shallow creep on the haiyuan fault gansu china revealed by sar interferometry interferometric synthetic aperture radar data are used to map the interseismic velocity field along the haiyuan fault system hfs at the north eastern boundary of the tibetan plateau two m 8 earthquakes ruptured the hfs in 1920 and 1927 but its 260 km long central section known as the tianzhu seismic gap remains unbroken since 1000 years the envisat sar data spanning the 2003 2009 period cover about 200 300 km2 along three descending and two ascending tracks interferograms are processed using an adapted version of roi pac the signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals mean line of sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise these maps show a dominant left lateral motion across the hfs and reveal a narrow 35 km long zone of high velocity gradient across the fault in between the tianzhu gap and the 1920 rupture we model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion the inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments the average creep rate is 5 mm yr 1 comparable in magnitude with the estimated loading rate at depth suggesting no strain accumulation on this segment the modeled creep rate locally exceeds the long term rate reaching 8 mm yr 1 suggesting transient creep episodes the present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment

rise and fall of the southern santa cruz mountains california from fission tracks geomorphology and geodesy the santa cruz mountains are closely associated with a left bend along the right lateral san andreas fault the loma prieta area on the northeast side of the san andreas consists of fault bounded blocks that rise along active deeply rooted reverse and oblique slip faults six samples from a transect across this area yield concordant apatite fission track ages averaging 4 6 0 5 ma these ages date the time of cooling below 110 c and suggest that about 3 km of unroofing has occurred over the last 4 6 m y allowing for current elevations of about 1 km this suggests an average uplift rate of the order of 0 8 mm yr over the last 4 6 m y to further define the extent and distribution of this young uplift we used morphometric analyses of the youthful topography of the area steep drainage slopes and high local relief indicate that the area northeast of the san andreas forms a well defined zone of high uplift in contrast the region on the southwest side of the san andreas is characterized by broad upwarping and folding more subdued topography old fission track ages and mean quaternary uplift rates of 0 1 0 4 mm yr geodetic data 1906 san francisco earthquake and subsequent strain transients 20 years of interseismic deformation 1989 loma prieta earthquake and 2 years of post loma prieta earthquake strain show that the southern santa cruz mountains repeatedly rise and subside through a complex sequence of bay area deformation events an additional deformation element that involves reverse slip averaging 2 3 mm yr along the foothills thrust system must occur to explain the longer term uplift pattern in the loma prieta area since the late pliocene the asymmetry in deformation on opposite sides of the san andreas probably reflects the contrasting rock types on opposite sides of the fault the influence of preexisting structures and the interaction with neighboring faults of the san andreas system

evolution of ocean slicks under a rising wind 1 airborne synthetic aperture radar and high resolution infrared imagery are used to examine the evolution of natural ocean slicks during a period of freshening wind initially the slicks are of the order of 50 m in width and have radar and thermal contrasts of the order of 10 db and several 0 1 c while there can be over time a transient reorganization of surface film material by internal waves the slicks are observed to disperse and are no longer detectible after 5 h the dispersion occurs through the action of langmuir circulation which results in along wind streaks or windrows as well as other effects prominent streaks occur preferentially along a slick s upwind side the downwind side resembling more of a leading edge at least early on over time the leading edge develops a corrugated shape and gaps begin to develop within the slicks the formation of gaps is compared with a conceptual model by dysthe 2006 for the tearing of a surface film in the region of positive surface straining eventually the slicks break into elongated patches that co exist with windrows which is consistent with the effects of advective dispersion

new slip rate estimates for the creeping segment of the san andreas fault california new continuous and differential global positioning system gps measurements of recent slip rates and 30 yr alignment array offsets from the central creeping segment of the san andreas fault yield a maximum right lateral slip rate of 25 1 mm yr this slip rate is 20 slower than the 30 mm yr slip rate accepted for this segment of the fault and 35 slower than the 39 mm yr slip rate predicted between the sierra nevada great valley block and the pacific plate new continuous gps measurements between pairs of sites that flank the creeping segment at intersite distances of 1 0 km and 70 km give relative fault parallel slip rates of 28 2 and 30 2 mm yr respectively these observations indicate that right lateral deformation rates increase with distance from the fault possible explanations for the gradient observed in the geodetic data are elastic strain accumulation along the creeping segment or significant distributed deformation on off fault structures

long term monitoring of creep rate along the hayward fault and evidence for a lasting creep response to 1989 loma prieta earthquake we present results from over 30 yr of precise surveys of creep along the hayward fault along most of the fault spatial variability in long term creep rates is well determined by these data and can help constrain 3d models of the depth of the creeping zone however creep at the south end of the fault stopped completely for more than 6 years after the m7 1989 loma prieta earthquake lpeq perhaps delayed by stress drop imposed by this event with a decade of detailed data before lpeq and a decade after it we report that creep response to that event does indeed indicate the expected deficit in creep

locked and creeping patches along the hayward fault california the results of mapping locked and unlocked patches on the hayward fault using the local recurrence time method are compared with models derived from surface creep and geodetic measurements the data used were the m 1 0 earthquakes in the usgs catalog since 1981 0 located within 5 km of the fault trace along the central and southern segments agreement with the other methods is satisfactory the 1868 rupture except for its two ends is classified as a locked segment by our method in the northern part of the hayward fault the data are poor and the results of the three methods differ short local recurrence times in the northern part suggest that an asperity exists there which is capable of an m6 5 class event

distribution of aseismic slip rate on the hayward fault inferred from seismic and geodetic data 1 we solve for the slip rate distribution on the hayward fault by performing a least squares inversion of geodetic and seismic data sets our analysis focuses on the northern 60 km of the fault interferometric synthetic aperture radar insar data from 13 independent ers interferograms are stacked to obtain range change rates from 1992 to 2000 horizontal surface displacement rates at 141 bench marks are measured using gps from 1994 to 2003 surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion the fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface south of the city of hayward a steeply east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth the inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of 5 mm yr to a depth of 4 6 km the interferometric synthetic aperture radar insar data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath point pinole although the insar data might be complicated by a small dip slip component at this location a low slip rate patch of 1 mm yr is inferred beneath san leandro consistent with the source location of the 1868 earthquake we calculate that the entire fault is accumulating a slip rate deficit equivalent to a mw 6 77 0 05 per century however this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip

short term surface deformation on the northern hayward fault ca and nearby landslides using polarimetric sar interferometry polinsar in this study we analyze 25 radarsat 2 images from ascending and descending geometries to study the creep rate on the hayward fault and landslide motions near berkeley ca we applied a coherence optimization technique from polarimetric synthetic aperture radar interferometry polinsar to increase the accuracy of the measurements we resolve 3 5 mm year of motion along the hayward fault in agreement with earlier creep estimates we identify a potential motion on secondary fault northeast and parallel to the hayward fault which is creeping at a lower rate of 1 5 mm year in addition we identify a number of landslides along the hills east of the fault that agree with earlier results from advanced interferometric synthetic aperture radar sar analysis and field investigations we investigate four particular slope instabilities one of which was marked as moderately active and three as highly active by earlier field investigations the resolved along hill slope displacement is estimated at 23 mm year our results demonstrate that polinsar is an effective method to increase the interferometric coherence and provide improved resolution of deformation features associated with natural hazards

