earthquake locations using single station deep borehole recordings implications for microseismicity on the san andreas fault in southern california we have located and estimated source parameters of 109 earthquakes 0 to 5 m using seismograms recorded at 2 5 km depth in the cajon pass borehole southern california the borehole is about 4 km from the san andreas fault at the boundary between the locked almost aseismic mojave and san bernardino segments where the san jacinto fault approaches the san andreas this area is of interest both on account of its tectonic complexity and its high potential seismic hazard the clear relatively unattenuated downhole recordings are rotated to determine the incoming azimuth of the p wave and the delay time between the p and s arrivals is used to estimate the hypocentral distance the difference between the hypocenters located this way and the southern california seismic network scsn locations of the same earthquakes increases with hypocentral distance from the borehole of the earthquakes within 20 km of the borehole 95 are within 1 2 km of the scsn epicenters and all are within 5 km of the scsn depths all but three of the 58 earthquakes located here which the scsn did not record were within 20 km of the borehole and so the location errors are similar to those of scsn a and b quality events most of the small earthquakes occurred within the relatively active san jacinto fault zone but at least eight are located close to or within the san andreas fault zone the stress drops for these events range from 0 1 to 18 mpa these earthquakes appear similar to the other events in this study suggesting that the san andreas fault is similar to other faults at the scale of these small earthquakes the variation in stress drop and slip orientation of these small earthquakes suggests that larger scale heterogeneity continues to these small scales also the location of these small earthquakes in major established fault zones implies that earthquake source dimensions are not geometrically controlled by fault zone width

observation and scaling of microearthquakes from the taiwan chelungpu fault borehole seismometers summary microearthquakes with magnitude down to 0 3 were detected by the taiwan chelungpu fault drillingproject borehole seismometers tcdpbhs despitethelargecoseismic slipof12m atthedrillsiteduringthe1999chi chiearthquake ourstudiesshowverylittleseismicitynear the tcdpbhs drill site 6 yr after the chi chi main shock the microearthquakes clustered at a depth of 9 12 km where the chelungpu thrust fault turns from a 30 dipping into the horizontal decollement of the taiwan fold and thrust tectonic structure continuous gps surveys did not observe post slip deformation at the larger slip region and no seismicity was observed near the drill site therefore we suggest that the thrust belt above the decollement is locked during this interseismic period we further investigated source parameters of 242 microearthquakes by fitting 2 shaped brune source spectra to our observation data using a frequency independent q model we find that the static stress drop increases significantly with increasing seismic moment however due to the intense debate on this topic of scalingrelations and the related self similarity of earthquakes we further improve the data analysis and correct for path and site effects using the projected landweber deconvolution pld method for events within some clusters the pld method analyses the source time functions of the larger and the smaller event by an iterative technique as a result we received source dimensions and stress drops of larger events including path and site effect corrections the results from the pld method are less scattered and also show a positive relation between static stress drop and seismic moment we find a similar positive trend for the apparent stress scaling with seismic moment

high resolution seismic velocities and shallow structure of the san andreas fault zone at middle mountain parkfield california a 5 km long high resolution seismic imaging survey across the san andreas fault saf zone and the proposed san andreas fault observatory at depth safod drill site near parkfield california shows that velocities vary both laterally and vertically velocities range from 4 0 km sec probably correspond to granitic rock of the salinian block which is exposed a few kilometers southwest of the saf the depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed safod site we observe a prominent lateral low velocity zone lvz beneath and southwest of the surface trace of the saf the lvz is about 1 5 km wide at 300 m depth but tapers to about 600 m wide at 750 m depth at the maximum depth of the velocity model 750 m the lvz is centered approximately 400 m southwest of the surface trace of the saf similar velocities and velocity gradients are observed at comparable depths on both sides of the lvz suggesting that the lvz is anomalous relative to rocks on either side of it velocities within the lvz are lower than those of san andreas fault gouge and the lvz is also anomalous with respect to gravity magnetic and resistivity measurements because of its proximity to the surface trace of the saf it is tempting to suggest that the lvz represents a zone of fractured crystalline rocks at depth however the lvz instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the saf such a sliver of sedimentary rock implies fault strands on both sides and possibly within the sliver suggesting a zone of fault strands at least 1 5 km wide at a depth of 300 m tapering to about 600 m wide at 750 m depth fluids within the sedimentary sliver are probably responsible for observed low resistivity values

