stress triggering of large earthquakes complicated by transient aseismic slip episodes 1 we investigate how a static stress change caused by a nearby earthquake perturbs sliding processes on a fault and the time to the next earthquake for this purpose we model numerically a reverse fault in a two dimensional semi infinite and elastic medium partly stable and partly unstable under a rate and state dependent friction law when no stress change intervenes steady sliding spreads out of the deep zone of frictional stability and keeps expanding updip during the interseismic period aseismic slip episodes ases develop spontaneously within the region of steady sliding and repeat quasi periodically when steady sliding has spread beyond a certain critical length the nucleation length above the zone of frictional stability one of the ases undergoes rapid buildup and avalanches into high speed seismic sliding when a step like stress change intervenes ases of larger amplitudes arise and evolve similarly to their spontaneous counterparts until one of them avalanches into an earthquake if the stress change arrives not too early nor too late in the seismic cycle the basic picture is as follows delaying the stress step be it positive or negative delays the timing of the ase sequence and thereby postpones the next earthquake when the stress step is delayed to more than a certain extent however an earlier ase gets to evolve into an earthquake advancing its time discontinuously only in the final stage of a seismic cycle does a positive stress step systematically advance the next earthquake and a negative stress step systematically delay it

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

lower crustal structure in northern california implications from strain rate variations following the 1906 san francisco earthquake 1 it is well known that geodetic data from a single instant in time cannot uniquely characterize structure or rheology beneath active seismogenic zones nevertheless comparison of spatial and temporal variations in deformation rate with time dependent mechanical models can place valuable constraints on fault zone geometry and rheology we consider postseismic strain rate transients by comparing geodetic data from north of san francisco bay obtained between 1906 and 1995 to predictions from viscoelastic finite element models models include 1 an elastic plate over a viscoelastic half space 2 distributed shear within a viscoelastic layer 3 discrete shear zones within an otherwise elastic layer 4 discrete shear zones in combination with distributed viscoelastic shear and 5 midcrustal detachment surfaces we vary as applicable locking depth elastic thickness depth to the top and bottom of the distributed shear layer distributed shear relaxation time discrete shear zone relaxation time and discrete shear zone width the best fitting physically reasonable elastic plate over viscoelastic half space models 1 do a poor job simultaneously predicting spatial and temporal variations in the data the best fitting distributed shear models 2 do a poor job predicting spatial variations in the deformation rate although they fit the geodetic data recent findings from seismic reflection refraction studies in northern california argue against models with shallow subhorizontal detachments 5 models incorporating discrete shear zones 3 4 provide the best fit to the geodetic data and are consistent with seismic studies that argue for discrete fault zones extending through the entire crust

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

large scale characteristics of plate boundary deformations related to the post seismic readjustment of a thin asthenosphere summary the large scale response of an elastic lithosphere riding on a thin viscoelastic asthenosphere to periodically occurring ruptures at a transform or subduction type plate boundary is described approximately by appropriate limit cycle solutions for a plate foundation model introduced previously by rice the cyclic behaviour of thickness averaged displacements strains and strain rates their decay away from the plate boundary and a resolution into coseismic and post seismic alterations are obtained and their dependence on repeat time and a characteristic relaxation time investigated a comparison is made with existing periodic solutions for the surface deformations in a nur mavko half space model this suggests important effects due to viscosity stratification on post seismic rebound when earthquake repeat times exceed relevant relaxation times by at least one order of magnitude

strain accumulation in western united states this review is principally concerned with recent geodetic strain measure ments in western united states undertaken by the us geological survey as part of the earthquake studies program and as a consequence is heavily biased toward the author s own publications most of the publications reporting crustal strain measurements in western united states prior to about 1968 have been compiled in one volume national geodetic survey 1973 and more recent work with complete bibliographies is summarized in three successive quadrennial reports to the international union of geodesy and geophysics meade 1971 savage 1975 thatcher 1979b the following conventions are employed in this paper strain a dimensionless quantity is reported in units of 10 6 extension is taken as positive to distinguish between engineering and tensor shear strain we denote the former by y and quote the units as j lrad whereas the latter is denoted by e and given the units of j lstrain uncertainties in all cases are quoted as one standard deviation at the present time geodetic techniques furnish the only reliable measure of strain accumulation in the seismic regions of the world the superiority of geodetic techniques derives principally from the long baseline of the measurements difficulties in coupling fiducial marks to the earth have generally made short baseline measurements of strain accumulation questionable wyatt 1982 however long baselines introduce the dis advantage that the measured strain is not closely localized the current technique of measuring strain accumulation involves the

a note on adding viscoelasticity to earthquake simulators abstract here i describe how time dependent quasi static stress transfer can be implemented in an earthquake simulator code that is used to generate long synthetic seismicity catalogs most existing seismicity simulators use precomputed static stress interaction coefficients to rapidly implement static stress transfer in fault networks with typically tens of thousands of fault patches the extension to quasi static deformation which accounts for viscoelasticity of earth s ductile lower crust and mantle involves the precomputation of additional interaction coefficients that represent time dependent stress transfer among the model fault patches combined with defining and evolving additional state variables that track this stress transfer the new approach is illustrated with application to a california wide synthetic fault network

evolution of fault systems at a strike slip plate boundary a viscoelastic model a viscoelastic model of crustal deformation suggests that the formation and evolution of strike slip fault systems are strongly influenced by rheologic contrasts between the upper and lower crust when deformation is driven by a narrow zone of high shear in the mantle the presence of a low viscosity lower crustal layer underlying a primarily elastic upper crust widens the deformation zone with time and promotes the formation of a broadly distributed network of interacting faults within the upper crust in contrast the deformation zone in a primarily elastic crust is narrow encompassing a single plate bounding fault patterns of surface strain rate and seismicity are thus significantly more complex in the presence of a low viscosity lower crust due to interactions between faulting in the upper crust at short time scales and viscous behavior in the lower crust at long time scales

stress diffusion along rupturing plate boundaries an analysis is made of viscoelastic lithosphere asthenosphere coupling in the time dependent redistribution of stress along plate boundaries or other seismic lineaments following great earthquakes the study is based on a generalization by rice of elsasser s model of stress diffusion in which general elastic plane stress deformations are allowed in lithospheric plates which are coupled in an elementary way to a maxwellian viscoelastic asthenosphere solutions are developed which describe the large scale quasistatic distribution of thickness averaged in the lithosphere at or near stationary or travelling rupture zones modeled here by either crack like zones of fixed stress drop or dislocation type slip zones sudden ruptures shed load onto the asthenosphere which is gradually transferred back to the lithosphere by a slow relaxation process the spatial and temporal characteristics of the predicted stress alterations suggest a significant role of lithosphere asthenosphere coupling effects in triggering interactions of great earthquakes patterns of prolonged aftershock activity and the breaking of barriers or gaps by time dependent stressing