calculation of synthetic seismograms in a laterally varying medium by the boundary element discrete wavenumber method abstract we investigate the propagation of lg waves in laterally varying crustal structures by numerical simulation the method of calculation is formulated in terms of boundary integral equations where the green9s functions are evaluated by wavenumber summation the approach is well suited to study the propagation of seismic waves in a layered medium where the interfaces are flat in some regions and irregular in others we investigate the effect of a crustal fault with vertical offset and study the case of a lateral change in crustal thickness the results show that the lg wave amplitude is only slightly affected by the presence of these heterogeneities they confirm the robustness of lg wave propagation in presence of lateral heterogeneities observed in other numerical simulations they show that large scale geometric features of the crust cannot account alone for the strong attenuation of lg waves observed in many regions the results also suggest a possible relation between the level of lg wave coda and the degree of roughness of the moho they further indicate the importance of backscattering and suggest a possible use of the backscattered wave field to map strong crustal heterogeneities

fault zone connectivity slip rates on faults in the san francisco bay area california the slip rate of a fault segment is related to the length of the fault zone of which it is part in turn the slip rate of a fault zone is related to its connectivity with adjoining or contiguous fault zones the observed variation in slip rate on fault segments in the san francisco bay area in california is consistent with connectivity between the hayward calaveras and san andreas fault zones slip rates on the southern hayward fault taper northward from a maximum of more than 10 millimeters per year and are sensitive to the active length of the maacama fault

a review of numerical experiments on seismic wave scattering this paper reviews applications of the finite difference and finite element methods to the study of seismic wave scattering in both simple and complex velocity models these numerical simulations have improved our understanding of seismic scattering in portions of the earth where there is significant lateral heterogeneity such as the crust the methods propagate complete seismic wavefields through highly complex media and include multiply scattered waves and converted phases e g p tosv sv top body wave to surface wave the numerical methods have been especially useful in cases of moderate and strong scattering in complex media where multiple scattering becomes important progress has been made with numerical methods in understanding how near surface low velocity basin structures scatter surface waves and vertically incident body waves the numerical methods have proven useful in evaluating scattering of surface waves and body waves from topography of both the free surface and interfaces buried at depth numerical studies have demonstrated the importance of conversions from body waves to surface waves andvice versa when lateral heterogeneities and topographic relief are present in the uppermost crust recently several investigations have applied numerical methods to study seismic wave propagation in velocity models which vary randomly in space this stochastic approach seeks to understand the effects of small scale complexity in the earth which cannot be resolved deterministically these experiments have quantified the relationships between the statistical properties of the random heterogeneity and the measurable properties of high frequency 1 hz seismograms these simulations have been applied to the study of many features observed in actual high frequency seismic waves including the amplitude and time decay of seismic coda the apparent attenuation from scattering the dispersion of waveforms and the travel time and waveform variations across arrays of receivers

seismic tomography and deformation modeling of the junction of the san andreas and calaveras faults local earthquake p traveltime data is inverted to obtain a three dimensional tomographic image of the region centered on the junction of the san andreas and calaveras faults the resulting velocity model is then used to relocate more than 17 000 earthquakes and to produce a model of fault structure in the region these faults serve as the basis for modeling the topography using elastic dislocation methods the region is of interest because active faults join it marks the transition zone from creeping to locked fault behavior on the san andreas fault it exhibits young topography and it has a good spatial distribution of seismicity the tomographic data set is extensive consisting of 1445 events 96 stations and nearly 95 000 travel time readings tomographic images are resolvable to depths of 12 km and show significant velocity contrasts across the san andreas and calaveras faulls a low velocity zone associated with the creeping section of the san andreas fault and shallow low velocity sediments in the southern santa clara valley and nonhem salinas valley relocated earthquakes only occur where vp s kmls and indicate that portions of the san andreas and calaveras faults are non vertical although we cannot complctcly cxclude the possibility hat all or parr of this rcsults from ray uacing problems the new dips are more consistent with geological observations that dipping faults intcrscct thc surfacc where surface traces have been mapped the topographic modeling predicts extensive subsidence in regions characterized by shallow low velocity material presumably the result of recent sedimentation some details of the topography at the junction of the san andreas and calaveras faults are not consisknl with lhe modeling results suggesting that the current position or this triple junction has changed with time the model also predicts those parts of the fault subject to contraction or extension perpendicular to the fault strike and hence the sense of any dip slip component in each locality the relative vertical motion accross the fault is consistent with the fault dips found with the new hypocentral locations

the improved retrieval of coastal sea surface heights by retracking modified radar altimetry waveforms measuring sea surface height ssh using satellite altimetry in coastal from coasts and shallow water region has long been a challenge since the radar altimeter waveforms are often contaminated by complex coastal topography and do not conform to theoretical brown waveform shapes the land contamination or surface variation due to ocean dynamics induce spurious peaks in altimeter waveforms that deviate from brown s theoretical model as the altimeter footprint approaches or leaves the shoreline these spurious peaks should be mitigated to minimize the error in the determination of the leading edge and associated track offset in the waveform retracking process here we introduce a novel algorithm to modify coastal waveforms 0 5 7 km from coasts using 20 hz altimetry data thus improving coastal data coverage and accuracy we apply our processing algorithm and use various retrackers to compare retrieved coastal sshs in four study regions in north america using both envisat and jason 2 altimetry the retrieved altimetry data in the 1 7 km coastal zone indicate that the 20 threshold retracker with modified waveform has a rmse of 21 cm as compared with in situ tide gauge data which corresponds to a 63 improvement in accuracy compared to the use of the original deep ocean waveform retracker

slow strain release along the eastern marmara region offshore istanbul in conjunction with enhanced local seismic moment release abstract we analyze a large transient strainmeter signal recorded at 62 5 m depth along the southern shore of the eastern sea of marmara region in northwestern turkey this region represents a passage of stress transfer from the izmit rupture to the marmara seismic gap the strain signal was recorded at the esenkoy site by one of the icdp gonaf international continental drilling programme geophysical observatory at the north anatolian fault strainmeters on the armutlu peninsula with a maximum amplitude of 5 microstrain and lasting about 50 days the onset of the strain signal coincided with the origin time of a mw 4 4 earthquake offshore yalova which occurred as part of a seismic sequence including eight mw 3 5 earthquakes the mw 4 4 event occurred at a distance of about 30 km from esenkoy on june 25th 2016 representing the largest earthquake in this region since 2008 before the event the maximum horizontal strain was subparallel to the regional maximum horizontal stress derived from stress inversion of local seismicity during the strain transient we observe a clockwise rotation in the local horizontal strain field of about 20 the strain signal does not correlate with known environmental parameters such as annual changes of sea level rainfall or temperature the strain signal could indicate local slow slip on the cinarcik fault and thus a transfer of stress to the eastern marmara seismic gap

a satellite altimeter model for ocean slick detection about 5 of ku band altimeter ocean data are degraded by the occurrence of high radar return cross sections 0 usually called 0 blooms during blooms which occur during no or low wind conditions the mean altimeter waveform can significantly depart from the expected shape in about 60 of the cases the waveforms are distorted to such an extent that either the range tracker loses lock or the off nadir angle estimate becomes unrealistic the analysis of high data rate altimeter waveforms during bloom events reveals the presence of v shaped patterns similar to the ones observed during rain events these patterns trace small scale i e smaller than the altimeter footprint changes in surface backscatter such variations of surface roughness are commonly observed in sar images under low wind conditions on the basis of the experience gained through the analysis of high resolution altimeter waveforms in the presence of rain cell a model is developed to analyze the altimeter response to phenomena whose length scale is smaller than the altimeter footprint the model is applied to simple patterns linear slicks and circular patches as well as to realistic surface 0 estimated by sar it is also used to analyze bloom events in terms of surface slicks the model results shows that the small scale 0 variations explain the behavior of altimeter waveforms in bloom events the results also show that a good proportion of data during bloom events are still valid for estimating geophysical parameters as the brown model remains valid use of high resolution altimeter waveforms may also offer an interesting mean to study marine slick occurrence rates and type

