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

seismic and aseismic deformation associated with the 1952 kern county california earthquake and relationship to the quaternary history of the white wolf fault synthesis of geodetic geologic and seismic data from the white wolf fault california indicates that the fault separates an area of late quaternary and continuing rapid uplift in the tehachapi mountains and transverse ranges from even more rapid subsidence in the southern san joaquin valley on july 21 1952 rupture of the white wolf fault produced the ml 7 2 kern county earthquake we used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952 1953 horizontal shear strains to model the measured coseismic vertical displacements with an elastic dislocation model a curved fault trace with decreasing fault depth 27 to 10 km from the surface vertically to the base slip 3 to 1 m and dip 75 to 20 from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record the measured seismic moment the fault plane solution and the pattern of surface rupture two short releveled lines near the 1952 epicenter tilted 4 and 17 rad down to the north from 5 10 years before the earthquake the preseismic tilts differ significantly from ten other surveys of these lines left lateral fault crossing shear strain from 0 2 20 years before the quake was two times greater than both preseismic off fault strains and the post seismic fault crossing strains during the first seven years after the earthquake aseismic deformation was negligible from 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone rising first in the epicentral region and then at the northeast end of the fault this was unaccompanied by any surface fault slip reconstruction of the vertical separation on the white wolf fault from late quaternary and late miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty fold from 0 1 0 2 mm yr to 3 9 mm yr between the past 10 15 m y and the most recent 0 6 1 2 m y we estimate a 170 to 450 yr average recurrence interval for earthquakes on the white wolf fault with slip equivalent to that in 1952 the 1952 earthquake appears to be characteristic of the quaternary record of fault displacement in the increase in white wolf slip toward the san andreas fault the ratio of reverse to lateral slip 1 3 1 and the ratio of vertical fault slip to emergence of the hanging wall block 3 1 the 8500 m deep sedimentary basin on the down thrown block cannot be explained by repeated slip of the white wolf fault in an elastic medium

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

viscoelastic cycle model of interseismic deformation in the northwestern united states summary we apply a viscoelastic cycle model to a compilation of gps velocity fields in order to address the kinematics of deformation in the northwestern united states a viscoelastic cycle model accounts for time dependent deformation following large crustal earthquakes and is an alternative to block models for explaining the interseismic crustal velocity field building on the approach taken in pollitz et al we construct a deformation model for the entire western united states based on combined fault slip and distributed deformation and focus on the implications for the mendocino triple junction mtj cascadia megathrust and western washington we find significant partitioning between strike slip and dip slip motion near the mtj as the tectonic environment shifts from northwest directed shear along the san andreas fault system to east west convergence along the juan de fuca plate by better accounting for the budget of aseismic and seismic slip along the cascadia subduction interface in conjunction with an assumed rheology we revise a previous model of slip for the m 9 1700 cascadia earthquake in western washington we infer slip rates on a number of strike slip and dip slip faults that accommodate northward convergence of the oregon coast block and northwestward convergence of the juan de fuca plate lateral variations in first order mechanical properties e g mantle viscosity vertically averaged rigidity explain to a large extent crustal strain that cannot be rationalized with cyclic deformation on a laterally homogeneous viscoelastic structure our analysis also shows that present crustal deformation measurements particularly with the addition of the plate boundary observatory can constrain such lateral variations

