dislocation models of interseismic deformation in the western united states 1 the gps derived crustal velocity field of the western united states is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation the interseismic velocity field is constrained by 1052 gps velocity vectors spanning the 2500 km long plate boundary zone adjacent to the san andreas fault and cascadia subduction zone and extending 1000 km into the plate interior the gps data set is compiled from u s geological survey campaign data plate boundary observatory data and the western u s cordillera velocity field of bennett et al 1999 in the context of viscoelastic cycle models of postearthquake deformation the interseismic velocity field is modeled with a combination of earthquake sources on 100 known faults plus broadly distributed sources models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins viscoelastic relaxation from distributed faulting in the plate interior as well as lateral variations in depth averaged rigidity in the elastic lithosphere resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the san andreas fault saf system primary deformation characteristics are captured along the entire saf system eastern california shear zone walker lane the mendocino triple junction the cascadia margin and the plate interior up to 1000 km from the major plate boundaries

viscoelastic coupling model for the earthquake cycle driven from below in a linear system the earthquake cycle can be represented as the sum of a solution which reproduces the earthquake cycle itself viscoelastic coupling model and a solution that provides the driving force we consider two cases one in which the earthquake cycle is driven by stresses transmitted along the schizosphere and a second in which the cycle is driven from below by stresses transmitted along the upper mantle i e the schizosphere and upper mantle respectively act as stress guides in the lithosphere in both cases the driving stress is attributed to steady motion of the stress guide and the upper crust is assumed to be elastic the surface deformation that accumulates during the interseismic interval depends solely upon the earthquake cycle solution viscoelastic coupling model not upon the driving source solution thus geodetic observations of interseismic deformation are insensitive to the source of the driving forces in a linear system in particular the suggestion of bourne et al 1998 that the deformation that accumulates across a transform fault system in the interseismic interval is a replica of the deformation that accumulates in the upper mantle during the same interval does not appear to be correct for linear systems

modeling broadscale deformation in northern california and nevada from plate motions and elastic strain accumulation we present a first order method for modeling broadscale deformation consistent with both plate tectonic motions and elastic strain accumulation on plate boundary faults interseismic deformation is assumed to be a superposition of long term rigid body motions between faults defined by angular velocities of spherical plates and backslip on shallow locked portions of faults in an elastic half space this method is applied to 1993 2000 continuous gps data from 35 sites in a profile from the san francisco bay area northern california to eastern nevada deformation is consistent within the 1 mm yr 1 uncertainties of the estimated site velocities with a simple 10 parameter model using 6 rigid plates and 3 locked san andreas system faults

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

northwest basin and range tectonic deformation observed with the global positioning system 1999 2003 1 we use geodetic velocities obtained with the global positioning system gps to quantify tectonic deformation of the northwest basin and range province of the western united states the results are based on gps data collected in 1999 and 2003 across five new quasi linear networks in northern nevada northeast california and southeast oregon the velocities show 3 mm yr westward movement of northern nevada with respect to stable north america west of longitude 119 w the velocities increase and turn northwest parallel to sierra nevada great valley microplate motion and similar to velocities previously obtained to the south the observations are explained by a kinematic model with three domains that rotate around euler poles in eastern oregon and western idaho northeast california experiences internal dextral shear deformation 11 2 3 6 nstrain yr subparallel to pacific north america motion relative motions of the domains imply 2 5 mm yr approximately east west extension in northwest nevada and 1 4 mm yr approximately north south contraction near the california oregon border the northward decreasing approximately east west extension in northwest nevada is consistent with the northern termination of basin and range deformation faulting and characteristic topography no significant extension is detected in the oregon basin and range the oregon cascade arc moves north at 3 5 mm yr and is possibly influenced by the approximately eastward motion of the juan de fuca plate these results disagree with secular northwest trenchward motion of the oregon forearc inferred from paleomagnetic rotations south of latitude 43 however trenchward motion exists and is consistent with block rotations approximately east west basin and range extension and northwest sierra nevada translation

coseismic deformation from earthquake faulting on a layered spherical earth summary a method for calculating the static displacement field following earthquake faulting in a layered spherical earth is presented at shallow levels the earth s layering is characterized by sharp jumps in bulk and shear moduli at the conrad discontinuity and the moho and is therefore important to consider when evaluating crustal deformation the solution to the equations of static equilibrium is represented as a superposition of spheroidal and toroidal components that each depend on spherical harmonic degree and the moment tensor a method that has recently been applied to the problem of wave propagation on a layered spherical earth is here applied to the static deformation field by representing the point source in terms of discontinuities in the displacement stress vector the green s function for a particular source geometry is derived directly numerical tests are presented to verify the accuracy of the method and to illustrate the effects of sphericity and layering on the calculated deformation fields the effect of sphericity is generally less than about 2 per cent of maximum deformation within 100 km of an earthquake source at crustal depths comparisons between the deformation calculated on a spherical homogeneous earth and spherical layered earth show that up to 20 per cent errors would be introduced if the earth s layered structure were ignored the effect of layering is strongest for sources with a strong horizontal slip component

itrf2000 a new release of the international terrestrial reference frame for earth science applications 1 for the first time in the history of the international terrestrial reference frame the itrf2000 combines unconstrained space geodesy solutions that are free from any tectonic plate motion model minimum constraints are applied to these solutions solely in order to define the underlying terrestrial reference frame trf the itrf2000 origin is defined by the earth center of mass sensed by satellite laser ranging slr and its scale by slr and very long baseline interferometry its orientation is aligned to the itrf97 at epoch 1997 0 and its orientation time evolution follows conventionally that of the no net rotation nnr nuvel 1a model the itrf2000 orientation and its rate are implemented using a consistent geodetic method anchored over a selection of itrf sites of high geodetic quality ensuring a datum definition at the 1 mm level this new frame is the most extensive and accurate one ever developed containing about 800 stations located at about 500 sites with better distribution over the globe compared to past itrf versions but still with more site concentration in western europe and north america about 50 of station positions are determined to better than 1 cm and about 100 sites have their velocity estimated to at or better than 1 mm yr level the itrf2000 velocity field was used to estimate relative rotation poles for six major tectonic plates that are independent of the trf orientation rate a comparison to relative rotation poles of the nuvel 1a plate motion model shows vector differences ranging between 0 03 and 0 08 m y equivalent to approximately 1 7 mm yr over the earth s surface itrf2000 angular velocities for four plates relative to the pacific plate appear to be faster than those predicted by the nuvel 1a model the two most populated plates in terms of space geodetic sites north america and eurasia exhibit a relative euler rotation pole of about 0 056 0 005 m y faster than the pole predicted by nuvel 1a and located about 10 n 7 e more to the northwest compared to that model

multianalytical provenance analysis of eastern ross sea lgm till sediments antarctica petrography geochronology and thermochronology detrital data in order to reveal provenance of detrital sediments supplied by west antarctic ice sheet wais 19 glaciomarine cores of last glacial maximum age were analyzed from eastern ross sea and sulzberger bay analytical techniques included petrographic analysis of gravel sized clasts geochronology zircon u pb zrn upb and thermochronology apatite fission track aft of sand sized fractions petrographic analysis revealed a similarity with the lithologies presently exposed in western marie byrd land mbl with major roles played by low grade metamorphic rocks and granitoids furthermore zrn upb and aft data allowed to identify the ages of formation and cooling of sedimentary source area consisting of cambrian precambrian basement i e swanson formation in western mbl which underwent at least two episodes of magma intrusion migmatization and cooling during devonian carboniferous and cretaceous paleocene times scarcity of volcanic clasts in the region of ross sea along the front of west antarctica ice streams in association with the occurrence of aft oligocene pliocene dates suggests a localized tectonic exhumation of portions of mbl as already documented for the opposite side of west antarctic rift system in the transantarctic mountains furthermore a zrn upb and aft population of late triassic jurassic age indicates the presence of unexposed rocks that formed or metamorphosed at that time in the sedimentary source area which could be identified in mcayeal ice stream and bindschadler ice stream catchment areas

postseismic and interseismic displacements near a strike slip fault a two dimensional theory for general linear viscoelastic rheologies 1 we present an analytic solution for the deformation near an infinite strike slip fault in an elastic layer overlying a linear viscoelastic half space the theory is valid for any linear viscoelastic rheology and any earthquake sequence this is a generalization of the work of j c savage and colleagues which only holds for models with a uniform shear modulus maxwell viscoelastic lower half space and periodic rupture recurrence we demonstrate the theory for models of an elastic layer over a maxwell standard linear solid burgers and triviscous half space for each of these models we calculate example postseismic displacements interseismic displacements in a periodic earthquake sequence and interseismic displacements for a nonperiodic earthquake sequence our solution is for a simple geometry however the model presents an elegant tool to explore the evolution of displacements for relatively complex rheologies and rupture recurrence histories

