southern california permanent gps geodetic array continuous measurements of regional crustal deformation between the 1992 landers and 1994 northridge earthquakes the southern california permanent gps geodetic array pgga was established in 1990 across the pacific north america plate boundary to continuously monitor crustal deformation we describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes we compare displacement rates at four site locations with those reported by feigl et al 1993 which were derived from an independent set of global positioning system gps and very long baseline interferometry vlbi measurements collected over nearly a decade prior to the landers earthquake the velocity differences for three sites 65 100 km from the earthquake s epicenter are of order of 3 5 mm yr and are systematically coupled with the corresponding directions of coseismic displacement the fourth site 300 km from the epicenter shows no significant velocity difference these observations suggest large scale postseismic deformation with a relaxation time of at least 800 days the statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets two possible noise models fit the pgga data equally well as described in the companion paper by zhang et al this issue the pre landers data are too sparse and heterogeneous to derive a reliable noise model under a fractal white noise model for the pgga data we find that the velocity differences for all three sites are statistically different at the 99 significance level a white noise plus flicker noise model results in significance levels of only 94 43 and 88 additional investigations of the pre landers data and analysis of longer spans of pgga data could have an important effect on the significance of these results and will be addressed in future work

creep along the imperial fault southern california from gps measurements 1 in may of 1999 and 2000 we surveyed with global positioning system gps 46 geodetic monuments established by imperial college london in a dense grid half mile spacing along the imperial fault with three additional national geodetic survey sites serving as base stations these stations were previously surveyed in 1991 and 1993 the imperial college sites were surveyed in rapid static mode 15 20 min occupations while the ngs sites continuously received data for 10 h d 1 site locations were calculated using the method of instantaneous positioning and velocities were determined relative to one of the ngs base stations combining our results with far field velocities from the southern california earthquake center scec we fit the data to a simple elastic dislocation model with 35 mm yr 1 of right lateral slip below 10 km and 9 mm yr 1 of creep from the surface down to 3 km the velocity field is asymmetrical across the fault and could indicate a dipping fault plane to the northeast or a viscosity contrast across the fault

influence of the earthquake cycle and lithospheric rheology on the dynamics of the eastern california shear zone the eastern california shear zone is bounded by the high heat flow region of the basin and range province and the low heat flow region of the sierra nevada block this difference in thermal state influences the rheology of the lower crust upper mantle resulting in a viscosity contrast between the two regions we analyze the effect of such a contrast on the kinematics and dynamics of the shear zone with numerical models this viscosity contrast drives asymmetric strain accumulation in the upper crust producing an asymmetric surface velocity field an additional consequence of this strain pattern is the potential for asymmetric co seismic displacement during an earthquake

geodetic detection of active faults in s california a new analysis of velocities of geodetic mark ers straddling the san andreas fault system in southern california reveals that interseismic deformation is localized along a dozen sub parallel narrow belts of high shear strain rate that correlate well with active geologic fault segments andlocallywithconcentratedzonesofmicroseismicity high shearstrainrates 0 3 0 95strain year areobservednorth ward and southward of the san andreas fault s big bend whereas the big bend itself is characterized by a diuse low magnitudeshearstrainrate dilatationaldeformationisdif fuse and of relatively low magnitude 0 2 strain year with the highest contraction rates occurring in the ventura andlos angeles basins because no prior assumptions were made regarding the geology tectonics or seismicity of the region ouranalysisdemonstratesthatgeodeticobservations alone can be used to detect active fault segments

