the role of ice thickness and bed properties on the dynamics of the enhanced flow tributaries of bailey ice stream and slessor glacier east antarctica

accelerated stress buildup on the southern san andreas fault and surrounding regions caused by mojave desert earthquakes a sequence of four mw 6 earthquakes including the 1992 mw 5 7 3 landers and mw 5 7 1 hector mine earthquakes occurred in the mojave desert in the 1990s in close proximity to the southern san andreas fault inducing stress changes on several of its segments we calculate that coseismic slip combined with postseismic relaxation of viscous lower crust and or upper mantle has led to a coulomb stress increase of 2 3 3 5 bar on the san bernardino mountain segment of the southern san andreas fault between 1992 and 2001 with a projected increase of 3 6 4 9 bar by the year 2020 in comparison the calculated coseismic stress increase is 1 8 bar for this segment this accelerated buildup of stresses is predicted to bring the san bernardino mountain segment which last ruptured more than 190 yr ago closer to a potentially major rupture meanwhile we project a net stress decrease of as much as 23 5 bar between 1992 and 2020 for the western coachella valley segment if the fault is governed by low effective friction or an increase of 1 5 bar if the fault is governed by high effective friction coulomb stresses are calculated to decrease on the mojave segment by as much as 21 bar between 1992 and 2020 accelerated stress buildup is also predicted to occur on parts of the san jacinto elsinore and calico faults the pattern of the observed post landers aftershock clustering and the calculated coulomb stress buildup on the calico fault is similar to that noted in the hector mine region prior to the 1999 mw 5 7 1 earthquake these results imply that the stress changes caused by an earthquake may still play a role in triggering future quakes in neighboring crust many years later through viscoelastic processes

slip parallel seismic lineations on the northern hayward fault california a high resolution relative earthquake location procedure is used to image the fine scale seismicity structure of the northern hayward fault california the seismicity defines a narrow near vertical fault zone containing horizontal alignments of hypocenters extending along the fault zone the lineations persist over the 15 year observation interval implying the localization of conditions on the fault where brittle failure conditions are met the horizontal orientation of the lineations parallels the slip direction of the fault suggesting that they are the result of the smearing of frictionally weak material along the fault plane over thousands of years

spatial variations in stress within the 1987 whittier narrows california aftershock sequence new techniques and results the ml 5 9 whittier narrows california earthquake of october 1 1987 triggered a complex aftershock sequence the aftershocks had many different focal mechanisms including thrusting on e w striking north dipping planes similar to the main shock right lateral motion on nw se striking planes similar to the ml 5 3 aftershock on october 4 1987 thrusting on n s striking planes and left lateral motion on n s striking planes i attempt to interpret these mechanisms in terms of a stress field that may have caused them previously stress inversion methods based on focal mechanisms assumed that the stress field was spatially uniform in order to test for the spatially varying effect of the main shock s dislocation on the aftershocks focal mechanisms a new inversion technique was developed that solves for a uniform stress field that is superimposed on a given spatially varying stress field a series of simulations were performed to test both the new method and an older method that uses only the uniform component of the stress field these simulations show that both techniques work but will be limited by the amount of noise in the data and the accuracy to which the given spatially varying stress field resembles the actual spatially varying stresses most importantly the simulations show that the older methods do work when the stress field has a uniform component that is at least as large as the spatially varying component the simulations also show that when applying the older stress inversion techniques one can determine if the results are valid and the amount of spatially varying stresses present by examining the misfit between the data and the model application of these techniques to the whittier narrows aftershocks suggests that the uniform component of the stress field and the complex faulting corresponds to the response of an elastic halfspace to a simple regional n s compression the misfit between the data and the uniform model demonstrate the presence of spatially varying stresses at whittier narrows however their form as shown by the simulations can not be determined with this data set and these techniques

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

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

rayleigh pulse dynamic triggering of fault slip we present an experimental and numerical investigation of the interaction of a rayleigh r pulse with a partially contacting strike slip fault between similar and dissimilar materials this study is intended to offer an improved understanding of the earthquake rupture mechanisms the fault is subjected to static normal and shear prestresses utilizing two dimensional dynamic photoelasticity in conjunction with high speed cinematography the evolution of time dependent isochromatic fringe patterns contours of maximum in plane shear stress associated with rayleigh pulse fault interaction is experimentally recorded it is shown that fault slip instability can be triggered by a pulse that propagates along the fault interface at rayleigh wave speed about 90 of the relevant shear wave speed and that the direction of the static shear preloading has an influence on the initiation of fault slip for the numerical studies a finite difference wave propagation simulator swifd solids wave impact fracture damage is used for a quantitative analysis of the problem under different combinations of contacting materials dynamic rupture in laterally heterogeneous structures is discussed by considering the effect of the acoustic impedance ratio of the two contacting materials on the wave patterns the results indicate that upon fault rupture mach head waves which carry a relatively large amount of concentrated energy can be generated that propagate from the fault contact region into the acoustically softer material such mach waves can cause concentrated damage in a particular region located inside an adjacent acoustically softer area this type of damage concentration might be another possible reason for the generation of the damage belt in kobe japan on the occasion of the 1995 hyogo ken nanbu earthquake

