earthquake recurrence on the southern san andreas modulated by fault normal stress earthquake recurrence data from the pallett creek and wrightwood paleoseismic sites on the san andreas fault appear to show temporal variations in repeat interval these sites are located near cajon pass southern california where detailed mapping has revealed geomorphically and structurally expressed domains of alternating extension and contraction respectively associated with releasing and restraining bends of the san andreas fault we investigate the interaction between strike slip faults and auxiliary reverse and normal faults as a physical mechanism capable of producing such variations under the assumption that fault strength is a function of fault normal stress e g byerlee s law failure of an auxiliary fault modifies the strength of the strike slip fault thereby modulating the recurrence interval for earthquakes in our finite element model auxiliary faults are driven by stress accumulation near restraining and releasing bends of a strike slip fault earthquakes occur when fault strength is exceeded and are incorporated as a stress drop which is dependent on fault normal stress the model is driven by a velocity boundary condition over many earthquake cycles resulting synthetic strike slip earthquake recurrence data display temporal variations similar to observed paleoseismic data within time windows surrounding auxiliary fault failures although observed recurrence data for the two paleoseismic sites are too short to be definitive about the temporal variations or the physical mechanism responsible for it our simple model supports the idea that interaction between a strike slip fault and auxiliary reverse or normal faults can modulate the recurrence interval of events on the strike slip fault possibly producing short term variations in earthquake recurrence interval

holocene rate of slip and tentative recurrence interval for large earthquakes on the san andreas fault cajon pass southern california abstract detailed mapping of the san andreas fault zone where it crosses cajon creek southern california has revealed a number of late quaternary deposits and geomorphological features offset by the fault radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the san andreas fault four independent determinations of the slip rate on the san andreas fault yield an average rate of 24 5 3 5 mm yr for the past 14 400 years the similarity of the four values which span different intervals of time from 5900 to 14 400 years ago suggest that the slip rate has been constant during this period the slip rate confirms that the san andreas fault is accumulating slip faster than any other fault in the plate boundary throughout california also the sum of the slip rates on the san andreas 24 5 mm yr and the san jacinto 10 mm yr faults south of their junction is the same as the san andreas north of their junction 35 mm yr an excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 a d and tentative evidence for six earthquakes in about the last 1000 years both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries combined with the historic record this investigation indicates that the last major earthquake at cajon creek was probably at the beginning or middle of the 18th century while the record at cajon creek may suggest that the next major earthquake on the san andreas fault is overdue the data can be interpreted to indicate that the recurrence interval for earthquakes on the san andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century

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

strength asperities along oceanic transform faults a model for the origin of extensional earthquakes on the eltanin transform system abstract we use finite element simulations of the state of stress in the vicinity of the eltanin transform system to investigate the possible origin of earthquakes occurring on or in the immediate vicinity of transform segments and featuring normal faulting mechanisms with tension axes oriented approximately 45 away from the azimuth of plate motion in principle unexpected along transform faults in addition to solutions previously described in the literature we report on seven new such mechanisms including a large event with magnitude 6 in 2001 we model the transform segments as either weak i e featuring the same elastic properties as the ridge or strong i e featuring the same properties as the surrounding cooled oceanic lithosphere in addition we allow irregularities in the geometry of the transform contact in the form of either bends in the plate boundary jogs or asperities i e strong elements in an otherwise weak transform system we also consider the effect of oblique ridge push on the pacific plate we conclude that the seismic properties of the eltanin transform system are best described by the model of a weak transform in which a few strong asperities are embedded we speculate that their origin may be linked to the complex and recent history of readjustments in rates and directions of spreading notably to the south of the eltanin system

relationship between tectonism and volcanism along the great sumatran fault zone deduced by spot image analyses abstract satellite images provide evidence for numerous stepovers pull apart grabens and volcanic structures along the nw trending right lateral great sumatran fault zone high resolution spot images permit us to analyze the relationship between volcanic and tectonic structures this analysis reveals that releasing stepovers and pull aparts are ephemeral structures along the great sumatran fault zone and that the geometry of the strike slip fault evolves permanently through time the interpretative structural maps obtained from spot image analysis reveal that the formation of huge peculiarly shaped volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull apart basins are analogous to the circular ring faults of calderas in particular detailed study of the segments at the southernmost 150 km long termination of the great sumatran fault zone from northwest of ranau lake to semangka bay allows us to describe the development of the ranau collapse caldera this caldera is located within a pull apart basin bounded by a stepover its peculiar rectangular shape and its relatively large size about 200 km 2 are controlled by the evolution of the extinct ranau releasing stepover similarly the toba elliptical caldera one of the largest volcanic caldera in the world is elongated parallel to the present trace of the great sumatran fault and appears to be related to a releasing stepover associated with a wide pull apart basin that is not active at present