long term creep rate changes and their causes 1 we discuss measurements of the long term creep rate from the san juan bautista section of the san andreas fault these observations of aseismic slip span a period of approximately half a century making it the longest creep record ever measured along an active fault zone the records show systematic changes in creep rate over time scales of tens of years and we address potential causes of these changes in principle they may be caused either by changes in resistive forces within the fault zone or by variations in plate tectonic driving forces such driving force variations have recently been suggested to have occurred in the same area western north america and within the same time window these should give rise to observable effects in the creep data but we find no evidence for temporal changes in plate tectonic forcing at these time scales a natural explanation for the observed long term changes is elastic reloading of adjacent locked portions of the san andreas fault after large earthquakes and possibly aseismic strain release that occurs before the earthquakes

historic surface slip along the san andreas fault near parkfield california the parkfield earthquake prediction experiment is focusing close attention on the 44 km long section of the san andreas fault that last ruptured seismically in 1966 ms 6 0 the 20 km long central segment of the 1966 parkfield rupture extending from the mainshock epicenter at middle mountain southeastward to gold hill forms a 1 to 2 km salient northeastward away from the dominant n40 w strike following the 1966 earthquake afterslip aseismic slip has been nearly constant moderate parkfield earthquakes have recurred on average every 21 years since 1857 when a great earthquake m 8 ruptured at least as far north as the southern parkfield segment many measurements of slip have been made near parkfield since 1966 nevertheless much of the history of surface slip remained uncertain especially the total amount associated with the 1966 event in 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 parkfield rupture segment we interpret net slip on each feature as a sum of event slip sum of coseismic and rapid preseismic and postseismic slip from parkfield earthquakes and steady interseismic slip as measured over the last 20 years on nearby alinement arrays creep meters and trilateration lines we assumed for each site that event slip was identical for the 1922 1934 and 1966 parkfield events and that long term average rate of interseismic slip was constant between all events two fences on the southern segment southeast of gold hill indicate event slip of 13 and 15 cm and interseismic slip rate of 0 36 and 0 30 cm yr since 1959 and 1908 respectively at these sites redundant independent data support our assumption that both event and interseismic slip occur uniformly on the central segment near parkfield both the 1934 and the 1966 ruptures offset a bridge built in 1932 interseismic slip rate near the bridge has been about 1 1 cm yr since 1966 thus we deduce an average event slip of 31 cm for the 1934 and 1966 earthquakes on a parallel fault trace 1 km to the southwest slip was about 8 cm in 1966 thus total event slip summed across the entire fault zone near parkfield was nearly 40 cm on middle mountain 4 km north of the 1966 mainshock epicenter an offset fence indicates 17 cm of slip in 1966 and a 2 26 cm yr interseismic slip rate since circa 1946 thus the central segment of the 1966 rupture is characterized by much larger event slip 40 cm than both distal segments 15 cm this amount of surface slip per event is about twice what had been previously assumed larger 1966 surface slip in the central part of the rupture is geodetically compatible with a coseismic slip of 65 10 cm slip on a narrow buried asperity between middle mountain and gold hill that has been inferred from the depth distribution of early aftershocks assuming our characteristic surface slip model one can further deduce a deficit in slip since the great 1857 earthquake taking the long term slip rate as 3 3 cm yr the surface slip deficit is 3 0 2 m south of gold hill but only 0 3 0 3 m northward from parkfield

probabilistic seismic hazard in the san francisco bay area based on a simplified viscoelastic cycle model of fault interactions 1 we construct a viscoelastic cycle model of plate boundary deformation that includes the effect of time dependent interseismic strain accumulation coseismic strain release and viscoelastic relaxation of the substrate beneath the seismogenic crust for a given fault system time averaged stress changes at any point not on a fault are constrained to zero that is kinematic consistency is enforced for the fault system the dates of last rupture mean recurrence times and the slip distributions of the assumed repeating ruptures are key inputs into the viscoelastic cycle model this simple formulation allows construction of stress evolution at all points in the plate boundary zone for purposes of probabilistic seismic hazard analysis psha stress evolution is combined with a coulomb failure stress threshold at representative points on the fault segments to estimate the times of their respective future ruptures in our psha we consider uncertainties in a four dimensional parameter space the rupture peridocities slip distributions time of last earthquake for prehistoric ruptures and coulomb failure stress thresholds we apply this methodology to the san francisco bay region using a recently determined fault chronology of area faults assuming single segment rupture scenarios we find that future rupture probabilities of area faults in the coming decades are the highest for the southern hayward rodgers creek and northern calaveras faults this conclusion is qualitatively similar to that of working group on california earthquake probabilities but the probabilities derived here are significantly higher given that fault rupture probabilities are highly model dependent no single model should be used to assess to time dependent rupture probabilities we suggest that several models including the present one be used in a comprehensive psha methodology as was done by working group on california earthquake probabilities

earthquake potential along the northern hayward fault california the hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace dislocation models of the surface deformation adjacent to the hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at 7 millimeters per year to the bottom of the seismogenic zone along a 20 kilometer long northern fault segment microearthquakes with the same waveform repeatedly occur at 4 to 10 kilometer depths and indicate deep creep at 5 to 7 millimeters per year the difference between current creep rates and the long term slip rate of 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern hayward fault adjacent to the locked 1868 earthquake rupture which broke the southern 40 to 50 kilometers of the fault the potential for a major independent earthquake of the northern hayward fault might be less than previously thought

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

statistical characterization of geosat altimetry noise dependence on environmental parameters 1 sea surface height ssh measurements from the geosat satellite altimetry mission are analyzed to statistically characterize their noise process which is defined here as the small scale 50 km time varying component of the altimetry data this analysis is one component of a larger effort to extract meaningful information about seafloor roughness from the small scale fabric of the altimetric gravity field both small scale seafloor topography e g abyssal hills and ssh noise significantly influence the measured gravity fabric hence a global quantitative understanding of ssh noise is required to infer what portion of the gravity fabric is associated with the true geoid it is also anticipated that the ssh noise analysis will prove useful in designing improvements to altimetry processing algorithms the ssh data are first processed by differencing nearest neighbor tracks to minimize the static component of variability and then by high pass filtering to remove variability associated with mesoscale oceanic circulation autocovariance analysis reveals that ssh noise can be decomposed into uncorrelated white noise and correlated semiperiodic components three parameters are measured which characterize the variance of both components and the correlation length of the correlated component the global distribution of uncorrelated noise variance is closely correlated to significant wave heights directly linking the uncertainty of the altimeter return time to the roughness of the sea surface the variance and correlation length of the correlated component of noise can be associated with three separate environmental parameters sea ice precipitation rates and upper atmospheric variability associated with the subtropical jet stream as widely differing as these environmental processes are it is hypothesized that features like sea ice rain storms and atmospheric turbulence can cause sudden perturbations in the return time of the altimetry signal creating decaying oscillations associated with the onboard tracking algorithms

the mission link between the hayward and calaveras faults the left stepover between the calaveras and hayward faults gives rise to compressive strain that is accommodated by vertical deformation producing the highest hills bordering the east side of san francisco bay monument peak to mission peak the projection of the active trace of the hayward fault is separated 3 5 km from the junction of the northern and southern segments of the calaveras fault at calaveras reservoir for pure strike slip on the hayward and calaveras faults the stepover region is compressed in area at a rate given by the separation times the long term slip rate of 9 mm yr on the hayward fault assuming a seismogenic depth of 10 km the horizontal compression accumulated over 150 years is equivalent to a dip slip seismic moment of 2 8 1018 n m this moment if released in a single event would generate an earthquake of magnitude 6 3 due to dip slip alone dip slip may occur on buried faults throughout the stepover region the base of the southwest face of monument peak suggests a buried thrust or reverse fault dipping northeast the prominent scarp on mission peak has the same strike and sense of slip current microearthquakes between the calaveras and hayward faults align parallel to the mission fault but show only horizontal slip on a vertical fault plane a dipping fault is needed to satisfy the geometric constraint of the convergence therefore the mission fault zone must consist of two or more fault surfaces inclined at different dip angles as is observed in the san andreas fault zone in the loma prieta area horizontal slip is building up stress on the inclined fault which is currently locked growing compressive stress will increasingly impede horizontal slip until a thrust or reverse slip event occurs allowing then more horizontal slip to be transferred from the calaveras to the hayward fault the dip slip fault might be the unstable element governing slip in the larger system

