crack fusion dynamics a model for large earthquakes the physical processes of the fusion of small cracks into larger ones are nonlinear in character a study of the nonlinear properties of fusion may lead to an understanding of the instabilities that give rise to clustering of large earthquakes we have investigated the properties of simple versions of fusion processes to see if instabilities culminating in repetitive massive earthquakes are possible we have taken into account such diverse phenomena as the production of aftershocks the rapid extension of large cracks to overwhelm and absorb smaller cracks the influence of anelastic creep induced time delays healing the genesis of juvenile cracks due to plate motions and others a preliminary conclusion is that the time delays introduced by anelastic creep may be responsible for producing catastrophic instabilities characteristic of large earthquakes as well as aftershock sequences however it seems that nonlocal influences i e the spatial diffusion of cracks may play a dominant role in producing episodes of seismicity and clustering

a comparison between the tectonic stress measured in situ and stress parameters from induced seismicity at monticello reservoir south carolina during may and early june 1979 five three component digital seismographs were deployed near the monticello reservoir south carolina to determine the stress drops of reservoir induced earthquakes and compare them with earlier hydrofracture stress measurements of zoback and hickman this issue records for 327 events were obtained 10 of these events each recorded on four or more stations were analyzed to determine stress drops depths for the 10 events ranged from 0 5 to 1 4 km fault plane orientations were calculated from an inversion of the body wave displacements individual radiation pattern corrections were applied in calculating the moments at each station final estimates of seismic moment were obtained by averaging logarithmically the estimates from each component for each event an arithmetic moment was obtained from the individual estimates of the source radius and stress drop was then calculated once from the final estimates of seismic moment and source radius for each event moments of the 10 digitally recorded earthquakes ranged from 2 5 1017 to 2 7 1018 dyne cm for the p wave data and from 4 6 1017 to 4 6 1018 dyne cm for the s wave data the stress drops s wave data only generally ranged from about 0 2 to 4 bars four earthquakes were recorded by a strong motion instrument at the dam site magnitudes m for these events ranged from 2 8 to 3 0 moments taken as averages of the values from each component ranged from 1 7 to 3 4 1020 dyne cm and the stress drops from 13 to 92 bars large differences between the corner frequencies of the two components factors of 2 5 and 4 for two of the events correspond to a large relative uncertainty for these stress drop values nevertheless tens of bars of stress are apparently released for the largest events in the monticello sequence but only a few bars are released for the smaller magnitude events in comparison the in situ stress data measured in two holes give an estimate of shear stress of about 45 bars at 0 2 km depth and 30 to 70 bars at 1 km depth in the northern hole and from 15 20 bars near the surface to almost 70 bars at depth in the southern hole the variation in shear stress inferred from in situ measurements at the deeper levels indicates an inconsistency between the thrust faulting mechanisms of the earthquakes and the state of stress which is more consistent with thrust faulting at 0 2 km depth and strike slip or normal faulting below thus the estimates of shear stress from in situ measurements are of the same order as the stress drops of the m 2 8 3 0 events but about 1 order of magnitude larger than the stress drops obtained for the smaller events

broad band deconvolution of gdsn data summary advances in the theory of wave propagation and source mechanism have coincided with the recent deployment of a global network of digitally recording seismographs the gdsn global digital seismograph network we demonstrate that broad band records of body wave ground displacement and velocity with spectral information from several hertz to tens of seconds can be obtained by the simultaneous deconvolution of the long and short period components of the gdsn seismographs frequency dependent effects in body waves that may arise from source complexity or from propagation through the earth are much more apparent and measurable in the broad band records than in the original seismograms

finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

seismic and aseismic deformation associated with the 1952 kern county california earthquake and relationship to the quaternary history of the white wolf fault synthesis of geodetic geologic and seismic data from the white wolf fault california indicates that the fault separates an area of late quaternary and continuing rapid uplift in the tehachapi mountains and transverse ranges from even more rapid subsidence in the southern san joaquin valley on july 21 1952 rupture of the white wolf fault produced the ml 7 2 kern county earthquake we used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952 1953 horizontal shear strains to model the measured coseismic vertical displacements with an elastic dislocation model a curved fault trace with decreasing fault depth 27 to 10 km from the surface vertically to the base slip 3 to 1 m and dip 75 to 20 from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record the measured seismic moment the fault plane solution and the pattern of surface rupture two short releveled lines near the 1952 epicenter tilted 4 and 17 rad down to the north from 5 10 years before the earthquake the preseismic tilts differ significantly from ten other surveys of these lines left lateral fault crossing shear strain from 0 2 20 years before the quake was two times greater than both preseismic off fault strains and the post seismic fault crossing strains during the first seven years after the earthquake aseismic deformation was negligible from 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone rising first in the epicentral region and then at the northeast end of the fault this was unaccompanied by any surface fault slip reconstruction of the vertical separation on the white wolf fault from late quaternary and late miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty fold from 0 1 0 2 mm yr to 3 9 mm yr between the past 10 15 m y and the most recent 0 6 1 2 m y we estimate a 170 to 450 yr average recurrence interval for earthquakes on the white wolf fault with slip equivalent to that in 1952 the 1952 earthquake appears to be characteristic of the quaternary record of fault displacement in the increase in white wolf slip toward the san andreas fault the ratio of reverse to lateral slip 1 3 1 and the ratio of vertical fault slip to emergence of the hanging wall block 3 1 the 8500 m deep sedimentary basin on the down thrown block cannot be explained by repeated slip of the white wolf fault in an elastic medium

waveforms and spectra of preshocks and aftershocks of the 1979 imperial valley california earthquake evidence for fault heterogeneity we have compared digitally recorded waveforms of m l 2 0 2 8 earthquakes that occurred in two small areas along the imperial fault before and after it broke in the ml 6 6 imperial valley earthquake on october 15 1979 eight preshocks 1977 1979 from a 4 by 1 km area centered 4 km se of the mainshock epicenter have strikingly similar waveforms over the entire record length 30 s with an average peak cross correlation between seismograms of 0 74 the seismograms are well correlated at frequencies up to at least 4 hz this implies similar source mechanisms and hypocenters within of the 4 hz wavelengths i e 200 400 m five aftershocks from the same area show an average peak cross correlation between seismograms of only 0 23 any associated changes in mechanism must be small because they are not reflected in the first motion data analysis of frequency content of these events using bandpass filtering techniques showed no systematic temporal changes in spectral shape ten preshocks and 24 aftershocks from a 1 by 2 km source area centered along the fault 16 km nw of the mainshock epicenter were also studied first motion data suggest that all of the aftershocks and a swarm of six preshocks on december 7 9 1978 were associated with the main fault but that four earlier preshocks were not the six preshocks on december 7 9 1978 were tightly clustered as evidenced by the strong similarity of the waveforms most peak cross correlations 0 6 during this swarm the 8 to 16 hz spectral amplitude increased relative to the 1 to 2 hz spectral amplitude over the whole record length by about a factor of 3 suggesting a systematic increase in stress drop groups of like events are also present among the aftershocks in this data set the average peak correlation for pairs of aftershocks 0 43 is almost the same as that for pairs of preshocks 0 45 if all 10 preshocks are included however several sources appear to have been active simultaneously during the aftershock period so that no more than two to three consecutive aftershocks have maximum cross correlations 0 6 the highly localized sources characterized by waveform similarity may represent fault asperities or clusters of asperities our observations are consistent with a decrease in the number of these asperities as the weaker ones fail under increasing stress during the intervals between large earthquakes

seismic codas on the earth and the moon a comparison the phenomenon of the seismic coda which is composed of seismic energy delayed by scattering is seen on both the earth and the moon on the moon the scattered coda is very large relative to body wave arrivals with a delay of the time of maximum energy whereas on earth scattered codas are relatively small and show no delay of the energy maximum in both cases the form of the coda is controlled by three distance scales the mean free path l which is the average distance seismic energy travels before it is scattered the attenuation distance x which is the average distance seismic energy travels before it is attenuated and the source receiver distance r two coda models are discussed based on these parameters a strong scattering diffusion model and a weak scattering single scattering model a discussion of the diffusion scattering model indicates that if x l 1 diffusion scattering is an appropriate model but if x l 1 single scattering is the appropriate model within the appropriate range of r a survey of the literature indicates that for the frequency range 0 5 10 hz diffusion scattering is important in lunar codas but for the frequency range 1 25 hz single scattering is important in terrestrial codas another important effect of attenuation is the elimination of scattering paths much longer than x on the moon this means that seismic energy in the coda can only propagate directly in the near surface strong scattering zone between surface sources and the seismometer for source seismometer separations of the order of x l 12 otherwise scattering is limited to regions near the source and the receiver on earth this effect probably prevents multiple scattering

the estimation of green s function from local earthquake recordings and the modelling of the site response abstract a method is presented for the modelling of the site response from the deconvolution of local earthquake recordings the green s function for the propagation medium is obtained by using the assumption of minimum phase propagation for a stratified absorptive earth model as deconvolution criterion the application of this technique to theoretical data shows that local synthetic seismograms can be deconvolved with high accuracy the site response for an actual site bhb in the swabian jura southwest germany has been estimated by integrating over a number of green s functions obtained for different incidence angles thus enhancing the effects of the superficial layers and reducing the effects of the deeper structure the comparison between accelerograms modelled with the obtained average site response and an observed accelerogram yields a good fit in peak and rms acceleration and strong motion duration

