shallow creep on the haiyuan fault gansu china revealed by sar interferometry interferometric synthetic aperture radar data are used to map the interseismic velocity field along the haiyuan fault system hfs at the north eastern boundary of the tibetan plateau two m 8 earthquakes ruptured the hfs in 1920 and 1927 but its 260 km long central section known as the tianzhu seismic gap remains unbroken since 1000 years the envisat sar data spanning the 2003 2009 period cover about 200 300 km2 along three descending and two ascending tracks interferograms are processed using an adapted version of roi pac the signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals mean line of sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise these maps show a dominant left lateral motion across the hfs and reveal a narrow 35 km long zone of high velocity gradient across the fault in between the tianzhu gap and the 1920 rupture we model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion the inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments the average creep rate is 5 mm yr 1 comparable in magnitude with the estimated loading rate at depth suggesting no strain accumulation on this segment the modeled creep rate locally exceeds the long term rate reaching 8 mm yr 1 suggesting transient creep episodes the present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment

stress drop for three m 4 3 4 7 1992 1994 parkfield ca earthquakes in a previous paper fletcher and spudich 1998 we determined the slip distribution for the three m 4 3 to 4 7 parkfield events that occurred between oct 20 1992 and dec 20 1994 the rupture characteristics of these events are of particular interest because they are the largest events to occur at parkfield since the 1966 mainshock to further investigate the rupture process of these events we compute the static stress change or stress drop caused by the slip using a boundary integral method from quin and das 1989 adapted for the forward modeling case of computing stresses from a static slip distribution we find that the initial phase at or very near the hypocenter has the largest stress change in all cases this peak has a value of about 3 8 mpa for the oct 20 1992 event which also has the smallest magnitude of the three at 4 3 and about 45 mpa for the other nov 14 1993 and 60 mpa for the dec 20 1994 event the rest of the stress release is comparatively small and smooth for the nov 14 1993 event which is the deepest event and the most impulsive the pattern of stress release of the other two events oct 20 1992 and dec 20 1994 is more complex with a greater spatial extent the spatial distribution of stress is approximately similar to the spatial distribution of slip but with the larger initial peak in stress drop at the hypocenter

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

temporal changes of shallow seismic velocity around the karadere duzce branch of the north anatolian fault and strong ground motion we analyze temporal variations of seismic velocity along the karadere duzce branch of the north anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 mw7 4 izmit and mw7 1 duzce earthquakes the analysis employs 36 sets of highly repeating earthquakes each containing 4 18 events the events in each cluster are relocated by detailed multi step analysis and are likely to rupture approximately the same fault patch at different times the decay rates of the repeating events in individual clusters are compatible with the omori s law for the decay rate of regional aftershocks a sliding window waveform cross correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events we find clear step like delays in the direct s and early s coda waves sharp seismic velocity reduction immediately after the duzce main shock followed by gradual logarithmic type recoveries a gradual increase of seismic velocities is also observed before the duzce main shock probably reflecting post seismic recovery from the earlier izmit main shock the temporal behavior is similar at each station for clusters at various source locations indicating that the temporal changes of material properties occur in the top most portion of the crust the effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones and generally decrease with normal distance from the fault a strong correlation between the co seismic delays and intensities of the strong ground motion generated by the duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material

time lapse seismic tomography of an underground mining zone abstract in this study we determined p wave tomographic images of the yongshaba deposit an underground mining zone in guizhou province china by inverting arrival time data of micro seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during january to april 2014 using an event location technique and a travel time tomography method that can handle complex seismic discontinuities the damping parameter used in the damped least squares method is determined with the help of trade off curve to reveal internal p wave velocity changes of the study area under severe mining activities a time lapse data partition scheme is used to assess the human influence on the underground structure we introduce a parameter to measure the relative velocity changes in different periods the resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test our tomographic results obtained from the entire data set reveal a prominent low velocity low v zone and many obvious high velocity high v zones these features match well with the geological setting and the excavation plan carried out in the mine according to in site surveys the low v zone may reflect empty volumes stress releases and rock breaking and cracking caused by mining activities whereas the high v zones are probably the consequences of local stress concentrations caused by regional stress redistribution the time lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period indicating that the mining activities influenced the rock property and caused complex changes of the underground structure

slip parallel seismic lineations on the northern hayward fault california a high resolution relative earthquake location procedure is used to image the fine scale seismicity structure of the northern hayward fault california the seismicity defines a narrow near vertical fault zone containing horizontal alignments of hypocenters extending along the fault zone the lineations persist over the 15 year observation interval implying the localization of conditions on the fault where brittle failure conditions are met the horizontal orientation of the lineations parallels the slip direction of the fault suggesting that they are the result of the smearing of frictionally weak material along the fault plane over thousands of years

