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

characterization of the san jacinto fault zone near anza california by fault zone trapped waves we installed three 350 m long seismic arrays each array consisting of 12 three component stations across the coyote creek fault ccf clark valley fault cvf and buck ridge fault brf of the san jacinto fault zone sjfz near anza california to record fault zone trapped waves from microearthquakes we observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone the coda normalized amplitude spectra of trapped waves showed peaks at 4 7 hz which decreased sharply with the distance from the fault trace observations and three dimensional finite difference simulations of trapped waves revealed low velocity and low q waveguides on these active faults with the width of 75 100 m in which shear velocities are reduced by 25 30 from wall rock velocities and q values are 40 90 at depths between the surface and 18 km the locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the sjfz in a region with complicated slip planes near anza the low velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities the waveguide on the brf dips south westward to connect the waveguide on the cvf which dips northeastward this waveguide extends at the seismogenic depth through anza slip gap to another low velocity waveguide on the casa loma fault clf which has been delineated in our previous study of the sjfz using trapped waves li et al 1997 the waveguide on the ccf in coyote mountain is nearly vertical and disconnected from the clf at the south edge of anza gap we interpret the low velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the anza region

fault zone architecture of the san jacinto fault zone in horse canyon southern california a model for focused post seismic fluid flow and heat transfer in the shallow crust abstract we report results of a new study of the architecture of the san jacinto fault zone in horse canyon california where stream incision has exposed a nearly continuous outcrop of the fault zone at 0 4 km depth the fault zone at this location consists of a fault core transition zone damage zone and tonalitic wall rocks we collected and analyzed samples for their bulk and grain density geochemical data clay mineralogy and textural and modal mineralogy progressive deformation within the fault zone is characterized by mode i cracking subsequent shearing of already fractured rock and cataclastic flow grain comminution advances towards the strongly indurated cataclasite fault core damage progression towards the core is accompanied by a decrease in bulk and grain density and an increase in porosity and dilational volumetric strain palygorskite and mixed layer illite smectite clay minerals are present in the damage and transition zones and are the result of hydrolysis reactions the estimated percentage of illite in illite smectite increases towards the fault core where the illite smectite to illite conversion is complete suggesting elevated temperatures that may have reached 150 c chemical alteration and elemental mass changes are observed throughout the fault zone and are most pronounced in the fault core we conclude that the observed chemical and mineralogical changes can only be produced by the interaction of fractured wall rocks and chemically active fluids that are mobilized through the fault zone by thermo pressurization during and after seismic events based on the high element mobility and absence of illite smectite in the fault core we expect that the greatest water rock ratios occur within the fault core these results indicate that hot pore fluids circulate upwards through the fractured fault core and into the surrounding damage zone though difficult to constrain we speculate that the site studied during this investigation may represent the top of a narrow ephemeral hydrothermal circulation cell that dissipates heat generated from rupture events at deeper levels 4 km

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

estimated optical constants of the tagish lake meteorite the visible near infrared and mid infrared 0 325 m real and imaginary indices of refraction are derived from reflectance measurements of the tagish lake meteorite these are compared to some real and imaginary indices of refraction of the individual minerals composing the tagish lake meteorite from this comparison it is clear that the imaginary indices of several individual minerals contribute to the estimated imaginary index of this meteorite

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

dynamic rupture on a material interface with spontaneous generation of plastic strain in the bulk abstract we discuss 2d simulations of in plane ruptures on a fault between different solids in a model that includes dynamic generation of plastic strain off the fault the fault is governed by a regularized friction with a gradual response to abrupt changes of normal stress while plastic strain in the bulk is governed by a coulomb yielding criterion the occurrence of off fault damage stabilizes the slip velocity on the fault stable self sustaining ruptures propagate on the material interface as narrow unidirectional pulses in the direction of slip on the more compliant side of the fault plastic strain is generated only on the stiffer side of the fault the tensional quadrant in the direction of rupture propagation in a strip of approximately constant width that is correlated with the slip velocity on the fault simulations for various values of confining normal stress material contrast and cohesion in the bulk suggest that significant generation of off fault damage is limited to conditions that correspond to the top few kilometers of the crust the results are compatible with field observations of shallow asymmetric damage patterns in the structure of several large faults

