the evolution of regional seismicity between large earthquakes 1 we describe a simple model that links static stress coulomb modeling to the regional seismicity around a major fault unlike conventional coulomb stress techniques which calculate stress changes we model the evolution of the stress field relative to the failure stress background seismicity is attributed to inhomogeneities in the stress field which are created by adding a random field that creates local regions above the failure stress the inhomogeneous field is chosen such that when these patches fail the resulting earthquake size distribution follows a gutenburg richter law immediately following a large event the model produces regions of increased seismicity aftershocks where the overall stress field has been elevated and regions of reduced seismicity where the stress field has been reduced stress shadows the high stress levels in the aftershock regions decrease due to loading following the main event combined with the stress shadow from the main event this results in a broad seismically quiet region of lowered stress around the epicenter pre event seismicity appears as the original stress shadows finally fill as a result of loading the increase in seismicity initially occurs several fault lengths away from the main fault and moves inward as the event approaches as a result of this effect the seismic moment release in the region around the future epicenter increases as the event approaches synthetic catalogues generated by this model are virtually indistinguishable from real earthquake sequences in california and washington

evidence from precise earthquake hypocenters for segmentation of the san andreas fault in san gorgonio pass we use precise hypocenter patterns and focal mechanisms to investigate the presence or absence of a continuous strike slip fault at depth connecting the san bernardino strand of the san andreas fault with the coachella valley segment of the banning fault we inverted 560 000 arrival times from 23 000 earthquakes 1981 1993 for high quality hypocenters and three dimensional p wave velocity structure in a 1 by 2 area centered on the san gorgonio pass cross sectional plots of relocated earthquakes reveal an abrupt 5 to 7 km high step in the maximum depth of hypocenters the step riser defines a near vertical locally curved surface that extends westerly more than 60 km from the coachella valley segment of the san andreas fault to the san jacinto fault a hypothetical continuous vertical san andreas fault through san gorgonio pass would cross the step at an oblique angle we suggest that the step is the expression of the contact between different basement rock types juxtaposed by large scale right slip motion on the ancestral san andreas fault south of the step in peninsular ranges type basement intrusives brittle failure occurs down to about 20 km depth while north of the step in san bernardino type basement pelona schist brittle failure occurs to only about 13 km depth the step provides a piercing plane that should be offset about 3 km right laterally by an active continuous vertical san andreas fault within the resolution of our mapping the step is not offset in this manner implying either that there has not been a throughgoing vertical fault at depth that a throughgoing fault has not experienced enough slip to offset the step or that a throughgoing fault is not vertical and dips north over the top of the step hypocentral patterns and focal mechanisms indicate distributed deformation thrust normal and strike slip faulting over a large volume in the san gorgonio pass region there is no evidence of hypocenter or slip vector alignments that would indicate a throughgoing continuous near vertical san andreas fault in summary we find no evidence indicating a continuous fault at seismogenic depth connecting the san bernardino strand and coachella valley segment of the san andreas fault zone we speculate that this is because the 3 km of right slip on the san bernardino stand of the san andreas fault and coachella valley segment of the banning fault has not been sufficient to form a single new structure through the 15 to 20 km gap between the two previously unconnected segments this implies that large earthquake rupture on the san andreas fault may be inhibited from propagating through san gorgonio pass thus limiting the maximum magnitudes on the southern san andreas fault

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

local geoelectrical models of the martian subsurface for shallow groundwater detection using sounding radars received 15 february 2002 revised 20 july 2002 accepted 28 august 2002 published 6 march 2003 1 low frequency sounding radars should be able to probe the martian subsurface layers down to varying depths depending on the geoelectrical properties of the sounded sites we present in this work four frequency dependent geoelectrical models of the martian subsurface in the 1 20 mhz frequency band based on laboratory electromagnetic characterization of martian soil analogues those models correspond to local martian sites that we considered to be of particular interest in the search for water using mainly the ground penetrating radar gpr instrument of the netlander mission results and discussion are also valid for both sounding experiments marsis and sharad the four models of the martian subsurface are designed to represent terrains where recent fluviallike features suggest the presence of near subsurface ground ice and probably liquid water we performed measurements on volcanic and sedimentary materials that may be present on these sites under the appropriate geophysical conditions that may exist in those terrains we then simulated the backscattered radar echo arising from each site in the 2 mhz frequency band using the finite difference time domain fdtd algorithm in order to evaluate the instrument performances to probe the subsurface stratigraphy of each site our results confirm that the near subsurface rich iron oxide mineralogy controls the instrument performances in terms of penetration depth and signal to noise ratio in the 2 mhz frequency band we finally discuss the geophysical and geoelectrical sounding conditions that could lead to an ambiguous detection of shallow subsurface water on mars for the netlander gpr index terms 3210 mathematical geophysics modeling 1794 history of geophysics instruments and techniques 5144 physical properties of rocks wave attenuation 5109 physical properties of rocks magnetic and electrical properties keywords mars hydrology gpr sounding simulation fdtd

numerical simulation of the operation of the gpr experiment on netlander 1 the first objective of the ground penetrating radar gpr experiment on netlander is to investigate the geological structures of the martian subsurface the aim of this paper is to present initial results obtained in the first phase of a long term effort to build a numerical model of the gpr operation on mars and test dedicated signal processing algorithms on the simulated data the simulation is based on the use of a finite difference time domain method and we have pointed out some of its advantages that allow us to take into account complex features of the underground this model has given reliable estimates of the power budget of the radar for a simple but still representative model of the martian subsurface in addition several detailed features such as gradients and roughness at the interfaces were introduced to appraise their possible influence on the gpr performances in the frame of a simplified geometry of both the gpr antennas and the various underground interfaces a simple and first order method was developed and tested on simulated data to show the ability of the gpr to retrieve a three dimensional distribution of the underground reflectors based on this model and even with some rather crude hypothesis on the subsurface electromagnetic characteristics information on the direction and distances of the reflectors has been retrieved with a satisfactory approximation

local observations of the onset of a large earthquake 28 june 1992 landers california the landers earthquake mw 7 3 of 28 june 1992 had a very emergent onset the first large amplitude arrivals are delayed by about 3 sec with respect to the origin time and are preceded by smaller scale slip other large earthquakes have been observed to have similar emergent onsets but the landers event is one of the first to be well recorded on nearby stations we used these recordings to investigate the spatial relationship between the hypocenter and the onset of the large energy release and to determine the slip function of the 3 sec nucleation process relative location of the onset of the large energy release with respect to the initial hypocenter indicates its source was between 1 and 4 km north of the hypocenter and delayed by approximately 2 5 sec three station array analysis of the p wave shows that the large amplitude onset arrives with a faster apparent velocity compared to the first arrivals indicating that the large amplitude source was several kilometers deeper than the initial onset an ml 2 8 foreshock located close to the hypocenter was used as an empirical green s function to correct for path and site effects from the first 3 sec of the mainshock seismogram the resultant deconvolution produced a slip function that showed two subevents preceding the main energy release an mw 4 4 followed by an mw 5 6 these subevents do not appear anomalous in comparison to simple moderate sized earthquakes suggesting that they were normal events which just triggered or grew into a much larger earthquake if small and moderate sized earthquakes commonly detonate much larger events this implies that the dynamic stresses during earthquake rupture are at least as important as long term static stresses in causing earthquakes and the prospects of reliable earthquake prediction from premonitory phenomena are not improved

control of relative sea level and climate on coal character in the westphalian c atokan four corners formation central appalachian basin u s a abstract peat successions preserved as coal seams preserve high resolution records of ancient terrestrial water table base level fluctuations that are driven by changes in relative sea level and or paleoclimatic change the aim of this study is to establish the roles of relative sea level and climate in controlling water table fluctuations within coal seams from the fluviodeltaic westphalian c bolsovian atokan four corners formation of the central appalachian basin usa comparison of the characteristics of 10 coals in the four corners formation with accommodation trends identified from analysis of intra coal clastic strata indicates that their thickness and vitrinite inertinite ratio correlate readily with the accommodation setting of the depositional sequence in which they occur coals that accumulated in relatively high accommodation fourth order sequences are high in vitrinite thin and overlain by or intercalated with marine and lacustrine sediments the coals have simple internal organization suggesting that they span parts of single high resolution accommodation cycles coals that accumulated in lower accommodation fourth order sequences are high in inertinite thick and overlain by or intercalated with terrestrial sediments the coals have composite internal organizations typified by multiple high resolution accommodation cycles and characteristic abrupt discontinuities of coal facies representing periods of depositional hiatus the correlations between the composition and thickness of the coals and the magnitude of associated marine transgression imply that relative sea level change was the principal control governing bulk accommodation in these mires landward of the paleoshoreline high frequency accommodation cycles within the coals reflect water table fluctuations in the original mires and were driven by high frequency sea level and or climate change the results of this study provide considerable insight into the manner in which coal composition and stratigraphy at a single locality vary over time in response to changing accommodation and provide a means to predict coal seam quality in an established sequence stratigraphic framework