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

finite element model predictions of static deformation from dislocation sources in a subduction zone sensitivities to homogeneous isotropic poisson solid and half space assumptions 1 dislocation models can simulate static deformation caused by slip along a fault these models usually take the form of a dislocation embedded in a homogeneous isotropic poisson solid half space hipshs however the widely accepted hipshs assumptions poorly approximate subduction zone systems of converging oceanic and continental crust this study uses three dimensional finite element models fems that allow for any combination including none of the hipshs assumptions to compute synthetic green s functions for displacement using the1995 mw 8 0 jalisco colima mexico subduction zone earthquake and associated measurements from a nearby gps array as an example fem generated synthetic green s functions are combined with standard linear inverse methods to estimate dislocation distributions along the subduction interface loading a forward hipshs model with dislocation distributions estimated from fems that sequentially relax the hipshs assumptions yields the sensitivity of predicted displacements to each of the hipshs assumptions for the subduction zone models tested and the specific field situation considered sensitivities to the individual poisson solid isotropy and homogeneity assumptions can be substantially greater than gps measurement uncertainties forward modeling quantifies stress coupling between the mw 8 0 earthquake and a nearby mw 6 3 earthquake that occurred 63 days later coulomb stress changes predicted from static hipshs models cannot account for the 63 day lag time between events alternatively an fem that includes a poroelastic oceanic crust which allows for postseismic pore fluid pressure recovery can account for the lag time the pore fluid pressure recovery rate puts an upper limit of 10 17 m2 on the bulk permeability of the oceanic crust

thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height satellite images of large volcanic explosions reveal that the tops of volcanic eruption columns are much colder than the surrounding atmosphere1 we propose that this effect occurs whenever a mixture of hot volcanic ash and entrained air ascends sufficiently high into a stably stratified atmosphere although the mixture is initially very hot it expands and cools as the ambient pressure decreases we show that cloud top undercoolings in excess of 20 c may develop in clouds that penetrate the stratosphere this is consistent with observations of the 4 april 1982 eruption of el chichon1 and the 18 may 1980 eruption of mount st helens2 furthermore from our model results we predict that for a given cloud top temperature variations in the initial temperature of 100 200 c may correspond to variations in the column height of 5 10 km we deduce that the present practice of converting satellite based measurements of the temperature at the top of volcanic eruption columns to estimates of the column height will produce rather inaccurate results and should therefore be discontinued

detection of a locked zone at depth on the parkfield california segment of the san andreas fault the parkfield california segment of the san andreas fault is transitional in character between the creeping segment of the fault to the northwest and the locked carrizo plain segment to the southeast the rate of shallow fault slip decreases from 25 30 mm yr northwest of the epicenter of the 1966 parkfield earthquake to zero at the southeastern end of the 1966 rupture zone data from a network of trilateration lines spanning the san andreas fault near parkfield and extending to the pacific coast near san luis obispo shed light on the rate of fault slip at depth since the 1966 earthquake in this study average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the parkfield fault segment the fault is taken to be a vertical surface with unknown distribution of strike slip displacement in an elastic half space a striking result of the inversions is that all solutions providing acceptable fits to the data exhibit a locked zone essentially coincident with the rupture surface of the 1966 parkfield earthquake the data require that the locked zone extend nearly as far north as the 1966 epicenter however the vertical extent of the locked zone is not well resolved over much of the parkfield segment the fault is slipping faster at the earth s surface than it is at seismogenic depths in order to fit the trilateration measurements it is necessary to include a component of contraction normal to the trend of the san andreas the inversion results suggest a spatially uniform normal strain of 0 06 strain yr the orientation of the contraction is compatible with geologic and seismic evidence of active folding and reverse faulting in the region the magnitude of the contraction is consistent with convergence rates inferred from global plate motion models

the coseismic slip distributions of the 1940 and 1979 imperial valley california earthquakes and their implications geodetic arrays observed by the u s coast and geodetic survey span the imperial fault in southern california for the 1940 m 7 1 imperial valley earthquake a 1934 1941 triangulation network has sufficient resolution to allow inversion for the coseismic slip distribution on fault segments 5 to 25 km long extending from the surface to a depth of 9 km the estimated right lateral slip is 0 8 to 1 7 m on the northern 30 km of the main trace of the imperial fault 4 8 0 2 m on a 10 km long segment straddling the united states mexico border and 1 3 0 4 m on a southern 25 km long segment in mexico fixing this strike slip model and inverting 1940 leveling data only for dip slip yields 0 1 m of east side down dip slip the seismic moment for this model is m0 3 2 0 3 1019 n m the 1979 geodetic data set mostly elevation changes from leveling routes has insufficient resolution for inversion however it is possible to use this geodetic data set and results published by others to infer that the 1940 and 1979 earthquakes may be similar on the rupture zone common to both events our preferred 1940 model is similar to the 1979 geodetic results of crook 1984 on the segments where both networks have good resolution elevation changes from 1940 and 1979 leveling data are very similar thus the geodetic data corroborate the surface slip evidence of sharp 1982b that the 1940 and 1979 slip distributions are examples of characteristic slip on the northern imperial fault