seismic tomography of the southern california crust based on spectral element and adjoint methods we iteratively improve a 3 d tomographic model of the southern california crust using numerical simulations of seismic wave propagation based on a spectral element method sem in combination with an adjoint method the initial 3 d model is provided by the southern california earthquake center the data set comprises three component seismic waveforms i e both body and surface waves filtered over the period range 2 30 s from 143 local earthquakes recorded by a network of 203 stations time windows for measurements are automatically selected by the flexwin algorithm the misfit function in the tomographic inversion is based on frequency dependent multitaper traveltime differences the gradient of the misfit function and related finite frequency sensitivity kernels for each earthquake are computed using an adjoint technique the kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient based minimization algorithm the inversion involved 16 iterations which required 6800 wavefield simulations the new crustal model m 16 is described in terms of independent shear v s and bulk sound v b wave speed variations it exhibits strong heterogeneity including local changes of 30 per cent with respect to the initial 3 d model the model reveals several features that relate to geological observations such as sedimentary basins exhumed batholiths and contrasting lithologies across faults the quality of the new model is validated by quantifying waveform misfits of full length seismograms from 91 earthquakes that were not used in the tomographic inversion the new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment

the 2010 m w 7 2 el mayor cucapah earthquake sequence baja california mexico and southernmost california usa active seismotectonics along the mexican pacific margin the el mayor cucapah earthquake sequence started with a few foreshocks in march 2010 and a second sequence of 15 foreshocks of m 2 up to m4 4 that occurred during the 24 h preceding the mainshock the foreshocks occurred along a north south trend near the mainshock epicenter the mw 7 2 mainshock on april 4 exhibited complex faulting possibly starting with a m6 normal faulting event followed 15 s later by the main event which included simultaneous normal and right lateral strike slip faulting the aftershock zone extends for 120 km from the south end of the elsinore fault zone north of the us mexico border almost to the northern tip of the gulf of california the waveform relocated aftershocks form two abutting clusters each about 50 km long as well as a 10 km north south aftershock zone just north of the epicenter of the mainshock even though the baja california data are included the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border the spatial distribution of large aftershocks is asymmetric with five m5 aftershocks located to the south of the mainshock and only one m5 7 aftershock but numerous smaller aftershocks to the north further the northwest aftershock cluster exhibits complex faulting on both northwest and northeast planes thus the aftershocks also express a complex pattern of stress release along strike the overall rate of decay of the aftershocks is similar to the rate of decay of a generic california aftershock sequence in addition some triggered seismicity was recorded along the elsinore and san jacinto faults to the north but significant northward migration of aftershocks has not occurred the synthesis of the el mayor cucapah sequence reveals transtensional regional tectonics including the westward growth of the mexicali valley and the transfer of pacific north america plate motion from the gulf of california in the south into the southernmost san andreas fault system to the north we propose that the location of the 2010 el mayor cucapah as well as the 1992 landers and 1999 hector mine earthquakes may have been controlled by the bends in the plate boundary

p wave image of the peninsular ranges batholith southern california we invert earthquake p wave arrival times to image the 3d distribution of p wave velocities in the mesozoic peninsular ranges batholith and nearby areas in southern california there is a 3 velocity contrast between the eastern and western peninsular ranges at 4 and 20 km depth west side faster and a 1 to 1 5 velocity contrast across the san andreas fault zone south side faster in the san bernardino region at 4 to 14 km depth the san andreas velocity contrast is due to the juxtaposition of different rock types by slip along the fault zone the peninsular ranges batholith velocity contrast is due to a difference in rock composition across the batholith the maximum gradient in the crustal velocities is coincident with a compositional boundary within the batholith that reflects emplacement of the batholith across juxtaposed oceanic and continental crust quaternary fault development has been in part concentrated at this boundary