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

strain accumulation in the santa barbara channel 1970 1988 geodetic observations between 1970 and 1988 indicate appreciable strain accumulation in the santa barbara channel california eleven line length changes from a six station geodetic network spanning the eastern two thirds of the channel were determined from electronic distance measurements in 1970 1971 and global positioning system observations in 1987 1988 within this network the strains observed are spatially nonuniform in the easternmost channel the strain is nearly uniaxial with convergence of 6 4 0 9 mm yr oriented n25 e 5 this direction is consistent with the seismicity which is dominated by thrust mechanisms with p axes directed to the northeast in the central channel the strain is less well determined but appears to include a significant component of shear that is left lateral when resolved on an east west plane

prospects for larger or more frequent earthquakes in the los angeles metropolitan region far too few moderate earthquakes have occurred within the los angeles california metropolitan region during the 200 year long historic period to account for observed strain accumulation indicating that the historic era represents either a lull between clusters of moderate earthquakes or part of a centuries long interseismic period between much larger moment magnitude m w 7 2 to 7 6 events geologic slip rates and relations between moment magnitude average coseismic slip and rupture area show that either of these hypotheses is possible but that the latter is the more plausible of the two the average time between m w 7 2 to 7 6 earthquakes from a combination of six fault systems within the metropolitan area was estimated to be about 140 years

rupture patterns and preshocks of large earthquakes in the southern san jacinto fault zone we relocated the large 1937 1942 and 1954 earthquakes in the san jacinto fault zone the epicenters of the main shocks aftershocks and some preshocks were determined using empirical station corrections from recent small events in the study areas the 1937 ml 5 9 earthquake has an epicenter between the surface traces of the san jacinto and buck ridge faults and aftershocks suggest about 7 km of rupture predominantly to the northwest a significant increase in small earthquake activity occurred about formula yr before this event the 1954 m l 6 2 earthquake is located at the southeast end of the mapped trace of the san jacinto fault and aftershocks suggest about 15 km of rupture further southeast into an area of folded young sediments with no surface fault trace this event was preceded by a cluster of small earthquakes which occurred within an 8 hr period 10 weeks before the main event and in the eventual rupture zone the 1942 m l 6 3 earthquake is located southwest of the southeast end of the coyote creek fault large aftershocks of this event are spread over a 15 by 18 km area southwest of the coyote creek fault and are not associated with any one fault the relation of the 1942 event to the san jacinto fault zone is not simple

temporal changes in microseismicity and creep near parkfield california the 25 km long section of the san andreas fault near parkfield california ruptured in similar magnitude 6 earthquakes in 1881 1901 1922 1934 and 1966 on the basis of a number of lines of seismological evidence a section of the san andreas fault now termed the parkfield preparation zone has been identified as the locus of the next parkfield earthquake1 3 here we describe coincident changes in surface creep rates and deep seismicity near the parkfield preparation zone following the 2 may 1983 coalinga earthquake and suggest that both respond to the same stimuli these changes were concentrated near the point of initiation of the magnitude 6 characteristic parkfield earthquakes lending credence to the hypothesis that this section of the san andreas fault is characterized by a unique set of physical properties4 6 which make it ideal for earthquake prediction studies and which may also be useful for identifying hypocentral regions in other areas

modeling the configuration of hf electrical antennas for deep bistatic subsurface sounding in the frame of the european space agency s 2016 exomars mission the electromagnetic investigation of the subsurface eiss ground penetrating radar has been designed and developed to perform deep soundings of the martian subsurface from the surface the eiss is designed to take advantage of the potential for bistatic radar investigations of the martian subsurface between the fixed station lander and the mobile platform rover and to characterize the 3 d structure and stratigraphy of the subsurface at depths ranging from 100 m to a few kilometers out to a 1 km radius around the lander the eiss makes use of an electric dipole antenna made of two identical 35 m resistively loaded monopoles to transmit and also receive in a monostatic mode the high frequency signal however the eiss s most innovative capability is its potential for bistatic operation made possible by the accommodation of a small magnetic sensor on the rover as initially planned for the exomars mission which can measure the magnetic field all three components of the received waves whatever the direction and orientation of the rover the aim of this paper is to show that the two monopoles of the antenna must be deployed on the surface in nearly opposite directions but not aligned to ensure good volume coverage around the transmitter this paper is based on finite difference in time domain fdtd electromagnetic simulations the simulated data have been used to study the impact of the angle between these two monopoles on the instrument performance

detection of a locked zone at depth on the parkfield california segment of the san andreas fault the parkfield california segment of the san andreas fault is transitional in character between the creeping segment of the fault to the northwest and the locked carrizo plain segment to the southeast the rate of shallow fault slip decreases from 25 30 mm yr northwest of the epicenter of the 1966 parkfield earthquake to zero at the southeastern end of the 1966 rupture zone data from a network of trilateration lines spanning the san andreas fault near parkfield and extending to the pacific coast near san luis obispo shed light on the rate of fault slip at depth since the 1966 earthquake in this study average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the parkfield fault segment the fault is taken to be a vertical surface with unknown distribution of strike slip displacement in an elastic half space a striking result of the inversions is that all solutions providing acceptable fits to the data exhibit a locked zone essentially coincident with the rupture surface of the 1966 parkfield earthquake the data require that the locked zone extend nearly as far north as the 1966 epicenter however the vertical extent of the locked zone is not well resolved over much of the parkfield segment the fault is slipping faster at the earth s surface than it is at seismogenic depths in order to fit the trilateration measurements it is necessary to include a component of contraction normal to the trend of the san andreas the inversion results suggest a spatially uniform normal strain of 0 06 strain yr the orientation of the contraction is compatible with geologic and seismic evidence of active folding and reverse faulting in the region the magnitude of the contraction is consistent with convergence rates inferred from global plate motion models

interseismic deformation along the san andreas fault in southern california eight trilateration networks located along the san andreas fault in southern california have been surveyed 8 to 19 times within a 14 to 17 year interval between 1971 and 1992 the data measurements of distances between the same 10 to 32 pairs of geodetic monuments within a network in each of the surveys have been corrected for coseismic offsets from nearby earthquakes calculated from dislocation models the corrected data a total of 2027 measurements are displayed in the form of plots of measured distance versus time for each of the 167 lines measured the hypothesis that the interseismic deformation is steady is tested by examining whether deviations from linear fits to the data in the plots are within the range expected for observational error a significant deviation from steady deformation is found only for the network located neax palmdale california in that network many of the measurements made in the survey of early 1982 deviate from the trend defined by the measurements in other surveys the deviations are not of the form fixed proportional error one would generally expect from systematic survey error if the survey of early 1982 is excluded the remaining data at palmdale are consistent with steady deformation thus the apparent strain event observed near palmdale in 1982 was transient

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

how similar were the 1934 and 1966 parkfield earthquakes bakun and mcevilly 1984 showed that surface waves from the 1934 and 1966 parkfield earthquakes are remarkably similar this observation together with other seismic and geologic data led to the forecast of a repeat of a characteristic parkfield earthquake by 1993 in this paper we model near field geodetic measurements collected before and after the 1934 and 1966 parkfield earthquakes the slip in 1934 on the northwestern half of the rupture is poorly constrained due to the limited station coverage in this part of the rupture near the hypocenter the geodetic measurements permit the two earthquakes to have the same slip distribution on the other hand the data clearly show that the 1966 earthquake ruptured through the right step in the surface trace of the fault while the 1934 earthquake stopped northwest of the step if we assume only that the slip distribution is to some degree smooth we find that the centroid of the 1934 slip distribution could be as much as 10 km northwest of the centroid of the 1966 slip the data are consistent with but do not demand the largest slip in 1934 occurring in a region of low slip in 1966 the geodetically determined moments of the two earthquakes from smooth inversions are indistinguishable m0 4 4 0 4 1018 n m we also examine inversions in which we assume that the two earthquakes start identically and estimate how far down the fault the two earthquakes could have remained the same we find that the 1934 event could have had the same slip as in 1966 up to the fault step the 1934 earthquake must have stopped at the step whereas the 1966 event slipped southeast of the step in this quasi characteristic model m0 of the 1934 quake is only 60 of that in 1966 the difference presumably occurring as afterslip in 1966 if the 1934 geodetic moment was indeed less than in 1966 and the loading rate was constant this observation could explain the early occurrence of the 1934 earthquake

