crustal structure of a transform plate boundary san francisco bay and the central california continental margin wide angle seismic data collected during the bay area seismic imaging experiment provide new glimpses of the deep structure of the san francisco bay area block and across the offshore continental margin san francisco bay is underlain by a veneer 300 m of sediments beneath which p wave velocities increase rapidly from 5 2 km s to 6 0 km s at 7 km depth consistent with rocks of the franciscan subduction assemblage the base of the franciscan at 15 18 km depth is marked by a strong wide angle reflector beneath which lies an 8 to 10 km thick lower crust with an average velocity of 6 75 0 15 km s the lower crust of the bay area block may be oceanic in origin but its structure and reflectivity indicate that it has been modified by shearing and or magmatic intrusion wide angle reflections define two layers within the lower crust with velocities of 6 4 6 6 km s and 6 9 7 3 km s prominent subhorizontal reflectivity observed at near vertical incidence resides principally in the lowermost layer the top of which corresponds to the 6 s reflector of brocher et al 1994 rheological modeling suggests that the lower crust beneath the 6 s reflector is the weakest part of the lithosphere the horizontal shear zone suggested by furlong et al 1989 to link the san andreas and hayward calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust a transect across the continental margin from the paleotrench to the hayward fault shows a deep crustal structure that is more complex than previously realized strong lateral variability in seismic velocity and wide angle reflectivity suggests that crustal composition changes across major transcurrent fault systems pacific oceanic crust extends 40 50 km landward of the paleotrench but contrary to prior models probably does not continue beneath the salinian block a cretaceous arc complex that lies west of the san andreas fault in the bay area the thickness 10 km and high lower crustal velocity of pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby pioneer seamount the salinian block consists of a 15 km thick layer of velocity 6 0 6 2 km s overlying a 5 km thick high velocity 7 0 km s lower crust that may be oceanic crust cretaceous arc derived lower crust or a magmatically underplated layer the strong structural variability across the margin attests to the activity of strike slip faulting prior to and during development of the transcurrent pacific north american plate boundary around 29 ma

seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

distribution of individual wave overtopping volumes on mound breakwaters abstract conventional mound breakwaters are usually designed to withstand low mean wave overtopping discharges and a low proportion of overtopping waves pow existing formulas to estimate pow and maximum individual wave overtopping volume are usually based on tests with high pow this study is focused on mound breakwaters subjected to pow

stress sensitivity of fault seismicity a comparison between limited offset oblique and major strike slip faults we present a new three dimensional inventory of the southern san francisco bay area faults and use it to calculate stress applied principally by the 1989 m 7 1 loma prieta earthquake and to compare fault seismicity rates before and after 1989 the major high angle right lateral faults exhibit a different response to the stress change than do minor oblique right lateral thrust faults seismicity on oblique slip faults in the southern santa clara valley thrust belt increased where the faults were unclamped the strong dependence of seismicity change on normal stress change implies a high coefficient of static friction in contrast we observe that faults with significant offset 50 100 km behave differently microseismicity on the hayward fault diminished where right lateral shear stress was reduced and where it was unclamped by the loma prieta earthquake we observe a similar response on the san andreas fault zone in southern california after the landers earthquake sequence additionally the offshore san gregorio fault shows a seismicity rate increase where right lateral oblique shear stress was increased by the loma prieta earthquake despite also being clamped by it these responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones we can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes whereas in more limited offset faults fluids could rapidly escape the difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes since many of these events occur both inside and outside major fault zones

modeling and validation of a 3d velocity structure for the santa clara valley california for seismic wave simulations a 3d seismic velocity and attenuation model is developed for santa clara valley california and its surrounding uplands to predict ground motions from scenario earthquakes the model is developed using a variety of geologic and geophysical data our starting point is a 3d geologic model developed primarily from geologic mapping and gravity and magnetic surveys an initial velocity model is constructed by using seismic velocities from boreholes reflection refraction lines and spatial autocorrelation microtremor surveys this model is further refined and the seismic attenuation is estimated through waveform modeling of weak motions from small local events and strong ground motion from the 1989 loma prieta earthquake waveforms are calculated to an upper frequency of 1 hz using a parallelized finite difference code that utilizes two regions with a factor of 3 difference in grid spacing to reduce memory requirements cenozoic basins trap and strongly amplify ground motions this effect is particularly strong in the evergreen basin on the northeastern side of the santa clara valley where the steeply dipping silver creek fault forms the southwestern boundary of the basin in comparison the cupertino basin on the southwestern side of the valley has a more moderate response which is attributed to a greater age and velocity of the cenozoic fill surface waves play a major role in the ground motion of sedimentary basins and they are seen to strongly develop along the western margins of the santa clara valley for our simulation of the loma prieta earthquake