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

lateral heterogeneity in the upper oceanic crust at deep sea drilling project site 504 the lateral variability of the upper oceanic crust can be studied by mapping the amplitude or power content of arrivals from a dense array of identical sources to a single receiver in this paper we present the results from a borehole seismic experiment at deep sea drilling project site 504 in the eastern equatorial pacific seismic amplitude anomalies of 20 db and more are present with a horizontal correlation length of 1 3 km finite difference synthetic seismogram techniques are used to predict seismic wave propagation through realistic bathymetry basement relief and lateral variations within basement a three dimensional model of the velocity structure around the borehole can be constructed the resultant model has horizontal gradients greater than 2 0 s 1 in the upper crust these gradients are comparable to the vertical gradients normally associated with oceanic crust assuming that velocity anomalies in the upper crust reflect changes in porosity the resultant structure provides constraints on the hydrothermal circulation at the site in addition the finite difference modeling indicates that polarity reversals can be generated by lateral velocity variations near the source and receiver

travel time analysis of borehole seismic data a method is presented for reducing travel time data from multiple offset borehole seismic experiments to velocity depth structure the technique which treats simultaneously data from any number of depths in the borehole in addition to surface data is based on the linear inversion scheme of dorman and jacobson 1981 given the parameters p t p px p and p t p px p the method solves for depth to specified slownesses assuming linear gradients in velocity between slowness values a practical limitation on the use of surface travel time measurements to resolve velocity depth structure is the necessity of assuming a surface velocity this is not necessary for the case of borehole data for a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel time curve thus a direct measure of the uppermost velocity of a section can be obtained from the travel time data of a shallow borehole receiver estimation of velocities from the inflection points of deeper receivers improves the resolution of the velocity depth function which would be obtained from surface data alone the technique is applied to data from three borehole seismic experiments in oceanic crust the experiments were carried out in the western atlantic crustal age 110 m y the gulf of california crustal age 1 m y and the costa rica rift area crustal age 6 m y all three experiments show relatively high upper crustal velocities 4 0 km s suggesting that layer 2a is not present even in the very young crust all sites had over 100 m of sediment thickness and it is postulated that sediment thickness and sediment permeability not merely age govern the velocity of the upper crust by accelerating the cementation of fractures and cracks

long term fault creep observations in central california the u s geological survey usgs has been monitoring aseismic fault slip in central california for more than 10 years as part of an earthquake prediction experiment since 1968 the usgs creep network has grown from one creep meter at the cienega winery south of hollister to a 44 station network that stretches from hayward east of san francisco bay to palmdale in southern california in general the long term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period e g calaveras fault near hollister and san andreas fault between san juan bautista and bear valley these fault sections are the most difficult to characterize with a single long term slip rate in contrast sections of the faults that are seismically relatively quiet e g san andreas fault between bear valley and parkfield produce the steadiest creep records and are easiest to fit with a single long term slip rate appendix is available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j82 004 1 00 payment must accompany order

continuous gps observations across the hayward fault california 1991 1994 in september 1991 the u s geological survey began continuous operation of two permanent global positioning system gps sites near the hayward fault we use two and one half years of data from an 8 km baseline to investigate gps processing strategies errors in the time and frequency domains and the uncertainties of rates of change calculated from such data experiments with session lengths show that at least 6 hours of data should be used to obtain a precision of 2 to 4 mm experiments with broadcast and improved orbits show that the broadcast orbit is sufficient for this short baseline single frequency solutions have larger rms scatter for scenarios made mostly in daylight ionospheric delay systematically shortens the baseline length by 2 4 parts per million for l1 and 4 0 parts per million for l2 for the dual frequency results the rms scatter about the best fitting straight line is 2 1 mm for baseline length 2 2 mm for north 2 9 mm for east and 11 2 mm for vertical for winton relative to chabot the rates of change are 0 6 0 1 mm yr in length 2 8 0 1 mm yr in north 8 1 0 2 mm yr in east and 8 1 0 6 mm yr in vertical the baseline rate of change is consistent with right lateral shear across the hayward fault but is not consistent with the 3 5 mm yr predicted by the model of lienkaemper et al 1991 the large westward and downward motion of winton relative to chabot may be due to monument instability power spectra appear white at high frequencies the estimated standard deviations for periods shorter than 5 days are 1 2 mm for length and north 1 9 mm for east and 5 5 mm for vertical power spectral density increases only slightly as frequency decreases from 0 2 to 0 03 cycle per day and there is no distinct corner in particular no characteristic 1 2 signature of random walk monument noise emerges from the white noise estimated autocorrelations for length north east and vertical fall to about 0 1 for lag times shorter than 10 days and fluctuate about zero for lag times longer than about 25 days because the time series is short it is possible that random walk monument noise exists but is undetectable in the estimated power spectral density and autocorrelation functions we use the estimated full covariance matrix to calculate the standard deviations of the baseline rate of change for various sampling schemes the theoretical standard deviation of dl dt determined from 1 year of daily observations is 0 28 times that determined from 1 year of annual observations we can obtain a similar uncertainty from 2 years of measurements made every 30 days and better results from 5 years of annual measurements however daily measurements allow the detection and correction of offsets that sometimes occur with equipment or firmware changes

four dimensional transform fault processes progressive evolution of step overs and bends many bends or step overs along strike slip faults may evolve by propagation of the strike slip fault on one side of the structure and progressive shut off of the strike slip fault on the other side in such a process new transverse structures form and the bend or step over region migrates with respect to materials that were once affected by it this process is the progressive asymmetric development of a strike slip duplex consequences of this type of step over evolution include 1 the amount of structural relief in the restraining step over or bend region is less than expected 2 pull apart basin deposits are left outside of the active basin and 3 local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes this type of evolution of step overs and bends may be common along the dextral san andreas fault system of california we present evidence at different scales for the evolution of bends and step overs along this fault system examples of pull apart basin deposits related to migrating releasing right bends or step overs are the pliopleistocene merced formation tens of km along strike the pleistocene olema creek formation several km along strike along the san andreas fault in the san francisco bay area and an inverted colluvial graben exposed in a paleoseismic trench across the miller creek fault meters to tens of meters along strike in the eastern san francisco bay area examples of migrating restraining bends or step overs include the transfer of slip from the calaveras to hayward fault and the greenville to the concord fault ten km or more along strike the offshore san gregorio fold and thrust belt 40 km along strike and the progressive transfer of slip from the eastern faults of the san andreas system to the migrating mendocino triple junction over 150 km along strike similar 4d evolution may characterize the evolution of other regions in the world including the dead sea pull apart the gulf of paria pull apart basin of northern venezuela and the hanmer and dagg basins of new zealand d 2004 elsevier b v all rights reserved