moho orientation beneath central california from regional earthquake travel times table al is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 011 2 50 payment must accompany order analysis of relative pn travel times recorded by 238 stations of the u s geological survey central california seismic network calnet from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the coast ranges and sierra nevada foothills by dividing the calnet into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space the orientation of a planar moho beneath each subarray is calculated by inversion of relative pn arrival times the results show that the moho dips to the northeast between 3 and 6 beneath the coast ranges and 8 17 beneath the great valley and is horizontal beneath the foothills pn residuals west of the san andreas fault are relatively early by 0 35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness unexplained pn travel time variance improves by approximately 60 after correction for the travel time due to dip and mean pn residual at each station the pn data corrected for the travel time due to a dipping moho indicate an isotropic upper mantle velocity of 7 97 0 05 km s beneath the coast ranges great valley and foothills evidence of upper mantle velocity anisotropy in the azimuthal variation in pn apparent velocity corrected for dip is only marginally significant at the 5 level however large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust unmodeled higher order mantle structure and gradients in upper mantle velocity arrival times of cape mendocino earthquakes recorded in the coast ranges south of san francisco bay indicate lateral variations in upper mantle velocity the 7 4 km s apparent velocity estimate from travel times of the oroville earthquake recorded in the coast ranges out to distances greater than 400 km is anomalous in that pn is normally observed at distances greater than 200 km these arrivals may originate from updip propagation along a 6 9 km s layer in the lower crust of the coast ranges subparallel to the moho

postseismic deformation induced by brittle rock damage of aftershocks 1 large earthquakes are commonly followed by abundant aftershocks that are densely located around the coseismic rupture zone laboratory experiments indicate that microscopic brittle rock failures acoustic emission are associated collectively with a macroscopic damage related inelastic relaxation utilizing basic relations between local brittle failures and gradual inelastic strain in a viscoelastic damage rheology model we develop connections between aftershock decay rates and the aftershocks induced component of geodetic deformation the discussed mechanism is relevant for postseismic relaxation produced by sources located within the seismogenic zone and especially in regions that overlap locations of high aftershocks activity assuming the omori utsu decay rate for aftershocks we find that the temporal decay of the damage related postseismic relaxation follows a generalized power law relation with the standard omori utsu law as a limit case the results provide a way for estimating the separate contributions to observed postseismic displacements that stem from brittle failures in the seismogenic zone aftershocks and other aseismic processes using the obtained theoretical expectations we analyze postseismic displacements measured by gps stations around the north anatolian fault 3 months following the 1999 m7 4 izmit earthquake we find that the observed postseismic displacements decay slower than the aftershock seismicity based on our theoretical results we conclude that up to 50 of the measured surface displacements at near fault sites can be attributed to aftershock induced inelastic deformation in the seismogenic zone the remainder postseismic deformation can generally be explained by relaxation in the deeper ductile substrate

crustal deformation the earthquake cycle and models of viscoelastic flow in the asthenosphere summary the crustal deformation patterns associated with the earthquake cycle can depend strongly on the rheological properties of subcrustal material substantial deviations from the simple patterns for a uniformly elastic earth are expected when viscoelastic flow of subcrustal material is considered the detailed description of the deformation pattern and in particular the surface displacements displacement rates strains and strain rates depend on the structure and geometry of the material near the seismogenic zone in the past few years various viscoelastic models of crustal deformation have been published these models differ in their predictions concerning the temporal and spatial patterns of crustal deformation in some cases these differences are due to varying choices for the physical mechanism under study in other cases however the differences are the result of the details of the mathematical treatment or the choice of model parameters we seek to resolve the origin of some of these differences by analysing several different linear viscoelastic models with a common finite element computational technique the models involve strike slip faulting and include a thin channel asthenosphere model a model with a varying thickness lithosphere and a model with a viscoelastic inclusion below the brittle slip plane the calculations reveal that the surface deformation pattern is most sensitive to the rheology of the material that lies below the slip plane in a volume whose extent is a few times the fault depth if this material is viscoelastic the surface deformation pattern resembles that of an elastic layer lying over a viscoelastic half space when the thickness or breadth of the viscoelastic material is less than a few times the fault depth then the surface deformation pattern is altered and geodetic measurements are potentially useful for studying the details of subsurface geometry and structure distinguishing among the various models in best accomplished by making geodetic measurements not only near the fault but out to distances equal to several times the fault depth this is where the model differences are greatest these differences will be most readily detected shortly after an earthquake when viscoelastic effects are most pronounced for a thin channel asthenosphere model we have found that the predicted displacements are less than those for a half space asthenosphere this result is contrary to recently published results based on analytical approximations the deficiencies of the latter work result from ignoring material below the asthenosphere and in using thickness averaged deformation parameters although the displacements predicted for a thin channel asthenosphere are less than those for a half space asthenosphere the post seismic strain rates at intermediate distances from the fault are greater in an absolute value sense in the former model

precursory surface deformation in great plate boundary earthquake sequences abstract we present an analysis of time dependent precursory source processes and associated ground surface deformation prior to the great earthquake rupture of a long seismic gap zone along a transform plate boundary this work is based on a theoretical model proposed by li and rice 1983 and reviewed briefly here in the model thickness averaged stress transmission in the lithosphere is analyzed by a generalized elsasser plane stress model which includes coupling to a viscoelastic asthenosphere upward progression of preseismic rupture at each section along strike is analyzed as quasi static extension in local antiplane strain of an elastic brittle crack this provides the necessary boundary condition in the context of the line spring procedure along strike for the elsasser plate model the approach to instability is simulated using what are thought to be typical material and tectonic parameters it is found that prior to a large earthquake the shear strain increases rapidly near the seismic gap zone but tends to diminish at moderate distances from the plate boundary such a strain reversal phenomenon although difficult to measure at present due to its small magnitude may be helpful in the interpretation of geodetic measurements and in the planning of geodetic network locations for the purpose of earthquake prediction the acceleration of slip with consequent acceleration of stress increase in the upper brittle crust predicted by the model may provide a plausible explanation for nonlinear features associated with seismicity preceding some major events see e g raleigh et al 1982 a precursor time t prec during which anomalous strain rate at the seismic gap exceeds twice its background level is found to be in the range of a couple of months to 5 yr depending on the tectonic loading rate t prec varies approximately as r 1 3 where r is a dimensionless loading rate parameter this precursor time may be associated with physical anomalities related to rapid stress increase such as ground tilting water table variation and radon emission which precede some large earthquakes consistent with observations the analysis predicts longer precursor times for larger earthquakes

earthquake depths and the relation to strain accumulation and stress near strike slip faults in southern california earthquakes in the major fault zones are predominantly deep earthquakes in the crustal blocks bounding the fault zones are predominantly shallow in the san jacinto fault zone maximum earthquake depths correlate with surface heat flow these relations together with focal mechanisms geodetic strain measurements and fault zone models are consistent with the following ideas 1 interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes 2 the aseismic slip in a narrow fault zone in the brittle plastic transition region concentrates strain at the base of the brittle fault zone 3 deep earthquakes occur in thelower part of the brittle fault zone due to stick slip failure of highly stressed patches 4 background earhtquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding deep and stick slip shallow regions of the fault zone furthermore the difference in seismicity between the san jacinto and southern san andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases 5 shear stress may be lessmore in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks 6 in the stress field produced by plate motion and slip in the deep fault zone the upper brittle fault zone is not oriented favorably for shear failure lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation less

