structure of the california coast ranges and san andreas fault at safod from seismic waveform inversion and reflection imaging 1 a seismic reflection and refraction survey across the san andreas fault saf near parkfield provides a detailed characterization of crustal structure across the location of the san andreas fault observatory at depth safod steep dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the saf the resulting velocity model constrains the top of the salinian granite with great detail steep dip reflection seismic images show several strong amplitude vertical reflectors in the uppermost crust near safod that define an 2 km wide zone comprising the main saf and two or more local faults another prominent subvertical reflector at 2 4 km depth 9 km to the northeast of the saf marks the boundary between the franciscan terrane and the great valley sequence a deep seismic section of low resolution shows several reflectors in the salinian crust west of the saf two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along strike of the saf they represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust the deepest reflections from 25 km depth are interpreted as crust mantle boundary

possible evidence for subducted sedimentary materials beneath central california seismic reflection and refraction data from the coast ranges of central california west of the san andreas fault suggest the presence of a low velocity 4 5 5 0 km s wedge at a depth of about 15 22 km that dips east beneath rocks of both the franciscan assemblage and the salinian block we propose that the wedge represents sedimentary materials that were subducted in an offshore trench within the past 23 m y the refraction data also suggest the presence of a second low velocity zone that extends at least to mid crustal depths beneath the san andreas fault zone

development and applications of double difference seismic tomography double difference dd tomography is a generalization of dd location it simultaneously solves for the three dimensional velocity structure and seismic event locations dd tomography uses a combination of absolute and more accurate differential arrival times and hierarchically determines the velocity structure from larger scale to smaller scale this method is able to produce more accurate event locations and velocity structure near the source region than standard tomography which uses only absolute arrival times we conduct a stability and uncertainty analysis of dd tomography based on a synthetic data set currently three versions of the dd tomography algorithms exist tomodd tomofdd and tomoadd tomodd assumes a flat earth model and uses a pseudo bending ray tracing algorithm to find rays between events and stations while tomofdd uses a finite difference travel time algorithm and the curvature of the earth is considered both codes are based on a regularly distributed inversion grid with the former for a local scale and the latter for a regional scale in contrast tomoadd adapts the inversion mesh to match with the data distribution based on tetrahedral and voronoi diagrams we discuss examples of applying dd tomography to characterize fault zone structure image high resolution structure of subduction zones and determine the velocity structure of volcanoes

some key features of the strong motion data from the m 6 0 parkfield california earthquake of 28 september 2004 the 2004 parkfield california earthquake was recorded by an extensive set of strong motion instruments well positioned to record details of the motion in the near fault region where there has previously been very little recorded data the strong motion measurements obtained are highly varied with significant variations occurring over only a few kilometers the peak accelerations in the near fault region range from 0 13 g to over 1 8 g one of the highest acceleration recorded to date exceeding the capacity of the recording instrument the largest accelerations occurred near the northwest end of the inferred rupture zone these motions are consistent with directivity for a fault rupturing from the hypocenter near gold hill toward the northwest however accelerations up to 0 8 g were also observed in the opposite direction at the south end of the cholame valley near highway 41 consistent with bilateral rupture with rupture southeast of the hypocenter several stations near and over the rupturing fault recorded relatively weak motions consistent with seemingly paradoxical observations of low shaking damage near strike slip faults this event had more ground motion observations within 10 km of the fault than many other earthquakes combined at moderate distances peak horizontal ground acceleration pga values dropped off more rapidly with distance than standard relationships at close in distance the wide variation of pga suggests a distance dependent sigma may be important to consider the near fault ground motion variation is greater than that assumed in shakemap interpolations based on the existing set of observed data higher density of stations near faults may be the only means in the near future to reduce uncertainty in the interpolations outside of the near fault zone the variance is closer to that assumed this set of data provides the first case where near fault radiation has been observed at an adequate number of stations around the fault to allow detailed study of the fault normal and fault parallel motion and the near field s wave radiation the fault normal motions are significant but they are not large at the central part of the fault away from the ends the fault normal and fault parallel motions drop off quite rapidly with distance from the fault analysis of directivity indicates increased values of peak velocity in the rupture direction no such dependence is observed in the peak acceleration except for stations close to the strike of the fault near and beyond the ends of the faulting