seismic velocity variations along the rupture zone of the 1989 loma prieta earthquake california 1 we revisit the rupture zone of the 1989 mw6 9 loma prieta earthquake central california by developing high resolution three dimensional 3 d vp and vp vs models we apply the simul2000 inversion method and algorithm to a set of composite events which have greater number of picks per event and reduced random picking errors compared with traditional master events our final p wave velocity model generally agrees with previous studies showing a high velocity body of above 6 7 km s in the southeast rupture zone of the main shock the 3 dvp vs model however has different features with low vp vs in the upper crust and high vp vs anomalies in deeper layers of the rupture zone we interpret the low vp vs at shallow depths to be granitic rocks whereas at greater depths the areas of higher vp vs around 1 725 1 75 presumably are mafic rocks the resulting 3 d velocity model was used to improve absolute locations for all local events between 1984 and 2010 in our study area we then applied a similar event cluster analysis waveform cross correlation and differential time relocation methods to improve relative event location accuracy over 88 of the seismicity falls into similar event clusters a dramatic sharpening of seismicity patterns is obtained after using these methods the medians of the relative location uncertainties calculated by using the bootstrap approach are 5 m for horizontal and 8 m for vertical differential times from cross correlation are used to estimatein situnear sourcevp vsratio within each event cluster the high resolutionvp vs method confirms the trend of the velocity variations from the tomographic results although absolute values are slightly different

three dimensional vp and vp vs structure at loma prieta california from local earthquake tomography p and s p times from local earthquakes and p times from explosions at seismic stations in the vicinity of the 1989 loma prieta earthquake are inverted simultaneously to produce three dimensional images of vp and vp vs structure along the rupture zone the results confirm the presence of the major heterogeneities identified in previous studies using p waves only including high vp along the rupture in areas of significant slip and a low velocity wedge along the san andreas at the se edge of the rupture a change in rock properties evidenced by a significant vp vs increase equal to a 30 poisson s ratio increase from the middle crust to the upper crust may have been responsible for the upward termination of the main shock rupture we speculate that the deep high vp body that lies along the se portion of the rupture zone consists of two bodies based on model results indicating that the sw part of the body has relatively high vp vs while the ne part has relatively low vp vs

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

poisson s ratio values for rocks abstract compared to other basic mechanical properties of rocks poisson s ratio is an elastic constant of which the significance is generally underrated yet in rock mechanics there is a considerable number of diverse areas which require a prior knowledge or estimation of the value of poisson s ratio this paper examines the values and applications of poisson s ratio in rock mechanics following an historical account of the initial controversy whether it was a material constant or not the effects of poisson s ratio in the elastic deformation of materials intact rocks and rock masses are briefly reviewed also the reported values of poisson s ratio for some elements materials and minerals are compiled while typical ranges of values are presented for some rocks and granular soils finally poisson s ratio classifications are recommended for isotropic intact rocks

san jacinto fault zone guided waves a discrimination for recently active fault strands near anza california we deployed three 350 m long eight element linear seismic arrays in the san jacinto fault zone sjfz near anza california to record microearthquakes starting in august through december 1995 two arrays were deployed 18 km northwest of anza across the casa loma fault clf and the hot springs fault hsf strands of the sjfz the third array was deployed across the san jacinto fault sjf in the anza slip gap we observed fault zone guided waves characterized by low frequency large amplitudes following s waves at the clf array and the sjf array for earthquakes occurring within the fault zone however we did not observe guided waves at the hsf array for any events the amplitude spectra of these guided waves showed peaks at 4 hz at the clf and 6 hz at the sjf which decreased sharply with the distance from the fault trace in contrast no spectral peaks at frequency lower than 6 hz were registered at the hsf array we used a finite difference method to simulate these guided modes as 5 waves trapped in a low velocity waveguide sandwiched between high velocity wall rocks the guided mode data are adequately fit by a waveguide on the clf with the average width of 120 m and s velocity of 2 5 km s about 25 reduced from the s velocity of the surrounding rock this waveguide becomes 40 to 60 m wide with the 5 velocity of 2 8 km s in the anza slip gap on the other hand there is not a continuous waveguide on the hsf at depth locations of the events with guided modes suggest that the fault plane waveguide extends along the clf between the towns of san jacinto and anza dipping northeastward at 75 80 to a depth of about 18 km it becomes nearly vertical in the anza gap we speculate that the existence of a continuous low velocity waveguide on the clf can be caused by the rupture of the magnitude 6 9 earthquake on april 21 1918 occurring near the towns of san jacinto and hemet further the lack of a clear waveguide on the hsf suggests that it was not ruptured in this event