on the consistency of earthquake moment rates geological fault data and space geodetic strain the united states new and dense space geodetic data can now map strain rates over continental wide areas with a useful degree of precision stable strain indicators open the door for space geodesy to join with geology and seismology in formulating improved estimates of global earthquake recurrence in this paper 174 gps vlbi velocities map united states strain rates of 30 0 10 8 yr 1 with regional uncertainties of 5 to 50 per cent kostrov s formula translates these strain values into regional geodetic moment rates two other moment rates m seismic and m geologic extracted from historical earthquake and geological fault catalogues contrast the geodetic rate because m geologic m seismic and m geodetic derive from different views of the earthquake engine each illuminates different features in california the ratio of m geodetic to m geologic is 1 20 the near unit ratio points to the completeness of the region s geological fault data and to the reliability of geodetic measurements there in the basin and range northwest and central united states both m geodetic and m seismic greatly exceed m geologic of possible causes high incidences of understated and unrecognized faults probably drive the inconsistency the ratio of m seismic to m geodetic is everywhere less than one the ratio runs systematically from 70 80 per cent in the fastest straining regions to 2 per cent in the slowest although aseismic deformation may contribute to this shortfall i argue that the existing seismic catalogues fail to reflect the long term situation impelled by the systematic variation of seismic to geodetic moment rates and by the uniform strain drop observed in all earthquakes regardless of magnitude i propose that the completeness of any seismic catalogue hinges on the product of observation duration and regional strain rate slowly straining regions require a proportionally longer period of observation characterized by this product gamma distributions model statistical properties of catalogue completeness as proxied by the ratio of observed seismic moment to geodetic moment i find that adequate levels of completeness should exist in median catalogues of 200 to 300 year duration in regions straining 10 7 yr 1 comparable to southern california similar levels of completeness will take more than 20 000 years of earthquake data in regions straining 10 9 yr 1 comparable to the southeastern united states predictions from this completeness statistic closely mimic the observed m seismic to m godetic ratios and allow quantitative responses to previously unanswerable questions such as what is the likelihood that the seismic moment extracted from an earthquake catalogue of x years falls within y per cent of the true long term rate the combination of historical seismicity fault geology and space geodesy offers a powerful tripartite attack on earthquake hazard few obstacles block similar analyses in any region of the world

fault creep along the southern san andreas from interferometric synthetic aperture radar permanent scatterers and stacking 1 interferometric synthetic aperture radar insar provides a practical means of mapping creep along major strike slip faults the small amplitude of the creep signal 10 mm yr combined with its short wavelength makes it difficult to extract from long time span interferograms especially in agricultural or heavily vegetated areas we utilize two approaches to extract the fault creep signal from 37 ers sar images along the southern san andreas fault first amplitude stacking is utilized to identify permanent scatterers which are then used to weight the interferogram prior to spatial filtering this weighting improves correlation and also provides a mask for poorly correlated areas second the unwrapped phase is stacked to reduce tropospheric and other short wavelength noise this combined processing enables us to recover the near field 200 m slip signal across the fault due to shallow creep displacement maps from 60 interferograms reveal a diffuse secular strain buildup punctuated by localized interseismic creep of 4 6 mm yr line of sight los 12 18 mm yr horizontal with the exception of durmid hill this entire segment of the southern san andreas experienced right lateral triggered slip of up to 10 cm during the 3 5 year period spanning the 1992 landers earthquake the deformation change following the 1999 hector mine earthquake was much smaller 1 cm and broader than for the landers event profiles across the fault during the interseismic phase show peak to trough amplitude ranging from 15 to 25 mm yr horizontal component and the minimum misfit models show a range of creeping locking depth values that fit the data