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

influence of tropical cyclones on sea surface temperature seasonal cycle and ocean heat transport recent studies suggested that tropical cyclones tcs contribute significantly to the meridional oceanic heat transport by injecting heat into the subsurface through mixing here we estimate the long term oceanic impact of tcs by inserting realistic wind vortices along observed tcs tracks in a 1 2 resolution ocean general circulation model over the 1978 2007 period warming of tcs cold wakes results in a positive heat flux into the ocean oceanic heat uptake ohu of 480 tw consistent with most recent estimates however 2 5 of this ohu only compensates the heat extraction by the tcs winds during their passage another 2 5 of this ohu is injected in the seasonal thermocline and hence released back to the atmosphere during the following winter because of zonal compensations and equatorward transport only one tenth of the ohu is actually exported poleward 46 tw resulting in a marginal maximum contribution of tcs to the poleward ocean heat transport other usually neglected tc related processes however impact the ocean mean state the residual ekman pumping associated with tcs results in a sea level drop rise in the core northern and southern flanks of tc basins that expand westward into the whole basin as a result of planetary wave propagation more importantly tc induced mixing and air sea fluxes cool the surface in tc basins during summer while the re emergence of subsurface warm anomalies warms it during winter this leads to a 10 reduction of the sea surface temperature seasonal cycle within tcs basins which may impact the climate system

shattered rock and precarious rock evidence for strong asymmetry in ground motions during thrust faulting recent thrust faulting models both physical and numerical have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults in this article recent field evidence supporting this strong asymmetry is described earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1 g and 100 cm sec new shattered rock evidence on the hanging wall of thrust faults in southern california supports the occurrence of intense ground motions on the other hand the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in southern california indicate relatively low ground motions thus indicating strong asymmetry in ground motions near the fault trace for thrust faults some current ground motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall this could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the los angeles basin and elsewhere in the world the new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models if such detachment occurs it could also have important implications for understanding thrust fault rupture mechanics for example the paradox of large overthrusts and the heat flow paradox for thrusts

systematic stress variations in the southern san joaquin valley and along the white wolf fault implications for the rupture mechanics of the 1952 ms 7 8 kern county earthquake and contemporary seismicity the southern san joaquin valley ssjv stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes as well as local and possibly temporal variations that appear to be the result of the 1952 ms 7 8 kern county earthquake the regional maximum horizontal principal stress shmax orientation in the san joaquin valley systematically rotates from ne sw compression along the western margins of the valley to n s compression in the ssjv this n s shmax stress direction in the ssjv is consistent with active development of a e w trending structural fabric of fold axes and shallow thrust and the reverse dip slip motion observed during the kern county earthquake on the south southeast dipping white wolf fault contemporary seismicity 30 40 years after the mainshock clusters in two separate areas along the white wolf fault the southwest region where the 1952 earthquake nucleated and the northeast region near where the earthquake rupture terminated earthquakes in the southwest and northeast regions show a diversity of focal mechanisms that include strike slip reverse oblique and to a lesser extent normal slip inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike slip reverse stress regime with s1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake inversion of focal plane mechanisms in the northeast region indicates a strike slip stress regime with s2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength these results indicate near complete stress drops for fault planes associated with the 1952 earthquake and some of the contemporary earthquakes implying fault surfaces which are frictionally weak i e slip planes subparallel to principal stress planes based on the observed stress state in the southern san joaquin valley much of the seismicity may be the result of elevated fluid pressures within these active fault zones