induced microearthquake patterns in hydrocarbon and geothermal reservoirs six case studies the injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs by deploying sensors downhole data sets have been collected that consist of a few hundred to well over 10 000 induced events we find that most induced events cluster into well defined geometrical patterns in many cases we must apply high precision relative location techniques to observe these patterns at three sedimentary sites thin horizontal strands of activity are commonly found within the location patterns we believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress at a massive carbonate and two crystalline sites combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns in addition the fine scale seismicity patterns often evolve systematically with time at sedimentary sites migration of seismicity toward the injection point has been observed and may result from slip induced stress along fractures that initially have little resolved shear in such cases triggering events may be critical to generate high levels of seismic activity at one crystalline site the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures we hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs

southern surface rupture associated with the 1992 m7 4 landers earthquake did it all happen during the mainshock approximately three minutes after the magnitude 7 4 landers mainshock on 28 june 1992 a m5 7 aftershock occurred south of the mainshock epicenter with a location of 34 7 65 n 116 23 82 w this aftershock was recorded on an array of portable digital seismic instruments deployed in morongo valley 21 km southwest of the event although peak accelerations are found to differ by approximately 50 at stations with similar general site conditions within 500 m of each other there is good coherence of arrivals across the array for frequencies 1 hz we use the recordings to determine the apparent phase velocity and azimuth of propagation across the array and show that the event clearly ruptured to the south with a rupture length of 11 km and a rupture velocity of approximately 3 0 km s our results suggest that at least some of the mapped surface rupture south of the town of yucca valley 11 km in extent maximum displacement of 20 cm may have been associated with this aftershock if this is the case then the fault that produced the southern end of the landers mainshock the johnson valley fault need not continue at depth across the active left lateral east west trending pinto mountain fault

a look inside the san andreas fault at parkfield through vertical seismic profiling the san andreas fault observatory at depth pilot hole is located on the southwestern side of the parkfield san andreas fault this observatory includes a vertical seismic profiling vsp array vsp seismograms from nearby microearthquakes contain signals between the p and s waves these signals may be p and s waves scattered by the local geologic structure the collected scattering points form planar surfaces that we interpret as the san andreas fault and four other secondary faults the scattering process includes conversions between p and s waves the strengths of which suggest large contrasts in material properties possibly indicating the presence of cracks or fluids

ages and stable isotope compositions of secondary calcite and opal in drill cores from tertiary volcanic rocks of the yucca mountain area nevada stable isotope compositions of fracture and cavity filling calcite from the unsaturated zone of three drill cores at yucca mountain tertiary volcanic complex indicate that the water from which the minerals precipitated was probably meteoric in origin a decrease in 18 o in the calcite with depth is interpreted as being due to the increase in temperature in drill holes corresponding to an estimated average geothermal gradient of 34 per kilometer a few of the calcite samples and all of the opal samples yielded uranium series ages older than 400 000 yr although most of the calcite samples yielded ages between 26 000 and 310 000 yr the stable isotope and uranium series dates from precipitated calcite and opal of this reconnaissance study suggest a complex history of fluid movement through the volcanic pile and episodes of fracture filling predominantly from meteoric water during at least the past 400 000 yr