a source extent analysis of the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 a new method for estimating source extent parameters has been applied to the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 from the stations of the world wide standard seismograph network and canadian seismic network estimates of the duration of far field long period sh waves were made by computing the variance or second central moment 2 of instrument corrected attenuation corrected pulses seventeen measurements for the imperial valley event and 25 for the victoria event were made we inverted for the source duration t the fault length l and the directivity parameter d by least squares we find that for the imperial valley t 9 0 0 5 s d 130 43 km s and l 46 13 km and for the victoria event t 10 9 0 6 s and d 251 71 km s l is not well constrained for this second event the inversion gives 23 km the lower bound obtained by constraining 2 to be positive everywhere on the focal sphere although a value as high as 40 km would still be within the uncertainties the positive values of d for both events indicate that rupture propagation was predominantly to the northwest in order to estimate the degree of unilateralness we have computed the rupture mode index defined by r d lt which is 1 for a unilateral rupture and zero for a bilateral rupture we find a value of r 0 31 0 14 for the imperial valley event significantly less than unity which indicates that a southern component of rupture is also present assuming uniform slip we estimate the northern and southern fault segments to be 26 and 20 km respectively the presence of a bilateral component is supported by near field studies which have made use of stations along the southern half of the imperial fault for the victoria event due primarily to the uncertainty in l the value of r can vary from 0 6 to 1 with the value of 1 being most probable assuming that the rupture is unilateral we can obtain an estimate of fault length from the relation l d t which yields l 23 6 km geodetic data and the distribution of aftershocks are consistent with a unilateral rupture mode and the fault length that we have obtained values of seismic moment from measurements of the zeroth moment of the displacement pulse are found to be 5 8 0 4 for imperial valley and 4 8 0 4 for victoria in units of 1018 n m the presence of a southern component of rupture for the imperial valley event would suggest a closer and perhaps causal relationship to the victoria event which occurred 8 months later and about 60 km to the south in a tectonic framework we interpret these two earthquakes as the failure of adjacent transform faults and consider the possibility that a concurrent spreading event may have occurred in the vicinity of cerro prieto

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

the santa barbara california earthquake of 13 august 1978 the 5 1 m l santa barbara earthquake of 13 august 1978 occurred at 22h54m 52 8s utc the epicenter was located 3 km southeast of santa barbara at 34 23 9 n latitude and 119 40 9 w longitude with a focal depth of 12 7 km the main shock was followed between 13 august and 30 september by 373 aftershocks that were located with the caltech usgs array the aftershock zone extended 12 km wnw from the epicenter and was 6 km wide in the n s direction and it had a very clear temporal development during the first 20 min of activity all the aftershocks were located in a cluster 7 km wnw of the main shock epicenter during the next 24 hr the aftershock zone grew to 11 km in the wnw direction and 4 km in the n s direction during succeeding weeks the zone extended to 12 by 6 km this temporal spatial development relative to the main shock epicenter may indicate that the initial rupture propagated 7 km unilaterally to the wnw and the initial rupture plane may have been considerably smaller than the eventual aftershock zone this smaller area suggests that the stress drop may have been significantly greater than that derived from the final aftershock zone in cross section the aftershock hypocenters outline a nearly horizontal plane dipping 15 or less at 13 km depth the main shock focal mechanism indicates nne ssw compression and vertical extension the preferred fault plane strikes n80 w and dips 26 nne indicating north over south thrusting with a component of left lateral movement focal mechanisms for 40 aftershocks also indicate compression in the general n s direction for most of these events the north dipping nodal plane dips between 7 and 45 with most dipping 25 or more which is significantly steeper than the plane delineated by the hypocenters themselves these observations are consistent with a tectonic model in which much of the slip during the santa barbara earthquake occurred on a nearly horizontal plane the after shocks then might represent movement on a complex series of imbricate thrust faults that flatten into the plane of primary slip hence the santa barbara earthquake may be taken as evidence for mid crustal horizontal shearing in the western transverse ranges

long period surface waves of four western united states earthquakes recorded by the pasadena strainmeter long period surface waves recorded on the north south pasadena strainmeter are used to determine the seismic moments and fault parameters of the 19 may 1940 imperial valley california the 16 december 1954 dixie valley and fairview peak nevada and the 18 august 1959 hebgen lake montana earthquakes synthetic strain seismograms are matched with the observed strainmeter seismograms source parameters from the strainmeter modeling are more consistent with source parameters estimated from geodetic and geologic information than parameters estimated from short period 15 sec body wave data long period surface wave moment estimates agree well with geodetic estimates of moment but are 1 5 to 5 times greater than moments obtained from modeling of teleseismic body waves or geologic information the imperial valley earthquake is best modeled as consisting of 5 point sources along a fault 87 5 km in length with a strike rake and dip of 326 180 and 90 the moment for the earthquake was 4 8 10 19 n m the synthetic seismogram that best models the fairview peak and dixie valley earthquakes assumes that the fairview peak earthquake was twice the size of the dixie valley event moments of 5 9 to 13 10 19 and 3 to 6 5 10 19 n m are obtained for these events a moment of 1 5 10 20 n m is obtained for the hebgen lake earthquake love waves of this earthquake are best modeled by a fault striking 102 although surface faulting produced during the earthquake strikes 130

moment tensor solutions for the 24 november 1987 superstition hills california earthquakes the teleseismic long period waveforms recorded by the global digital seismograph network from the two largest superstition hills earthquakes are inverted using an algorithm based on optimal filter theory these solutions differ slightly from those published in the preliminary determination of epicenters monthly listing because a somewhat different improved data set was used in the inversions and a time dependent moment tensor algorithm was used to investigate the complexity of the main shock the foreshock origin time 01 54 14 5 m b 5 7 m s 6 2 had a scalar moment of 2 3 1025 dyne cm a depth of 8 km and a mechanism of strike 217 dip 79 rake 4 the main shock origin time 13 15 56 4 m b 6 0 m s 6 6 was a complex event consisting of at least two subevents with a combined scalar moment of 1 0 1026 dyne cm a depth of 10 km and a mechanism of strike 303 dip 89 rake 180

influence of static stress changes on earthquake locations in southern california earthquakes induce changes in static stress on neighbouring faults that may delay hasten or even trigger subsequent earthquakes1 10 the length of time over which such effects persist has a bearing on the potential contribution of stress analyses to earthquake hazard assessment but is presently unknown here we use an elastic half space model11 to estimate the static stress changes generated by damaging magnitude m 5 earthquakes in southern california over the past 26 years and to investigate the influence of these changes on subsequent earthquake activity we find that in the 1 5 year period following a m 5 earthquake any subsequent nearby m 5 earthquake almost always ruptures a fault that is loaded towards failure by the first earthquake after this period damaging earthquakes are equally likely to rupture loaded and relaxed faults our results suggest that there is a short period of time following a damaging earthquake in southern california in which simple coulomb failure stress models could be used to identify regions of increased seismic hazard

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

gpr a ground penetrating radar for the netlander mission in the coming decade several missions are planned that will land on the surface of mars landers or instrumented geophysical stations among the scientific objectives of these projects one of the most important will be to unravel the many unknowns in the geological and hydrological history of the planet the netlander mission offers a unique opportunity to explore the interior of mars its subsurface its atmosphere and its distant environment from four landing sites that will be selected to offer a variety of different geophysical conditions we have thus proposed to fly on these four landers a ground penetrating radar gpr to explore the geological characteristics of the subsurface and search for water reservoirs down to a depth which may be sufficient to allow a possible detection of liquid water we provide in this paper a short description of this radar which is based on a new concept to allow a 3 d imaging of the subsurface by determining the range and direction of the underground reflectors in order to access to deep layers it will operate at a low frequency of 2 mhz some results obtained by a numerical modeling of the radar operation in an electromagnetic model of the martian subsurface are presented in order to illustrate the main capabilities of the radar in the last section preliminary results from an initial field test are reported in addition to its primary goal as a ground penetrating radar the gpr will also be operated on mars as an ionospheric sounder and in a passive mode as a hf receiver to measure the radio electric background

dislocation model for aseismic crustal deformation at hollister california geodetic observations at many active plate margins reveal relatively steady aseismic motion during the time between major earthquakes we model the aseismic motion by the relative motion between several blocks that suffer frictional resistance in the upper crust frictional drag is represented by uniform dislocation on a fault surface we assume that friction occurs only on the upper brittle zone of each block boundary and that frictionless sliding occurs below this zone using a bayesian inversion procedure we invert geodetic data to determine the parameters of the block and dislocation model parameters to be estimated include the two horizontal components of velocity for each block and the dislocation rate dip angle slip angle and width of each rectangular fault patch for hollister we assume five blocks and nine rectangular fault patches rates of length change on 92 lines observed by trilateration make up the data triangulation leveling very long baseline interferometry satellite ranging and global positioning system data can also be easily included in our model our inversion procedure uses prior estimates of all block and fault parameters based on geological and seismological data the block parameters are estimated from geological observations of fault displacement rates while the widths of the fault patches are estimated from earthquake hypocentral depths we choose prior estimates of the dislocation rate to match observed creep rates the estimated block motion is well resolved by the geodetic data and agrees well with the geological estimates this suggests that the block motion is steady on time scales from 10 years to 1 m y the net motion across the san andreas calaveras fault system is 38 3 mm yr oriented s38 e thus some additional displacement possibly offshore is needed to match the rate of 56 mm yr predicted by plate tectonic models the depth of the transition from frictional to free sliding varies considerably from 0 4 km on the southern calaveras to 11 km on the central san andreas fault two segments of the san andreas north of 36 50 and south of 36 40 have the greatest potential for moderate to large earthquakes