evolution of the stress field near the arava basin located along the dead sea fault system as revealed by joint sets abstract to reveal the development of the stress field near the arava basin located along the dead sea fault system we characterized joint sets that were exposed mainly in the santonian and eocene chalk rock outcropping in the study area in total 2600 strike orientations of single layer joints slj and multilayer joints mlj mirrored four main stress fields 1 hnw j1 326 2 hnnw j2 343 3 hnne j3 033 and 4 hn j4 360 the results showed that the relatively lower values of the fracture spacing ratio fsr and their standard deviations in j1 implied that j1 was formed during the burial stage of the eocene chalk formations the inconsistency between the statistical parameters of the log normal distributions of joint spacing and fsr raised doubt about the applicability of the saturation joint density model in our studied area the joint sets characteristics suggest that the regional hnw operated during the eocene during the miocene the regional stress field changed clockwise to hnnw probably in association with the early stage of transform motion along the dead sea fault in the pliocene pleistocene the hnne and hn local stress fields both operated and were accompanied by a tensional stress field that can be identified in the intensive normal faulting evolved along the western arava basin margins

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

modest movements spectacular fabrics in an intracontinental deep crustal strike slip fault striding athabasca mylonite zone nw canadian shield geometry and strain partitioning within lower crustal intraplate strike slip shear zones can be extremely complex compared with analogous structural levels of interplate strike slip shear zones sited at plate margins striding athabasca mylonite zone canadian shield is a spectacular ca 500 km long granulite facies continental intraplate shear zone the shear zone is composed of middle archean granulite facies annealed mylonites ca 3 13 ga and late archean ca 2 62 2 60 ga granulite facies ribbon mylonite belts which thread a sinuous course along a chain of crustal scale lozenges cored by relatively stiff rocks of mafic to intermediate composition to the northeast the mylonites form a n s trending 5 10 km thick dextral strike slip belt to the southwest this bifurcates into a pair of conjugate strike slip shear zones overlain by a contemporaneous dip slip shear zone striding athabasca mylonite zone was kinematically inefficient as a strike slip fault and cannot have accommodated large wallrock displacements nevertheless spectacular granulite facies ribbon mylonites were formed throughout the shear zone reflecting the very high temperatures ca 850 1000 c high recrystallization rate strain rate ratios and the transpressive nature of the deformation wk 1 possibly accommodated by significant volume loss by magma migration

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

the equivalent strength of geometrical barriers to earthquakes we present a quantitative framework for evaluating the influence of non planar fault geometry on repeated seismic ruptures we model quasistatic ruptures on a non planar fault trace imbedded in a two dimensional elastic medium under in plane strain because of the presence of fault segments that are not parallel to the regional shear stress i e bends the apparent strength at a given point on the fault is not fixed but fluctuates with normal stress compressional features behave as increasingly strong barriers to fracture unless the stored normal stress is released in order to unlock the fault since slip on the fault itself cannot get rid of the normal stress this is achieved through the action of off fault morphological features such as secondary faulting folding and vertical motions that we introduce parametrically in the form of an aseismic relaxation the apparent strength of a fault bend will stabilize in a narrow interval of values after repeated ruptures characterized by a non dimensional hardness parameter whereby the relaxation rate is scaled by the tectonic loading rate on a fault structure having several small widely separated bends three families of events can be identified whose frequency and magnitude depend on the hardness relaxation parameter and the geometry small events that cluster in the tension zones of the bends intermediate size ruptures involving a single interbend segment and large ruptures that break through bends and link on or more interbend segments large multi segment events are most likely to occur for low values of the hardness i e fast relaxation and slow loading rate regions with compressional features act as barriers that stop most ruptures stress is stored at these sites until they themselves break and initiate motion on the smoother long reaches of the fault

active faulting near the cajon pass well southern california implications for the stress orientation near the san andreas fault the dosecc cajon pass well is located 4 km ne of the san andreas fault in a region that includes a diversity of secondary structures active faults close to the well are subparallel to the right lateral san andreas fault but exhibit left lateral and normal slip the stress orientation measured in the well is not consistent with right lateral slip on the san andreas fault or the stresses inferred from focal mechanisms of earthquakes within 10 km of the fault but may drive the e striking slightly normal left lateral cleghorn fault the closest active fault to the well if the in situ stress is everywhere consistent with the highly variable styles of active secondary faulting like the cleghorn the orientation of stresses along the san andreas fault may be quite variable