variations of the velocity contrast and rupture properties of m6 earthquakes along the parkfield section of the san andreas fault summary we investigate the seismic velocity contrast across the san andreas fault saf in the parkfield area using fault zone head waves fzhw that propagate along the bimaterial fault interface and direct p waves we systematically analyse large data sets of near fault waveforms recorded by several seismic networks over the period 1984 2005 clear fzhw are observed at many stations on the ne side of the fault in the creeping section of the saf north of middle mountain mm this indicates the presence of a sharp bimaterial interface and that the ne side of the fault has lower seismic velocities in that region the obtained p wave velocity contrast is about 5 10 per cent north of mm and it systematically decreases to 0 2 per cent near gold hill gh the along strike variations of the velocity contrast are consistent with geological observations of a sliver of high velocity rock immediately to the ne of the saf near gh associated with the gh fault and existing 3 d seismic tomography results the obtained imaging results offer an explanation for the mixed rupture directions of the m6 type parkfield earthquakes the strong velocity contrast around mm is expected to produce a preferred propagation direction to the se for earthquakes that nucleate near mm e g the 1934 and 1966 parkfield earthquakes in contrast the near zero velocity contrast and multiple fault branches near gh imply that earthquakes that nucleate near gh e g the 2004 parkfield earthquake are not expected to have a preferred propagation direction to the se and are likely to propagate in directions that are controlled by other factors such as structural and stress heterogeneities the observed systematic reduction of the velocity contrast along the saf from nw of mm to se of gh provides a dynamic arrest mechanism for earthquakes that nucleate in the northern part of the parkfield section and propagate to the se and a dynamic arrest mechanism for earthquakes that nucleate in the southern section and propagate to the nw

source parameters of microearthquakes on an interplate asperity off kamaishi ne japan over two earthquake cycles summary we have estimated the source parameters of interplate earthquakes in an earthquake cluster off kamaishi ne japan over two cycles of m 4 9 repeating earthquakes the m 4 9 earthquake sequence is composed of nine events that occurred since 1957 which have a strong periodicity 5 5 0 7 yr and constant size m4 9 0 2 probably due to stable sliding around the source area asperity using p and s wave traveltime differentials estimated from waveform cross spectra three m 4 9 main shocks and 50 accompanying microearthquakes m1 5 3 6 from 1995 to 2008 were precisely relocated the source sizes stress drops and slip amounts for earthquakes of m2 4 or larger were also estimated from corner frequencies and seismic moments using simultaneous inversion of stacked spectral ratios relocation using the double difference method shows that the slip area of the 2008 m 4 9 main shock is co located with those of the 1995 and 2001 m 4 9 main shocks four groups of microearthquake clusters are located in and around the mainshock slip areas of these two clusters are located at the deeper and shallower edge of the slip areas and most of these microearthquakes occurred repeatedly in the interseismic period two other clusters located near the centre of the mainshock source areas are not as active as the clusters near the edge the occurrence of these earthquakes is limited to the latter half of the earthquake cycles of the m 4 9 main shock similar spatial and temporal features of microearthquake occurrence were seen for two other cycles before the 1995 m5 0 and 1990 m5 0 main shocks based on group identification by waveform similarities stress drops of microearthquakes are 3 11 mpa and are relatively constant within each group during the two earthquake cycles the 2001 and 2008 m 4 9 earthquakes have larger stress drops of 41 and 27 mpa respectively these results show that the stress drop is probably determined by the fault properties and does not change much for earthquakes rupturing in the same area the occurrence of microearthquakes in the interseismic period suggests the intrusion of aseismic slip causing a loading of these patches we also found that some earthquakes near the centre of the mainshock source area occurred just after the earthquakes at the deeper edge of the mainshock source area these seismic activities probably indicate episodic aseismic slip migrating from the deeper regions in the mainshock asperity to its centre during interseismic periods comparison of the source parameters for the 2001 and 2008 main shocks shows that the seismic moments 1 04 1016 nm and 1 12 1016 nm for the 2008 and 2001 earthquakes respectively and source sizes radius 570 m and 540 m for the 2008 and 2001 earthquakes respectively are comparable based on careful phase identification and hypocentre relocation by constraining the hypocentres of other small earthquakes to their precisely located centroids we found that the hypocentres of the 2001 and 2008 m 4 9 events are located in the southeastern part of the mainshock source area this location does not correspond to either episodic slip area or hypocentres of small earthquakes that occurred during the earthquake cycle

streaks multiplets and holes high resolution spatio temporal behavior of parkfield seismicity 1 double difference locations of 8000 earthquakes from 1969 2002 on the parkfield section of the san andreas fault reveal detailed fault structures and seismicity that is although complex highly organized in both space and time distinctive features of the seismicity include 1 multiple recurrence of earthquakes of the same size at precisely the same location on the fault multiplets implying frictional or geometric controls on their location and size 2 sub horizontal alignments of hypocenters along the fault plane streaks suggestive of rheological transitions within the fault zone and or stress concentrations between locked and creeping areas 3 regions devoid of microearthquakes with typical dimensions of 1 5 km holes one of which contains the m6 1966 parkfield earthquake hypocenter these features represent long lived structures that persist through many cycles of individual events