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

effects of three dimensional velocity structure on the seismicity of the 1984 morgan hill california aftershock sequence abstract a three dimensional velocity model for the area surrounding the 24 april 1984 morgan hill earthquake has been developed by simultaneously inverting local earthquake and refraction arrival time data this velocity model corresponds well to the surface geology of the region predominantly showing a low velocity region associated with the sedimentary sequence to the southwest of the madrone springs fault this low velocity sequence extends to a depth of approximately 5 km previously constructed geologic cross sections for this region had suggested that these sediments extended to a depth of less than 2 km relocation of 2 179 hypocenters with the three dimensional velocity model did not show great differences when compared to locations done with a one dimensional velocity model with station corrections in both cases the calaveras fault is steep but has a noticeable dip to the northeast and the seismicity does not intersect the surface along the trace of the calaveras fault in the southern half of the rupture zone the greatest differences between the two sets of locations was that the locations using the three dimensional velocity model were deeper and fewer earthquakes were located within the low velocity region the focal mechanisms were also determined for 946 earthquakes using both the one dimensional and three dimensional earth models both earth models yield similar focal mechanisms for these earthquakes

stress orientations from well bore breakouts in the coalinga region borehole breakouts are caused by unequal stress concentrations around a borehole resulting in shear failure of the borehole wall and creating hole elongation perpendicular to the maximum horizontal in situ stress breakouts in tertiary sedimentary rocks were identified in 10 holes in the west coalinga cantua creek and west haven oil fields in the western san joaquin valley the deeper and more reliable 1 5 to 3 7 km breakouts were oriented northwest indicating northeast compressive stress perpendicular to the major fold axes in the san joaquin valley and at a 70 90 angle to the major strike slip faults in the coast ranges this stress direction is consistent with a reverse slip component on northwest trending faults in the region and with the focal mechanisms reported for the may 1983 coalinga earthquake the existing literature on stress measurements in central and southern california indicates that on a regional scale the stress regime is transitional between thrust and strike slip faulting a hydraulic fracture test in the kettleman hills oil field at 3 km depth indicates that this stress regime exists in the upper crust near coalinga as well

comments on near surface stress and displacement in a layered elastic crust and stress in a stratified crust overlying a buried screw dislocation by p c leary

a seismic refraction profile across the san andreas sargent and calaveras faults west central california in 1981 the united states geological survey recorded a seismic refraction profile across the southern santa cruz mountains in west central california to examine the shallow velocity structure of this seismogenic region this 40 km long profile which consisted of three shotpoints extended northeastward from near watsonville california to coyote lake crossing the san andreas sargent and calaveras faults this entire region is characterized by a highly heterogeneous upper crust west of watsonville 1 km of alluvium with a velocity of 2 12 km sec overlies a basement with a velocity 5 45 km sec the abrupt deepening of basement by 1 5 km just east of watsonville at a subsurface fault suggests that the zayante fault to the north and the vergeles fault to the south are connected the tertiary sediment at the san andreas fault is 2 5 km thick and has a velocity of 3 34 km sec the san andreas fault is not marked by any distinctive seismic velocity features whereas a 1 km wide low velocity zone is found at both the sargent and calaveras faults east of the sargent fault the basement surface forms a broad anticlinal structure with velocities ranging from 5 4 to 6 0 km sec from the anticlinal crest basement dips to the east beneath the santa clara valley and reaches a maximum depth of 1 km on the east side the overlying alluvium has a velocity of 2 7 km sec at the crest of the basement anticlinal structure a vertical low velocity zone coincident with exposed serpentine provides strong evidence of faulting

a seismic refraction study of the santa clara valley and southern santa cruz mountains west central california abstract two seismic refraction profiles were recorded in the santa clara valley region of central california to determine the upper crustal seismic velocity structure a reversed 8 km long profile was recorded across the valley near gilroy and an unreversed 38 km long profile was recorded in the southern santa cruz mountains the data indicate that the valley is wedge shaped in cross section with the basement dipping 10 to the east the alluvial fill of the valley has a maximum thickness of 1 5 km and has an average compressional wave velocity of 2 3 km sec the top of the basement has a velocity of 4 3 km sec the velocity reaches 6 0 km sec at a depth of 2 km the calaveras fault zone located due east of the valley was crossed by a short unreversed profile it appears to be characterized by a low velocity v p 2 7 km sec to a depth of 2 5 km the southern santa cruz mountain profile indicates a 4 6 km sec near surface refractor and a layer with a velocity of 6 1 km sec at 2 km depth thus considered together the similarity in velocity structure determined from the two profiles indicates that the franciscan rocks of the santa cruz mountains continue beneath the valley the presence of high compressional wave velocities 6 0 km sec at 2 km depth in this franciscan terrain can best be explained by an abundance of metabasalts greenstones at that depth these seismic observations indicate that an area of at least 120 km 2 is underlain by this high velocity basaltic material therefore the upper crustal structure in the study area differs dramatically from the franciscan terrain of the diablo range where greywackies and metagreywackies v p 5 4 km sec dominate in the uppermost crust