comparison of geodetic and geologic data from the wasatch region utah and implications for the spectral character of earth deformation at periods of 10 to 10 million years the wasatch fault and adjacent fault zones provide an opportunity to compare present day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows ranging from the last millennium up to 10 myr the three easternmost gps sites of the basin and range geodetic network bargen at this latitude define a 130 km wide region spanning three major normal faults extending east west at a total rate of 2 7 0 4 mm yr with an average regional strain rate estimated to be 21 4 nstrain yr about twice the basin and range average on the wasatch fault the vertical component of the geologic displacement rate is 1 7 0 5 mm yr since 6 ka 0 6 mm yr since 130 ka and 0 5 0 7 mm yr since 10 ma however it appears likely that at the longest timescale rates slowed over time from 1 0 to 1 4 mm yr between 10 and 6 ma to 0 2 to 0 3 mm yr since 6 ma the cumulative vertical displacement record across all three faults also shows time variable strain release ranging from 2 to 4 mm yr since 10 ka to 1 mm yr averaged over the past 130 kyr conventional earthquake recurrence models reid type behavior would require an accordingly large variation in strain accumulation or loading rate on a 10 kyr timescale for which there appears to be no obvious geophysical explanation alternatively seismic strain release given a wide range of plausible constitutive behaviors for frictional sliding may be clustered on the 10 kyr timescale resulting in the high holocene rates with comparatively low uniform strain accumulation rates on the 100 kyr timescale wallace type behavior the latter alternative combined with observations at the million year timescale and the likelihood of a significant contribution of postseismic transients implies maxima of spectral amplitude in the velocity field at periods of 10 myr variations in tectonic loading 10 kyr clustered strain release and of 100 years postseismic transients if so measurements of strain accumulation and strain release may be strongly timescale dependent for any given fault system

crustal strain near the big bend of the san andreas fault analysis of the los padres tehachapi trilateration networks california in the region of the los padres tehachapi geodetic network the san andreas fault saf changes its orientation by over 30 degree from n 40 degree w close to that predicted by plate motion for a transform boundary to n 73 degree w the strain orientation near the saf is consistent with right lateral shear along the fault with maximum shear rate of 0 38 plus minus 0 01 mu rad yr at n 63 degree w in contrast away from the saf the strain orientations on both sides of the fault are consistent with the plate motion direction with maximum shear rate of 0 19 plus minus 0 01 mu rad yr at n 44 degree w the strain rate does not drop off rapidly away from the fault and thus the area is fit by either a broad shear zone below the saf or a single fault with a relatively deep locking depth the fit to the line length data is poor for locking depth d less than 25 km for d of 25 km a buried slip rate of 30 plus minus 6 mm yr is estimated the authors also estimated buried slip for models that include the garlock and big pine faults in addition to the saf slip rates on other faults are poorly constrained bymore the los padres tehachapi network the best fitting garlock fault model had computed left lateral slip rate of 11 plus minus 2 mm yr below 10 km buried left lateral slip of 15 plus minus 6 mm yr on the big pine fault within the western transverse ranges provides significant reduction in line length residuals however deformation there may be more complicated than a single vertical fault a subhorizontal detachment on the southern side of the saf cannot be well constrained by these data the authors investigated the location of the saf and found that a vertical fault below the surface trace fits the data much better than either a dipping fault zone located south of the surface trace less

kinematics of transpressional deformation in the eastern san francisco bay region california inversions of earthquake focal mechanisms for brittle strain show that the dextral hayward fault forms the western boundary of a distinct seismotectonic domain in the eastern san francisco bay region east of the hayward fault the seismogenic strain field is characterized by subhorizontal extension oriented northwest southeast and subhorizontal shortening oriented northeast southwest major folds and thrust faults east of the hayward fault are oriented normal to the direction of maximum shortening but have formed in a predominantly strike slip tectonic setting and exhibit a right stepping en echelon geometry typical of dextral wrench folds the direction of maximum right lateral shear strain in this domain is n20 w which generally is parallel to the strike of the dextral calaveras greenville and concord faults but oriented 15 25 more northerly than the strike of the san andreas and hayward faults to the west the clockwise rotation of maximum right lateral shear strain from west to east across the hayward fault is confirmed by geodetic measurements the kinematic consequences of this variation in regional strain for deformation east of the hayward fault include the following 1 dextral faults must form left restraining contractional stepovers in order to maintain continuity and conserve slip along strike thus creating localized fold and thrust belts 2 dextral faulting accommodates a component of shortening normal to the boundary between the pacific plate and the sierra nevada central valley microplate thus obviating the need for a laterally continuous boundary parallel zone of thrust faulting along the eastern margin of the coast range coast ranges sierran block boundary zone to accommodate plate normal motion at the latitude of the east bay domain the transpressional kinematics of the east bay domain may be present in other obliquely convergent orogenic belts and they have implications for seismic hazard assessment in the highly urbanized san francisco bay area

