serial ruptures of the san andreas fault carrizo plain california revealed by three dimensional excavations 1 it is poorly known if fault slip repeats regularly through many earthquake cycles well documented measurements of successive slips rarely span more than three earthquake cycles in this paper we present evidence of six sequential offsets across the san andreas fault at a site in the carrizo plain using stream channels as piercing lines we opened a latticework of trenches across the offset channels on both sides of the fault to expose their subsurface stratigraphy we can correlate the channels across the fault on the basis of their elevations shapes stratigraphy and ages the three dimensional excavations allow us to locate accurately the offset channel pairs and to determine the amounts of motion for each pair we find that the dextral slips associated with the six events in the last millennium are from oldest to youngest 5 4 0 6 8 0 0 5 1 4 0 5 5 2 0 6 7 6 0 4 and 7 9 0 1 m in this series three and possibly four of the six offset values are between 7 and 8 m the common occurrence of 7 8 m offsets suggests remarkably regular but not strictly uniform slip behavior age constraints for these events at our site combined with previous paleoseismic investigations within a few kilometers allow a construction of offset history and a preliminary evaluation of slip and time predictable models the average slip rate over the span of the past five events between a d 1210 and a d 1857 has been 34 mm yr not resolvably different from the previously determined late holocene slip rate and the modern geodetic strain accumulation rate we find that the slip predictable model is a better fit than the time predictable model in general earthquake slip is positively correlated with the time interval preceding the event smaller offsets coincide with shorter prior intervals and larger offset with longer prior intervals

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

stress slip and earthquakes in models of complex single fault systems incorporating brittle and creep deformations numerical simulations of slip evolution along a cellular vertical strike slip fault in an elastic half space are performed for several models representing discrete fault systems embedded in three dimensional elastic continua the geometry and imposed boundary conditions correspond approximately to the central san andreas fault the simulations incorporate brittle and creep deformations in series the net fault zone deformation rate is the sum of creep rate and frictional slip rate brittle fault properties are given by various distributions of earthquake stress drops on failing segments numerical cells the assumed distributions represent two idealized situations corresponding to different extreme states along an evolutionary path of a fault 1 a strongly disordered state characterized by a wide range of size scales representing immature fault zones and extended spatial domains and 2 a relatively regular state having a narrow range of size scales representing mature highly slipped faults the assumed creep properties are identical in all cases these are prescribed in terms of coefficients characterizing a power law dependency of creep slip rate on stress the combined brittle creep process and employed parameters lead to an overall pine tree stress depth profile with a brittle ductile transition depth of about 12 5 km and variable stress along strike profiles with brittle creep transition around 65 km nw of the 1857 rupture the spatial patterns of simulated hypocenters are statistically similar to observed data the results indicate that the range of size scales characterizing strong fault zone heterogeneities has important manifestations on the seismic response of a fault system a narrow range of size scales leads to frequency size statistics of earthquakes resembling the characteristic earthquake distribution and quasi periodic temporal distribution of large events as in the seismic gap hypothesis on the other hand a wide range of size scales leads to gutenberg richter power law frequency size statistics and random or clustered temporal distribution of large events the simulations demonstrate that treatment of the various observed forms of frequency size and temporal statistics of earthquakes can be unified through the concept of range of size scales characterizing fault zone heterogeneities this has a clear physical interpretation in terms of structural properties of a given fault zone or broad lithospheric domain and is supported by observed earthquake and fault data in some simulated cases the frequency size statistics of small earthquakes fall sharply below the self similar gutenberg richter line the results indicate that small earthquakes prepare the fault for the occurrence of a large event by smoothing during gradual tectonic loading the long wavelength components of stress on the fault this is done through short wavelength stress roughening associated with the numerous ruptures of the small events the above pattern of smoothing roughening of long short wavelengths of stress on the fault is reversed during large scale ruptures of the big events