anisotropy repeating earthquakes and seismicity associated with the 2008 eruption of okmok volcano alaska 1 we use shear wave splitting sws analysis and double difference relocation to examine temporal variations in seismic properties prior to and accompanying magmatic activity associated with the 2008 eruption of okmok volcano alaska using bispectrum cross correlation a multiplet of 25 earthquakes is identified spanning five years leading up to the eruption each event having first motions compatible with a normal fault striking ne sw cross correlation differential times are used to relocate earthquakes occurring between january 2003 and february 2009 the bulk of the seismicity prior to the onset of the eruption on 12 july 2008 occurred southwest of the caldera beneath a geothermal field earthquakes associated with the onset of the eruption occurred beneath the northern portion of the caldera and started as deep as 13 km subsequent earthquakes occurred predominantly at 3 km depth coinciding with the depth at which the magma body has been modeled using geodetic data automated sws analysis of the okmok catalog reveals radial polarization outside the caldera and a northwest southeast polarization within we interpret these polarizations in terms of a magma reservoir near the center of the caldera which we model with a mogi point source sws analysis using the same input processing parameters for each event in the multiplet reveals no temporal changes in anisotropy over the duration of the multiplet suggesting either a short term or small increase in stress just before the eruption that was not detected by gps or eruption triggering by a mechanism other than a change of stress in the system

variations in rupture process with recurrence interval in a repeated small earthquake in theory and in laboratory experiments friction on sliding surfaces such as rock glass and metal increases with time since the previous episode of slip1 this time dependence is a central pillar of the friction laws widely used to model earthquake phenomena2 3 on natural faults other properties such as rupture velocity4 5 porosity and fluid pressure6 11 may also vary with the recurrence interval eighteen repetitions of the same small earthquake separated by intervals ranging from a few days to several years allow us to test these laboratory predictions in situ the events with the longest time since the previous earthquake tend to have about 15 larger seismic moment than those with the shortest intervals although this trend is weak in addition the rupture durations of the events with the longest recurrence intervals are more than a factor of two shorter than for the events with the shortest intervals both decreased duration and increased friction are consistent with progressive fault healing during the time of stationary contact

wide scale detection of earthquake waveform doublets and further evidence for inner core super rotation summary we report on more than 100 earthquake waveform doublets in five subduction zones including an earthquake nest in bucaramanga colombia each doublet is presumed to be a pair of earthquakes that repeat at essentially the same location these doublets are important for studying earthquake physics as well as temporal changes of the inner core particularly our observation from one south sandwich islands ssi doublet recorded at station ink in canada shows an inner core traveltime change of 0 1 s over 6 yr confirming the innercore differential motion occurring beneath central america observations from one aleutian islands doublet recorded at station bosa in south africa and from one kuril islands doublet recorded at station bdfb in brazil show an apparent inner core traveltime change of 0 1 s over 7 yr and 6 yr respectively providing evidence for the temporal change of inner core properties beneath central asia and canada respectively on the other hand observations from one tonga fiji solomon islands doublet recorded at station ptga in brazil and from one bucaramanga doublet recorded at station wrab in australia and station chto in thailand show no little temporal change no more than 0 005 syr 1 if any of inner core traveltimes for the three corresponding ray paths for which the path in the inner core is nearly parallel to the equatorial plane such a pattern of observations showing both presence and possible absence of inner core traveltime change can be explained by the geometry and relative directions of ray path lateral velocity gradient and inner core particle motion due to an eastward super rotation of a few tenths of a degree per year

fine structure of p wave velocity distribution along the atotsugawa fault central japan abstract a seismic experiment with five explosive sources and 396 seismic stations was conducted in august 2005 in the atotsugawa fault area central japan the 396 seismic stations were located on a survey line with a length of 47 km the profile line was located along the atotsugawa fault the average spacing of the seismic stations was about 119 m the spatially high density linear array was used to reveal the seismic structure of the fault zone the fine structure along the atotsugawa fault was obtained at depths shallower than 6 km a large lateral variation of p wave velocity which was consistent with the geological structure was found at the shallowest layer the heterogeneous variation with a velocity range of 5 7 km s 6 2 km s was obtained at the second layer the second layer was divided into three parts the western part of the research area was determined to be high velocity the low velocity area was detected at the central part at the east end of the profile line the p wave velocity was higher than that of the central part the low velocity area coincided with a low seismicity area the depth section of the reflection profile showed a boundary located at a depth of around 13 km the boundary was roughly consistent with the cut off depth of the seismicity of the microearthquakes the high seismic activity area was characterized to be reflective