identification and correlation of visible tephras in the lake suigetsu sg06 sedimentary archive japan chronostratigraphic markers for synchronising of east asian west pacific palaeoclimatic records across the last 150 ka abstract the lake suigetsu sg06 sedimentary archive from honshu island central japan provides a high resolution palaeoenvironmental record including a detailed record of explosive volcanism from japan and south korea thirty visible tephra are recorded within the 73 m long sg06 core which spans the last 150 ka here we describe and characterise these tephras based on major element glass composition which is useful for the identification and correlation of these tephras and the age models of the records in which they are found utilising the large number of radiocarbon measurements n 600 from terrestrial plant macrofossils in the lake suigetsu sg06 record we are able to provide precise and accurate ages for the tephras from eruptions within the last 50 ka glass compositional data of some of the largest eruptions from japan k ah at aso 4 aso a aso d and ata sampled at proximal outcrops are also presented these data show that the major element glass chemistry is distinctive for many of the visible sg06 tephra units and allows some of the layers to be correlated to known eruptions from volcanoes in japan and south korea namely k ah sg06 0967 u oki sg06 1288 at sg06 2650 aso 4 sg06 4963 sg06 4979 k tz sg06 5181 aso abcd sg06 5287 and ata sg06 5181 the following ages were obtained for the sg06 tephra units 3 966 4 064 cal ka bp 95 4 probability range for the sg06 0588 tephra 10 242 10 329 cal ka bp 95 4 probability range for sg06 1293 19 487 112 sg06 2012 ka bp 2 for sg06 1965 28 425 194 sg06 2012 ka bp 2 for sg06 2504 28 848 196 sg06 2012 ka bp 2 for sg06 2534 29 765 190 sg06 2012 ka bp 2 for sg06 2601 29 775 191 sg06 2012 ka bp 2 for sg06 2602 43 713 156 sg06 2012 ka bp 2 for sg06 3485 46 364 202 sg06 2012 ka bp 2 for sg06 3668 49 974 337 sg06 2012 ka bp 2 for sg06 3912 50 929 378 sg06 2012 ka bp 2 for sg06 3974 and improved ages for two of the most important tephra markers across japan the k ah 7 165 7 303 cal ka bp at 95 4 probability range sg06 0967 and at tephra 30 009 189 sg06 2012 ka bp at 2 sg06 2650

fault slip and seismic moment of the 1700 cascadia earthquake inferred from japanese tsunami descriptions 1 the 1700 cascadia earthquake attained moment magnitude 9 according to new estimates based on effects of its tsunami in japan computed coseismic seafloor deformation for hypothetical ruptures in cascadia and tsunami modeling in the pacific ocean reports of damage and flooding show that the 1700 cascadia tsunami reached 1 5 m heights at seven shoreline sites in japan three sets of estimated heights express uncertainty about location and depth of reported flooding landward decline in tsunami heights from shorelines and post 1700 land level changes we compare each set with tsunami heights computed from six cascadia sources each source is vertical seafloor displacement calculated with a three dimensional elastic dislocation model for three sources the rupture extends the 1100 km length of the subduction zone and differs in width and shallow dip for the other sources ruptures of ordinary width extend 360 670 km to compute tsunami waveforms we use a linear long wave approximation with a finite difference method and we employ modern bathymetry with nearshore grid spacing as small as 0 4 km the various combinations of japanese tsunami heights and cascadia sources give seismic moment of 1 9 1022 n m equivalent to moment magnitude 8 7 9 2 this range excludes several unquantified uncertainties the most likely earthquake of moment magnitude 9 0 has 19 m of coseismic slip on an offshore full slip zone 1100 km long with linearly decreasing slip on a downdip partial slip zone the shorter rupture models require up to 40 m offshore slip and predict land level changes inconsistent with coastal paleoseismological evidence

mechanical deformation model of the western united states instantaneous strain rate field summary we present a relationship between the long term fault slip rates and instantaneous velocities as measured by global positioning system gps or other geodetic measurements over a short time span the main elements are the secularly increasing forces imposed by the bounding pacific and juan de fuca jdf plates on the north american plate viscoelastic relaxation following selected large earthquakes occurring on faults that are locked during their respective interseismic periods and steady slip along creeping portions of faults in the context of a thin plate system in detail the physical model allows separate treatments of faults with known geometry and slip history faults with incomplete characterization i e fault geometry but not necessarily slip history is available creeping faults and dislocation sources distributed between the faults we model the western united states strain rate field derived from 746 gps velocity vectors in order to test the importance of the relaxation from historic events and characterize the tectonic forces imposed by the bounding pacific and jdf plates relaxation following major earthquakes m 8 0 strongly shapes the present strain rate field over most of the plate boundary zone equally important are lateral shear transmitted across the pacific north america plate boundary along 1000 km of the continental shelf downdip forces distributed along the cascadia subduction interface and distributed slip in the lower lithosphere post earthquake relaxation and tectonic forcing combined with distributed deep slip constructively interfere near the western margin of the plate boundary zone producing locally large strain accumulation along the san andreas fault saf system however they destructively interfere further into the plate interior resulting in smaller and more variable strain accumulation patterns in the eastern part of the plate boundary zone much of the rightlateral strain accumulation along the saf system is systematically underpredicted by models which account only for relaxation from known large earthquakes this strongly suggests that in addition to viscoelastic cycle effects steady deep slip in the lower lithosphere is needed to explain the observed strain rate field

source parameters for the 1952 kern county earthquake california a joint inversion of leveling and triangulation observations coseismic leveling and triangulation observations are used to determine the faulting geometry and slip distribution of the july 21 1952 mw 7 3 kern county earthquake on the white wolf fault a singular value decomposition inversion is used to assess the ability of the geodetic network to resolve slip along a multisegment fault and shows that the network is sufficient to resolve slip along the surface rupture to a depth of 10 km below 10 km the network can only resolve dip slip near the fault ends the preferred source model is a two segment right stepping fault with a strike of 51 and a dip of 75 sw the epicentral patch has deep 6 27 km left lateral oblique slip while the northeastern patch has shallow 1 12 5 km reverse slip there is nearly uniform reverse slip epicentral 1 6 m northeast 1 9 m with 3 6 m of left lateral strike slip limited to the epicentral patch the seismic moment is m0 9 2 0 5 1019 n m mw 7 2 the signal to noise ratio of the leveling and triangulation data is reduced by 96 and 49 respectively the slip distribution from the preferred model matches regional geomorphic features and may provide a driving mechanism for regional shortening across the comanche thrust and structural continuity with the scodie seismic lineament to the northeast

paleoenvironmental changes in the japan sea during the last 85 000 years five distinct changes in the paleoenvironment of the japan sea within the last 85 000 years are revealed from the sedimentary record of a piston core recovered from the oki ridge changes in both surface and deepwater conditions are registered by changes in lithology calcium carbonate content organic carbon content oxygen and carbon isotope ratios and microfossil assemblages including calcareous nannoplankton diatoms radiolaria and foraminifera between 85 and 27 ka the warm tsushima current did not flow into the japan sea and cold surface water conditions prevailed environments at the seafloor fluctuated between dysaerobic to weakly oxic conditions between 27 and 20 ka freshwater input to the japan sea probably from the huang ho river in china stratified the water column and the severe anoxic conditions eliminated most benthic fauna between 20 and 10 ka the cold oyashio current flowed into the japan sea through the tsugaru strait reestablishing deepwater ventilation shallow water benthic assemblages of the north pacific ocean subsequently colonized the japan sea and occupied the vacant niches of the deep basins between 10 and 8 ka the foraminifer compensation level fcl gradually rose to a depth shallower than 1000 m and bottom conditions changed from dysaerobic to oxic at 10 ka the warm tsushima current started to flow into the japan sea through the tsushima strait to establish the modern oceanographic regime which has existed since 8 ka the eustatic sea level during the last glacial maximum was above the sill depths 130 m of the tsushima and tsugaru straits assuming that tectonic movements at these straits were negligible for the last 20 ka