coseismic slip resolution and post seismic relaxation time of the 1999 chi chi taiwan earthquake as constrained by geological observations geodetic measurements and seismicity summary we analysed the surface displacements estimated by global positioning system gps mea surements and systeme probatoire d observation de la terre spot image correlation and the seismicity change induced by the mw 7 6 1999 september 20 chi chi earthquake we first modelled the seismogenic zone for which we introduced a realistic geologically constrained ramp and flat geometry and estimated by joint inversion of the gps and spot data the co seismic slip distribution stressing the resolution of the model parameters the fair adjustment indicates that the ramp and flat geometry is consistent with the geodetic measurement the cal culated coseismic slip increases both towards the surface and northwards with maximum slip of approximately 15 m in good agreement with the observed surface rupture and previously ob tained slip models based on seismic waveforms this distribution may be represented to a good approximation by a linear increase of slip with a constant azimuth in the plate convergence direction special emphasis is given to the analysis of the model resolution which sharply decreases with depth in spite of the exceptionally well distributed and high quality inverted data this resolution decrease expected for any slip distribution based on surface observations only led us then to examine the seismicity as independent data to test our model following the static stress triggering hypothesis the model is accurately defined in terms of coulomb stress change in the central part of the rupture where the fault geometry is well constrained by seismic profile in contrast at the southern and northern ends it is less appropriate however the static stress triggering approach is not precise enough to determine whether fault geometry or slip distribution is less suitable we thus conclude that the analysis of seismicity in terms of coulomb stress change might not allow us to examine small scale features of seismogenic zones finally we discuss post seismic relaxation from the temporal variation of seismicity rate in contrast to the relaxation time of 0 23 0 90 yr inferred from post seismic permanent gps measurements we found that the decay of aftershock sequence follows omori s law with an 8 14 yr relaxation time we suggest that this discrepancy results from the coexistence of several different processes not all associated with the production of aftershocks

coulomb stress accumulation along the san andreas fault system 1 stress accumulation rates along the primary segments of the san andreas fault system are computed using a three dimensional 3 d elastic half space model with realistic fault geometry the model is developed in the fourier domain by solving for the response of an elastic half space due to a point vector body force and analytically integrating the force from a locking depth to infinite depth this approach is then applied to the san andreas fault system using published slip rates along 18 major fault strands of the fault zone gps derived horizontal velocity measurements spanning the entire 1700 200 km region are then used to solve for apparent locking depth along each primary fault segment this simple model fits remarkably well 2 43 mm yr rms misfit although some discrepancies occur in the eastern california shear zone the model also predicts vertical uplift and subsidence rates that are in agreement with independent geologic and geodetic estimates in addition shear and normal stresses along the major fault strands are used to compute coulomb stress accumulation rate as a result we find earthquake recurrence intervals along the san andreas fault system to be inversely proportional to coulomb stress accumulation rate in agreement with typical coseismic stress drops of 1 10 mpa this 3 d deformation model can ultimately be extended to include both time dependent forcing and viscoelastic response

interseismic coupling and refined earthquake potential on the hayward calaveras fault zone interseismic strain accumulation and fault creep is usually estimated from gps and alignment arrays data which provide precise but spatially sparse measurements here we use interferometric synthetic aperture radar to resolve the interseismic deformation associated with the hayward and calaveras faults hf and cf in the east san francisco bay area the large 1992 2011 sar data set permits evaluation of short and long wavelength deformation larger than 2 mm yr without alignment of the velocity field to a gps based model our time series approach in which the interferogram selection is based on the spatial coherence enables deformation mapping in vegetated areas and leads to refined estimates of along fault surface creep rates creep rates vary from 0 2 mm yr on the northern cf to 14 2 mm yr on the central cf south of the hf surface junction we estimate the long term slip rates by inverting the long wavelength deformation and the distribution of shallow slip due to creep by inverting the remaining velocity field this distribution of slip reveals the locations of locked and slowly creeping patches with potential for a m6 8 0 3 on the hf near san leandro a m6 6 0 2 on the northern cf near dublin a m6 5 0 1 on the hf south of fremont and a m6 2 0 2 on the central cf near morgan hill with cascading multisegment ruptures the hf rupturing from berkeley to the cf junction could produce a m6 9 0 1 the northern cf a m6 6 0 1 the central cf a m6 9 0 2 from the junction to gilroy and a joint rupture of the hf and central cf could produce a m7 1 0 1