anisotropy of magnetic susceptibility in diamagnetic limestones reveals deflection of the strain field near the dead sea fault northern israel abstract to exploit the potential of anisotropy of magnetic susceptibility ams as a tool to estimate the strain field around major faults we measured the ams of calcite bearing diamagnetic rocks that crop out next to the dead sea fault dsf in northern israel through integrated magnetic and geochemical methods we found that the rocks are almost pure calcite rocks and therefore the magnetic fabric is primarily controlled by preferred crystallographic orientation pco with the minimum principal ams axes k 3 parallel to calcite c axes we applied a separation procedure in several samples with high fe content in order to calculate the ams anisotropy parameters and compare them to pure diamagnetic rocks aarm thermo susceptibility curves and irm were used to characterize the magnetic phases we found that for fe content below 500 ppm the ams is mostly controlled by the diamagnetic phase and showed that differences in the degree of anisotropy p up to 3 p 1 005 to 1 023 and in anisotropy difference k up to 0 25 10 6 si in diamagnetic rocks are related to differences of strain magnitudes the spatial distribution of the magnetic fabrics indicates n s maximum shortening parallel to the strike of the hula western border fault hwbf one of the main strands of the dsf in northern israel the anisotropy parameters suggest that the strain magnitudes increase eastward with the proximity to the hwbf these results suggest that the strain field near the hwbf is locally deflected as a consequence of the dsf activity in light of the fault weakness model and geological setting of the study area we suggest that the area accommodates dominant transtension during the pleistocene the present study demonstrates the useful application of ams measurements in iron free limestones as recorders of the strain field near plate boundaries

stress distribution and failure in anisotropic rock near a bend on a weak fault large transform faults thin skinned thrust faults and listric normal faults often contain restraining and releasing bends that can alter the state of stress in the adjacent crust during faulting episodes crustal rock commonly has anisotropic mechanical properties due to the presence of sedimentary layering subsidiary fractures and faults schistosity or other foliation we present an analytical solution for the stress distribution in anisotropic rock produced by sliding on a wavy frictionless surface the frictionless surface represents the limiting case of a weak fault the fault shape treated is either a sinusoid or a periodic array of isolated restraining and releasing bends the rheological behavior of the rock is that of an incompressible linear viscous fluid with an orthotropic anisotropy characterized by a greater viscosity for shortening and extension than for shear in the principal directions of anisotropy our results illustrate that stress and flow associated with a bend in a fault will extend to much greater distances from the fault when the medium is anisotropic the magnitude of the stress perturbation increases with degree of anisotropy and decreases with radius of curvature of the fault surface principal stress directions tend to align parallel to the principal directions of anisotropy except in the immediate region of the fault bend mean stress and maximum shear stress magnitudes vary along the fault in a cellular manner with multiple maxima near a bend stress concentrations emanate from the bend and are elongate in directions parallel to the principal directions of anisotropy for isotropic rock locations and orientations of shear failure near restraining bends are distinctly different from those near releasing bends in contrast with application of an anisotropic failure criterion as the magnitude of anisotropy increases the patterns of shear failure at a restraining and a releasing bend become increasingly similar the model may help explain field observations from dip slip and strike slip regimes that indicate a complex stress history and local stress reorientation adjacent to a bend in a fault

paleostress inversion a multi parametric geomechanical evaluation of the wallace bott assumptions wallace 1951 and bott 1959 were the first to introduce the idea that the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface however this simplified hypothesis is questionable since fault mechanical interactions may induce slip reorientations earlier numerical geomechanical models confirmed that the slickenlines slip vectors are not necessarily parallel to the maximum resolved shear stress but are consistent with local stress perturbations this leads us to ask as to what extent the wallace and bott simplifications are reliable as a basis hypothesis for stress inversion from fault slip data here a geomechanical multi parametric study using a 3d boundary element method covering i fault geometries such as intersected faults or corrugated fault surfaces ii the full range of andersonian state of stress iii fault friction iv fault fluid pressure v half space effect and vi rock properties is performed in order to understand the effect of each parameter on the misfit angle between geomechanical slip vectors and the resolved shear stresses it is shown that significant misfit angles can be found under specific configurations invalidating the wallace and bott assumptions even though fault friction tends to minimize the misfit we therefore conclude that in such cases stress inversions based on fault slip data should be interpreted with care

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

attached or not attached evidence from crustal stress observations for a weak coupling of the vrancea slab in romania abstract the crustal stress pattern of romania provides key insights into whether the vrancea slab with its seismogenic volume between 70 and 175 km depth is still coupled to the crust and thus acts as a stress guide or whether it is already in a state of detachment from the crust knowledge of the state of the slab under vrancea is particularly critical because the slab attached to the crust can result in future strong earthquake occurrence in the crust and even in the currently aseismic zone between 40 km and 70 km depth potentially causing severe damage our analysis of the contemporary tectonic stress observations in the context of potential stress sources and the comparison with numerical modelling shows that the crustal stress pattern in romania is heterogeneous and does not contain a long wave length stress pattern that would be expected if there is a strong present day coupling between the subducted slab and the upper plate or if lateral plate boundary forces would control the regional stress pattern therefore we conclude that the crustal stress pattern of romania is characterised by small differential horizontal stresses where local stress sources third order effects are responsible for the observed heterogeneity of stress orientations and that the subducted slab under vrancea is only weakly coupled to the crust