evidence for water filled cracks in earthquake source regions 1 we identify lowered vp vs ratios near earthquake source regions in southern california using observations from a seismic tomography model and high resolution local vp vs estimates using waveform cross correlation data from within similar event clusters the median tomographic vp vs ratio is 1 716 0 008 at all the relocated crustal earthquake locations compared to the background median value of 1 729 0 007 for the tomography model although the error estimates overlap slightly the median in situ vp vs ratio of 1 673 0 022 within the similar event clusters suggests that tomographic studies are overestimating vp vs at source regions interpretation of vp vs anomalies is complicated by the scatter in values obtained for individual clusters and in comparisons to absolute vp and vs velocities in the tomography model however the low vp vs ratios measured for the seismicity clusters are hard to explain with known rocks and suggest the presence of water filled cracks with several percent porosity in earthquake source regions in southern california which likely has an effect on faulting and earthquake activity

evolving geometrical and material properties of fault zones in a damage rheology model we discuss numerical simulations of evolving fault zone structures in a 3 d lithospheric model with a seismogenic crust governed by a damage rheology that accounts for large strain associated with permanent brittle deformation results for the initial propagation of an existing narrow damage zone subjected to oblique loading exhibit strong asymmetry of the evolving damage with respect to the initial fault orientation and predict out of plane directions of the propagating damage zones the orientations of the simulated damage zones agree with analytical expectations based on fracture mechanics for the directions of wing cracks generated at the tips of a crack under mixed mode loading lithosphere scale numerical simulations for the long term evolution of a large strike slip fault zone produce initially a system of stepping en echelon segments associated with the asymmetric generation of new damage zones the simulated fault zone segments evolve with continuing deformation to a throughgoing localized structure large scale perturbation in the geometry of the moho interface together with the regional heat regime can reduce considerably the degree of localization of the fault zone structure and the associated deformation fields

application of high resolution dem data to detect rock damage from geomorphic signals along the central san jacinto fault abstract we analyze geomorphic properties extracted from lidar and srtm shuttle radar topography mission data to test whether the damage zone along the central san jacinto fault sjf zone can be resolved with remotely sensed data in a quantitative fashion the sjf is one of the most active faults in southern california with well expressed geomorphology and a fast slip rate as seen in the geology and by gps we use arcmap and the taudem toolbox to compare several morphometric parameters including drainage density dd on both sides of the fault using a 1 km and a 5 km buffer for the lidar and srtm data respectively we also analyze the spatial patterns of dd near the fault using two different definitions of spatial dd the high resolution of the lidar data allows us to focus on a single fault eliminating the effects of parallel nearby faults from the lidar data we find that the highest dd values occur in areas between two fault strands followed generally by rocks on the northeast side of the fault with the lowest dd values occurring on the southwest side of the fault the srtm data shows a band of high dd values centered on the main fault trace with 1 km width our results indicate that there is a strong correlation between drainage density and proximity to the fault with zones of structural complexity along the fault displaying the highest dd we interpret this to largely be an effect of degree of rock damage as these are areas that are expected to be more damaged and field observations support this contention if we are correct then it appears that the northeast side of the sjf is generally more damaged south of the trifurcation area there is evidence that the signal is reversed on the larger scale with more damage on the southwest side of the fault inferred from the srtm data possibly caused by extension between the coyote creek and clark faults the implications of the observed asymmetry could be geological evidence for rupture propagation direction because a preferred propagation direction is predicted to produce asymmetric damage structure that would be recorded in the volume of rock surrounding a fault