ages of late holocene earthquakes on the central garlock fault near el paso peaks california a trench across the central garlock fault within a small playa adjacent to el paso mountains revealed buried fissures and fault scarps that provide strong evidence for five surface rupturing earthquakes within the past 5 kyr weaker evidence suggested the possibility of three additional events within this same time period these observations indicate an average recurrence interval of 700 to 1200 years nearly all 28 radiocarbon dates on samples of detrital charcoal wood and shell are in correct stratigraphic order these dates constrain the ages of the individual faulting events and indicate that the recurrence interval is not periodic but is quite irregular individual recurrence intervals range from 190 to 1545 years if all eight events are actual earthquakes or from 190 to 3405 years if only the five well documented events represent earthquakes the two most recent earthquakes occurred within the past 550 years with preferred ages of about a d 1790 and a d 1600 the a d 1790 event ruptured a narrower width of the fault zone and may have been a smaller earthquake than the others the radiocarbon dates from the trench also indicate that the sedimentation rate on the playa has fluctuated within the past 5 kyr low sedimentation rates 0 1 to 0 35 mm yr and infrequent floods 1 per century prevailed from about 2000 b c to a d 100 and from a d 500 to 1300 during the remaining intervals within the past 5 kyr the sedimentation rate averaged 1 0 to 1 7 mm yr and major floods occurred two to four times per century the changes in sedimentation rate and flood frequency may reflect climatic changes or they may simply reflect changes in drainage patterns

the plasmapause response to the southward turning of the imf derived from sequential euv images 1 we statistically examined the plasmapause response to the southward turning of the interplanetary magnetic field imf using sequential global images of the plasmasphere in order to understand how the convection electric field propagates to the inner magnetosphere the extreme ultraviolet euv imager on the imager for magnetopause to aurora global exploration satellite clearly observed inward motion of the plasmapause driven by the southward turning of the imf we surveyed the euv data in the 2000 2001 period and found 16 events using the sequential euv images we calculated the plasmapause radial velocity and then estimated the time development of the convection electric field at the plasmapause epp epp and the solar wind electric field derived from the measurement by the ace satellite had very similar variations each other but there surely was a time lag consequently our research indicates that the plasmapause response to the southward turning of the imf takes 10 30 min this timescale suggests that the convection electric field penetrates from the magnetopause to the inner magnetosphere through the ionosphere

complexity of seismicity due to highly rate dependent friction we study a simple antiplane fault of finite length embedded in a homogeneous isotropic elastic solid in order to understand the origin of seismic source heterogeneity in the presence of nonlinear rate dependent friction all the mechanical properties of the medium and friction are assumed to be homogeneous starting from a heterogeneous initial stress distribution we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for more than 20 000 events in some cases the style of seismicity produced by this model is determined by a control parameter which measures the degree of rate dependence of friction for classical friction models with rate independent friction no complexity appears and seismicity is perfectly periodic for weakly rate dependent friction seismicity becomes slightly nonperiodic but most events are still characteristic earthquakes when friction is highly rate dependent seismicity is completely irregular and ruptures of all sizes occur inside the fault highly rate dependent friction destabilizes the healing process producing premature healing of slip and partial stress drop premature healing causes rupture to take the form of narrow propagating slip episodes the so called heaton s 1990 pulses partial stress drop produces large variations in the state of stress which in turn produce earthquakes of different sizes we make the conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity

morphologic dating of scarps formed by repeated slip events along the san andreas fault carrizo plain california morphologic dating of fault scarps determines late cenozoic fault activity by comparing observed topographic profiles with those determined using a calibrated hillslope development model we postulate that the material transport rate along the profile is a function only of local slope and is transport limited morphologic dating of hillslopes bounded by continuously dropping boundaries or cut by continuously slipping faults is used to determine the material transport rate constant the time since the downdrop was initiated or the fault began to slip we calibrated the hillslope development model on the southwest facing scarp southeast of wallace creek along the san andreas fault saf in the carrizo plain california the scarp has been exposed by right lateral offset of a southeast sloping shutter ridge located on the southwest side of the saf and by vertical offset related to secondary deformation we assume that all the observed offset occurs after initial exposure of the scarp by passage of the shutter ridge forward modeling of profile development yielded a mass diffusivity of 8 6 0 8 m2 kyr 1 normal fault slip rates were determined for two graben bounding faults in the northern elkhorn hills in the southeastern carrizo plain by applying the calibrated to the degradation of the scarps to determine the scarp age one fault scarp began to form about 12 kyr ago and the other about 63 kyr ago those ages and estimates of the dip slip along the faults result in slip rates of 1 2 mm yr 1