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

seismicity around parkfield correlates with static shear stress changes following the 2003 mw6 5 san simeon earthquake 1 earthquakes trigger other earthquakes but the physical mechanism of the triggering is currently debated most studies of earthquake triggering rely on earthquakes listed in catalogs which are known to be incomplete around the origin times of large earthquakes and therefore missing potentially triggered events here we apply a waveform matched filter technique to systematically detect earthquakes along the parkfield section of the san andreas fault from 46 days before to 31 days after the nearby 2003 mw6 5 san simeon earthquake after removing all possible false detections we identify 8 times more earthquakes than in the northern california seismic network catalog the newly identified events along the creeping section of the san andreas fault show a statistically significant decrease following the san simeon main shock which correlates well with the negative static stress changes i e stress shadow cast by the main shock in comparison the seismicity rate around parkfield increased moderately where the static stress changes are positive the seismicity rate changes correlate well with the static shear stress changes induced by the san simeon main shock suggesting a low friction in the seismogenic zone along the parkfield section of the san andreas fault

rupture characteristics of the three m 4 7 1992 1994 parkfield earthquakes slip on the san andreas fault was determined for three m 4 7 earthquakes using a tomographic inverse system beroza and spudich 1988 to invert seismic source time functions stfs from s waves stfs were obtained by deconvolving mainshock accelerograms by those from collocated smaller earthquakes accelerograms were from the u s geological survey parkfield small aperture array upsar and from a distributed array of digital accelerometer stations at parkfield eight or nine stfs are used in each of the three inversions stfs are typically symmetrical pulses with a duration of about 0 3 0 5 s in the inversion mainshock rise time was set to 0 05 s and we allowed the rupture time to vary slightly from a constant rupture velocity of approximately 0 85 rupture for all three events which are located in or close to the middle mountain preparation zone or box mmb quickly reaches a local maximum in slip and then propagates outward to peaks ridges or plateaus in the slip distribution slip for the october 20 1992 event located just inside the southern edge of the mmb propagates from an initial spike north and updip along a curving ridge for about 2 km the initial spike continued to grow in the november 14 1993 event located north of the october 20 1992 event just beneath the hypocenter of the 1966 parkfield earthquake which shows little directivity although there is a smaller patch of slip updip and to the south in contrast rupture for the december 20 1994 event located just south of the october 20 1992 event propagated north and slightly updip creating a rough plateau in slip a few kilometers wide on a side directivity for this event also is to the north directivity for all three events points in the approximate direction of the 1966 hypocenter small pulses which comprise a coda are found on the stfs for several seconds after the initial impulsive event several tests based on the assumption that the average of all stfs from upsar for each event is an estimate of the true slip at the source suggest that the codas in the stfs are s waves from a long duration source rather than uncorrected site response an initiation phase is found on the array average for the november 14 1993 and december 20 1994 events these precursory phases are the result of a spike in slip at the hypocenter a value of 2 4 4 mm is obtained for d c the slip weakening distance by interpreting the initial spike as a critical patch the few aftershocks for the october 20 1992 event are distributed to the north and updip of the mainshock but the november 14 1993 event had a strong burst of aftershock activity that propagated to the north of its hypocenter at roughly the same depth aftershocks of the december 20 1994 event are mostly updip the november 14 1993 event had the simplest slip distribution appeared to be the most impulsive and had the most active aftershock sequence and the greatest depth if the eventual parkfield earthquake initiates near the 1966 hypocenter then the directivity of the three events studied here will have pointed to it however it is certainly possible that both the initiation of characteristic parkfield shocks and the directivity of smaller events are controlled by fault properties on a larger scale such as by fault bends or jogs

high resolution image of calaveras fault seismicity 1 by measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35 km section of the calaveras fault this represents 92 of all seismicity since 1984 and includes the rupture zone of the m 6 2 1984 morgan hill california earthquake the relocated seismicity forms highly organized structures that were previously obscured by location errors there are abundant repeating earthquake sequences as well as linear clusters of earthquakes large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30 year time interval suggesting that these portions of the fault are either locked or creeping the area of greatest slip in the morgan hill main shock coincides with the most prominent of these voids suggesting that this part of the fault may be locked between large earthquakes we find that the calaveras fault at depth is extremely thin with an average upper bound on fault zone width of 75 m given the location error however this width is not resolvably different from zero the relocations reveal active secondary faults which we use to solve for the stress field in the immediate vicinity of the calaveras fault we find that the maximum compressive stress is at a high angle only 13 from the fault normal supporting previous interpretations that this fault is weak

a high frequency secondary event during the 2004 parkfield earthquake by using seismic records of the 2004 magnitude 6 0 parkfield earthquake we identified a burst of high frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation we imaged this event in three dimensions by using a waveform back projection method as well as by timing distinct arrivals visible on many of the seismograms the high frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions indicating that slip may have grown abruptly at this point the time history obtained from full waveform back projection suggests a rupture velocity of 2 5 kilometers per second energy estimates for the subevent together with long period slip inversions indicate a lower average stress drop for the northern part of the parkfield earthquake compared with that for the region near its hypocenter which is in agreement with stress drop estimates obtained from small magnitude aftershocks

the long recurrence intervals of small repeating earthquakes may be due to the slow slip rates of small fault strands observations since 1998 have revealed that repeating earthquakes and particularly small repeating earthquakes occur less often than expected given their seismically derived slip and the regional fault slip rate here we test the hypothesis that small repeaters occur infrequently because they occur on fault segments or strands with low slip rates we analyze the recurrence interval moment scaling of earthquake sequences near parkfield california we find that closely spaced sequences which likely occur on the same fault strand and respond to the same slip rate follow a scaling consistent with seismic slip rates while widely spaced sequences which likely occur on different strands follow a scaling consistent with the previous counter intuitive observations these results suggest that spatially varying slip rates could create the recurrence interval scaling though we cannot exclude other explanations