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

optimization of the antennas of the eiss radar designed to perform deep martian subsurface sounding the electromagnetic investigation of subsurface eiss instrument has been designed to get information on the morphology and geological structure of the martian subsurface and search for potential liquid water reservoirs eiss is an impulse radar operating from the martian surface at central frequencies of a few megahertz in order to perform deep sounding of the subsurface particular attention has been paid to the design of the instrument s electrical antennas that need to transmit and receive the electromagnetic waves without distortion over the instrument s full frequency bandwidth i e from 1 to 7 mhz to fit into the mass and volume allocation wire antennas made of identical loaded dipoles have been selected they will be deployed on the surface by a mechanical mortarlike device the resistive profile along the monopoles needs to be carefully chosen to ensure a proper behavior of the antenna once they are deployed on the surface this paper deals with the performance of these electrical antennas and the optimization of their resistive profile the study of the impact of the subsurface is performed on simulated data obtained by a numerical finite difference in time domain code when available experimental data show the good agreement with the theory the studies performed show that a slightly suboptimal resistive profile will allow a correct operation of the instrument at the surface of mars for any landing site

modeling of postseismic relaxation following the great 1857 earthquake southern california abstract repeated first order triangulation measurements made from 15 monuments in central and southern california are used to explore the structure of the crust and upper mantle beneath southern california as well as to estimate the distribution of slip during the great 1857 earthquake the monuments are divided into two subnetworks with repeated observations covering the respective time intervals from 1884 to 1923 and from 1898 to 1923 the angle changes occurring within these times are modeled as a sum of two contributions postseismic displacements following the 1857 earthquake and plate tectonic loading the postseismic displacements have been calculated using a elastic viscoelastic coupling model that for any fault geometry yields the exact displacements on a spherically stratified earth the southern california crust and upper mantle are modeled as an upper elastic plate underlain by a viscoelastic asthenosphere for several different combinations of structural parameters inversions are performed for the slip distribution of the 1857 earthquake in general the best fits to the data are achieved with a triangular slip distribution an elastic plate thickness of 16 km a crustal thickness of 16 to 33 km and a mantle viscosity of 0 4 to 0 8 10 19 pa s although these inversions identify the best rheological models they lead to observed residuals that are inconsistent with a normally distributed data set a preferred model of slip is constructed that accounts for the budget of total slip during the earthquake cycle and satisfies a normally distributed data set the preferred slip distribution is similar in shape to the mapped surface slip distribution but is greater in amplitude by a factor of 1 5 to 2 in addition the san jacinto fault which has no known surface slip associated with the 1857 earthquake is inferred to have accommodated 1 to 3 m of slip during the 1857 earthquake and appears to be the favored site for tectonic strain accumulation since 1857 inferred fault slip exceeding known surface slip implies either unrecognized secondary faulting or a slip deficit at the surface relative to the slip at depth

the san andreas fault system through the transverse ranges as illuminated by earthquakes the pattern of seismogenic faulting in the san gorgonio pass san bernardino basin area of the san andreas fault saf zone is mapped from about 7000 quality selected focal mechanisms 1981 nov 92 they are derived from phase data of the southern california network by a relocation procedure based on location dependent station corrections and by a grid search procedure about 3 4 of the mechanisms have been interpreted as planes of rupture or slip planes that delineate many distinct faults one of these faults is right lateral dips steeply northeast and is continuous trough the san gorgonio pass area we interpret it as the main branch of the saf a very large earthquake seems possible on this portion of the saf because of its continuity and because of the unusually deep reach of the seismicity 23 km southwest of this fault a volume of diffuse and persistent seismicity has a sharp downward cut off which may reflect a basal detachment dipping 20 northeast and intersecting the saf along the deepest seismicity northeast of the saf the floor of the seismicity is shallower by as much as 10 km stress characteristics are derived from slip planes in 10 selected subregions the regime is transpressional near the constricting bend of the saf at san gorgonio pass it is dominated by horizontal extension in the san bernardino basin area and it is again transpressional in the eastern san gorgonio mountains north of the cucamonga thrust the broad features of this zonation may be related to the intersection of the saf with the pinto mountain fault and with the san jacinto fault a change of fault kinematics in the yucaipa cluster coincides with the 1992 landers and big bear main shocks and may be a manifestation of static stress change

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

application of the global positioning system to crustal deformation measurements 3 result from the southern california borderlands five years of measurements from the global positioning system gps satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern california gps provides the first practical technique to make precise geodetic measurements in the region the geodetic network is situated along the california coastline from vandenberg 120 6 w 34 6 n to san diego with additional sites on santa cruz san nicolas santa catalina santa rosa and san clemente islands the precision of horizontal interstation vectors is subcentimeter and the interstation vector rate between ovro and vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation no significant motion is observed in the western santa barbara channel between vandenberg and santa rosa island 0 5 1 6 mm yr where the quoted uncertainties are one standard deviation motions in the eastern santa barbara channel are consistent with compressional deformation of 6 1 mm yr at n16 3 e this motion is in agreement with seismicity and an independent geodetic analysis for the period 1971 1987 larsen 1991 san clemente island is moving relative to san diego at the rate of 5 9 1 8 mm yr at a direction of n38 20 w the motion between san nicolas island and san clemente island 0 8 1 5 mm yr is insignificant

seismic behavior of the southern san andreas fault zone in the northern coachella valley california comparison of the 1948 and 1986 earthquake sequences moderate earthquakes in 1948 and 1986 that occurred along the southern san andreas fault zone are examined to help understand the subsurface geometry and seismic behavior of active fault segments in the northern coachella valley the 1986 north palm springs earthquake occurred between the banning and mission creek traces of the san andreas fault zone based on data from both portable and permanent stations aftershocks of the 1986 event define a nearly planar surface that strikes about n60 to 70 w is about 15 km long and dips northeast from near the banning surface trace at about 45 to 50 relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34 n00 26 116 w36 34 and a focal depth of 10 4 km the first motion focal mechanism for the mainshock indicates pure right slip on a plane dipping northeast at 40 to 45 with a strike of n60 w previous moment tensor solutions e g hartzell 1989 exhibit significant oblique reverse slip on a plane with a more westerly strike and a seismic moment of 1 4 to 1 7 1018 n m mw 6 1 actual and synthetic wood anderson recordings indicate a local magnitude of ml 6 0 the 1948 desert hot springs earthquake produced waveforms similar to 1986 although amplitudes for 1948 are typically 20 to 30 larger the 1948 mainshock is relocated at 33 n55 2 116 w28 9 with a focal depth of 12 km a revised magnitude of ml 6 3 and an estimated seismic moment of 2 to 3 1018 n m based on arrival time information from close 20 km portable stations deployed in 1948 to 1949 the 1948 aftershock distribution is about 15 km long 8 to 10 km wide and abuts the 1986 aftershock sequence to the southeast along the banning fault the 1948 aftershock zone is consistent with a focal mechanism that exhibits predominantly right slip motion rake 169 on a plane that strikes n55 w and dips more steeply northeast at 60 to 70 in 1986 the horizontal extent of the rupture may have been controlled by secondary cross faults and possible changes in fault geometry while the down dip extent may have been controlled by the presence at depth of a regional detachment in 1948 the rupture likely had a steeper dip than in 1986 and extended southeast as far as the northern indio hills where the surface trace of the banning fault undergoes an approximate 7 change in strike these results indicate that the banning fault is nonvertical is likely segmented according to fault dip as well as fault strike and has been the primary locus of recent moderate sized earthquake activity in the northern coachella valley