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

an apparent shear zone trending north northwest across the mojave desert into owens valley eastern california strain rates measured at four geodetic networks in eastern california situated between northern owens valley and transverse ranges along a small circle drawn about the pacific north america pole of rotation are remarkably consistent each exhibits 0 14 mu rad yr simple right lateral engineering shear strain accumulation across the local vertical plane tangent to the small circle local faults e g owens valley garlock helendale traversing these networks are not as closely aligned with the vertical planes of maximum shear strain accumulation as is the local tangent to the small circle a fifth network slightly east of the small circle shows no significant strain accumulation thus a shear zone trending n35 degree w from near the eastern end of the big bend of the san andreas fault to northern owens valley is indicated by these data this corresponds to the eastern california shear zone proposed on geological evidence by dokka and travis the shear zone carries approximately 8 mm yr of the pacific north america relative plate motion from the san andreas fault north northwest across the mojave desert into owens valley and the northern basin and range province the shear zone observed at the surface may be a manifestation of a through going subcrustal fault

kinematics of the southwestern u s deformation zone inferred from gps motion data 1 we have estimated the surface deformation field of the southwestern u s deformation zone in terms of the velocity gradient field and surface creep simultaneously by inversion of 497 geodetic velocities the model shows aseismic fault motion consistent with aseismic creep measurements and a sense of motion consistent with geological observations we deduce that our surface deformation field shows distributed deformation in a zone around the faults containing shear strains and rotations the eastern california shear zone acts as a distinct fault zone bounded by more rigid blocks the faults within the zone partition the shear motion while right lateral shear strain rates and clockwise rotations are concentrated between the bounding faults in the same way the san jacinto and southern san andreas faults act as bounding faults of a fault zone the mojave desert is dominated by right lateral shear whereas the western transverse ranges wtr is subjected to contraction we find significant localization of deformation east of the san andreas fault between the transverse ranges and the san francisco bay we attribute this to a relative rigidity contrast across the fault of 1 8 finally a moment deficit analysis shows an accumulation of moment deficit between 1973 and 2000 corresponding to a mw 6 1 6 3 earthquake along the san andreas fault just north of the big bend around the imperial and southern san andreas faults and in the san francisco bay area along the hayward and southern calaveras faults

strain accumulation across the central nevada seismic zone 1973 1994 five trilateration networks extending for 280 km along the central nevada seismic zone 1915 pleasant valley m 7 3 1954 dixie valley m 6 8 1954 stillwater m 6 8 1954 rainbow mountain m 6 6 1954 fairview peak m 7 1 and 1932 cedar mountain m 7 2 have been surveyed 6 times since 1973 to determine deformation along the zone within the precision of measurement the deformation appears uniform along the zone and is described by the principal strain rates 0 036 0 008 strain yr n60 w 3 and 0 031 0 008 strain yr n30 e 3 extension reckoned positive the observed strain rates are consistent with simple right lateral tensor shear at the rate of 0 033 strain yr across a shear zone striking n15 w this central nevada shear zone appears to be the northward continuation of the eastern california shear zone the orientation of the strike slip and normal slip ruptures within the central nevada seismic zone are consistent with principal stress axes parallel to the measured principal strain rate axes space based geodetic measurements very long baseline interferometry indicate that the relative motion accommodated across the basin and range province west of ely nevada is about 9 1 1 5 mm yr n16 w 8 dixon et al 1995 notice that the right lateral shear zone postulated to explain deformation in the central nevada seismic zone is properly oriented to accommodate that relative motion however a 135 km effective width of the shear zone would be required to accommodate all of the 9 1 mm yr relative motion at the strain rates observed in the nevada seismic zone only about 3 mm yr of that relative motion is accommodated within the span of the trilateration networks

