activity of the offshore newport inglewood rose canyon fault zone coastal southern california from relocated microseismicity an offshore zone of faulting approximately 10 km from the southern california coast connects the seismically active strike slip newport inglewood fault zone in the los angeles metropolitan region with the active rose canyon fault zone in the san diego area relatively little seismicity has been recorded along the offshore newport inglewood rose canyon fault zone although it has long been suspected of being seismogenic active low angle thrust faults and quaternary folds have been imaged by seismic reflection profiling along the offshore fault zone raising the question of whether a through going active strike slip fault zone exists we applied a waveform cross correlation algorithm to identify clusters of microseismicity consisting of similar events analysis of two clusters along the offshore fault zone shows that they are associated with nearly vertical north northwest striking faults consistent with an offshore extension of the newport inglewood and rose canyon strike slip fault zones p wave polarities from a 1981 event cluster are consistent with a right lateral strike slip focal mechanism solution manuscript received 23 july 2003

block models of crustal motion in southern california constrained by gps measurements 1 we estimate slip rates on major active faults in southern california using a block model constrained by global positioning system measurements of interseismic deformation the block model includes the effects of block rotation and elastic strain accumulation consistent with a simple model of the earthquake cycle our estimates of the right lateral strike slip rate on the san andreas fault vary by at least a factor of 5 from a high of 35 9 0 5 mm yr in the carrizo plain to a low of 5 1 1 5 mm yr through the san bernadino segment shortening across the puente hills thrust and left lateral slip on the raymond hill fault are consistent with both thickening and escape tectonics in the los angeles basin discrepancies between geodetic and geologic slip rate estimates along the san andreas and san jacinto faults as well as in the eastern california shear zone may be explained by a temporal change in fault system behavior we find no substantial evidence for long term postseismic relaxation and infer that the viscosity of the lower crust upper mantle may be relatively high 1019 pa s

present day strain accumulation and slip rates associated with southern san andreas and eastern california shear zone faults 1 we present the first results from a dense network of 36 campaign and 46 continuous gps stations located in the eastern transverse ranges province etr a transition zone between the southernmost san andreas fault ssaf and eastern california shear zone ecsz we analyzed the campaign data together with available data from continuous gps stations for the period 1994 2009 we used the gps velocity estimates to constrain elastic block models to investigate fault loading rates representing four hypotheses characterized by different fault block geometries fault block scenarios include blocks bounded by the east striking left lateral pinto mountain blue cut and chiriaco faults of the etr blocks bounded by a right lateral north northwest striking structure the landers mojave earthquake line that cuts obliquely across the etr and mapped mojave desert faults and combinations of these end member hypotheses each model implies significantly different active fault geometries block rotation rates and slip rates for etr and ecsz structures all models suggest that ssaf slip rate varies appreciably along strike generally consistent with rates derived from tectonic geomorphology and paleoseismology with a maximum of 23 mm yr right lateral along the southernmost coachella valley strand decreasing systematically to 10 mm yr right lateral through the san gorgonio pass region slip rate estimates for the san jacinto fault are 12 mm yr for all models tested all four models fit the data equally well in a statistical sense qualitative comparison among models and consideration of geologic slip rates and other independent data reveals strengths and weaknesses of each model

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

how fault evolution changes strain partitioning and fault slip rates in southern california results from geodynamic modeling in southern california the pacific north america relative plate motion is accommodated by the complex southern san andreas fault system that includes many young faults 2 ma the initiation of these young faults and their impact on strain partitioning and fault slip rates are important for understanding the evolution of this plate boundary zone and assessing earthquake hazard in southern california using a three dimensional viscoelasto plastic finite element model we have investigated how this plate boundary fault system has evolved to accommodate the relative plate motion in southern california our results show that when the plate boundary faults are not optimally configured to accommodate the relative plate motion strain is localized in places where new faults would initiate to improve the mechanical efficiency of the fault system in particular the eastern california shear zone the san jacinto fault the elsinore fault and the offshore dextral faults all developed in places of highly localized strain these younger faults compensate for the reduced fault slip on the san andreas fault proper because of the big bend a major restraining bend the evolution of the fault system changes the apportionment of fault slip rates over time which may explain some of the slip rate discrepancy between geological and geodetic measurements in southern california for the present fault configuration our model predicts localized strain in western transverse ranges and along the dextral faults across the mojave desert where numerous damaging earthquakes occurred in recent years

constancy of geologic slip rate along the central garlock fault implications for strain accumulation and release in southern california summary we present new slip rate estimates for the central garlock fault the incised edge of an alluvial fan deposit with a 10be cosmogenic radionuclide age of 13 3 5 9 1 1 ka is offset 70 7 m yielding a slip rate of 5 3 1 0 2 5 mm yr 1 a stream thalweg that is deeply incised into this surface is offset a lesser amount 58 4 m providing the lower bound for this rate climate considerations however suggest that the deep incision of this thalweg may have occurred at a younger date specifically at the end of the younger dryas period at 11 5 ka or at the onset of a summer monsoon rainfall pattern at 8 10 ka if the deep incision was related to one of these climate events then the resulting slip rate would be 5 1 0 3 and 6 6 1 2 mm yr 1 respectively all of the slip rates are consistent with previous geologic slip rates most of which suggest a latest pleistocene holocene slip rate of 5 7 mm yr 1 these geologic slip rates however contrast markedly with much slower at most half as fast short term rates of elastic strain accumulation constrained by geodesy the new slip rate thus adds to a growing body of evidence indicating that the garlock fault is experiencing a period of transient lack of strain accumulation in which the lower crust and mantle beneath the fault is deforming at a rate that is much slower than its long term average rate these observations suggest that the garlock fault experiences two modes of strain accumulation the current slow mode in which strain accumulates very slowly along the fault and fast mode during which the fault must store and release elastic strain energy at much faster rates than the long term average to account for the relatively rapid geologic slip rates measured along the central part of the fault

a numerical study of strike slip bend formation with application to the salton sea pull apart basin how stepovers of strike slip faults connect to form bends is a question important for understanding the formation of push up ranges restraining bends and pull apart basins releasing bends we investigated the basic mechanics of this process in a simple three dimensional viscoelastoplastic finite element model our model predicts localized plastic strain within stepovers that may eventually lead to the formation of strike slip bends major parameters controlling strain localization include the relative fault strength geometry of the fault system and the plasticity model assumed using the drucker prager plasticity model in which the plastic yield strength of the crust depends on both shear and normal stresses our results show that a releasing bend is easier to develop than a restraining bend under similar conditions these results may help explain the formation of the salton sea pull apart basin in southern california 0 5 0 1 ma ago when the stepover between the imperial fault and the san andreas fault was connected by the brawley seismic zone