order and diversity in the modes of circum pacific earthquake recurrence recurrence characteristics of great circum pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture as noted by others the pattern of moment release is typically very irregular with strong concentrations in a few isolated regions of a much larger aftershock zone despite this complexity rupture nucleation is notably systematic with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release this generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip well documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well ordered while source dimensions slip and cumulative moment release vary considerably from cycle to cycle on none of the segments studied do earthquakes consistently recur in nearly identical events instead individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events despite these differences the duration of the seismic cycle is approximately uniform furthermore when strain release occurs in a sequence of large earthquakes these events take place towards the end of the cycle and occur with increasing rupture length and magnitude these ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults the ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength the irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event although these mechanistic associations are indirect and tentative the features of recurrent behavior documented here have implications for long term earthquake hazard assessment that are not dependent on the models proposed to explain them chief among these are the cycle to cycle differences among gap filling events and the absence of shocks that fill major slip deficient regions early in the seismic cycle

a simple explanation for the occurrence of the 1911 morgan hill earthquake in the stress shadow of the 1906 san francisco earthquake we propose a new model to overcome an apparent stress shadow paradox the stress shadow of the 1906 san francisco earthquake was so effective in inhibiting earthquake activity that no m 6 earthquakes occurred in the san francisco bay area for more than 70 years after the 1906 earthquake with one exception the 1911 morgan hill earthquake how could this earthquake occur only 5 years after the 1906 earthquake in an area where earthquake activity should have been inhibited our model is characterized by localized locked patches on the morgan hill segment of the calaveras fault surrounded by steadily creeping patches according to this model the tectonic stressing rate on the morgan hill segment is so high that the stress shadow caused by the 1906 earthquake can be dissipated within several years this simple model is consistent with both the surface deformation data and the apparent earthquake recurrence at morgan hill in contrast to two previous models which are either poorly constrained or have difficulty explaining such observations

fault trace complexity cumulative slip and the shape of the magnitude frequency distribution for strike slip faults a global survey summary we examine whether the shape of the magnitude frequency distribution for strike slip faults is described by the gutenburg richter relationship log n a bm or by the characteristic earthquake model by analysing a data set of faults from california mexico japan new zealand china and turkey for faults within regional seismic networks curves of the form log n yrpl a bm where n yr is the number of events per year equal to magnitude m are fit to the instrumental record of seismicity and geological data are used to estimate independently the size and recurrence rate of the largest expected earthquakes that would rupture the total length of the fault extrapolation of instrumentally derived curves to larger magnitudes agrees with geological estimates of the recurrence rate of the largest earthquakes for only four of the 22 faults if uncertainties in curve slope are considered and significantly underestimates the geological recurrence rates in the remaining cases also if we predict the seismicity of the faults as a function of fault length and slip rate and the predicted seismicity is distributed in accord with the gutenburg richter relationship we find the predicted recurrence rate to be greater than the observed recurrence rates of smaller earthquakes along most faults if individual fault zones satisfy the gutenburg richter relationship over the long term our observations imply that during the recurrence interval of the largest expected earthquakes the recurrence of lesser sized events is not steady but rather strongly clustered in time however if the instrumental records provide an estimate of the long term rate of small to moderate earthquakes along the faults our observations imply that the faults generally exhibit a magnitude frequency distribution consistent with the characteristic earthquake model also we observe that the geometrical complexity of strike slip faults is a decreasing function of cumulative strike slip offset the four faults we observe to be consistent with the gutenburgrichter relationship are among those characterized by the least amount of cumulative slip and greatest fault trace complexity we therefore suggest that the ratio of the recurrence rate of small to large earthquakes along a fault zone may decrease as slip accumulates and the fault becomes smoother

time dependent model of creep on the hayward fault from joint inversion of 18 years of insar and surface creep data 1 spatial and temporal variations of aseismic fault creep influence the size and timing of large earthquakes along partially coupled faults to solve for a time dependent model of creep on the hayward fault we invert 18 years of surface deformation data 1992 2010 obtained by interferometric processing of 52 and 50 sar images acquired by the ers1 2 and envisat satellites respectively and surface creep data obtained at 19 alinement and 4 creepmeter stations for multi temporal analysis of the sar data we developed a method for identifying stable pixels using wavelet multi resolution analysis we also implement a variety of wavelet based filters for reducing the effects of environmental artifacts using a reweighted least squares approach we inverted the interferometric data to generate a time series of surface deformation over the san francisco bay area with a precision of better than a few millimeters to jointly invert the insar displacement time series and the surface creep data for a time dependent model of fault creep we use a robust inversion approach combined with a kalman filter the time dependent model constrains a zone of high slip deficit that may represent the locked rupture asperity of past and future m 7 earthquakes we identify several additional temporal variations in creep rate along the hayward fault the most important one being a zone of accelerating slip just northwest of the major locked zone we estimate that a slip rate deficit equivalent to mw 6 3 6 8 has accumulated on the fault since the last event in 1868

finite difference elastic wave propagation in 2d heterogeneous transversely isotropic media1 the velocity stress formulation for propagation of elastic seismic waves through 2d heterogeneous transversely isotropic media of arbitrary orientation is presented the equations are recast into a finite difference scheme and solved numerically using fourth order spatial operators and a second order temporal operator on a staggered grid absorbing free surface and symmetry boundary conditions have been implemented test cases compare well with other published solutions synthetic seismograms are calculated over two idealized models i vertical fractures in granite with a dolerite sill reflector and ii a dipping anisotropic shale comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future

evidence for surface rupture in 1868 on the hayward fault in north oakland and major rupturing in prehistoric earthquakes wgcep90 estimated the hayward fault to have a high probability 0 45 in 30 yr of producing a future m7 bay area earthquake this was based on a generic recurrence time and an unverified segmentation model because there were few direct observations for the southern fault and none for the northern hayward fault to better constrain recurrence and segmentation of the northern hayward fault we trenched in north oakland unexpectedly we observed evidence of surface rupture probably from the m7 1868 earthquake this extends the limit of that surface rupture 13 km north of the segmentation boundary used in the wgcep90 model and forces serious re evaluation of the current two segment paradigm although we found that major prehistoric ruptures have occurred here we could not radiocarbon date them however the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated ad 1640 1776

strain accumulation along the san andreas fault system east of san francisco bay california prescott w h and lisowski m 1983 strain accumulation along the san andreas fault system east of san francisco bay california in p vyskocil a m wassef and r green editors recent crustal movements 1982 tectonophysics 97 41 56 the occurrence of several large earthquakes to the east of san francisco bay during historical times and present high levels of microseismicity indicate that a significant part of the relative plate motion may be occurring east of san francisco bay furthermore the hayward fault is known to be slipping aseismically at the surface and the calaveras fault may be slipping aseismically also these facts raise an important question is the observed creep rate accommodating all of the east bay deformation or is there a significant amount of strain accumulating along these faults several small survey networks 2 km diameter located along the hayward and calaveras faults have been measured occasionally since 1965 recent observations of these and other networks have been made by the u s geological survey these observations imply a surface slip rate on the hayward fault at fremont hayward berkeley and richmond of about 6 mm yr on the calaveras fault north of the hayward calaveras fault junction surface slip rates have been determined from only four data sets three of which give a rate of 3 mm yr the u s geological survey annually measures 32 longer lines 10 30 km in the east bay observations of these lines extend back to 1977 for most and to 1970 for some of the unes the observed creep rates and the data for the longer east bay lines provide constraints on the amount and position of deeper slip on the hayward and calaveras faults after correcting for line length changes due to fault slip we calculated the strain accumulation rate the shear strain rate parallel to east bay faults is 0 07 0 02 strain yr a rate well below that of other areas along the san andreas fault system suggesting that creep is relieving a large part of the strain in this area