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

preliminary velocity and resistivity models of the loma prieta earthquake region a preliminary three dimensional velocity model of the loma prieta epicentral region in the santa cruz mountains of california has been derived using raypaths from aftershocks recorded by the u s g s seismic network in addition a magnetotelluric sounding profile was completed prior to the earthquake and a two dimensional resistivity model computed the velocity and resistivity models include a low resistivity 3 10 ohm m low velocity 3 3 5 8 km s wedge of material between the zayante and san andreas faults that extends to depths of over 8 km and is interpreted to be largely tertiary marine sedimentary rocks a high resistivity 1000 ohm m high velocity 6 4 6 7 km s zone between the san andreas and sargent faults may represent largely mafic intrusive rocks

simultaneous inversion of active and passive source datasets for 3 d seismic structure with application to tasmania 1 refraction wide angle reflection and teleseismic arrival time data are combined in a simultaneous inversion for 3 d lithospheric p wavespeed and moho geometry beneath tasmania southeast australia a new iterative non linear tomography scheme which incorporates a novel grid based method of traveltime prediction is used to obtain the solution model synthetic resolution tests demonstrate that the crucial issue of trade off between interface depth and layer velocity is satisfactorily resolved in contrast to previous results from separate inversions of the active and passive source datasets the new images reveal a zone of elevated wavespeed beneath the cambrian mt read volcanics and indicate that both crustal thinning and elevated wavespeeds occur beneath northeast tasmania which supports the case for the existence of a prior passive margin otherwise most major inferences from the previous studies including evidence for remnant subduction and crustal shortening are supported by the new results

large scale right slip displacement on the east san francisco bay region fault system california implications for location of late miocene to pliocene pacific plate boundary a belt of northwardly younging neogene and quaternary volcanic rocks and hydrothermal vein systems together with a distinctive cretaceous terrane of the franciscan complex the permanente terrane exhibits about 160 to 170 km of cumulative dextral offset across faults of the east san francisco bay region esfbr fault system the offset hydrothermal veins and volcanic rocks range in age from 01 ma at the northwest end to about 17 6 ma at the southeast end in the fault block between the san andreas and esfbr fault systems where volcanic rocks are scarce hydrothermal vein system ages clearly indicate that the northward younging thermal overprint affected these rocks beginning about 18 ma the age progression of these volcanic rocks and hydrothermal vein systems is consistent with previously proposed models that relate northward propagation of the san andreas transform to the opening of an asthenospheric window beneath the north american plate margin in the wake of subducting lithosphere the similarity in the amount of offset of the permanente terrane across the esfbr fault system to that derived by restoring continuity in the northward younging age progression of volcanic rocks and hydrothermal veins suggests a model in which 80 110 km of offset are taken up 8 to 6 ma on a fault aligned with the bloomfield tolay franklin concord sunol calaveras faults an additional 50 70 km of cumulative slip are taken up 6 ma by the rogers creek hayward and concord franklin sunol calaveras faults an alternative model in which the permanente terrane is offset about 80 km by pre miocene faults does not adequately restore the distribution of 8 12 ma volcanic rocks and hydrothermal veins to a single northwardly younging age trend if 80 110 km of slip was taken up by the esfbr fault system between 8 and 6 ma dextral slip rates were 40 55 mm yr such high rates might occur if the esfbr fault system rather than the san andreas fault acted as the transform margin at this time major transpression across the boundary between the pacific and north american plates at about 3 to 5 ma would have resulted in the transfer of significant slip back to the san francisco peninsula segment of the san andreas fault since that time the esfbr fault system has continued to slip at rates of 11 14 mm yr if this interpretation is valid the esfbr fault system was the pacific north american plate boundary between 8 and 6 ma and this boundary has migrated both eastward and westward with time in response to changing plate margin geometry and plate motions