block kinematics of the pacific north america plate boundary in the southwestern united states from inversion of gps seismological and geologic data 1 the active deformation of the southwestern united states 30 41 n is represented by a finite number of rotating elastic plastic spherical caps gps derived horizontal velocities geologic fault slip rates transform fault azimuths and earthquake derived fault slip vector azimuths are inverted for block angular velocities creep on block bounding faults permanent strain rates within the blocks and the rotations of 11 published gps velocity fields into to a common north american reference frame gps velocities are considered to be a combination of rigid block rotations recoverable elastic strain rates resulting from friction on block bounding faults and nonrecoverable strain rates resulting from slip on faults within the blocks the resulting pacific north america angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the gulf of california earthquake slip vectors in the gulf of california and alaska and gps velocities along coastal california and within the pacific basin published fault slip rates are satisfied except in the southern mojave desert where the motion of the mohave block relative to north america is faster than can be explained by mapped faults the largest blocks the sierra nevada great valley and the eastern basin and range show permanent strain rates after removing elastic strain of only a few nanostrain per year demonstrating approximately rigid behavior observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short term strains evident in gps velocities are largely elastic in only about 20 of the region is distributed deformation needed to match the data indicating that a plate tectonic style description of the deformation of the western united states is plausible most blocks rotate about vertical axes at approximately the same rate as the pacific relative to north america suggesting that locally spin rates are communicated from block to block arguing against both floating block and ball bearing mechanisms of block rotation the similarities of the blocks spin rates to that of the pacific suggests that the pacific strongly influences their motions through edge tractions however it is shown that the blocks cannot rotate about the pacific north america pole without spinning counter to the sense of pacific north america shear unlike some other broad plate boundaries in the western united states vertical axis rotations take up very little of the slip rate budget across the region

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

slip rates and off fault deformation in southern california inferred from gps data and models 1 in southern california fault slip rate estimates along the san andreas fault saf and garlock fault from geodetically constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates the sum of geodetic model slip rates across the eastern california shear zone is higher than the geologic sum however the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real or attributable to epistemic uncertainties in the two types of estimates we further examine uncertainties in geodetically derived slip rate estimates on major faults in southern california by conducting a suite of inversions with four kinematic models long term rigid elastic block models constrained by the geologic slip rates cannot fit the present day gps derived velocity field deforming permanent off fault strain elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present day gps derived velocity field with 28 33 of the total geodetic moment rate occurring as distributed deformation off of the major faults models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the major southern california faults with ranges of elastic viscoelastic 29 34 30 37 mm yr for the carrizo saf segment 20 24 20 32 mm yr for the mojave saf segment 14 17 18 22 mm yr for the coachella saf segment 13 19 14 22 mm yr for the san jacinto fault and 5 11 5 11 mm yr for the western garlock fault

the central and southern elsinore fault zone southern california we examine hypocenter patterns geomorphic features total slip determinations and slip rate information to define the active elements of the southern and central portions of the right lateral elsinore fault zone in southern california in addition to the elsinore fault the seismicity patterns define a continuous fault between about 33 n and 33 5 n 7 to 12 km east of the elsinore fault that corresponds to the aguanga agua tibia and earthquake valley faults this part of the fault zone is double stranded total slip and current slip rate are apparently distributed between the two strands slip transfer between the strands occurs at the ends of the eastern strand over two steps with topographic signatures reflecting the step geometries in a right lateral fault zone a contractional left step occupied by palomar mountain and an extensional right step within the vallecito fish creek basin we suggest that the two strands of the double stranded portion of the fault zone have different ages and the proportion of slip carried by each strand may have changed through time hypocenter patterns define several northeast striking faults within the fault zone we attribute a 1940 ml 5 1 earthquake to one of them

stress modulation on the san andreas fault by interseismic fault system interactions during the interseismic phase of the earthquake cycle between large earthquakes stress on faults evolves in response to elastic strain accumulation driven by tectonic plate motions because earthquake cycle processes induce non local stress changes the interseismic stress accumulation rate on one fault is influenced by the behavior of all nearby faults using a geodetically constrained block model we show that the total interseismic elastic strain field generated by fault interactions within southern california may increase stressing rates on the mojave and san bernardino sections of the san andreas fault within the big bend region by as much as 38 relative to estimates from isolated san andreas models assuming steady fault system behavior since the c e 1857 fort tejon earthquake shear stress accumulated on these sections due only to interaction with faults other than the san andreas reaches 1 mpa 3 times larger than the coseismic and postseismic stress changes induced by recent southern california earthquakes stress increases along big bend sections coincide with the greatest earthquake frequency inferred from a 1500 yr long paleoseismic record and may affect earthquake recurrence intervals within geometrically complex fault systems including the sections of the san andreas fault closest to metropolitan los angeles

seismotectonics of the northern elsinore fault zone southern california earthquake hypocenters and focal mechanisms within and adjacent to the elsinore temecula trough are used to determine the contemporary pattern of subsurface deformation and fault geometry along the northern elsinore fault zone instrumentally recorded earthquakes with magnitudes m l 1 5 1979 to 1989 were relocated using a velocity model derived from previous seismic refraction and velocity inversion studies in the peninsular ranges single event focal mechanisms were determined for all events with at least 10 p wave first motions our preferred hypocentral locations combined with gravity modeling and additional geologic information suggest considerable curvature and oblique motion along the glen ivy north fault the fault at depth curves westwards away from the more linear surface trace and changes dip from near vertical at lake elsinore to 70 s and then 70 to 75 sw along the base of the santa aria mountains along the wildomar and willard faults earthquakes tend to cluster and focal mechanisms exhibit a mixed pattern of strike and oblique sup motion particularly near the extrapolated intersection with the e w oriented murrieta hot springs fault distinct aseismic zones extending to depths of 10 km correlate with the two known areas of active subsidence in the elsinore temecula trough average p axis from earthquake focal mechanisms for the 74 events analyzed both on and off the fault zone trends n01 w 17 or about 45 to 55 from the various nw striking segments of the northern elsinore fault zone the observed pattern of oblique slip and nonvertical dips along active segments of the northern elsinore fault zone is consistent with their young age 2 5 ma and their small cumulative horizontal offset 15 km and with a model in which oblique strain has yet to be partitioned between vertical strike slip and subparallel low angle dip slip faults as is observed along segments of the san andreas fault zone in central california