progressive strain localization in a major strike slip fault exhumed from midseismogenic depths structural observations from the salzach ennstal mariazell puchberg fault system austria 1 analysis of a strike slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high strain fault core we focus on the ennstal segment of the 400 km long salzach ennstal mariazell puchberg semp strike slip fault system in the eastern alps which accommodated 60 km of left lateral displacement during oligo miocene time macroscopic and microscopic observations reveal a zoned fault featuring a high strain core at least 10 m wide within a fault zone that is at least 150 m wide grain size distribution analysis shows how the ennstal segment of the semp evolved our data reveal a 10 m wide high strain fault core characterized by a power law relationship of grain sizes d2 2 0 bordered by a 54 m wide transition zone where the largest and smallest grains are characterized by two power law relationships d2 2 0 and 1 6 respectively this zone is in turn bordered by a region with grain sizes that show a single low strain power law relationship of d2 1 6 we interpret these relationships to be the result of concentrated shear overprinting an initial low strain power law grain size distribution before strain localized to the core this is consistent with the theory that faults mature by smoothing geometrical complexities forming a highly localized high strain fault core the data do not support the idea that damage forms primarily in response to dynamic stresses during seismic rupture although they do suggest that this mechanism may operate within tens of meters of the fault once it has developed its zoned structure

bonneville basin shoreline records of large lake intervals during marine isotope stage 3 and the last glacial maximum abstract lacustrine shoreline depositional features in the bonneville basin western north america were investigated to chronicle paleolake expansion and contraction in response to climate change based on stratigraphy sedimentology geomorphology and radiocarbon dating this study identifies multiple intermediate to large lake intervals during marine isotope stage 3 and the last glacial maximum timing and magnitude of the bonneville basin lake size change are compared with those of hemispheric scale climate change events reported from the greenland ice sheet southwestern north america winter precipitation and east asian monsoon 18 o records the amount of precipitation received by surface water systems in the bonneville basin appears to have increased significantly during the greenland stades throughout the last half of mis 3 and early mis 2 the lake also underwent rapid large amplitude oscillations during the last glacial maximum

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

relations among fault behavior subsurface geology and three dimensional velocity models the development of three dimensional p wave velocity models for the regions surrounding five large earthquakes in california has lead to the recognition of relations among fault behavior and the material properties of the rocks that contact the fault at seismogenic depths regions of high moment release appear to correlate with high seismic velocities whereas rupture initiation or termination may be associated with lower seismic velocities these relations point toward a physical understanding of why faults are divided into segments that can fail independently an understanding that could improve our ability to predict earthquakes and strong ground motion

high resolution subducting slab structure beneath northern honshu japan revealed by double difference tomography the high resolution seismic velocity structure of the subducting slab beneath northern honshu japan has been obtained by double difference tomography capitalizing on the existence of two planes of seismicity the upper plane lies in the region with average to high vp vs ratios 1 72 1 85 which may correspond to the transformations of metabasalt and metagabbro to blueschist the lower plane is associated with very low vp vs ratios 1 6 1 7 in sharp contrast with high vp vs ratios 1 8 1 85 in the region between the two planes these features may be explained by forsterite enstatite h2o formation from serpentine dehydration in the lower plane and partial hydration of the region between the two planes our results provide direct constraints for petrologic modeling of the subducting slab