paleoseismic studies of the san francisco peninsula segment of the san andreas fault zone near woodside california a late holocene slip rate is proposed for the san francisco peninsula segment of the san andreas fault zone at the filoli center located between the town of woodside and upper crystal springs reservoir california at this site the primary trace of the san andreas fault zone traverses n35 w across the head of a large alluvial fan offsetting buried stream channel deposits one deposit with a minimum calibrated accelerator mass spectrometry ams radiocarbon age of 2070 120 years b p is offset 30 2 m yielding an average geologic slip rate of 17 4 mm yr 1 for the late holocene a stratigraphically lower channel sequence with a minimum calibrated ams radiocarbon age of 330 200 years b p is offset 4 1 0 5 m we interpret that this older sequence records both the penultimate earthquake on the peninsula segment accompanied by 1 6 0 7 m of dextral slip and the 2 5 m of slip that occurred in 1906 on the basis of radiocarbon analyses and historical descriptions we propose that the pre 1906 earthquake occurred in june 1838 our interpretations suggest that the san francisco peninsula segment of the san andreas fault zone may rupture in earthquakes of at least two magnitudes 1 a large magnitude mw 8 event with a recurrence of 250 320 years that involves the north coast san francisco peninsula and santa cruz mountains segments as occurred in 1906 and 2 a mw 7 0 7 4 1838 type event that ruptures only the san francisco peninsula segment the apparent difference in slip rate of 7 7 mm yr 1 between the north coast segment 24 3 mm yr 1 and the peninsula segment 17 4 mm yr 1 suggests a possible slip rate gradient on the northern california section of the san andreas fault

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

holocene history of the san andreas fault between crystal springs reservoir and san andreas dam san mateo county california abstract the geologic part of a seismic safety investigation of san andreas dam focused on a two mile reach of the fault between the dam and crystal springs reservoir historical documents and detailed mapping revealed that the 1906 faulting caused 7 to 9 ft of right shear in the left eastern abutment and occurred on nearly straight traces that experienced a maximum of 9 ft of slip 90 per cent of which was concentrated within a zone 100 ft wide radiocarbon dates on detrital charcoal from two sites of ponded alluvium established that the 1906 traces have been active for at least the last 3 000 yr deflected stream channels further indicate that the 1906 traces have been the locus of right slip for the last 6500 yr and perhaps for all of holocene time detailed geomorphic and stratigraphic analyses of one site showed that five 1906 magnitude slip events have occurred within the last 1130 160 radiocarbon years from which a minimum strain rate of 1 2 cm yr and an average recurrence frequence of 224 25 yr can be calculated trenches excavated across the 1906 traces of the san andreas fault revealed that the zone of active faulting is marked by a slow upwelling of highly sheared rock materials this apparent vertical movement of fault gouge when coupled with right shear and differential stream erosion has produced most of the distinctive landforms of the rift valley

finite frequency traveltime tomography of san francisco bay region crustal velocity structure summary seismic velocity structure of the san francisco bay region crust is derived using measurements of finite frequency traveltimes a total of 57 801 relative traveltimes are measured by cross correlation over the frequency range 0 5 1 5 hz from these are derived 4862 summary traveltimes which are used to derive 3 d p wave velocity structure over a 341 140 km2 area from the surface to 25 km depth the seismic tomography is based on sensitivity kernels calculated on a spherically symmetric reference model robust elements of the derived p wave velocity structure are a pronounced velocity contrast across the san andreas fault in the south bay region west side faster a moderate velocity contrast across the hayward fault west side faster moderately low velocity crust around the quien sabe volcanic field and the sacramento river delta very low velocity crust around lake berryessa these features are generally explicable with surface rock types being extrapolated to depth 10 km in the upper crust generally high mid lower crust velocity and high inferred poisson s ratio suggest a mafic lower crust