magnetostratigraphy and biochronology of the san timoteo badlands southern california with implications for local pliocene pleistocene tectonic and depositional patterns the san timoteo badlands expose nearly 2000 m of nonmarine sedimentary rocks ideally situated temporally and spatially to address questions regarding major tectonic events that occurred in southern california from latest miocene to medial pleistocene time these events include development of the san andreas san jacinto fault system and uplift of the transverse ranges the badlands sequence also spans three of the four most recent north american land mammal ages and therefore coincides with important trans beringian and neotropical mammal dispersal events no other terrestrial sequence in north america records relatively continuous deposition over this interval of time magnetostratigraphy and mammalian biochronology provide a temporal framework that allows greater resolution of the geologic and biotic events recorded within the sequence than previous studies on this region data show that the sequence extends from chron c3an 2n ca 6 3 ma at the lowest part of the section in the study area to within the brunhes chron or younger than 0 78 ma near the top refined age estimates for geologic and biotic events recorded within the sequence indicate that 1 a peninsular ranges basement source south of the badlands region dominated deposition from before 6 3 ma to about 4 6 ma after which time the provenance shifted to a san gabriel mountains basement source from the north 2 deposition of san gabriel mountains type material continued until as recently as ca 0 70 ma although sediments from the san bernardino mountains appeared in the northwesternmost area of the badlands ca 1 5 ma 3 the late hemphillian mount eden local fauna is dated as 5 6 ma compared with previous estimates of 5 0 5 4 ma 4 the early irvingtonian el casco local fauna is 1 3 1 4 ma and 5 the shutt ranch local fauna is ca 0 7 0 99 ma paleomagnetic data show that the badlands region may be slightly rotated in a counterclockwise direction contrary to previous models predicting clockwise rotation in right lateral shear zones

variable rates of late quaternary strike slip on the san jacinto fault zone southern california three strike slip displacements of strata with known approximate ages have been measured at two locations on the san jacinto fault zone minimum horizontal offset between 5 7 and 8 6 km in no more than 0 73 m y northeast of anza indicates 8 12 mm yr average slip rate since late pleistocene time two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the coyote creek fault in western imperial valley horizontal slip of 1 7 m has been calculated for the youngest sediment of lake cahuilla since its deposition 275 510 yr b p the corresponding slip rate is 2 8 5 0 mm yr right lateral offset of 10 9 m measured on a buried stream channel older than 5060 yr b p but younger than 6820 yr b p yields average slip rates for the intermediate time periods 400 to 6000 yr b p of 1 2 mm yr the average rates of slip for these three time intervals suggest a relatively quiescent period for the san jacinto fault zone from about 4000 b c to about 1600 a d to the extent that long term variations in seismic activity of major strike slip faults elsewhere are known fluctuations in slip rate for the san jacinto fault would not appear to be abnormal if the san jacinto and adjacent segments of the san andreas fault alternately assume dominant roles in absorbing motion between the pacific and american plates perhaps even more recently than 400 yr ago the san andreas fault south of the transverse ranges expressed most of the motion but has since become relatively inactive

a 3 d viscoelastoplastic model for simulating long term slip on non planar faults summary the geometrical complexity of non planar faults is known to affect not only fault slip rates but also crustal deformation and seismicity in the surrounding region but implementing non planar faults in numerical models has been challenging we have developed a 3 d viscoelastoplastic finite element model to simulate long term steady state fault slip along conceptual restraining bends and crustal deformation around them by incorporating plastic yielding in both fault zones and the surrounding crust this model can predict the quasi steady state stress field and strain partitioning while avoiding pathological stress build up encountered in traditional viscoelastic models here we detail the formulation of this model and explore major model parameters the model results show that restraining bends tend to impede fault slip increase shear stress in the surrounding crust and localize strain in a pair of belts off the fault zone these effects are enhanced by high viscosity in the lower crust and upper mantle high aspect ratio of bend width over fault length fast relative motion of fault blocks and high fault friction these model results may help explain the diffuse deformation surrounding the big bend of the san andreas fault and the more confined deformation around the lebanon bend of the dead sea fault

a fault based model for crustal deformation fault slip rates and off fault strain rate in california we invert global positioning system gps velocity data to estimate fault slip rates in california using a fault based crustal deformation model with geologic constraints the model assumes buried elastic dislocations across the region using uniform california earthquake rupture forecast version 3 ucerf3 fault geometries new gps velocity and geologic slip rate data were compiled by the ucerf3 deformation working group the result of least squares inversion shows that the san andreas fault slips at 19 22 mm yr along santa cruz to the north coast 25 28 mm yr along the central california creeping segment to the carrizo plain 20 22 mm yr along the mojave and 20 24 mm yr along the coachella to the imperial valley modeled slip rates are 7 16 mm yr lower than the preferred geologic rates from the central california creeping section to the san bernardino north section for the bartlett springs section fault slip rates of 7 9 mm yr fall within the geologic bounds but are twice the preferred geologic rates for the central and eastern garlock inverted slip rates of 7 5 and 4 9 mm yr respectively match closely with the geologic rates for the western garlock however our result suggests a low slip rate of 1 7 mm yr along the eastern california shear zone and southern walker lane our model shows a cumulative slip rate of 6 2 6 9 mm yr across its east west transects which is 1 mm yr increase of the geologic estimates for the off coast faults of central california from hosgri to san gregorio fault slips are modeled at 1 5 mm yr similar to the lower geologic bounds for the off fault deformation the total moment rate amounts to 0 88 1019 n m yr with fast straining regions found around the mendocino triple junction transverse ranges and garlock fault zones landers and brawley seismic zones and farther south the overall california moment rate is 2 76 1019 n m yr which is a 16 increase compared with the ucerf2 model online material table of geological slip rates

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

paleoseismology and global positioning system earthquake cycle effects and geodetic versus geologic fault slip rates in the eastern california shear zone published slip rates for the owens valley fault zone in eastern california based on geodetic data and elastic half space models 5 7 mm yr are faster than longer term geologic rates 2 3 mm yr we use global positioning system data spanning the central owens valley a more realistic rheological model with an elastic upper crust over a viscoelastic lower crust and upper mantle and paleoseismic data from adjacent faults to show that this difference could reflect earthquake cycle effects we estimate a long term rate 2 1 0 7 mm yr and earthquake recurrence interval 2750 350 1000 yr from the geodetic data both in agreement with independent geologic estimates

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

geodetic slip rates in the southern san andreas fault system effects of elastic heterogeneity and fault geometry 1 we use high resolution interferometric synthetic aperture radar and gps measurements of crustal motion across the southern san andreas fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern san andreas and san jacinto faults two possibilities have been proposed to explain this observation large contrasts in crustal rigidity across the faults or an alternate fault geometry such as a dipping san andreas fault or a blind segment of the san jacinto fault we evaluate these possibilities using a two dimensional elastic model accounting for heterogeneous structure computed from the southern california earthquake center crustal velocity model cvm h 6 3 the results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates however we find that small changes in the location of faults at depth can strongly impact the results our preferred model includes a san andreas fault dipping northeast at 60 and two active branches of the san jacinto fault zone in this case we infer nearly equal slip rates of 18 1 and 19 2mm yr for the san andreas and san jacinto fault zones respectively these values are in good agreement with geologic measurements representing average slip rates over the last 10 10 years implying steady long term motion on these faults