interpretation of s waves generated by near surface chemical explosions at safod a series of near surface chemical explosions conducted at the san andreas fault observatory at depth safod were recorded by high frequency downhole receiver arrays in separate experiments in november 2003 and may 2005 the 2003 experiment involved 100 kg shots detonated along a 46 km long line hole ryberg line centered on safod and recorded by 32 three component geophones in the pilot hole between 0 8 and 2 0 km depth the 2005 experiment involved 36 kg shots detonated at parkfield area seismic observatory paso stations at 1 8 km offset recorded by 80 three component geophones in the main hole between the surface and 2 4 km depth these data sample the downgoing seismic wavefield and constrain the shallow velocity and attenuation structure as well as the first order characteristics of the source using forward modeling on a velocity structure designed for the near field both observed p and s wave energy for the paso shots are identified with the travel times expected for direct and or reflected phases larger offset recordings from shots along the hole ryberg line reveal substantial sv and sh energy especially southwest of safod from the source as indicated by p to s amplitude ratios the generated sv energy is interpreted to arise chiefly from p to s conversions at subhorizontal discontinuities this provides a simple mechanism for often observed low p to s amplitude ratios from nuclear explosions in the far field as originating from strong near field wave conversions online material figures of observed and synthetic seismogram record sections snapshots of synthetic wavefields and models of crustal structure

determining safod area microearthquake locations solely with the pilot hole seismic array data 1 in august 2002 an array of 32 three component geophones was installed in the san andreas fault observatory at depth safod pilot hole ph at parkfield ca as an independent test of surface observation based microearthquake locations we have located such events using only data recorded on the ph array we then compared these locations with locations from a combined set of ph and parkfield high resolution seismic network hrsn observations we determined the uncertainties in the locations as they relate to errors in the travel time picks and the velocity model by the bootstrap method based on the ph and combined locations we find that the c2 cluster to the northeast of the ph has the smallest location uncertainties events in this cluster also have the most similar waveforms and largest magnitudes this confirms earlier suggestions that the c2 cluster is a promising target for the safod main hole

rupture characteristics and tomographic source imaging of ml 3 earthquakes near anza southern california the rupture characteristics of several ml 3 earthquakes near anza california are determined from time domain analysis of path corrected displacement pulses these pulses are corrected for propagation effects apparent attenuation scattering site resonance using the waveforms of adjacent small events ml 2 1 as empirical green s functions the earthquakes studied occurred during two swarm sequences near cahuilla in an area characterized by swarm activity at shallow depth 5 km the data consist of seismograms recorded by the anza seismic network a high dynamic range local array with digital telemetry the waveforms of adjacent small events ml 2 1 are deconvolved from those of the ml 3 events to yield the source time functions of the larger earthquakes the ml 3 earthquakes display a variety of rupture modes similar to those reported for large shocks but over much shorter time scales and distances events with single and multiple ruptures are observed relative locations of the subevents are obtained from azimuthal differences in the time separation of pulses in the deconvolved displacement waveforms some events as small as ml 2 4 display unilateral rupture propagation as manifested by azimuthal variations in the pulse widths of the deconvolved displacement pulses a tomographic back projection technique is applied to the deconvolved displacement pulses to image the slip velocity on the fault as a function of time after nucleation and distance along the fault using a one dimensional fault model the tomographic inversion for one of the events ml 2 4 reveals a unilateral rupture with rupture velocity about equal to the shear wave velocity vs and fault length of 300 m a similar inversion for an ml 3 7 event resolves two subevents separated by about 200 m and yields an average rupture velocity of 0 8vs these rupture velocities are similar to those reported for large earthquakes indicating that rupture velocity is independent of seismic moment down to 3 1 1019 dyn cm the smallest event studied there are clear variations in pulse width for neighboring events with similar seismic moments implying significant differences in static stress drops factor of five variations in stress drop 11 61 bars are calculated for events within 200 m of each other thus the stress and or strength along this fault zone varies considerably over distances of hundreds of meters