geodetic measurement of crustal deformation on the san andreas hayward and calaveras faults near san francisco california analysis of a geodetic network of 115 lines crossing the san andreas hayward and calaveras faults in the vicinity of san francisco bay and measured repeatedly between 1970 and 1980 has revealed details about the accommodation of relative plate motion in this area the most striking result is that the deformation is not uniformly distributed across the area in the east bay along the hayward and calaveras faults all motion appears to take place as slip directly on the fault with no accumulation of strain in the adjacent crust on both the calaveras and the hayward faults the rate obtained for the 1970 to 1980 period agrees with geologic rates spanning a few million years and with creep rates spanning a few decades the hayward fault slip rate is 7 1 mm yr the calaveras fault slip rate is 7 1 mm yr with perhaps half of this slip distributed across a zone a few kilometers wide probably as inelastic deformation of weak near surface material the absence of strain accumulation in the east bay is surprising since the hayward and calaveras faults have been the site of large earthquakes in the past a block located east of the calaveras fault and south of the las positas fault has been rotating clockwise at a rate of 0 3 0 1 rad yr with very little internal deformation along the san francisco peninsula no detectable slip occurs less than 1 5 mm yr at the surface but appreciable strain is accumulating near fault shear strain rates are 0 6 0 1 strain yr engineering with direction n47 w 9 the slip rate near the san andreas fault is 12 2 3 9 mm yr distributed across a broad zone the relative motion across the whole region during the period 1970 1980 is 32 1 7 4 mm yr supplement available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j81 011 1 00 payment must accompany order

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22

delta plain coal deposits from the than formation of the early cretaceous saurashtra basin gujarat western india abstract the than formation of the early cretaceous saurashtra basin consists of an interbedded assemblage of sandstone claystone carbonaceous shale and coal facies small scale crossbedded and laminated mudstone siltstone sr lm facies are considered to have formed by down current migration of small scale subaqueous bedforms and overbank deposits of distributary channels the planar and trough crossbedded sandstone lithofacies sp st formed by migration of sand dunes and transverse bars in distributary channels laminated to ripple laminated siltstone f1 facies with or without coaly stringers alternate with fine clastics are suggestive of flood deposits and abandoned interchannel plain of delta distributaries the muddy carbonaceous shale fm facies represents deposition largely in low lying interchannel plains and backswamps peat coal pc facies are interpreted as in situ deposits in quiet water conditions of sedimentation across a lake the integrated results of lithofacies texture and palaeocurrent analysis suggest that than strata were deposited in a prograding major delta distributary complex in lower and upper delta plain environments

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

constitutive relations for fault slip and earthquake instabilities constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults the constitutive relations discussed include simple rate independent slip weakening models in which shear strength degrades with ongoing slip to a residual frictional strength and also more realistic but as yet less extensively applied slip rate and surface state dependent relations for the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip seeking values consistent with the current slip rate models of crustal faults range from simple single degree of freedom spring slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere

the rupture mechanism of the coyote lake earthquake of 6 august 1979 inferred from near field data the coyote lake earthquake ml 5 7 occurred within the dense network of seismic instruments of central california and was recorded by numerous accelerograph stations two of these stations located within the fault zone itself provide particularly remarkable records of the rupture process we use these data obtained in the immediate vicinity of the fault and the broadband seismograms recorded at berkeley about 100 km away from the epicenter to infer the velocity of propagation of the rupture the extent of the fractured area the rupture front geometry and other characteristics of the fracture mechanism in order to extract this information from the data we model the earthquake as a propagating dislocation starting at the hypocenter and spreading radially we compute the resulting ground motion at the receiver sites using the discrete wavenumber representation of green s functions bouchon 1981 the results yield a rupture velocity of 2 6 km sec a fault length of about 14 km and a fault slip of 15 to 20 cm they also show that the fault extends to shallower depth than indicated by the aftershocks the most important result is the finding that slip at any given location on the fault takes place in a very short time span and that simple uniform dislocation models give a good description of the rupture process

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

interactions between the landers and hector mine california earthquakes from space geodesy boundary element modeling and time dependent friction the m w 7 1 hector mine earthquake happened 7 years after the 1992 m w 7 3 landers earthquake within a system of right lateral strike slip faults comprising the eastern california shear zone because estimates of recurrence intervals of m 7 earthquakes on these faults range anywhere from 1500 to 50 000 years the close temporal spacing of these two earthquakes suggests that there is some interaction between the two current models of simple coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause and effect relationship the interaction between the computed normal and right lateral shear stress reductions induced by the landers earthquake at the hector mine hypocenter is unclear we use a combination of space geodesy boundary element modeling and computer modeling of time dependent fault friction to investigate the interaction between the two earthquakes we compare stress changes on the hector mine rupture plane induced by the landers earthquake with the detailed slip distribution inferred using a combination of gps and insar data the slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events for each earthquake the azimuth of the remote maximum compressive stress was approximately 17 6 and the magnitude of the shear stress drop was 8 1 mpa on the landers rupture and 10 2 mpa on the hector mine rupture using a simple spring and slider model as a proxy for the rate and state frictional response of the hector mine faults to landers earthquake imposed stress steps we find that while a decrease in normal stress at the hector mine hypocenter would by itself have caused the hector mine earthquake to nucleate immediately the simultaneous decrease in shear stress may have caused a delay in the peak shear loading consideration of an acceptable range of rate and state friction parameters secular strain rates and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years