evaluating the use of high frequency radar coastal currents to correct satellite altimetry 1 coastal altimeter waveforms may differ from the ones in the open ocean either from rapid changes in the sea state or the presence of land within the satellite altimeter footprint the optimal retracking method for an individual track may turn out to be a combination of several retrackers and may depend on the sea state the coastal high frequency radar hfr ocean surface currents hourly interpolated with a resolution up to 2 km and an offshore range up to 150 km are evaluated to validate the altimeter sea surface height ssh measurements a method to derive hfr ssh mapped with a varying spatial scale optimal interpolation from the hfr velocities has been implemented evaluated mainly in the regions farther than 25 km off the u s west coast the hfr ssh shows good agreement with jason 1 2 altimetry products over the years 2008 and 2009 three jason 2 pistach retrackers and one generic open ocean retracker have been analyzed using the traditional 1 hz sampling rate nearshore an experimental reprocessing of the 20 hz range measurements is also tested to check for a gain in along track spatial resolution referencing to the hfr ssh indicate the need to have several retrackers available even over the continental shelf with ice3 fitting better during bloom events and mle 4 or red3 for high sea states these studies demonstrate the value of hfr as a potential tool to correct coastal altimeter ssh refine their spatial resolution and provide some insight into the altimeter behavior as a function of ocean conditions

ground motion modeling of hayward fault scenario earthquakes part i construction of the suite of scenarios abstract we construct kinematic earthquake rupture models for a suite of 39 m w 6 6 7 2 scenario earthquakes involving the hayward calaveras and rodgers creek faults we use these rupture models in 3d ground motion simulations as discussed in part ii aagaard et al 2010 to provide detailed estimates of the shaking for each scenario we employ both geophysical constraints and empirical relations to provide realistic variation in the rupture dimensions slip heterogeneity hypocenters rupture speeds and rise times the five rupture lengths include portions of the hayward fault as well as combined rupture of the hayward and rodgers creek faults and the hayward and calaveras faults we vary rupture directivity using multiple hypocenters typically three per rupture length yielding north to south rupture bilateral rupture and south to north rupture for each rupture length and hypocenter we consider multiple random distributions of slip we use two approaches to account for how aseismic creep might reduce coseismic slip for one subset of scenarios we follow the slip predictable approach and reduce the nominal slip in creeping regions according to the creep rate and time since the most recent earthquake whereas for another subset of scenarios we apply a vertical gradient to the nominal slip in creeping regions the rupture models include local variations in rupture speed and use a ray tracing algorithm to propagate the rupture front although we are not attempting to simulate the 1868 hayward fault earthquake in detail a few of the scenarios are designed to have source parameters that might be similar to this historical event

inferred depth of creep on the hayward fault central california a relation between creep rate at the surface trace of a fault the depth to the bottom of the creeping zone and the rate of stress accumulation on the fault is derived from weertman s 1964 friction model of slip on a fault a 5 1 km depth for the creeping zone on the hayward fault is estimated from the measured creep rate 5 mm yr at the fault trace and the rate of stress increase on the upper segment of the fault trace inferred from geodetic measurements across the san francisco bay area although fault creep partially accommodates the secular slip rate on the hayward fault a slip deficit is accumulating equivalent to a magnitude 6 6 earthquake on each 40 km segment of the fault each century thus the current behavior of the fault is consistent with its seismic history which includes two moderate earthquakes in the mid 1800 s

the greenville fault preliminary estimates of its long term creep rate and seismic potential abstract once assumed to be locked we show that the northern third of the greenville fault gf creeps at 2 mm yr based on 47 yr of trilateration net data this northern gf creep rate equals its 11 ka slip rate suggesting a low strain accumulation rate in 1980 the gf easternmost strand of the san andreas fault system east of san francisco bay produced an m w 5 8 earthquake with a 6 km surface rupture and dextral slip growing to 2 cm on cracks over a few weeks trilateration shows a 10 cm post 1980 transient slip ending in 1984 analysis of 2000 2012 crustal velocities on continuous global positioning system stations allows creep rates of 2 mm yr on the northern gf 0 1 mm yr on the central gf and 0 mm yr on its southern third modeled depth ranges of creep along the gf allow 5 25 aseismic release greater locking in the southern two thirds of the gf is consistent with paleoseismic evidence there for large late holocene ruptures because the gf lacks large 1 km discontinuities likely to arrest higher 1 m slip ruptures we expect full length 54 km ruptures to occur that include the northern creeping zone we estimate sufficient strain accumulation on the entire gf to produce m w 6 9 earthquakes with a mean recurrence of 575 yr while the creeping 16 km northern part has the potential to produce an m w 6 2 event in 240 yr it may rupture in both moderate 1980 and large events these two dimensional model estimates of creep rate along the southern gf need verification with small aperture surveys online material green net data file

historic creep rate and potential for seismic slip along the hayward fault california the hayward fault is considered the most likely source of one or more major earthquakes in the san francisco bay area in the next few decades historically at least one and probably two major earthquakes about m 6 8 occurred along the hayward fault one in 1836 and another in 1868 little is known about the 1836 event but the 1868 earthquake was accompanied by a surface rupture that extended as much as 41 km along the southern part of the fault although the amount of surface slip in 1868 is uncertain right slip including afterslip reached at least several centimeters and possibly several decimeters in places this paper documents the spatial variation of creep rate along the hayward fault since the 1868 earthquake creep aseismic fault slip occurs over at least 66 km and may extend over the fault s entire 82 km length of which about 13 km lies underwater creep rate seems nearly constant over decades but short term variations occur we derive creep rate mainly from our own systematic surveying of offset cultural features curbs fences and buildings on each feature we solve directly for accumulated creep by using multiple linear regression creep rate mostly falls in the range of 3 5 6 5 mm yr but systematic variation occurs along strike fault segments with distinctly higher and lower rates generally correspond to parts of the fault most salient from the overall average alinement of the fault most distinctive is a 4 km long section near the south end of the fault that creeps at about 9 mm yr such a high rate has occurred there at least since the 1920s and probably since the 1868 earthquake as indicated by an offset railroad track built in 1869 we suggest that this 9 mm yr slip rate may approach the long term or deep slip rate that controls average recurrence interval between major earthquakes if so assuming an elastic rebound model the potential for slip in large earthquakes below the surficial creeping zone is now 1 1 m in the southern 1868 segment of the fault and 1 4 m in the northern 1836 segment subtracting surface creep rates from a long term slip rate of 9 mm yr gives present potential for surface slip in large earthquakes of up to 0 8 m with an average of 0 6 m in the northern segment and 0 4 m in the southern segment we present a simple hypothesis for rupture potential that is compatible with historic creep rate microseismicity distribution and geodetic data if seismic rupture occurs on segments 41 km long by 10 km deep 7 km fully locked 3 km creeping today s potential for seismic moment release is 1 4 1019 and 1 1 1019 n m for both 1836 and 1868 segments respectively and 2 5 1019 n m for both segments jointly converting moment to magnitude gives ml 6 8 in the northern segment ml 6 7 in the southern segment and ml 7 0 for simultaneous rupture of both