an investigation of the regional variations and frequency dependence of anelastic attenuation in the mantle under the united states in the 0 5 4 hz band summary studies of teleseismic p and s wave amplitudes and spectra in the 0 5 4 hz band show large variations in the attenuative properties of the upper mantle under the united states the data indicate that attenuation is greatest under the south western united states including but not confined to the basin and range province the lowest attenuation prevails under the north central shield regions the north eastern part of the country consisting of new england and possibly including a larger area along the eastern seaboard is characterized by moderate attenuation in the mantle the level of the high frequency energy in short period seismic waves and the differences between q values derived from short and long period data indicate that q is frequency dependent the form of frequency dependence of t compatible with the data in the 0 5 4 hz range does not allow a rapid decrease of t with increasing frequency rather it supports a gradual decrease covering the broader 0 1 4 hz range the curves of t versus frequency for shield to shield and mixed shield to western united states type paths are parallel with an average difference of 0 2 s in t in the short period band but may diverge towards the long period band for both curves t p is below 1 s for shield to shield paths t p must be below 0 5 s at 1 hz

san andreas fault earthquake chronology and lake cahuilla history at coachella california the southernmost 100 km of the san andreas fault has not ruptured historically it is imperative to determine its rupture history to better predict its future behavior this paleoseismic investigation in coachella california establishes a chronology of at least five and up to seven major earthquakes during the past 1100 yr this chronology yields a range of average recurrence intervals between 116 and 221 yr depending on assumptions with a best estimate average recurrence interval of 180 yr the most recent earthquake occurred c 1690 more than 300 yr ago suggesting that this stretch of the fault has accumulated a large amount of tectonic stress and is likely to rupture in the near future assuming the fault follows a stress renewal model this study also establishes the timing of the past 5 6 highstands of ancient lake cahuilla since a d 800 we found that earthquakes do not tend to occur at any particular stage in the lake cycle

regional variation and frequency dependence of qβ in the crust of the united states approximate lg attenuation coefficient values are determined for paths in eastern north america at periods between 2 and 4 sec where no data were previously available these new data together with reported values at a period of 1 sec are consistent with values predicted by a frequency dependentent q of the crust in which q varies as 0 2 the new data are inconsistent with values which would be predicted by models having prominent maxima or minima in q 1 within the period range 1 to 5 sec lg attenuation coefficients were computed for the frequency independent crustal q models of cheng and mitchell 1981 for the basin and range province and colorado plateau of the western united states the predicted values at 1 sec for the colorado plateau and basin and range province are about twice as large and three times as large respectively as those predicted for the eastern united states recently reported values of q for 1 sec lg in those regions are consistent with the attenuation coefficients and q values of lg predicted by the colorado plateau and basin and range province models a model with a small degree of frequency dependence of q would be consistent but is not required by the data

strong motion modeling of the imperial valley earthquake of 1979 twelve three component strong motion displacement records are modeled for the 1979 imperial valley earthquake to recover the distribution of slip on the imperial fault plane the final model for which point source responses are calculated by a discrete wavenumber finite element technique uses a structure with gradients in material properties rather than layers the effects of a velocity gradient are investigated by comparing synthetics with a layer over a half space model using generalized rays it is shown that a uniform fault rupture model on a rectangular fault plane does not explain the data the preferred fault model has slip concentrated below 5 km in the basement material and between the epicenter 5 km south of the international border and highway 80 within this region there appears to be two localized areas of larger dislocations one just north of the border near bonds corner and a second under interstate 8 at meloland overpass a major arrival associated with large amplitude vertical accelerations up to 1 7 g is identified in the el centro array records this arrival has an s p time of approximately 2 3 sec at many of the array stations and is modeled as originating from a localized source 8 km to the south of the array the moment is estimated to be 5 0 10 25 dyne cm from the strong motion records which is consistent with teleseismic body wave estimates the preferred fault model is strike slip with a 90 dip the average strike is 143 however to explain vertical waveforms near the fault trace a corrugated or wiggly fault plane is introduced the average rupture velocity is in the range 2 5 to 2 7 km sec 0 8 to 0 9 times the basement shear wave velocity the preferred model has unilateral rupture propagation to the north although the data would allow a small amount of propagation to the south the estimated stress drop for the entire fault plane is only 5 to 10 bars however the stress drop over the more localized sources is about 200 bars the fault model is consistent with the pattern of seismicity and observations of aseismic creep in the imperial valley and suggests that the southern half of the imperial fault acts as a locked section which breaks periodically

palaeomagnetism of the silurian lipeon formation nw argentina and the gondwana apparent polar wander path summary a palaeomagnetic study was carried out on ferriferous beds of the early silurian lipeon formation in nw argentina 24 5 s 69 9 w samples were collected at 19 sites in four different localities detailed thermal demagnetization has permitted the identification of three different magnetic components all components are carried by haematite as deduced from irm curves and microscope analyses component a is interpreted to be of recent origin due to its coincidence with the geomagnetic dipole direction and its association with post diagenetic platy haematite component b with exclusive positive inclinations was found in two out of four sampling localities its possible significance is discussed on the basis of an indeterminate result of the fold test and its correlation with the degree of deformation in the samples the third isolated component c was present in the other two localities it passed both fold and reversal tests and is clearly associated with oolitic haematite of early diagenetic origin a palaeomagnetic pole of possible early silurian age was computed on the basis of mean site directions of component c 74 9 s 214 8 e a95 9 9 k 47 n 6 this new pole implies lower latitudes for gondwana in the silurian than were previously proposed and suggests a modification of the gondwana apwp a reversed polarity option for the early middle palaeozoic path is suggested as a possible alternative

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

estimates of q in central asia as a function of frequency and depth using the coda of locally recorded earthquakes digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the hindu kush region of afghanistan were used to determine the attenuation factors of the s wave coda qc and primary s waves q an anomalously rapid decay of the coda shortly after the s wave arrival observed also in a study of coda in central asia by rautian and khalturin 1978 seems to be due primarily to depth dependent variations in qc in particular we deduce the average qc in the crust and uppermost mantle 100 km depth is approximately four times lower than the deeper mantle 400 km depth over a wide frequency range 0 4 to 24 hz further while qc generally increases with frequency at any depth the degree of frequency dependence of qc depends on depth except at the highest frequency studied here 48 hz the magnitude of qc at a particular frequency increases with depth while its frequency dependence decreases for similar depths determinations of q and qc agree suggesting a common wave composition and attenuation mechanism for s waves and codas comparison of these determinations of qc in afghanistan with those in other parts of the world shows that the degree of frequency dependence of qc correlates with the expected regional heterogeneity such a correlation supports the prejudice that qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments

stochastic model of earthquake fault geometry summary an earthquake fault pattern is assumed to consist of a system of infinitesimal elementary dislocation loops after an initial dislocation subsequent ruptures occur according to a critical branching process the position of each secondary dislocation loop is randomly shifted along the initial fault plane from the location of the main shock in addition the orientations of the fault plane and slip vector of the secondary dislocations are rotated according to a three dimensional cauchy distribution which has one intrinsic scaling parameter in their turn the secondary dislocations produce new dependent shocks according to the same law the process cascades indefinitely we simulate an earthquake fault system by the monte carlo method this fault pattern is both visually and statistically similar to real earthquake faults the model yields statistical spatial distributions that correspond to those of the analysis of the catalogue data of real earthquakes we incorporate in our scheme the time magnitude model of earthquake occurrence developed earlier by the monte carlo method we then simulate synthetic sequences that reproduce all known magnitude space time statistical properties of real earthquake catalogues the results of these simulations make us question the suitability of some terms that are commonly used in the theory of an earthquake source for example as a consequence of the validity of the stochastic model the concepts like an individual earthquake fault plane or fault surface crack tip or fault tip and friction do not have an unambiguous definition

the november 29 1978 oaxaca mexico earthquake a large simple event the recent oaxaca mexico earthquake november 29 1978 mw 7 6 ms 7 8 seismic moment m0 3 2 1027 dyne cm is of special interest because of its location within a predetermined seismic gap the event excited long period 100 200 s multiple rayleigh and love waves which were well recorded by the wwssn these data along with p wave first motion data and p waveforms were used to constrain the source mechanism the results indicate an oblique thrust mechanism consistent with subduction of the cocos plate to the northeast beneath mexico dip 14 strike n90 w rake 54 hence this event is indeed of the type anticipated by ohtake et al 1977 a local network of stations installed in a joint university of mexico california institute of technology program began operation 20 days prior to the mainshock gonzalez 1979 mcnally et al 1979 mcnally 1981 ponce et al 1980 the 43 foreshocks of magnitude m 2 8 were recorded by the network in a period of 20 days prior to the mainshock these events show an interesting spatial and temporal pattern which culminates in the last 1 8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake this pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity with supplemental stations the network continued operation until december 12 1978 singh et al 1980 and provided good hypocentral control for the more than 169 aftershocks of magnitude m 2 8 the area of the aftershock zone determined from these events is 5525 km2 85 km by 65 km in spite of the large size of the event p waves for the oaxaca event indicate an extremely simple source at the period range of the wwssn long period seismographs this simplicity suggests that the p waves were generated by a limited portion of the rupture surface perhaps by the breaking of a fault zone asperity this result may be further supported by the discrepancy between the larger surface wave moment compared with that determined from body waves such simplicity also appears to be the case for the 1965 and 1968 mexican earthquakes on the adjacent eastern and western ends of the oaxaca aftershock zone respectively chael et al 1980 this type of body wave simplicity for a large subduction zone earthquake is an important characteristic of the mode of strain release along some subducting plate boundaries

the montenegro yugoslavia earthquake of april 15 1979 source orientation and strength long period teleseismic p s and rayleigh waves and geologic considerations indicate that the montenegro earthquake involved thrust faulting on a plane striking nearly parallel to the adriatic coast and dipping ca 15 toward the yugoslav mainland there is some support from modeling of body waves recorded on long period wwssn instruments for a focal depth of 22 km but the possibility of a multiple source and the difficulty of matching some of the detailed characteristics of the p and s wave forms reduce our confidence in the determination of the depth fortunately the source orientation and moment of the event are not sensitive functions of the depth the long period 256 s moment was 4 6 10 19 nm 4 6 10 26 dyne cm the moment obtained by fitting the first cycle of p and s waves recorded on wwssn long period instruments is about four times smaller this increase of moment with period is consistent with spectral estimates of the moment from sh waves recorded at sro and asro stations

analysis of the el centro differential array for the 1979 imperial valley earthquake two methods of analysis have been applied to the el centro differential array data from the 1979 imperial valley earthquake spectral ratios of array averages were developed as one way of estimating the spatial averaging of ground motion this spatial averaging will result in a reduction in the high frequency motion experienced by a large rigid foundation that is assumed to move as a single unit as an alternative covariance measurements in several frequency bands were made to illustrate how the fraction of the power propagating as single traveling wave decreases with increasing distance between stations base averaging reduction factors determined from these analyses show that reductions of 10 to 30 per cent would be expected for this particular earthquake for foundations of about 50 m in dimension and for frequencies above 20 hz the effect is frequency dependent and does not show any significant reduction at frequencies below 5 hz for this particular situation at a distance from the rupture surface of about 5 km the reduction is more pronounced for horizontal components than for vertical components

lateral variations of high frequency seismic wave propagation at regional distances across the turkish and iranian plateaus in this study we investigate crustal and uppermost mantle physical properties that characterize some of the continental plateaus of the middle east this is done as part of a larger effort to map and compare high frequency wave propagation at regional distances across the earth s continental plateaus thousands of short period wwssn seismograms recorded at stations located in the middle east and produced by earthquakes with epicentral distances less than about 20 were examined visually in an effort to study lateral variations of high frequency 0 5 2 hz seismic wave propagation across this area particularly to the north of the zone of continental collision between the africa arabian and eurasian plates variations of frequencies and amplitudes of sn and lg relative to p are examined and mapped throughout the region and this work is supplemented by a study of velocities of pn sn and lg sn amplitude variations are very striking in this area an important observation of this study is that sn propagates efficiently beneath a major part of the turkish and iranian plateaus sn is strongly attenuated however in the northernmost portion of the plateaus south of the black and caspian seas and in an area between the two seas these regions are characterized in general by active tectonism including volcanism faulting and folding however this active tectonism is not restricted to the areas of high sn attenuation but appears to extend beneath other parts of the iranian and turkish plateaus patterns of lateral variations in the propagation of lg are not as consistent as those for sn lg propagates efficiently across turkey iran and adjacent regions but the lg waves that cross the turkish and iranian plateaus are weak and have relatively long predominant periods of about 2 5 s the lg phase is not observed when the path of propagation crosses the southern caspian and the black seas consistent with the evidence of oceanic type crustal structure beneath these seas lg is also not observed from subcrustal events located in the hindu kush region the velocity of pn beneath most of the plateau areas and particularly in the regions of sn attenuation ranges between 8 0 and 8 2 km s velocities for sn and lg throughout the middle east are about 4 5 and 3 4 km s respectively it is possible to interpret the efficient sn propagation and the relatively normal uppermost mantle p and s velocities beneath a major part of the turkish iranian plateau as indicating a partial underthrusting of the arabian continental plate beneath the iranian and turkish continental blocks alternatively the mapped regions of efficient sn propagation and high sn attenuation may result from a differential cooling of a past thermal anomaly in this case the isostatic compensation of the plateaus could be due to an anomalously hot uppermost mantle that developed behind the mid to upper tertiary subduction zone s in this region and to a subsequent crustal shortening after the collision of the arabian and eurasian plates our results cannot uniquely confirm the validity of the above proposed models

new palaeomagnetic data from central iran and a triassic palaeoreconstruction new pole positions for triassic and cretaceous times have been obtained from volcanic and sedimentary sequences in central iran these new results confirm the general trend of the apparent polar wander path apwp of the central east iran microplate ceim from the triassic through the tertiary as published by soffel and forster 1983 1984 two new palaeopoles for the triassic of the ceim have been obtained limestones and tuffs from the nakhlak region yield a mean direction of 094 0 25 0 n 12 k 4 1 95 24 7 after bedding correction corresponding to a palaeopole position of 310 8 e 3 9 s and volcanic rocks from the sirjan regions yield a mean direction of 114 5 35 1 n 44 k 45 9 95 3 2 after bedding correction and a palaeopole position of 295 8 e 10 3 n combining these with the two previously published results yields a new palaeopole position of 317 5 e 12 7 n for the triassic of the ceim thus confirming that large counterclockwise rotations of the ceim have occurred since the triassic time new results have also been obtained from cretaceous limestones from the saghand region of the ceim the mean direction of 340 7 26 3 n 33 k 44 3 95 3 8 and the corresponding palaeopole position of 283 1 e 64 4 n is in agreement with previously determined cretaceous palaeopole positions of the ceim furthermore results have also been obtained from triassic dolomite limestone sandstone and siltstone from the natanz region which is located to the west of the ceim a total of 161 specimens from 44 cores taken at five sites gave a mean direction of the five sites at 033 3 25 1 n 5 k 69 0 95 9 3 and a palaeopole position of 167 2 e 53 7 n they pass the positive fold test of mcelhinny 1964 on the level of 99 confidence this pole position is in fairly good agreement with the mean triassic pole position of the turan plate 149 e 49 n it indicates that the area of natanz has not undergone the large counterclockwise rotation relative to the turan plate since the triassic which has been shown for the ceim a triassic palaeogeographic reconstruction of iran arabia gondwana and the turan plate eurasia is also presented

analysis of states of stress between provinces of constant stress recent studies have indicated the existence of crustal provinces or domains within which the state of stress shows little apparent horizontal variation between provinces the state of stress may undergo substantial changes often over remarkably short distances as analyzed here the transition in stress state between adjacent domains is caused by shear tractions acting on the base of the elastic brittle layer and is mediated by a stress field within a zone between the provinces that can be determined by solving the equations of force equilibrium and stress compatibility the component of horizontal stress oriented perpendicular to the provincial boundary changes smoothly across the transition zone from one province to the other whereas the horizontal stress oriented parallel to the boundary shows smaller but discontinuous changes the horizontal component of shear stress is unlikely to change from one province to another because of the implausible nature of the required basal traction thus in this regard states of stress in adjacent domains appear to be coupled the coincidence of a number of stress transition zones with boundaries defined by heat flow suggests that in many cases the basal tractions are the result of thermally induced mass transport below the elastic brittle layer in the direction of increasing horizontal compressive stress this analysis provides a quantitative relationship between changes in the stress field measured in the upper portion of the crust and shear stresses acting on the base of the elastic brittle layer

seismic imaging of the site response using microearthquake recordings part i method abstract microearthquake signals from widely distributed geographical regions have been noticed repeatedly to be strongly affected by local site effects the corresponding seismograms contain important information on the individual propagation path a single station inversion method is presented which allows the imaging of the subsurface impedance structure from locally recorded sh waves records the technique is based on the kunetz claerbout equation which in its original form states the relationship between the transmission response and the reflection response for p waves in a horizontally layered medium and vertical incidence claerbout 1968 by adapting claerbout9s formulation to the transmission problem for sh waves under oblique incidence microearthquake recordings can be used to calculate corresponding pseudo reflection seismograms which in turn can be inverted for the impedance structure by exploiting levinson recursion limitations in the high frequency content of the signals due to the combined effects of absorption recording system and bandlimited source signals as well as the influence of noise drastically reduce the resolutional power of the inversion procedure a number of different deconvolution techniques have been tested with synthetic seismograms for the applicability to reduce these effects good results have been obtained with the blind deconvolution technique of scherbaum and stoll 1985 utilizing the minimum phase property of the green9s function from the analysis of synthetic data it can be expected that for realistic source receiver situations and short period recording systems the main reflectors down to a depth of several hundreds of meters could be resolved

source mechanism and surface wave excitation for two earthquakes in northern baja california mexico summary two earthquakes of comparable local magnitude from different regions of northern baja california the 1975 july 17 pino solo earthquake ml 5 1 from the peninsular ranges region and the 1976 december 7 mesa de andrade earthquake ml 5 3 from the colorado river delta region are studied in detail to determine possible causes for the observed stronger excitation of surface waves by earthquakes from the delta region relative to earthquakes of comparable local magnitude in the peninsular ranges region data from distant stations are complemented with data from aftershock studies using local arrays and for the mesa de andrade earthquake with data from two strong motion stations the strong motion records for ihe mesa de andrade earthquake show that it consisted of two events 45 s apart which produced maximum recorded horizontal accelerations of 0 21 and 0 24g respectively at a distance of 18 km from the epicentre synthetic seismograms are used to help determine the depth of these events and their source time functions both events are relatively simple the second event is sharp and impulsive and could be thought of as an aftershock since it has a considerably smaller moment than the first event even though its recorded acceleration was higher comparison of measured parameters shows that while the local magnitude of the mesa de andrade earthquake is only 0 2 units larger than the pino solo earthquake and its source dimension is 2 2 times larger source areas 4 8 times larger its moment is larger by a factor of 6 2 8 5 this is approximately explained by the cy2 scaling law however the near source spectra and accelerations recorded on the strong motion accelerograph at riito at a distance of about 18 km are considerably larger than predicted by the scaling law this is probably a results of high stress drop asperities on the fault surface although near field focusing directivity may also have contributed

strong motion prediction using mathematical modeling techniques abstract the parkfield california earthquake of 1966 produced the first recording of ground motion in the immediate vicinity of an earthquake fault the simplicity of displacement pulse observed at station 2 aroused great interest among seismologists working in the area of earthquake source mechanism since then numerous theoretical works on simulating strong motion have been carried out starting with the use of a simple kinematic model assuming a uniform slip over a fault plane the assumption of uniform slip had to be modified to explain the strong motion observed during the parkfield earthquake of 1966 the borrego mountain earthquake of 1968 the san fernando earthquake of 1971 the imperial valley earthquake of 1979 the el asnam earthquake of 1980 and others through these studies the simulation technique has been advanced to include a more realistic medium in the beginning most methods used green9s function for an unbounded homogeneous medium then the free surface effect the effect of a sedimentary layer as well as laterally heterogeneous basin structure have been included in the simulation the above simulation studies were however restricted to relatively low frequency waves represented in a displacement or velocity seismogram in order to predict an acceleration seismogram dominated by high frequency waves hybrid models have been proposed in which gross features of rupture propagation are specified deterministically but the details of the process are described by a stochastic model specified by a small number of parameters the application of these models to several california earthquakes revealed encouraging results that some of the key parameters of the model are stable among earthquakes mathematical modeling techniques will play an important role in the prediction of strong motion for a great earthquake in california as well as for major earthquakes outside california based on the records of moderate earthquakes already acquired in california

source parameters and scaling laws of the 1978 swabian jura southwest germany aftershocks the 3 september 1978 swabian jura southwest germany earthquake mwa 5 7 was followed by a large number of aftershocks which have been recorded with a local network of five portable seismic stations the seismic moments fault radii and the static stress drops have been determined from the sh displacement spectra using the brune 1970 source model the data set is consistent with the gutenberg richter energy magnitude relation below a wood anderson magnitude of about 4 the corner frequencies increase only slowly with decreasing magnitudes no corner frequency higher than 15 hz has been observed in the magnitude range down to 0 8 correspondingly the high frequency decay slopes show a strong increase when the corner frequencies are approaching the maximum frequency this prevents the use of slope data for q determinations in terms of the madariaga 1977 crack model the data show a strong influence of source complexities on the smaller events

analysis of the ground accelerations radiated by the 1980 livermore valley earthquakes for directivity and dynamic source characteristics the strong motion accelerograph recordings of the 24 january 1980 main shock and the 27 january 1980 aftershock of the livermore valley earthquake sequence are analyzed for systematic variations with azimuth or station location the variation of the peak accelerations with epicentral azimuth is apparently reversed for the two events the main shock accelerations are larger to the south and the aftershock accelerations are larger to the northwest we eliminate the site effects by forming the ratio of the peak accelerations recorded at the same station after correcting for the epicentral distance this analysis indicates that source directivity caused a total variation of a factor of 10 in the peak accelerations comparison of this variation with the spatial extent of the aftershock sequences suggests that the strong directivity in the radiated accelerations is the result of unilateral ruptures in both events the accelerograms recorded at 10 stations within 35 km of the events were digitized to analyze the azimuthal variation of the rms acceleration the peak velocity and the radiated energy flux the variation of rms acceleration correlates almost exactly with the variation of the peak accelerations this correlation is analyzed using both deterministic and stochastic models for the acceleration waveforms the peak velocities corrected for epicentral distance vary with azimuth by a factor of 5 for both events while the radiated energy flux varies by a factor of 30 for the main shock and 15 for the aftershock the peak velocities are strongly correlated with the radiated energy flux the radiated seismic energies are estimated to be 2 6 0 9 1020 dyne cm for the main shock and 1 5 0 3 1020 dyne cm for the aftershock

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

the 1886 charleston south carolina earthquake and the appalachian detachment several lines of evidence from new and existing data are consistent with an extended fault slip associated with the 1886 earthquake in south carolina back slip over a portion of the appalachian detachment approximately corresponding to the coastal plain of south carolina fits some of the data including coseismic and post seismic strain indicators a small seismic source for the august 31 1886 main shock cannot be ruled out and a large but only partially seismic fault slip is possible during this event intensities of mm viii occurred over a wide area of the coastal plain and are not limited to the charleston summerville area where the traditional small source for this earthquake is located near source effects such as ground fissuring liquefaction changes in line of sight foreshocks and aftershocks occurred over a wide area of the coastal plain and neighboring inner piedmont which we tentatively refer to as the meizoseismal area this area is sharply bounded by the savannah river toward the southwest and approximately by the north carolina south carolina state border toward the northeast these boundaries are not associated with any known structure transverse to the strike of the appalachians during the 14 years prior to 1886 seismicity was concentrated around the extended meizoseismal area while the seismicity within that area was unusually low suggesting a premonitory doughnutlike pattern the intensity distribution in the far and near field of the 1886 event and of the great indian detachment earthquakes e g the 1905 m 8 and the 1934 m 8 3 events are qualitatively and quantitatively similar this is consistent with comparable source dimensions since well known intermediate magnitude earthquakes in india and in eastern north america exhibit similar intensity patterns and indicate similar seismic attenuation in the two cratonic environments several meters of coseismic 1886 northwest compressive strain near summerville are indicated by buckling of rails a large postseismic normal displacement is suggested by releveling data 1935 1968 across the brevard fault zone updip northwest of the presumed 1886 detachment slip these strains are consistent with a southeastward slip of the detached crystalline sheet associated with the 1886 earthquake available data indicate that in the southern appalachians the updip transition from maximum horizontal to least horizontal compressive stress in the dip direction perpendicular to structural trends occurs somewhere in the piedmont and may coincide with the steepest portion of the detachment the stress field may also contain a time dependent component in the sediments of the coastal plain orthogonal sets of subvertical clastic dikes oriented along and across the appalachian strike direction suggest one or more interchange of the principal stress axes we infer from coseismic dry fissuring and liquefaction that clastic dikes filled from above and from below were also generated during the 1886 event prehistoric clastic dikes along with other structures in the sediments may have been generated during prehistoric earthquakes similar to the 1886 event stress reversals corresponding to earthquake cycles may characterize the stress field within detached sheets or wedges that move primarily by great detachment earthquake ruptures appendices are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 008 1 00 payment must accompany order

variable rupture mode of the subduction zone along the ecuador colombia coast three large earthquakes occurred within the rupture zone of the 1906 colombia ecuador earthquake m w 8 8 in 1942 m s 7 9 1958 ms 7 8 and 1979 m s 7 7 we compared the size and mechanism of these earthquakes by using long period surface waves tsunami data and macroseismic data the 1979 event is a thrust event with a seismic moment of 2 9 10 28 dyne cm and represents subduction of the nazca plate beneath south america the rupture length and direction are 230 km and n40 e respectively examination of old seismograms indicates that the 1906 event is also a thrust event which ruptured in the northeast direction the seismic moment estimated from the tsunami data and the size of the rupture zone is 2 10 29 dyne cm the 1942 and 1958 events are much smaller about 1 5 to 1 10 of the 1979 event in the seismic moment than the 1979 event we conclude that the sum of the seismic moments of the 1942 1958 and 1979 events is only formula of that of the 1906 event despite the fact that the sequence of the 1942 1958 and 1979 events ruptured approximately the same segment as the 1906 event this difference could be explained by an asperity model in which the fault zone is held by a discrete distribution of asperities with weak zones in between the weak zone normally behaves aseismically but slips abruptly only when it is driven by failure of the asperities a small earthquake represents failure of one asperity and the rupture zone is pinned at both ends by adjacent asperities so that the effective width and the amount of slip are relatively small a great earthquake represents failure of more than one asperity and consequently involves much larger width and slip

analysis of peak ground motion in terms of a model of inhomogeneous faulting measurements of peak ground velocity v and acceleration a for earthquakes and mine tremors with local magnitudes ranging from 0 8 to 6 4 have been analyzed in terms of a model of inhomogeneous faulting the fault model involves the failure of a circular asperity of radius ri surrounded by a previously faulted annular region of outer radius r0 the failure of the asperity results in seismic radiation with a characteristic frequency proportional to 1 ri followed by lower frequency radiation proportional to 1 r0 as static equilibrium is regained over the larger region for r0 ri 1 both v and a are associated primarily with the failure of the asperity but also depend on the large scale source parameters specifically v r0 r 0 10 r0 ri 0 15 and a r 0 30 r0 ri 2 0 45 where is the shear wave velocity is the overall stress drop is the modulus of rigidity and is density the terms involving r0 ri correspond to the high frequency radiation associated with the failure of the asperity and the other terms indicate the peak parameters due to the broad scale readjustment observations of peak ground motion for events with seismic moments ranging from 5 1016 to 1026 dyn cm indicate that r0 ri is normally in the range of 1 10 and appears to be independent of earthquake size the inhomogeneous fault model also yields convenient expressions for the small scale displacement and stress drop of the asperity failure as well as for the level of regional stress available to cause slip

three dimensional spontaneous rupture propagation and implications for the earthquake source mechanism summary we consider spontaneous cracks spreading out areally over a fault plane in an infinite medium as a realistic earthquake source model the boundary integral equation technique described in das is used to determine the displacements and stresses everywhere on the crack plane for faults spreading out symmetrically in all four quadrants over infinite planes of constant yield strength we find that the terminal rupture velocity viiicr in the direction of purely anti plane rupture is given by viiicr in the purely in plane direction of rupture propagation we find the terminal rupture velocity viicr to be given by viicr or viicr 0 5 depending on the yield strength we also find that crack propagation in the purely antiplane direction does not influence crack propagation in the purely in plane direction and vice versa for infinitely long shear cracks of finite width the slip at a point is found to grow more slowly after the arrival of a shear wave diffracted from the nearer crack edge the slip virtually ceases after the shear arrival from the further crack edge this implies that for such faults the slip is controlled by the fault width for a rectangular fault in an infinite medium our final dynamic solution is in closer agreement with the static elliptical crack solution of eshelby than with the static rectangular dislocation model solution of chinnery we find the relationship between the average slip and average dynamic stress drop e on such a rectangular fault of half width w to be e c w where c 0 7 the slip at an interior point is again controlled by the fault width for faults that are much longer than wide so that the slip may stop in the region where the fault initiated before the completion of the rupture process this implies that two rectangular faults of varying lengths but of the same width have the same slip for the same average stress drop a fact clearly contradicted by observations that fault slip increases with earthquake size for great earthquakes occurring along the same fault this contradiction is resolved if stress drops are also bigger for bigger earthquakes

slip in the 1857 and earlier large earthquakes along the carrizo plain san andreas fault the moment magnitude mw 7 9 fort tejon earthquake of 1857 with a 350 kilometer long surface rupture was the most recent major earthquake along the south central san andreas fault california based on previous measurements of its surface slip distribution rupture along the 60 kilometer long carrizo segment was thought to control the recurrence of 1857 like earthquakes new high resolution topographic data show that the average slip along the carrizo segment during the 1857 event was 5 3 1 4 meters eliminating the core assumption for a linkage between carrizo segment rupture and recurrence of major earthquakes along the south central san andreas fault earthquake slip along the carrizo segment may recur in earthquake clusters with cumulative slip of 5 meters

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

analysis of near source static and dynamic measurements from the 1979 imperial valley earthquake gross features of the rupture mechanism of the 1979 imperial valley earthquake ml 6 6 are inferred from qualitative analysis of near source ground motion data and observed surface rupture a lower bound on the event s seismic moment of 2 5 1025 dyne cm is obtained by assuming that the average slip over the whole fault plane equals the average surface rupture 40 5 cm far field estimates of moment suggest an average slip over the fault plane of 105 cm from which a static stress drop of 11 bars is obtained an alternative slip model consistent with the far field moment has 40 5 cm of slip in the upper 5 km of the fault and 120 cm of slip in the lower 5 km this model suggests a static stress drop of 39 bars from the lower estimate of 11 bars an average strain drop of 32 strain is derived this strain drop is four times greater than the strain that could have accumulated since the 1940 el centro earthquake based on measured strain rates for the region hence a major portion of the strain released in the 1979 main shock had been accumulated prior to 1940 unusually large amplitude 500 to 600 cm sec2 vertical accelerations were recorded at stations e05 e06 e07 e08 eda of the ei centro array and the five stations of the differential array near eda although the peak acceleration of 1705 cm sec2 at e06 is probably amplified by a factor of 3 due to local site conditions these large amplitude vertical accelerations are unusual in that they are evident on only a few stations all of which are near the fault trace and at about the same epicentral range two possible explanations are considered first that they are due to a direct p wave generated from a region about 17 km north of the hypocenter or second that they are due to a pp phase that is unusually strong in the imperial valley due to the large p wave velocity gradient in the upper 5 km of the imperial valley based on the distribution of both the horizontal and vertical offsets it is likely that the rupture went beyond stations e06 and e07 during the main shock by exploiting the antisymmetry of the parallel components of particle velocity between e06 and e07 and by examining polarization diagrams of the particle velocity at e06 and e07 an average rupture velocity in the basement of 2 5 to 2 6 km sec between the hypocenter and station e06 is obtained in addition several lines of evidence suggest that the imperial fault dips about 75 to the ne

structures of the zagros fold thrust belt in iran the zagros fold thrust belt in iran forms the external part of the zagros active orogenic wedge it includes a sequence of heterogeneous latest neoproterozoic phanerozoic sedimentary cover strata 7 to 12 km thick and composed of alternating incompetent and competent layers overlying precambrian crystalline basement with a complex pre zagros structural fabric balancing structures of the cover statigraphic units in the zagros fold thrust belt requires in sequence and out of sequence involvement of the precambrian basement in the deformation six detailed balanced and retrodeformable cross sections which are constructed based on geological and geophysical data across various sectors of the belt show fault bend and fault propagation folds interpreted to have formed by slip on their subjacent thrusts out of sequence basement rooted thrusts as the interpretations in the constructed cross sections suggest have breached the cover basement interface and using incompetent cover strata for propagation cut across the cover structures and have created associated new folds superimposed upon the pre existing structures this style of deformation which has resulted in structural complexity of the belt characterizes a 200 to 300 km wide zone of distributed partly synchronous deformation with along strike and across strike variations it also implies that the zagros fold thrust belt as the external part of the orogenic wedge is still in its subcritical condition with internal deformation to achieve a critical taper in creating structural complexity in addition to out of sequence thrusting mechanically weak layers evaporites and mudstones of the cover strata have played a significant role by providing several detachment horizons shortening estimates across the belt are variable based on the constructed cross sections and their restorations minimum shortening estimates range from 16 percent to 30 percent in different sectors of the belt

levinson inversion of earthquake geometry sh transmission seismograms in the presence of noise the kunetz claerbout equation for the acoustic transmission problem in a layered medium in its original form establishes the relation between the transmission and the reflec tion response for p waves in an horizontally layered medium and with vertical incidence it states that the reflection seismogram due to an impulsive source at the surface is one side of the autocorrelation of the seismogram due to an impulsive source at depth and a surface receiver by adapting claerbout s formulation to the transmission of sh waves the kunetz claerbout equation also holds for reflection and transmission coefficients dependent on the incident angle thus earthquake geometry sh transmission seismograms can be used to caculate corresponding pseudoreflection seismograms which in turn can be inverted for the impedance structure using the levinson algorithm if the average incidence angle is known a geometrical correction on the resulting impedance model can improve the resolution of layer thicknesses in contrast to the inversion of reflection seismograms the levinson algorithm is shown to yield stable results for the inversion of transmission seismograms even in the presence of additive noise this noise stabilization is inherent to the kunetz claerbout equation results of inverted sh wave microearthquake seismograms from the swabian jura sw germany seismic zone obtained at recording site hausen im tal have been compared with sonic log data from nearby exploration drilling at trochtelfingen the agreement of the main structural elements is fair to a depth of several hundred metres

nonlinear seismic response of soft clay sites a nonlinear stress strain model utilizing ramberg osgood idealization of backbone curve and allowing for its progressive degradation due to cyclic loading is used to conduct site response studies at two soft clay sites the sites selected are 1 southern pacific building san francisco and 2 station no 119 site tokyo japan the subsurface conditions and the material properties used in analyses are presented the computer program dcharm used for nonlinear analysis allows for progressive degradation of the backbone curve for comparison the results obtained from program shake using equivalent linear idealization are also presented the program dcharm is shown to provide ground response results closer to the recorded data than shake the results obtained indicate that substantially higher accelerations and durations of earthquakes are needed to cause significant degradation of soft clay

variability of displacement at a point implications for earthquake size distribution and rupture hazard on faults abstract to investigate the nature of earthquake magnitude distributions on faults we compare the interevent variability of surface displacement at a point on a fault from a composite global data set of paleoseismic observations with the variability expected from two prevailing magnitude frequency distributions the truncated exponential model and the characteristic earthquake model we use forward modeling to predict the coefficient of variation cv for the alternative earthquake distributions incorporating factors that would effect observations of displacement at a site the characteristic earthquake model with a characteristic magnitude range of 0 25 produces cv values consistent with the data cv 0 5 only if the variability for a given earthquake magnitude is small this condition implies that rupture patterns on a fault are stable in keeping with the concept behind the model this constraint also bears upon fault rupture hazard analysis which for lack of point specific information has used global scaling relations to infer variability in average displacement for a given size earthquake exponential distributions of earthquakes from m 5 to the maximum magnitude give rise to cv values that are significantly larger than the empirical constraint a version of the model truncated at m 7 however yields values consistent with a larger cv 0 6 determined for small displacement sites although this result allows for a difference in the magnitude distribution of smaller surface rupturing earthquakes it may reflect in part less stability in the displacement profile of smaller ruptures and or the tails of larger ruptures online material global surface displacement data and identified sources of uncertainty

on tsunami nucleation ii an instantaneous modulated line source abstract tsunami generation on a spherically symmetric self gravitating and elastic earth is investigated using normal mode summations and moment tensor line sources wave cancellation in a line source of length l substantially eliminates tsunamis whose horizontal wavenumber component along the fault exceeds 3 l this results in a preferential beaming of tsunami radiation perpendicular to the fault strike beaming from a source of fixed length occurs in the same fashion irrespective of slip mechanism tsunami radiation patterns from finite sources are thus more uniform than those from point sources the largest far field sea waves from any long shallow fault are likely to be found within 20 azimuth of the perpendicular bisecting great circle although line sources subordinate distinctions in radiation patterns focal mechanism is still a critical element in determining the extent of tsunami excitation the ratio of maximum tsunami amplitude from strike and dip slips of equal moment are 1 10 for faults 400 km in length finite faulting weakens strike slip tsunamis relative to dip slip tsunamis because the beaming and natural radiation directions of strike slips are severely misaligned using an empirical relation linking source process time fault length and moment i predict tsunami energies in the range 1010 1017 j for earthquake moments spanning 2 1018

the effect of crustal structure on the regional phases pg and lg at the nevada test site the effect of local structure on the characteristics of the regional phases pg and lg was investigated by analyzing events at epicentral distances less than 10 from the nevada test site the stations used are on markedly different geological structures three on granitic intrusions two on pahute mesa a feature overlaying a buried volcanic caldera and finally four on yucca flats a deep alluvial valley the crustal effect on amplitudes is quite significant for both lg and pg yucca flats gives amplitudes as much as a full order of magnitude higher than the stations on granite while pahute mesa stations are 0 7 0 8 magnitude units higher in amplitude than the granite sites besides gross amplitude effects the local crust also induces prolonged ringing at yucca flats resulting in the lengthening of the signal wave trains the observed site amplification effects show no clear dependence on the azimuths of arrivals indicating that the phenomenon is due to local structures and is not caused by lateral refraction of surface wave modes the observed amplification is similar in magnitude for the vertical component of both pg and lg thus not affecting the pg lg amplitude ratio a possible discriminant between earthquakes and explosions at regional distances finite difference theoretical simulations using two dimensional models of the yucca basin reproduce in a qualitative sense the ringing effect observed at the yucca flats any attempt at matching characteristics of pg and lg with synthetic seismograms computed for laterally homogeneous layered structures is apt to lead to false conclusions about the source and path properties unless the recording sites used are carefully chosen for small crustal effects

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

moment tensor inversion of complex earthquakes summary moment tensor inversion methods can be applied with success in the determination of source properties of simple earthquakes however these methods utilize the assumption of a point source which is inadequate for modelling many complicated shallow earthquakes for complex earthquakes an inversion using finite faulting models is desirable but the number of parameters involved requires that a good starting model be found or that independent constraints be placed on some of the parameters a method is presented for low pass filtering both the data and green s functions passing only signals with wavelengths greater than the dimension of the entire fault the filter tends to smooth complications in the waveforms and allows application of the point source moment tensor inversion this method is applied to body waves from the 1978 thessaloniki greece earthquake the 1971 san fernando earthquake and to a multiple point source synthetic model of the san fernando event for the thessaloniki event although a multiple source mechanism has been suggested inversion results before and after filtering were essentially identical indicating that a point source mechanism is sufficient in modelling the long period teleseismic body waves in the case of the san fernando earthquake the point source green s functions were incapable of simultaneously modelling the p and sh waves inversion of p waves alone resulted in extreme parameter resolution problems but allowed constraint in one axis of the moment tensor and suggested an overall source time function inversion of a synthetic san fernando data set yielded similar results but allowed an investigation of the shortcomings of the method under controlled circumstances although the results may require substantial interpretation the method presented represents a simple first step in the analysis of complex earthquakes

seismic imaging of the site response using microearthquake recordings part ii application to the swabian jura southwest germany seismic network the subsurface site structure of the swabian jura southwest germany seismic network has been modeled from the inversion of locally recorded sh wave microearthquake seismograms the observed records have been deconvolved from the effects of apparent source pulse broadening due to the combined influence of frequency bandlimited source signals absorption peg leg multiple reflections and the recording system following the procedure of scherbaum and stoll 1985 corresponding pseudo reflection seismograms were calculated based on the kunetz claerbout relation for sh waves and inverted using a levison recursion algorithm the final structural models have been optimized in terms of numbers of layers and depth resolution using ferber s 1985 noise stabilization technique the resulting impedance models for the swabian jura southwest germany seismic network show some strongly correlated reflectors in the uppermost layers which are well understood in terms of the local stratigraphy the comparison of the inverted impedance model hsn with the p wave velocity log from a nearby exploration drilling trochtelfingen beb shows a high degree of similarity of the dominating reflectors down to a depth of several hundred meters as a consequence the coda generating process for the early part of the coda of the observed swabian jura microearthquake seismograms wood anderson magnitude mwa 3 0 can be completely interpreted as an effect of the local subsurface structure

displacement and geometrical characteristics of earthquake surface ruptures issues and implications for seismic hazard analysis and the process of earthquake rupture there now exist about three dozen historical earthquakes for which in vestigators have constructed maps of earthquake rupture traces accompanied by de scriptions of the coseismic slip observed along the fault strike the maps and slip distributions are compiled here to place observational bounds on aspects of seismic hazard analysis and fault mechanics analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults the observations also give support to the ideas that there exists a process zone or volume of about 3 4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes to this may be added the observation that the form of earthquake surface slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike online material maps of surface ruptures digitized values and curve fits to surface slip distributions and notes and references for tables 1 and 2

regional relationships among earthquake magnitude scales various magnitude scales commonly used and the interrelationships among them are reviewed it is shown that problems exist with all of the magnitude scales being used in the united states when using regional catalogs for example it is often necessary to determine how the reported magnitudes were determined often such information is not available although the potential errors are quite large both the ms and the mb scales were designed to be universal scales however both ms and mb magnitudes are often determined beyond the applicable range of the equations used to define the two scales furthermore the ms magnitudes are not generally available for moderate to small earthquakes in most earthquake catalogs the mb magnitudes are more generally available than ms values however there is also much greater variation in the way mb is determined in particular a significant change in the mb scale occurred in the early 1960 s when the world wide standard seismograph network wwssn was established this change in instrumentation used to determine mb values had a significant effect on estimated magnitudes post 1960 values are lower and the saturation level of the mb scale the older longer period instruments recorded larger mb magnitudes than can be recorded with the wwssn instruments in addition great care must be taken when selecting the mb magnitudes of western u s earthquakes because the values are often in considerable error owing to the fact that they were determined at distances less than 25 and were not properly corrected for attenuation in the upper mantle or asthenosphere the seismic body wave magnitude mb of an earthquake is strongly affected by regional variations in the q structure composition and physical state within the earth therefore because of differences in attenuation of p waves between the western and eastern united states a problem arises when comparing mb values for the two regions a regional mb magnitude bias exists which depending on where the earthquake occurs and where the p waves are recorded can lead to magnitude errors as large as unit there is also a significant difference between mb and ml values for earthquakes in the western united states an empirical link between the mb of an eastern u s earthquake and the ml of an equivalent western earthquake is given by ml 0 57 0 92 mb east this result is important when comparing ground motion between the two regions and for choosing a set of real western u s earthquake records to represent eastern earthquakes

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

earthquake magnitude scales the use and meaning of earthquake magnitude scales are examined common misunderstandings regarding local magnitude surface wave magnitude and body wave magnitude are discussed and clarified it is shown that no single magnitude will adequately describe the size of an earthquake and that different geographic regions pose different requirements inter relations between seismic moment and the various types of magnitude are examined magnitudes are recommended for equations or curves which specify the strong ground motion acceleration velocity displacement or response spectra as a function of distance from the earthquake source

the drift history of iran from the ordovician to the triassic new late ordovician and triassic palaeomagnetic data from iran are presented these data in conjunction with data from the literature provide insights on the drift history of iran as part of cimmeria during the ordovician triassic a robust agreement of palaeomagnetic poles of iran and west gondwana is observed for the late ordovician earliest carboniferous indicat ing that iran was part of gondwana during that time data for the late permian early early tri assic indicate that iran resided on subequatorial palaeolatitudes clearly disengaged from the parental gondwanan margin in the southern hemisphere since the late early triassic iran has been located in the northern hemisphere close to the eurasian margin this northward drift brought iran to cover much of the palaeotethys in approximately 35 ma at an average plate speed ofc 7 8 cm year 21 and was in part coeval to the transformation of pangaea from an irvin gian b to a wegenerian a type configuration

repeat times of large earthquakes implications for earthquake mechanics and long term prediction information on the time intervals between large earthquakes is now available for several fault segments along plate boundaries in japan alaska california cascadia and turkey when dates in a sequence are known historically as along much of the nankai trough they provide information on the natural intrinsic variability of the rupture process most sets of repeat times however are dominated by paleoseismic determinations of dates of older large earthquakes which contain measurement uncertainties in addition to intrinsic variability a bayesian technique along with prior information on measurement uncertainties is used to make maximum likelihood estimates of intrinsic repeat time and its normalized standard deviation the coefficient of variation cv it is these intrinsic parameters and their uncertainties that are most useful for understanding the mechanics of earthquakes and for prediction for timescales of a few decades our estimates of intrinsic cv are small 0 to 0 25 for several very active fault segments where deformation is relatively simple large events do not appear to be missing in historic and paleoseismic records and data are available at or near major asperities and away from the ends of rupture zones cv is larger for regions of multibranched faulting overlapping slip near the ends of rupture zones and for data from uplifted terraces at subduction zones a poisson process is an inferior characterization of all of the 11 segments we examined scenarios used by recent working groups that assume either poissonian behavior or renewal processes with cv of 0 5 0 2 for the most active fault segments in the san francisco bay area likely lead to incorrect 30 year probability estimates the hayward fault and perhaps the peninsular segment of the san andreas fault in the san francisco bay area appear to be advanced in their buildup of stress that will be released in future large earthquakes multibranched faulting may account for why the predicted tokai earthquake in japan has not occurred as of 2006 parkfield earthquakes from 1857 to 2004 were characterized by the largest uncertainty of the sequences we studied cv 0 37 which may account for the failure of past predictions the large cv for parkfield fits our hypothesis that relatively weak fault segments are characterized by more irregular earthquake recurrence paleoseismic data from coastal sites along the cascadia subduction zone are characterized by cvs of about 0 3

preseismic and coseismic deformation associated with the coyote lake california earthquake supplement is available with entire article on microfiche orderfrom american geophysical union 2000 florida avenue n w washington d c 20006 document j79 008 01 00 payment mustaccompany order the coyote lake earthquake ml 5 9 august 6 1979 epicenter about 100 km southeast of san francisco occurred on the calaveras fault within a geodetic network that had been surveyed annually since 1972 to monitor strain accumulation the rupture surface as defined by aftershocks is a vertical rectangle 20 km in length extending from a depth of 4 km to about 12 km the observed deformation of the geodetic network constrains the average slip to be about 0 33 0 05 m right lateral although the geodetic data furnished an exceptionally detailed picture of the preearthquake deformation no significant premonitory anomaly associated with the coyote lake earthquake can be identified

a scattering model to explain seismic q observations in the lithosphere between 1 and 30 hz observations of lithospheric q for shear waves in the range 1 30 hz reported in the literature are examined using a scattering model consisting of randomly distributed spheres for the single scattering approximation i have assumed 1 q 1 q sub i gv omega where q sub i is the intrinsic q due to anelasticity v is the velocity of shear waves omega is angular frequency and g is the turbidity g is taken as a constant in this paper since g integral n a sigmada where n a da is the number of scattering spheres of radius a unit volume and sigma is the scattering cross section for a sphere this suggests that geometrical scattering dominates this will be true for ka 1 where k is wavenumber based on this criterion and the observation that q is large i e g small at a frequency of 0 05 hz a is of the order of magnitude 1 10 km fits of observations of shear wave q from japan and central asia indicate that q sub i 2000 500 for the lithosphere of both regions g 0 01 km for the japan observations and g 0 005 km sup 1 for central asia

hypocenter for the 1979 imperial valley earthquake using p and s wave arrival times with the laterally varying p wave velocity structure derived from analysis of a refraction survey of the imperial valley a hypocenter is ascertained for the october 15 1979 imperial valley earthquake latitude 32 sup 0 39 50 n longitude 115 sup 0 19 80 w depth 8 0 km time 23 16 54 40 gmt

magnitude the relation of ml to mblg abstract the m blg magnitude developed by nuttli 1973 for the central united states is adapted to other regions of the world by incorporating a specific term for the spatial coefficient of anelastic attenuation using region specific values m blg values are determined for earthquakes occurring near the wwssn stations bks gsc and dug a comparison of the m blg values with published m l magnitudes shows that these two magnitude scales are essentially equivalent between magnitudes 3 to 5 because of this observation we now have a direct way of comparing eastern and western united states earthquake sizes for the purpose of adapting western strong motion data bases to the east and also for understanding teleseismic magnitude biases

regionalization of crustal coda q in the continental united states a contour map of crustal q sub 0 values for the entire united states is presented based on a scattering model to explain the coda waves of local and near regional earthquakes these coda q sub 0 values are in good agreement with q of lg waves data are obtained from over 250 local earthquakes with most magnitudes between 3 0 and 5 0 recorded by 25 wwssn and 27 lrsm stations throughout the continental united states these two sets of data provide a range of frequencies from 0 5 to 3 5 hz a frequency dependence of q is observed in the range of frequencies considered a power law dependence of the form q q sub 0 f f sub 0 sup eta is assumed the value of frequency dependence eta is found to be maximum in the tectonically active western united states and least in the stable regions of the central and south central united states the lowest q sub 0 values are obtained in the western united states with values ranging from 140 to 200 in the coastal plains average crustal q sub 0 values for the basin and range province vary from 200 to 300 increasing gradually to 400 in the colorado plateau region q sub 0 valuesmore in the columbia plateau vary from 200 to 400 q sub 0 values increase very rapidly along the central and southern rocky mountains from 400 to 800 east of the rocky mountains q sub 0 values increase gradually in the interior plains to a maximum value of around 1300 in the mississippi embayment region farther east q sub 0 values decrease gradually to an almost constant value of 1000 along the appalachian mountains coastal regions of eastern and northeastern united states have a crustal q sub 0 value between 900 to 700 regions of north central and gulf coastal plains show low q sub 0 values ranging from 600 to 400 and less the source factor b f sub p a measure of the intensity of scattering was obtained for several different regions of the continental united states regions of high q sub 0 values exhibit low intensity of scattering less

complexity of rupture in large strike slip earthquakes in turkey complexity of rupture propagation has an important bearing on the state of stress along the earthquake fault plane and on the prediction of strong ground motion in the near field by studying far field body waveforms recorded by wwssn long period seismograms it has been possible to investigate the degree of complexity of several turkish earthquakes the results which are obtained by matching synthetic p waveforms to observed data indicate that the july 22 1967 mudurnu valley earthquake m s 7 1 is a complex event which can be explained by the superposition of elementary sources with variable amplitudes and source time sequence history in this regard it is very similar to the february 4 1976 guatemala earthquake m s 7 5 a comparison of these two events indicates that their source time series ranges from 5 to ca 20 s and regardless of the total moment of the earthquake the moment of the individual events is bounded at around 5 10 26 dyn cm the november 24 1976 e turkey earthquake m s 7 3 on the other hand has a complexity which cannot be explained by such a simple model in this respect it may be more similar to the tangshan china earthquake and as such may involve significant thrust normal or other complications to its faulting mechanism than the strike slip mechanism of the p wave first motion data the source time history for the 1967 mudurnu valley event is used to illustrate its significance in modeling strong ground motion in the near field the complex source time series of the 1967 event predicts greater amplitudes 2 5 larger in strong ground motion than a uniform model scaled to the same size for a station 20 km from the fault such complexity is clearly important in understanding what strong ground motion to expect in the near field of these and other continental strike slip faults such as the san andreas

long period mechanism of the 8 november 1980 eureka california earthquake the seismic moment and source orientation of the 8 november 1980 eureka california earthquake m s 7 2 are determined using long period surface and body wave data obtained from the sro asro and ida networks the favorable azimuthal distribution of the recording stations allows a well constrained mechanism to be determined by a simultaneous moment tensor inversion of the love and rayleigh wave observations the shallow depth of the event precludes determination of the full moment tensor but constraining m zx m zy 0 and using a point source at 16 km depth gives a major double couple for period t 256 sec with scalar moment m 0 1 1 10 27 dyne cm and a left lateral vertical strike slip orientation trending n48 2 e the choice of fault planes is made on the basis of the aftershock distribution this solution is insensitive to the depth of the point source for depths less than 33 km using the moment tensor solution as a starting model the rayleigh and love wave amplitude data alone are inverted in order to fine tune the solution this results in a slightly larger scalar moment of 1 28 10 27 dyne cm but insignificant 5 changes in strike and dip the rake is not well enough resolved to indicate significant variation from the pure strike slip solution additional amplitude inversions of the surface waves at periods ranging from 75 to 512 sec yield a moment estimate of 1 3 0 2 10 27 dyne cm and a similar strike slip fault orientation the long period p and sh waves recorded at sro and asro stations are utilized to determine the seismic moment for 15 to 30 sec periods a deconvolution algorithm developed by kikuchi and kanamori 1982 is used to determine the time function for the first 180 sec of the p and sh signals the sh data are more stable and indicate a complex bilateral rupture with at least four subevents the dominant first subevent has a moment of 6 4 10 26 dyne cm summing the moment of this and the next three subevents all of which occur in the first 80 sec of rupture yields a moment of 1 3 10 27 dyne cm thus when the multiple source character of the body waves is taken into account the seismic moment for the eureka event throughout the period range 15 to 500 sec is 1 3 0 2 10 27 dyne cm