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

thermal nature of mantle upwellings below the ibero western maghreb region inferred from teleseismic tomography independent models of p wave and s wave velocity anomalies in the mantle derived from seismic tomography help to distinguish thermal signatures from those of partial melt volatiles and compositional variations here we use seismic data from sw europe and nw africa spanning the region between the pyrenees and the canaries in order to obtain a new s sks relative arrival time tomographic model of the upper mantle below iberia western morocco and the canaries similar to previous p wave tomographic results the s wave model provides evidence for 1 subvertical upper mantle low velocity structures below the canaries atlas ranges and gibraltar arc which are interpreted as mantle upwellings fed by a common lower mantle source below the canaries and 2 two low velocity anomalies below the eastern rif and betics that we interpret as the result of the interaction between quasi toroidal mantle flow induced by the gibraltar slab and the mantle upwelling behind it the analysis of teleseismic p wave and s wave arrival time residuals and the conversion of the low velocity anomalies to temperature variations suggest that the upwellings in the upper mantle below the canaries atlas ranges and gibraltar arc system may be solely thermal in nature with temperature excesses in the range 100 350 c our results also indicate that local partial melting can be present at lithospheric depths especially below the atlas ranges the locations of thermal mantle upwellings are in good agreement with those of thinned lithosphere moderate to high heat flow measurements and recent magmatic activity at the surface

postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the southern california mantle i reexamine the lower crust and mantle relaxation following two large events in the mojave desert the 1992 m7 3 landers and 1999 m7 1 hector mine california earthquakes time series from continuous gps sites out to 300 km from the ruptures are used to constrain models of postseismic relaxation crustal motions in the mojave desert region are elevated above background for several years following each event to account for broadscale relaxation of the lower crust and mantle the burgers body model is employed involving transient and steady state viscosities joint afterslip postseismic relaxation modeling of the gps time series up to one decade following the hector mine earthquake reveals a significant rheological contrast between a northwest trending southwest domain that envelopes the san andreas fault system and western mojave desert and an adjacent northeast domain that envelopes the landers and hector mine rupture areas in the central mojave desert the steady state viscosity of the northeast domain mantle asthenosphere is inferred to be 4 times greater than that of the southwest domain this pattern is counter to that expected for regional heat flow which is higher in the northeast domain but it is explicable by means of a nonlinear rheology that includes dependence on both strain rate and water concentration i infer that the southwest domain mantle has a relatively low steady state viscosity because of its high strain rate and water content the relatively low mantle water content of the northeast domain is interpreted to result from the continual extraction of water through igneous and volcanic activity over the past 20 myr

ice age as a trigger of active quaternary volcanism and tectonism abstract the stress accumulation within the crust caused by the surface mass redistribution associated with the glaciation deglaciation cycle during the quaternary was numerically evaluated in order to examine the relationship between active quaternary volcanism and tectonism in island arc areas and ice age the vertical gradient of horizontal stress difference in the lithosphere for a meltwater of 130 m in equivalent sea level reaches a maximum value of 0 8 mpa km which is corresponding to the equivalent buoyancy of about 100 kg m3 for magma filled cracks for an earth model with a lithospheric layer of 20 30 km thickness and with a viscosity greater than 1023 pa s the changes in stress difference during the stages of deglaciation of 10 000 years amount to 13 mpa for both the top and bottom of the thin lithosphere thus the additional stress difference within the crust may be effective for island arc areas with thin lithospheric thickness we therefore speculate that the stress accumulation associated with ice age may be an important trigger and or accelerator on the active quaternary volcanism and tectonism for the areas along the circum pacific

geometry of the 1954 fairview peak dixie valley earthquake sequence from a joint inversion of leveling and triangulation data in 1954 four earthquakes greater than ms 6 0 occurred within a 30 km radius and in a period of 6 months elevation and angle changes calculated from repeated leveling and triangulation surveys which span the coseismic period provide constraints on the fault geometries and coseismic slip of the faults which were activated the quality of the coseismic geodetic data is assessed corrections are applied to the leveling data for subsidence due to groundwater withdrawal in the fallon area and a rod miscalibration error of 150 30 ppm is isolated in leveling surveys made in 1967 the leveling and triangulation observations are then simultaneously inverted using the single value decomposition svd inversion method to determine fault geometries and coseismic slip using svd it is possible to determine on which faults slip is resolvable given the data distribution the faults are found to dip between 50 and 80 and extend to depths of 5 to 14 km the geodetically derived slip values are generally equal to or greater than the maximum observed displacement along the surface scarps where slip is resolvable the geodetic data indicates the 1954 sequence contained a significant component of right lateral slip this is consistent with the n15 w trending shear zone which geodetic surveys have detected in western nevada