interaction of the san andreas fault creeping segment with adjacent great rupture zones and earthquake recurrence at parkfield three dimensional finite element calculations are employed to study interactions in space and time between the creeping segment of the san andreas fault in central california and the adjacent currently locked zones of the 1857 and 1906 great earthquakes vertically the model consists of an elastic upper crust over a maxwell viscoelastic region representing the entire lower crust or a narrower horizontal detachment layer and a stiffer and more viscous upper mantle the crust has a single vertical fault extending to the top of the mantle at 25 km depth in zones along strike corresponding to the 1857 and 1906 events the top 12 5 km of the fault is locked against slip except in great earthquakes below the locked zones and everywhere along the creeping region between them the fault is freely slipping the model parameters are compatible with seismological and geological observations and with a ratio of maxwell relaxation time to the relaxing layer thickness in the range 1 to 2 yr km as established by li and rice 1987 and fares and rice 1988 based on fits to geodetic data along the san andreas fault an imposed constant far field shear motion and periodic 1857 and 1906 type earthquakes generate slip rates along the creeping fault segment that evolve in time throughout the entire earthquake cycle shortly after an adjacent great earthquake slip rates in the creeping zone are higher than the far field velocity while later in the cycle they are lower hence time dependency should be accounted for when measurements of fault slip are used to estimate the plate motion if parkfield earthquakes are a response to a time dependent loading of the type simulated here their recurrence interval would tend to lengthen with time since the 1857 event thus the hypothesis of characteristic periodic earthquakes at parkfield may not provide the best estimate of the occurrence time of the next event using for example the statistics of past events and assuming that parkfield earthquakes are a response to a slip deficit near middle mountain and that the elastic crustal layer is 17 5 km thick we find that the next event is predicted for about 1992 9 years if the lower crust is a 7 5 km thick layer having a material relaxation time of 15 years and 1995 11 years if the 7 5 km thick lower crust is characterized by a relaxation time of 7 5 years these values may be compared to the 1988 7 years estimate based on periodicity in time the modeling results also indicate that the interaction between the 1857 and 1906 rupture zones is small

stress transfer in earthquakes hazard estimation and ensemble forecasting inferences from numerical simulations abstract observations indicate that earthquake faults occur in topologically complex multi scale networks driven by plate tectonic forces we present realistic numerical simulations involving data mining pattern recognition theoretical analyses and ensemble forecasting techniques to understand how the observable space time earthquake patterns are related to the fundamentally inaccessible and unobservable dynamics numerical simulations can also help us to understand how the different scales involved in earthquake physics interact and influence the resulting dynamics our simulations indicate that elastic interactions stress transfer combined with the nonlinearity in the frictional failure threshold law lead to the self organization of the statistical dynamics producing 1 statistical distributions for magnitudes and frequencies of earthquakes that have characteristics similar to those possessed by the gutenberg richter magnitude frequency distributions observed in nature and 2 clear examples of stress transfer among fault activity described by stress shadows in which an earthquake on one group of faults reduces the coulomb failure stress on other faults thereby delaying activity on those faults in this paper we describe the current state of modeling and simulation efforts for virtual california a model for all the major active strike slip faults in california noting that the working group on california earthquake probabilities wgcep uses statistical distributions to produce earthquake forecast probabilities we demonstrate that virtual california provides a powerful tool for testing the applicability and reliability of the wgcep statistical methods furthermore we show how the simulations can be used to develop statistical earthquake forecasting techniques that are complementary to the methods used by the wgcep but improve upon those methods in a number of important ways in doing so we distinguish between the official forecasts of the wgcep and the research quality forecasts that we discuss here finally we provide a brief discussion of future problems and issues related to the development of ensemble earthquake hazard estimation and forecasting techniques

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

a re examination of the 1947 manix california earthquake sequence and comparison to other sequences within the mojave block the 10 april 1947 manix california earthquake m l 6 2 is the largest earthquake to have occurred in the mojave block in historic time left lateral movement was observed along the manix fault strike of n70 e following the earthquake however the large aftershocks of the sequence as located by richter and nordquist 1951 align themselves along a trend of n30 w this led richter 1958 to conclude that rupture during the main shock was along the trend of the aftershocks and that the surface faulting was a secondary feature in this study relocation of the larger aftershocks of the sequence verifies the previously observed trend of n30 w suggesting the aftershocks occurred along a fault conjugate to the manix fault body waveform modeling of the main shock indicates that it was composed of two subevents the location of the second subevent with respect to the first subevent suggests unilateral rupture to the southwest along the manix fault although the focal mechanisms of both events are consistent with either faulting along the manix fault or a conjugate fault both subevents occurred at relatively shallow depths 5 to 7 km a study of other recent earthquakes of m l 3 0 within the mojave block indicates that the majority of earthquakes in the region occur at shallow depths 7 km although the heat flow in the area is 10 to 20 mw m2 lower than in surrounding regions where deeper seismicity is present

source parameters of the 1933 long beach earthquake regional seismographic network and teleseismic data for the 1933 m l 6 3 long beach earthquake sequence have been analyzed both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the newport inglewood fault the focal mechanism had a strike of 315 dip of 80 to the northeast and rake of 170 relocation of the foreshock main shock aftershock sequence using modern events as fixed reference events shows that the rupture initiated near the huntington beach newport beach city boundary and extended unilaterally to the northwest to a distance of 13 to 16 km the centroidal depth was 10 2 km the total source duration was 5 sec and the seismic moment was 5 10 25 dyne cm which corresponds to an energy magnitude of m w 6 4 the source radius is estimated to have been 6 6 to 7 9 km which corresponds to a brune stress drop of 44 to 76 bars both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents when the slip estimate from the seismic moment of 85 to 120 cm is compared with the long term geological slip rate of 0 1 to 1 0 mm yr along the newport inglewood fault the 1933 earthquake has a repeat time on the order of a few thousand years

seismicity and fault interaction southern san jacinto fault zone and adjacent faults southern california implications for seismic hazard the southern san jacinto fault zone is characterized by high seismicity and a complex fault pattern that offers an excellent setting for investigating interactions between distinct faults this fault zone is roughly outlined by two subparallel master fault strands the coyote creek and clark san felipe hills faults that are located 2 to 10 km apart and are intersected by a series of secondary cross faults seismicity is intense on both master faults and secondary cross faults in the southern san jacinto fault zone the seismicity on the two master strands occurs primarily below 10 km the upper 10 km of the master faults are now mostly quiescent and appear to rupture mainly or solely in large earthquakes our results also indicate that a considerable portion of recent background activity near the april 9 1968 borrego mountain rupture zone m l 6 4 is located on secondary faults outside the fault zone we name and describe the palm wash fault a very active secondary structure located about 25 km northeast of borrego mountain that is oriented subparallel to the san jacinto fault system dips approximately 70 to the northeast and accommodates right lateral shear motion the vallecito mountain cluster is another secondary feature delineated by the recent seismicity and is characterized by swarming activity prior to nearby large events on the master strand the 1968 borrego mountain and the april 28 1969 coyote mountain m l 5 8 events are examples of earthquakes with aftershocks and subevents on these secondary and master faults mechanisms from those earthquakes and recent seismic data for the period 1981 to 1986 are not simply restricted to strike slip motion dipslip motion is also indicated teleseismic body waves long period p and sh of the 1968 and 1969 earthquakes were inverted simultaneously for source mechanism seismic moment rupture history and centroid depth the complicated waveforms of the 1968 event m o 1 2 10 19 nm are interpreted in terms of two subevents the first caused by right lateral strike slip motion in the mainshock along the coyote creek fault and the second by a rupture located about 25 km away from the master fault our waveform inversion of the 1969 event indicates that strike slip motion predominated releasing a seismic moment of 2 5 10 17 nm nevertheless the right lateral nodal plane of the focal mechanism is significantly misoriented 20 with respect to the master fault and hence the event is not likely to be associated with a rupture on that fault from this and other examples in southern california we conclude that cross faults may contribute significantly to seismic hazard and that interaction between faults has important implications for earthquake prediction

in the shadow of 1857 the effect of the great ft tejon earthquake on subsequent earthquakes in southern california the great 1857 fort tejon earthquake is the largest earthquake to have hit southern california during the historic period we investigated if seismicity patterns following 1857 could be due to static stress changes generated by the 1857 earthquake when post 1857 earthquakes with unknown focal mechanisms were assigned strike slip mechanisms with strike and rake determined by the nearest active fault 13 of the 13 southern california m 5 5 earthquakes between 1857 and 1907 were encouraged by the 1857 rupture when post 1857 earthquakes in the transverse ranges with unknown focal mechanisms were assigned reverse mechanisms and all other events were assumed strike slip 11 of the 13 earthquakes were encouraged by the 1857 earthquake these results show significant correlations between static stress changes and seismicity patterns the correlation disappears around 1907 suggesting that tectonic loading began to overwhelm the effect of the 1857 earthquake early in the 20th century