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

stable rapid rate of slip since inception of the san jacinto fault california 1 in california where the san jacinto fault sjf and san andreas fault saf accommodate the majority of the dextral shear deformation between the pacific and north american plates initiation of the sjf led to an apparent decline in the slip rate of the saf previous studies suggest that since then slip rate has covaried between these faults possibly due to changes in fault strength variation in topographic loading along a fault or the development of new faults and that presently the sjf is the dominant plate boundary structure however we dated displaced sedimentary deposits and landforms over three distinct time intervals since 700 ka and our results imply a constant slip rate of 12 1 3 4 2 6 mm yr this rate is similar to the fault s lifetime rate and from rates derived from geodesy suggesting that since the sjf initiated its slip rate has remained relatively stable and does not exceed that of the saf

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

transient strain accumulation and fault interaction in the eastern california shear zone satellite synthetic aperture radar interferometry reveals transient strain accumulation along the blackwater little lake fault system within the eastern california shear zone the surface strain map obtained by averaging eight years 1992 2000 of earth resource satellite ers radar data shows a 120 km long 20 kmwide zone of concentrated shear between the southern end of the 1872 owens valley earthquake surface break and the northern end of the 1992 landers earthquake surface break the observed shear zone is continuous through the garlock fault which does not show any evidence of left lateral slip during the same time period a dislocation model of the observed shear indicates right lateral slip at 7 6 3 mm yr on a vertical fault below 5 km depth a rate that is two to three times greater than the geologic rates estimated on northwest trending faults in the eastern mojave area this transient slip rate and the absence of resolvable slip on the garlock fault may be the manifestation of an oscillatory strain pattern between interacting conjugate fault systems

geometrical impact of the san andreas fault on stress and seismicity in california 1 most large earthquakes in northern and central california clustered along the main trace of the san andreas fault saf the north american pacific plate boundary however in southern california earthquakes were rather scattered here we suggest that such along strike variation of seismicity may largely reflect the geometrical impact of the saf using a dynamic finite element model that includes the first order geometric features of the saf we show that strain partitioning and crustal deformation in california are closely related to the geometry of the saf in particular the big bend is shown to reduce slip rate on southern saf and cause high shear stress and strain energy over a broad region in southern california and a belt of high strain energy in the eastern california shear zone

slip rate of the western garlock fault at clark wash near lone tree canyon mojave desert california the precise tectonic role of the left lateral garlock fault in southern california has been controversial three proposed tectonic models yield significantly different predictions for the slip rate history orientation and total bedrock offset as a function of distance along strike in an effort to test these models we present the first slip rate estimate for the western garlock fault that is constrained by radiocarbon dating a channel referred to here as clark wash incised into a latest pleistocene alluvial fan has been left laterally offset at least 66 6 m and no more than 100 m across the western garlock fault indicating a left lateral slip rate of 7 6 mm yr 95 confidence interval of 5 3 10 7 mm yr using dendrochronologically calibrated radiocarbon dates the timing of aggradational events on the clark wash fan corresponds closely to what has been documented elsewhere in the mojave desert suggesting that much of this activity has been climatically controlled the range front fault located a few hundred meters northwest of the gar lock fault has probably acted primarily as a normal fault with a holocene rate of dip slip of 0 4 0 7 mm yr the record of prehistoric earthquakes on the garlock fault at this site though quite possibly incomplete suggests a longer interseismic interval 1200 2700 yr for the western garlock fault than for the central garlock fault the relatively high slip rate determined here indicates that the western and central segments of the garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data the high rate of motion on the western garlock fault is most consistent with a model in which the western garlock fault acts as a conjugate shear to the san andreas fault other mechanisms involving extension north of the garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault but they cannot explain a high rate of slip on the western garlock fault

incorporating and reporting uncertainties in fault slip rates 1 quantitative slip rate estimates are essential to understanding crustal deformation processes and assessing seismic hazard computing slip rates requires two fundamental ingredients estimates of the age of an offset landforrn or deposit and displacement along the fault of interest because both of these measures contain uncertainty slip rates are inherently uncertain methods to compute and report slip rates have not been standardized and therefore slip rate data are presented inconsistently and are frequently ambiguous in particular slip rate uncertainty is often insufficiently characterized we present a rigorous probabilistic approach to computing and reporting intermediate to long term fault slip rates additionally we have developed freely available software that provides standard age and displacement uncertainty models we demonstrate the method using recent observations from the neodani and death valley fish lake valley fault zones and we compare slip rates determined using this approach with previously published results

role of the continental margin in the global carbon balance during the past three centuries the global continental margins are important sites of deposition and regeneration of terrestrial organic carbon and of calcium carbonate produced in situ the material balance of organic carbon in the coastal zone determines to a large extent its role as either a source or sink of co2 relative to the atmosphere we explore the role of the continental margin in the global cycling of carbon during the recent geologic past using a new model totem terrestrial ocean atmosphere ecosystem model we conclude that during the past 300 yr the coastal zone has been a site of relatively stable calcium carbonate deposition as well as a site of greater input recycling and storage of terrestrial organic carbon as a result more organic carbon in the coastal zone is remineralized than produced in situ by photosynthesis i e the coastal zone is net heterotrophic and this reduces its sink strength for anthropogenic co2 continuation of this trend in the future will weaken the ability of the coastal zone to act as a sink for the rising anthropogenic co2 in the atmosphere reversal of the carbon biogeochemical balance from net remineralization heterotrophy to net production autotrophy may occur in those continental margin sections where bioproduction is enhanced by elevated inorganic nutrient inputs e g coastal upwelling zones and or where there is efficient carbon storage

southern san andreas san jacinto fault system slip rates estimated from earthquake cycle models constrained by gps and interferometric synthetic aperture radar observations we use ground geodetic and interferometric synthetic aperture radar satellite observations across the southern san andreas saf san jacinto sjf fault systems to constrain their slip rates and the viscosity structure of the lower crust and upper mantle on the basis of periodic earthquake cycle maxwell viscoelastic finite element models key questions for this system are the saf and sjf slip rates the slip partitioning between the two main branches of the sjf and the dip of the saf the best fitting models generally have a high viscosity lower crust 10 21 pa s overlying a lower viscosity upper mantle 10 19 pa s we find considerable trade offs between the relative time into the current earthquake cycle of the san jacinto fault and the upper mantle viscosity with reasonable assumptions for the relative time in the earthquake cycle the partition of slip is fairly robust at around 24 26 mm a for the san jacinto fault system and 16 18 mm a for the san andreas fault models for two subprofiles across the saf sjf systems suggest that slip may transfer from the western coyote creek branch to the eastern clark superstition hills branch of the sjf from nw to se across the entire system our best fitting model gives slip rates of 2 3 12 9 12 9 and 17 3 mm a for the elsinore coyote creek clark and san andreas faults respectively where the large uncertainties in the slip rates for the sjf branches reflect the large uncertainty in the slip rate partitioning within the sjf system