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

evolution of the southern san joaquin basin and mid tertiary transitional tectonics central california a cenozoic tectonic and sedimentary history is proposed for the southern san joaquin basin ssjb and tehachapi mountains that evolved adjacent to the plate margin off central california seismic reflection borehole field biostratigraphic and paleomagnetic data are integrated into geologic and fault structure maps cross sections and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence timing and distribution of complex tectonically linked events the largely buried structures and strata in the ssjb preserve an unusually complete record of the mid tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the pliocene significant structural relief existing across both extensional and contractile features is preserved in the subsurface and an active fold thrust belt propagates basinward along the margin of the u shaped tejon embayment the cenozoic evolution of the ssjb reflects the regional deformation of central california as different tectonic events followed each other along the adjacent north american plate margin five oligocene miocene basin phases are identified in the ssjb 1 late oligocene early miocene extensional subsidence with high and low angle normal faulting accompanied by volcanism and deposition of coarse syntectonic conglomerates 2 middle miocene uplift 3 later mid miocene transtensional subsidence to lower bathyal depths 4 alternating subsidence and uplift until the late miocene and 5 flexural subsidence due to pliocene to recent contraction reconstructions of mid tertiary california place the southern san joaquin tehachapi extensional terrane as a paleotectonic block located between the western mojave terrane then to the east and the western california terrane then to the south and west regional extension occurred during a long transition period between convergent and transform boundaries along the north american plate margin significant slip along the san andreas transform postdates this extensional event its origin apparently coincided with a regional middle miocene uplift event indicating that the san andreas fault is younger than previously supposed the tertiary ssjb has subsided due to extension transtension crustal tilting and thrust related loading the maricopa subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the sierra nevada and locally has subsided beyond 12 km in contrast the relatively stable tejon embayment underlain by sierran crystalline crust achieved its maximum subsidence in the miocene since pliocene time the ssjb has filled and continued to deepen after the basin was tectonically shut off from the pacific ocean

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

where is the real transform boundary in california 1 the zone of highest geodetically defined strain rate in california does not everywhere coincide with the surface trace of the san andreas fault saf to determine whether this reflects the pattern of long term permanent deformation we analyzed the velocity field on swaths across the transform located so as to avoid intersections among the major fault strands slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model representing the interseismic strain accumulation our slip rates compare well with current geologic estimates which suggests that the present day velocity field is representative of long term motions we find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the saf and has an overall trend closer to the relative plate motion vector than the saf most sections of the saf take up less than half of the total slip rate and slip is transferred from one part of the system to another in a way that suggests that the saf should not be considered as a unique locator of the plate boundary up to half of the total displacement takes place on faults outside the high strain rate zone distributed over several hundred kilometers on either side our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere around which stress and strain rate decrease on a length scale controlled by the length of the transform

aftershocks of the 1952 kern county california earthquake sequence we have studied the seismograms recorded at the historic berkeley brk and pasadena pas stations for 20 aftershocks of the 21 july 1952 kern county earthquake sequence these events in the magnitude range of m w 4 5 to 5 6 are too small to be studied teleseismically yet they are important for better understanding the tectonics of the southern sierra nevada and the tehachapi mountains on scale recordings of moderate sized events from this important earthquake sequence were first scanned digitized and then subjected to waveform modeling using a seismic moment tensor inverse procedure in particular the long period three component galitzen instrument at brk and the 6 sec wood anderson at pas provided very high quality seismograms that could be analyzed in this manner these two sites have been continuously operated from 1887 and 1927 respectively and both are current sites of state of the art broadband high dynamic range instrumentation first motion polarities reported by bath and richter 1958 were used as additional constraints in the estimation of source parameters there is considerable variability in the three component seismograms of the 1952 aftershocks which in turn result in a diversity of focal mechanisms the majority of the solutions are northwest striking reverse mechanisms that likely occurred on various mapped thrust faults in the hanging block of the mainshock there are several events with northeast striking left lateral mechanisms that are consistent with the strike of the white wolf fault as well as several normal slip events the results of this study indicate that there are a variety of active fault structures adjacent to the white wolf garlock and san andreas faults in this region