local magnitudes and apparent variations in seismicity rates in southern california redetermination of local magnitudes for moderate earthquakes recorded by the southern california seismographic network scsn from 1932 to 1990 has shown that the magnitudes have not been consistently determined over that time period the amplitudes of ground velocities recorded on wood anderson instruments were systematically overestimated prior to 1944 compared to present reading procedures leading to a significant overestimation of local magnitudes in addition the change from human to computerized estimation of event magnitude from a suite of amplitudes in 1975 led to slightly lower event magnitudes for the time after 1975 compared to the time before these changes contribute to an apparently higher rate of seismicity in the 1930s and 1940s than later in the catalog which had been interpreted as a decrease in seismicity rate after the 1952 kern county m w 7 5 earthquake wood anderson amplitudes have been reread and consistent magnitudes recalculated using uniform procedures for all earthquakes with a catalog magnitude of 4 5 and greater within the scsn from 1932 to 1943 and those with a catalog magnitude of 4 8 and greater from 1944 to 1990 so as to create a complete list of all earthquakes with a modern local magnitude of 5 0 or greater using these new magnitudes we find that the rate of m l 5 0 and greater earthquakes in southern california over this 59 year period to be poissonian with no changes in rate significant above the 90 level from this rate in any 30 year period the poissonian probability of a m 6 earthquake is 99 7 the probability of an m 7 earthquake is 65 and the probability of an m 8 event is 18

earthquakes as a coupled shear stress high pore pressure dynamical system the migration coalescence and localization of slip seismicity and zones of high pore pressure are modeled using a porosity reduction mechanism to drive pore pressure within a fault zone in excess of hydrostatic increased pore pressure in discrete cells creates zones of low effective stress which induces slip that may propagate to surrounding cells depending on the local state of stress at slip stress is transferred using the solution for a rectangular dislocation in an elastic half space and pore pressures are redistributed by conserving fluid mass using simple assumptions about fault rheology and permeability it is shown that the interaction between shear stress and effective stress evolves to a state of earthquake clustering with repeated events locked zones and large variations in fault strength the model evolves from a uniform shear stress state on a strong fault to a heterogeneous shear stress state on a weak fault

large earthquake occurrence affected by small stress changes abstract many observations suggest that relatively small changes in stress may trigger or inhibit earthquakes this result is surprising because the perturbing stress is only a small fraction of the stress drop from the affected earthquake we investigate this phenomenon by simulating the earthquake fault with a numerical model that is physically realistic a fault is not homogeneous and therefore the failure criteria will vary over the surface of the fault we model fault heterogeneity as simple coulomb failure on smaller but mostly uniform areas that comprise the fault as a result of multiple and repeated failures over these small areas the fault can have a very uneven stored stress we show that it is possible for small changes in confining stress of a few tenths of a bar to advance or retard the occurrence of large earthquakes by decades because such stress changes can be generated by nearby earthquakes and over much longer time scales by postseismic strain diffusion from large distant earthquakes estimates of the probabilities of earthquake occurrence must take into account these stress perturbations in order to be reliable

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

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

a study on seismicity in the yunnan region by using the multidimensional stress release model abstract the long term seismic activity in the yunnan region has attracted much attention due to frequent occurrences of disastrous earthquakes we investigated the seismicity of the yunnan region using the multidimensional stress release model msrm to evaluate the long term seismic hazard msrm combines three factors that affect earthquake occurrence i e initial stress background tectonic loading and coseismic stress transfer and can simulate and forecast the spatiotemporal seismicity in this study a complete catalog comprising 28 strong earthquakes m 6 5 after 1900 in the yunnan region was investigated using msrm we calculated the spatiotemporal variations of seismicity rates and found that strong earthquakes occur at locations with higher seismic activity comparisons between the seismicity rates at the epicenter locations immediately before and after strong earthquakes show that the seismicity rates in the vicinity of the epicenters significantly decrease after strong earthquakes the estimated seismicity rates at the ending time january 1 2025 suggest that strong earthquake potential in the section between kunming and huaning of the xiaojiang fault and the northeastern section of the lijiang xiaojinhe fault should receive more attention while the seismicity in the southwestern part decreases and the seismicity in the middle segment of the red river fault remains low

reconstruction of a channel offset in 1857 by the san andreas fault near cholame california abstract the u s geological survey has formally forecast the next moderate m 6 parkfield earthquake for 1988 5 yr sieh and jahns 1984 forecast that a m 7 earthquake is likely to accompany the next parkfield earthquake the latter forecast rests on interpretation of 3 to 4 m of 1857 slip on eight gullies near cholame and slip loading rate of 3 5 cm yr since 1857 but lienkaemper 1987 interprets 1857 slip here as 4 to 8 m these eight gullies yield larger slip when one includes slip on secondary faults at three gullies and one picks gully centerlines judging the post 1857 alteration differently at five sites our goal in this paper is to show our interpretation of slip on one gully deemed the least ambiguous of these eight as they appear in large scale 1966 aerial photographs we interpret slip as 2 m larger at this key site suggesting a much lower likelihood of the next parkfield earthquake triggering a larger cholame earthquake our detailed studies of many sites showing 1857 slip are continuing and will be published later using 1966 aerial photos we infer the shape of this key gully before fault offset by eliminating effects of later natural and artificial processes of alteration on 1986 photos right lateral offset seems 3 5 0 5 m but on 1966 photos we measure 5 7 0 7 m differencing precise 1966 and 1986 topographic maps shows that little incision occurred but soil deposited up to a 1 m thickness on the left bank probably drifted downslope from agricultural disking in the adjacent field this soil deposit forced the channel bottom 2 m away from the left bank upstream from fault only causing offset in 1986 to appear 2 m smaller than in 1966 this 2 m alteration presumably occurred before sieh 1978 interpreted 3 3 m slip here

earthquakes in the los angeles metropolitan region a possible fractal distribution of rupture size although there is debate on the maximum size of earthquake that is possible on any of several known fault systems in the greater los angeles metropolitan region it is reasonable to assume that the distribution of earthquakes will follow a fractal distribution of rupture areas for this assumption and an overall slip rate for the region of approximately 1 centimeter per year roughly one magnitude 7 4 to 7 5 event is expected to occur every 245 to 325 years a model in which the earthquake distribution is fractal predicts that additionally there should be approximately six events in the range of magnitude 6 6 in this same span of time a higher rate than has occurred in the historic record

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

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

source study of the 1906 san francisco earthquake all quality teleseismic recordings of the great 1906 san francisco earthquake archived in the 1908 carnegie report by the state earthquake investigation commission were scanned and digitized first order results were obtained by comparing complexity and amplitudes of teleseismic waveforms from the 1906 earthquake with well calibrated similarly located more recent earthquakes 1979 coyote lake 1984 morgan hill and 1989 loma prieta earthquakes at nearly co located modern stations peak amplitude ratios for calibration events indicated that a localized moment release of about 1 to 1 5 10 27 dyne cm was responsible for producing the peak the teleseismic body wave arrivals at longer periods 50 to 80 sec we found spectral amplitude ratios of the surface waves require a total moment release between 4 and 6 10 27 dyne cm for the 1906 earthquake comparable to previous geodetic and surface wave estimates thatcher 1975 we then made a more detailed source analysis using morgan hill s body waves as empirical green s functions in a finite fault subevent summation the morgan hill earthquake was deemed most appropriate for this purpose as its mechanism is that of the 1906 earthquake in the central portion of the rupture from forward and inverse empirical summations of morgan hill green s functions we obtained a good fit to the best quality teleseismic waveforms with a relatively simple source model having two regions of localized strong radiation separated spatially by about 110 km assuming the 1906 epicenter determined by bolt 1968 this corresponds with a large asperity on the order of the loma prieta earthquake in the golden gate san francisco region and one about three times larger located northwest along strike between point reyes and fort ross this model implies that much of the 1906 rupture zone may have occurred with relatively little 10 to 20 sec radiation consideration of the amplitude and frequency content of the 1906 teleseismic data allowed us to estimate the scale length of the largest asperity to be less than about 40 km with rough constraints on the largest asperity size and magnitude we produced a suite of estimated synthetic ground velocities assuming a slip distribution similar to that of the loma prieta earthquake but with three times as much slip for purposes of comparison with the recent abundant loma prieta strong motion data set we moved the largest 1906 asperity into loma prieta region peak ground velocity amplitudes are substantially greater than those recorded during the loma prieta earthquake and are comparable to those predicted by the attenuation relationship of joyner and boore 1988 for a magnitude m w 7 7 earthquake