complexity of the deep san andreas fault zone defined by cascading tremor seismic tremors can be used to distinguish plate boundaries analysis of tremors occurring deep beneath the san andreas fault may identify the boundary between the north american plate and the preserved remnant of a subducted slab

joint inversion for vp vs and vp vs at safod parkfield california we refined the three dimensional 3 d vp vs and vp vs models around the san andreas fault observatory at depth safod site using a new double difference dd seismic tomography code tomoddps that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential p s and s p times this new method is able to provide a more robust vp vs model than that from the original dd tomography code tomodd obtained simply by dividing vp by vs for the new inversion waveform cross correlation times for earthquakes from 2001 to 2002 were also used in addition to arrival times from earthquakes and explosions in the region the vp values extracted from the model along the safod trajectory match well with the borehole log data providing in situ confirmation of our results similar to previous tomographic studies the 3 d structure around parkfield is dominated by the velocity contrast across the san andreas fault saf in both the vp and vs models there is a clear low velocity zone as deep as 7 km along the saf trace compatible with the findings from fault zone guided waves there is a high vp vs anomaly zone on the southwest side of the saf trace that is about 1 2 km wide and extends as deep as 4 km which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the salinian basement rock to the southwest the relocated earthquakes align beneath the northeast edge of this high vp vs zone we carried out a 2 d correlation analysis for an existing resistivity model and the corresponding profiles through our model yielding a classification that distinguishes several major lithologies

joint inversion of body wave arrival times and surface wave dispersion for three dimensional seismic structure around safod we incorporate body wave arrival time and surface wave dispersion data into a joint inversion for three dimensional p wave and s wave velocity structure of the crust surrounding the site of the san andreas fault observatory at depth the contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations we find that the trade off between fitting the two data types controlled by the weighting defines a clear optimal solution varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type all the acceptable solutions yield structures with similar primary features but the smaller scale features change substantially when there is a lower relative weight on the surface wave data it appears that the solution over fits the body wave data leading to a relatively rough vs model whereas for the optimal weighting we obtain a relatively smooth model that is able to fit both the body wave and surface wave observations adequately

episodic tremors and deep slow slip events in central california abstract observations of episodic tremor and slip ets events have given insight into the seismic hazards along subduction zones by demonstrating that strain release was possible from slow slip events sses here we show evidences that recurring episodes of nonvolcanic tremors nvt along the san andreas fault saf in the parkfield california area are reminiscent of seismic events associated with ets and sses observed in subduction zones nonetheless the parkfield episodes are not uniformly periodic through time but exhibit systematic differences between aperiodic and periodic recurrences that are related to the 2004 m6 0 parkfield earthquake and the long term saf slip rate by constraining the locations of the nvt episodes their duration and the correlated response between episodes and repeating earthquake activity along the saf we propose a simple sse model for the 52 parkfield episodes observed in 9 7 yr our findings suggest that 10 day period slow slips corresponding to maximum slips of 7 8 mm occur on a saf parallel patch that is 25 km long and 15 km wide centered at 25 km depth sse moment magnitudes range between 5 0 and 5 4 and imply a stress drop of 10 kpa we also show that seismicity patterns above the sse zone show correspondence with the sse timing and this emphasizes the importance of nvt episodes as in situ measurements to inferred fault slip transients below the seismogenic zone finally we compare the source parameters of the saf sses with sses of other regions and we show that they follow similar scalings

compositional variations of syntectonic white mica in low grade ignimbritic mylonite the silla del gobernador shear zone sgsz coastal range of central chile 328s is a low grade mylonite generated from a triassic acidic ignimbritic protolith different microstructures e g quarter mats and pressure shadow structures associated with pressure solution and solution transfer processes and well preserved porphyroclasts can be observed white mica generation appears to be favoured by reaction softening from dissolved quartz and feldspar porphyroclasts according to the mylonite fabrics three different types of white mica can be established a b and c representing mylonites with evidences of relatively low intermediate and high strain conditions deformation mechanisms suggest low grade 350 400 8c conditions for mylonitic deformation in the presence of fluids consistent with the temperature estimations based on chlorite chemistry the geological setting of the sgsz also suggests low pressure 3 kbar conditions no major element mobility with respect to protolith composition was observed in low strained mylonites whereas for the highly strained mylonites a marked k2o secondary input and higher fo2 conditions during deformation are suggested the chemical composition of the neoformed white mica correlates with strain intensity the high strained mylonites contain phengitic celadonitic micas whereas lowstrained mylonites have paragonitic and muscovitic micas white mica geobarometry in highly deformed rocks yields pressure values with no geological significance because the metasomatic k input favour their high k content independent of the confining pressure thus the white mica composition in the mylonites of the sgsz is controlled by the whole rock geochemistry and is related to strain conditions