sound velocity measurements of casio 3 perovskite to 133 gpa and implications for lowermost mantle seismic anomalies abstract we report the measurements of aggregate shear velocity v s of casio 3 perovskite capv at high pressure p between 32 and 133 gpa and room temperature t on the basis of brillouin spectroscopy the sample had a tetragonal perovskite structure throughout the experiments the measured p v s data show the shear modulus and its pressure derivative at ambient condition to be g 0 115 8 gpa and g 1 20 respectively the zero pressure shear velocity is determined to be v s0 5 23 km s in good agreement with the previous estimate inferred from the ultrasonic measurements on ca si ti o 3 perovskite at 1 bar our experimental results are broadly consistent with the earlier calculations on tetragonal capv but exhibit lower velocity at equivalent pressure such tetragonal capv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle while primitive mantle includes certain amount of capv a depleted peridotite former harzburgite layer in subducted oceanic lithosphere is deficient in capv and enriched in ferropericlase in the lower mantle such harzburgite exhibits 0 9 faster v s and 0 7 slower bulk sound velocity v at the lowermost mantle p t conditions if capv is present in the tetragonal form in the surrounding mantle the observed fast v s and slow v anomalies in the d layer underneath the circum pacific region might be attributed in large part in the presence of subducted harzburgitic materials

regional deformation near palmdale california 1973 1983 the tehachapi trilateration network spans the intersection of the san andreas and garlock faults in southern california in the big bend region of the san andreas fault analysis of data from 1973 1983 shows strain differences between the northwest and southeast regions of the network and slip at depth on both faults the palmdale network spanning the san andreas fault entirely within the tehachapi network showed increases of about 1 strain in both east west and north south extension in late 1979 the tehachapi strains also jumped at this time but the magnitude of the increase was only about one third that of palmdale the principal strain rates for tehachapi over the time interval 1973 1983 are e 1 0 08 0 01 strain yr and e 2 0 12 0 01 strain yr with the 1 axis directed n76 e strains were also computed for two subregions the principal strain rates for the southeast tehachapi region are e 1 0 14 0 02 strain yr and e 2 0 16 0 02 strain yr with the 1 axis directed n73 e this result differs significantly from the principal strain rates at palmdale e 1 0 21 0 02 strain yr e 2 0 17 0 01 strain yr with the 1 axis directed n71 e for the northwest tehachapi region e 1 0 11 0 02 strain yr and e 2 0 06 0 02 strain yr with the 1 axis directed n93 e

the 1952 kern county california earthquake a case study of issues in the analysis of historical intensity data for estimation of source parameters abstract seismic intensity data based on first hand accounts of shaking give valuable insight into historical and early instrumental earthquakes comparing an observed intensity distribution to intensity prediction models based on modern calibration events allows the magnitude to be estimated for many historic earthquakes magnitude estimates can also potentially be refined for earthquakes for which limited instrumental data are available however the complicated nature of macroseismic data and the methods used to collect and interpret the data introduce significant uncertainties in this paper we illustrate these challenges and possible solutions using the 1952 kern county california earthquake as a case study published estimates of its magnitude vary from mw 7 2 7 5 making it possibly the second largest in california during the 20th century we considered over 1100 first hand reports of shaking supplemented with other data and inferred the magnitude in several ways using intensity prediction equations yielding a preferred intensity magnitude mi 7 2 0 2 where the uncertainty reflects our judgement the revised intensity distribution reveals stronger shaking on the hanging wall south of the surface expression of the white wolf fault than on the footwall characterizing the magnitude and shaking distribution of this early instrumental earthquake can help improve estimation of the seismic hazard of the region such reinterpreted intensities for historic earthquakes combined with u s geological survey usgs did you feel it data for more recent events can be used to produce a uniform shaking dataset with which earthquake hazard map performance can be assessed