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

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

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

gravity magnetic and high precision relocated seismicity profiles suggest a connection between the hayward and calaveras faults northern california 1 gravity magnetic and seismicity data profiled across the hayward fault zone were generated as part of ongoing studies to help determine the geologic and tectonic setting of the san francisco bay region these data combined with previous geophysical studies that indicate that the hayward fault zone dips 75 ne near san leandro and follows a preexisting structure reveal a possible direct connection between the seismogenic portion of the hayward and calaveras faults at depth although the relocated seismicity data are regional in nature they suggest that the dip of the hayward fault zone may vary from near vertical in the northwestern part of the fault to about 75 ne at san leandro in the central part of the fault to about 50 ne in the southeastern part of the fault gravity and magnetic data profiled across the hayward fault zone were processed using standard geophysical techniques cross sections of high precision relocated hypocenters were constructed along each profile from the northwestern to the southeastern end of the hayward fault zone profiles and cross sections are referenced to pinole point where the hayward fault enters san pablo bay and are spaced 2 5 km apart topographic profiles shown on the seismicity cross sections were generated using u s geological survey usgs 7 5 min 30 m digital elevation models relocation of seismicity data was accomplished using a regional double difference method the double difference method incorporates ordinary travel time measurements and cross correlation of p and s wave differential travel time measurements relative locations between earthquakes have hypocentral errors of about 100 m horizontally and 250 m vertically absolute location uncertainties were not determined but are probably dramatically improved compared to the usgs s northern california seismic network catalog data

slip in the 1868 hayward earthquake from the analysis of historical triangulation data despite the importance of the hayward fault for earthquake hazards in the san francisco bay area neither the rupture length nor the average coseismic slip in the most recent major earthquake which occurred in 1868 is well known we estimate slip in the 1868 earthquake by analyzing triangulation data collected between 1853 1860 and 1876 1891 we apply a denuisancing procedure that projects out the dependence of that data on station coordinates allowing their displacements to be estimated directly the best fit to the data is obtained with a 52 km long by 10 km deep rupture that extends from warm springs to near berkeley considerably longer than the 30 km of reported surface breakage the estimated slip for an earthquake of this length is 1 9 0 4 m and the seismic moment is m0 3 0 0 7 1019 n m corresponding to mw 7 0 the data are inconsistent with regionally high strain rate prior to the 1906 earthquake as had been previously suggested the triangulation data record northeastward displacement of station loma prieta presumably as a result of the 1865 earthquake the fault normal displacement is not easily explained by slip on either the san andreas fault or the 1989 loma prieta rupture surface this observation is however consistent with an mw 6 3 4 thrust event northeast of the san andreas fault in the southern santa cruz mountains thrust belt

long term creep rates on the hayward fault evidence for controls on the size and frequency of large earthquakes the hayward fault hf in california exhibits large m w 6 5 7 1 earthquakes with short recurrence times 161 65 yr probably kept short by a 26 78 aseismic release rate including postseismic its interseismic release rate varies locally over time as we infer from many decades of surface creep data earliest estimates of creep rate primarily from infrequent surveys of offset cultural features revealed distinct spatial variation in rates along the fault but no detectable temporal variation since the 1989 m w 6 9 loma prieta earthquake lpe monitoring on 32 alinement arrays and 5 creepmeters has greatly improved the spatial and temporal resolution of creep rate we now identify significant temporal variations mostly associated with local and regional earthquakes the largest rate change was a 6 yr cessation of creep along a 5 km length near the south end of the hf attributed to a regional stress drop from the lpe ending in 1996 with a 2 cm creep event north of there near union city starting in 1991 rates apparently increased by 25 above pre lpe levels on a 16 km long reach of the fault near oakland in 2007 an m w 4 2 earthquake initiated a 1 2 cm creep event extending 10 15 km along the fault using new better constrained long term creep rates we updated earlier estimates of depth to locking along the hf the locking depths outline a single 50 km long locked or retarded patch with the potential for an m w 6 8 event equaling the 1868 hf earthquake we propose that this inferred patch regulates the size and frequency of large earthquakes on hf online material 2007 event creep models plots of creepmeter data maps and cross sections of relocated microearthquakes and active fault traces and iterative solutions for depth of creep

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

earthquake deformation cycle on the san andreas fault near parkfield california six moderate earthquakes are thought to have ruptured the parkfield segment of the san andreas fault since 1857 the similar characteristics of the three seismically recorded events together with the quasi regular 22 year recurrence interval have led to the forecast of a similar event in 1988 5 years in this study the potential for the hypothesized earthquake is assessed by estimating the time required for interseismic straining to recover the strain energy released by the most recent june 1966 parkfield earthquake for a simple model of the earthquake cycle this interval is proportional to the ratio of the 1966 seismic moment to the interseismic moment deficit rate baseline lengths measured before and after the 1966 earthquake are inverted to estimate the magnitude and distribution of the combined coseismic and transient postseismic slip although the geodetic data determine only broad spatial averages of the fault slip they do require that slip at depth exceeded the surface offsets measured in 1966 inversions with extreme prior models are calculated to assess the range of seismic moment and moment deficit rates consistent with the available geodetic data inversions assuming only that the slip distributions are to some degree smooth yield strain energy recovery intervals of 14 25 years extreme values range from as little as 6 years to as much as 29 years in comparison historic recurrence intervals of parkfield earthquakes range from 12 to 32 years these results are consistent with the expectation of a magnitude 5 6 earthquake near parkfield by 1993

a review of finite difference methods for seismo acoustics problems at the seafloor understanding seismic wave propagation in realistic seafloor environments is essential for many problems in marine seismology finite difference methods are becoming increasingly popular in solving propagation problems as the limitations of other techniques which apply only at high frequency or for flat lying media become fully appreciated the seafloor problem a high contrast in poisson s ratio at a rough sharp interface is particularly challenging and many published formulations fail to solve it accurately the purpose of this paper is to summarize the published finite difference formulations for the elastic wave equation and to outline their applicability to seafloor problems

surface faulting along the superstition hills fault zone and nearby faults associated with the earthquakes of 24 november 1987 the m 6 2 elmore desert ranch earthquake of 24 november 1987 was associated spatially and probably temporally with left lateral surface rupture on many northeast trending faults in and near the superstition hills in western imperial valley three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the superstition hills fault zone as far as 9 km the maximum observed surface slip 12 5 cm was on the northern of the three the elmore ranch fault at a point near the epicenter twelve hours after the elmore ranch earthquake the m 6 6 superstition hills earthquake occurred near the northwest end of the right lateral superstition hills fault zone surface rupture associated with the second event occurred along three strands of the zone here named north and south strands of the superstition hills fault and the wienert fault for 27 km southeastward from the epicenter in contrast to the left lateral faulting which remained unchanged throughout the period of investigation the right lateral movement on the superstition hills fault zone continued to increase with time a behavior that was similar to other recent historical surface ruptures on northwest trending faults in the imperial valley region we measured displacements over 339 days at as many as 296 sites along the superstition hills fault zone and repeated measurements at 49 sites provided sufficient data to fit with a simple power law data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes the maximum right lateral slip at 1 day was about 50 cm near the south central part of the north strand of superstition hills fault and the predicted maximum final displacement is probably about 112 cm at imler road near the center of the south strand of the superstition hills fault the overall distributions of right lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture the average estimated final right lateral slip for the superstition hills fault zone is about 54 cm the average left lateral slip for the conjugate faults trending northeastward is about 23 cm the southernmost ruptured member of the superstition hills fault zone newly named the wienert fault extends the known length of the zone by about 4 km the southern half of this fault south of new river expressed only vertical displacement on a sinuous trace the maximum vertical slip by the end of the observation period there was about 25 cm but its growth had not ceased photolineaments southeast of the end of new surface rupture suggest continuation of the superstition hills fault zone in farmland toward mexico

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

fault creep and microseismicity on the hayward fault california implications for asperity size 1 the hayward fault is documented to undergo significant creep with some patches accommodating 50 or more of the long term fault displacement in spite of this the fault has also experienced moderate to large earthquakes by comparing the patterns of microseismicity observed on the fault with models of fault zone creep we can investigate the long term displacement deformation history of the fault in terms of the relative roles of aseismic creep fault slip accommodated through microseismicity and strain accumulation slip deficit we find that microseismicity on the hayward fault produces a negligible percentage of the seismic moment dissipated on the fault combining seismicity with our fault creep models allows us to calculate the size of asperities on the creeping fault for small asperities associated with repeating earthquakes on the hayward fault the rupture areas of these asperities range from 20 to 60 m2

upper crustal velocity structure in the rose area of the east pacific rise we describe a method to constrain the seismic velocity structure of the upper 500 800 m of young oceanic crust conventional interpretations of seismic refraction profiles provide little or no information about this shallowmost crust the method described here requires the assumption of a linear velocity gradient and utilizes measured values of range travel time and slowness of the observed refracted waves results are reported from several refraction profiles that sample crust between 0 and 4 m y in age on the flanks of the east pacific rise at 12 n these data give an average gradient in the uppermost 800 m of the crust of 3 5 s 1 and a velocity at the seafloor of 2 5 km s ray path and travel time modeling show that the refracted waves sampling the uppermost part of the crust are effectively masked by reflections from the seafloor

finite difference seismic modeling of axial magma chambers we tested the feasibility of using finite difference methods to model seismic propagation at 10 hz through a two dimensional representation of an axial magma chamber with a thin liquid lid our technique produces time series of displacement or pressure at seafloor receivers to mimic a seismic refraction experiment and snapshots of p and s energy propagation the results indicate that our implementation is stable for models with sharp velocity contrasts and complex geometries we observe a high energy downward traveling shear phase observable only with borehole receivers that would be useful in studying the nature and shape of magma chambers the ability of finite difference methods to model high order wave phenomena makes this method ideal for testing velocity models of spreading axes and for planning near axis drilling of the east pacific rise in order to optimize the benefits from shear wave imaging of sub axis structure

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

present tectonic motion across the coast ranges and san andreas fault system in central california geodetic results from very long baseline interferometry vlbi satellite laser ranging slr and the global positioning system gps are used to estimate angular velocities between the sierran microplate pacific plate and north american plate the sierra pacific pole of rotation lies nearer to the san andreas fault than does the pacific north america pole of rotation and leads to different tectonic implications than if the latter is used the angular velocities show that the san andreas fault system and central california coast ranges accommodate motion of 39 2 mm yr mainly by strike slip faulting all confidence limits following signs in this paper are 95 confidence limits fault normal motion is small is mainly convergent at rates up to 3 3 1 0 mm yr and varies along the coast but is divergent at 2 6 1 2 mm yr across san pablo bay and associated topographic lows across which the sierran and central valley watershed drains to the pacific ocean the mountain ranges tend to be larger where the fault normal convergence rates are larger the low convergence rate 0 5 1 8 mm yr normal to the san andreas fault in the carrizo plain differs sharply from that previously inferred 8 2 1 2 mm yr and 4 9 1 6 mm yr by feigl et al 1993 the difference is due to differences between their and our elastic strain accumulation models and between how their and our pacific plate reference frames are defined the ranges in most places require a minimum of 4 2 1 m y of fault normal convergence at the present rate to attain their present cross sectional area if erosion is neglected more if it is not the amount of convergence previously estimated from a balanced cross section across the diablo range in central california requires 10 8 3 m y of convergence at the present rate the former is consistent with widely held views about the onset of the coast range orogeny but the latter is not both are consistent however with the recent plate reconstructions by s cande j stock and colleagues which indicate that pacific plate motion relative to north america changed to a more convergent direction 20 25 clockwise of its prior direction at ca 8 to 6 ma and not at 3 5 ma as had been previously inferred the inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the san andreas fault of 0 7 4 7 from which we infer that the sierran microplate changed motion relative to north america at the same time ca 8 to 6 ma as did the pacific plate we further infer that the motion accommodated across the great basin must also have changed at the same time we also examine the hypothesis that stable sliding occurs along the san andreas fault and other northwest striking strike slip faults in central california where the fault normal convergence rate is low or negative and that these faults are unstable where the fault normal convergence rate is high such a relationship appears to hold in general but fails in detail in particular there are substantial sections of fault with small inferred rates of fault normal convergence across which the san andreas fault is locked moreover the creeping section of the san andreas fault i e the section between parkfield and the calaveras junction is the locus of greater fault normal convergence 3 2 1 4 mm yr than is the locked part of the fault 0 5 1 8 mm yr south of parkfield thus this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault

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

finite difference seismograms for laterally varying marine models summary the method of finite differences is applied to the elastic wave equation to generate synthetic seismograms for laterally varying seafloor structures the results are compared with borehole seismic data from the gulf of california deep sea drilling project site 485 in which lines were shot over flat and rough topography the significant new phenomenon observed in both the synthetic seismograms and the field data is the generation of a double head wave due to the interaction of the incident wavefront with the side of a hill and the flat seafoor adjacent to the hill in these models the hills are on the order of a seismic wavelength in height and steep velocity gradients occur over distances comparable to wavelengths ray theoretical methods would not be suitable for studying such structures true amplitude record sections are obtained by the finite difference method which show for these models that the head wave generated at the flat seafloor adjacent to the hill is lower in amplitude than if the hill were not present and is lower in amplitude than the head wave generated at the hill a second feature which is important for borehole receivers is the existence of the direct wave root in the upper basement this energy occurs below the sharp interface when the direct wave impinges on the interface from above there is no corresponding snell s law ray path for this energy and the energy is evanescent with depth in the lower medium the properties of both the double head wave and the direct wave root are clearly demonstrated in the finite difference snapshot displays

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

holocene slip rate of the hayward fault at union city california measured offsets of well dated alluvial fan deposits near the masonic home in union city constrain holocene slip rate of the hayward fault between 7 and 9 mm yr our best minimum geologic slip rate over the past 8 27 0 08 kyr i e 8270 years is 8 0 0 7 mm yr a steep stream its channel cut into bedrock flows southwest out of the east bay hills crosses the fault and deposits its load on an alluvial fan we cut two 5 m deep fault parallel trenches 20 30 m southwest of the main fault through the crest of the fan walls of the trenches reveal a series of nested distributary channel fills these channels had cut into old surfaces that are indicated by paleosols developed on flood silts we distinguished many channel fills by their shape clast size flow direction elevation and relation to paleosols enabling us to correlate them between both trenches two distinct episodes of fan deposition occurred during the holocene reconstructing the apex positions of these fan units indicates that about 42 6 m and 66 6 m of fault slip has occurred since their inceptions at about 4 58 0 05 ka and 8 27 0 05 ka respectively we lowered the age and age uncertainty of the younger unit from earlier reports based on new multiple radiocarbon dates the 4 58 ka slip rate of 9 2 1 3 mm yr is not significantly different at 95 confidence from the 8 27 ka slip rate of 8 0 0 7 mm yr because current regional strain rates are fully consistent with neogene plate tectonic rates lisowski et al 1991 and the historic surface rate of creep in union city is only 4 7 0 1 mm yr galehouse 1994 the larger 8 mm yr holocene slip rate implies that strain is now accumulating on a locked zone at depth the 8 mm yr rate is probably minimal because earlier trenching evidence nearby implies that some unknown additional amount of fault deformation occurs outside of the narrow fault zone assumed in measuring slip lienkaemper et al 1991 suggest that the fast creep rate of 9 mm yr measured near the southern end of the hayward fault may underestimate the deep slip rate because 1868 surface slip occurred there in addition to the continuing fast creep if the historic deep slip rate equals the long term rate then the 9 mm yr creep rate reflects the minimum seismic loading rate of the hayward fault better than the 8 mm yr holocene rates do