mechanical fault interaction within the los angeles basin a two dimensional analysis using mechanical efficiency 1 mechanical models examine deformation within eight different structural cross sections proposed by davis et al 1989 and shaw and suppe 1996 along a northeast southwest transect across the los angeles basin california horizontal contraction of the models constrained by geodetic measurements yields varying dip slip rates along frictionally sliding faults within the different cross sections mechanical efficiency analysis using effective stiffness and strain energy density assesses the overall fault system deformation as well as the partitioning of work between fault slip and host rock strain the cross section interpreted by shaw and suppe 1996 has the best fit to paleoseismically determined slip rates and the greatest mechanical efficiency greatest proportion of work toward fault slip however this model produces excessive reverse slip along the newport inglewood fault a modified fault configuration with a wedge or blind puente hills thrust fault rather than a ramp detachment configuration better matches the paleoseismic data with slightly lower mechanical efficiency slip rates in the mechanical models based on interpretations of shaw and suppe 1996 have much closer match to the geologically determined rates than those estimated from kinematic models this difference is due to 1 differing time spans of slip rate estimates and 2 deformable rather than rigid host rock in the mechanical models the mechanical efficiency analysis provides quantitative indicators of overall fault system deformation including the cumulative effect of interaction between individual faults assessment of effective stiffness and strain energy density furthers our understanding of two dimensional fault interactions in the los angeles basin and offers great potential for future applications

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

initiation of the san jacinto fault and its interaction with the san andreas fault insights from geodynamic modeling the san andreas fault saf is the pacific north american plate boundary yet in southern california a significant portion of the relative plate motion is accommodated by the san jacinto fault sjf here we investigate the initiation of the sjf and its interaction with the saf in a three dimensional visco elasto plastic finite element model the model results show that the restraining bend of the southern saf causes strain localization along the sjf thus may have contributed to its initiation slip on the sjf tends to reduce slip rate on the saf and enhance deformation in the eastern california shear zone the initiation of the sjf and its interaction with the saf reflect the evolving plate boundary zone as it continuously seeks the most efficient way to accommodate the relative plate motion

magnetic lineations in the central indian basin for the period a24 a21 a study in relation to the indian ocean triple junction trace abstract a detailed study of magnetics and bathymetry was carried out in the central indian basin between latitudes 9 s and 16 30 s and longitudes 73 e and 79 30 e the observed east west trending magnetic anomalies are identified as anomalies 21 to 25 the magnetic lineation pattern revealed the presence of a new fracture zone at 75 45 e trending n12 e this fracture zone runs parallel to the 86 e fracture zone and appears to change its direction south of 20 s latitude the differential offsets observed in the identified magnetic anomalies across the new fracture zone are explained in terms of the evolution of the triple junction the study also confirms the strike of the 79 e fracture zone and its southern extension through the study area

how much can off fault deformation contribute to the slip rate discrepancy within the eastern california shear zone kinematic assumptions of geodetic inversions for fault slip require that the slip sums to the plate boundary velocity this assumption neglects permanent off fault deformation which could account for discrepancies between geologic and geodetic estimates we use three dimensional mechanical models to assess if unaccounted permanent strain surrounding faults could contribute to slip rate discrepancies across disconnected faults within the mojave desert california usa portion of the eastern california shear zone ecsz we modified fault configurations derived from the southern california earthquake center community fault model to better represent the disconnected nature of active faults in the ecsz south of the garlock fault the models with revised fault geometry produce slip rates that better match geologic strike slip rates thus validating the revisions within these models off fault deformation accounts for 40 23 of the total strain across the ecsz this suggests that a significant portion of the discrepancy between the geologic and geodetically modeled slip rates in the ecsz could be due to the geodetic inversion model assumption of zero permanent off fault deformation

inception of the eastern california shear zone and evolution of the pacific north american plate boundary from kinematics to geodynamics 1 the san andreas fault saf is the transform boundary between the pacific and the north american plates yet up to 25 of the relative plate motion is now accommodated by the eastern california shear zone ecsz here we investigate the inception of the ecsz and its geodynamic interactions with the saf using a 3 d viscoelastoplastic finite element model for a given fault configuration of the plate boundary zone the model simulates long term slip on the faults and plastic strain outside the faults our results show that the formation of the big bend of the saf around 5 12 ma impeded fault slip and localized strain along the ecsz causing its inception development of the ecsz was further enhanced by the activation of the garlock fault gf and lithospheric weakening caused by the encroachment of the basin and range extension similarly the san jacinto fault sjf in southern california developed along a belt of localized strain which resulted from the formation of the restraining bend along the san bernardino mountains segment of the saf 2 myr ago once activated the sjf reduced slip on both the southern saf and the ecsz across the mojave desert these results indicate causative relationship between the saf and the ecsz the inception of the ecsz and other young faults is the consequence of the evolving saf plate boundary zone that continuously adjusts itself to accommodate the relative plate motion

reconciling geologic and geodetic model fault slip rate discrepancies in southern california consideration of nonsteady mantle flow and lower crustal fault creep in southern california slip rates derived from geodesy constrained elastic models are lower than geologic rates along the mojave and san bernardino segments of the san andreas fault and the garlock fault in contrast the summed geodetic rate across the mojave eastern california shear zone ecsz is significantly higher than the summed geologic rate we show that geodetic and geologic slip rates in southern california can be reconciled using a viscoelastic earthquake cycle model that explicitly incorporates time dependent deformation due to nonsteady interseismic fault creep in the lower crust and viscous flow in the upper mantle to reconcile geologic and geodetic model rates our model requires that the southern san andreas fault and the garlock fault are in the late stages of the earthquake cycle resulting in lower current deformation rates than the cycle averaged rate our model implies that the ecsz and the san jacinto faults are in the early stages of the earthquake cycle resulting in high current deformation rates

is there a discrepancy between geological and geodetic slip rates along the san andreas fault system previous inversions for slip rate along the san andreas fault system safs based on elastic half space models show a discrepancy between the geologic and geodetic slip rates along a few major fault segments in this study we use an earthquake cycle model representing an elastic plate over a viscoelastic half space to demonstrate that there is no significant discrepancy between long term geologic and geodetic slip rates the california statewide model includes 41 major fault segments having steady slip from the base of the locked zone to the base of the elastic plate and episodic shallow slip based on known historical ruptures and geologic recurrence intervals the slip rates are constrained by 1981 secular velocity measurements from gps and l band intereferometric synthetic aperture radar a model with a thick elastic layer 60 km and half space viscosity of 1019pa s is preferred because it produces the smallest misfit to both the geologic and the geodetic data we find that the geodetic slip rates from the thick plate model agrees to within the bounds of the geologic slip rates while the rates from the elastic half space model disagree on specific important fault segments such as the mojave and the north coast segment of the san andreas fault the viscoelastic earthquake cycle models have generally higher slip rates than the half space model because most of the faults along the safs are late in the earthquake cycle so today they are moving slower than the long term cycle averaged velocity as governed by the viscoelastic relaxation process