late neogene relative motions of the pacific and north america plates in this paper a new set of finite rotations describing the relative motion of the pacific and north america plates during the last 10 m y incorporating recently published studies of the pacific antarctic antarctic africa and africa north america plate boundaries is presented these finite rotations show that changes have occurred in pacific western north america motion at 2 48 ma and between 3 40 and 3 9 ma resulting in increased compression along the pacific north america plate boundary the most significant change in relative motion was of the latter age during this change the predicted motion of the pacific plate along the california coast changes from transform to transpressive due to a clockwise rotation of the relative convergence vector by 12 this timing of the onset of transpression agrees well with a variety of geologic data along the california plate boundary including the onset of compressive deformation in onshore and offshore sedimentary basins formation of reverse faults and anticlines which are parallel to strike slip faults of the previous more westerly directed azimuth of relative motion a change in the orientation of the san andreas fault and formation of a set of new more northerly trending strike slip faults in this model this change in relative motion is caused by a change in the absolute motion of the pacific plate due to the detachment of a slab beneath the fiji basin between 3 4 and 3 9 ma the detachment of this slab and the resultant change in overall pacific plate torque resulted in a noncollisional orogeny along the california plate boundary this study shows that minor adjustments in the motion of large oceanic plates such as the pacific plate can have profound consequences on the preserved geologic record copyright 1991 by the american geophysical union

three dimensional structure influences on the strong motion wavefield of the 1989 loma prieta earthquake abstract the accurate calculation of ground motions for future large earthquakes depends on detailed knowledge of three dimensional 3d geologic structure and the earthquake source process as well as sufficient computational resources in this article we describe the results of finite difference simulations for the 1989 loma prieta ca earthquake with a 3d seismic velocity model for the san francisco bay region and a heterogeneous slip model of the source additionally we explore the sensitivity of the synthetics to the major geologic structures in the velocity model the san francisco bay region particularly the loma prieta region is a unique area for the study of 3d wave propagation because of the pronounced lateral velocity contrasts across the strike slip faults of the region understanding the effects of such long wavelength structure is doubly important when considering a source located close to such a contrast our simulations show that the lateral velocity contrast across the san andreas fault saf would be expected to substantially affect the propagation of elastic waves radiated from a source in the loma prieta region indeed we find that the refraction of energy by the saf serves to reduce ground motions at stations located along the san francisco peninsula and the quaternary and tertiary alluvial basins of the san francisco bay region are found to amplify and extend the duration of ground motions in santa clara valley livermore valley and san pablo bay we find that the 3d model as currently defined accurately describes the spatial variation of peak ground velocity for frequencies less than 0 5 hz which suggests that this model may be used to estimate ground motions for future earthquake scenarios

dextral translation and progressive emergence of the pleistocene merced basin and implications for timing of initiation of the san francisco peninsula segment of the san andreas fault

a crustal velocity model for locating earthquakes in monterey bay california the seismicity of the monterey bay displays a sparse distribution of events with a majority in northern monterey bay on the san gregorio fault the paucity of near shore and offshore seismic recording instruments and the use of velocity models from inland regions for earthquake hypocentral locations and focal mechanisms have led to uncertainties and inaccuracies for seismic events on major faults that crosscut the bay new three component seismic data were acquired during 1997 1999 by the monterey bay aquarium research institute mbari margin seismology project using ocean bottom digital corehole and standard seismometers and coastal digital reftek instruments obtained from iris passcal by the university of california santa cruz we have analyzed phases from earthquakes in the monterey bay vicinity located by these instruments and used them to supplement the adjacent coastal stations of the permanent northern california seismic network a new one dimensional velocity model for the region requires slow velocities from 2 to 6 km that we attribute to sheared granites observed in the salinian block in the center of monterey bay velocities in the 10 16 km layer are consistent with continental crustal velocities a sharp increase in velocity at 16 km suggests a boundary that results from underplating of oceanic crust this underplated zone appears to extend to a depth of 27 30 km where we observe normal upper mantle velocities near 8 0 km sec new details from the ocean bottom corehole and coastal reftek instruments for events along the sgf and monterey bay fault zones hint at local fine scale structures and have implications for tectonic history and plate reconstruction interpretation