three dimensional modeling of near fault ground motions with nonplanar rupture models and topography case of the 2004 parkfield earthquake 1 we study the applicability of deterministic strong ground motion simulations at very near fault distances for a subvertical strike slip fault model corresponding to the 2004 m6 parkfield california earthquake in the frequency range up to 1 hz theoretical modeling under the assumptions of a planar rupture and 1 d medium shows that as a consequence of the s wave radiation pattern the particle motion for such close stations should be almost linear in the fault normal fn direction having fault parallel fp and vertical v components of almost zero however as shown on the parkfield earthquake recordings observed particle motions are rather circular with peak velocities at fp and v components comparable to those at fn components we investigate several realistic features that could explain this controversy namely nonplanar fault realistic three dimensional 3 d medium and the topography of the area we test and quantify these hypotheses using discrete wave number and discontinuous galerkin modeling methods applying 1 d and 3 d velocity structures respectively and two nonplanar rupture models we compare the synthetic and observed particle motions and peak velocity ratios and conclude that deviations from a planar rupture geometry in reasonable bounds for the parkfield fault and the influence of topography only partially explain the behavior of the observed seismograms on the contrary the heterogeneous 3 d velocity structure significantly reduces the synthetic peak ratios to values closer to 1 and provides rather circular particle motions therefore the 3 d velocity model is crucial to obtain realistic estimates of ground motions at near fault distances and is more important than the detailed fault geometry or topography in the parkfield area

nonvolcanic tremors deep beneath the san andreas fault we have discovered nonvolcanic tremor activity i e long duration seismic signals with no clear p or s waves within a transform plate boundary zone along the san andreas fault near cholame california the inferred epicentral region of the 1857 fort tejon earthquake moment magnitude 7 8 the tremors occur between 20 to 40 kilometers depth below the seismogenic zone the upper 15 kilometers of earth s crust where earthquakes occur and their activity rates may correlate with variations in local earthquake activity

locating nonvolcanic tremors beneath the san andreas fault using a station pair double difference location method 1 it has been a challenging task to locate nonvolcanic tremors because of the lack of impulsive wave arrivals a station pair double difference dd location method is developed to determine absolute tremor locations by directly using the station pair travel time differences measured from cross correlating tremor waveform envelopes multiple tremors are located together for inverting for station corrections to take into account velocity model inaccuracy the new method is applied to tremors in the parkfield region of central california between 27 july 2001 and 21 february 2009 compared to the tremor catalog locations determined from a grid search location method most of the newly located tremors are located at depths between 20 and 35 km well below the seismogenic zone in the area the tremors beneath cholame ca are more clearly separated into two zones laterally distributed across the san andreas fault with most tremors occurring to the southwest and exhibiting a periodic pattern of occurrence the new tremor locations help better delineate the spatial and temporal distribution of tremor activity and therefore are helpful for better understanding tremor origin and process

precise tremor source locations and amplitude variations along the lower crustal central san andreas fault 1 we precisely locate 88 tremor families along the central san andreas fault using a 3d velocity model and numerous p and s wave arrival times estimated from seismogram stacks of up to 400 events per tremor family maximum tremor amplitudes vary along the fault by at least a factor of 7 with by far the strongest sources along a 25 km section of the fault southeast of parkfield we also identify many weaker tremor families which have largely escaped prior detection together these sources extend 150 km along the fault beneath creeping transitional and locked sections of the upper crustal fault depths are mostly between 18 and 28 km in the lower crust epicenters are concentrated within 3 km of the surface trace implying a nearly vertical fault a prominent gap in detectible activity is located directly beneath the region of maximum slip in the 2004 magnitude 6 0 parkfield earthquake