style and rate of holocene slip palos verdes fault southern california recent studies of the palos verdes fault in the los angeles outer harbor characterize the fault s holocene activity the studies included areal and local geophysical surveys as well as drilling sampling and dating of the subsurface materials the studies identify the fault s location in the shallow subsurface document evidence of holocene strike slip quantify dip slip versus sediment age and quantify the net slip where an early holocene paleochannel crosses the fault the slip rate for the palos verdes fault is constrained to between 2 7 and 3 mm yr for the past 7 8 to 8 ka because the new evidence shows that the palos verdes fault has one of the highest slip rates in the los angeles basin the studies provide significant new insight into the regional tectonics and the seismic risk of the los angeles region using this rate of slip and segmentation models for the fault the coastal los angeles region can expect an mw 7 to 7 2 event about every 400 to 900 years from this potential seismic source

chapter 5 extension and contraction within an evolving divergent strike slip fault complex the san andreas and san jacinto fault zones at their convergence in southern california

constraints on fault slip rates of the southern california plate boundary from gps velocity and stress inversions summary we use global positioning system gps velocities and stress orientations inferred from seismicity to invert for the distribution of slip on faults in the southern california plate boundary region of particular interest is how long term slip rates are partitioned between the indio segment of the san andreas fault saf the san jacinto fault sjf and the san bernardino segment of the saf we use two new sets of constraints to address this problem the first is geodetic velocities from the southern california earthquake center s scec crustal motion map version 3 by shen et al which includes significantly more data than previous models the second is a regional model of stress field orientations at seismogenic depths as determined from earthquake focal mechanisms while gps data have been used in similar studies before this is the first application of stress field observations to this problem we construct a simplified model of the southern california fault system and estimate the interseismic surface velocities using a backslip approach with purely elastic strain accumulation following meade et al in addition we model the stress orientations at seismogenic depths assuming that crustal stress results from the loading of active faults the geodetically derived stressing rates are found to be aligned with the stress orientations from seismicity we therefore proceed to invert simultaneously gps and stress observations for slip rates of the faults in our network we find that the regional patterns of crustal deformation as imaged by both data sets can be explained by our model and that joint inversions lead to better constrained slip rates in our preferred model the sjf accommodates 15 mm yr 1 and the indio segment of the saf 23 mm yr 1 of right lateral motion accompanied by a low slip rate on the san bernardino segment of the saf anomalous fault segments such as around the 1992 m w 7 3 landers surface rupture can be detected there observed stresses deviate strongly from the long term loading as predicted by our simple model evaluation of model misfits together with information from palaeoseismology may provide further insights into the time dependence of strain accumulation along the san andreas system

elevated shear zone loading rate during an earthquake cluster in eastern california we compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern california shear zone ecsz during a cluster of major earthquake activity new slip rates are presented for six dextral faults that compose the ecsz in the central mojave desert these rates were determined from displaced alluvial fans dated with cosmogenic 10 be and from a displaced lava flow dated with 40 ar 39 ar we find that the sum geologic mojave ecsz slip rate 6 2 1 9 mm yr is only half the present day geodetically measured velocity of 12 2 mm yr these rates account for cumulative fault slip and geodetic observations that span the 60 km wide shear zone therefore this difference cannot be attributed to postseismic relaxation redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

simulating the recent evolution of the southern big bend of the san andreas fault southern california 1 the southern big bend of the san andreas fault has been interpreted to have successively abandoned two strands the mission creek and mill creek strands of the san andreas fault during the past 1 ma before taking up activity along its present day configuration this evolution is simulated within three dimensional models to explore the hypothesis that fault systems evolve to increase mechanical efficiency the three dimensional boundary element method models are validated by comparison of modeled fault slip rates and uplift rates with geologic data the model results match well the geologic strike slip rates and uplift patterns overestimation of reverse slip rates along the northern san bernardino mountains may reflect model assumptions of fault geometry the partitioning of slip among active faults changes between the three phases of southern san andreas fault evolution revealing 1 a trade off in strike slip rates between the san jacinto and san andreas faults 2 a trade off between strike slip rates on the san andreas fault and reverse slip along faults associated with uplifting the san bernardino mountains and 3 a trade off in strike slip rates along the san andreas and strike slip rates along the eastern california shear zone mechanical efficiency of the entire model increases from the mission creek to mill creek fault configuration and decreases from the mill creek configuration to the present day configuration the decrease in mechanical efficiency with the transition to the present day fault geometry may reflect gaps in our understanding of tectonic loading and or fault geometry

long term fault slip rates distributed deformation rates and forecast of seismicity in the western united states from joint fitting of community geologic geodetic and stress direction data sets 1 the long term average velocity field of the western united states is computed with a kinematic finite element code community data sets include fault traces geologic offset rates geodetic velocities principal stress directions and euler poles there is an irreducible minimum amount of distributed permanent deformation which accommodates one third of pacific north america relative motion in california much of this may be due to slip on faults not included in the model all data sets are fit at a common rms level of 1 8 datum standard deviations experiments with alternate weights fault sets and euler poles define a suite of acceptable community models in pseudoprospective tests fault offset rates are compared to 126 additional published rates not used in the computation 44 are consistent another 48 have discrepancies under 1 mm a 1 and 8 have larger discrepancies updated models are then computed novel predictions include dextral slip at 2 3 mm a 1 in the brothers fault zone two alternative solutions for the mendocino triple junction slower slip on some trains of the san andreas fault than in recent hazard models and clockwise rotation of some domains in the eastern california shear zone long term seismicity is computed by assigning each fault and finite element the seismicity parameters coupled thickness corner magnitude and spectral slope of the most comparable type of plate boundary this long term seismicity forecast is retrospectively compared to instrumental seismicity the western united states has been 37 below its long term average seismicity during 1977 2008 primarily because of temporary reduced activity in the cascadia subduction zone and san andreas fault system