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

stress near geometrically complex strike slip faults application to the san andreas fault at cajon pass southern california slip on an undulatory strike slip fault induces predictable residual stresses in the adjacent crust elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries across fault parallel planes domains of reverse normal right lateral and left lateral residual stresses are induced near the bends in the fault these residual stresses increase in magnitude from one slip event to the next typically reaching failure level after several events a two dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip induced stresses and fourier synthesis allows for the solution with arbitrary but small fault distortions maximum fault normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault reduced to 2 e times the maximum value at a distance equal to 1 wavenumber this fault parallel residual shear stress oscillates between dextral and sinistral along the fault with maximum magnitude adjacent to the maximum excursions in the fault trace fault parallel residual shear stress is maximum at a distance from the fault of 1 wavenumber where its magnitude is 1 e times the maximum normal stress on the fault two and three dimensional finite element analyses extend the analytic model they account for depth decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations application of the model to the san andreas fault in the cajon pass region produces the complex distribution of fault parallel normal reverse right lateral and left lateral structures recognized near the main trace left lateral stresses on planes subparallel to the san andreas fault at the cajon pass well are predicted by this model

in situ stress orientation near the san andreas fault preliminary results to 2 1 km depth from the cajon pass scientific drillhole stress induced wellbore breakouts were analyzed in the cajon pass scientific drillhole 4 3km ne of the san andreas fault to determine the orientation of the principal horizontal stresses using borehole televiewer digitized data in the depth interval 1750 2115 m the mean orientation of the maximum horizontal stress is 073 the scatter in the orientation of breakouts is as much as 37 and variations occur with wavelengths of few hundred meters in this interval reflecting either true changes in stress orientation with depth or the variability of rock anisotropy all the measured orientations however result in left lateral shear stress resolved onto planes parallel to the san andreas fault contrary to its post miocene right lateral slip of 300 km the stress orientation is consistent however with reported left lateral displacement across the cleghorn fault 3 km northeast of and possibly subparallel to the san andreas near cajon pass since the pleistocene the cajon pass stress orientation may reflect the current state of stress across the san andreas fault in which case it should change prior to right lateral slip on this fault or a shallow local perturbation to the san andreas stress field in which case it should rotate couterclockwise with depth determination of the stress orientations at greater depths in the cajon pass drillhole will better sample the near field of the san andreas fault and help resolve this question

slip heterogeneity on a corrugated fault abstract slip heterogeneity reflects the fundamental physics of earthquake rupture and has been attributed to strong fault patches termed asperities or barriers we propose that variations in fault surface orientation due to slip parallel corrugations may act as geometric asperities and barriers generating variations in incremental i e due to a single earthquake slip across a fault surface we evaluate this hypothesis using observations from the arkitsa normal fault exposure in central greece a scan of the arkitsa fault surface with 1 m spatial resolution and mm scale precision reveals corrugations made up of 1 5 m wide synforms antiforms and nearly planar fault sections with long axes that extend in the slip direction across the entire exposed surface the surface is thus more than an order of magnitude smoother in the direction of slip than in the perpendicular direction slip perpendicular profiles exhibit nearly self similar scaling across the range of observed wavelengths 2 50 m whereas slip parallel profiles are significantly smoother at shorter wavelengths

closing up structures alternatives to pull apart basins the effect of bends in the north anatolian fault turkey fault blocks passing bends or stepovers in a fault zone must adapt their margins to the uneven fault trace two cases of adaption are distinguished for extensional bends or stepovers transtension 1 the fault margins close up behind a single bend knickpoint of a strike slip fault and a closing up structure new term arises or 2 fault block margins are extended behind a releasing bend double bend or stepover parallel to the displacement and a pull apart basin originates the dosing up described here is accomplished by acute angled synthetic strike slip faults that dissect the straight fault in front of a knickpoint to form a zig zag block boundary behind it crustal extension is also involved in the closing up structure but in a different way from typical pull apart basins the closing up structure illustrated was developed behind an extensional knickpoint in the north anatolian fault west of lake abant nw turkey where the process of closing up continues to this day the kinematic model of this closing up structure is supported by displacements and ruptures observed during the 1967 mudurnu valley earthquake and the 1957 abant earthquake

what do the cajon pass stress measurements say about stress on the san andreas fault comment on in situ stress measurements to 3 5 km depth in the cajon pass scientific research borehole implications for the mechanics of crustal faulting by mark d zoback and john h healy

effect of surface morphology on the dissipation during shear and slip along a rock rock interface that contains a visco elastic core high resolution topography measurements of the vuache sillingy fault alps france reveal a characteristic roughness of the fault zone we investigate the effect of roughness on the rheology of a planar shear configuration by using a model system consisting of a visco elastic layer embedded into a rigid solid the model is discussed in the context of several geological cases a damage fault zone a fault smeared with a clay layer and a shear zone with strain weakening using both analytical approaches and finite element simulations we calculate to linear order the relation between wall roughness and the viscous dissipation in the fault zone as well as the average shear rate