interpretation of gravity data in a complex volcano tectonic setting southwestern nevada this regional gravity study based on an irregular 2 km data grid was conducted during the past few years at yucca mountain southern nye county nevada as part of a program to locate a suitable repository for high level nuclear waste about 100 surface rock samples three borehole gamma gamma logs and one borehole gravity study provide excellent density control a nearly linear increase in density of 0 26 g cm3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain isostatic and 2 0 g cm3 bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects respectively the bare mountain gravity high with an isostatic anomaly maximum of 48 mgal is connected with a greater gravity high over the funeral mountains to the southwest together these highs result from a continuous block of dense metamorphosed precambrian and paleozoic rocks that stretches across much of the walker lane from the east edge of death valley to bare mountain the calico hills gravity high appears more likely to originate from a northeast trending buried ridge of paleozoic rocks that extends southwestward beneath busted butte 5 km southeast of the proposed repository where two and three dimensional modeling indicates that the pre cenozoic rocks lie less than 1000 m beneath the surface tuff at least 4000 m thick fills a large steep sided depression in the pretuff rocks beneath yucca mountain and crater flat the gravity low and the thick tuff section lie within a large collapse area that includes the crater flat timber mountain silent canyon caldera complexes gravity lows in crater flat itself are interpreted to coincide with the source areas of the prow pass member the bullfrog member and the tram member of the crater flat tuff these source areas add nearly 350 km2 to the previously recognized extent of the local caldera complexes southward extension of the broad gravity low associated with crater flat into the amargosa desert is evidence for sector graben type collapse segments related to the formation of the timber mountain caldera and superimposed on the other volcanic and extensional structures within crater flat

seismic tomography of the southern california plate boundary region from noise based rayleigh and love waves we use cross correlations of ambient seismic noise between pairs of 158 broadband and short period sensors to investigate velocity structure over the top 5 10 km of the crust in the southern california plate boundary region around the san jacinto fault zone sjfz from the 9 component correlation tensors associated with all station pairs we derive dispersion curves of rayleigh and love wave group velocities the dispersion results are inverted first for rayleigh and love waves group velocity maps on a 1 5 1 5 km2 grid that includes portions of the sjfz the san andreas fault saf and the elsinore fault we then invert these maps to 3d shear wave velocities in the top 7 km of the crust the distributions of the rayleigh and love group velocities exhibit 2 azimuthal anisotropy with fast directions parallel to the main faults and rotations in complex areas the reconstructed 3d shear velocity model reveals complex shallow structures correlated with the main geological units and strong velocity contrasts across various fault sections along with low velocity damage zones and basins the sjfz is marked by a clear velocity contrast with higher v s values on the ne block for the section se of the san jacinto basin and a reversed contrast across the section between the san jacinto basin and the saf velocity contrasts are also observed along the southern parts on the saf and the elsinore fault with a faster southwest block in both cases the region around the salton trough is associated with a significant low velocity zone strong velocity reductions following flower shape with depth are observed extensively around both the sjfz and the saf and are especially prominent in areas of geometrical complexity in particular the area between the sjfz and the saf is associated with an extensive low velocity zone correlated with diffuse seismicity at depth and a similar pattern including correlation with deep diffuse seismicity is observed on a smaller scale in the trifurcation area of the sjfz these results augment local earthquake tomography images that have low resolution in the top few km of the crust and provide important constraints for studies concerned with behavior of earthquake ruptures generation of rock damage and seismic shaking hazard in the region