instability model for recurring large and great earthquakes in southern california the locked section of the san andreas fault in southern california has experienced a number of large and great earthquakes in the past and thus is expected to have more in the future to estimate the location time and slip of the next few earthquakes an earthquake instability model is formulated the model is similar to one recently developed for moderate earthquakes on the san andreas fault near parkfield california in both models unstable faulting the earthquake analog is caused by failure of all or part of a patch of brittle strain softening fault zone in the present model the patch extends downward from the ground surface to about 12 km depth and extends 500 km along strike from parkfield to the salton sea the variation of patch strength along strike is adjusted by trial until the computed sequence of instabilities matches the sequence of large and great earthquakes sincea d 1080 reported by sieh and others the last earthquake was them 8 3 ft tejon event in 1857 the resulting strength variation has five contiguous sections of alternately low and high strength from north to south the approximate locations of the sections are 1 parkfield to bitterwater valley 2 bitterwater valley to lake hughes 3 lake hughes to san bernardino 4 san bernardino to palm springs and 5 palm springs to the salton sea sections 1 3 and 5 have strengths between 53 and 88 bars sections 2 and 4 have strengths between 164 and 193 bars patch section ends and unstable rupture ends usually coincide although one or more adjacent patch sections may fail unstably at once the model predicts that the next sections of the fault to slip unstably will be 1 3 and 5 the order and dates depend on the assumed length of an earthquake rupture in about 1700

the earthquake deformation cycle at the nankai trough southwest japan geodetic measurements made landward of the nankai trough site of great subduction zone thrust earthquakes in 1854 and 1946 provide a uniquely detailed picture of the strain buildup process supply constraints on the mechanism of strain accumulation and allow for improved estimates of earthquake recurrence provided the two most recent movement cycles are similar the observations dating from about 1890 may be used to reconstruct a single complete deformation cycle coseismic strain release postseismic transients interseismic strain accumulation very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary persists for at least 30 years and shows a clear tendency to become longer wavelength with increasing time the transient movements have two timescales the first of about a year or less corresponds to deformation largely uplift concentrated close to the coseismic fault and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane the second longer timescale is associated with a diffusion like spread of the deformation further landward an effect qualitatively similar to that first predicted by elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces although average post 1890 tilt rates exceed late quaternary and holocene averages by at least a factor of three because of the nonlinearity of strain buildup and the significant permanent deformation simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3 strict application of the time predictable model assumed correct overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be estimated

low frequency currents at the northern shelf edge of the grand banks the structure and generation mechanisms of low frequency currents at the northern shelf edge region of the grand banks are investigated using data from a bottom mounted acoustic doppler current profiler adcp and an array of conventional current meters collected during the canadian atlantic storms program ii the vertical structure of the current is determined from an empirical orthogonal function analysis of the adcp data the analysis shows that over the shelf edge the low frequency currents are predominantly barotropic and polarized in the long shelf direction there is little coherence between the local wind and the barotropic current while baroclinic currents are moderately coherent with the local wind in the interior of the shelf the currents and the local wind are highly coherent the difference in response to wind forcing over the shelf edge and in the shelf interior suggests that currents in the shelf interior are wind driven while currents over the shelf edge are generated by mechanisms other than direct wind forcing possible generation mechanisms for the barotropic long shelf low frequency currents are discussed including meanders and eddy formation in the labrador current and continental shelf waves generated by distant storms

implications of an elastic analysis of in situ stress measurements near the san andreas fault twenty nine measurements of in situ stress obtained with the hydraulic fracturing technique near palmdale california are the basis of an elastic analysis of the state of stress in the mojave desert adjacent to the san andreas fault the measurements were made at depths extending from 80 to 849 m and at distances from the fault between 2 and 34 km the elastic solution indicates a state of deviatoric stress typical for continents in that the inferred depth gradient of the maximum shear stress is about 7 9 mpa km extrapolation yields an average shear stress in the upper 14 km of the crust of about 56 mpa a result that is higher than estimates of the average shear stress on the san andreas fault based on the analysis of heat flow data this finding is consistent however with estimates of fault strength based on laboratory determinations of the coefficient of friction for samples of san andreas fault gouge if the regional state of deviatoric stress is limited by the strength of the fault zone if so then the coefficient of friction of the san andreas fault zone inferred from the stress field results is about 0 45 the state of stress does not appear to vary systematically with distance from the san andreas fault although considerable localized variation is observed the observations suggest an upper bound of about 0 1 mpa km for the horizontal gradient of the maximum shear stress in the direction perpendicular to the san andreas fault a result that implies a corresponding limit of about 1 4 mpa on the shear traction applied to the base of the seismogenic layer finally we demonstrate the potential application of in situ stress data to the direct assessment of accumulated slip which could be released in a large earthquake we show that on the basis of a model involving a locked fault extending to about 22 km the total fault slip below the locked portion is less than 13 m a more comprehensive set of stress data could permit the estimation of an even lower bound

the determination of displacement fields from geodetic data along a strike slip fault observations of angles or distances between stations of a geodetic network are commonly used to infer information about the movement of the surface of the earth the absence of any observations external to the network leads to an ambiguous displacement field existing techniques for eliminating this ambiguity are all unsatisfactory in some respect the best technique an inner coordinate solution is not appropriate for networks located in a strike slip fault environment the inner coordinate solution zeros the rotation of all stations about their center of mass along a strike slip fault like the san andreas however motion normal to the fault is less likely than motion parallel to the fault the solution presented here an outer coordinate solution finds the rotation of the network that minimizes the components of displacement normal to the fault since motion along a strike slip fault is generally expected to be parallel to the fault the displacements obtained with the outer coordinate solution are more reasonable than those obtained with other techniques examination of a trilateration network near san francisco bay demonstrates the large effect that the choice of adjustment technique can have on the inferred relative motion of the two sides of the fault the inner coordinate solution gave a rate of about 1 mm yr whereas the preferred outer coordinate solution rate was 36 mm yr

geodetic measurement of horizontal deformation in the northern san francisco bay region california analysis of geodolite observations in the coast ranges of northern california indicates that relative plate motion is spread over a wide zone little or no deformation occurs to the southwest of the san andreas fault itself to the northeast of the san andreas fault a region transected by the parallel rodgers creek and maacama faults 25 6 mm yr of right lateral slip is distributed over about 60 km there is a peak shear strain of 0 6 0 1 rad yr near the san andreas fault elsewhere in the deforming zone shear strain rates are 0 3 0 04 rad yr at the northeastern boundary of the study area the strain rate is less than 0 2 rad yr except for the high strain rate near the san andreas fault there is no evidence of any localization of deformation to the surface traces of the faults nor is there any evidence of creep or aseismic slip on any of the faults in the area the data indicate that the relative motion between the pacific and north american plate occurs primarily to the northeast of the san andreas fault the observations are inconsistent with models that include slip at depth on only the san andreas fault the data can be fit equally well by models with slip at depth on all three of the major faults or with motion distributed across the entire zone from the san andreas fault to the napa valley fault preliminary analysis of observations to the farallon islands about 40 km southwest of the san andreas fault suggests that little or no additional relative motion is accommodated on that side of the san andreas fault in particular it is unlikely that the 20 mm yr difference between the plate motion rate about 55 mm yr and the san andreas fault system rate about 35 mm yr can be explained by deformation between the san andreas fault and the farallon islands

