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

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

waveform modeling of strong motion data for the saguenay earthquake of 25 november 1988 abstract the magnitude 6 0 saguenay earthquake of 25 november 1988 in quebec canada was one of the largest recorded in eastern north america ena during the past 50 years and it has provided important high quality data for estimation of earthquake hazard in ena ground motions for the event recorded by both eastern canadian telemetered and strong motion networks were anomalously large and unexpectedly rich in high frequency energy as compared with most other events having the same m b or moment magnitudes displacement waveforms derived from the eastern canadian strong motion acceleration data have been modeled in detail in an attempt to explain these anomalies the modeling has provided strong evidence that the surface on which the faulting occurred is relatively long and narrow with its long axis nearly horizontal and situated immediately above an internal crustal discontinuity that lies at a depth of about 30 km the faulting process was essentially antiplane and unilateral with the crack starting at the northwest end of the long axis and propagating in the negative strike direction towards the southeast and slip durations at individual points on the fault were short of the order of 0 2 sec compared with the time taken for the crack to propagate from its starting point to the far end of the fault about 2 5 sec in the model the inferred short slip durations are consistent with the inferred fault geometry and both are consistent with normal static and dynamic stress drops i e within a factor of 2 of 100 bars the narrow elongate fault geometry and associated short slip durations are responsible for the enhanced high frequency radiation as compared with normal events having the same seismic moment release the associated spectra differ greatly from that of the brune spectral model with the former having at least two corner frequencies whereas the latter has only one the unilateral fault propagation resulted in a strong azimuthal variation of waveform characteristics with the largest amplitudes and shortest durations concentrated towards the south and southeast and the smallest amplitudes and longest durations in the opposite directions as the majority of strong motion instruments and closer ectn stations were located within the former azimuthal range the anomalous high frequency content and large observed amplitudes are accentuated by the station distribution with respect to the directivity characteristics of the source another significant factor likely to have contributed to the observed peak amplitudes at least in some instances is an indicated superposition of direct waves from the source with reflections from the inferred mid crustal discontinuity elongate source geometry and associated effects such as those described above may not be uncommon for large thrust intraplate earthquakes and may have significant design implications for critical structures such as nuclear power plants

modeling of source rupture characteristics for the saguenay earthquake of november 1988 the magnitude mb 6 0 saguenay earthquake of 25 november 1988 in quebec canada was one of the largest to have occurred in eastern north america during the past half century recorded high frequency ground motions exceeded anticipated values for an event of its size by a factor of 10 on both the regional network and strong motion instruments two proposed explanations for the discrepancy are 1 that the source was a rare high stress drop event and 2 that it was an asymmetrical fractional stress drop rupture involving only normal effective stresses in this article detailed fault slip models are derived to fit characteristics of strong motion displacement velocity and acceleration data the results establish that the effective rupture stress was normal less than 100 bars that the fault rupture was highly asymmetrical with respect to the point of rupture initiation and that the average slip time for points within the rupture area approx 0 2 sec was considerably less than that associated with the standard brune 1970 source spectral model the rupture area developed in a number of episodes each widening or lengthening the previously ruptured area which may explain the short average slip time the results indicate that the widely used assumption in hazard analyses that earthquake spectra are adequately represented by the standard brune 1970 complete stress drop model may be seriously unreliable for prediction of strong ground motion in eastern north america

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

high frequency ground motion scaling in the pacific northwest and in northern and central california abstract large data sets of vertical and horizontal seismograms from the pacific northwest seismic network northern california seismic network and berkeley digital seismic network are used to study the high frequency 0 25 16 hz ground motion scaling characteristics in washington oregon northern california 39 n to 42 n and 119 w to 124 w and central california 35 n to 39 n and 118 w to 123 w we used peak filtered ground velocities to characterize the propagation excitation and site terms the regression results for propagation were modeled using a geometrical spreading function g r and a frequency dependent attenuation q f q 0 f for the pacific northwest the best q model that fits the observation is expressed by q f 280 f 0 55 the eastern central california and western central california results are parameterized with q f 280 f 0 50 and q f 240 f 0 35 respectively the northern california results are not easy to model and require using a frequency dependent and a frequency dependent geometrical spreading the geometrical spreading effect for the frequencies higher than 5 hz is very strong in that region the excitation terms for the small events studied were modeled using a brune s source model an average stress drop of 30 bars was obtained for the pacific northwest the northern california average value for stress drop is 90 bars the observations of eastern central california were modeled with a stress drop of 49 bars while the best fit for the western part required 80 bars the range of the values obtained for q g r and indicates that the ground motion parameters for one region should not be used for another as an example our results show that the ground motion amplitude due to an m w 5 0 earthquake at a distance of 50 km is different by a factor of 1 5 between the pacific northwest and eastern central california