change of apparent segmentation of the san andreas fault around parkfield from space geodetic observations across multiple periods sequences of earthquakes are commonly represented as a succession of periods of interseismic stress accumulation followed by coseismic and postseismic phases of stress release because the recurrence time of large earthquakes is often greater than the available span of space geodetic data it has been challenging to monitor the evolution of interseismic loading in its entire duration here we analyze large data sets of surface deformation at different key episodes around the cholame parkfield and creeping segments of the san andreas fault that show evidence of significant deceleration of fault slip during the interseismic period we compare the average fault slip rates before and after the 2004 mw6 parkfield earthquake in the 1986 2004 and 2006 2012 periods respectively avoiding 2 years of postseismic deformation after 2004 using a combination of gps data from the plate boundary observatory the southern california earthquake center crustal motion map and the bay area velocity unification networks and interferometric synthetic aperture radar from the advanced land observing satellite alos and envisat satellites we show that the area of coupling at the transition between the parkfield and cholame segments appears larger later in the interseismic period than it does earlier on while strong plate coupling is uniform across the parkfield and cholame segments in the 1986 2004 period creep occurs south of the 2004 epicenter after 2006 making segmentation of the san andreas fault south of parkfield more clearly apparent these observations indicate that analyses of surface deformation late in the earthquake cycle may overestimate the area of plate coupling a fault surface creeping much below plate rate may in some case be a region that does not promote earthquake nucleation but rather just be at a slower stage of its evolution our analysis also shows signs of large variation of slip velocity above and below plate rate in the creeping segment indicating that cycles of weakening and hardening can also be at play in dominantly aseismic areas

seismicity and fault geometry of the san andreas fault around parkfield california and their implications abstract fault geometry is a consequence of tectonic evolution and it provides important information on potential seismic hazards we investigated fault geometry and its properties in parkfield california on the basis of local seismicity and seismic velocity residuals refined by an adaptive velocity hypocentral parameter inversion method the station correction terms from the hypocentral parameter inversion present characteristic seismic velocity changes around the fault suggesting low seismic velocities in the region east of the fault and high seismic velocities in the region to the west large seismic velocity anomalies are observed at shallow depths along the whole fault zone at depths of 3 8 km seismic velocity anomalies are small in the central fault zone but are large in the northern and southern fault zones at depths 8 km low seismic velocities are observed in the northern fault zone high seismicity is observed in the southwest fracture zone which has developed beside the creeping segment of the san andreas fault the vertical distribution of seismicity suggests that the fault has spiral geometry dipping ne in the northern region nearly vertical in the central region and sw in the southern region the rapid twisting of the fault plane occurs in a short distance of approximately 50 km the seismic velocity anomalies and fault geometry suggest location dependent piecewise faulting which may cause the periodic m6 events in the parkfield region

spatio temporal evolution of aseismic slip along the haiyuan fault china implications for fault frictional properties article history we use 20 years of synthetic aperture radar acquisitions by the ers and envisat satellites to investigate the spatial and temporal variations of strain rates along the 35 km long creeping section of the haiyuan fault at the north eastern boundary of the tibetan plateau we then use the derived displacements to infer the fault s frictional properties and discuss the relationship between creep and the seismic behavior of the fault located in between a millennial seismic gap and the 1920 m8 surface rupture trace this section has an average creep rate of 5 1m m yr about the interseismic loading rate the comparison of average surface velocity profiles derived from sar interferometry across the creeping section reveals a creep rate increase and or a creep migration to shallower depth between the 1990s and the 2000s we apply a smoothed time series analysis scheme on envisat insar data to investigate the creep rate variations during the 2004 2009 time period our analysis reveals that the creep rate accelerated in 2007 although data resolution does not allow to better constrain the onset of creep acceleration and its amplitude both decadal and short term transient behaviors are coeval with the largest earthquakes m 4 5 along the fault segment in recent years from the precise mapping of the surface fault trace we use the fault strike variations and the mohr circle construction to compute the along strike distribution of the friction coefficient along the creeping segment and compare it with the observed distribution of the creep rate we find that the creep rate scales logarithmically with the friction coefficient in agreement with the rate and state friction law in a rate strengthening regime the estimated value of log v 2 10 3 indicates that the earthquakes occurring along the creeping section cannot be the cause for a significant change in the overall segment s creep rate and that the recorded micro seismicity is most likely creep driven finally given the size and frictional properties of the creeping section we estimate based on previous models of dynamic rupture simulations a 0 20 probability for a rupture to break through this section together with the geometrical configuration of the haiyuan fault these results suggest that the creeping segment may act as a persistent barrier to earthquake propagation

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

depth dependent strength of the fault gouge at the atotsugawa fault central japan a possible mechanism for its creeping motion abstract from the friction experiments of the atotsugawa fault gouge using a tri axial apparatus under several pressure temperature conditions at depths down to 9 km we found an increase in the coefficient of friction from 0 50 to 0 58 as depth increases from 1 km to 9 km based on this result we proposed a fault slip model which explains the creeping motion of the atotsugawa fault in this model stress on the fault attains to the low frictional strength in the shallow part earlier than that in the deep so that this difference results in a creeping slip in the shallow depth during inter seismic period the model could successfully describe the presence of a creeping section along the atotsugawa fault at shallow depth with its creeping rate similar to the field observation on the fault