late neogene motion of the pacific plate oceanic crust between 38 s and 65 s along the pacific antarctic ridge records a complete history of neogene pacific antarctic motion we have modeled magnetic stages corresponding to portions of isochrons 1 2 2a 3 3a 4 and 5 from 17 high quality marine magnetic traverses of the ridge to determine the neogene finite rotation poles for this plate pair we chose to model velocities over time intervals of about 0 5 m y because this greatly increased the number of estimates of pacific antarctic plate velocity our analysis shows that a change in pacific antarctic relative motion occurred between chron 2a 3 40 ma and the beginning of chron 3 time 3 86 ma we used 203 estimates of pacific antarctic spreading velocity to calculate a 0 3 4 ma and 3 86 10 3 ma euler pole combined with tracks of seamounts that record the passage of the pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the pacific plate with respect to the hotspot frame of reference using these euler poles to calculate pacific antarctic africa north america circuit euler poles show that this recent change in pacific plate absolute motion corresponds to a change from strike slip to transpressive motion along the coastal california pacific north america plate boundary interestingly this age of increased tectonism agrees in time with deformation in new zealand and northern japan

three dimensional upper crustal velocity structure beneath san francisco peninsula california this paper presents new seismic data from and crustal models of the san francisco peninsula in much of central california the san andreas fault juxtaposes the cretaceous granitic salinian terrane on its west and the late mesozoic early tertiary franciscan complex on its east on san francisco peninsula however the present day san andreas fault is completely within a franciscan terrane and the pilarcitos fault located southwest of the san andreas marks the salinian franciscan boundary this circumstance has evoked two different explanations either the pilarcitos is a thrust fault that has pushed franciscan rocks over salinian rocks or the pilarcitos is a transform fault that has accommodated significant right lateral slip in an effort to better resolve the subsurface structure of the peninsula faults we established a temporary network of 31 seismographs arrayed across the san andreas fault and the subparallel pilarcitos fault at 1 2 km spacings these instruments were deployed during the first 6 months of 1995 and recorded local earthquakes air gun sources set off in san francisco bay and explosive sources travel times from these sources were used to augment earthquake arrival times recorded by the northern california seismic network and were inverted for three dimensional velocity structure results show lateral velocity changes at depth 0 5 7 km that correlate with downward vertical projections of the surface traces of the san andreas and pilarcitos faults we thus interpret the faults as high angle to vertical features constrained to a 70 110 dip range from this we conclude that the pilarcitos fault is probably an important strike slip fault that accommodated much of the right lateral plate boundary strain on the peninsula prior to the initiation of the modern day san andreas fault in this region sometime after about 3 0 m y ago

evidence for the collision of the salinian block with the franciscan subduction zone california recent detailed field mapping near pilarcitos lake san francisco peninsula california indicates that a subduction related suture between the salinian block continental fragment and the franciscan subduction complex may be preserved in the area in this region a ne dipping mylonite zone separates salinian rocks and structurally overlying franciscan rocks asymmetric augen pressure shadows and and s surfaces in the mylonite suggest down dip movement and a ne over sw sense of shear indicating the thrusting of franciscan over salinian rocks the mylonite is part of a 1 km wide zone of ductile thrust faulting these structures may have formed as a result of a collision between the salinian block and the franciscan subduction zone franciscan rocks being thrust westward over the salinian block because paleocene rocks are involved in the thrusting and clasts of the mylonitized rocks are present in upper paleocene and eocene in the area the age of thrusting is bracketed at approximately 50 60 ma th

contrasts in compliant fault zone properties inferred from geodetic measurements in the san francisco bay area author s materna k burgmann r abstract 2016 american geophysical union all rights reserved in crustal fault zones regions of damaged rock characterized by reduced elastic shear modulus can influence patterns of near field interseismic deformation in order to study these compliant fault zones cfzs and how they might develop over the lifetimes of faults we compare two fault segments with contrasting fault age and lithology along the san andreas fault in the san francisco bay area new geodetic measurements of the interseismic velocity fields at each location are used to constrain fault zone parameters through a markov chain monte carlo method at black mountain in the santa cruz mountains of the san francisco peninsula we do not find evidence for a compliant fault zone instead we find that the geodetic data are more consistent with a model of a single fault in a homogeneous elastic half space at lake san andreas a younger fault segment 35nkm farther north we find evidence for a compliant fault zone about 3 4 1 1 1 4nkm wide containing a shear modulus of about 40 of the shear modulus of the surrounding rock we also find that the best fitting cfz model at this location unlike the best fitting homogeneous half space model has a locking depth that agrees well with the observed depth of microseismicity based on differences in fault age cumulative displacement and lithology between black mountain and lake san andreas we infer that lithology plays an important and in this case perhaps a dominant role in the accumulation of fault zone damage structures and the development of cfzs over the lifetime of a fault