forecast model for large and great earthquakes in southern california an earthquake instability model is used to compute theoretical fault slip and ground surface deformation before possible future earthquakes on the san andreas fault in southern california the quasistatic model represents the earth s crust by an elastic half space and the san andreas imperial cerro prieto and san jacinto faults by flat rectangular surfaces whose strikes approximate the actual local strikes the seismogenic zone of the san andreas fault is represented by a brittle strain softening patch extending from the ground surface to 11 km depth and from parkfield to the salton sea 500 km the strength of the patch varies along strike and unstable failures of parts of the patch are earthquake analogs failed areas heal immediately after instability for the model to reproduce the lengths slips and times of earthquakes since 1080 a d reported by k sieh and others the patch must be divided into five sections along strike the sections have alternately low and high strength with the two strongest sections being roughly coincident with the major bends of the san andreas fault ten to twenty years before unstable slippage of patch areas portions of the patch bottom begin to fail stably aseismically the stable failure causes accelerating fault slip at depth and at the fault trace as well as anomalous changes in lengthening rates of trilateration lines in the model simulation the next two sections to fail unstably in the future are just north of the salton sea and just south of parkfield rupture lengths are about 100 km and 40 km at favorable locations precursory anomalies of fault slip and trilateration line length become large enough to be detected in field data a few years before each earthquake

earthquakes as self organized critical phenomena a cellular automation model with threshold elements previously proposed by bak et al 1988 which generates 1 f noise and self organizes into a critical state is studied as a model for earthquake occurrence the model explains some of fractal features of earthquakes such as the power law distribution of the size of the earthquake the model is modified so that every earthquake may trigger aftershocks the modified trigger model explains other fractal features of earthquakes such as the power law decay of aftershock frequency and the power law decay of aftershock frequency and the power law correlation of the hypocenter distribution

stratigraphic evidence for seven meters of dextral slip on the san andreas fault during the 1857 earthquake in the carrizo plain the smallest geomorphic offsets along a 35 km section of the san andreas fault in the carrizo plain vary from 7 to 10 m our three dimensional excavation of alluvial deposits a few km southeast of wallace creek confirms that at least 6 6 to 6 9 m of dextral slip occurred there during the latest large earthquake in 1857 dates on detrital charcoal suggest that the last event prior to the 1857 earthquake occurred before a date within the range a d 1305 to 1623 the 3 m range in smallest offsets along this portion of the fault may reflect either a 3 m variation in slip along the san andreas in 1857 or 2 to 3 m of slip during an event prior to 1857 observations made after the recent landers earthquake are compatible with the hypothesis of large local variations in slip during a single earthquake but do not explain the cause of such variations off fault dextral rotations would be one plausible explanation however paleoseismic data in the carrizo plain are too sparse to allow rejection of the alternative hypothesis that slip in the event prior to 1857 was only 2 to 3 m an amount of slip which would be several times too small to fit a time predictable model

morphologic patterns of stream channels on the active yishi fault southern shandong province eastern china implications for repeated great earthquakes in the holocene abstract stream channels totaling 427 on the active yishu fault were systematically measured in the field three categories of deflected stream channels were identified these were the r l and u groups which correspond to channels that are right laterally deflected left laterally deflected and those from which fault displacement cannot be determined respectively of the measured channels 48 7 are group u 37 7 group r and 13 6 group l each group has four different patterns that are strongly affected by local conditions such as channel density width of crush zone slope angle and stream incision and sedimentation by introducing a parameter the deflection angle which is defined as the angle between the fault slip direction on the downstream block and the downstream flow direction at least three paleoslip events preceding the 1668 m 8 5 earthquake at zuoshan were identified and their occurrence times estimated they were all within holocene time but the recurrence intervals seem to have become longer over the past 10 000 years the recurrence intervals appeared to be doubled from 2000 years to about 4000 years over the same period each faulting event was associated with 4 9 m right lateral displacement on the fault if the typical maximum displacement associated with each event is 8 m then the slip rate has decreased from 4 mm yr to about 2 mm yr however considering the possibility that the maximum displacement during each event may not be located at the same place then the recurrence interval on the whole rupture section could be underestimated because in that case the recurrence interval would not be a simple inverse relationship of the average slip rate