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

constraining fault constitutive behavior with slip and stress heterogeneity 1 we study how enforcing self consistency in the statistical properties of the preshear and postshear stress on a fault can be used to constrain fault constitutive behavior beyond that required to produce a desired spatial and temporal evolution of slip in a single event we explore features of rupture dynamics that 1 lead to slip heterogeneity in earthquake ruptures and 2 maintain these conditions following rupture so that the stress field is compatible with the generation of aftershocks and facilitates heterogeneous slip in subsequent events our three dimensional finite element simulations of magnitude 7 events on a vertical planar strike slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the dynamic stress drop initial shear stress and breakdown work fracture energy are spatially heterogeneous in these models the breakdown work is on the order of mj m 2 which is comparable to the radiated energy these conditions producing slip heterogeneity also tend to produce narrower slip pulses independent of a slip rate dependence in the fault constitutive model an alternative mechanism for generating these confined slip pulses appears to be fault constitutive models that have a stronger rate dependence which also makes them difficult to implement in numerical models we hypothesize that self consistent ruptures could also be produced by very narrow slip pulses propagating in a self sustaining heterogeneous stress field with breakdown work comparable to fracture energy estimates of kj m 2

the influence of normal fault geometry on igneous sill emplacement and morphology magma flow within the subsurface is heavily influenced by the pre existing structure of the upper crust during continental rifting normal faults modify the geometry of igneous networks by providing preferential pathways for the intrusion of magma however the way in which magma intrudes into fault planes is poorly understood here we quantitatively document the relationship between fault architecture and intrusion distribution and geometry using three dimensional seismic reflection data from the exmouth sub basin offshore northwest australia inclined segments of saucer shaped sills intrude several faults along convex into the hangingwall fault plane corrugations we suggest that stress field perturbations associated with the fault plane corrugations provide suitable conditions for fault reactivation as magma conduits pre existing faults also modify sill geometries through the offset of stratigraphic horizons that may be preferentially intruded potentially resulting in the formation of a new sill or the development of minor intrusive steps this work emphasizes the importance of the pre existing structural template in controlling the growth and final geometry of intrusive networks

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

oceanic transform earthquakes with unusual mechanisms or locations relation to fault geometry and state of stress in the adjacent lithosphere on oceanic transforms most earthquakes are expected to occur on the principal transform displacement zone ptdz and to have strike slip mechanisms consistent with transform parallel motion we conducted a search for transform earthquakes departing from this pattern on the basis of source mechanisms and locations taken from the harvard centroid moment tensor catalogue and the bulletin of the international seismological centre respectively events with unusual mechanisms occur on several transforms we have determined the source mechanisms and centroid depths of 10 such earthquakes on the st paul s marathon owen heezen tharp menard and rivera transforms from inversions of long period body waveforms relative locations of earthquakes along these transforms have been determined with a multiple event relocation technique much of the anomalous earthquake activity on oceanic transforms is associated with complexities in the geometry of the ptdz or the presence of large structural features that may influence slip on the fault reverse faulting earthquakes occur at a compressional bend in the owen transform in the area of mount error and at the st paul s transform near st peter s and st paul s rocks a normal faulting earthquake on the heezen transform is located at the edge of a pull apart basin marking an extensional offset of the fault normal faulting earthquakes along the tharp menard and rivera transforms may also be related to extensional offsets some events with unusual mechanisms occur outside of the transform fault zone however and do not appear to be related to fault zone geometry for instance earthquakes with mechanisms indicating reverse faulting on ridge parallel fault planes are located near the ridge transform intersections of the st paul s and the marathon transforms possible additional contributors to the occurrence of anomalous earthquakes include recent changes in plate motion differential lithospheric cooling and the development of a zone of weakness along the fault zone but we do not find strong evidence to confirm the influence of these processes

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

what can gps data tell us about the dynamics of post seismic deformation summary this paper describes differences in time varying post seismic deformation due to after slip and viscoelastic relaxation following large strike slip earthquakes and how these differences may be exploited to characterize the configuration and rheology of aseismically deforming material in the subsurface the analysis involves two steps first near field time dependent post seismic deformation characteristics of a typical mw 7 4 strike slip earthquake is defined based on analysis of gps data from three recent earthquakes secondly this earthquake is modelled assuming uniform slip along a rectangular surface and several classes of after slip and viscoelastic relaxation models that can reproduce the evolution of early post seismic displacements with time at a near field reference point are developed postseismic displacements and velocities away from the reference point where the differences are greatest and thus most likely to be distinguished with gps are compared i find that displacements from a judiciously designed network of continuous or frequently occupied campaign mode gps sites are sufficiently precise to distinguish linear viscoelastic relaxation from after slip on a vertical surface extending the coseismic rupture furthermore both the thickness and viscosity of a relaxing linearly viscoelastic layer may be identified to maximize what post seismic gps surveys can tell us particularly concerning potential relaxation of low viscosity layers in the crust and or upper mantle some gps sites should be located along strike beyond the rupture tip also far field gps sites should be occupied as frequently as sites close to the rupture