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

source parameters of a m4 8 and its accompanying repeating earthquakes off kamaishi ne japan implications for the hierarchical structure of asperities and earthquake cycle 1 we determine the source parameters of a m4 9 0 1 characteristic earthquake sequence and its accompanying microearthquakes at 50 km depth on the subduction plate boundary offshore of kamaishi ne japan the microearthquakes tend to occur more frequently in the latter half of the recurrence intervals of the m4 9 0 1 events our results show that the microearthquakes are repeating events and they are located not only around but also within the slip area for the 2001 m4 8 event from the hierarchical structure of slip areas and smaller stress drops for the microearthquakes compared to the m4 8 event we infer the small repeating earthquakes rupture relatively weak patches in and around the slip area for the m4 8 event and their activity reflects a stress concentration process and or change in frictional property healing at the area we also infer the patches for the m4 9 0 1 and other repeating earthquakes undergo aseismic slip during their interseismic period

seimological studies at parkfield v characteristic microearthquake sequences as fault zone drilling targets studies at very high resolution of microearthquakes at parkfield california since 1987 reveal a systematic organization in space and time dominated by clustering of nearly identical regularly occurring microearthquakes characteristic events on 10 to 20 m wide patches within the fault zone more than half of the 4000 events in our 1987 to 1996 catalog exhibit this trait in general recurrence intervals 0 5 to 2 yr scale with the magnitude of the repeating events for the on scale range mw 0 2 to 1 3 in this study the similar waveforms superimposed locations quasi periodic recurrence and uniform size of these characteristic events permit relative hypocenter location accuracy of meters and predictable occurrence times within windows of a few months clustered characteristic events occur at depths as shallow as about 3 km and these are feasible targets for deep scientific drilling and observation at the focus of a subsequent small earthquake within an active plate boundary fault zone at parkfield the achievable location accuracy to which a hypocenter can be specified as well as the predictability of its occurrence time appear to be uniquely favorable for in situ fault zone measurements

inversion of gps data for spatially variable slip rate on the san andreas fault near parkfield ca we analyze gps data collected from 1991 1998 at 35 sites near the parkeld segment of the san andreas fault inverting the resultant site velocities for the distri bution of interseismic slip rate on the san andreas reveals an area of low slip rate on thefault extendingfrom between middle mountain and carr hill to southeast of gold hill this slip rate patternis similar to thatfoundbyharris and segall 1987 using trilateration data collected between 1966 and 1984 we infer a deep slip rate 33 mm yr and depth of the transition between seismogenic and non seismogenic slip 14km thatagree betterwithindependentgeologic ev idence than those found in the 1987 study in contrast to harris and segall 1987 wendnoevidenceoffault normal contraction

characterization of aftershock fault plane orientations of the 1999 izmit turkey earthquake using high resolution aftershock locations 1 joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high resolution images of seismically active regions in this study we focus on the location accuracy of aftershocks of the 1999 mw 7 4 izmit nw turkey earthquake we obtained a new velocity model for the region and depicted its improvement on absolute locations in terms of uncertainty and misfit two well developed aftershock clusters located in the akyazi area and karadere duzce region were analyzed in detail based on a waveform cross correlation approach that allowed improving the location accuracy by a factor of 6 relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of 300 m

diversity of fault zone damage and trapping structures in the parkfield section of the san andreas fault from comprehensive analysis of near fault seismograms summary we perform comprehensive analyses of trapped waves and signals of damaged fault zone rocks associated with time delays of body waves along the parkfield section of the san andreas fault saf waveforms generated by thousands of earthquakes and recorded by near fault stations in several permanent and temporary deployments are examined with attention to the possible influence of a lithology contrast across the fault on signals of the low velocity damage zone clear candidate trapped waves are identified at only three stations mmnb and two of the other near fault stations flip and pies further to the nw clear candidate trapped waves are not seen at any of the near fault stations to the se of mmnb the locations of the events generating good candidate trapped waves at mmnb and the other two stations are not distributed broadly in space but clustered in a small number of locations moreover events that generate clear candidate trapped waves at one station do not typically generate trapped waves at the other two stations these observations imply that the damage zone is highly variable along strike and that a coherent connected waveguide does not exist for distances along strike larger than at most 3 5 km the distance between stations synthetic waveform fits for observed trapped waves at stations mmnb and flip indicate that the most likely parameters of the trapping structures at these locations are widths of about 150 m depths of about 3 km velocity reductions of 30 40 per cent and q values of 10 40 synthetic calculations of trapped waves demonstrate that if there is a contrast of seismic velocity across the fault the trapped waves are delayed relative to the s wave arrival trapped waves at station mmnb and to a lesser extent also at stations flip and pies in the nw section show this characteristic this suggests a lithology contrast in the top few km at these locations in agreement with results from tomography and studies of head waves in the parkfield area at several mini across fault arrays where trapped waves are not observed a low velocity damage zone is detected from the delay in the arrival time of body wave phases relative to a nearby off fault station the observed delay of the s wave is greater than the p wave delay consistent with the existence of a damage zone with poisson ratio of about 0 33 the observations of time delays without trapped waves indicate that parts of the damage zone are insufficiently coherent to generate trapped waves a broader damage zone may exist in the region between the saf and the south west fracture zone the results highlight the diversity of damage structures along the 40 km of the saf examined in this study and imply that fault imaging based on data at single sites does not necessarily apply to a larger section