depth dependent earthquake focal mechanism orientation evidence for a weak zone in the lower crust the traction free boundary condition across the earth s surface provides an opportunity for studying the relationship between stress orientation and earthquake focal mechanisms because it requires alignment of principal stress axes with vertical and horizontal orientations a survey of earthquake focal mechanisms in northern california shows that their principal axes are also closely aligned with the vertical and the horizontal in the upper few kilometers of earth s crust thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well the focal mechanism alignment can also be characterized by the relative magnitude of the off diagonal elements mxz and myz of the seismic moment tensor we find significant and systematic depth variations in the horizontal moment tensor element ms which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principal stress and focal mechanism axes values of ms near earth s surface are small but increase with depth to a maximum between 5 and 8 km at greater depths there is a gradual decrease which suggests decreasing horizontal shear traction toward the base of the seismogenic zone we interpret this tendency of axes to become oriented near the base of the seismogenic zone and its expression in ms as the signature of a weak zone in the lower crust if correct this observation would have important implications for the mechanics of lithospheric deformation

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

crustal deformation in great california earthquake cycles periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model a depth l h extending downward from the earth s surface at a transform boundary between uniform elastic lithospheric plates of thickness h is locked between earthquakes it slips an amount consistent with remote plate velocity vpl after each lapse of earthquake cycle time tcy lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress the plates are coupled at their base to a maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions compatible with plate velocity are transmitted to the surface plates the coupling is described approximately through a generalized elsasser model we argue that the model gives a more realistic physical description of tectonic loading including the time dependence of deep slip and crustal stress buildup throughout the earthquake cycle than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone parameters of the model are chosen to fit seismic and geologic constraints and the apparent time dependence of surface strain rates along presently locked traces of the 1857 and 1906 san andreas ruptures we fix vpl 35 mm yr tcy 160 years and l 9 11 km based on earthquake nucleation depths the geodetic data are then found to be described reasonably within the context of a model that is locally uniform along strike and symmetric about a single san andreas fault strand by lithosphere thickness h 20 30 km and elsasser relaxation time tr 10 16 years we therefore suggest that the asthenosphere appropriate to describe crustal deformation on the earthquake cycle time scale lies in the lower crust and perhaps crust mantle transition zone and has an effective viscosity between about 2 1018 and 1019 pa s depending on the thickness assigned to the asthenospheric layer predictions based on the chosen set of parameters are also consistent with data on variations of contemporary surface strain and displacement rates as a function of distance from the 1857 and 1906 rupture traces although the fit is degraded by asymmetry relative to the fault and by slip on adjacent fault strands

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

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

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

the near source ground motion of the 6 august 1979 coyote lake california earthquake a finite fault striking n24 w and extending to a depth of 10 km is proposed to explain the strong ground motion data for the 6 august 1979 coyote lake california earthquake m l 5 9 our source model suggests that right lateral faulting initiated at a depth of 8 km and ruptured toward the south with a velocity of 2 8 km sec this unilateral rupture can explain the large displacement recorded south of the epicenter however the waveform coherency across an array south and southwest of the epicenter suggests that the rupture length is less than 6 km the maximum dislocation is about 120 cm in a small area near the hypocenter and the total moment is estimated to be 3 5 10 24 dyne cm an abrupt stopping phase which corresponds to a deceleration of right lateral motion can explain the high peak acceleration recorded at array station 6 the stress drop in the hypocentral area is about 140 bars the average stress drop over the entire rupture surface is 30 bars the preferred finite source model can predict the p n1 waveforms and the beginning features in the teleseismic seismograms no clear arrivals can be observed in the near source data for the possible second and third smaller events suggested by nabelek personal communication