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

determination of the tectonic stress tensor from slip along faults that obey the coulomb yield condition a new method to calculate the stress tensor associated with slip along a population of faults is derived here the method incorporates two constraints first the stresses in the slip direction satisfy the coulomb yield criterion and second the slip occurs in the direction of maximum shear stress along the fault the computations provide the complete stress tensor normalized by the vertical stress and evaluation of the mean coefficient of friction and the mean cohesion of the faults during the time of faulting the present method is applied to three field cases dixie valley nevada wadi neqarot israel and yuli microearthquakes taiwan it is shown that the coefficieet of friction of the three cases varies between 0 1 to 0 8 further it is demonstrated that previous stress inversion methods implicitly assume zero friction along the faults

aftershocks due to property variations in the fault zone a mechanical model abstract studies of many large strike slip earthquakes have shown that the seismicity rate increases in quadrants in which the coulomb stress is increased by the earthquake rupture this mechanism explains the concentration of aftershocks near the tips of the fault rupture but cannot explain the aftershocks on the sides of the rupture zone where the aftershocks are commonly abundant we suggest that these aftershocks may be induced by variations of fault zone properties not included in the previous models in a finite element model that treats the rupture plane as a zone with heterogeneous physical properties as suggested by field observations and laboratory experiments the coulomb stress changes triggered by the mainshock no longer uniformly decrease on the sides of the rupture zone but show complex patterns of negative and positive changes these results may explain some of the aftershocks on the sides of the mainshock rupture

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

a kinematic model of southern california we propose a kinematic model for southern california based on late quaternary slip rates and orientations of major faults in the region internally consistent motions are determined assuming that these faults bound rigid blocks relative to north america most of california west of the san andreas fault is moving parallel to the san andreas fault through the transverse ranges and not parallel to the motion of the pacific plate this is accomplished by counterclockwise rotation of california south of the san andreas fault and by the westward movement of central california north of the gar lock fault the velocities of the blocks are calculated along several paths in southern california that begin in the mojave desert and end off the california coast a path that crosses the western transverse ranges accumulates the accepted relative north america pacific plate velocity whereas paths to the north and south result in a significant missing component of motion this implies the existence of a zone of active deformation in southern california that is interpreted to include the western transverse ranges and northwest trending predominately strike slip faults close to the coast both north and south of the transverse ranges strain on this system accounts for about a third of the total north america pacific plate motion

nearly frictionless faulting by unclamping in long term interaction models in defiance of direct rock friction observations some transform faults appear to slide with little resistance in this paper finite element models are used to show how strain energy is minimized by interacting faults that can cause long term reduction in fault normal stresses unclamping a model fault contained within a sheared elastic medium concentrates stress at its end points with increasing slip if accommodating structures free up the ends then the fault responds by rotating lengthening and unclamping this concept is illustrated by a comparison between simple strike slip faulting and a mid ocean ridge model with the same total transform length calculations show that the more complex system unclamps the transforms and operates at lower energy in another example the overlapping san andreas fault system in the san francisco bay region is modeled this system is complicated by junctions and stepovers a finite element model indicates that the normal stress along parts of the faults could be reduced to hydrostatic levels after 60 100 k y of system wide slip if this process occurs in the earth then parts of major transform fault zones could appear nearly frictionless

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

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

fast deformation processes and eruptive activity at mount etna italy the seismic and deformation patterns observed on mount etna before and during the 1991 1993 eruption the third largest since the seventeenth century in terms of lava volume are consistent with the regional tectonic framework of eastern sicily the pattern of the stress field acting on the intermediate and lower crust was defined at the local scale by focal mechanisms of microearthquakes occurring at depths between 10 and 25 km beneath the volcano they provide evidence for a strike slip compressional stress regime with the maximum compressive component acting approximately n s the fault plane solutions and the spatial and temporal distribution of seismicity indicate that usually sinistral shear ruptures occur along approximately ne sw trending fault zones while dip slip ruptures affect approximately nnw sse trending fault zones the latter include the avenues along which magma ascended during the 1991 1993 eruption seismic observations indicate a local inversion of the stress field acting on the upper crust depth 10 km underlying mount etna which was initiated less than 2 months before the eruptive event and disappeared with its end this is consistent with a local tensile regime that favored the magma ascent through the shallow crust the events preceding the eruption and accompanying its onset tilt anomalies and seismic swarms and those occurring shortly after its beginning a mainshock aftershock seismic sequence and associated coseismic tilts provide important evidence for understanding the dynamics of the two main volcano tectonic structures ne sw and nnw sse trending fault zones and associated intrusive mechanisms on etna the shape and location of the eruption feeding dike have been modeled from ground deformation data this approximately nnw sse modeled dike the seismicity and the position of the fractures are consistent with the regional stress field characterized by 1 oriented approximately n s the geophysical data presented and analyzed in the present paper strongly suggest an overall regional tectonic control as well as an active role for the intruding magma in the dynamics of the volcano