a first deep seismic survey in the sea of marmara deep basins and whole crust architecture and evolution abstract increased source strength streamer length and dense spatial coverage of seismic reflection profiles of the seismarmara leg 1 allow to image the deep structure of the marine north marmara trough nmt on the strike slip north anatolian fault naf west of the destructive izmit 1999 earthquake a reflective lower crust and the moho boundary are detected they appear upwarped on an e w profile from the southern central basin eastwards towards more internal parts of the deformed region thinning of the upper crust could use a detachment suggested from an imaged dipping intracrustal reflector that would allow upper crustal material to be dragged from beneath it and above the lower crust accounting for the extensional component but also southwest motion of the southern margin of the nmt sections across the eastern half of the nmt crossing the cinarcik and imrali basins reveal several faults that are active reaching into the basement and have varying strike and proportions of normal and strike slip displacement they might be viewed as petals of a large scale negative flower structure that spreads over a width of 30 km at surface and is rooted deeper in the lithosphere under the central basin a very thick sediment infill is revealed and its extensional bounding faults are active and imaged as much as 8 km apart down to 6 km depth we interpret them as two deep rooted faults encompassing a foundering basement block rather than being merely pulled apart from a jog in a strike slip above a decollement the deep basin lengthening would account for only a modest part of the proposed 60 km finite motion since 4 myr along the same direction oblique to the nmt that sidesteps the shear motion from its two ends thus differential motion occurred much beyond the deep basins like subsidence involving the nmt bounding faults and the intracrustal detachments the complex partitioned motion localized on active faults with diverse natures and orientations is suggested to represent the overburden deformation induced from horizontal plane simple shear occurring in depth at lithospheric scale and in front of the north anatolian fault when it propagated through the region

fault structure and mechanics of the hayward fault california from double difference earthquake locations 1 the relationship between small magnitude seismicity and large scale crustal faulting along the hayward fault california is investigated using a double difference dd earthquake location algorithm we used the dd method to determine high resolution hypocenter locations of the seismicity that occurred between 1967 and 1998 the dd technique incorporates catalog travel time data and relative p and s wave arrival time measurements from waveform cross correlation to solve for the hypocentral separation between events the relocated seismicity reveals a narrow near vertical fault zone at most locations this zone follows the hayward fault along its northern half and then diverges from it to the east near san leandro forming the mission trend the relocated seismicity is consistent with the idea that slip from the calaveras fault is transferred over the mission trend onto the northern hayward fault the mission trend is not clearly associated with any mapped active fault as it continues to the south and joins the calaveras fault at calaveras reservoir in some locations discrete structures adjacent to the main trace are seen features that were previously hidden in the uncertainty of the network locations the fine structure of the seismicity suggests that the fault surface on the northern hayward fault is curved or that the events occur on several substructures near san leandro where the more westerly striking trend of the mission seismicity intersects with the surface trace of the aseismic southern hayward fault the seismicity remains diffuse after relocation with strong variation in focal mechanisms between adjacent events indicating a highly fractured zone of deformation the seismicity is highly organized in space especially on the northern hayward fault where it forms horizontal slip parallel streaks of hypocenters of only a few tens of meters width bounded by areas almost absent of seismic activity during the interval from 1984 to 1998 when digital waveforms are available we find that fewer than 6 5 of the earthquakes can be classified as repeating earthquakes events that rupture the same fault patch more than one time these most commonly are located in the shallow creeping part of the fault or within the streaks at greater depth the slow repeat rate of 2 3 times within the 15 year observation period for events with magnitudes around m 1 5 is indicative of a low slip rate or a high stress drop the absence of microearthquakes over large contiguous areas of the northern hayward fault plane in the depth interval from 5 to 10 km and the concentrations of seismicity at these depths suggest that the aseismic regions are either locked or retarded and are storing strain energy for release in future large magnitude earthquakes

hydrogeochemical evaluation of groundwater of bhaktapur municipality nepal the present study aims to evaluate groundwater hydrogeochemistry of bhaktapur municipality nepal with respect to water types chemical elements and their statistical variability samples were collected in pre monsoon april june monsoon july august and post monsoon october november seasons in 2007 laboratory analysis of samples revealed inter seasonal variability primarily attributed to hydrogeochemical processes all or most of the samples exceeded the world health organization who guideline value glv for iron conductivity and turbidity while none of the samples exceeded who glv for hardness and arsenic with minor fraction exceeding glv for chloride content higher ammonia content in well water and highest phosphate concentration occurred for stone spout in pre monsoon season principle component analysis highlighted three principle components while the dendrogram divided the sampling sites into four and two clusters respectively for well water and stone spout water sampling sites saturation index computed through wateq4f model revealed that ferrihydrite siderite strengite and vivianite minerals were in under saturated state while goethite was in supersaturated condition for each type of samples well water and stone spout water in both pre and post monsoon seasons depicting their tendency to precipitate high degree of supersaturation in goethite mineral indicated response to temperature gradient of the reactions while under saturation for other minerals indicated the effect of dilution