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

slip complexity in earthquake fault models we summarize studies of earthquake fault models that give rise to slip complexities like those in natural earthquakes for models of smooth faults between elastically deformable continua it is critical that the friction laws involve a characteristic distance for slip weakening or evolution of surface state that results in a finite nucleation size or coherent slip patch size h models of smooth faults using numerical cell size properly small compared to h show periodic response or complex and apparently chaotic histories of large events but have not been found to show small event complexity like the self similar power law gutenberg richter frequency size statistics this conclusion is supported in the present paper by fully inertial elastodynamic modeling of earthquake sequences in contrast some models of locally heterogeneous faults with quasi independent fault segments represented approximately by simulations with cell size larger than h so that the model becomes inherently discrete do show small event complexity of the gutenberg richter type models based on classical friction laws without a weakening length scale or for which the numerical procedure imposes an abrupt strength drop at the onset of slip have h 0 and hence always fall into the inherently discrete class we suggest that the small event complexity that some such models show will not survive regularization of the constitutive description by inclusion of an appropriate length scale leading to a finite h and a corresponding reduction of numerical grid size

direct geological evidence for prior earthquakes on the 1981 corinth fault central greece we present the preliminary results of geomorphic and trench investigations at the eastern end of a scarp produced by the 1981 corinth earthquake sequence at this site the 1981 rupture produced average vertical displacements of 0 6 0 7 m maximum 1 3 m that in part occurred along a cumulative scarp which displaces the fan surface by up to 5 m based on this cumulative scarp displacing the holocene fan surface we estimate a vertical slip rate of 1 mm yr that translates to 1 mm yr n s extension we found evidence for two individual pre 1981 earthquakes which occurred on the basis of preliminary radiocarbon dating on two paleosols shortly after a d 590 and a d 1295 each of these events produced a vertical displacement ranging between 0 5 and 1 2 m hence comparable to the 1981 scarp height this suggests that coseismic slip may have been characteristic at this location over the past three seismic cycles following this hypothesis and assuming a periodic strain release a maximum recurrence interval of 700 yr is calculated these results indicate that the 1981 surface breaking faults accommodate only part of the regional extension deduced from the geodetic measurements

earthquake cycle fault zones and seismicity patterns in a rheologically layered lithosphere we study the coupled evolution of earthquakes and faults in a model consisting of a seismogenic upper crust governed by damage rheology over a viscoelastic substrate the damage rheology has two types of functional coefficients 1 a generalized internal friction separating states associated with material degradation and healing and 2 damage rate coefficients for positive degradation and negative healing changes the evolving damage modifies the effective elastic properties of material in the upper crust as a function of the ongoing deformation this simulates the creation and healing of fault systems in the upper seismogenic zone in addition to the vertically averaged thin sheet approximation we introduce a green function for three dimensional elastic half space for the instantaneous component of deformation the formulation accounts in an internally consistent manner for evolving deformation fields evolving fault structures aseismic energy release and spatiotemporal seismicity patterns these developments allow us to simulate long histories of crustal deformation and to study the simultaneous evolution of regional earthquakes and faults for various model realizations to focus on basic features of a large strike slip fault system we first consider a simplified geometry of the seismogenic crust by prescribing initial conditions consisting of a narrow damage zone in an otherwise damage free plate for this configuration the model generates an earthquake cycle with distinct interseismic preseismic coseismic and postseismic periods model evolution during each period is controlled by a subset of physical properties which may be constrained by geophysical geodetic rock mechanics and seismological data in the more generic case with a random initial damage distribution the model generates large crustal faults and subsidiary branches with complex geometries the simulated statistics depend on the space time window of the observational domain the results indicate that long healing timescale h describing systems with relatively long memory leads to the development of geometrically regular fault systems and the characteristic frequency size earthquake distribution conversely short h relatively short memory leads to the development of a network of disordered fault systems and the gutenberg richter earthquake statistics for intermediate values of h the results exhibit alternating overall switching of response from periods of intense seismic activity and the characteristic earthquake distribution to periods of low seismic activity and gutenberg richter statistics