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

slip partitioning of the calaveras fault california and prospects for future earthquakes examination of main shock and microearthquake data from the calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8 9 km near the base of the zone of microearthquakes the spatial pattern of pre main shock microseismicity surrounding the coyote lake and morgan hill hypocenters is similar to the pattern of the post main shock microseismicity microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones estimates of the fault regions which slipped during the coyote lake and morgan hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic we propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes from the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the calaveras fault from an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites for the next m 5 earthquakes the first zone is located near gilroy and was last ruptured by a m5 2 earthquake in 1949 the second zone is located south of calaveras reservoir and north of the 1988 m5 1 alum rock earthquake it has not slipped seismically since at least 1903 and the size of the aseismic region is sufficiently large to sustain a m5 5 earthquake

crustal structure of the southern calaveras fault zone central california from seismic refraction investigations a magnitude 5 7 earthquake on 6 august 1979 within the calaveras fault zone near coyote lake of west central california motivated a seismic refraction investigation in this area a northwest southeast profile along the fault as well as two fan profiles across the fault were recorded to examine the velocity structure of this region the analysis of the data reveals a complicated upper crustal velocity structure with strong lateral variations in all directions the near surface layers consist of recent sediments with seismic p wave velocities of 2 6 to 3 2 km sec these are underlain by rocks of the great valley sequence which have an average velocity of 4 5 km sec the great valley sequence is present along the whole profile depths range from 4 3 to 4 8 km in the northwest near anderson lake and in the southeast of the profile line near hollister in the middle of the profile near coyote lake however this layer thins and we find a laterally limited higher velocity layer 5 1 km sec between depths of 2 8 to 4 8 km the high velocity zone which coincides with a gravity high also correlates spatially with the hypocentral area of the 6 august earthquake and its aftershocks and may therefore represent an asperity on the fault velocities within the fault zone were determined from the fan profiles near anderson lake a pronounced delay of first arrivals on the fan records indicates a vertical 1 to 2 km wide near surface low velocity zone along the fault near coyote lake the delays observed in the fan records correlate with two subsurface en echelon fault planes which have been previously identified from lineations in the seismicity pattern the structure of the lower crust is similar to the neighboring diablo range a 8 to 9 km thick upper crustal layer with a seismic velocity of 5 7 to 6 3 km sec is underlain by a 3 km thick layer with velocity 6 8 km sec in accordance with previous studies of the diablo range there are indications of a pronounced lower crustal low velocity zone between a depth of 17 and 23 km the presence of this low velocity zone suggests that higher velocity igneous rocks of the middle crust have been thrust over sedimentary rocks

west margin of north america a synthesis of recent seismic transects abstract a comparison of the deep structure along nine recent transects of the west margin of north america shows many important similarities and differences common tectonic elements identified in the deep structure along these transects include actively subducting oceanic crust accreted oceanic arc or oceanic like lithosphere of mesozoic through cenozoic ages cenozoic accretionary prisms mesozoic accretionary prisms backstops to the mesozoic prisms and undivided lower crust not all of these elements are present along all transects in this study nine transects including four crossing subduction zones and five crossing transform faults are plotted at the same scale and vertical exaggeration v e 1 1 using the above scheme for identifying tectonic elements the four subduction zone transects contain actively subducting oceanic crust cenozoic accretionary prisms and bodies of basaltic rocks accreted in the cenozoic including remnants of a large oceanic plateau in the oregon and vancouver island transects rocks of age and composition eocene basalt similar to the oceanic plateau are currently subducting in southern alaska where they are doubled up on top of pacific oceanic crust and have apparently created a giant asperity or impediment to subduction most of the subduction zone transects also contain mesozoic accretionary prisms and two of them vancouver island and alaska also contain thick tectonically underplated bodies of late mesozoic early cenozoic oceanic lithosphere interpreted as fragments of the extinct kula plate in the upper crust most of the five transform fault transects all in california reflect 1 tectonic wedging of a mesozoic accretionary prism into a backstop which includes mesozoic early cenozoic forearc rocks and mesozoic ophiolitic arc basement rocks and 2 shuffling of the subduction margin of california by strike slip faulting in the lower crust they may reflect migration of the mendocino triple junction northward seen in rocks east of the san andreas fault and cessation of farallon plate subduction seen in rocks west of the san andreas fault in northern california lower crustal rocks east of the san andreas fault have oceanic crustal velocity and thickness and contain patches of high reflectivity they may represent basaltic rocks magmatically underplated in the wake of the migration of the mendocino triple junction or they may represent stalled subducted fragments of the farallon gorda plate the latter alternative does not fit the accepted slabless window model for the migration of the triple junction this lower crustal layer and the moho are offset at the san andreas and maacama faults in central california a similar lower crustal layer is observed west of the san andreas fault west of the continental slope it is pacific oceanic crust but beneath the continent it may represent either pacific oceanic crust stalled subducted fragments microplates of the farallon plate or basaltic rocks magmatically underplated during subduction of the pacific farallon ridge or during breakup of the subducted farallon plate the transect in southern california is only partly representative of regional structure as the structure here is 3 dimensional in the upper crust a mesozoic prism has been thrust beneath crystalline basement rocks of the san gabriel mountains and mojave desert in the mid crust a bright reflective zone is interpreted as a possible master decollement that can be traced from the fold and thrust belt of the los angeles basin northward to at least the san andreas fault a moho depression beneath the san gabriel mountains is consistent with downwelling of lithospheric mantle beneath the transverse ranges that appears to be driving the compression across the transverse ranges and los angeles basin

