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

systematic survey of high resolution b value imaging along californian faults inference on asperities understanding and forecasting earthquake occurrences is presumably linked to understanding the stress distribution in the earth s crust this cannot be measured instrumentally with useful coverage however the size distribution of earthquakes quantified by the gutenberg richter b value is possibly a proxy to differential stress conditions and could therewith act as a crude stress meter wherever seismicity is observed in this study we improve the methodology of b value imaging for application to a high resolution 3 d analysis of a complex fault network in particular we develop a distance dependent sampling algorithm and introduce a linearity measure to restrict our output to those regions where the magnitude distribution strictly follows a power law we assess the catalog completeness along the fault traces using the bayesian magnitude of completeness method and systematically image b values for 243 major fault segments in california we identify and report b value structures revisiting previously published features e g the parkfield asperity and documenting additional anomalies e g along the san andreas and northridge faults combining local b values with local earthquake productivity rates we derive probability maps for the annual potential of one or more m6 events as indicated by the microseismicity of the last three decades we present a physical concept of how different stressing conditions along a fault surface may lead to b value variation and explain nonlinear frequency magnitude distributions detailed spatial b value information and its physical interpretation can advance our understanding of earthquake occurrence and ideally lead to improved forecasting ability

new slip rate estimates for the creeping segment of the san andreas fault california new continuous and differential global positioning system gps measurements of recent slip rates and 30 yr alignment array offsets from the central creeping segment of the san andreas fault yield a maximum right lateral slip rate of 25 1 mm yr this slip rate is 20 slower than the 30 mm yr slip rate accepted for this segment of the fault and 35 slower than the 39 mm yr slip rate predicted between the sierra nevada great valley block and the pacific plate new continuous gps measurements between pairs of sites that flank the creeping segment at intersite distances of 1 0 km and 70 km give relative fault parallel slip rates of 28 2 and 30 2 mm yr respectively these observations indicate that right lateral deformation rates increase with distance from the fault possible explanations for the gradient observed in the geodetic data are elastic strain accumulation along the creeping segment or significant distributed deformation on off fault structures

interplate quasi static slip off sanriku ne japan estimated from repeating earthquakes 1 space time distribution of quasi static slip on the plate boundary off sanriku northeastern ne japan was estimated by using small repeating earthquakes we found 2509 repeating earthquakes occurring on the plate boundary for the last 17 years based on the interpretation that these events are caused by repeating slips of small asperities surrounded by stable sliding area we regarded these repeating events as creepmeters embedded on the plate boundary the results show that deeper portions of the plate boundary are sliding almost stably and earthquakes with magnitudes more than 6 are accompanied by quasi static slips in the surrounding areas

stability and uncertainty of finite fault slip inversions application to the 2004 parkfield california earthquake the 2004 parkfield california earthquake is used to investigate stability and uncertainty aspects of the finite fault slip inversion problem with different a priori model assumptions we utilize records from 54 strong ground motion stations and 13 continuous 1 hz sampled geodetic instruments two inversion procedures are compared a linear least squares subfault based methodology and a nonlinear global search algorithm these two methods encompass a wide range of the different approaches that have been used to solve the finite fault slip inversion problem for the parkfield earthquake and the inversion of velocity or displacement waveforms near surface related site response top 100 m frequencies above 1 hz is shown to not significantly affect the solution results are also insensitive to selection of slip rate functions with similar duration and to subfault size if proper stabilizing constraints are used the linear and nonlinear formulations yield consistent results when the same limitations in model parameters are in place and the same inversion norm is used however the solution is sensitive to the choice of inversion norm the bounds on model parameters such as rake and rupture velocity and the size of the model fault plane the geodetic data set for parkfield gives a slip distribution different from that of the strong motion data which may be due to the spatial limitation of the geodetic stations and the bandlimited nature of the strong motion data cross validation and the bootstrap method are used to set limits on the upper bound for rupture velocity and to derive mean slip models and standard deviations in model parameters this analysis shows that slip on the northwestern half of the parkfield rupture plane from the inversion of strong motion data is model dependent and has a greater uncertainty than slip near the hypocenter

direct test of static stress versus dynamic stress triggering of aftershocks 1 aftershocks observed over time scales of minutes to months following a main shock are plausibly triggered by the static stress change imparted by the main shock dynamic shaking effects associated with passage of seismic waves from the main shock or a combination of the two we design a direct test of static versus dynamic triggering of aftershocks by comparing the near field temporal aftershock patterns generated by aseismic and impulsive events occurring in the same source area the san juan bautista california area is ideally suited for this purpose because several events of both types of m 5 have occurred since 1974 we find that aftershock rates observed after impulsive events are much higher than those observed after aseismic events and this pattern persists for several weeks after the event this suggests that at least in the near field dynamic triggering is the dominant cause of aftershocks and that it generates both immediate and delayed aftershock activity