surface seismic and electrical methods to detect fluids related to faulting in the absence of drilling surface based geophysical methods are necessary to observe fault zones and fault zone physical properties at seismogenic depths these in situ physical properties can then be used to infer the presence and distribution of fluids along faults although such observations are by nature indirect and become less exact with greater depth multiple observations of a range of such geophysical properties as compressional and shear seismic velocity vp and vs vp v5 ratio related to poisson s ratio resistivity and attenuation in and adjacent to fault zones offer the greatest hope of making inferences of the fault zone geometry fluids in the fault zone and fluid reservoirs in the surrounding crust for simple geometries fault zone guided waves can provide information on fault zone width and velocities for faults of the order of 200 m wide to address the question of whether a narrow fault zone can be imaged well enough at depths of seismic rupture to infer the presence of anomalously high fluid rock ratios we present synthetic seismic tomography and magnetotelluric examples for an ideal case of a narrow fault zone with a simple geometry large changes in material properties and numerous earthquakes within the fault zone a synthetic 0 5 km wide fault zone with 20 velocity reduction is well imaged using local earthquake tomography when sequential velocity inversions are done the true fault width is found even to 9 km depth although the calculated amplitude of the velocity reduction is lower than the actual amplitude vp vs is as well determined as vp magnetotelluric imaging of a synthetic fault zone shows that a conductive fault zone can be well imaged within the upper 10 km further a narrow 1 km very low resistivity 3 ohm m fault core can be imaged within a broad 5 km low resistivity 10 ohm m fault zone illustrating that regions of a fault containing large quantities of interconnected fluids within a broader conductive fault zone should be detectable thus variations in fluid content and fluid pressure can be inferred from electrical and seismic methods but there will always be uncertainty in these inferences due to the trade off with other factors such as intrinsic variations in porosity mineralogy and pore geometry the best approach is combined modeling of varied seismic and electrical data

waveform tomography of crustal structure in the south san francisco bay region 1 we utilize a scattering based seismic tomography technique to constrain crustal structure around the southern san francisco bay region sfbr this technique is based on coupled traveling wave scattering theory which has usually been applied to the interpretation of surface waves in large regional scale studies using fully three dimensional kernels this technique is here applied to observed p s and surface waves of intermediate period 3 4 s dominant period observed following eight selected regional events we use a total of 73 seismograms recorded by a u s geological survey short period seismic array in the western santa clara valley the berkeley digital seismic network and the northern california seismic network modifications of observed waveforms due to scattering from crustal structure include positive or negative amplification delay and generation of coda waves the derived crustal structure explains many of the observed signals which cannot be explained with a simple layered structure there is sufficient sensitivity to both deep and shallow crustal structure that even with the few sources employed in the present study we obtain shallow velocity structure which is reasonably consistent with previous p wave tomography results we find a depth dependent lateral velocity contrast across the san andreas fault saf with higher velocities southwest of the saf in the shallow crust and higher velocities northeast of the saf in the midcrust the method does not have the resolution to identify very slow sediment velocities in the upper approximately 3 km since the tomographic models are smooth at a vertical scale of about 5 km