seismic energy release in mexican subduction zone earthquakes we estimated radiated seismic energy es from 18 shallow thrust mexican subduction zone earthquakes 4 1022 m 1 1 1028 dyne cm 11 h 37 km using digital accelerograms from the guerrero accelerograph array and neglecting stations with large site effects es is computed by integrating squared velocity spectra after applying geometrical spreading and q corrections we discarded epicentral recordings for the largest michoacan event we find that log es m 4 152 0 275 which gives a median value of es m and apparent stress a of 7 1 10 5 and 24 bars respectively the median es m value is in accordance with gutenberg and richter s g r 1956 formula for es in which es m 5 10 5 is implicit worldwide es m data where es is computed from local records mostly fall between 5 10 5 and 5 10 4 for events with m 1022 dyne cm on the other hand es m values generally lie between 5 10 6 and 5 10 5 if es is estimated from teleseismic records especially anomalous are the es m data from kikuchi and fukao 1988 for large and great earthquakes which fall near 5 10 6 thus while es from local data suggests that the g r relation seldom overestimates seismic energy release the teleseismic data point to the contrary the cause of this discrepancy may lie in the difficulty of resolving incoherent radiation from the fault and inappropriate choice of t in the analysis of teleseismic data

source parameters of the 23 april 1992 m 6 1 joshua tree california earthquake and its aftershocks empirical green s function analysis of geos and terrascope data source parameters of the m 6 1 23 april 1992 joshua tree mainshock and 86 m 1 8 to 4 9 aftershocks are determined using an empirical green s function methodology for the aftershocks deconvolved p and s wave spectra are calculated for 126 pairs of closely spaced events recorded on portable geos stations s wave spectra from the two horizontal components are averaged the deconvolved spectra are fit by a ratio of omega square source models yielding an optimal least squares corner frequency for both the large and the small event in each pair we find no resolved difference between the inferred p and s wave corner frequencies using the standard brune model for stress drop we also find no resolved nonconstant scaling of stress drop with moment although we also conclude that detailed scaling systematics would be difficult to resolve in particular a weak increase of stress drop with moment over a limited moment magnitude cannot be ruled out for magnitudes smaller than m 3 to 3 5 the inferred stress drop values will be limited by the maximum observable corner frequency value of 60 hz for the mainshock source time functions are obtained from mainshock recordings at three terrascope stations pfo pas and gsc using an m 4 3 foreshock as an empirical green s function the results indicate a fairly simple single pulse source time function with clear south to north directivity and an inferred rupture radius of 5 to 6 km the deconvolved source time functions are inverted to obtain a finite rupture model that gives a robust estimate of rupture dimension early aftershocks are found to lie along the perimeters of regions with high mainshock slip the inferred mainshock stress drop value 56 bars is within the range determined for the aftershocks our derived mainshock source spectra do not show resolvable deviation from the omega square model

tomographic images of p wave velocity variation at parkfield california tomographie inversion is applied io delay times from local earthquakes to image three dimensional velocity variations near parkfield california the 25 20 square km region is represented by nearly cubic blocks of 0 5 km per side arrival times of p waves from 551 local earthquakes with depths of 0 to 15 km were used as sources producing 3135 rays covering the target region the data were recorded on low noise downhole seismographs a conjugate gradient method is used to invert the resulting sparse system of simultaneous equations to diminish the effects of noisy data the laplacian of the model parameters is constrained to be small within horizontal layers providing smoothing of the model the resolution of the model is estimated by calculating point spread functions at blocks of interest estimates of standard errors of the model parameters are calculated by the jackknife statistical procedure the results of the inversion show correlation with some of the local geological and geophysical features station corrections removed the long wavelength anomaly associated with the contrast of the salinian block southwest of the san andreas fault versus the franciscan to the northeast a velocity low located a few kilometers northwest of parkfield depth 2 5 3 5 km appears to lie along the gradient of the large bouguer gravity anomaly associated with the parkfield syncline the south southeastward extension of the low velocities may relate to reflections observed on the parkfield consortium for continental reflection profiling cocorp lines we speculate on the geological meaning of these features and interpret them either as part of the local strike slip tectonics or a shallow crustal detachment the correlation of higher velocity features and seismic activity may indicate that earthquakes are occurring in more competent zones while aseismic slip takes place in zones of lower velocity less competent rocks