use of empirical green s functions spectral ratios and kinematic source models for simulating strong ground motion abstract ground motions for large and moderately large earthquakes at short and moderate distances are particularly important for seismic hazard estimation in eastern north america ena very few direct observations of such ground motions have been obtained however because of the sparsity of recording sites and the relatively low rates of occurrence of large earthquakes inside the region estimation of strong ground motion must therefore rely heavily on theoretical models to extend empirical results obtained from small earthquakes and from the few larger ones for which reliable data are available because of the generally large distances between recording stations the main source of useful data comes from lg wave trains observed at relatively large distances for the two largest earthquakes to have occurred near populated regions of southeastern canada during the past decade spectral ratios of the lg wave trains of the mainshocks with respect to those of their aftershocks are found to depend almost entirely upon the source radiation characteristics of the sources alone this result is utilized to derive elastodynamically based kinematic rupture models that are consistent with the empirical spectral ratio data such models provide a firm physical basis from which to infer the most probable spectral characteristics for future large earthquakes in the region in converse application it is shown that spectral ratios obtained from such models along with empirical seismograms from small earthquakes can be used to accurately simulate strong ground motions at short and moderate as well as large distances as such small event seismograms are relatively plentiful the problem of reliable strong ground motion estimation is therefore reduced to that of obtaining reliable representative source spectral models the solution of this latter problem must continue to depend upon whatever empirical data are available and upon appropriately detailed theoretical modeling

rupture histories of eastern north american earthquakes abstract digital worldwide records of p and sh waves are used to invert for rupture histories of the recent larger eastern north american earthquakes the data include a broad bandwidth to facilitate recovery of both source details and total moment the following events are studied 9 january 1982 new brunswick 5 october and 23 december 1985 nahanni 25 november 1988 saguenay and 25 december 1989 ungava earthquakes we employ a finite fault waveform inversion scheme that discretizes the slip history as a function of position on the fault this formulation allows estimation of rise time and slip velocity in addition to the spatial distribution of slip static stress drops averaged over the entire rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western united states earthquakes estimates of stress drop for spatially limited asperities are in the range of a few hundred bars with the value for the saguenay earthquake asperity being the largest approaching 1 kbar the variation in stress drop is considerable but no evidence is seen for a scaling relation in which stress drop increases with moment maximum slip velocities on the fault also have a wide range from 50 cm sec to perhaps greater then 200 cm sec with the saguenay and 5 october nahanni earthquakes lying at the upper end of this range of the events studied the saguenay earthquake is unique in terms of its greater depth spatially concentrated source and large asperity stress drop

anatomy of a small intraplate earthquake a dissection of its rupture characteristics using regional data summary the magnitude mblg 5 0 mont laurier earthquake of 1990 october 19 in quebec canada was one of the largest to have occurred in eastern north america during the past decade high frequency ground motions recorded on regional network instruments exceeded values anticipated for an event of its size by a factor of 3 a commonly favoured explanation for the discrepancy is that the source was a rare high stress event in this paper detailed fault slip models are derived to fit waveform and spectral characteristics of the regional data the results establish that the effective rupture stress was normal about 100 bars that the fault rupture developed asymmetrically and that the average slip time for points inside the rupture area approx 0 1 s was significantly less than that associated with the standard brune 1970 source spectral model the rupture area developed in at least four distinct episodes each extending the previously ruptured area taken together with similar results for the mblg 6 5 saguenay earthquake of 1988 november the results indicate that a widely used assumption in hazard analyses that earthquake spectra are adequately represented by the standard brune spectral model is unreliable for the interpretation and prediction of strong ground motion