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

rupture history of the 1984 morgan hill california earthquake from the inversion of strong motion records near source strong motion velocity records and teleseismic short period p waveforms are modeled to obtain the spatial and temporal distribution of slip for the 1984 morgan hill earthquake both forward modeling and constrained least squares inversion techniques are used to interpret the strong motion velocity waveforms in the frequency range of approximately 0 2 to 2 0 hz these data support a nearly unilateral rupture to the southeast with a rupture propagation velocity of nine tenths of the local s wave velocity the majority of the slip occurs over a fault length of 25 km and to a first approximation can be interpreted as two main source regions each with an extent of about 5 km with their centers separated by about 12 km however each of the sources has detailed structure of its own and a simple two point source model is not an accurate representation of the morgan hill earthquake the second source occurs about 4 5 sec after the first and is approximately 3 times larger the maximum dislocation on the fault plane is about 1 m the total moment of the earthquake is estimated to be 2 1 10 25 dyne cm the morgan hill earthquake offers convincing evidence for very inhomogeneous slip and stress distributions on shallow strike slip faults

subsurface structure and kinematics of the calaveras hayward fault stepover from three dimensional vp and seismicity san francisco bay region california the calaveras and hayward faults are major components of the san andreas fault system in the san francisco bay region dextral slip is presumed to transfer from the calaveras fault to the hayward fault in the mission hills region an area of uplift in the contractional stepover between the two faults here the estimated deep slip rates drop from 15 to 6 mm yr on the calaveras fault and slip begins on the hayward fault at an estimated 9 mm yr a lineament of microseismicity near the mission fault links the seismicity on the calaveras and hayward faults and is presumed to be related directly to this slip transfer however geologic and seismologic evidence suggest that the mission fault may not be the source of the seismicity and that the mission fault is not playing a major role in the slip transfer we perform a joint inversion for hypocenters and the 3d p wave velocity structure of the stepover region using 477 earthquakes we find strong velocity contrasts across the calaveras and hayward faults corroborated by geologic gravity and aeromagnetic data detailed examination of two seismic lineaments in conjunction with the velocity model and independent geologic and geophysical evidence suggests that they represent the southern extension of a northeasterly dipping hayward fault that splays off the calaveras fault directly accounting for the deep slip transfer the mission fault appears to be accommodating deformation within the block between the hayward and calaveras faults thus the calaveras and hayward faults need to be considered as a single system for developing rupture scenarios for seismic hazard assessments online material 3d interactive visualizations of the mission and alum rock hypocenters

internal structure of the san jacinto fault zone at blackburn saddle from seismic data of a linear array local and teleseismic earthquake waveforms recorded by a 180 m long linear array bb with seven seismometers crossing the clark fault of the san jacinto fault zone northwest of anza are used to image a deep bimaterial interface and core damage structure of the fault delay times of p waves across the array indicate an increase in slowness from the southwest most bb01 to the northeast most bb07 station automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves the observed fault zone head waves imply that the clark fault in the area is a sharp bimaterial interface with lower seismic velocity on the southwest side the moveout between the head and direct p arrivals for events within 40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3 2 per cent a constant moveout for events beyond 40 km to the southeast is due to off fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance the lack of head waves from events beyond 20 km to the northwest is associated with structural complexity near the hemet stepover events located in a broad region generate fault zone trapped waves at stations bb04 bb07 waveform inversions indicate that the most likely parameters of the trapping structure are width of 200 m s velocity reduction of 30 40 per cent with respect to the bounding blocks q value of 10 20 and depth of 3 5 km the trapping structure and zone with largest slowness are on the northeast side of the fault the observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest this inference is consistent with results of other geological and seismological studies

slip partitioning and deformation cycles close to major faults in southern california evidence from small scale faults geometry and kinematics of small scale faults adjacent to large seismically active faults have been analyzed for a large region of southern california the sampled population of small scale faults is scattered in orientation and slip but possesses distinct trends two important characteristics are first the paucity of small scale faults that mimic orientations and mechanisms of the adjacent master faults and second the large variation in slip mechanisms of fault planes oriented e w and n s in order to test the similarity of the sampled fault population to present day deformations data on geometry and slip of the small scale faults have been used for stress inversions these results are compared with those derived from focal mechanisms of low magnitude earthquakes in the same region stress orientations derived from small scale faults are spatially heterogeneous but horizontal axes of stress maintain n s and e w trends over the entire region the similarities between stress inversions from small faults and seismicity support the conclusion that most of the sampled faults are recent and formed during cyclical events of fracturing in a relatively strong crust adjacent to weak master faults