surface deformation and tectonic setting of taiwan inferred from a gps velocity field 1 we have determined the present day surface deformation of taiwan by computing the velocity gradient field and fault slip from 143 gps velocity vectors in southern taiwan the derived strain and rotation rates and fault slips are indicative of lateral extrusion toward the south in northern taiwan we infer the onset of gravitational collapse which is induced by the on land extension of the okinawa trough in the eastern central range the observed inverted nw se extension is consistent with geological observations and high heat flow measurements this could be the result of exhumation of crustal material the model further shows a significant decrease in slip rate northward along the longitudinal valley fault at 23 7 n the northern coastal range shows high strain rates and two oppositely rotating blocks by combining the surface deformation model with seismicity data and seismic tomography we are able to propose a coherent model for the present day tectonic activity both seismicity and tomography show further evidence for active southward propagating exhumation of a crustal slice in the eastern central range offshore east taiwan we deduce strong evidence of a southward propagating crustal tear fault accommodating most of the philippine sea plate eurasian plate convergence the tear is the crustal response to incipient northwestward subduction of the philippine sea plate thus the ryukyu trench is bending southward becoming almost perpendicular to the convergence direction while subduction of the philippine sea plate continues in this setting a sudden rapid southward propagation of the afore mentioned tear is conceivable

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

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

active displacement transfer and differential block motion within the central walker lane western great basin velocities determined for 50 global positioning system sites within the central walker lane indicate differential motion among tectonic blocks forming a boundary zone between the great basin extensional province and the sierra nevada the velocity field is related to displacement transfer from the owens valley and furnace creek fault systems of east ern california to transtensional structures of the walker lane and extensional faults of the central nevada seismic belt block boundaries are sharp and appear to be inherited from pre tertiary crustal structure the block geometries exert strong influence on dif ferential displacements concentrated along boundaries as belts of divergent transcurrent and convergent motion the aggregate velocity accounts for about 25 of the relative motion between the pacific and north american plates about 5 mm yr of the motion is localized along the eastern margin of the sierra nevada whereas about 10 mm yr is stepped 100 km east along a belt of east northeast trending transtensional faults that merge with northwest trending transcurrent structures of the walker lane about 6 mm yr of the velocity field is transferred to north northeast trending extensional faults of the central nevada seismic belt the heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and n408w directed shear

stress coupling between earthquakes in northwest turkey and the north aegean sea we have investigated the coulomb stress interactions of 29 earthquakes m s greater than or equal to 6 0 that have occurred in the region of northwest turkey and north aegean sea since 1912 of these events 23 may be related to earlier events and 16 are clearly related to earlier events all events after 1967 are related to previous events events in the early part of our time interval that show no correlation could be related to historical events as yet unidentified in some cases faults that have received a stress reduction from earlier events are prepared for an event by an earthquake occurring a few years before that creates a local coulomb stress rise thus regions of coulomb stress shadow can become regions where a damaging earthquake may occur the relation between smaller events and the coulomb stress distribution is less clear but may be related to poor data quality and practical limitations of our modeling technique nonetheless there are 4 times as many events per unit area in regions of enhanced stress than where stress is reduced we discuss the contemporary distribution of coulomb stress and argue that it is possible to identify the likely locations of future damaging earthquakes including identifying the most likely candidate faults

aseismic creep along the san andreas fault northwest of parkfield ca measured by radar interferometry ers 1 radar images acquired 14 months apart studied by differential radar interferometry show the wide area distribution of aseismic creep along the fault segment northwest of parkfield california a sharp discontinuity in the interferometric phase of less than 2 cm equivalent line of sight displacement extends over 80 km in the differential interferogram coincident with the mapped trace of the active san andreas fault and consistent with the expected and measured fault motion although patterns of strain associated with the transition from locked to creeping are not clearly identifiable a decrease in creep displacement from northwest to southeast along the fault is visible the observations are in agreement with a model of elastic deformation constrained by in situ data that supports a maximum expected deformation signature of 10 mm across the image