analysis of nonvolcanic tremor on the san andreas fault near parkfield ca using u s geological survey parkfield seismic array 1 reports by nadeau and dolenc 2005 that tremor had been detected near cholame valley spawned an effort to use upsar u s geological survey parkfield seismic array to study characteristics of tremor upsar was modified to record three channels of velocity at 40 50 sps continuously in january 2005 and ran for about 1 month during which time we recorded numerous episodes of tremor one tremor on 21 january at 0728 was recorded with particularly high signal levels as well as another episode 3 days later both events were very emergent had a frequency content between 2 and 8 hz and had numerous high amplitude short duration arrivals within the tremor signal here using the first episode as an example we discuss an analysis procedure which yields azimuth and apparent velocity of the tremor at upsar we then provide locations for both tremor episodes the emphasis here is how the tremor episode evolves twelve stations were operating at the time of recording slowness of arrivals was determined using cross correlation of pairs of stations the same method used in analyzing the main shock data from 28 september 2004 a feature of this analysis is that 20 s of the time series were used at a time to calculate correlation the longer windows resulted in more consistent estimates of slowness but lower peak correlations these values of correlation peaks of about 0 25 however are similar to that obtained for the s wave of a microearthquake observed peaks in slowness were traced back to source locations assumed to lie on the san andreas fault our inferred locations for the two tremor events cluster near the locations of previously observed tremor south of the cholame valley tremor source depths are in the 14 24 km range which is below the seismogenic brittle zone but above the moho estimates of error do not preclude locations below the moho however the tremor signal is very emergent but contains packets that are several times larger than the background tremor signal and lasts about 5 s these impulsive wavelets are similar to low frequency earthquakes signals seen in japan but appear to be broader band rather than just higher in low frequency energy they may be more appropriately called high energy tremor het het signals at upsar correlate well with the record of this event from station ghib of the hrsn borehole array at parkfield and hets typically have a higher cross correlation coefficient than the rest of the tremor event the amplitudes of a large het are consistent with a magnitude of 0 1 when compared with a m2 3 event that had about the same epicenter polarizations of the tremor episode at upsar are mostly just north of east both linearity and azimuth evolve over time suggesting a change in tremor source location over time and linearity is typically higher at the hets

seismotectonics and fault structure of the california central coast abstract i present and interpret new earthquake relocations and focal mechanisms for the california central coast the relocations improve upon catalog locations by using 3d seismic velocity models to account for lateral variations in structure and by using relative arrival times from waveform cross correlation and double difference methods to image seismicity features more sharply focal mechanisms are computed using ray tracing in the 3d velocity models seismicity alignments on the hosgri fault confirm that it is vertical down to at least 12 km depth and the focal mechanisms are consistent with right lateral strike slip motion on a vertical fault a prominent newly observed feature is an 25 km long linear trend of seismicity running just offshore and parallel to the coastline in the region of point buchon informally named the shoreline fault this seismicity trend is accompanied by a linear magnetic anomaly and both the seismicity and the magnetic anomaly end where they obliquely meet the hosgri fault focal mechanisms indicate that the shoreline fault is a vertical strike slip fault several seismicity lineations with vertical strike slip mechanisms are observed in estero bay events greater than about 10 km depth in estero bay however exhibit reverse faulting mechanisms perhaps reflecting slip at the top of the remnant subducted slab strike slip mechanisms are observed offshore along the hosgri san simeon fault system and onshore along the west huasna and rinconada faults while reverse mechanisms are generally confined to the region between these two systems this suggests a model in which the reverse faulting is primarily due to restraining left transfer of right lateral slip

incorporating fault zone head wave and direct wave secondary arrival times into seismic tomography application at parkfield california 1 we present a three dimensional 3d p wave velocity vp model of the parkfield region that utilizes existing p wave arrival time data including fault zone head waves fzhws and data from direct wave secondary arrivals dwsas the first arrival and dwsa travel times are obtained as the global and local minimum travel time paths respectively the inclusion of fzhws and dwsas results in as much as a 5 and a 10 increase in the across fault velocity contrast respectively for the vp model at parkfield relative to that of thurber et al 2006 viewed along strike three pronounced velocity contrast regions are observed a pair of strong positive velocity contrasts sw fast one nw of the 1966 parkfield earthquake hypocenter and the other se of the 2004 parkfield earthquake hypocenter and a strong negative velocity contrast ne fast between the two hypocenters the negative velocity contrast partially to entirely encompasses peak coseismic slip estimated in several slip models for the 2004 earthquake suggesting that the negative velocity contrast played a part in defining the rupture patch of the 2004 parkfield earthquake following ampuero and ben zion 2008 the pattern of velocity contrasts is consistent with the observed bilateral rupture propagation for the 2004 parkfield earthquake although the velocity contrasts also suggest bilateral rupture propagation for the 1966 parkfield earthquake the fault is creeping to the nw here i e exhibiting velocity strengthening behavior thus it is not surprising that rupture propagated only se during this event

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

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake