models are not correct for the complicated propagational phenomena associated with realistic crustal structure the inversion is usually performed by removing short periods and inverting only selected portions of the records in this article we introduce a source estimation technique that allows for better use of the entire broadband record when only imperfect green s function are available the procedure desensitizes the timing between the principal crustal arrivals by fitting portions of the green s functions independently the preferred source is the orientation that minimizes the average l1 and l2 norms in term of the four parameters m 0 strike dip and rake the l2 error emphasizes the longer period surface waves while the l1 error weights the shorter period body waves we obtain estimates of duration by modeling long period and broadband signals the depth determination can be obtained directly from the surface reflected phases sp mp or ss ms or by cycling through the depth dependent green s functions in addition to the source parameters we obtain t phase alignments shifts that can be used as green s function corrections for relocating other events or as a guide to deriving new crustal models

ml m0 as a regional seismic discriminant the m b m s ratio determined by teleseismic observations has proven to be an effective discriminant for explosive sources tend to be significantly richer in short period energy than are earthquakes unfortunately this method is limited by the detection threshold of teleseismic surface waves however recent advances in instrumentation allowing low amplitude surface wave measurements coupled with new analytical techniques make it feasible to use regional waveform data to determine the long period source excitation level of low magnitude events we propose using the ratio of m l local magnitude to m 0 scalar seismic moment as an analogous regional discriminant we applied this criterion to a data set of 299 earthquakes and 178 explosions and found that this ratio seems to be diagnostic of source type for a given m 0 the m l of an explosion is more than 0 5 magnitude units larger than that of an earthquake this separation of populations with respect to source type can be attributed to the fact that m l is a short period 1 hz energy measurement whereas seismic moment is determined from long period body wave phases period 4 s and surface waves 10 to 40 sec using regional stations with sources 200 to 600 km away the effective threshold for magnitude measurements for this discriminant is found to be m l 3 1 for earthquakes and m l 3 6 for explosions this method does require the determination of regional crustal models and path calibrations from master events or by other means

broadband modelling of regional seismograms imperial valley to pasadena summary this paper addresses broadband wave propagation along a corridor from imperial valley to pasadena california the path consists of 50 km of slow basin structure with a shallow moho followed by 250 km of relatively normal southern california structure approaching pasadena events occurring in the valley produce extended body wave codas which carry a dispersive waveform imprint along with longer period rayleigh waves many features of these seismograms at periods greater than a few seconds can be modelled by applying a finite difference technique to a 2 d structure shorter periods display complex behaviours not easily modelled by present techniques especially for shallow events events occurring in the normal structure produce broadband sh waveforms that can be modelled analytically if some extra empirically derived alignment parameters determined from calibration events are used these parameters are determined by adjusting traveltime differentials between the decomposed wavefield s ss etc for example delays in ss relative to s correspond to local corrections for sediment cover for a particular source region source complexity such as asperities and directivity become increasingly important at shorter periods as demonstrated by numerical experiments and observation

modeling of strong ground motions from the 16 september 1978 tabas iran earthquake we have examined the adequacy of the published ks3 and l1 slip distribution models developed for the m s 7 4 16 september 1978 tabas earthquake hartzell and mendoza 1991 by modeling the high frequency accelerograms recorded at dayhook boshrooyeh and tabas stations the agreement in peak ground acceleration pga and duration between data and simulated accelerograms was chosen as the criterion for the model adequacy the fault was specified with a seismogenic rupture area 95 km long and 45 km wide we used a semi empirical simulation method in which accelerograms from an aftershock from the 1979 imperial valley earthquake were used to represent the radiation pattern of p sv or sh waves for a source receiver geometry the fault surface is divided into many subfaults the contributions from these subfaults weighted by the slip amounts were lagged and summed to simulate the accelerograms of the main event we computed accelerograms assuming constant rake and variable rake on the fault plane the former was simulated using the sum slip model i e the slip model representing the vector sum of the slip components at every point on the fault and the latter was simulated by summing the accelerograms generated by the slip models of the dip slip and strike slip faults the influence of the rise time of the main event was investigated the ks3 asperity model produces results marginally better than the l1 asperity model the duration is predicted consistent with the data the peak amplitudes of the simulated ground motions remain within a factor of 1 5 and 3 of the recorded data at dayhook and boshrooyeh respectively for the variable rake and within a factor for 2 5 at both stations for the fixed rake angle ks3 model the large pga recorded on the tabas accelerograms could not be reproduced which warrants additional study a notable agreement was obtained in the response spectra at dayhook this study demonstrates that accelerograms generated by an earthquake in one tectonic region can be transported to another tectonic region so that they can be used to represent the empirical radiation pattern of the subfaults in simulating ground motions for a large event

focal depths for small to moderate earthquakes mn 2 8 in western quebec southern ontario and northern new york regional depth phase modeling is a useful technique to constrain earthquake focal depths for events within sparse networks we demonstrate how regional depth phases can be used to obtain focal depths for hundreds of earthquakes with m n 2 8 that occurred from 1980 to 2004 in southern ontario western quebec and northern new york state we discuss the development of regional depth phases with distance their use in focal depth determinations by matching waveforms to synthetics and possible sources of error in general focal depths can be determined to within about 3 km when there is at least one record at regional distances with clearly identifiable depth phases such as spmp or spg in southwestern ontario and northern new york focal depths range from 2 to 15 km in areas of western quebec and along the ottawa river valley in ontario focal depths range from 2 to 25 km more than half of the earthquakes in a cluster of activity near maniwaki in western quebec are deeper than 20 km all earthquakes deeper than 20 km occurred north of about 45 3 n there is an intriguing tendency for earthquake depths to cluster near 8 km 12 km 15 km and 22 km another interesting observation is that there are several earthquake pairs or small groups with very similar epicenters but quite different depths the most active regions in western quebec and along the ottawa river valley tend to exhibit a relatively wide range of focal depths along broad zones that trend roughly northwest suggesting that seismicity is occurring along diffuse northwest trending faults throughout the crust the wide range of focal depths may indicate the potential for a larger maximum scale of fault rupture and serve as a contemporary marker of locations where large events have occurred in the recent geologic past

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region

dynamical aspects of the flow through the strait of belle isle abstract we present an analysis of 50 days of current meter and sea revel data collected in a long narrow strait connecting the gulf of st lawrence to the atlantic ocean the dynamical balances implied by a scale analysis of the equation of motion are compared with the data for semidiurnal and diurnal tides and for low frequency flows the main result being that the near surface currents along the strait are as expected in geostrophic balance with the sea level slope across the strait the flow appears to be driven by the sea level difference between opposite ends of the strait produced by large scale meteorological forcing and a regression model involving acceleration and friction suggests a spindown time of 1 1 days the near bottom currents are significantly less than those new the surface at both levels the currants are reasonably uniform across the channel apart from the possibility of nearshore intensification at the lower level the vertical and horizontal structure of the low frequency curre

a re examination of the 1925 charlevoix quebec earthquake the 1 march 1925 charlevoix earthquake is one of the largest earthquakes known to have occurred in southeastern canada because it occurred in one of the most active seismic zones of southeastern canada and near now heavily populated regions understanding this earthquake may have implications for seismic hazard assessments of the region previous attempts have been made to study this earthquake using modern waveform analysis techniques but the source parameters could be only partially constrained due to a lack of good data recently a number of seismograms that had not been previously studied were rediscovered warranting a second look at the earthquake the best overall solution is strike 42 7 dip 53 7 rake 105 10 depth 10 km seismic moment 3 1 2 5 1025 dyne cm mw 6 2 ms 6 2 0 3 mb 6 5 0 4 source duration 5 sec and stress drop 35 bars the focal mechanism is consistent with many recent earthquakes in the charlevoix seismic zone but inconsistent with the regional stress field

geology and hydrocarbon accumulations columbus basin offshore trinidad the columbus basin situated on the eastern shelf of trinidad lies at the eastern extremity of a belt of severe deformation along the northern boundary of south america that has been affected by compressional and wrench tectonics in pliocene pleistocene time two major structural trends are present in the columbus basin a series of east northeast trending anticlines and north northwest oriented normal faults the basin was filled during late miocene to holocene time with sediments deposited by an ancestral orinoco river draining a hinterland to the southwest the pliocene pleistocene section which contains the hydrocarbon accumulations in the columbus basin was deposited in three coarsening upward sedimentary sequences followed by a late pleistocene transgressive sequ nce traps for hydrocarbon accumulation were formed by an easterly trending pliocene pleistocene wrench system with associated east northeast oriented anticlines combined with north northwest oriented normal faults oil was sourced in the late miocene lower cruse formation whereas gas was derived both from pliocene pleistocene pro delta shales and as a late high temperature phase of lower cruse hydrocarbon generation the north northwest faults formed migration conduits from the oil source rock to pliocene pleistocene reservoirs the temporal relationship of faulting to oil generation is a major factor affecting the distribution of oil and gas the size of hydrocarbon accumulations is limited to some extent by a lack of an effective hydrocarbon seal particularly in the western half of th basin

source retrieval from broadband regional seismograms hindu kush region abstract a method of relocation and source characterization of small earthquakes using one modern regional station is developed and tested first we model teleseismic body waves of two events which are used as masters short period depth phases pp and sp are used to establish the epicentral depth and the events are relocated using calibrated stations and a mantle model derived for this region tip the events moved upward by 12 and 31 km respectively the regional waveforms recorded at the iris station gar from the best determined source are forward modeled to establish a local crustal model a four layered model with a thickness of 65 km proves effective synthetics from this crustal model can then be compared with the data from other events where the depths are poorly known i e pde depths of 33 km forward modeling of the short period phases allows better estimates of depth and makes relocation possible next we perform a long period inversion of whole and partial waveform data to obtain source mechanism and moment and repeat the procedure if necessary this technique is applied to events in the tectonically active pamir hindu kush region to test its usefulness five crustal events sampling various azimuths are presented as examples of relocation and determination of source mechanisms from small events the smallest event has a moment of 5 1 1022 dyn cm for the seven events we studied the average depth correction to pde is 19 km and the location correction is 11 km on average the method can be used to identify earthquakes and thus it lowers the threshold below present mb ms discrimination since these events are too small to be seen teleseismically secondly these events can be used as masters in the calibration of other systems case based event characterizations etc

focal depth determination for moderate and small earthquakes by modeling regional depth phases spg spmp and spn there are three usable regional depth phases spg spmp and spn and their corresponding reference phases pg pmp and pn the differential time between each depth phase and its reference phase can be used to estimate earthquake focal depth we have developed a method to determine focal depth for moderate and small earthquakes by using a regional depth phase modeling rdpm method we used a default focal mechanism to generate the differential times for all earthquakes to estimate the reliability of the modeled focal depths we compared our solutions with those obtained by other methods and found the consistency is good because the focal depths estimated by rdpm are model dependent we tested the extent of the dependency and found that a 10 error in the crustal model may generate a 10 15 error in the modeled depth the absolute error is determined by the error in the crustal model and the focal depth itself we found that earthquake location errors have only a small effect on the modeled focal depths by analyzing synthetic and observed waveforms we found a distance window within which spmp and pmp are well developed and within the window the p portion of the waveform is relatively simple and spmp and pmp are easy to identify we also demonstrated that the assumptions of spmp and pmp are correct that regional depth phases are not developed or not discernible in some regions and that regional depth phases have special features that can be used to identify those phases