postseismic strain following the 1989 loma prieta earthquake from gps and leveling measurements postseismic deformation in the 5 years following the 1989 loma prieta earthquake has been measured with the global positioning system and precise leveling postearthquake velocities at distances greater than 20 km from the coseismic rupture are not significantly different from those observed in the 20 years prior to the earthquake however velocities at stations within 20 km of the rupture exceed preearthquake rates and exhibit unanticipated contraction normal to the strike of the san andreas fault system a combination of forward modeling and nonlinear optimization suggests that the observed postseismic deformations were caused by aseismic oblique reverse slip averaging 2 9 cm yr on the san andreas fault and or the loma prieta rupture zone and 2 4 cm yr reverse slip along a buried fault within the foothills thrust belt the best fitting sources of postseismic deformation are all located at depths of less than 15 km we find no evidence for accelerated flow or shear below the loma prieta rupture in the first 5 years following the earthquake the inferred postseismic slip is likely to have been caused by the coseismic stress change updip of the 1989 rupture

a theory of intraplate tectonics the theory of plate tectonics assumes a rigid behavior of tectonic plates and therefore fails to account for observed intraplate deformation a new theory of intraplate tectonics is developed to calculate the first order intraplate deformation induced by horizontal displacement of deformable plate boundaries it is based on simple assumptions that link the well established directions of relative plate motion to the displacement and deformation fields within a plate interior adjacent to three types of deformable plate boundaries inward outward and tangential displaced boundaries the theory predicts the direction of intraplate displacement displacement rate strain and stress fields in terms of small circles great circles and 45 loxodromes around the pole of rotation of two adjacent plates the 45 loxodromes are two orthogonal sets of directions clockwise and counterclockwise that intersect both small and great circles at 45 the principal axis of the maximum horizontal stress follows small circles for inward displaced boundaries great circles for out ward displaced boundaries and loxodromes for tangential displaced boundaries the theoretical predictions are systematically compared with more than 4000 reliable observed directions of maximum horizontal stress provided by the world stress map project zoback 1992 the theory indicates that the first order intraplate deformation is predominantly induced by horizontal forces acting on plate boundaries and by buoyancy forces that arise from lateral density variations between mid ocean ridges and plate interiors ridge push the simplicity of the predictions and their good agreement with observations suggests that intraplate deformation should be investigated in the pole of rotation spherical coordinate system

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

viscoelastic earthquake cycle models with deep stress driven creep along the san andreas fault system 1 we develop a two dimensional boundary element earthquake cycle model including deep interseismic creep on vertical strike slip faults in an elastic lithosphere coupled to a viscoelastic asthenosphere uniform slip on the upper part of the fault is prescribed periodically to represent great strike slip earthquakes below the coseismic rupture the fault creeps in response to lithospheric shear stresses within a narrow linear viscous fault zone the model is applied to the gps contemporary velocity field across the carrizo plain and northern san francisco bay segments of the san andreas fault as well as triangulation measurements of postseismic strain following the 1906 san francisco earthquake previous analysis of these data using conventional viscoelastic coupling models without stress driven creep segall 2002 shows that it is necessary to invoke different lithosphere asthenosphere rheology in northern and southern california in order to explain the data we show that with deep stress driven interseismic creep on the san andreas fault the data can be explained with the same rheology for northern and southern california we estimate elastic thickness in the range 44 100 km 95 confidence level fault zone viscosity per unit width of 0 5 8 2 1017 pa s m and asthenosphere relaxation time of 24 622 years 0 1 2 9 1020 pa s for northern and southern california we estimate a slip rate of 21 27 mm yr and recurrence time of 188 315 years for the northern san francisco bay san andreas fault and slip rate of 32 42 mm yr with recurrence time of 247 536 years for the carrizo plain

a reevaluation of the isostatic rebound of lake bonneville recent data pertaining to the uplift of the provo and bonneville shorelines of the late pleistocene lake bonneville are evaluated in a reanalysis using a rigorous model of deformation of a plane earth comprising an elastic layer overlying a viscoelastic channel the new observations constrain the wavelength and amplitude of the uplift of two shorelines formed at two different epochs and they partly resolve the ambiguities in the plausible mechanical parameters found in previous solutions the results indicate that the effective thickness of the elastic lithosphere is 28 to 30 km equal to the seismically determined crustal thickness in the region the estimate of the upper mantle viscosity in the region varies from 1 5 3 4 lo19 pa s to 2 1 5 8 l018 pa s depending on whether the upper mantle is assumed to be of uniform viscosity or the flow is confined to a narrow channel in the asthenosphere the predicted rate of present uplift at the center of the desiccated lake is 0 06 mm yr and 0 03 mm yr for the shallow channel and uniform mantle models respectively these values are beyond the first order leveling accuracies and the question of the existence of a narrow channel in the region cannot be resolved with the available observations

geodetic exploration of the elastic properties across and within the northern san andreas fault zone abstract relying on densely spaced gps data collected between 1996 and 2007 and on insar data spanning the 1992 2001 time period we explore the effects of lateral rheological heterogeneities across the san andreas fault near point reyes on the interseismic velocity field we evaluate heterogeneous elastic dislocation models including an asymmetric model with contrasting rigidities on either side of the san andreas fault and models with a narrow compliant fault zone that lead to high near fault strain rates we show that we cannot resolve a compliant zone from interseismic surface displacement measurements unless independent constraints on the locking depth exist we infer an 18 1 mm yr 1 slip rate on the san andreas fault with a 10 2 km locking depth and a 40 higher rigidity to the sw of the fault at bodega bay gps and edm data suggest a compliant zone on the saf with a 50 to 60 rigidity decrease but 25 km further south we find no evidence for a weak fault zone this suggests substantial variability in fault zone properties along the san andreas fault

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

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

reconciling viscoelastic models of postseismic and interseismic deformation effects of viscous shear zones and finite length ruptures we have developed a suite of earthquake cycle models for strike slip faults to investigate how finite ruptures and lithosphere scale viscous shear zones affect interseismic deformation in particular we assess whether localized and stationary interseismic deformation and large scale rapidly decaying postseismic transients may be explained with models incorporating either or both of these features models incorporating viscous shear zones give more stationary interseismic deformation than layered half space maxwell viscoelastic models producing similar early postseismic deformation in the near field this tendency is accentuated when the effective viscosity per unit width of the shear zone increases either with depth or with interseismic time models with finite 200 km long ruptures produce time dependent deformation similar to that from models with infinitely long ruptures though with smaller magnitude and somewhat more localized surface velocity fluctuations models incorporating a stiff lithosphere a low viscosity mantle asthenosphere and a crust to lithosphere scale viscous shear zone can replicate postseismic and interseismic deformation typical of large strike slip earthquakes such models require depth dependent power law or transient rheologies for the viscous shear zone material in the crust as long as the effective viscosity per unit shear zone width is approximately 1015 pa s m at crustal depths during the postseismic interval below the moho the shear zone effective viscosity per unit width must be higher throughout the seismic cycle at least over a short depth interval for example if shear zone viscosity per unit width is 5 1016 pa s m in the mantle lithosphere a model with a 50 km thick lithosphere and asthenosphere effective viscosities of 1 to 5 1018 pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation

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

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

a nonlinear least squares inverse analysis of strike slip faulting with application to the san andreas fault a nonlinear weighted least squares inverse analysis was performed for a synthetic elastic layer over viscoelastic half space model of strike slip faulting and an inversion of strain rate data was attempted for the locked portions of the san andreas fault in california the model parameters were elastic layer thickness h fault locking depth d earthquake recurrence time t average shear modulus of the elastic layer and viscoelastic half space half space viscosity and coseismic offset du only the ratio of viscosity to shear modulus can be resolved and thus shear modulus was held constant based on an eigenvector analysis of synthetic data it is not possible to resolve all of the five remaining parameters with the current observations which are very close to the fault and vary only with time since the last earthquake a suite of inversions was therefore performed for the san andreas holding recurrence time and locking depth fixed at particular values for each inversion these inversions resulted in nearly the same data misfit thus a range of estimated values rather than a particular set of values was found for the remaining three parameters inversions on data from the northern san andreas resulted in predicted parameter ranges similar to those produced by inversions on data from the whole fault the addition of observations which vary with distance from the fault should help to constrain these ranges and perhaps allow resolution of recurrence time and locking depth

observation of diffusion processes in earthquake populations and implications for the predictability of seismicity systems scale invariance either in space or in time has been shown in many papers to characterize earthquake distributions unfortunately little work has been dedicated to looking at the general space time scaling invariance of seismicity systems even though a better understanding of how the two domains spatial and temporal link together could help the development of the stochastic dynamical modeling of earthquake populations in this paper we report the observation of diffusion processes of temporally correlated seismic activity for three different data sets a mine creighton mine canada the long valley caldera in eastern california and a 7 year period of recorded seismic activity in southern california the observed subdiffusion processes are indicative of the general space time scaling of the system taking the form of a slow power law growth r t similar to t h of the mean distance r t between the main event arid the temporally correlated afterevents occuring after a delay t h is found on average to be small 0 1 for creighton mine 0 22 for the long valley caldera and 0 22 for the southern california main events with magnitude greater than or equal to 1 5 but fluctuates significantly from one main event to the other the diffusion is found to be intermittent non gaussian and multiscaling and except for the long valley caldera a systematic correlation between the sizes of the main event and subsequent afterevents and the growth exponent h is observed while classical viscous relaxation models e g elastic listhosphere plastic asthenosphere coupling or fluid flow triggered by sudden changes in pore pressure have been proposed to characterize this relaxation by homogeneous i e nonintermittent normal h 0 5 diffusion processes the direct implication of the reported results is that seismicity systems at spatial scales from meters to hundreds of kilometers and small microearthquakes in a mine to intermediate magnitudes relax spatiotemporally in a nonelastic way revealing the stochastic space time scale invariant nature of such systems since these diffusion processes correspond to a loss of information with time on the location of the main event they can be used to investigate the limits of predictability at all spatial scales of seismicity systems in terms of the spatiotemporal clustering of temporally correlated earthquakes

the 1984 morgan hill california earthquake the morgan hill california earthquake magnitude 6 1 of 24 april 1984 ruptured a 30 kilometer long segment of the calaveras fault zone to the east of san jose although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected no short term precursors were noted and so the time of the 1984 earthquake was not predicted unilateral rupture propagation toward the south southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion including an exceptionally large horizontal acceleration of 1 29g at a site on a dam abutment near the southeast end of the rupture zone

stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle an elementary mechanical model of a coupled subduction zone is presented it is used to study the transfer of extensional stresses along the underthrusting plate and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate during the earthquake cycle the model assumes an elastic plate which deforms one dimensionally has shear drag interactions treated approximately by the elsasser procedure with a maxwellian viscoelastic mantle and is subject to steady gravitational sinking forces great thrust earthquakes are simulated as sudden stress relieving displacements at the contact zone between the slab and overriding plate the model allows analysis of the space and time variable stresses in the main thrust zone the adjacent oceanic plate and the subducting slab over each earthquake cycle stress variations leading to outer rise events are discussed as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone based on simulations of periodic great earthquakes the results of simulated slow progression of deformation slip from below into the thrust contact zone are also presented such preslip occurring over a time scale that is small in comparison with the whole seismic cycle e g of the order of a tenth or so is shown to transiently slow the rate of buildup of or even relieve extensional stresses in the slab in the absence of preslip those stresses build up steadily in time during the latter part of the cycle the results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world the observations include tensional outer rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle normal tensional intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab

microplate versus continuum descriptions of active tectonic deformation whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi continuous flow has important implications for lithospheric dynamics fault mechanics and earthquake hazard assessment actively deforming regions of the western united states central asia japan and new zealand show features that argue for both styles of movement but new observations are necessary to determine which is most appropriate and at what scale the description applies geologic geodetic seismic and paleomagnetic measurements tend to sample complementary aspects of the deformation field so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases provided the total relative motion across each region is known and the distribution of active faults is well mapped determination of fault slip rates can provide potentially decisive constraints reconnaissance geological studies supply useful slip rate estimates but precise values depend upon detailed intensive investigation of individual sites geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long term deformation pattern earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere especially the strength and thickness of the elastic crust relative to the ductile lithosphere and dynamical models can provide contrasting forecasts of observable features with testable consequences

kinematics of the pacific north america plate boundary zone northern california we measured motions of 54 sites in an east west transect across northern california at 38 40 north by global positioning system gps observations over a 4 year we estimate the total slip rate on the san andreas fault system to be 39 6 0 6 1 5 mm yr 68 6 upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters our best fitting model fits the fault parallel velocities with a mean square error of 1 04 and the following estimated fault slip rates all in mm yr san andreas 17 4 3 1 2 5 ma acama 13 9 2 8 4 1 and bartlett springs 8 2 1 9 2 1 the data are fit best by models in which the san andreas fault is locked to 14 9 7 1 12 5 km the ma acama fault locked to 13 4 4 8 7 4 km except for shallow creep in the upper 5 km and the bartlett springs fault creeping at all depths the ma acama fault most likely poses a significant seismic hazard as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake we find little evidence for contraction across coast ranges except at western edge of great valley where 1 3 mm yr of shortening is permitted by the data no strain is observed within the great valley or sierra nevada except that associated with right lateral strike slip on the san andreas fault system this is consistent with models of the pacific north america plate boundary zone in which the relative plate motion is partitioned into two domains one strike slip and one dominantly extensional separated by the elastically deforming sierra nevada great valley block

a review of heterogeneous materials and their implications for relationships between kinematics and dynamics in continents 1 the variation of mechanical properties as a function of depth in the lithosphere controls the relationship between surface deformation and whole lithosphere deformation where mechanical competence elastic strength or viscous stiffness is a continuous function with depth surface deformation can be used to constrain either force balance or rheological parameters for the lithosphere however where competence is vertically discontinuous the surface deformation and deformation at depth may have a range of relationships including complete independence therefore both inversions and forward models designed to test either rheology or stress state are highly sensitive to the a priori choice of a mechanical model