significant lateral dip changes may have limited the scale of the 2015 mw 7 8 gorkha earthquake the 2015 mw 7 8 gorkha earthquake has drawn interest due to its complex fault geometry both geodetic and geologic studies have focused on the dip variations in this study we invert the coseismic geodetic data for the 2 d dip variations of the earthquake the best fit model confirms that the dip varies with depth and suggests that there is a significant lateral dip anomaly along strike the depth dependent dip variation suggests that the earthquake ruptured a ramp flat fault the shallow ramp may have prevented the rupture breaking through the surface in addition a lateral large dip anomaly is found in the northeastern corner of the slip area which supports the previous findings of inferred interseismic fault coupling coseismic high frequency radiations and the aftershock mechanisms this lateral dip anomaly is likely associated with local tearing within the indian slab it may have blocked the east southeastward rupture propagations of the gorkha earthquake implying important controls on the earthquake scale and the spatial limits of ruptures

predicted variation of stress orientation with depth near an active fault application to the cajon pass scientific drillhole southern california preliminary measurements of the stress orientation at a depth of 2 km in the cajon pass scientific drillhole about 3 6 km from the san andreas fault in southern california indicate that the local direction of maximum compression is nearly normal to the fault zone the measurements have been interpreted to indicate that the regional orientation of the maximum compression is normal to the fault and taken as evidence for a very weak fault the orientation of the regional maximum compression expected from plate tectonic arguments is about 66 ne from the strike of the fault geodetic data indicate that the orientation of maximum compressive strain rate is about 43 ne from the strike of the fault and show nearly pure right lateral shear acting parallel to the fault these apparent conflicts in the inferred orientation of the axis of maximum compression may be explained in part by a model in which the fault zone is locked over a depth interval in the range of 2 5 to 15 km but is very weak above and below that interval an analytic solution for this model predicts orientations of the principal stresses that are nearly parallel or normal to the fault for observations made close to the fault and above and below the locked zone but an orientation consistent with plate tectonic estimates at depths corresponding to that of the locked interval this solution does require however a few mm yr of creep at the surface on the san andreas or nearby sub parallel faults such as the san jacinto which has not yet been observed or a shallow zone near the faults of distributed deformation

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

earthquake recurrence time variations with and without fault zone interactions summary the macroscopic behaviour of a fault zone can be described by a classic coulomb friction law and static kinetic rheology the critical failure stress leading to the activation of a single shear zone segment then depends on the normal stress acting on the future fault plane it is known that the seismic cycle on a main fault patch can be modulated by surrounding fault activations leading to interaction with the main segment or by other external stress perturbations the results presented here stem from analytical assessments and 2 d numerical elastic models they show that an earthquake recurrence time variation with continuing deformation system development is not necessarily connected to fault interaction processes under the above assumptions instead aperiodicity can result from a non ideal orientation of the rupture plane in the simplest possible homogeneous stress eld even when constant loading rate and frictional parameters are assumed this eiect can still be found in the multifault geometries we studied where the interaction of segments determines the seismicity patterns of the system to varying extents our results suggest that a stable earthquake periodicity on non changing fault sets will seldom be found in nature we propose a reconsideration of the ideas leading to a hypothesized steady seismic cycle in earthquake prediction models

the morphology of strike slip faults examples from the san andreas fault california fault zones are seldom straight nor are fractures within them continuous permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip these strain fields may be manifest as vertical deformation near the fault strain fields that increase area result in subsidence and strain fields that reduce area result in uplift although these effects are modified by erosion and deposition the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike graphical views of strain fields associated with simple vertical strike slip faults are derived using boundary element methods if the fault is straight and parallel to the applied shear strain the hills and valleys resulting from finite fault rupture are separated by the fault trace this is rarely observed in nature instead strike slip faults cut through hills and valleys apparently of their own making we show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments we test our theory with examples from parts of the san andreas fault system in california the morphologies of the parkfield region and the southern san andreas fault in the coachella valley are shown to be consistent with plausible distributions of slip on mapped surface faults the location of the salton sea appears to be a consequence of a reduction of slip on the san andreas fault toward the brawley seismic zone