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

caribbean tectonics and relative plate motions during the last century three different ways of interpreting the tectonic evolution of the gulf of mexico and the caribbean have been proposed taking into account the bailey willis school of a permanent pre jurassic deep sea basin the edward suess school of a subsided continental terrain and the alfred wegener school of continental separation the present investigation is concerned with an outline of an interpretation which follows that of pindell and dewey 1982 an attempt is made to point out ways in which the advanced hypotheses can be tested the fit of africa north america and south america is considered along with aspects of relative motion between north and south america since the early jurasic attention is given to a framework for reconstructing caribbean plate evolution the evolution of the caribbean the plate boundary zones of the northern and southern caribbean and the active deformation of the caribbean plate

teleseismic body waveforms and receiver structures beneath seismic stations summary in this paper we describe a practical method for determining crustal structure by comparing observations of teleseismic body waves with synthetics constructed using generalized ray theory layered crustal structure beneath a seismic station strongly influences teleseismic body waves arriving in the 50 s or so after the initial p however the effect of crustal structure differs significantly for vertical component and radialcomponent records for any proposed crustal structure we develop an approximate theoretical formulation to predict the radial component seismogram as a function of the vertical component observations then we compare the observed and predicted radial component seismograms in the time domain and vary the proposed crustal structure using a grid search scheme until we obtain an optimum match the theoretical approximation is complete to second order in the reflection and transmission coefficients and is accurate to within a few per cent for modelling teleseismic waveforms we demonstrate the feasibility of this second order radial vertical comparison sorvec method by testing it on synthetically generated waveforms having complicated source properties and significant levels of background noise we illustrate the application to real data by determining the crustal structure beneath two seismic stations in tibet lzh and amdo beneath station lzh the crustal thickness is about 65 km beneath station amdo the crustal thickness is about 75 km and there is a low velocity layer in the lower crust

upper mantle compressional velocity structure beneath the northwest atlantic ocean a compressional velocity model for the northwest atlantic ocean is derived by modeling waveform and travel time data from long period and short period world wide standard seismograph network and canadian network station recordings a 90 km thick lid with a velocity of 8 1 km s at the top gradually increasing to 8 3 km s at the bottom is obtained by fitting the travel time data of first arrivals and waveform data of almost purely oceanic paths at distances of 8 20 triplication p waveform data constrain the structure below the lid a distinct but smooth low velocity zone is located at a depth of 170 km combining these results with the shear wave structure derived by grand and helmberger 1984a for the same region we suggest an olivine rich mineralogy in the upper 100 km and partial melting possibly extending as deep as 300 km based on low seismic velocities and high vp vs

the 25 november 1988 saguenay quebec earthquake source parameters and the attenuation of strong ground motion the saguenay earthquake of 25 november 1988 occurred close to the southern margin of the saguenay graben in southern quebec it was caused by almost purely dip slip faulting centered at a depth of 26 km with a p axis oriented northeast southwest this faulting mechanism is similar to those of the larger historical earthquakes in eastern north america but the focal depth is substantially greater than all but one of these events the seismic moment estimated from regional pni waves and teleseismic long period body waves is 5 10 24 dyne cm corresponding to a moment magnitude of 5 8 the source duration of the earthquake is estimated to be 1 8 sec corresponding to a stress drop of 160 bars which is not significantly higher than the average stress drop of 120 bars estimated from previous large earthquakes in eastern north america in order to simultaneously match the recorded ground motion amplitudes of strong motion acceleration strong motion velocity and teleseismic short period and long period body waves it is necessary to use a source function having a complex shape that implies the presence of asperities and larger local stress drops the large set of strong motion recordings of the saguenay earthquake has been used to validate a procedure for estimating strong ground motion attenuation based on a simple wave propagation model the most important feature of the recorded strong motions is that their peak amplitudes do not decay significantly with distance inside 120 km but then decay abruptly beyond 120 km profiles of recorded accelerograms with absolute times indicate that at distances beyond 64 km the peak ground motions are due to strong postcritical reflections from velocity gradients in the lower crust the principal shear wave arrivals and the variation of their peak amplitudes with distance were reproduced in synthetic seismograms generated using a regional crustal structure model the critical distances for the postcritical reflections were short because of the deep focal depth of the event causing the elevation of ground motion amplitudes out to 120 km similar studies of earthquakes in other regions of eastern north america indicate that the strength of the postcritical reflections and the distance ranges over which they are dominant are controlled by the focal depth and crustal structure regional variations in crustal structure thus give rise to predictable regional variations in strong ground motion attenuation