three component sonde orientation in a deviated well in well seismics when operating with a three component tool particle velocities are measured in the sonde coordinate system but are often needed in other systems e g source bound or geographic when the well is vertical a change from the three orthogonal components of the sonde to another orthogonal coordinate system can be performed through one rotation around the vertical axis and if necessary another one around a horizontal axis hardage 1983 if the well is deviated the change of coordinate system remains easy in the case when the source is located at the vertical of the sonde or in the case when the source stands in the vertical plane defined by the local well axis in the general case offset vsps or walkaways or when looking for unknown sources such as microseismic emissions induced by hydraulic fracturing coordinate rotation may still be performed provided that we first get back to a situation in which one of the axes is vertical

shear wave anisotropy in the parkfield varian well vsp abstract a vertical seismic profile vsp survey was run to 1334 m depth in the instrumented varian well 1 4 km from the san andreas fault trace at parkfield california to test the sensor string shortly after its permanent installation the cable subsequently failed near the 1000 m level so the test survey represents the deepest data acquired in the study a shear wave vibrator source was used at three ofsets and two orthogonal orientations and the data have been processed for p and s wave velocities and for s wave velocity anisotropy velocities are well determined 3 3 and 1 9 km sec respectively at the deeper levels and the s waves are seen clearly to be split by anisotropy below about 400 m some 8 per cent velocity difference is apparent between polarizations parallel to and perpendicular to the san andreas fault faster and slower respectively and the difference seems to decrease with distance from the fault suggesting that the cause may be the fabric of the fault zone repeated surveys at the 1000 m depth are being conducted as part of the parkfield monitoring program

a complex linear least squares method to derive relative and absolute orientations of seismic sensors summary determining the relative orientation of the horizontal components of seismic sensors is a common problem that limits data analysis and interpretation for several acquisition setups including linear arrays of geophones deployed in borehole installations or ocean bottom seismometers deployed at the seafloor to solve this problem we propose a new inversion method based on a complex linear algebra approach relative orientation angles are retrieved by minimizing in a least squares sense the l2 norm between the complex traces hodograms of adjacent pairs of sensors this methodology can be applied without restrictions only if the wavefield recorded by each pair of sensors is very similar in most cases it is possible to satisfy this condition by low pass filtering the recorded waveforms the main advantage of our methodology is that in the complex domain the relative orientations of seismic sensors can be viewed as a linear inverse problem which ensures that the preferred solution corresponds to the global minimum of a misfit function it is also possible to use simultaneously more than one independent data set other seismic events to better constrain the solution of the inverse problem furthermore by a computational point of view our method results faster than the relative orientation methods based on waveform cross correlation after several tests on synthetic data sets we applied successfully our methodology to different types of real data these applications include the alignment of borehole sensors relative to a vertical seismic profiling vsp acquisition and the orientation of ocean bottom seismometers obs relative to a neighbouring land station of known orientation using land stations the absolute orientation of obs can be retrieved finally as a last application we checked the correct orientation for land stations of a seismological array in germany

introduction to special section preparing for the san andreas fault observatory at depth 7209 seismology earthquake dynamics andmechanics 7230seismology seismicityandseismotectonics 8010structural geology fractures and faults citation hickman s m zoback and w ellsworth 2004 introduction to specialsection preparing for the san andreas fault observatory atdepth geophys res lett 31 l12s01 doi 10 1029 2004gl020688