triggering of 1812 santa barbara earthquake by a great san andreas shock implications for future seismic hazards in southern california we study the evolution of the stress field over the last 200 years in southern california using the stress buildup associated with major faults and stress drops associated with great earthquakes in this report we calculate the change in the coulomb failure function cff associated with the great wrightwood earthquake of dec 8 1812 on the san andreas fault for the region near santa barbara that experienced a large shock 13 days later the rupture length constrained by paleoseismic data is used for the wrightwood earthquake of 1812 a uniform displacement of 3 5 m from the surface to seismogenic depths is assumed the geologic setting and focal mechanisms of earthquakes in the santa barbara region are dominated by thrust left lateral strike slip faulting and combinations of the two for our preferred rupture model of the earthquake of dec 8 the santa barbara shock of dec 21 1812 occurred in a region of 0 01 to 0 1 mpa cff for reasonable fault orientations indicating that it may have been advanced by years to decades and triggered by the great san andreas earthquake 13 days earlier we also calculate cff on typical kinds of faults in southern california for a future candidate great earthquake along the san bernardino and coachella valley segments of san andreas fault this earthquake produces large positive values of cff for shallow dipping thrust faults beneath the greater los angeles area and for nw trending right lateral strike slip faults in three regions this suggests that shocks of magnitude similar to the m 7 1 1812 santa barbara earthquake could be triggered by the next great event on the southern san andreas fault

earthquakes and associated tectonics in a part of coastal central california seismicity along a corridor 65 km wide and 100 km long across the central coast ranges of california southwest from the san andreas fault zone is used to determine tectonic properties of the three constituent provinces a region of horizontal shear along the san andreas fault zone a moderately active belt of compressional faulting along the coastline and an intermediate low seismicity zone fault plane mechanisms of 33 earthquakes in the corridor indicate that the three provinces are distinguished by characteristic tectonic features directions of p compression axes fault types and fault slip directions each interprovince transition appears to be less than 12 km wide across the boundary between the san andreas fault zone and the low seismicity region just sw of the san andreas zone p axes are rotated about 45 and the dominant right lateral slip directions also change by about 45 across the boundary between the low seismicity and the compressive regions near the nacimiento fault a small rotation occurs in p axes and slips change from dominantly horizontal shear to the ne to oblique reverse to the sw a remarkable result is that almost all inferred faults in the corridor can be interpreted as having moderate to steeply ne dipping planes along which horizontal components of slip are right lateral and vertical components have the ne side moving up the base of the seismogenic zone is about 12 km deep throughout the corridor it is suggested that the seismicity and stress patterns in each of the three provinces reflect the relative strengths of upper crustal rocks in the seismogenic zone the largest recorded earthquake ml 6 0 in the corridor since 1941 is the bryson earthquake of 1952 a reanalysis of the epicenter was made based on the calibration event technique in which readings of several newly collected seismograms were included the revised location is just sw of the nacimiento fault zone about 6 km sw of bryson displacement is right lateral along a 55 ne dipping plane that strikes n55 w in conformance with san andreas type faulting

a 100 year average recurrence interval for the san andreas fault at wrightwood california evidence for five large earthquakes during the past five centuries along the san andreas fault zone 70 kilometers northeast of los angeles california indicates that the average recurrence interval and the temporal variability are significantly smaller than previously thought rapid sedimentation during the past 5000 years in a 150 meter wide structural depression has produced a greater than 21 meter thick sequence of debris flow and stream deposits interbedded with more than 50 datable peat layers fault scarps colluvial wedges fissure infills upward termination of ruptures and tilted and folded deposits above listric faults provide evidence for large earthquakes that occurred in a d 1857 1812 and about 1700 1610 and 1470

evolution of the stress field in southern california and triggering of moderate size earthquakes a 200 year perspective changes in stress in southern california are modeled from 1812 to 2025 using as input 1 stress drops associated with six large 7 0 7 5 earthquakes through 1995 and 2 stress buildup associated with major faults with slip rates 3 mm yr as constrained by geodetic paleoseismic and seismic measurements evolution of stress and the triggering of moderate to large earthquakes are treated in a tensoffal rather than a scalar manner we present snapshots of the cumulative coulomb failure function acff as a function of time for faults of various strike dip and rake throughout southern california we take acff to be zero everywhere just prior to the great shock of 1812 we find that about 95 of those well located m 6 earthquakes whose mechanisms involve either strike slip or reverse faulting are consistent with the coulomb stress evolutionary model that is they occurred in areas of positive acff the interaction between slow moving faults and stresses generated by faster moving faults significantly advanced the occurrence of the 1933 long beach and 1992 landers events in their earthquake cycles coulomb stresses near major thrust faults of the western and central transverse ranges have been accumulating for a long time future great earthquakes along the san andreas fault especially if the san bernardino and coachella valley segments rupture together can trigger moderate to large earthquakes in the transverse ranges as appears to have happened in the santa barbara earthquake that occurred 13 days after the great san andreas shock of 1812 maps of current acff provide additional guides to long term earthquake prediction

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

centroid moment tensor solutions for april june 1992 abstract centroid moment tensor solutions are presented for 245 earthquakes that occurred during the second quarter of 1992 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward the solution for an earthquake under mindanao philippines on 9 may represents the 10 000th event in the cmt catalogue

an estimation of the electrical characteristics of planetary shallow subsurfaces with tapir antennas in the frame of the netlander program we have developed the terrestrial and planetary investigation by radar tapir imaging ground penetrating radar to explore the martian subsurface at kilometric depths and search for potential water reservoirs this instrument which is to operate from a fixed lander is based on a new concept which allows one to image the various underground reflectors by determining the direction of propagation of the reflected waves the electrical parameters of the shallow subsurface permittivity and conductivity need to be known to correctly determine the propagation vector in addition these electrical parameters can bring valuable information on the nature of the materials close to the surface the electric antennas of the radar are 35 m long resistively loaded monopoles that are laid on the ground their impedance measured during a dedicated mode of operation of the radar depends on the electrical parameters of soil and is used to infer the permittivity and conductivity of the upper layer of the subsurface this paper presents an experimental and theoretical study of the antenna impedance and shows that the frequency profile of the antenna complex impedance can be used to retrieve the geoelectrical characteristics of the soil comparisons between a numerical modeling and in situ measurements have been successfully carried over various soils showing a very good agreement

full wave calibration of time and frequency domain ground penetrating radar in far field conditions full wave modeling of ground penetrating radar gpr data using green s functions for wave propagation in planar layered media and antenna characteristic global reflection and transmission functions for describing far field antenna effects including antenna medium interactions has shown a great potential for nondestructive characterization of soils and materials the accuracy of the retrieved parameters in the gpr data inversion depends on the accuracy of the gpr external calibration in this research we studied the stability and the repeatability of two different gpr systems namely frequency and time domain systems a combination of a vector network analyzer and 800 5200 mhz horn antenna was used as a frequency domain gpr fd gpr whereas a gssi gpr system using a 900 mhz bowtie antenna was used as a time domain gpr td gpr both gpr systems including their antennas were calibrated several times using measurements with the antennas at different heights over a perfect electric conductor pec in the laboratory as well as over a water layer in addition measurements were performed over a thin water layer and a relatively thick sandy soil layer as validating medium the results showed that the fd gpr is relatively stable while the td gpr presents a significant drift which can be accounted for using corrections based on the air direct coupling waves free space measurements water and pec based calibrations provided very similar results for the gpr calibration functions inversions for the water layer and the sandy soil layer provided reliable results and showed a high degree of the repeatability for both radar systems the error on the calibration based on inaccurate antenna heights over pec showed the significant errors on the inversion results for the directive antenna horn antenna but less error for the bowtie antenna this analysis demonstrated the general validity of the proposed far field radar modeling approach not only with respect to frequency and time domain radars but as well with respect to the calibrating medium

an imaging hf gpr using stationary antennas experimental validation over the antarctic ice sheet terrestrial and planetary imaging radar tapir is an innovative high frequency ground penetrating radar gpr developed in the frame of the martian netlander mission to probe the subsurface down to kilometric depths unlike most gprs tapir is able to image underground reflectors with stationary antennas in this paper after a brief presentation of the instrument we describe the method developed to interpret data collected during the radar of netlander in terre ade acutelie raneta field survey in antarctica this method consists of retrieving the direction of arrival of each detected echo through the measurement of five components of the electromagnetic field the three magnetic components and the horizontal components of the electric field thus both the range and the direction of each individual reflection or diffraction due to the ice bedrock interface are resolved we validated this method on finite difference time domain numerically simulated data for different subsurface configurations before applying it to raneta observations in particular the irregular topography of the bedrock in two sounding sites was revealed we discuss the accuracy of our results