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

the 2001 bhuj earthquake tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation 1 the january 26 2001 bhuj earthquake mw 7 6 is one of the most catastrophic indian earthquakes we have investigated the 3 d seismic velocity and poisson s ratio structures of the bhuj source area to understand the probable cause of triggering the earthquake we used 1948 p and 1865 s wave high quality arrival times from 331 aftershocks recorded at a temporary seismic network significant variations up to 5 in velocity and 10 in poisson s ratio are revealed in the aftershock area the mainshock is located in a distinctive zone characterized by high vp low vs and high poisson s ratio in the depth range of 20 to 30 km and extending 15 to 30 km laterally this feature is very similar to that of the 1995 kobe earthquake zhao et al 1996 the anomaly may be due to a fluid filled fractured rock matrix which might have contributed to the initiation of the bhuj earthquake

3d velocity structure around the source area of the 1999 chi chi taiwan earthquake before and after the mainshock 3d v p and v p v s models for the source area before and after the chi chi earthquake are derived from a tomographic inversion of seismic travel time data from the taiwan central weather bureau seismic network cwbsn resulting 3d velocity models reveal a strong lateral variation in the crust particularly at a depth range of 5 to 25 km along the trend of the dipping chelungpu thrust in general low v p and high v p v s anomalies are found in and around the source volume of the chi chi earthquake a volume centered around the entire 100 km 40 km chelungpu rupture surface we pay special attention to the crustal properties and their changes due to the chi chi event and the association of the crustal anomalies with seismicity combining the results with those of other geological and geophysical surveys the crustal anomalies are interpreted as images of a highly fractured and fluid filled chi chi source region fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the chi chi earthquake the v p models obtained before and after the chi chi event show some interesting heterogeneities regions of low v p anomaly appear to have further expanded with the occurrence of the chi chi earthquake the v p v s model shows high anomalies near the source rupture especially near where the rupture is believed to have nucleated after the chi chi event the v p v s model shows a substantial change that may reflect an outward shift of the fluid filled fractured source region this shift is shown by a general expansion in v p v s anomalies to a broader region e g where two intensive aftershock clusters occurred and a significant v p v s reduction to more normal values near the chelungpu rupture we attribute these changes in v p and v p v s anomalies as well as the expansion and shift of the anomalous regions to be a response to the localized stress change caused by the slip of the chi chi rupture the location of the seismic events is correlated with that of the crustal v p and v p v s anomalies large events appear to occur in the transition zones of both v p and v p v s anomalies this finding may be of use in the identification of weakened crustal regions in a seismic zone thus suggesting where impending large earthquakes are likely to occur all input data and initial and final models of this article are given as electronic files in the cd attached to the end of this issue

3 d velocity structure of the san jacinto fault zone near anza california i p waves summary seismic arrival times from microearthquakes ml 4 on the san jacinto fault near anza california are used to find spatial variations in the seismic velocity that are related to the crustal structure of the fault zone preliminary modelling of the 1 d p wave velocity structure of the upper 25 km of crust reveals that most of the variation in velocity is lateral rather than depth dependent the traveltime anomalies due to lateral structure can be partially compensated for by applying station corrections however the variance of the traveltime residuals is still 2 25 times larger than the variance of the picking error the spatially correlated residuals show that this variance is due to localized velocity anomalies and that the data require further modelling using a 3 d velocity structure because the 3 d inverse problem is non unique smoothness constraints are applied to find the model that has the minimum structure required to fit the data to the picking error where a smooth model is defined such that the gradient of the velocity perturbation from the original 1 d model is small with small non zero station corrections a 3 d velocity model can be found that fits the data well the structure is well resolved from 3 to 9 km depth where lateral perturbations of up to 7 per cent are determined independently of the trade off between station corrections and poorly resolved near surface structure the model shows a horizontal gradient with overall faster velocities in the north east side of the fault zone at 3 6 km depth the signature of the fault zone is evident in the lower velocities beneath the surface trace of the fault however at 9 km depth higher seismic velocities are found extending into the fault zone from the north east block this higher velocity region occurs where there is a distinct lack of seismicity on the fault there is also a localized feature in the south west of the modelled region that is more than 10 km from the main trace of the fault with velocities 3 per cent slower than average