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

estimation of multivalued arrivals in 3d models using wavefront construction part i1 a new 3d wavefield modelling approach based on dynamic ray tracing is presented this approach is called wavefront construction and it can be used in 3d models with constant or smoothly varying material properties s and p velocity and density separated by smooth interfaces wavefronts consisting of rays arranged in a triangular network are propagated stepwise through the model at each time step the differences in a number of parameters are checked between each pair of rays on the wavefront new rays are interpolated whenever this difference between pairs of rays exceeds some predefined maximum value a controlled sampling of the wavefront at all time steps is thus obtained receivers are given multiple event values by interpolation when the wavefronts pass them the strength of the wavefront construction method is that it is robust and efficient

orientation of three component geophones in the san andreas fault observatory at depth pilot hole parkfield california to identify and constrain the target zone for the planned safod main hole through the san andreas fault saf near parkfield california a 32 level three component 3c geophone string was installed in the pilot hole ph to monitor and improve the locations of nearby earthquakes the orientation of the 3c geophones is essential for this purpose because ray directions from sources may be determined directly from the 3d particle motion for both p and s waves due to the complex local velocity structure rays traced from explosions and earthquakes to the ph show strong ray bending observed azimuths are obtained from p wave polarization analysis and ray tracing provides theoretical estimates of the incoming wave field the differences between the theoretical and the observed angles define the calibration azimuths to investigate the process of orientation with respect to the assumed velocity model we compare calibration azimuths derived from both a homogeneous and 3d velocity model uncertainties in the relative orientation between the geophone levels were also estimated for a cluster of 36 earthquakes that was not used in the orientation process the comparison between the homogeneous and the 3d velocity model shows that there are only minor changes in these relative orientations in contrast the absolute orientations with respect to global north were significantly improved by application of the 3d model the average data residual decreased from 13 to 7 supporting the importance of an accurate velocity model we explain the remaining residuals by methodological uncertainties and noise and with errors in the velocity model

microearthquake spectra from the anza california seismic network site response and source scaling abstract we analyzed spectra of local microearthquakes recorded by the anza california seismic network to isolate the effects of site response and to investigate the scaling of source parameters for small earthquakes spectra of microearthquakes m m o 19 dyne cm at anza have shapes characteristic of the receiver sites and are generally independent of the source region thus the site response is a major conditioner of the observed spectral shape to remove the effects of site response from the spectra of a m 3 event and isolate its source spectrum we divided by the spectra of an adjacent aftershock used as an empirical green9s function event the spectral ratios indicate that the apparent corner frequencies of small earthquakes m o 19 dyne cm observed at even the high f max stations on hard rock are much lower than the source corner frequencies the spectral ratios are consistent with stress drop remaining constant with decreasing seismic moment for events with moments as small as 10 18 dyne cm the spectral ratios display remarkable agreement between sites which showed vast differences in their original spectra indicating that the spectral division effectively removed the site response the source spectrum of the m 3 event has a high frequency spectral fall off of about 2 an apparent dependence of high frequency fall off with seismic moment in the original spectra can also be explained by the effects of site response the difference between the p and s wave corner frequencies and high frequency roll offs in the observed spectra for these events is the result of the site response and is not a source property the shapes of the spectra of microearthquakes at anza can largely be explained by attenuation at shallow depth with a frequency independent q for some sites near surface resonances are also apparent in the spectra of microearthquakes it is indicated by t values determined for each site that q p q s for the shallow low q layer further evidence of low near surface q is observed in the anomalous spectra of an unusually shallow earthquake source depth 1 km in the network the spectra from this shallow event are depleted in high frequency energy at most stations relative to those of deeper events this observation can be explained by a low q surficial zone for stations of the network situated on alluvium this low q layer has a maximum thickness of about 3 km and maximum p and s wave q values of 30 to 50

earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod 1 arrival time data from 453 local earthquakes and 6 explosions at parkfield ca are inverted for earthquake locations and three dimensional vp and vp vs structure the structure is dominated by the velocity contrast across the saf with the southwest side about 20 25 faster consistent with previous studies nearly all the earthquakes occur almost directly beneath the fault trace we find high vp vs anomalies that correlate with low resistivity features in a magnetotelluric model that are interpreted to represent fluids we locate a magnitude 2 earthquake that is a potential target event for the final stage of safod drilling yielding a depth of 3 1 km below surface and an epicenter 100 m southwest of the fault trace nonlinear analyses indicate 95 confidence relative and absolute uncertainties on the order of 500 700 m vertically and 200 300 m horizontally for this target earthquake

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