a new multilayered visco elasto plastic experimental model to study strike slip fault seismic cycle nowadays technological advances in satellite imagery measurements as well as the development of dense geodetic and seismologic networks allow for a detailed analysis of surface deformation associated with active fault seismic cycle however the study of earthquake dynamics faces several limiting factors related to the difficulty to access the deep source of earthquake and to integrate the characteristic time scales of deformation processes that extend from seconds to thousands of years to overcome part of these limitations and better constrain the role and couplings between kinematic and mechanical parameters we have developed a new experimental approach allowing for the simulation of strike slip fault earthquakes and analyze in detail hundreds of successive seismic cycle model rheology is made of multilayered visco elasto plastic analog materials to account for the mechanical behavior of the upper and lower crust and to allow simulating brittle ductile coupling postseismic deformation phase and far field stress transfers the kinematic evolution of the model surface is monitored using an optical system based on subpixel spectral correlation of high resolution digital images first results show that the model succeed in reproducing the deformation mechanisms and surface kinematics associated to the main phases of the seismic cycle indicating that model scaling is satisfactory these results are comforted by using numerical algorithms to study the strain and stress distribution at the surface and at depth along the fault plane our analog modeling approach appears then as an efficient complementary approach to investigate earthquake dynamics

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

slip patterns and earthquake populations along different classes of faults in elastic solids numerical simulations of slip instabilities on a vertical strike slip fault in an elastic half space are performed for various models belonging to two different categories the first category consists of inherently discrete cellular fault models such are used to represent fault systems made of segments modeled by numerical cells that can fail independently of one another their quasi independence is assumed to provide an approximate representation of strong fault heterogeneity due to geometric or material property disorder that can arrest ruptures at segment boundaries the second category consists of models having a well defined continuum limit these involve a fault governed by rate and state dependent friction and are used to evaluate what types of property heterogeneity could lead to the quasi independent behavior of neighboring fault segments assumed in the first category the cases examined include models of a cellular fault subjected to various complex spatial distributions of static to kinetic strength drops and models incorporating rate and state dependent friction subjected to various spatial distributions of effective stress normal stress minus pore pressure the results indicate that gradual effective stress variations do not provide a sufficient mechanism for the generation of observed seismic response strong and abrupt fault heterogeneity as envisioned in the inherently discrete category is required for the generation of complex slip patterns and a wide spectrum of event sizes strong fault heterogeneity also facilitates the generation of rough rupture fronts capable of radiating high frequency seismic waves the large earthquakes in both categories of models occur on a quasi periodic basis the degree of periodicity increases with event size and decreases with model complexity however in all discrete segmented cases the models generate nonrepeating sequences of earthquakes and the nature of the large quasi periodic events is highly variable the results indicate that expectations for regular sequences of earthquakes and or simple repetitive precursory slip patterns are unrealistic the frequency size fs statistics of the small failure episodes simulated by the cellular fault models are approximately self similar with b 1 2 and ba 1 where b and ba are b values based on magnitude and rupture area respectively for failure episodes larger than a critical size however the simulated statistics are strongly enhanced with respect to self similar distributions defined by the small events this is due to the fact that the stress concentrated at the edge of a rupture expanding in an elastic solid grows with the rupture size when the fault properties e g geometric irregularities are characterized by a narrow range of size scales the scaling of stress concentrations with the size of the failure zone creates a critical rupture area terminating the self similar earthquake statistics in such systems events reaching the critical size become on the average unstoppable and they continue to grow to a size limited by a characteristic model dimension when however the system is characterized by a broad spectrum of size scales the above phenomena are suppressed and the range of apparent self similar fs statistics is broad and characterized by average b and ba values of about 1 the simulations indicate that power law extrapolations of low magnitude seismicity will often underestimate the rate of occurrence of moderate and large earthquakes the models establish connections between features of fs statistics of earthquakes range of self similar regimes local maxima and structural properties of faults dominant size scales of heterogeneities dimensions of coherent brittle zones the results suggest that observed fs statistics can be used to obtain information on crustal thickness and fault zone structure

earthquake failure sequences along a cellular fault zone in a three dimensional elastic solid containing asperity and nonasperity regions numerical simulations of earthquake failure sequences along a discrete cellular fault zone are performed for a three dimensional 3 d model representing approximately the central san andreas fault the model consists of an upper crust overlying a lower crust and mantle region together defining an elastic half space with a vertical half plane fault the fault contains a region where slip is calculated on a uniform grid of cells governed by a static kinetic friction law and regions where slip is prescribed so as to represent tectonic loading aseismic fault creep and adjacent great earthquakes the computational region models a 70 km long and 17 5 km deep section of the san andreas fault to the nw of the great 1857 rupture zone different distributions of stress drops on failing computational cells are used to model asperity parkfield asperity and nonasperity fault regions the model is inherently discrete and corresponds to a situation in which a characteristic size of geometric disorder within the fault i e cell size here a few hundreds of meters is much larger than the nucleation size of the order of tens of centimeters to tens of meters based on slip weakening or state evolution slip distances the computational grid is loaded by a constant plate motion imposed at the lower crust upper mantle and creeping fault regions and by a staircase slip history imposed at the 1857 and 1906 rupture zones stress transfer along and outside the fault due to the imposed loadings and failure episodes along the computational grid is calculated using 3 d elastic dislocation theory the resulting displacement field in the computational region is compatible with geodetic and seismological observations only when the asperity and nonasperity regions are characterized by significantly different average stress drops the frequency magnitude statistics of the simulated failure episodes are approximately self similar for small events with b 1 2 the b value of statistics based on rupture area ba is about 1 but are strongly enhanced with respect to self similarity for events larger than a critical size this is interpreted as a direct manifestation of our 3 d elastic stress transfer calculations beyond certain rupture area and potency seismic moment divided by rigidity release values the event is usually unstoppable and it continues to grow to a size limited by a characteristic model dimension this effect is not accounted for by cellular automata and block spring models in which the adopted simplified stress transfer laws fail to scale properly with increasing rupture size the simulations suggest that local maxima in observed frequency magnitude statistics correspond to dimensions of coherent brittle zones such as the width of the seismogenic layer or the length of a fault segment bounded by barriers the analysis indicates that a single cell size representing approximately a single scale of geometric disorder cannot induce self similarity in a 3 d elastic model over a broad range of magnitudes a representation of geometric disorder covering a range of scales may thus be required to generate a wide domain of self similar gutenberg richter statistics our simulations show a great diversity in the mode of failure of the parkfield asperity the earthquakes themselves define an irregular sequence of events the modeling like many other discrete fault models suggests that expectations for periodic parkfield earthquakes and or simple precursory patterns repeating from one event to the other are unrealistic

multiple magma sources involved in marginal sea formation pb sr and nd isotopic evidence from the japan sea region the cenozoic basaltic rocks in sikhote alin and sakhalin exhibit a change over 55 m y from subduction zone related to continental rift related volcanism as the japan sea opened between eastern sikhote alin and the japan arc the temporal geochemical trends in volcanic rocks erupted before and during the opening of the japan sea suggesting that there was a change in magma source from the lithosphere to the depleted asthenosphere as the japan sea opening progressed two geochemical groups can be identified within the basalts that postdate the japan sea opening tholeiitic basalts and alkalic basalts as the tholeiitic basalts have emi type signatures we propose that the tholeiites had a significant contribution from an emi type precambrian subcontinental lithospheric mantle the alkalic basalts yielding geochemical characteristics similar to enriched oceanic island basalts may have been derived from the subcontinental asthenospheric mantle the opening of the japan sea was triggered by lateral migration of asthenospheric mantle from beneath northeast china toward the japan arc

regional deformation near palmdale california 1973 1983 the tehachapi trilateration network spans the intersection of the san andreas and garlock faults in southern california in the big bend region of the san andreas fault analysis of data from 1973 1983 shows strain differences between the northwest and southeast regions of the network and slip at depth on both faults the palmdale network spanning the san andreas fault entirely within the tehachapi network showed increases of about 1 strain in both east west and north south extension in late 1979 the tehachapi strains also jumped at this time but the magnitude of the increase was only about one third that of palmdale the principal strain rates for tehachapi over the time interval 1973 1983 are e 1 0 08 0 01 strain yr and e 2 0 12 0 01 strain yr with the 1 axis directed n76 e strains were also computed for two subregions the principal strain rates for the southeast tehachapi region are e 1 0 14 0 02 strain yr and e 2 0 16 0 02 strain yr with the 1 axis directed n73 e this result differs significantly from the principal strain rates at palmdale e 1 0 21 0 02 strain yr e 2 0 17 0 01 strain yr with the 1 axis directed n71 e for the northwest tehachapi region e 1 0 11 0 02 strain yr and e 2 0 06 0 02 strain yr with the 1 axis directed n93 e

strain localization along the san andreas fault consequences for loading mechanisms the orientation of the near fault strain accumulation associated with the relative motion between the pacific and north american plates is better predicted by the orientation of the local faulting than by the orientation of the regional plate motion this observation follows from consideration of shear strain results from geodetic studies ranging from the imperial valley to shelter cove along the san andreas fault system this set of results includes calculations from regions where the faulting and plate motion are considerably oblique to each other both the localization of the strain associated with pacific north american motion and the orientation of the associated strain can be simply caused by failure of the lithosphere as opposed to localization of plate driving forces two types of models for the rheology and geometry of lithospheric failure have been prevalent in the geodetic literature 1 viscoelastic models which are dominated by horizontal layering of the crust and 2 elastic half space models with geometry which is dominantly vertical horizontal models can accommodate the observed strain results only if postseismic relaxation times are long compared to the repeat times of large earthquakes vertical models produce strains which are parallel to the local structure at all times if the horizontal detachments of the viscoelastic models exist they must be at depths 2 3 times the seismogenic thickness geologic studies also indicate that major transcurrent features extend as ductile shear zones to depths much larger than would be expected in horizontal models these considerations lead us to favor models with vertical geometries the vertical structure of these models can be reconciled with laboratory measurements of rock strength through the strain softening processes associated with flow of composite materials such as quartzo feldspathic rocks independent of the horizontal rheological layering in the starting low strain material progressive deformation will lead to preferential weakening in the plane of shear and to the development of a shear zone which will always be weaker than the surrounding material thus shear zones such as the san andreas system develop vertical tabular geometries which extend far below the brittle plastic transition and which are mechanically persistent

a physical basis for time clustering of large earthquakes we develop a theory that links stress interaction between earthquakes and the occurrence of temporal clustering coseismic static stress change in the vicinity 50 km of large earthquakes suggests that perturbations of 0 1 to 1 bars may affect the occurrence of other earthquakes at larger distances interactions also seem to exist four m 8 earthquakes have occurred in mongolia on distant faults 400 km during the last century also paleoseismic observations documenting much longer time periods display a time clustering of major events we demonstrate with simple mechanical concepts that postseismic stress relaxation magnifies the coseismic stress change and has a major effect on fault interaction during the seismic cycle in the simple case where two distant faults are coupled the probabilistic occurrence of triggered earthquakes may increase dramatically due to long range postseismic coupling

postseismic deformation following the landers earthquake california 28 june 1992 accelerated strain followed the landers and big bear earthquakes returning to the normal rate only after a period of several months we observed this strain throughout most of southern california using the global positioning system gps three gps receivers operating continuously in fixed positions at pinyon flat jet propulsion laboratory pasadena and goldstone all recorded postseismic deformation in a relative sense in addition we established 16 sites where we deployed portable receivers occasionally over a period of about 6 months near the rupture zones of the earthquakes anomalous postseismic displacements ranged from 55 mm near the epicenter to a few millimeters far from the fault we modeled the displacements using dislocation theory as due to variable slip on the faults that were displaced at the times of the earthquakes the model suggests that the postseismic strain released the equivalent of about 15 of the seismic moment of the mainshock while the strain released from the upper 10 km is about the same as what can be explained by direct effects of aftershocks the major contribution of strain release comes from the lower layer below 10 km depth significant afterslip or viscous relaxation must have occurred below 10 km depth to explain the observed deformation more than 100 km from the fault one interpretation is that high stress on the margin of the co seismic rupture zone drives the rupture to extend itself into urbroken rock below and along the initial rupture zone

the role of an intracrustal asthenosphere on the behavior of major strike slip faults measurements of strain accumulation adjacent to the san andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike slip faults has been attributed to the damping effect of a viscoelastic asthenosphere however to explain the narrowness of the zone on the san andreas fault it is necessary to conclude that the thickness of the lithosphere is 10 20 km this does not appear to be consistent with the relatively low surface heat flow measurements in this paper an alternative four layer model is proposed to explain the behavior of the san andreas fault an upper elastic plate extends to a depth of about 15 km the depth of the deepest seismicity on and adjacent to the fault beneath this upper elastic plate is a soft intracrustal asthenosphere that exhibits a viscoelastic behavior the viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz which is known to have a very soft rheology or the presence of water that can lead to pressure solution creep or to both effects beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks and beneath this second elastic layer is the asthenosphere there is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in california and elsewhere we show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the san andreas fault

post seismic relaxation and earthquake triggering in the southern adriatic region summary an attempt at quantifying post seismic relaxation triggered by decoupling earthquakes along the eastern thrusting border of the adriatic plate southern dinarides is carried out by finite element modelling with a model constituted by an elastic lithosphere riding on a viscous asthenosphere in particular it is investigated the possibility that the above phenomenon is responsible for the fact that in the last two centuries most major earthquakes in the southern dinarides ms 6 have been followed within a few years by intense mainly tensional earthquakes in southern italy i e the zone lying on the opposite margin of the adriatic plate this analysis has been applied to the last example of the supposed seismic interrelation i e the triggering 1979 april 15 montenegro event ms 6 7 and the presumably induced 1980 november 23 irpinia earthquake in the southern apennines ms 6 9 results indicate that the strain induced in the southern apennines by the triggering event has significant amplitude since it largely exceeds the effect of earth tides and the principal stress axes are consistent with those of southern apenninic earthquakes the order of magnitude of the time delay between the montenegro and irpinia events 1 6 yr could be explained by assuming that earthquake triggering is most probable when the highest values of the induced strain rate reach the southern apennines in particular this interpretation predicts the observed time delay when a model diffusivity of 400 m2 s 1 is assumed the constraints that this diffusivity value may pose on the structural and rheological features of the crust upper mantle system in the study area are discussed it is shown that the effects of the montenegro event on the present velocity field are comparable to though systematically lower than the velocities suggested by geodetic observations in the italian region this suggests that geodynamic interpretations of geodetic data given without taking into account possible transient effects on the kinematic pattern as those related to post seismic relaxation may be incorrect experiments carried out by tentatively simulating the presence of subducted lithosphere along the western margin of the adriatic plate as a lateral variation of diffusivity have shown that this structural feature may emphasize e w tensional strains in the southern apennines

inference of multiple earthquake cycle relaxation timescales from irregular geodetic sampling of interseismic deformation abstract characterizing surface deformation throughout a full earthquake cycle is a challenge due to the lack of high resolution geodetic observations of duration comparable to that of characteristic earthquake recurrence intervals 250 10 000 years here we approach this problem by comparing long term geologic slip rates with geodetically derived fault slip rates by sampling only a short fraction 0 001 0 1 of a complete earthquake cycle along 15 continental strike slip faults geodetic observations provide snapshots of surface deformation from different times through the earthquake cycle the timing of the last earthquake on many of these faults is poorly known and may vary greatly from fault to fault assuming that the underlying mechanics of the seismic cycle are similar for all faults geodetic observations from different faults may be interpreted as samples over a significantly larger fraction of the earthquake cycle than could be obtained from the geodetic record along any one fault alone as an ensemble we find that geologically and geodetically inferred slip rates agree well with a linear relation of 0 94 0 09 to simultaneously explain both the ensemble agreement between geologic and geodetic slip rate estimates with observations of rapid postseismic deformation we consider the predictions from simple two layer earthquake cycle models with both maxwell and burgers viscoelastic rheologies we find that a two layer burgers model with two relaxation timescales is consistent with observations of deformation throughout the earthquake cycle whereas the widely used two layer maxwell model with a single relaxation timescale is not suggesting that the earthquake cycle is effectively characterized by a largely stress recoverable rapid postseismic stage and a much more slowly varying interseismic stage

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

reconciling rapid strain accumulation with deep seismogenic fault planes in the ventura basin california global positioning system measurements across the east central ventura basin transverse ranges southern california before the nearby 1994 northridge earthquake show high strain rates interpreting this rapid strain accumulation using the usual model of deep slip on a dislocation in a uniform elastic half space requires slip to extend to within 5 km of the surface such shallow slip is difficult to reconcile with the substantial coseismic displacement at depths from 7 to 19 km during the northridge earthquake here we model the displacement and velocity fields throughout the earthquake cycle using a two dimensional finite element model with a viscoelastic rheology displacements are driven by far field and basal velocity boundary conditions and by imposed periodic earthquakes on the thrust faults bounding the basin the thrust faults rupture through an elastic upper crust to a depth of 15 km after a transient stage during which stresses and strains build up to quasi equilibrium values the behavior of the model becomes periodic the sum of the coseismic displacement divided by the repeat interval plus the average interseismic velocity is equal to the geologic velocity the temporal variation in surface velocity depends mainly on the elsasser relaxation time proportional to the product of the maxwell time of the lower crust and the ratio of the thicknesses of the entire crust and viscoelastic lower crust we are able to match the observed high strain rate only if we include the observed variations in elastic modulus associated with the deep basin sediments the model reconciles geologic geodetic and seismological observations of deformation there are trade offs among the far field convergence rate the elsasser time the earthquake repeat time and the time into the earthquake cycle acceptable convergence rates range from 8 mm yr for a relaxation time of the lower crust of 300 years to 12 mm yr for a 30 year relaxation time

lateral offsets and revised dates of large prehistoric earthquakes at pallett creek southern california recent excavation and new radiocarbon dates of sediments at pallett creek are the basis for new conclusions regarding the late holocene history of the san andreas fault systematic dissection of a 50 m long 15 m wide 5 m deep volume of earth centered on the fault enables documentation in three dimensions of fault patterns lateral offsets and vertical deformation associated with large earthquakes of the past the excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 a d with an average recurrence interval of about 145 years prehistoric slip events that occurred in 1720 50 1550 70 1350 50 1080 65 and 845 75 a d have lateral offsets that are comparable to those of the most recent great earthquake of 1857 thus all of these events represent earthquakes of large magnitude the lateral offsets of two other events in 935 85 and 1015 100 a d are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events the new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years on the basis of these data the probability of a large earthquake with surficial fault rupture at this site is between 0 2 and 5 during 1984 and 7 and 60 by the year 2000

crustal deformation across and beyond the los angeles basin from geodetic measurements we combine 6 years of global positioning system gps data with 20 years of trilateration data and a century of triangulation taped distance and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater los angeles region we interpolate the velocities to construct a regional strain rate map beyond the los angeles basin our results generally agree with the model proposed by the working group on california earthquake probabilities in 1995 important regional findings of this study are as follows 1 there is a significant n s convergence and e w extension about 0 22 and 0 17 0 05 strain yr respectively for the two components along the southern frontal fault system of the san gabriel mountains 2 the crustal deformation around the big bend of the san andreas fault saf cannot be explained solely by wrench style motion across the saf remaining motion could be part of a nw se extension which is the response to nfsw compression in the central transverse ranges region alternatively it could be caused by left lateral faulting on an oblique blind thrust beneath the san gabriel mountains 3 low strain rates axe found along the elsinore fault and newport inglewood 4 north south compression decreases from the raymond hill fault westward to the santa monica fault there is little east west extension along this fault system

nucleation and growth of dip slip faults in a stable craton we analyze the conditions for nucleation of a dip slip fault in intact crust and its subsequent growth using linear elastic fracture mechanics and the finite element method we assume a fault can be modeled as a mode ii shear crack in a layered elastic crust and we investigate fault behavior under conditions of a variable dip and shear fracture energy two shear fracture energy gradients gc were evaluated both have surface values of 1 105 j m2 and increase linearly to values of 1 106 or 1 107 j m2 at 15 km depth the base of the seismogenic zone we assume that the vertical and horizontal components of the gravitational stress are both equal to the lithostatic load effectively removing them from the calculation of shear stress on the fault strain required for failure varied with respect to shear fracture energy gradient and rupture direction but not with dip rupture upward from the base of the seismogenic zone required more than twice the initial strain to cause fault growth and would result in very large earthquakes rupture downward in models associated with the higher gc gradient resulted in stable growth of the fault that is additional strain was needed to allow for continuous fault growth from the surface to the base of the seismogenic zone we suggest that fault nucleation and growth are a stable process which would preferentially occur by rupturing downward under stable conditions rerupturing of an established fault may initiate at depth or the surface depending on the degree of healing between rupture events and the tectonic stress available a vast majority of shallow crustal earthquakes nucleate at the base of the seismogenic zone and rupture upwards this suggests that most events are a reactivation of an old established fault and not growth of a new or developing fault