postseismic variations in seismic moment and recurrence interval of repeating earthquakes in laboratory experiments longer stationary contact time leads to larger seismic moment during repeated ruptures however not all observations in natural fault systems agree with the prediction we analyze a subset of 34 m 0 4 2 1 repeating earthquake sequences res from 1987 to 2009 at parkfield to examine the variation of their recurrence properties in space and time following a 2004 m6 earthquake many of the repeating events have greatly reduced recurrence intervals tr that systematically increase with time in addition to this change in timing we also find systematic changes in seismic moment mo where most sequences experienced an immediate increase in mo and subsequent decay as tr approached pre quake durations the res at shallower depth tend to have a larger range in both tr and mo whereas deeper res show smaller variation the shallowest res with the greatest magnitude m1 8 2 1 among the events we studied reveal a large variation in tr but small variation in mo these observations are qualitatively consistent with earthquake simulations in 3d continuum fault models with rate and state dependent friction in the models res are produced on velocity weakening patches surrounded by velocity strengthening fault areas the models show that the degree of postseismic variation in mo and tr is a function of radius r and nucleation zone size h of the velocity weakening patch a ratio of r h 1 produces negative mo tr slopes whereas larger ratios of r h yield weak positive slopes given the same nucleation size h i e the same frictional properties and effective normal stress smaller radii and hence smaller seismic moments result in negative mo tr slopes whereas larger radii and hence larger moments lead to weak positive mo tr slopes which are consistent with observations conversely with only a small percentage of its slip accumulated seismically a small asperity appears to grow in mo under high loading rate which is contrary to the view that mo should decrease due to a reduced strength recovery time

distribution of aseismic slip rate on the hayward fault inferred from seismic and geodetic data 1 we solve for the slip rate distribution on the hayward fault by performing a least squares inversion of geodetic and seismic data sets our analysis focuses on the northern 60 km of the fault interferometric synthetic aperture radar insar data from 13 independent ers interferograms are stacked to obtain range change rates from 1992 to 2000 horizontal surface displacement rates at 141 bench marks are measured using gps from 1994 to 2003 surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion the fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface south of the city of hayward a steeply east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth the inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of 5 mm yr to a depth of 4 6 km the interferometric synthetic aperture radar insar data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath point pinole although the insar data might be complicated by a small dip slip component at this location a low slip rate patch of 1 mm yr is inferred beneath san leandro consistent with the source location of the 1868 earthquake we calculate that the entire fault is accumulating a slip rate deficit equivalent to a mw 6 77 0 05 per century however this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip

repeating earthquake activities associated with the philippine sea plate subduction in the kanto district central japan a new plate configuration revealed by interplate aseismic slips abstract we detect repeating earthquakes associated with the philippine sea plate subduction to reveal the plate configuration in the kanto district we find 140 repeating earthquake groups with 428 events by waveform similarity analysis most repeating earthquakes in the eastern part of the kanto district occur with a regular time interval they have thrust type focal mechanisms and are distributed near the upper surface of the philippine sea plate these observations indicate that the repeating earthquakes there occur as a repetition of ruptures on the isolated patches distributed on the plate boundary owing to the concentration of stress caused by aseismic slips in the surrounding areas this shows that the distributions of repeating earthquakes suggest the aseismic slips in the surrounding areas of small patches we determine spatial distributions of repeating earthquakes in the eastern part of the kanto district and find that they correspond to the upper boundary of the philippine sea plate that is the upper boundary of the oceanic crust layer of the philippine sea plate the plate geometry around choshi is newly constrained by repeating earthquake data and a rather flat geometry in the eastern part of the kanto district is revealed the obtained geometry suggests uplift of the philippine sea plate due to the collision with the pacific plate beneath choshi repeating earthquakes in the western part of the kanto district have extremely shorter recurrence times and their focal mechanisms are not of the thrust types these repeating earthquakes are classified as burst type activity and likely to occur on the preexistent fault planes which are distributed around the collision zone between the philippine sea plate and the inland plate the variation among the repeating earthquake activities in the kanto district indicates that regular repetition of repeating earthquakes is possible only on the plate boundary with a smooth and simple geometry

empirical green s function study of the january 17 1994 northridge california earthquake the january 17 1994 northridge main shock was analyzed using an empirical green s function approach to estimate the spatial and temporal distribution of fault slip far field source time functions obtained from seismograms recorded by the berkeley digital seismic network bdsn and terrascope stations reveal the distributed nature of the source and the presence of a second large subevent the location of the fault beneath the san fernando valley as well as the updip directivity of the rupture contributed to the locally heavy damage due to this earthquake