viscoelastic deformation for a clustered earthquake cycle 1 the clustering of earthquakes in time on the same fault affects the rate and pattern of interseismic deformation we develop a simple analytic viscoelastic model of the surface velocity field through a clustered earthquake cycle by superposing the velocities of individual earthquake cycles of constant period but varying phase velocity profiles prior to and after an earthquake show a wider range of behavior than they do for earthquake cycles with constant repeat times these new types of behavior provide possible explanations for discrepancies between geologic estimates of long term fault slip rates and slip rate estimates from geodetic data

strain accumulation at yucca mountain nevada 1983 1998 a 14 station 50 km aperture geodetic array centered on the proposed radioactive waste disposal site at yucca mountain nevada was surveyed in 1983 1984 1993 and 1998 to determine the rate of strain accumulation there the coseismic effects of the 1992 ms 5 4 little skull mountain earthquake which occurred within the array were calculated from a dislocation model and removed from the data the measured principal strain accumulation rates determined over the 1983 1998 interval are e1 2 12 nanostrain yr n87 w 12 and e2 22 12 nanostrain yr n03 e 12 extension reckoned positive and quoted uncertainties are standard deviations the n65 w extension rate is 2 12 nanostrain yr significantly less than the 1991 1997 n65 w rate of 50 9 nanostrain yr reported by wernicke et al 1998 the implied maximum right lateral engineering shear strain accumulation rate is e1 e2 23 10 nanostrain yr a marginally significant rate almost half e1 6 nanostrain yr n90 w e2 6 nanostrain yr n00 e and 12 nanostrain yr of the measured strain rate can be attributed to strain accumulation on the death valley furnace creek 50 km distant and hunter mountain panamint valley 90 km distant faults the residual strain rate after the removal of those fault contributions is not significant at the 95 confidence level

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

gps derived strain in northwestern california termination of the san andreas fault system and convergence of the sierra nevada great valley block contribute to southern cascadia forearc contraction abstract gps derived velocities 1993 2002 in northwestern california show that processes other than subduction are in part accountable for observed upper plate contraction north of the mendocino triple junction mtj region after removing the component of elastic strain accumulation due to the cascadia subduction zone from the station velocities two additional processes account for accumulated strain in northern california the first is the westward convergence of the sierra nevada great valley sngv block toward the coast and the second is the north northwest impingement of the san andreas fault system from the south on the northern california coastal region in the vicinity of humboldt bay sierra nevada great valley block motion is northwest toward the coast convergent with the more northerly north northwest san andreas transform fault parallel motion in addition to the westward converging sierra nevada great valley block san andreas transform parallel shortening also occurs in the humboldt bay region approximately 22 mm yr of distributed pacific sngv motion is observed inland of cape mendocino across the northern projections of the maacama and bartlett springs fault zones but station velocities decrease rapidly north of cape mendocino the resultant 6 10 mm yr of san andreas fault parallel shortening occurs above the southern edge of the subducted gorda plate and at the latitude of humboldt bay part of the san andreas fault parallel shortening may be due to the viscous coupling of the southern edge of the gorda plate to overlying north american plate we conclude that significant portions of the upper plate contraction observed north of the mtj region are not solely a result of subduction of the gorda plate but also a consequence of impingement of the western edge of the sierra nevada great valley block and growth of the northernmost segments of the san andreas fault system

mantle flow beneath a continental strike slip fault postseismic deformation after the 1999 hector mine earthquake two recent large earthquakes in the mojave desert california the magnitude 7 3 1992 landers and magnitude 7 1 1999 hector mine earthquakes have each been followed by elevated crustal strain rates over periods of months and years geodetic data collected after the hector mine earthquake exhibit a temporally decaying horizontal velocity field and a quadrant uplift pattern opposite to that expected for localized shear beneath the earthquake rupture we interpret the origin of this accelerated crustal deformation to be vigorous flow in the upper mantle in response to the stress changes generated by the earthquake our results suggest that transient flow in the upper mantle is a fundamental component of the earthquake cycle and that the lower crust is a coherent stress guide coupling the upper crust with the upper mantle

strain accumulation and rotation in western oregon and southwestern washington 1 velocities of 75 geodetic monuments in western oregon and southwestern washington extending from the coast to more than 300 km inland have been determined from gps surveys over the interval 1992 2000 the average standard deviation in each of the horizontal velocity components is 1 mm yr 1 the observed velocity field is approximated by a combination of rigid rotation euler vector relative to interior north america 43 40 n 0 14 119 33 w 0 28 and 0 822 0 057 myr 1 clockwise quoted uncertainties are standard deviations uniform regional strain rate eee 7 4 1 8 een 3 4 1 0 and enn 5 0 0 8 nstrain yr 1 extension reckoned positive and a dislocation model representing subduction of the juan de fuca plate beneath north america subduction south of 44 5 n was represented by a 40 km wide locked thrust and subduction north of 44 5 n by a 75 km wide locked thrust

dislocation model for aseismic crustal deformation at hollister california geodetic observations at many active plate margins reveal relatively steady aseismic motion during the time between major earthquakes we model the aseismic motion by the relative motion between several blocks that suffer frictional resistance in the upper crust frictional drag is represented by uniform dislocation on a fault surface we assume that friction occurs only on the upper brittle zone of each block boundary and that frictionless sliding occurs below this zone using a bayesian inversion procedure we invert geodetic data to determine the parameters of the block and dislocation model parameters to be estimated include the two horizontal components of velocity for each block and the dislocation rate dip angle slip angle and width of each rectangular fault patch for hollister we assume five blocks and nine rectangular fault patches rates of length change on 92 lines observed by trilateration make up the data triangulation leveling very long baseline interferometry satellite ranging and global positioning system data can also be easily included in our model our inversion procedure uses prior estimates of all block and fault parameters based on geological and seismological data the block parameters are estimated from geological observations of fault displacement rates while the widths of the fault patches are estimated from earthquake hypocentral depths we choose prior estimates of the dislocation rate to match observed creep rates the estimated block motion is well resolved by the geodetic data and agrees well with the geological estimates this suggests that the block motion is steady on time scales from 10 years to 1 m y the net motion across the san andreas calaveras fault system is 38 3 mm yr oriented s38 e thus some additional displacement possibly offshore is needed to match the rate of 56 mm yr predicted by plate tectonic models the depth of the transition from frictional to free sliding varies considerably from 0 4 km on the southern calaveras to 11 km on the central san andreas fault two segments of the san andreas north of 36 50 and south of 36 40 have the greatest potential for moderate to large earthquakes

modeling of postseismic relaxation following the great 1857 earthquake southern california abstract repeated first order triangulation measurements made from 15 monuments in central and southern california are used to explore the structure of the crust and upper mantle beneath southern california as well as to estimate the distribution of slip during the great 1857 earthquake the monuments are divided into two subnetworks with repeated observations covering the respective time intervals from 1884 to 1923 and from 1898 to 1923 the angle changes occurring within these times are modeled as a sum of two contributions postseismic displacements following the 1857 earthquake and plate tectonic loading the postseismic displacements have been calculated using a elastic viscoelastic coupling model that for any fault geometry yields the exact displacements on a spherically stratified earth the southern california crust and upper mantle are modeled as an upper elastic plate underlain by a viscoelastic asthenosphere for several different combinations of structural parameters inversions are performed for the slip distribution of the 1857 earthquake in general the best fits to the data are achieved with a triangular slip distribution an elastic plate thickness of 16 km a crustal thickness of 16 to 33 km and a mantle viscosity of 0 4 to 0 8 10 19 pa s although these inversions identify the best rheological models they lead to observed residuals that are inconsistent with a normally distributed data set a preferred model of slip is constructed that accounts for the budget of total slip during the earthquake cycle and satisfies a normally distributed data set the preferred slip distribution is similar in shape to the mapped surface slip distribution but is greater in amplitude by a factor of 1 5 to 2 in addition the san jacinto fault which has no known surface slip associated with the 1857 earthquake is inferred to have accommodated 1 to 3 m of slip during the 1857 earthquake and appears to be the favored site for tectonic strain accumulation since 1857 inferred fault slip exceeding known surface slip implies either unrecognized secondary faulting or a slip deficit at the surface relative to the slip at depth