modelling aftershock migration and afterslip of the san juan bautista california earthquake of october 3 1972 abstract the san juan bautista earthquake of october 3 1972 m l 4 8 located along the san andreas fault in central california initiated an aftershock sequence characterized by a subtle but perceptible tendency for aftershocks to spread to the northwest and southeast along the fault zone the apparent dimension of the aftershock zone along strike increased from about 7 10 km within a few days of the earthquake to about 20 km eight months later in addition the mainshock initiated a period of accelerated fault creep which was observed at 2 creep meters situated astride the trace of the san andreas fault within about 15 km of the epicenter of the mainshock the creep rate gradually returned to the preearthquake rate after about 3 yrs both the spreading of the aftershocks and the rapid surface creep are interpreted as reflecting a period of rapid creep in the fault zone representing the readjustment of stress and displacement following the failure of a stuck patch or asperity during the san juan bautista earthquake numerical calculations suggest that the behavior of the fault zone is consistent with that of a material characterized by a viscosity of about 3 6 10 14 p although the real rheology is likely to be more complicated in this model the mainshock represents the failure of an asperity that slips only during earthquakes aftershocks represent the failure of second order asperities which are dragged along by the creeping fault zone

recent state of stress change in the walker lane zone western basin and range province united states the nw to north trending walker lane zone wlz is located along the western boundary of the northern basin and range province with the sierra nevada this zone is distinguished from the surrounding basin and range province on the basis of irregular topography and evidence for both normal and strike slip holocene faulting inversion of slip vectors from active faults historic fault offsets and earthquake focal mechanisms indicate two distinct quaternary stress regimes within the wlz both of which are characterized by a consistent wnw 3 axis these are a normal faulting regime with a mean 3 axis of n85 9 w and a mean stress ratio r value r 2 1 3 1 of 0 63 0 74 and a younger strike slip faulting regime with a similar mean 3 axis n65 70 w and r values ranging between 0 1 and 0 2 this younger regime is compatible with historic fault offsets and earthquake focal mechanisms both the extensional and strike slip stress regimes reactivated inherited mesozoic and cenozoic structures and also produced new faults the present day strike slip stress regime has produced strike slip normal oblique slip and normal dip slip historic faulting previous workers have explained the complex interaction of active strike slip oblique and normal faulting in the wlz as a simple consequence of a single stress state with a consistent wnw 3 axis and transitional between strike slip and normal faulting maximum horizontal stress approximately equal to vertical stress or r 0 in both regimes with minor local fluctuations the slip data reported here support previous results from owens valley that suggest deformation within temporally distinct normal and strike slip faulting stress regimes with a roughly constant wnw trending 3 axis zoback 1989 a recent change from a normal faulting to a strike slip faulting stress regime is indicated by the crosscutting striae on faults in basalts 300 000 years old and is consistent with the dominantly strike slip earthquake focal mechanisms and the youngest striae observed on faults in plio quaternary deposits geologic control on the timing of the change is poor it is impossible to determine if there has been a single recent absolute change or if there is rather an alternating or cyclical variation in stress magnitudes our slip data in particular the cross cutting normal and strike slip striae on the same fault plane are inconsistent with postulated simple strain partitioning of deformation within a single regional stress field suggested for the wlz by wesnousky and jones 1994 the location of the wlz between the deep seated regional extension of the basin and range and the right lateral strike slip regional tectonics of the san andreas fault zone is probably responsible for the complex interaction of tectonic regimes in this transition zone in early to mid tertiary time the wlz appears to have had a similarly complex deformational history in this case as a back arc or intra arc region accommodating at least part of the right lateral component of oblique convergence as well as a component of extension

objective estimation of spatially variable parameters of epidemic type aftershock sequence model application to california the etas model is widely employed to model the spatio temporal distribution of earthquakes generally using spatially invariant parameters we propose an efficient method for the estimation of spatially varying parameters using the expectation maximization em algorithm and spatial voronoi tessellation ensembles we use the bayesian information criterion bic to rank inverted models given their likelihood and complexity and select the best models to finally compute an ensemble model at any location using a synthetic catalog we also check that the proposed method correctly inverts the known parameters we apply the proposed method to earthquakes included in the anss catalog that occurred within the time period 1981 2015 in a spatial polygon around california the results indicate significant spatial variation of the etas parameters we find that the efficiency of earthquakes to trigger future ones quantified by the branching ratio positively correlates with surface heat flow in contrast the rate of earthquakes triggered by far field tectonic loading or background seismicity rate shows no such correlation suggesting the relevance of triggering possibly through fluid induced activation furthermore the branching ratio and background seismicity rate are found to be uncorrelated with hypocentral depths indicating that the seismic coupling remains invariant of hypocentral depths in the study region additionally triggering seems to be mostly dominated by small earthquakes consequently the static stress change studies should not only focus on the coulomb stress changes caused by specific moderate to large earthquakes but also account for the secondary static stress changes caused by smaller earthquakes