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

discordant 14c ages from buried tidal marsh soils in the cascadia subduction zone southern oregon coast peaty tidal marsh soils interbedded with estuarine mud in late holocene stratigraphic sequences near coos bay oregon may have been submerged and buried during great m 8 subduction earthquakes smaller localized earthquakes or by nontectonic processes radiocarbon dating might help distinguish among these alternatives by showing that soils at different sites were submerged at different times along this part of the cascadia subduction zone but comparison of conventional 14c ages for different materials from the same buried soils shows that they contain materials that differ in age by many hundreds of years errors in calibrated soil ages represent about the same length of time as recurrence times for submergence events 150 500 yr this similarity precludes using conventional 14c ages to distinguish buried soils along the southern oregon coast accelerator mass spectrometer 14c ages of carefully selected macrofossils from the tops of peaty soils should provide more precise estimates of the times of submergence events

how regularly do earthquakes recur a synthetic seismicity model for the san andreas fault we have been attempting to improve estimates of long term earthquake recurrence probabilities along the san andreas fault by means of synthetic seismicity calculations the calculations are based on the concept of fault segmentation and incorporate the physics of static dislocation theory forecasts constructed from synthetic seismicity are robust in that they embody regional seismicity information over several units of magnitude they tie together in a physical manner a spectrum of fault segment features such as length strength characteristic magnitude mean repeat time and slip rate they can reasonably account for fault segment interactions and they are formulated from a catalog which can be extended as long as needed to be statistically significant we contend earthquake recurrence is more aperiodic than previously thought and as a result probabilities of major san andreas earthquakes should be revised downward

effects of geological complexities on coseismic displacement hints from 2d numerical modelling by means of a 2d finite element procedure we tested how heterogeneities at the scale of seismogenic fault affect the displacement we defined one or more slip distributions for two typical normal faulting earthquakes in the central apennines computed the displacement occurred within different structures including lateral heterogeneities and compared the different displacement profiles to isolate the effect of the crustal structure to understand at what magnitude the heterogeneities affect the observation significantly we compared the predicted coseismic displacement with gps and dinsar data for the colfiorito 1997 earthquake we find that heterogeneities significantly affect the observable horizontal coseismic displacement for the larger magnitudes whereas for smaller quakes they affect horizontal displacement close to the fault trace only

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

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

a physical model for earthquakes 2 application to southern california the purpose of this paper is to apply ideas developed in a previous paper to the construction of a detailed model for earthquake dynamics in southern california the basis upon which the approach is formulated is that earthquakes are perturbations on or more specifically fluctuations about the long term motions of the plates this concept is made mathematically precise by means of a fluctuation hypothesis which states that all physical quantities associated with earthquakes can be expressed as integral expansions in a fluctuating quantity called the offset phase while in general the frictional stick slip properties of the complex interacting faults should properly come out of the underlying physics a simplification is made here and a simple spatially varying friction law is assumed together with the complex geometry of the major active faults an assumed spatially varying earth rheology the average rates of long term offsets on all the major faults and the friction coefficients one can generate synthetic earthquake histories for comparison to the real data the result is a set of slip time histories for all of the major faults which are similar to data obtained by geologic trenching studies however the patterns of earthquake occurrence are evidently chaotic in the technical sense meaning that although there is an element of periodicity to the events the patterns shift change and evolve with time time scales for pattern evolution seem to be of the order of a thousand years for average recurrance intervals of a hundred years or so in the synthetic slip histories for the model faults a series of events on the big bend is identified which resembles the pattern of events leading up to the 1857 fort tejon earthquake one of the events is similar enough to the 1857 event that it is taken as a model of that earthquake the horizontal surface deformation 130 years after the model event is calculated for comparison to the data observed by various methods during the past decade in southern california for example horizontal surface rates of strain and velocities are calculated and compared to u s geological survey trilateration data to very long baseline interferometry line length changes and to laser ranging data on the san andreas fault experiment baseline with a few exceptions agreement is good

six similar sequential ruptures of the san andreas fault carrizo plain california we document the precise sizes but not the dates of the six latest offsets across the san andreas fault at wallace creek california three and perhaps four of these including the latest in 1857 show dextral offset of 7 5 8 m the third and fourth offsets however are just 1 4 and 5 2 m the predominance of similar offsets for the latest six events suggests that the fundamental properties of the fault system that control slip size do not vary greatly from event to event the large offsets imply that ruptures involving this site are typically more than 200 km long

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

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