strain accumulation near yucca mountain nevada 1993 1998 a 50 km aperture geodetic network centered on the proposed high level radioactive waste disposal site at yucca mountain nevada was surveyed with gps in 1993 and 1998 the average deformation rate across the area is described by the principal strain rates 22 8 8 8 nstrain yr 1 n77 6 w 13 5 and 8 8 11 9 nstrain yr 1 n12 5 e 13 5 extension reckoned positive and a clockwise rotation rate about a vertical axis of 9 6 7 4 nrad yr 1 relative to fixed north america quoted uncertainties are standard deviations those strain rates are consistent with the geodetic strain rates 2 12 nstrain yr 1 n87 12 w and 22 12 nstrain yr 1 n03 12 e previously reported by savage et al 1999 for the 1983 1998 interval and with the low extension rate 5 20 nstrain yr 1 marrett et al 1998 inferred from the geologic record none of those strain rates is consistent with the 50 9 nstrain yr 1 n65 w extension rate for the area reported by wernicke et al 1998

present day kinematics of the eastern california shear zone from a geodetically constrained block model we use global positioning system gps data from 1993 2000 to determine horizontal velocities of 65 stations in eastern california and western nevada between 35 and 37 n we relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km the velocity of the sierra nevada block with respect to nevada is 11 1 0 3 mm yr with slip partitioned across the death valley 2 8 0 5 mm yr panamint valley 2 5 0 8 mm yr and airport lake owens valley 5 3 0 7 4 6 0 5 mm yr faults the western mojave block rotates at 2 1 0 8 my clockwise with 3 7 0 7 mm yr of left lateral motion across the western garlock fault we infer 11 2 mm yr of right lateral motion across the mojave region of the eastern california shear zone

asymmetry in elastic properties and the evolution of large continental strike slip faults 1 we use geodetic studies to quantify several cases of significant asymmetry in interseismic and coseismic effects along large continental strike slip faults using simple two dimensional edge dislocation models we first show that asymmetric elastic loading characterizes the present main marmara fault a portion of the north anatolian fault along the northern margin of the sea of marmara the ratio of asymmetry there is about 10 this ratio is even larger about 30 along the northern sumatra fault near lake toba caldera we then examine two profiles near point reyes and point arena across the northern san andreas fault that have been previously proposed as affected by asymmetry both in interseismic and coseismic effects we show that an asymmetry ratio of 1 6 in interseismic loading exists near point arena with the southwest side of the fault being more rigid than the northeast one on the other hand we do not find significant asymmetry for the point reyes profile that was previously described as highly asymmetric we examine coseismic motion during the 1906 earthquake along the same two profiles ratios of 1 2 and 1 7 are found for the point arena and point reyes profiles respectively we discuss the possible causes of asymmetry contrasts in seismic velocity in the brittle portion suggest ratios generally not exceeding 2 5 for the dynamic rigidity in the upper brittle section larger ratios may involve other complex causes such as differences between static and dynamic rigidities contrasts in rheology in the deeper creeping sections and postseismic transients we conclude that asymmetry should be systematically included within the parameters to be inverted when dealing with the mechanics of large scale strike slip faults

gravitational viscoelastic postseismic relaxation on a layered spherical earth viscoelastic relaxation of a ductile asthenosphere underlying a purely elastic plate is a strong candidate process for explaining anomalous rates of crustal deformation observed following large earthquakes the nongravitational treatment of pollitz 1992 which is valid on a global scale and includes the effects of compressibility is here extended to permit the calculation of gravitational viscoelastic relaxation for a specified spherically layered viscoelastic rheology following an earthquake in an elastic layer the simple approximations we adopt make the resulting treatment particularly suitable for near field calculations for an asthenosphere with a maxwell rheology the effect of gravitational coupling is manifested only many relaxation times after the earthquake as obtained by previous investigators its effect is generally to speed up the long wavelength component of the relaxation process and attenuate the overall vertical displacement pattern several subtle features common to the relaxation behavior from several different fault types thrust rift and strike slip are identified the effect of gravitational coupling on horizontal displacements is consistent with flexure of the upper elastic plate driven by the corresponding effects on the vertical displacement stress diffusion away from the source region generally exhibits pulse like behavior which is dispersive in both space and time if the asthenosphere is confined to a relatively narrow channel then the dispersion branches governing relaxation are radically altered and stress diffusion effects far from the coseismic rupture zone exhibit a complicated time dependence reflecting the competing tendencies of toroidal and spheroidal mode relaxation

transient rheology of the uppermost mantle beneath the mojave desert california abstract geodetic data indicate that the m7 1 hector mine california earthquake was followed by a brief period a few weeks of rapid deformation preceding a prolonged phase of slower deformation we find that the signal contained in continuous and campaign global positioning system data for 2 5 years after the earthquake may be explained with a transient rheology quantitative modeling of these data with allowance for transient linear biviscous rheology in the lower crust and upper mantle demonstrates that transient rheology in the upper mantle is dominant its material properties being typified by two characteristic relaxation times 0 07 and 2 years the inferred mantle rheology is a jeffreys solid in which the transient and steady state shear moduli are equal consideration of a simpler viscoelastic model with a linear univiscous rheology 2 fewer parameters than a biviscous model shows that it consistently underpredicts the amplitude of the first 3 months signal and allowance for a biviscous rheology is significant at the 99 0 confidence level another alternative model deep postseismic afterslip beneath the coseismic rupture predicts a vertical velocity pattern opposite to the observed pattern at all time periods considered despite its plausibility the advocated biviscous rheology model is non unique and should be regarded as a viable alternative to the non linear mantle rheology model for governing postseismic flow beneath the mojave desert

stress rates in the central cascadia subduction zone inferred from an elastic plate model gps vectors and surface tilt and uplift rates from northwestern oregon and southwestern washington are inverted to estimate rates of stress changes along the cascadia thrust fault and base of the overriding plate using a finite thickness elastic plate model the data are fit by elevated shear stress and coulomb failure function cff rates within 80 km of the trench by contrast an elastic half space dislocation model does not fit as well and predicts significant amounts of locking and elevated cff rates near the coast

changes in vegetation climate and snowfall regime since the late pleistocene in a snowy mountainous region of central japan although the most crucial factor shaping vegetation history is temperature snowfall regime is also important in regions of heavy snow pollen analysis in a mire in a snowy mountainous region of ce

pollen event stratigraphy of the varved sediment of lake suigetsu central japan from 15 701 to 10 217 sg vyr bp suigetsu varve years before present description interpretation and correlation with other regions high resolution pollen data average interval between samples 15 years are reported on part of a varved sediment core from lake suigetsu japan spanning the interval 15 701 to 10 217 sg vyr bp suigetsu varve years before present this new record is compared with a previously proposed event stratigraphy based on pollen based reconstructed changes of mean annual temperature the deglacial climate history reconstructed at lake suigetsu resembles that observed in the north atlantic although the major boundaries of pollen zones are asynchronous with those in the north atlantic event stratigraphy by several centuries the onset of the late glacial interstadial occurred earlier in japan than in the north atlantic this demonstrates that the climate in japan was closely linked to the low latitude pacific sea surface temperatures that first reacted to orbital forcing conversely the onset of the subsequent cold reversal phase in japan lagged that of the north atlantic younger dryas by several centuries the duration of this cold phase was about the same as the younger dryas event but the amplitude was much reduced 4 2 c in mean annual temperature these findings support the hypothesis that this pan hemispheric cooling event was triggered by north atlantic forcing most probably by a meltwater pulse and an associated change in the north atlantic thermohaline circulation however the mechanism which transmitted the change in the north atlantic to the far east is unknown

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

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

resolution of fault slip along the 470 km long rupture of the great 1906 san francisco earthquake and its implications data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture because of varying network aperture and station density slip resolution is very uneven although slip is determined within uncertainties of 1 0 m along 60 of the fault constraints are poor on the remaining mostly offshore portions of the rupture slip decreases from maxima of 8 6 and 7 5 m at shelter cove and tomales bay to 4 5 m near mount tamalpais and 2 7 m at loma prieta the geodetically derived slip distribution is in poor agreement with estimates based on analysis of s wave seismograms probably because these waves register only 20 30 of the total seismic moment obtained from longer period surface waves consideration of a range of fault geometries for 1906 slip near loma prieta indicates right lateral motions lie between 2 3 and 3 1 m these values are considerably greater than the 1 5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based this factor along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts casts doubt on some claims of success in predicting the 1989 m 6 9 loma prieta earthquake

a physical model for strain accumulation in the san francisco bay region summary strain accumulation in tectonically active regions is generally a superposition of the effects of background tectonic loading steady state dislocation processes such as creep and transient deformation in the san francisco bay region sfbr the most uncertain of these processes is transient deformation which arises primarily in association with large earthquakes as such it depends upon the history of faulting and the rheology of the crust and mantle which together determine the pattern of longer term decade scale post seismic response to earthquakes we utilize a set of 102 gps velocity vectors in the sfbr in order to characterize the strain rate field and construct a physical model of its present deformation we first perform an inversion for the continuous velocity gradient field from the discrete gps velocity field from which both tensor strain rate and rotation rate may be extracted the present strain rate pattern is well described as a nearly uniform shear strain rate oriented approximately n34 w 140 nanostrain yr 1 plus a n56 e uniaxial compression rate averaging 20 nanostrain yr 1 across the shear zone we fit the velocity and strain rate fields to a model of time dependent deformation within a 135 km wide arcuate shear zone bounded by strong pacific plate and sierra nevada block lithosphere to the sw and ne respectively driving forces are purely lateral consisting of shear zone deformation imposed by the relative motions between the thick pacific plate and sierra nevada block lithospheres assuming a depth dependent viscoelastic structure within the shear zone we account for the effects of steady creep on faults and viscoelastic relaxation following the 1906 san francisco and 1989 loma prieta earthquakes subject to constant velocity boundary conditions on the edges of the shear zone fault creep is realized by evaluating dislocations on the creeping portions of faults in the fluid limit of the viscoelastic model a priori plate boundary pb parallel motion is set to 38 mm yr 1 ag rid search based on fitting the observed strain rate pattern yields a mantle viscosity of 1 2 10 19 p as and a pb perpendicular convergence rate of 3m m yr 1 most of this convergence appears to be uniformly distributed in the pacific sierra nevada plate boundary zone

postseismic relaxation across the central nevada seismic belt 1 two gps geodetic surveys across the basin and range br province of western north america have detected an anomalous compression east of the central nevada seismic belt cnsb the first network installed by the u s geological survey along an east west trending transect across the br at approximately 40 n consisted of 63 monuments observed campaign style in 1992 1996 and 1998 the second network installed by the california institute of technology consisted of 50 continuously operating gps receivers 18 of which were roughly along the same transect as the campaign survey we have used a viscoelastic postseismic relaxation model of the four largest earthquakes in the cnsb on the pleasant valley fault in 1915 the cedar mountain fault in 1932 and on the fairview peak and dixie valley faults in 1954 to interpret the geodetic velocities observed across the cnsb a model of postseismic relaxation with a lower crustal maxwell viscosity of 5 50 1018 pa s can explain the apparent compression regardless of the assumed mantle viscosity the geodetic velocities that we determined after removing the postseismic contribution indicate that within current gps detection limits the br is stable east of the cnsb and is undergoing rapid right lateral shear west of the cnsb

rheology of the lithosphere inferred from postseismic uplift following the 1959 hebgen lake earthquake 1 we have modeled the broad postseismic uplift measured by geodetic leveling in the epicentral area of the 1959 mw 7 3 hebgen lake montana earthquake a normal faulting event in the northern basin and range province to fit the observed uplift we calculate synthetic postseismic deformation using the relaxation response of a gravitational viscoelastic earth to the earthquake for a model with an elastic plate overlying a viscoelastic half space we find that the elastic thickness is 38 8 km which is close to the local crustal thickness the half space viscosity is estimated at 4 1018 0 5 pa s the leveling data do not require a viscous lower crust but permit a lower bound viscosity of 1020 pa s the observed broad uplift cannot be explained by physically plausible afterslip on and below the coseismic fault however local deformation across the coseismic surface rupture requires shallow afterslip reaching the surface the postseismic deformation induced by the estimated viscoelastic structure decays exponentially with a time constant of 15 years because of coupling between the elastic layer and the viscoelastic substrate this relaxation time is significantly longer than the 2 year maxwell relaxation time of the viscous half space itself our result suggests the importance of postseismic relaxation in interpreting high precision global positioning system velocities for example our model results suggest that postseismic transient velocities from both the 1959 hebgen lake and the 1983 mw 6 9 borah peak earthquakes are currently as large as 1 2 mm yr

a three dimensional semianalytic viscoelastic model for time dependent analyses of the earthquake cycle 1 exploring the earthquake cycle for large complex tectonic boundaries that deform over thousands of years requires the development of sophisticated and efficient models in this paper we introduce a semianalytic three dimensional 3 d linear viscoelastic maxwell model that is developed in the fourier domain to exploit the computational advantages of the convolution theorem a new aspect of this model is an analytic solution for the surface loading of an elastic plate overlying a viscoelastic half space when fully implemented the model simulates 1 interseismic stress accumulation on the upper locked portion of faults 2 repeated earthquakes on prescribed fault segments and 3 the viscoelastic response of the asthenosphere beneath the plate following episodic ruptures we verify both the analytic solution and computer code through a variety of 2 d and 3 d tests and examples on the basis of the methodology presented here it is now possible to explore thousands of years of the earthquake cycle along geometrically complex 3 d fault systems

relationship of geodetic velocities to velocities in the mantle 1 we examine the relationship of the velocities at the surface i e geodetic velocities to the velocities in the upper most mantle throughout a seismic cycle we model the rheology of the lower crust and mantle as linear viscoelastic and vary the relative viscosities between the lower crust and mantle as well as the distribution of viscosities and secular velocities in the mantle we find that the geodetic velocities are related to the transient velocities at depth the strength of the relationship decreases as the contrast between the lower crust and mantle viscosity increases in these models we find that the geodetic velocities are not related to the secular velocities at depth

detailed japan sea paleoceanography during the last 25 kyr constraints from ams dating and δ18o of planktonic foraminifera abstract we have reconstructed paleoceanographic changes in the japan sea for the period from the last glacial maximum lgm to the present using expanded paleoclimate proxy records from two cores together with previously published results from three others a suite of new accelerated mass spectrometer ams 14 c dates provides chronologies that differ from earlier age models and suggest correlation between conditions in the japan sea changes in the north pacific and global sea level and climate changes we present evidence that reduced salinity of the japan sea surface water during the late pleistocene was due primarily to the increased importance of the precipitation evaporation balance at a time when lowered sea level had reduced water exchange with the pacific to a few percent of the present value

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

holocene slip rate and earthquake recurrence on the honey lake fault zone northeastern california the honey lake fault zone a major right lateral fault in the basin and range province of eastern california is one of a broad system of faults that accommodate some of the relative motion between the pacific and north american plates these right lateral faults may have significantly higher slip rates and thus greater earthquake hazards than the normal faults for which the basin and range is more commonly known in the honey lake valley the fault forms a 50 km long zone of landforms typical of active strike slip faults right lateral offset of an incised creek channel is used to estimate a holocene slip rate of between 1 1 and 2 6 mm yr a fault exposure in holocene alluvium shows evidence for at least four late holocene surface faulting earthquakes

strain accumulation and rotation in western nevada 1993 2000 1 the positions of 44 gps monuments in an array extending from the sierra nevada at the latitude of reno to near austin nevada have been measured several times in the 1993 2000 interval the western half of the array spans the walker lane belt whereas the eastern half spans the central nevada seismic zone cnsz the principal strain rates in the walker lane belt are 29 6 5 3 nstrain yr 1 n88 4 e 5 4 and 12 8 6 0 nanostrain yr 1 n01 6 w 5 4 extension reckoned positive and the clockwise as seen from above the earth rotation rate about a vertical axis is 13 6 4 0 nrad yr 1 the quoted uncertainties are standard deviations the motion in the walker lane belt can then be represented by a zone striking n35 w subject to 16 8 4 9 nstrain yr 1 extension perpendicular to it and 19 5 4 0 nstrain yr 1 right lateral simple shear across it the n35 w strike of the zone is the same as the direction of the local tangent to the small circle drawn about the pacific north america pole of rotation the principal strain rates for the cnsz are 46 2 11 0 nstrain yr 1 n49 9 w 6 0 and 13 6 6 1 nstrain yr 1 n40 1 e 6 0 and the clockwise rotation rate about a vertical axis is 20 3 6 3 nrad yr 1 the motion across the cnsz can then be represented by a zone striking n12 e subject to 32 6 11 0 nstrain yr 1 extension perpendicular to it and 25 1 6 3 nstrain yr 1 right lateral simple shear across it the n12 e strike of the zone is similar to the strikes of the faults rainbow mountain fairview peak and dixie valley within it

viscoelastic shear zone model of a strike slip earthquake cycle i examine the behavior of a two dimensional 2 d strike slip fault system embedded in a 1 d elastic layer schizosphere overlying a uniform viscoelastic half space plastosphere and within the boundaries of a finite width shear zone the viscoelastic coupling model of savage and prescott 1978 considers the viscoelastic response of this system in the absence of the shear zone boundaries to an earthquake occurring within the upper elastic layer steady slip beneath a prescribed depth and the superposition of the responses of multiple earthquakes with characteristic slip occurring at regular intervals so formulated the viscoelastic coupling model predicts that sufficiently long after initiation of the system 1 average fault parallel velocity at any point is the average slip rate of that side of the fault and 2 far field velocities equal the same constant rate because of the sensitivity to the mechanical properties of the schizosphere plastosphere system i e elastic layer thickness plastosphere viscosity this model has been used to infer such properties from measurements of interseismic velocity such inferences exploit the predicted behavior at a known time within the earthquake cycle by modifying the viscoelastic coupling model to satisfy the additional constraint that the absolute velocity at prescribed shear zone boundaries is constant i find that even though the time averaged behavior remains the same the spatiotemporal pattern of surface deformation particularly its temporal variation within an earthquake cycle is markedly different from that predicted by the conventional viscoelastic coupling model these differences are magnified as plastosphere viscosity is reduced or as the recurrence interval of periodic earthquakes is lengthened application to the interseismic velocity field along the mojave section of the san andreas fault suggests that the region behaves mechanically like a 600 km wide shear zone accommodating 50 mrn yr fault parallel motion distributed between the san andreas fault system and eastern california shear zone in the viscoelastic coupling model savage and prescott 1978 time dependent deformation around a strike slip fault zone is realized by loading subsequent rupture and relaxation of a fault present in a schizosphere elastic upper crust overlying a ductile plastosphere assuming a two dimensional 2 d geometry periodic rupture of the fault and identical slip interseismic deformation is obtained as the superposition of viscoelastic relaxation of the plastosphere summed over the infinity of past periodic earthquakes this model yields far field interseismic motions equal to 1 2 of the average slip rate v0 imposed on the fault thus even when no external mechanism is invoked to load the fault i e when the locked portion of the fault penetrates the entire schizosphere the model satisfies the expectation of rigid plate like behavior in the far field in addition when periodic coseismic offsets are factored in the model predicts that the time averaged fault parallel velocity of each point is vol2 depending on location

reservoir induced deformation and continental rheology in vicinity of lake mead nevada lake mead is a large reservoir in nevada formed by the construction of the 221 m high hoover dam in the black canyon of the colorado river the lake encompasses an area of 635 km2 and the total volume of the reservoir is 35 5 km3 filling started in february 1935 on the basis of a first order leveling in 1935 several levelings were carried out to measure the deformation induced by the load of the reservoir subsidence in the central parts of the lake relative to the first leveling was around 120 mm 1941 218 mm 1950 and 200 mm 1963 the subsidence pattern clearly shows relaxation of the underlying basement due to the water load of the lake which ceased after 1950 modeling of the relaxation process by means of layered viscoelastic compressible flat earth models with a detailed representation of the spatial and temporal distribution of the water load shows that the thickness of the elastic crust underneath lake mead is 30 3 km the data are also consistent with a 10 km thick elastic upper crust and a 20 km thick viscoelastic lower crust with 1020 pa s as a lower bound for its viscosity the subcrust has an average viscosity of 1018 0 2 pa s a surprisingly low value the leveling data constrain the viscosity profile down to 200 km depth

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

strain accumulation rates in the san francisco bay area 1972 1989 maps of the strain accumulation rate in the san francisco bay area have been constructed from trilateration observations extending from about 1972 until the loma prieta earthquake in late 1989 the observations were corrected to remove offsets imposed by shallow fault creep and by four m 6 earthquakes that occurred in the bay area during that time interval the bay area was divided into 32 contiguous polygons and the uniform in both space and time strain rates that best explain the changes in the corrected earthquake and shallow fault creep offsets removed distances within each polygon were calculated in a coordinate system with the 1 axis directed n58 e and the 2 axis n32 w perpendicular and parallel to the local tangent to the small circle drawn about the pacific sierra nevada pole of rotation the averages of these 32 strain rates each weighted by the area of the polygon are e11 9 2 7 4 e12 160 7 4 6 and e22 8 2 6 2 nanostrain yr where extension is reckoned positive and quoted uncertainties are standard deviations as expected from the pacific sierra nevada relative plate motion the overall strain rate is predominantly right lateral shear across a vertical plane striking n32 w the net increase in the 12 225 km2 area of the trilateration network is only 212 110 m2 yr which arises from almost equal extensions in the n32 w and n58 e directions within the network the strain rates vary from polygon to polygon significant areal dilatation rates are observed in almost of the individual polygons and the n32 w extension rates tend to be negative to the west of the hayward rodgers creek fault trend and positive east of it the n58e extension rate is positive in 22 out of 32 polygons a proportion that is significantly larger than would be expected by chance if the n58 e extension rate were zero or negative the pre 1989 strain accumulation across the eventual site of the loma prieta rupture involves fault normal contraction as well as right lateral shear consistent with the rupture mechanism

strain accumulation across the carrizo segment of the san andreas fault california impact of laterally varying crustal properties 1 major strike slip faults juxtapose geologically dissimilar terrain which may vary in mechanical properties leading to an asymmetric pattern of strain accumulation we present new gps data on the carrizo segment of the san andreas fault separating the salinian block southwest of the fault from franciscan terrane northeast of the fault to better quantify asymmetric strain accumulation we also present a series of finite element models to investigate the possible role of variable elastic layer thickness and material properties of the upper crust the geodetic data are well fit with a simple model comprising a weak upper crustal zone 10 25 km wide northeast of the fault this model is also consistent with geologic data on the distribution of major rock types and corresponding laboratory data on their material properties as well as paleoseismic seismic and magnetotelluric data using this model we estimate a long term average over several seismic cycles slip rate for the san andreas fault of 36 1 5 2 mm yr in agreement with the known holocene rate within uncertainties and a viscosity for the combined lower crust upper mantle of 2 5 1019 pa 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

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

postseismic mantle relaxation in the central nevada seismic belt holocene acceleration of deformation and postseismic relaxation are two hypotheses to explain the present day deformation in the central nevada seismic belt cnsb discriminating between these two mechanisms is critical for understanding the dynamics and seismic potential of the basin and range province interferometric synthetic aperture radar detected a broad area of uplift 2 to 3 millimeters per year that can be explained by postseismic mantle relaxation after a sequence of large crustal earthquakes from 1915 to 1954 the results lead to a broad agreement between geologic and geodetic strain indicators and support a model of a rigid basin and range between the cnsb and the wasatch fault

deformation across the pacific north america plate boundary near san francisco california we have detected a narrow zone of compression between the coast ranges and the great valley and we have estimated slip rates for the san andreas rodgers creek and green valley faults just north of san francisco these results are based on an analysis of campaign and continuous global positioning system gps data collected between 1992 and 2000 in central california the zone of compression between the coast ranges and the great valley is 25 km wide the observations clearly show 3 8 1 5 mm yr 1 of shortening over this narrow zone the strike slip components are best fit by a model with 20 8 1 9 mm yr 1 slip on the san andreas fault 10 3 2 6 mm yr 1 on the rodgers creek fault and 8 1 2 1 mm yr 1 on the green valley fault the pacific sierra nevada great valley motion totals 39 2 3 8 mm yr 1 across a zone that is 120 km wide at the latitude of san francisco standard deviations are one the geodetic results suggest a higher than geologic rate for the green valley fault the geodetic results also suggest an inconsistency between geologic estimates of the san andreas rate and seismologic estimates of the depth of locking on the san andreas fault the only convergence observed is in the narrow zone along the border between the great valley and the coast ranges

surface faulting and paleoseismic history of the 1932 cedar mountain earthquake area west central nevada and implications for modern tectonics of the walker lane the 1932 cedar mountain earthquake m s 7 2 was one of the largest historical events in the walker lane region of western nevada and it produced a complicated strike slip rupture pattern on multiple quaternary faults distributed through three valleys primary right lateral surface ruptures occurred on north striking faults in monte cristo valley small scale lateral and normal offsets occurred in stewart valley and secondary normal faulting occurred on north northeast striking faults in the gabbs valley epicentral region a reexamination of the surface ruptures provides new displacement and fault zone data maximum cumulative offset is estimated to be 2 7 m and newly recognized faults extend the maximum width and end to end length of the rupture zone to 17 and 75 km respectively a detailed quaternary allostratigraphic chronology based on regional alluvial geomorphic relationships tephrochronology and radiocarbon dating provides a framework for interpreting the paleoseismic history of the fault zone a late wisconsinan alluvial fan and piedmont unit containing a 32 36 ka tephra layer is a key stratigraphic datum for paleoseismic measurements exploratory trenching and radiocarbon dating of tectonic stratigraphy provide the first estimates for timing of late quaternary faulting along the cedar mountain fault zone three trenches display evidence for six faulting events including that in 1932 during the past 32 36 ka radiocarbon dating of organic soils interstratified with tectonically ponded silts establishes best fit ages of the pre 1932 events at 4 5 12 15 and 18 ka each with 2 ka uncertainties on the basis of an estimated cumulative net slip of 6 12 m for the six faulting events minimum and maximum late quaternary slip rates are 0 2 and 0 7 mm yr respectively and the preferred rate is 0 4 0 5 mm yr the average recurrence interseismic interval is 3600 yr the relatively uniform thickness of the ponded deposits suggests that similar size characteristic rupture events may characterize late quaternary slip on the zone a comparison of event timing with the average late quaternary recurrence interval indicates that slip has been largely regular periodic rather than temporally clustered to account for the spatial separation of the primary surface faulting in monte cristo valley from the epicenter and for a factor of two to three disparity between the instrumentally and geologically determined seismic moments associated with the earthquake we hypothesize two alternative tectonic models containing undetected subevents either model would adequately account for the observed faulting on the basis of wrench fault kinematics that may be associated with the walker lane the 1932 cedar mountain earthquake is considered an important modern analogue for seismotectonic modeling and estimating seismic hazard in the walker lane region in contrast to most other historical events in the basin and range province the 1932 event did not occur along a major range bounding fault and no single throughgoing basement structure can account for the observed rupture pattern the 1932 faulting supports the concept that major earthquakes in the basin and range province can exhibit complicated distributive rupture patterns and that slip rate may not be a reliable criterion for modeling seismic hazard

geodetic estimates of fault slip rates in the san francisco bay area bourne et al 1998 have suggested that the interseismic velocity profile at the surface across a transform plate boundary is a replica of the secular velocity profile at depth in the plastosphere on the other hand in the viscoelastic coupling model the shape of the interseismic surface velocity profile is a consequence of plastosphere relaxation following the previous rupture of the faults that make up the plate boundary and is not directly related to the secular flow in the plastosphere the two models appear to be incompatible if the plate boundary is composed of several subparallel faults and the interseismic surface velocity profile across the boundary known each model predicts the secular slip rates on the faults which make up the boundary as suggested by bourne et al the models can then be tested by comparing the predicted secular slip rates to those estimated from long term offsets inferred from geology here we apply that test to the secular slip rates predicted for the principal faults san andreas san gregorio hayward calaveras rodgers creek green valley and greenville faults in the san andreas fault system in the san francisco bay area the estimates from the two models generally agree with one another and to a lesser extent with the geologic estimate because the viscoelastic coupling model has been equally successful in estimating secular slip rates on the various fault strands at a diffuse plate boundary the success of the model of bourne et al 1998 in doing the same thing should not be taken as proof that the interseismic velocity profile across the plate boundary at the surface is a replica of the velocity profile at depth in the plastosphere

seismic cycle and rheological effects on estimation of present day slip rates for the agua blanca and san miguel vallecitos faults northern baja california mexico 1 geodesy can be used to infer long term fault slip rates assuming a model for crust and upper mantle rheology we examine the sensitivity of fault slip rate estimates to assumed rheology for the agua blanca and san miguel vallecitos faults in northern baja california mexico part of the pacific north america plate boundary zone the agua blanca fault is seismically quiet but offset alluvial fans indicate young activity current seismicity is confined to the nearby san miguel vallecitos fault a small offset fault better aligned with plate motion gps measurements between 1993 and 1998 suggest that both faults are active with a combined slip rate of 4 8 mm yr 1 regardless of rheological model however slip rate estimates for the individual faults are sensitive to assumed rheology elastic half space models yield 2 3 mm yr 1 for the agua blanca fault and somewhat faster rates for the san miguel vallecitos fault 2 4 mm yr 1 with uncertainties of about 1 mm yr 1 models incorporating viscoelastic rheology and seismic cycle effects suggest a faster slip rate for the agua blanca fault 6 1 mm yr 1 and a slower rate for the san miguel vallecitos fault 1 1 mm yr 1 in better agreement with geological data but these rates are sensitive to assumed rheology numerical simulations with a finite element model suggest that for similar rheological and friction conditions slip on the san miguel vallecitos fault should be favored due to better alignment with plate motion long term faulting processes in the larger offset agua blanca fault may have lowered slip resistance allowing accommodation of motion despite misalignment with plate motion index terms 1206 geodesy and gravity crustal movements interplate 8155 1236 geodesy and gravity rheology of the lithosphere and mantle 8160 8107 tectonophysics continental neotectonics keywords fault slip rates and the seismic cycle northern baja california

transient rheology of the upper mantle beneath central alaska inferred from the crustal velocity field following the 2002 denali earthquake the m7 9 2002 denali earthquake alaska is one of the largest strike slip earthquakes ever recorded the postseismic gps velocity field around the 300 km long rupture is characterized by very rapid horizontal velocity up to 300 mm yr for the first 0 1 years and slower but still elevated horizontal velocity up to 100 mm yr for the succeeding 1 5 years i find that the spatial and temporal pattern of the displacement field may be explained by a transient mantle rheology representing the regional upper mantle as a burghers body 1 infer steady state and transient viscosities of 1 2 8 x 10 18 pa s and 2 1 0 x 10 17 pa s respectively corresponding to material relaxation times of 1 3 and 0 05 years the lower crustal viscosity is poorly constrained by the considered horizontal velocity field and the quoted mantle viscosities assume a steady state lower crust viscosity that is 7 1 systematic bias in predicted versus observed velocity vectors with respect to a fixed north america during the first 3 6 months following the earthquake is reduced when all velocity vectors are referred to a fixed site this suggests that the post denali gps time series for the first 1 63 years are shaped by a combination of a common mode noise source during the first 3 6 months plus viscoelastic relaxation controlled by a transient mantle rheology

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

finite element models of stress orientations in well developed strike slip fault zones implications for the distribution of lower crustal strain finite element models are used to examine the effects of strike slip earthquakes on stresses in an elastic layer overlying a finite width viscoelastic shear zone in the lower crust the overall dimensions of the model are 300 km wide 400 km long and 50 km deep three geometries for the lower crustal shear zone are considered 1 a viscoelastic half space approximation with a shear zone that extends to the model boundaries 300 km in width 2 a wide shear zone model 70 km in width and 3 a narrow shear zone model 10 km in width earthquakes are simulated with a fault plane that slips without friction at the desired time step and is centered above the shear zone extending to a depth of 15 km and running the length of the mesh far field plate velocity boundary conditions are enforced at the model edges so that stress on the fault evolves naturally a coulomb type failure criterion based on the average shear stress on the fault is set such that the earthquake cycle is 250 years the models are run until a limit cycle is reached and transient stresses have decayed away we focus on the maximum changes in the stress field during the earthquake cycle by examining stresses before and immediately after each earthquake in addition to comparisons of the separate components of the stress tensor we present results in the form of maximum compressive stress orientations plunge angles of the principal stress axes and beach ball diagrams that facilitate visualizing the full tensor stresses are concentrated in the upper crust where it overlies the finite width viscoelastic shear zone which causes the plunge angles of the principal stress axes to rotate from andersonian orientations of 90 and 0 to angles that approach 45 in the lower crust our results suggest that an examination of stress orientations in the upper and middle crust from borehole breakouts and focal mechanisms may provide insight as to the distribution of strain in the lower crust and may eventually allow us to distinguish between localized and distributed deformation models for the lower crust in active strike slip zones

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

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

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

a model of the earthquake cycle along the san andreas fault system for the past 1000 years 1 we simulate 1000 years of the earthquake cycle along the san andreas fault system by convolving best estimates of interseismic and coseismic slip with the green s function for a point dislocation in an elastic plate overlying a viscoelastic half space interseismic slip rate is based on long term geological estimates while fault locking depths are derived from horizontal gps measurements coseismic and postseismic deformation is modeled using 70 earthquake ruptures compiled from both historical data and paleoseismic data this time dependent velocity model is compared with 290 present day geodetic velocity vectors to place bounds on elastic plate thickness and viscosity of the underlying substrate best fit models rms residual of 2 46 mm yr require an elastic plate thickness greater than 60 km and a substrate viscosity between 2 1018 and 5 1019 pa s these results highlight the need for vertical velocity measurements developed over long time spans 20 years our numerical models are also used to investigate the 1000 year evolution of coulomb stress stress is largely independent of assumed rheology but is very sensitive to the slip history on each fault segment as expected present day coulomb stress is high along the entire southern san andreas because there have been no major earthquakes over the past 150 300 years animations s1 and s2 of the time evolution of vector displacement and coulomb stress are available as auxiliary material

variations in creep rate along the hayward fault california interpreted as changes in depth of creep variations in surface creep rate along the hayward fault are modeled as changes in locking depth using 3d boundary elements model creep is driven by screw dislocations at 12 km depth under the hayward and other regional faults inferred depth to locking varies along strike from 4 12 km 12 km implies no locking our models require locked patches under the central hayward fault consistent with a m6 8 earthquake in 1868 but the geometry and extent of locking under the north and south ends depend critically on assumptions regarding continuity and creep behavior of the fault at its ends for the northern onshore part of the fault our models contain 1 4 1 7 times more stored moment than the model of burgmann et al 2000 45 57 of this stored moment resides in creeping areas it is important for seismic hazard estimation to know how much of this moment is released coseismically or as aseismic afterslip

active faulting in the walker lane 1 deformation across the san andreas and walker lane fault systems accounts for most relative pacific north american transform plate motion the walker lane is composed of discontinuous sets of right slip faults that are located to the east and strike approximately parallel to the san andreas fault system mapping of active faults in the central walker lane shows that right lateral shear is locally accommodated by rotation of crustal blocks bounded by steep dipping east striking left slip faults the left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area including rattlesnake and garfield flats teels columbus and rhodes salt marshes and queen valley the benton springs and petrified springs faults are the major northwest striking structures currently accommodating transform motion in the central walker lane right lateral offsets of late pleistocene surfaces along the two faults point to slip rates of at least 1 mm yr the northern limit of northwest trending strike slip faults in the central walker lane is abrupt and reflects transfer of strike slip to dip slip deformation in the western basin and range and transformation of right slip into rotation of crustal blocks to the north the transfer of strike slip in the central walker lane to dip slip in the western basin and range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long lived feature of the deformation field the complexity of faulting and apparent rotation of crustal blocks within the walker lane is consistent with the concept of a partially detached and elastic brittle crust that is being transported on a continuously deforming layer below the regional pattern of faulting within the walker lane is more complex than observed along the san andreas fault system to the west the difference is attributed to the relatively less cumulative slip that has occurred across the walker lane and that oblique components of displacement are of opposite sense along the walker lane extension and san andreas contraction respectively despite the gross differences in fault pattern the walker lane and san andreas also share similarities in deformation style including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike slip and dip slip faults

slip on the san andreas fault at parkfield california over two earthquake cycles and the implications for seismic hazard parkfield california which experienced m 6 0 earthquakes in 1934 1966 and 2004 is one of the few locales for which geodetic observations span multiple earthquake cycles we undertake a comprehensive study of deformation over the most recent earthquake cycle and explore the results in the context of geodetic data collected prior to the 1966 event through joint inversion of the variety of parkfield geodetic measurements trilateration two color laser and global positioning system including previously unpublished two color data we estimate the spatial distribution of slip and slip rate along the san andreas using a fault geometry based on precisely relocated seismicity although the three most recent parkfield earthquakes appear complementary in their along strike distributions of slip they do not produce uniform strain release along strike over multiple seismic cycles since the 1934 earthquake more than 1 m of slip deficit has accumulated on portions of the fault that slipped in the 1966 and 2004 earthquakes and an average of 2 m of slip deficit exists on the 33 km of the fault southeast of gold hill to be released in a future perhaps larger earthquake it appears that the fault is capable of partially releasing stored strain in moderate earthquakes maintaining a disequilibrium through multiple earthquake cycles this complicates the application of simple earthquake recurrence models that assume only the strain accumulated since the most recent event is relevant to the size or timing of an upcoming earthquake our findings further emphasize that accumulated slip deficit is not sufficient for earthquake nucleation online material model fault geometry fit to the data for the inversions and model resolution

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

present day vertical deformation of the cascadia margin pacific northwest united states we estimate present day uplift rates along the cascadia subduction zone in california oregon and washington in the pacific northwest united states by utilizing repeated leveling surveys and tide gauge records these two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast uplift rates are extended inland through east west leveling lines that connect the north south line along the coast to the north south line along the inland valleys just west of the cascades the results are summarized as a contour map of present day uplift rates for the western pacific northwest we find that rates of present day uplift vary latitudinally along the coast and inland valleys as well as longitudinally along transects connecting the coast to the inland valleys long term tidal records of neah bay astoria and crescent city indicate uplift of land relative to sea level of 1 6 0 2 0 0 0 2 and 0 9 0 2 mm yr respectively 1 standard error unlike previous estimates of relative sea level change at astoria we adjust for discharge effects of the columbia river including human management influences after approximating an absolute framework by using 1 8 0 1 mm yr to compensate for global sea level rise results indicate that much of the western pacific northwest is rising at rates between 0 and 5 mm yr the most rapid uplift rates are near the coast particularly near the olympic peninsula the mouth of the columbia river cape blanco and cape mendocino two axes of uplift are identified one trends northeast from the southwest oregon coast and the other trends south southeasterly from the olympic peninsula to the columbia river the puget sound vicinity and a small east west region from the north central oregon coast to the inland willamette valley are subsiding at rates up to 1 mm yr we interpret the overall pattern of rapid present day uplift to be generated by interseismic strain accumulation in the subduction zone this interseismic elastic strain accumulation implies significant seismic hazard

crustal deformation across the sierra nevada northern walker lane basin and range transition western united states measured with gps 2000 2004 1 global positioning system gps data collected in campaigns in 2000 and 2004 were processed and interpreted with other gps data in the western basin and range province to provide new constraints on the rate style and pattern of deformation of the central and northern walker lane wl which lies near the western boundary of the basin and range across the central wl near 38n latitude the velocities with respect to north america increase westward by 10 mm yr inducing dextral shear farther north between 40 and 41n latitude a western zone of 7 mm yr relative motion undergoes dextral shear and an eastern zone of 3 mm yr relative motion undergoes extension and shear these data show that the northern wl is essentially a dextral shear zone experiencing minor net dilatation ed 2 6 0 8 nstrain yr near most holocene normal faults dilatation inferred from the velocity field is not greater than the uncertainties however near the central nevada seismic belt we detect significant dilatation expressed as extension in a direction approximately normal to the range fronts ed 23 0 3 9 nstrain yr some of which is attributable to transient postseismic deformation following large historic earthquakes a block model constrained by velocities corrected for transient effects shows that the sum of dextral slip rates across the honey lake warm springs east pyramid fault system and mohawk valley faults is 7 mm yr the wl is a zone whose width and dilatation rate increase northwestward consistent with counterclockwise rotation of the sierra nevada microplate and transfer of deformation into the pacific northwest

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

slicing up the san francisco bay area block kinematics and fault slip rates from gps derived surface velocities received 26 october 2004 revised 11 february 2005 accepted 10 march 2005 published 16 june 2005 i observations of surface deformation allow us to determine the kinematics of faults in the san francisco bay area we present the bay area velocity unification bavu bay view a compilation of over 200 horizontal surface velocities computed from campaign style and continuous global positioning system gps observations from 1993 to 2003 we interpret this interseismic velocity field using a three dimensional block model to determine the relative contributions of block motion elastic strain accumulation and shallow aseismic creep the total relative motion between the pacific plate and the rigid sierra nevada great valley sngv microplate is 37 9 0 6 mm yr 1 directed toward n30 4 w 0 8 at san francisco 2 fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data notable right lateral slip rates in mm yr 1 san gregorio fault 2 4 1 0 west napa fault 4 0 3 0 zone of faulting along the eastern margin of the coast range 5 4 1 0 and mount diablo thrust 3 9 1 0 of reverse slip and 4 0 0 2 of right lateral strike slip slip on the northern calaveras is partitioned between both the west napa and concord green valley fault systems the total convergence across the bay area is negligible poles of rotation for bay area blocks progress systematically from the north america pacific to north america sngv poles the resulting present day relative motion cannot explain the strike of most bay area faults but fault strike does loosely correlate with inferred plate motions at the time each fault initiated