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

active faulting geometry and stress pattern near complex strike slip systems along the maghreb region constraints on active convergence in the western mediterranean the maghreb region from tunisia to gibraltar is a key area in the western mediterranean to study the active tectonics and stress pattern across the africa eurasia convergent plate boundary in the present study we compile comprehensive data set of well constrained crustal stress indicators from single focal mechanism solutions formal inversion of focal mechanism solutions and young geologic fault slip data based on our and published data analyses stress inversion of focal mechanisms reveals a first order transpression compatible stress field and a second order spatial variation of tectonic regime across the maghreb region with a relatively stable s hmax orientation from east to west therefore the present day active contraction of the western africa eurasia plate boundary is accommodated by 1 e w strike slip faulting with reverse component along the eastern tell and saharan tunisian atlas 2 a predominantly ne trending thrust faulting with strike slip component in the western tell part and 3 a conjugate strike slip faulting regime with normal component in the alboran rif domain this spatial variation of the present day stress field and faulting regime is relatively in agreement with the inferred stress information from neotectonic features according to existing and newly proposed structural models we highlight the role of main geometrically complex shear zones in the present day stress pattern of the maghreb region then different geometries of these major inherited strike slip faults and its related fractures v shaped conjugate fractures horsetail splays faults and riedel fractures impose their component on the second and third order stress regimes neotectonic and smoothed present day stress map mean s hmax orientation reveal that plate boundary forces acting on the africa eurasia collisional plates control the long wavelength of the stress field pattern in the maghreb the current tectonic deformations and the upper crustal stress field in the study area are governed by the interplay of the oblique plate convergence i e africa eurasia lithosphere mantle interaction and preexisting tectonic weakness zones

rough faults distributed weakening and off fault deformation 1 we report systematic spatial variations in fault rocks along nonplanar strike slip faults cross cutting the lake edison granodiorite sierra nevada california sierran wavy fault and lobbia outcrops of the adamello batholith in the italian alps lobbia wavy fault in the case of the sierran fault pseudotachylyte formed at contractional fault bends where it is found as thin 1 2 mm fault parallel veins epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends we argue that the presence of fluids as illustrated by this example does not necessarily preclude the development of frictional melt in the case of the lobbia fault pseudotachylyte thickness varies along the length of the fault but the pseudotachylyte veins thicken and pool in extensional bends we conduct a quantitative analysis of fault roughness microcrack distribution stress and friction along the lobbia fault numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness resulting in an overall weaker fault than suggested by the corresponding static friction coefficient the models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip rather than being confined to the region close to the dynamically propagating crack tip

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

the work budget of rough faults abstract faults in nature have measurable roughness at many scales and are not planar as generally idealized we utilize the boundary element method to model the geomechanical response of synthetic rough faults in an isotropic linear elastic continuum to external tectonic loading in terms of the work budget faults are generated with known fractal roughness parameters including the root mean square slope a measure of roughness amplitude and the hurst exponent h a measure of geometric self similarity energy within the fault models is partitioned into external work w ext internal elastic strain energy w int gravitational work w grav frictional work w fric and seismic energy w seis results confirm that w ext or work done on the external model boundaries is smallest for a perfectly planar fault and steadily increases with increasing this pattern is also observed in w int the energy expended in deforming the host rock the opposite is true for gravitational work or work done against gravity in uplifting host rock as well as with frictional work or energy dissipated with frictional slip on the fault and w seis or seismic energy released during slip events effects of variation in h are not as large as for but w grav w fric and w seis increase with increasing h with w int and w ext decreasing across the same range remarkably however for a narrow range of roughness amplitudes which are commonly observed along natural faults the total work of the system remains approximately constant while slightly larger than the total work of a planar fault faults evolve toward the most mechanically efficient configuration therefore we argue that this range of roughness amplitudes may represent an energy barrier preventing faults from removing asperities and evolving to smooth planar discontinuities a similar conclusion is drawn from simulations at relatively shallow depths with results showing that shallower faults have larger energy barriers and can be mechanically efficient at higher roughness amplitudes

the velocity field along the san andreas fault in central and southern california the velocity field within a 100 km broad zone centered on the san andreas fault between the mexican border and san francisco bay has been inferred from repeated surveys of trilateration networks in the 1973 1989 interval the velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the san andreas fault in the transverse ranges of southern california across strike profiles of the fault parallel component of velocity exhibit the expected sigmoidal shape whereas across strike profiles of the fault normal component of velocity are flat and featureless no significant convergence upon the fault is observed even along the big bend sector of the fault simple dislocation models can explain most of the features of the observed velocity field but those explanations are not unique about 35 mm yr of relative plate motion is accounted for within the span of the trilateration networks geologic studies indicate that the secular slip rate on the san andreas fault is about 35 mm yr the agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes the relative motion observed across the san andreas fault 35 mm yr plus that observed across the eastern california shear zone 8 mm yr accounts for most 43 mm yr of the observed north america pacific relative plate motion 47 mm yr