evolution of moderate seismicity in the san francisco bay region 1850 to 1993 seismicity changes related to the occurrence of large and great earthquakes the rate of seismic activity of moderate size m 5 5 earthquakes in the san francisco bay sfb region has varied considerably during the past 150 years as measured by the rate of seismic moment release seismic activity in the sfb region is observed to accelerate prior to m 7 0 earthquakes in 1868 1906 and 1989 and then to decelerate following them we examine these seismicity changes in the context of the evolution of the stress field in the sfb region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace we use a coulomb failure function cff to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the sfb region we find that the occurrence of a large or great earthquake creates a stress shadow a region where the stress driving earthquake deformation is decreased interseismic strain accumulation acts to reverse this process gradually bringing faults in the sfb region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible as the stress shadow generated by a large or great earthquake disappears it migrates inward toward the fault associated with that large or great event the observed changes in the rate of occurrence of moderate earthquakes in the sfb region are broadly consistent with this model in detail the decrease in seismicity throughout most of the sfb region and a localized increase in the monterey bay region following the great 1906 earthquake is consistent with our predicted stress changes the timing and location of moderate size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the sfb region those earthquakes that are most inconsistent with our stress evolution model including the 1911 earthquake southeast of san jose are found to occur in regions where dip slip faulting is common in addition to strike slip the 1906 earthquake brought that zone of dip slip faulting closer to failure suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike slip one similar to the 1984 morgan hill earthquake the occurrence of activity on faults very close to the san andreas such as the lake elsman earthquakes of 1988 and 1989 appear to be associated with the last disappearence of the stress shadow on the loma prieta segment of the san andreas fault thus events of that type may represent an intermediate term precursor to a large earthquake such as the 1989 loma prieta event much of the moderate size earthquake activity in the sfb region appears to be modulated in time by the buildup and release of stress in large and great earthquakes a tensorial approach to earthquake prediction i e taking into account changes in the components of the stress tensor has several advantages over examining scalar changes such as those in seismic activity and moment release rates this tensorial approach allows for both activation and quiescence but in different subregions prior to as well as after large earthquakes

a re examination of historic earthquakes in the san jacinto fault zone california the high level of seismic activity and the potential for large earthquakes in the san jacinto fault zone southern california make it desirable to have accurate locations and source parameters for as many previous events as possible prior to the installation of a dense seismic network in this region earthquakes were located using only a few stations with generally poor azimuthal coverage resulting in considerable uncertainty in the locations we relocate and obtain moment estimates for historic pre wwssn earthquakes in the western imperial valley by comparing the waveforms and travel times with recent earthquakes in the region all the events are in the m l 5 5 to 6 5 range the historic earthquakes of interest occurred in 1937 1942 and 1954 we use the 1968 borrego mountain 1969 coyote mountain and 1987 elmore ranch earthquakes as calibration events we employ regional and teleseismic data from continuously operating stations with pasadena debilt berkeley ottawa and st louis recording most of the events the waveforms imply that all the events are almost pure strike slip events on vertical or near vertical faults approximate values for the strikes were obtained and are within the range of observed strikes for well studied earthquakes in this region the earthquakes are relocated by comparing s p and surface wave s travel times of historic events with the presumably well located recent events the relocations require only a small change in location for the 1954 event and a larger adjustment in the 1942 epicenter it also appears that the 1969 earthquake may have been mislocated the moment estimates are obtained by direct comparison of the maximum amplitudes the moment estimates imply that the 1968 and not the 1942 earthquake is the largest to have occurred in the region this century previous magnitude estimates suggested the 1942 event was larger

contrasts between source parameters of m 5 5 earthquakes in northern baja california and southern california summary earthquake source processes are determined for four historic events that occurred in baja california between 1915 and 1954 these results are combined with previous studies of recent and historic seismicity to compare earthquake processes along the san jacinto imperial cerro prieto and northern baja fault systems of southern california and northern mexico observed regional differences in rupture complexity fault rupture lengths and aftershock foreshock activity appear to be related to variations in heat flow and total fault offset there appear to be no systematic variations in stress drop as measured from far field observations between earthquakes along the imperial cerro prieto fault zone and those within the peninsular ranges of northern baja california slip rates estimated from historical seismicity between 1915 and 1992 fall 1 5 cm yr 1 short of the estimated relative plate motion between the pacific plate and north america for this region

modeling of short period surface wave propagation in southern california abstract for the 1992 landers mainshock the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong motion observations that cannot be easily interpreted by a simplified point source model in a laterally homogeneous medium both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3d medium surface wave synthetics are constructed using surface wave gaussian beam method which allows for a mild degree of lateral heterogeneity we analyze the amplitude and waveform data of the landers strong motion recordings in southern california in order to identify the path effects and the effect of an extended source the overall source parameters of the landers earthquake are obtained by comparing synthetics with recordings at near source stations which lie within relatively homogeneous path structures this procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms the surface wave phase velocity map is modified to fit the data recorded at larger hypocentral distances the final improved model generates a very good agreement between observations and synthetics in the presence of lateral heterogeneity the focusing defocusing and multipath interference are found to contribute significantly to the strong motion shaking in a 3d basin

historic earthquakes 1918 to 1923 and an assessment of source parameters along the san jacinto fault system body waveform modeling is used to determine the source parameters of two earthquakes that occurred along the northern san jacinto fault system in 1918 and 1923 the 1918 earthquake parameters are consistent with a 20 to 25 km rupture along the claremont fault at a depth of 7 5 km limited data for the 1923 event are more consistent with rupture on the san jacinto fault than rupture along the san andreas fault or a buried cross fault although the greatest damage associated with the earthquake occurred closer to the san andreas fault using the results of this study and previous studies of historic earthquakes along the southern san jacinto fault the relationships between rupture length and magnitude focal depth and heat flow and other parameters are examined in an effort to estimate the source properties of future events that are expected along currently quiescent portions of the fault zone one important observation is that rupture lengths along the san jacinto fault are shorter for an equivalent moment magnitude than those along the imperial valley mexicali fault system the higher heat flow of the imperial valley region may be responsible for this difference

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years

source characteristics of earthquakes along the southern san jacinto and imperial fault zones 1937 to 1954 abstract body waveform inversion techniques are used to study the source parameters of four earthquakes occurring between 1937 and 1954 along the southern san jacinto and imperial faults 1937 buck ridge 1940 imperial valley 1942 borrego mountain and 1954 salada wash events all earthquakes had simple rupture histories with the exception of the 1940 imperial valley main shock which consisted of at least four subevents whose relative locations indicate unilateral rupture toward the southeast earthquakes in regions of high heat flow 80 mw m 2 had focal depths near the base of the seismogenic zone 8 to 10 km the 1937 buck ridge earthquake located in a region of lower heat flow however appears to have occurred at a shallow 3 2 km depth the location mechanism and aftershock distribution for the 1942 borrego mountain earthquake suggest it could have occurred along the split mountain fault a recently identified northeast trending cross fault located between the elsinore and coyote creek faults or along an unnamed fault that parallels the trend of the coyote creek fault moment and rupture length estimates obtained from this study agree well with estimates obtained in previous studies that used different data sets

geodetic monitoring of the southern san andreas fault california 1980 1991 five geodetic arrays 10 to 40 km aperture located along the san andreas fault have been surveyed frequently several times in most years over the 1980 1991 interval to detect possible fluctuations in the deformation rate in each survey of an array the distances between the same four to seven pairs of geodetic monuments were measured the distances measured with corresponding standard deviation range from 8 4 km 3 4 mm to 38 km 8 2 mm the data are displayed as plots of measured distance as a function of time linear fits in such plots furnish a satisfactory representation of the data that is the scatter of the data about the linear fits is within the range expected for the estimated standard deviations in measurement however there are coherent low amplitude within the observational error fluctuations apparent in some of the measured distances those fluctuations need not be tectonic effects but rather may be either random patterns or artifacts introduced by systematic drift in instrument calibration or wander of the geodetic monuments we conclude that the measurements are consistent with steady deformation of the arrays over the 1980 1991 interval and we find no convincing evidence in the data for fluctuations in the rate of deformation

radar soundings of the subsurface of mars the martian subsurface has been probed to kilometer depths by the mars advanced radar for subsurface and ionospheric sounding instrument aboard the mars express orbiter signals penetrate the polar layered deposits probably imaging the base of the deposits data from the northern lowlands of chryse planitia have revealed a shallowly buried quasi circular structure about 250 kilometers in diameter that is interpreted to be an impact basin in addition a planar reflector associated with the basin structure may indicate the presence of a low loss deposit that is more than 1 kilometer thick