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

evolution of the gorda escarpment san andreas fault and mendocino triple junction from multichannel seismic data collected across the northern vizcaino block offshore northern california the gorda escarpment is a north facing scarp immediately south of the mendocino transform fault the gorda juan de fuca pacific plate boundary between 126 w and the mendocino triple junction it elevates the seafloor at the northern edge of the vizcaino block part of the pacific plate 1 5 km above the seafloor of the gorda juan de fuca plate to the north stratigraphy interpreted from multichannel seismic data across and close to the gorda escarpment suggests that the escarpment is a relatively recent pop up feature caused by north south compression across the plate boundary close to 126 w the vizcaino block acoustic basement shallows and is overlain by sediments that thin north toward the gorda escarpment these sediments are tilted south and truncated at the seafloor by contrast in a localized region at the eastern end of the gorda escarpment close to the mendocino triple junction the top of acoustic basement dips north and is overlain by a 2 km thick wedge of pre 11 ma sedimentary rocks that thickens north toward the gorda escarpment this wedge of sediments is restricted to the northeast corner of the vizcaino block unless the wedge of sediments was a preexisting feature on the vizcaino block before it was transferred from the north american to the pacific plate the strong spatial correlation between the sedimentary wedge and the triple junction suggests the entire vizcaino block with the san andreas at its eastern boundary has been part of the pacific plate since significantly before 11 ma

the velocity field along the san andreas fault in central and southern california the velocity field within a 100 km broad zone centered on the san andreas fault between the mexican border and san francisco bay has been inferred from repeated surveys of trilateration networks in the 1973 1989 interval the velocity field has the appearance of a shear flow that remains parallel to the local strike of the fault even through such major deflections as the big bend of the san andreas fault in the transverse ranges of southern california across strike profiles of the fault parallel component of velocity exhibit the expected sigmoidal shape whereas across strike profiles of the fault normal component of velocity are flat and featureless no significant convergence upon the fault is observed even along the big bend sector of the fault simple dislocation models can explain most of the features of the observed velocity field but those explanations are not unique about 35 mm yr of relative plate motion is accounted for within the span of the trilateration networks geologic studies indicate that the secular slip rate on the san andreas fault is about 35 mm yr the agreement between these two estimates implies that most of the strain accumulation is elastic and will be recovered in subsequent earthquakes the relative motion observed across the san andreas fault 35 mm yr plus that observed across the eastern california shear zone 8 mm yr accounts for most 43 mm yr of the observed north america pacific relative plate motion 47 mm yr

earthquake travel time tomography of the southern santa cruz mountains control of fault rupture by lithological heterogeneity of the san andreas fault zone the 1989 loma prieta earthquake occurred along the stretch of the san andreas fault zone within the southern santa cruz mountains that last failed as a major earthquake in 1906 the southeastern end of the 1989 rupture marks the transition from stable aseismic slip on the central creeping section of the san andreas fault to unstable failure on the locked 1906 segment we investigate this transition and the rupture characteristics of the 1989 earthquake using a three dimensional p wave velocity model of the southern santa cruz mountains section of the fault zone the velocity model was determined by joint progressive inversion of 5422 p wave arrival times from 173 earthquakes which included aftershocks of the 1989 earthquake and premainshock background events recorded by the u s geological survey central california network this velocity model was used to relocate 424 background earthquakes the loma prieta mainshock and 292 aftershocks the velocity model is calibrated in terms of lithology using laboratory data for california coast ranges rock types and available seismic refraction profiles and is shown to be in good agreement with the surface geology the model images a large anomalous high velocity body at midcrustal depths within the rupture zone of the 1989 earthquake that the available evidence suggests might have gabbroic or other mafic composition on the basis of the relationship of the lithological features interpreted from the velocity model to the seismicity and surface creep we propose a model in which the high velocity body is primarily responsible for the transition from stable to unstable fault slip at pajaro gap the active plane of the san andreas fault cuts throughout the body the fault system attempts to circumvent this barrier by transferring slip to secondary faults including splay faults that have propagated along the frictionally favorable contact between the high velocity rock mass and franciscan country rocks however the near arrest of stable sliding causes stress to concentrate within the body and the high strength unstable contact within it evolves from a barrier to the asperity that failed in the 1989 earthquake the general features of the 1989 rupture predicted by this asperity model agree with several rupture histories computed for the earthquake the model implies that as proposed by other workers the loma prieta earthquake did not involve a repeat of the 1906 slip which has an important bearing on earthquake recurrence estimates for the santa cruz mountains segment of the fault