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

post orogenic thermal reset of the pingtan dongshan metamorphic belt se china insights from zircon fission track and u pb double dating abstract zircon double dating technique for u pb and fission track ft ages is used to reassess the cooling conditions for the pingtan dongshan metamorphic belt pdmb three cretaceous gneissic bodies from the chinmen area and two dioritic plutons from the zhangzhou igneous complex zic in two sides of the changle nanao fault are studied u pb dating results define the magma emplacement of gneisses at 144 134 ma and dioritic rocks at 102 99 ma and ft dating results give ages of 96 86 ma and 87 ma for corresponding samples combining 40ar 39ar mineral ages reported previously the fast cooling curve 86 c myr 100 96 ma of zic plutons matches near surface intrusives however those for deep seated chinmen gneisses are problematic as their resultant conditions are similar to zic samples a recent concept favors the early to late cretaceous tecto magmatic evolution in the coastal se china involving episodes of orogenic magmatism 147 130 ma and shallow magmatism under post orogenic extension 110 85 ma as the pingtan island in northern pdmb is considered as a remnant piece of colliding fragment the cooling rate known for gneissic bodies in this island 35 c myr 130 120 ma can better represent the record of this collision on the basis of these features we advocate a magmatic quiescence at 130 120 ma representing collision between pdmb and a concealed microcontinent which resulted in rapid uplift of this metamorphic belt cooling curves obtained for chinmen gneisses are suggested to have further subjected to thermal reset by high temperature magmatism taking place during the post orogenic stage

spectral estimates of seismic shear waves the scaling of seismic sources according to brune 1970 1971 with a stress drop of 100 bars allows accurate prediction of average fourier spectra of seismic shear waves in the far field for the frequency band 1 to 10 hz random vibration theory under the assumption of stationary response accurately predicts average linear elastic response spectrum amplitudes in this same frequency band these conclusions are reached through comparison of theoretical estimates with strong motion observations during a wide range of california earthquakes local magnitudes 4 0 to 7 2 distances 9 to 130 km thus these models are an accurate physically based method of estimating high frequency strong ground motion for a range of earthquake magnitudes and distances and offer the advantage over purely empirical methods of a physical basis for extrapolating to predict ground motions for magnitudes and distances which are poorly documented with data

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

attenuation of strong ground motion in canada from a random vibrations approach abstract theoretical relations are developed for the attenuation of horizontal peak ground acceleration pga and velocity pgv in canada with emphasis on eastern canada the theoretical basis is a composite of several approaches at intermediate epicentral distances results from random vibration theory hanks and mcguire 1981 boore 1983 which can be used to describe pga and pgv in terms of shear radiation implied by brune9s 1970 source model are applied to canadian earthquakes motions are extended into the near source region using arguments concerning magnitude scaling joyner and boore 1981 at distances greater than two crustal thicknesses ground motions are attributed largely to the lg phase and are attenuated accordingly nuttli 1982 the resulting attenuation relations for pga and pgv in eastern canada are in excellent agreement with strong motion data from moderate eastern canadian and central united states earthquakes compared to empirical attenuation relationships based on modified mercalli intensity the theoretical model predicts a stronger dependence of peak ground motions on magnitude in other respects amplitude levels and distance decay empirical and theoretical predictions are similar the model predicts that eastern events will have higher peak accelerations but similar peak velocities in comparison to their western counterparts of the same moment magnitude this arises from the relative enrichment of eastern earthquakes in higher frequencies

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

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

earthquake source spectra in eastern north america abstract a new model for average earthquake s wave source spectra in eastern north america ena is derived the model is consistent with fundamental elasto dynamic principles as well as the familiar spectral scaling laws appropriate to sources having geometrical and dynamical similarity observed characteristics of s wave spectra in ena are fully explained as simple consequences of directivity effects entailed by the classical crack rupture model kostrov 1964 madariaga 1976 with normal effective stresses of the order of 100 bars and fractional stress drop brune 1970 abnormally high stress drops are not required recognition of the fundamental role of directivity explains most features of observed data that are currently interpreted as anomalous among other things neglect of such effects is shown to be responsible for the conclusion by boatwright and choy 1992 and boore and atkinson 1992 that high frequency spectral levels for the magnitude 6 saguenay earthquake of 1988 are anomalously large other important implications of the new model are that some previously inferred systematic variations in brune stress drop with magnitude are simply artifacts of the invalid use of the standard brune 1970 1971 model assumption and that current estimates of q in ena which depend on this assumption are likely to be significantly too large for high frequencies associated uncertainties allow the possibility that effective stress does not vary significantly with magnitude for earthquakes having magnitudes in the range of 4 to 7 the new model suggests that some currently favored source spectra may under estimate spectral amplitudes of future larger earthquakes in ena by as much as a factor of 6 for some frequencies the proposed new spectra are practically identical with those of the brune standard model for an effective stress of 100 bars for frequencies up to about twice the brune corner frequency but then increase to values 3 times larger over a range of about 4 in frequency as such the high frequency spectral levels are equivalent to standard brune model values for earthquakes of one full magnitude unit larger with consequent important implications for seismic hazard estimates in the region