source parameters of 1978 tabas and 1979 qainat iran earthquakes from long period surface waves the teleseismic records of three strong earthquakes which inflicted heavy damage and loss of many thousands of lives in northeastern iran during 1978 and 1979 are analyzed the observed r 2 and r 3 phases of the 16 september 1978 tabas earthquake across the ida network are inverted to derive the gross source parameters of the main shock the synthetic seismograms computed for the inferred source model are compared with observations the agreement is generally good at most of the stations however unexpected asymmetrical observations at esk cmo kip and pfo stations cannot be explained by directivity alone possible factors responsible for these observations may include regional heterogeneities or the departure of fault plane from simple planar geometry the resulting focal mechanism agrees with nearly pure reverse mechanism with a nw se strike the seismic moment estimate for this earthquake is about 1 5 10 27 dyne cm m w 7 4 the seismic moment of the same earthquake obtained from long period p wave observations at six wwssn stations is smaller than the above estimate by a factor of 2 starting with the geologic field data of the qainat earthquakes of 14 and 27 november 1979 the r 2 and g 2 phases on the ultra long period instruments at pasadena and berkeley are analyzed to estimate their seismic source moments the estimated values are 1 0 10 26 dyne cm for the 14 november and 8 0 10 26 dyne cm for the 27 november 1979 events their corresponding moment magnitudes are 6 6 and 7 2 respectively

a source study of the thessaloniki northern greece 1978 earthquake sequence summary the thessaloniki northern greece earthquake sequence appears to have occurred along faults forming a graben structure this graben situated in the border region between the serbomacedonian massif and the vardar zone is bounded to the south west by clearly exposed north west striking north east dipping normal faults relative hypocentre determinations fault plane solutions surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes the source parameters of the mainshock mb 6 1 ms 6 4 were determined by computing body wave synthetic seismograms in the time domain and comparing them with the observed this modelling constrained the orientation of faulting determined by the p wave fault plane solution it also constrained the source depth to 6 mp 2 km similar depths were calculated by a relative relocation method for the other three large events of the sequence the dislocation time function required for the far field had a total duration of 9 mp 1 5 s the body wave moment was 5 2 mp 1 8 times 1025 dyne cm for a fault length of 35 km and a fault width of 17 km both estimated from the aftershock distribution the static stress drop was found to be 4 bar an accelerograph record of the mainshock shows two distinct events 3 4 s apart these two events were unresolved by the long period data in the far field but the unusually long duration of the mainshock time function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time seismic energy released in this fashion can account for waveform complexities observed at some stations

fixed recurrence and slip models better predict earthquake behavior than the time and slip predictable models 1 repeating earthquakes 1 the behavior of individual events in repeating earthquake sequences in california taiwan and japan is better predicted by a model with fixed inter event time or fixed slip than it is by the time and slip predictable models for earthquake occurrence given that repeating earthquakes are highly regular in both inter event time and seismic moment the time and slip predictable models seem ideally suited to explain their behavior taken together with evidence from the companion manuscript that shows similar results for laboratory experiments we conclude that the short term predictions of the time and slip predictable models should be rejected in favor of earthquake models that assume either fixed slip or fixed recurrence interval this implies that the elastic rebound model underlying the time and slip predictable models offers no additional value in describing earthquake behavior in an event to event sense but its value in a long term sense cannot be determined these models likely fail because they rely on assumptions that oversimplify the earthquake cycle we note that the time and slip of these events is predicted quite well by fixed slip and fixed recurrence models so in some sense they are time and slip predictable while fixed recurrence and slip models better predict repeating earthquake behavior than the time and slip predictable models we observe a correlation between slip and the preceding recurrence time for many repeating earthquake sequences in parkfield california this correlation is not found in other regions and the sequences with the correlative slip predictable behavior are not distinguishable from nearby earthquake sequences that do not exhibit this behavior

source dynamics of the 1979 imperial valley earthquake from near source observations of ground acceleration and velocity two sets of observations obtained during the 15 october 1979 imperial valley earthquake ms 6 9 are presented the data suggest different dynamic characteristics of the source when viewed in different frequency bands the first data set consists of the observed residuals of the horizontal peak ground accelerations and particle velocity from predicted values within 50 km of the fault surface the residuals are calculated from a nonlinear regression analysis of the data campbell 1981 to the following empirical relationships pga a 1 r c 1 d 1 pgv a 2 r c 2 d 2 in which r is the closest distance to the plane of rupture the so calculated residuals are correlated with a positive scalar factor signifying the focusing potential at each observation point the focusing potential is determined on the basis of the geometrical relation of the station relative to the rupture front on the fault plane the second data set consists of the acceleration directions derived from the windowed time histories of the horizontal ground acceleration across the el centro differential array ecda the horizontal peak velocity residuals and the low pass particle acceleration directions across ecda require the fault rupture to propagate northwestward the horizontal peak ground acceleration residuals and the high frequency particle acceleration directions however are either inconclusive or suggest an opposite direction for rupture propagation the inconsistency can best be explained to have resulted from the incoherence of the high frequency radiation which contributes most effectively to the registration of pga a test for the sensitivity of the correlation procedure to the souce location is conducted by ascribing the observed strong ground shaking to a single asperity located 12 km northwest of the hypocenter the resulting inconsistency between the peak acceleration and velocity observations in relation to the focusing potential is accentuated the particle velocity of delta station mexico in either case appears abnormally high and disagrees with other observations near the southeastern end of the fault trace from the observation of a nearly continuous counterclockwise rotation of the plane of p wave particle motion at ecda the average rupture velocity during the first several seconds of source activation is estimated to be 2 0 to 3 0 km sec a 3 km upper bound estimate of barrier dimensions is tentatively made on the basis of the observed quasiperiodic variation of the polarization angles

the oroville earthquakes a study of source characteristics and site effects half space modeling in the time domain of broadband seismic records from the oroville aftershock sequence demonstrates that these events have simple sources systematic scaling between the far field source duration and seismic moment for the aftershocks studied here and other california earthquakes confirms brune s 1970 source model and shows that the stress drops for california earthquakes range between 10 and 100 bars these events also show no trend in stress drop as a function of size for 3 0 or m sub l or 6 4 analysis of the static deformation associated with the main shock shows that the slip occurred on a fault 81 km sup 2 in area corresponding to the zone defined by the first 3 days of aftershocks teleseismic studies indicate that the seismic source area was 50 km sup 2 corresponding to a zone completely free of aftershocks in the middle of the aftershock zone both results imply a stress drop of 29 39 bars and seismic moment between 5 7 and 8 7 x 10 sup 24 dyn cm the evidence suggests that faulting occurred seismically on the central portion of the fault and propagated out slowly enlarging the fault plane complicated waveforms recorded for the same events at nearby sites on alluvium indicatemore the critical role that propagation through complex structure plays in determining the character of the observed seismic signal at close in stations modeling of these records show that dipping structure may account for the observed waveform complexities less

simulation of ground accelerations for the may 1980 mammoth lakes california earthquakes abstract strong motion accelerograms from the may 1980 mammoth lakes earthquake sequence are used to support the technique of estimating near source ground motion by the summation of existing records an sma 1 record recovered 3 km from the epicenter of the first magnitude m l 6 earthquake in the mammoth lakes sequence 25 may 16 33 44 utc is simulated by the summation of records from later events the source time history on which this summation is based comes from a study of the teleseismic p waves the summation method has several distinct advantages it is easy to apply inexpensive costly wave propagation codes are not needed and source to receiver propagation effects are automatically included the method however requires a good data base from which to perform the summation

pseudoreflection profiling method an efficient complement to cdp method1 a method for pseudoreflection profiling prp is presented by using transmission seismograms this method produces time sections similar to those obtained using the common depth point cdp method pseudoreflection profiles are obtained using the following procedure equivalent transmission responses at sites on soft deposits are derived by wiener filter estimation using seismograms acquired on the sites and on the basement rock these responses are substituted into the claerbout kunetz equation to obtain the pseudoreflection profiles a 3d subsurface model is produced when the prp method is applied to explosion seismograms the time sections thus obtained are consistent with those obtained using the cdp method in a neighbouring area the maximum station interval for obtaining coherent profiles with this method is estimated to be one quarter of the wavelength of the transmitted waves this technique should be used in combination with another method such as the cdp method because it has two weak points the velocity structure cannot be obtained directly and undesirable waves cannot be eliminated through processing

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

paleomagnetism of red beds of early devonian age from central iran abstract paleomagnetic results are reported from 13 sites of red beds of early devonian age from central iran detailed paleomagnetic analyses were carried out two types of partial progressive demagnetization were applied one using alternating magnetic fields the other heating these procedures resulted in the detection of the characteristic remanences with a mean direction with d 24 2 i 1 3 95 10 1 the paleomagnetic pole is located at 51 3 n 163 7 w if one shifts the iranian landmass to its most likely position in the gondwana configuration then the position of the paleomagnetic pole coincides with the alternative polar wander path 14 15 which crossed south america in early middle paleozoic times