strain accumulation across the eastern california shear zone at latitude 36 30 n the motion of a linear array of monuments extending across the eastern california shear zone ecsz has been measured from 1994 to 1999 with the global positioning system the linear array is oriented n54 e perpendicular to the tangent to the local small circle drawn about the pacific north america pole of rotation and the observed motion across the ecsz is approximated by differential rotation about that pole the observations suggest uniform deformation within the ecsz strike n23 w 26 nstrain yr 1 extension normal to the zone and 39 nstrain yr 1 simple right lateral shear across it with no significant deformation in the two blocks the sierra nevada mountains and southern nevada on either side the deformation may be imposed by right lateral slip at depth on the individual major fault systems within the zone if the slip rates are death valley furnace creek fault 3 2 0 9 mm yr 1 hunter mountain panamint valley fault 3 3 1 6 mm yr 1 and owens valley fault 6 9 1 6 mm yr 1 however this estimate of the slip rate on the owens valley fault is 3 times greater than the geologic estimate

evidence for post early miocene initiation of movement on the garlock fault from offset of the cudahy camp formation east central california the cudahy camp formation located in the el paso basin east central california consists of a 350 m thick volcanic and sedimentary sequence of early miocene age previous investigators determined from paleocurrent indicators that the source for these rocks was to the south and southeast but failed to suggest a specific location for that source restoration of 64 km of left slip on the garlock fault places cudahy camp rocks north and northwest of the early miocene eagle crags volcanic field the unique geochemical nature of basalts found in both locations coincident ages of volcanic rocks identical clast content of volcanic tuffs and identical stratigraphic sequence indicate that the source for the cudahy camp formation was the eagle crags volcanic field this is the first documented occurrence of miocene rocks offset by the garlock fault because the cudahy camp rocks and their sources appear to be offset by the full displacement on the garlock fault we conclude that movement on the garlock started after earliest middle miocene time further constraining initiation of movement on the garlock fault adds significantly to fully understanding the evolution of the western north american plate boundary in the middle to late cenozoic

stratigraphic record of pleistocene faulting and basin evolution in the borrego badlands san jacinto fault zone southern california sedimentary rocks in the borrego badlands southern california contain a record of pleistocene crustal deformation during initiation and evolution of the san jacinto fault zone we used detailed geologic stratigraphic and paleomagnetic analysis to determine the age and geometry of the deposits and reconstruct the history of fault controlled sedimentation in this area the base of the 300 to 500 m thick ocotillo formation is a paraconformity to abrupt conformable contact that records a brief hiatus followed by rapid progradation of coarse alluvial sediment over lacustrine facies of the borrego formation at 1 05 0 03 ma this coincides with regional scale progradation of ocotillo formation sand and gravel and appears to record initiation of strike slip faults in the southwestern salton trough at ca 1 1 ma thickness trends clast compositions paleocurrents and distribution of paleosols provide evidence for initiation of the east coyote mountain fault at ca 1 05 ma followed by onset of nne ward basin tilting obliquely toward the santa rosa segment of the clark fault at ca 1 0 ma stratigraphic omission of the ocotillo formation and progressively older units southwest of the coyote creek fault beneath the fonts point sandstone provides evidence that tilting to the northnortheast was related in part to growth of the san felipe anticline during deposition of the ocotillo formation map and stratigraphic data suggest that the coyote creek fault in the western borrego badlands postdates ocotillo deposition and thus appears to have propagated southeast into the study area at ca 0 6 ma the fonts point sandstone is a thin sheetlike alluvial deposit that records the end of deposition and onset of transpressive deformation in the borrego badlands the base of the fonts point sandstone changes from a conformable contact in a narrow belt southeast of the inspiration point fault where it is dated at 0 6 0 02 ma to an angular unconformity on the folded ocotillo formation northwest of the fault the pattern of stratal truncation records initiation of the inspiration point fault at ca 0 6 ma this coincides with a major structural reorganization in the san jacinto fault zone that initiated the modern phase of north south shortening and erosion in the southwestern salton trough

initiation of the southern elsinore fault at 1 2 ma evidence from the fish creek vallecito basin southern california 1 strike slip faults west of the san andreas fault saf accommodate more than half of the modern pacific north america plate motion in southern california yet their age kinematic evolution and long term slip rates are incompletely understood the southern elsinore fault is part of this zone and occupies the boundary between mesozoic crystalline rocks of the peninsular ranges and late cenozoic sedimentary rocks of the salton trough depocenter the fish creek vallecito basin a recently inverted western sub basin of the salton trough contains a thick succession of plio pleistocene sedimentary rocks that preserve a record of crustal subsidence and uplift in response to evolving tectonic controls deposits older than 1 2 ma accumulated during transtension and subsidence in the hanging wall of the low angle west salton detachment fault a fanning dip interval high in the section dated at 1 2 1 0 ma accumulated during tilting caused by earliest motion on the elsinore and earthquake valley faults total offset of 1 2 km yields a lifetime slip rate of about 1 2 mm yr for the southern elsinore fault similar to a late pleistocene rate in the coyote mountains but slower than published estimates for the northern elsinore fault this difference likely results from long term transfer of slip from the northern elsinore fault to the san felipe and san jacinto fault zones farther east initiation of the elsinore fault at 1 2 ma coincides with a major tectonic reorganization that terminated slip on the detachment fault and formed the modern network of strike slip faults west of the saf

latest pleistocene and holocene slip rate for the san bernardino strand of the san andreas fault plunge creek southern california implications for strain partitioning within the southern san andreas fault system for the last 35 k y an alluvial succession on the northeast side of the san bernardino strand of the san andreas fault includes distinctive aggradational and degradational features that can be matched with correlative features on the southwest side of the fault key among these are 1 a terrace riser on the northeast side of the fault that correlates with an offset channel wall on the southwest side of the fault and forms a basis for slip estimates for the period ca 35 ka to the present and 2 a small alluvial fan on the southwest side of the fault that has been matched with its most likely source gullies on the northeast side of the fault and forms a basis for slip estimates for the last 10 5 k y slip rate estimates for these two separate intervals are nearly identical the rate for the older feature is most likely between 8 3 and 14 5 mm yr with a 95 confidence interval of 7 0 15 7 mm yr the rate for the younger feature is most likely between 6 8 and 16 3 mm yr with a 95 confidence interval of 6 3 18 5 mm yr these rates are only half the previously published slip rate for the san andreas fault 35 km to the northwest in cajon pass a rate that traditionally is extrapolated southeastward along the san bernardino section of the fault results from plunge creek suggest that about half of the 25 mm yr rate at cajon pass transfers southeastward to the san jacinto fault as proposed by other workers on the basis of regional geologic relations these results indicate that the discrepancy between latest quaternary slip rates and present day rates of strain accumulation across the san bernardino section of the san andreas fault from geodesy can be largely explained by slip transfer between faults leading to spatial variation in rate along the san andreas fault nonetheless the latest pleistocene and holocene slip rate at plunge creek is still somewhat faster than rates inferred for the san bernardino section of the san andreas fault based on elastic block modeling of geodetic data and may be more appropriate than those rates for hazard estimation