a rare foreshock sequence of the 20 january 2007 odaesan korea earthquake to measure the existence of preseismic velocity changes 1 dilatancy theory and laboratory studies predict that there may be changes in the velocity of seismic waves in the earth s crust preceding large earthquakes due to cracks opening up in response to stress changes these changes however have been extremely difficult to find in the field because we do not know where or when the next earthquake will strike often there is not sufficient instrumentation to capture such a signal if it exists with active sources but natural sources such as repeating earthquakes offer hope of measuring a preseismic signal to date however they have not measured a preseismic signal perhaps due to insufficient temporal sampling especially if the signal is short lived we examine a rare foreshock sequence of nine near repeating events with fine temporal sampling all occurring within 3 days of the mw 4 6 main shock with the last event occurring an hour before the nearest event occurred within 114 m of the main shock hypocenter this presents a unique opportunity to measure preseismic velocity changes at the depth of the main shock which is 9 4 km because the foreshocks are not exact repeats slight position differences bias the velocity change measurements the locations of these events are known precisely however which allows for us to correct for these biases by treating the events as a source array improving the measurement precision by up to an order of magnitude we observe no apparent preseismic velocity change signal but we are able to place an upper bound on its existence ranging from 0 01 to 0 08

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

across fault velocity gradients and slip behavior of the san andreas fault near parkfield a long lasting question in earthquake physics is why slip on faults occurs as creep or dynamic rupture we compute passive measurements of the seismic p wave velocity gradient across the san andreas fault near parkfield where this transition of slip mode occurs at a scale of a few kilometers unbiased measurements are obtained through the application of a new bayesian local earthquake tomographic code that avoids the imposition of any user defined initial velocity contrast across the fault or any damping scheme that may cause biased amplitude in retrieved seismic velocities we observe that across fault velocity gradients correlate with the slip behavior of the fault the p wave velocity contrast decays from 20 in the fault section that experience dynamic rupture to 4 in the creeping section suggesting that rapid change of material properties and attitude to sustain supra hydrostatic fluid pressure are conditions for development of dynamic rupture low vp and high vp vs suggest that fault rheology at shallow depth is conversely controlled by low frictional strength material

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

variability of earthquake nucleation in continuum models of rate and state faults and implications for aftershock rates using two continuum models of rate and state faults one with a weaker patch and the other with rheological transition from steady state velocity weakening to velocity strengthening friction we simulate several scenarios of spontaneous earthquake nucleation plausible for natural faults investigate their response to static shear stress steps and infer the corresponding aftershock rates overall nucleation processes at weaker patches behave similarly to theories based on spring slider models with some notable deviations in particular nucleation and aftershock rates are affected by normal stress heterogeneity in the nucleation zone nucleation processes at rheological transitions behave differently producing complex slip velocity histories nonmonotonic responses to static stress changes and aftershock rates with pronounced peaks and seismic quiescence for such processes positive stress steps sometimes delay nucleation of seismic events by inducing aseismic transients that relieve stress and postpone seismic slip superposition of the complex aftershock response for spatially variable stress changes results in omori s law for a period of time followed by seismic quiescence such behavior was observed at the base of the seismogenic zone near the 1984 morgan hill earthquake we show that the computed aftershock rates are linked to unperturbed slip velocity evolution in the nucleation zone and construct simplified analytical scenarios that explain some features of the response the qualitative differences that we find between the two nucleation models indicate that aftershock response of rate and state faults to static stress changes would depend on the conditions under which nucleation occurs on natural faults and may be different from predictions based on spring slider models

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

inference of coseismic slip via joint inversion of gps and aftershock data the 2004 parkfield example 1 in many coseismic slip inversions the number of model parameters is much larger than that of the independent observations and the problem is extremely underdetermined it is thus instructive to incorporate additional data sets into the slip inversion in this study i describe a new approach for coseismic slip inversion whereby both gps displacements and first day aftershock rate changes are used jointly to constrain the solution the joint inversion incorporates the dieterich s aftershock model which adopts a constitutive friction that depends logarithmically on the sliding rate the method is applied to the 2004 parkfield earthquake the joint inversion not only provides resolving power of slip at depths inaccessible to gps only inversions but it also helps to gain insight on the fault mechanical properties i show that the data are consistent with the adopted aftershock model being the dominant mechanism for aftershock production along the parkfield segment and i obtain an upper bound on the friction dependence on the log of rate of fault patches that have experienced aftershock activity a consequence of the irregular aftershock distribution is that the slip distribution is extremely nonsmooth with the aftershock zones acting as barriers

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