contemporary vertical velocity of the central basin and range and uplift of the southern sierra nevada 1 we estimate the present day vertical velocity field in the central basin and range and sierra nevada mountains and consider causative processes we analyzed data from 16 continuously operating global positioning system stations finding that relative vertical velocity precision wrms scatter is 0 25 mm yr these data demonstrate that the southern sierra nevada is experiencing uplift of 0 5 mm yr relative to a local reference frame in southern nevada and 1 mm yr relative to stations in the northern mojave desert as a possible source of these motions we explored post seismic viscoelastic relaxation following the 1872 owens valley and 1952 kern county earthquakes with models of laterally homogeneous lower crust and upper mantle viscosity post seismic deformation from these two earthquakes cannot entirely explain the data set lateral variations in viscosity or contributions from other regional earthquakes may also be important

p and sh velocity structure in the upper mantle beneath northeast china evidence for a stagnant slab in hydrous mantle transition zone abstract using high dense regional body waves for three deep earthquakes that occurred around russia china border we investigate both s and p wave velocity structures in the mantle transition zone beneath northeast china and northern part of north china craton where the northwestern pacific plate is imaged to subhorizontally lie above the 660 km discontinuity we observe an increasing trend of s p travel time residuals along the epicentral distance within a distance range of 11 16 5 indicating a velocity anomaly in mtz we seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector both sh and p data suggest a 140 20 km high velocity layer lying above a slightly depressed and broad 660 km discontinuity shear velocity reduction of 2 5 in the deeper part of the transition zone is required to compensate for the significantly large relative time between ab and cd triplicate branches and the increased trending of s p travel time residuals as well the mtz as a whole is featured by low shear velocity and high v p v s ratio a water rich mantle transition zone with 0 2 0 4 wt of h 2 o may account for the discrepancy between the observed v p and v s velocity structures our result supports the scenario of a viscosity dominated stagnant slab with an increased thickness of 140 km which was caused by the large viscosity contrast between the lower and upper mantles the addition of water and eastward trench retreat might facilitate stagnation of the subducting pacific slab beneath northeast china

geodetic measurements of horizontal strain near the white wolf fault kern county california 1926 1993 the white wolf fault located north of the big bend segment of the san andreas fault is the ne sw trending left lateral oblique reverse fault responsible for the ms 7 8 1952 kern county earthquake we combined global positioning system gps measurements with historical triangulation and trilateration data to determine changes in the strain rate over 7 decades 1926 1993 we reanalyzed the historical geodetic data and calculated an elevated preseismic 1926 1952 maximum shear strain rate of 0 62 0 16 strain yr across the white wolf fault the maximum shear strain rate decreased with distance toward the garlock fault to 0 09 0 08 strain yr in the decade following the earthquake 1952 1963 the near fault was high 0 85 0 23 strain yr and decreased to 0 23 0 13 strain yr across the garlock fault in 1993 we resurveyed many of the same monuments with gps receivers to estimate fault crossing and off fault strain rates for the preceding 30 years across the white wolf fault the maximum shear strain rate dropped to 0 19 0 07 strain yr the azimuths of the maximum principal strain rates for the 1963 1993 epoch rotate from a fault normal orientation 57 15 across the white wolf fault to 11 3 e across the garlock fault

birth of a fault connecting the kern county and walker pass california earthquakes a band of seismicity transects the southern sierra nevada range between the northeastern end of the site of the 1952 m w moment magnitude 7 3 kern county earthquake and the site of the 1946 m w 6 1 walker pass earthquake relocated earthquakes in this band which lacks a surface expression better delineate the northeast trending seismic lineament and resolve complex structure near the walker pass mainshock left lateral earthquake focal planes are rotated counterclockwise from the strike of the seismic lineament consistent with slip on shear fractures such as those observed in the early stages of fault development in laboratory experiments we interpret this seismic lineament as a previously unrecognized incipient currently blind strikeslip fault a unique example of a newly forming structure

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