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

estimation of rupture processes of the 2008 wenchuan earthquake from joint analyses of two regional seismic arrays article i nfo article history we estimated the source rupture of the 2008 wenchuan earthquake ms 8 china based on a back projection of seismic waves to their source plane using data from regional broadband arrays in taiwan and northern vietnam observations from these arrays located at different azimuths were processed to eval uate the spatio temporal rupture behavior of the fault the seismic energy spot converted from windowed array waveforms was back projected to the rupture plane of the earthquake to image the instantaneous slip on the fault plane rupture processes were reconstructed based on the imaged time dependent seismic energy radiating from the earthquake fault plane the high station density of both arrays enabled a detailed examination of the rupture processes the results indicate that the 2008 wenchuan earthquake had a rupture duration of approximately 100 s with major asperities radiating energy at 50 s after the initiation of rupture the location of asperities is in good agreement with sections of the earthquake fault along which major dam age occurred such as along the yingxiu beichuan fault the radiated seismic energy shows a complex spatial distribution on the fault plane our analysis indicates that the rupture initiated at the epicenter and extended to the northeast for approximately 280 km the average rupture velocity of this earthquake was approxi mately 2 8 km s

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

southern california deformation southern california is one of the most seismically active places in the united states and has therefore been of long standing interest to geoscientists in their perspective jackson et al discuss recent work to collect geodetic data on the motion and earthquake potential of the faults in southern california

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

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

the missing complexity in seismically imaged normal faults what are the implications for geometry and production response the impact of geometric uncertainty on across fault flow behaviour at the scale of individual intra reservoir faults is investigated in this study a high resolution digital elevation model dem of a faulted outcrop is used to construct an outcrop scale geocellular grid capturing high resolution fault geometries 5 m scale seismic forward modelling of this grid allows generation of a 3d synthetic seismic cube which reveals the corresponding seismically resolvable fault geometries 12 5 m scale construction of a second geocellular model based upon the seismically resolvable fault geometries allows comparison with the original outcrop geometries running fluid flow simulations across both models enables us to assess quantitatively the impact of outcrop resolution versus seismic resolution fault geometries upon across fault flow the results suggest that seismically resolvable fault geometries significantly underestimate the area of across fault juxtaposition relative to realistic fault geometries in turn this leads to overestimates in the sealing ability of faults and inaccurate calculation of fault plane properties such as transmissibility multipliers tms

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

inversion of relative motion data for estimates of the velocity gradient field and fault slip abstract a new method is presented for a purely kinematic analysis of relative motion data of crustal deformation we start by developing a general integral formulation that links the relative motion between two positions to the velocity gradient field next fault slip is explicitly incorporated leading to the final observation equation this equation provides a new complete and in practice exact description of the time dependent relation between surface motion and the underlying kinematics the combination of equations belonging to pairs of observation sites e g of a geodetic network leads in a natural way to a linear inverse problem from which the velocity gradient field and slip on active faults can be estimated the method is successfully tested in a synthetic experiment

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

shortening and thickening of metropolitan los angeles measured and inferred by using geodesy geodetic observations using the global positioning system gps and other techniques record a high rate of north south shortening in an east southeast trending 5 40 km wide belt in northern metropolitan los angeles california downtown los angeles is observed to be converging upon the southern san gabriel mountains at 6 mm yr aside from the elastic strain that will be released during earthquakes rupturing the san andreas and san jacinto faults east west lengthening across northern metropolitan los angeles is minor 2 5 mm yr therefore north south shortening is accommodated mainly by vertical crustal thickening

contemporary kinematics of the western united states determined from earthquake moment tensors very long baseline interferometry and gps observations using moment tensors from earthquakes between 1850 and 1995 we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western u s plate boundary zone the velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5 of the nuvel ia pacific north american plate motion direction the magnitude of the earthquake related velocity is 62 of the nuvel 1a total pacific north american plate motion if earthquake occurrence is a random poisson like process then the large formal errors associated with the model velocity estimates are an indication that the 144 year interval is too short to define the long term seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long term moment release rate associated with both the seismic and aseismic deformation to spatially resolve the deficit of the earthquake moment release in the last 144 years we have also estimated the total long term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities both with and without the nuvel 1a pacific north american rigid plate motion constraint we find a systematic difference between the southern california earthquake center scec geodetic velocity map and nuvel 1a north american reference frames with a pole at 38 8 n 124 2 w 0 33 m y 1 this difference is significant at the 99 confidence level and indicates that 4 7 mm yr of ne sw convergence is absorbed in offshore areas of southern california if both the scec velocity map and nuvel 1a correctly describe the relative motion between the pacific and north america plates deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long term model strain rate tensors are largest in areas east of san francisco bay in the transverse ranges and along the southern san andreas fault and san jacinto fault the deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an mw 7 earthquake elsewhere the accumulated strain in the last 144 years is small except in northern mojave desert along the garlock fault and in the region southwest of the southern san andreas san jacinto fault zone within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an mw 6 5 7 0 earthquake

block versus continuum deformation in the western united states abstract the relative role of block versus continuum deformation of continental lithosphere is a current subject of debate continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies in contrast block models are favored by geologic studies of displacement in places like asia a problem in this debate is a lack of data from which unequivocal conclusions may be reached in this paper we apply the techniques of study used in regions such as the alpine himalayan belt to an area with a wealth of instrumental data the western united states by comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60 of the pacific north america motion across the plate boundary in california and nevada has occurred seismically and 40 aseismically the san francisco bay area shows similar partitioning between seismic and aseismic deformation and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth in some cases this deformation can be located on creeping surface faults but elsewhere it is spread over a several kilometer wide zone adjacent to the fault these superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere our results support the dominant role of non continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle our conclusions apply only to the regions where the data are good and even within the western united states i e the basin and range deformation styles remain poorly resolved nonetheless we maintain that block motion is the deformation style of choice for those continental regions where the data are best

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

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

heightened odds of large earthquakes near istanbul an interaction based probability calculation we calculate the probability of strong shaking in istanbul an urban center of 10 million people from the description of earthquakes on the north anatolian fault system in the marmara sea during the past 500 years and test the resulting catalog against the frequency of damage in istanbul during the preceding millennium departing from current practice we include the time dependent effect of stress transferred by the 1999 moment magnitude m 7 4 izmit earthquake to faults nearer to istanbul we find a 62 15 probability one standard deviation of strong shaking during the next 30 years and 32 12 during the next decade

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

crustal velocity field near the big bend of california s san andreas fault we use geodetic data spanning the 1920 1992 interval to estimate the horizontal velocity field near the big bend segment of california s san andreas fault saf more specifically we estimate a horizontal velocity vector for each node of a two dimensional grid that has a 15 min by 15 min mesh and that extends between latitudes 34 0 n and 36 0 n and longitudes 117 5 w and 120 5 w for this estimation process we apply bilinear interpolation to transfer crustal deformation information from geodetic sites to the grid nodes the data include over a half century of triangulation measurements over two decades of repeated electronic distance measurements a decade of repeated very long baseline interferometry measurements and several years of global positioning system measurements magnitudes for our estimated velocity vectors have formal standard errors ranging from 0 7 to 6 8 mm yr our derived velocity field shows that 1 relative motion associated with the saf exceeds 30 mm yr and is distributed on the earth s surface across a band 100 km wide that is roughly centered on this fault 2 when velocities are expressed relative to a fixed north america plate the motion within our primary study region has a mean orientation of n44 w 2 and the surface trace of the saf is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5 and 10 counterclockwise relative to these contours and 3 large strain rates shear rates 150 nrad yr and or areal dilatation rates 150 nstr yr exist near the garlock fault near the white wolf fault and in the ventura basin