fault network modeling of crustal deformation in california constrained using gps and geologic observations abstract we have developed a kinematic fault network model of crustal deformation in an elastic half space surface deformation is calculated using this model assuming each fault segment slipping beneath a locking depth each fault segment connects to its adjacent elements with slip vector continuity imposed at fault nodes or intersections the degree of the constraints determines whether deformation is block like or not we apply this model to invert gps observations for slip rates on major faults in california with geological rate constraints based on the f test result we find that lesser block like models fit the data significantly better than the strictly block like model our final inversion shows a slip rate varying from 20 to 23 mm yr along the northern san andreas from the santa cruz to the north coast segment slip rates vary from 9 to 13 mm yr along the hayward to the maacama fault segment and from 15 to 3 mm yr along the central calaveras to the west napa fault segment for the central california creeping zone the result suggests a depth dependent creep rate with an average of 22 mm yr over the top 5 km and 32 mm yr underneath from the mojave to san bernardino mountain segments we also find a significant decrease in slip rate along the san andreas in comparison with the geologic rates in contrast to a significant increase in slip rate on faults along the eastern california shear zone along the southern san andreas slip rates vary from 21 to 25 mm yr from the coachella valley to imperial valley segments slip rates range from 0 to 3 mm yr across the western transverse ranges faults which is consistent with the regional crustal thickening overall slip rates derived from geodetic observations correlate strongly with the geologic slip rates statistically suggesting high compatibility between geodetic and geologic observations

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

temporally constrained tectonic rotations derived from magnetostratigiraphic data implications for the initiation of the garlock fault california examination of declination anomalies within a well controlled chronologic framework can permit precise delineation of the progression of a tectonic rotation in the past declination anomalies from numerous spatially separated and individually dated localities have been synthesized in order to define regional progressive rotations rarely however have magneto stratigraphic data from a single locality been used successfully to chronicle a sequential rotation that resulted from movement along a nearby fault magnetostratigraphic studies from the el paso basin in southern california reveal progressively smaller amounts of counterclockwise rotation for strata dated between 11 and 7 ma two tests indicate that this is a temporally controlled rather than a spatially controlled rotation this rotation is interpreted to have resulted from sinistral shear along the garlock fault as such it appears to delimit the initiation of miocene movement along the western segment of this major strike slip fault

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

paleoseismicity of the north branch of the newport inglewood fault zone in huntington beach california from cone penetrometer test data abstract application of cone penetrometer testing cpt is a promising method for studying subsurface fault zones in stratified unconsolidated sediment where trenching is not feasible analysis of data from 72 cpts spaced 7 5 to 30 0 m apart and 9 borings indicates that the north branch fault the active strand of the newport inglewood fault zone nifz in huntington beach has generated at least three and most likely five recognizable surface ruptures in the past 11 7 0 7 ka additional smaller earthquakes similar to the m w 6 4 1933 long beach earthquake may also have occurred but would not be recognizable with this method the minimum right lateral holocene slip rate of the nifz in the study area is estimated to be 0 34 to 0 55 mm yr the actual slip rate may be significantly higher

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

kinematic modeling of fault slip rates using new geodetic velocities from a transect across the pacific north america plate boundary through the san bernardino mountains california 2015 american geophysical union all rights reserved campaign gps data collected from 2002 to 2014 result in 41 new site velocities from the san bernardino mountains and vicinity we combined these velocities with 93 continuous gps velocities and 216 published velocities to obtain a velocity profile across the pacific north america plate boundary through the san bernardino mountains we modeled the plate boundary parallel horizontal deformation with 5 14 parallel and one obliquely oriented screw dislocations within an elastic half space our rate for the san bernardino strand of the san andreas fault 6 5 3 6mm yr is consistent with recently published latest quaternary rates at the 95 confidence level and is slower than our rate for the san jacinto fault 14 1 2 9mm yr our modeled rate for all faults of the eastern california shear zone ecsz combined 15 7 2 9mm yr is faster than the summed latest quaternary rates for these faults even when an estimate of permanent off fault deformation is included the rate discrepancy is concentrated on faults near the 1992 landers and 1999 hector mine earthquakes the geodetic and geologic rates agree within uncertainties for other faults within the ecsz coupled with the observation that postearthquake deformation is faster than the pre 1992 deformation this suggests that the ecsz geodetic geologic rate discrepancy is directly related to the timing and location of these earthquakes and is likely the result of viscoelastic deformation in the mantle that varies over the timescale of an earthquake cycle rather than a redistribution of plate boundary slip at a timescale of multiple earthquake cycles or longer

the palos verdes terraces california bathtub rings from a buried reverse fault uplift of the palos verdes peninsula has long been associated with a northwest trending southwest dipping reverse fault unfortunately the palos verdes hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension orientation or earthquake potential in this paper we investigate the tectonic style and slip rate of the palos verdes hills fault and the uplift history of the palos verdes hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration elevations of 211 terrace remnants constrain a fault model with 3 0 to 3 7 mm yr 1 of oblique dextral reverse slip on a fault dipping 67 at 6 to 12 km depth beneath the peninsula if the rate was constant through time fault inception would have occurred 2 4 3 0 ma we propose that the largest credible earthquakes on the fault have magnitude 6 and could revisit every 2000 years

an age constraint on gulf of california rifting from the santa rosalia basin baja california sur mexico marine rocks of the santa rosalia basin baja california sur were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on gulf of california rifting progressive demagnetization of samples from the boleo formation the earliest marine sequence overlying the deeply eroded basement reveals a high coercivity characteristic remanent magnetization chrm in addition to a low coercivity overprint the chrm appears to be a primary magnetic remanence with stratigraphically bound normal and reversed polarity directions a single 40 ar 39 ar isotopic age of 6 76 0 90 ma 2 was obtained for the cinta colorada a tephra deposit of reversed paleomagnetic polarity within the boleo formation the age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed polarity intervals of the geomagnetic polarity time scale gpts in the 2 range 5 86 7 66 ma the interval with the highest probability is 6 57 6 94 ma in conjunction with the isotopic age preliminary magnetostratigraphy of the boleo formation is correlated with the gpts in order to further delineate the onset of marine sedimentation the most likely correlation yields an age of 6 93 7 09 ma gpts subchron c3bn for the base of the boleo formation and 6 14 6 27 ma for the top this correlation indicating an average sedimentation rate of 28 4 cm k y could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the boleo formation however even if the isotopic age of the cinta colorada is used as the only age constraint the result is consistent with data from the northern gulf of california and shows that rifting started much earlier than the ca 3 6 ma commencement of sea floor spreading at the mouth of the gulf of california the deposition of the boleo formation was probably related to an early phase of gulf rifting caused by a change in pacific north american plate motions