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

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions the achankovil shear belt of the southern indian peninsular shield is prominent on landsat images it coincides with the boundary between charnockites of the madurai block to the north and khondalites of the trivandrum block to the south and with a major change in the aeromagnetic pattern that can be traced across southern india field investigations reveal a major shear zone along the southwestern edge of the achankovil shear belt the tenmala shear zone rocks in the tenmala shear zone include charnockite garnet biotite gneiss garnet sillimanite gneiss cordierite gneiss and granite kinematic indicators include stretched and asymmetric garnet feldspar and quartz porphyroclasts shear bands asymmetric folds extensional and contractional composite structures hook folds on rotated and deformed gash veins and lineations kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear with a minor component of reverse shear textures as well as mineral assembl

stochastic prediction of ground motion and spectral response parameters at hard rock sites in eastern north america empirical predictions of ground motions from large eastern north american earthquakes are hampered by a lack of data for such events for this reason most prediction techniques have been based at least in part on data from the seismically active and well instrumented western north america concentrating on the prediction of response spectra on hard rock sites we have used a relatively new theoretical technique that does not require western data to make ground motion predictions for eastern north america this method often referred to as the stochastic model has its origins in the work of hanks and mcguire who treat high frequency motions as filtered random gaussian noise for which the filter parameters are determined by a seismological model of both the source and the wave propagation the model has been successfully applied to the predictions of ground motions in the western united states and to short period magnitudes from large to great earthquakes worldwide for our application the essential parameters of the model are estimated by using existing data from small to moderate eastern north american earthquakes a crucial part of the model is the relation between seismic moment and corner frequency the relation proposed in 1983 by nuttli for mid plate earthquakes leads to predictions of ground motions that are lower than available data by a factor of about 4 on the other hand a constant stress parameter of 100 bars gives model predictions in good accord with the data to aid in applications the ground motion predictions are given in the form of regression equations for earthquakes of magnitude 4 5 to 7 5 at distances within 100 km of the source the explanatory variables are hypocentral distance and moment magnitude m because predictions are often required in terms of m g rather than m we have used the theoretical model to establish a relation between the two magnitudes the predicted relation agrees with the sparse data available although the large uncertainties in the observed magnitudes for the larger events as well as the sensitivity of the theoretical magnitude to the attenuation model make it difficult to discriminate between various source scaling models

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

evaluation of models for earthquake source spectra in eastern north america there have been several relations proposed in the last few years to describe the amplitudes of ground motion in eastern north america ena these relations differ significantly in their assumptions concerning the amplitude and shape of the spectrum of energy radiated from the earthquake source in this article we compare ground motions predicted for these source models against the sparse ena ground motion database the source models evaluated include the two comer models of boatwright and choy 1992 atkinson 1993a haddon 1996 and joyner 1997a b and the one comer model of brune as independently implemented by frankel et al 1996 and by toro et al 1997 the database includes data from ena mainshocks of m 4 and historical ena earthquakes of m 5 5 for a total of 110 records from 11 events of 4 m 7 3 all recorded on rock we also include 24 available rock records from 4 large earthquakes in other intraplate regions con clusions are checked to determine whether they are sensitive to the addition of these non ena data the atkinson source model as implemented in the ground motion relations of atkinson and boore 1995 is the only model that provides unbiased ground motion predictions over the entire period band of interest from 0 1 to 10 sec the source models of frankel et al 1996 toro et al 1997 and joyner 1997a b all provide unbiased ground motion estimates in the period range from 0 1 to 0 5 sec but over estimate motions at periods of 1 to 10 sec the haddon 1996 source model over predicts motions at all periods by factors of 2 to 10 these conclusions do not change significantly if data from non ena intraplate regions are excluded although the ten dency of all models toward overprediction of long period amplitudes becomes more pronounced the tendency of most proposed ena source models to overestimate long period motions is further confirmed by an evaluation of the relationship between ms a measure of the spectrum at 20 sec period and moment magnitude a worldwide catalog of shallow continental earthquakes triep and sykes 1996 is compared to the ms m relations implied by each of the source models the atldnson source model is consistent with these data while other proposed ena models overpredict the average ms for a given m the implications of mmi data from historical earthquakes are also addressed by exploiting the correlation between felt area and high frequency source spectral level high frequency spectral amplitudes as specified by the atkinson and boore 1995 frankel et al 1996 toro et al 1997 and joyner 1997a b source models equal or exceed the levels inferred from the felt areas of most of the large ena events with the noteable exception of the saguenay earthquake by contrast high frequency spectral amplitudes specified by the haddon 1996 source model agree with the felt area of the saguenay earthquake but overpredict the felt areas of nearly all other large events in general models that fit the saguenay data be it intensity data strong ground motion data regional seismographic data or teleseismic data will not fit the data from the remaining earthquakes a source model derived from the california database suitably modified for re gional differences in crustal properties is also evaluated this model is not signifi

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

comparison of source scaling relations of eastern and western north american earthquakes source scaling relations have been obtained for earthquakes in eastern north america and other continental interiors and compared with a relation obtained for earthquakes in western north america the scaling relation for eastern north american earthquakes was constructed from measurements of seismic moment and source duration obtained by the waveform modeling of seismic body waves the events used include nine events of m blg magnitude 4 7 to 5 8 that occurred after 1960 and four earlier events with magnitudes between 5 5 and 6 6 the scaling relation for events in other continental interiors was used for comparative purposes and to provide constraints for large magnitudes detailed analysis of the uncertainties in the scaling relations has allowed the resolution of two important issues concerning the source scaling of earthquakes in eastern north america first the source characteristics of earthquakes in eastern north america and other continental interiors are consistent with constant stress drop scaling and are inconsistent with nonconstant scaling models such as that of nuttli 1983 second the stress drops of earthquakes in eastern north america and other continental interiors are not significantly different from those of earthquakes in western north america and have median values of approximately 100 bars the source parameters of earthquakes in eastern north america are consistent with a single constant stress drop scaling relation whereas the source parameters of earthquakes in western north america are much more variable and show significant departures from an average scaling relation in which stress drop decreases slightly with seismic moment

an investigation into earthquake ground motion characteristics in eastern north america abstract a random vibration model of the hanks mcguire type is used to predict peak ground motions at rock sites in eastern north america the assumed geometric decay and distance dependent duration approximate the propagation of direct body waves at short distances and lg waves at regional distances the model predicts peak acceleration and velocities as well as response spectra and magnitude for a given seismic moment and corner frequency to predict ground motions for a given lg magnitude m lg the model is first used to calculate the seismic moment corresponding to that magnitude assuming a source scaling law then knowing the moment and corner frequency the model is used to calculate peak ground motions available data from strong motion recordings and from the ectn and lrsm networks modified to estimate horizontal ground motions on rock where appropriate are used to verify the model9s assumption and predictions modified mercalli intensity data are also used ground motions predicted by the model with a stress drop of 50 to 200 bars agree with the ground motion data but the m lg values computed by the model for given seismic moments do not agree with nuttli9s 1983 moment versus m lg data for large earthquakes resolution of the latter disagreement awaits the collection of instrumental data from large events in eastern north america

three dimensional finite difference simulation of fault dynamics rectangular faults with fixed rupture velocity we analyze three dimensional finite difference solutions for a simple shear crack model of faulting to determine the effects of fault length and width on the earthquake slip function the fault model is dynamic with only rupture velocity fault dimensions and dynamic stress drop prescribed the numerical solutions are accurate for frequencies up to 5 hz and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies near the hypocenter the slip velocity exhibits a square root singularity whose intensity increases with hypocentral distance at distances greater than the fault width w growth of the velocity intensity ceases and the slip function becomes nearly invariant with distance along the fault length closed form expressions are developed for the dependence of static slip s slip rise time tr and slip velocity intensity v on fault geometry along the center line of a long narrow fault at hypocentral distances exceeding w these expressions reduce to s w tr 0 5 w vr and v w 2 vr where is the dynamic stress drop the shear modulus and vr the rupture velocity the numerical results imply that uniform dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high frequency ground motion relative to low frequency ground motion a further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width but will be independent of rupture length