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

measurements of the strain field associated with episodic creep events on the san andreas fault at san juan bautista california over the past 6 years repeated strain steps recorded on a tensor strainmeter installed close to the san andreas fault trace at san juan bautista in california correlated closely with episodic creep events registered on a nearby creepmeter the strain events were remarkably similar in character were of about 1 hour duration and were followed within hours to days by creep events of 2 6 days duration the episodic slip of a source region from 200 m to 500 m in depth and at most a few kilometers in length is consistent with the observed strain and creep data we postulate that the additional stress transferred to this region by the loma prieta earthquake is giving rise to an increased rate of accumulation of shear strain around a locked patch beneath the creep source region thus accounting for the increased frequency of the creep strain events following the loma prieta earthquake

under the hood of the earthquake machine toward predictive modeling of the seismic cycle advances in observational laboratory and modeling techniques open the way to the development of physical models of the seismic cycle with potentially predictive power to explore that possibility we developed an integrative and fully dynamic model of the parkfield segment of the san andreas fault the model succeeds in reproducing a realistic earthquake sequence of irregular moment magnitude m w 6 0 main shocks including events similar to the ones in 1966 and 2004 and provides an excellent match for the detailed interseismic coseismic and postseismic observations collected along this fault during the most recent earthquake cycle such calibrated physical models provide new ways to assess seismic hazards and forecast seismicity response to perturbations of natural or anthropogenic origins

characteristic repeating earthquakes in an arc continent collision boundary zone the chihshang fault of eastern taiwan abstract creeping crustal faults often generate a large number of microearthquakes and less commonly they may also produce major earthquakes that rupture the brittle crust the chihshang fault in eastern taiwan is characterized by such behavior and has been known to undergo 2 3 cm yr surface creep making it one of the most active creeping thrust faults known in the world it gives an excellent opportunity to study how a creeping fault can generate large earthquakes however the understanding of fault behavior at depth in this area has been limited due to sparse sampling from seismic and geodetic stations in this study we determine and evaluate a population of repeating earthquakes to improve the understanding of deep fault deformation we propose a repeating sequence identification scheme in the region where the station coverage is sparse and one sided using this identification method we found 30 m 2 3 characteristic repeating sequences under the northern half of the chihshang fault at 7 23 km depth slip estimates from these sequences indicate an average slip rate of 3 cm yr which is consistent with the rate inferred from surface geodetic measurements we infer that the 30 km long chihshang fault is creeping along its northern half and locked in the south consistent with the occurrence of the 2003 ml 6 4 earthquake on the southern fault section

creep and quakes on the northern transition zone of the san andreas fault from gps and insar data 1 the san juan bautista sjb segment forms the northern transition zone of the creeping section of the san andreas fault it is an area of moderate seismicity the largest instrumentally recorded earthquakes have been m5 5 however historic records suggest six m 6 earthquakes occurred near the sjb segment between 1840 and 1899 it is also an area that has experienced several slow earthquakes we perform a joint inversion of gps and insar data to determine the current rate and distribution of interseismic creep we find two low slip locked segments at mid seismogenic depths that may represent source regions for the 19th century earthquakes we find that the sjb segment is currently accumulating strain energy at the rate of one m6 3 6 7 earthquake per century at this rate the 1840 1899 sequence released 135 510 years of accumulated moment deficit

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

seismicity rate immediately before and after main shock rupture from high frequency waveforms in japan 1 we analyze seismicity rate immediately before and after 82 main shocks with the magnitudes ranging from 3 to 5 using waveforms recorded by the hi net borehole array in japan by scrutinizing high frequency signals we detect 5 times as many aftershocks in the first 200 s as in the japan meteorological agency catalogue after correcting for the changing completeness level immediately after the main shock the aftershock rate shows a crossover from a slower decay with an omori s law exponent p 0 58 0 08 between 20 and 900 s after the main shock to a faster decay with p 0 92 0 04 after 900 s the foreshock seismicity rate follows an inverse omori s law with p 0 73 0 08 from several tens of days up to several hundred seconds before the main shock the seismicity rate in the 200 s immediately before the main shock appears steady with p 0 35 0 50 these observations can be explained by the epidemic type aftershock sequence etas model and the rate and state model for a heterogeneous stress field on the main shock rupture plane alternatively nonseismic stress changes near the source region such as episodic aseismic slip or pore fluid pressure fluctuations may be invoked to explain the observation of small p values immediately before and after the main shock

a review of tephra transport and dispersal models evolution current status and future perspectives abstract tephra transport models try to predict atmospheric dispersion and sedimentation of tephra depending on meteorology particle properties and eruption characteristics defined by eruption column height mass eruption rate and vertical distribution of mass models are used for different purposes from operational forecast of volcanic ash clouds to hazard assessment of tephra dispersion and fallout the size of the erupted particles a key parameter controlling the dynamics of particle sedimentation in the atmosphere varies within a wide range largest centimetric to millimetric particles fallout at proximal to medial distances from the volcano and sediment by gravitational settling on the other extreme smallest micrometric to sub micrometric particles can be transported at continental or even at global scales and are affected by other deposition and aggregation mechanisms different scientific communities had traditionally modeled the dispersion of these two end members volcanologists developed families of models suitable for lapilli and coarse ash and aimed at computing fallout deposits and for hazard assessment in contrast meteorologists and atmospheric scientists have traditionally used other atmospheric transport models dealing with finer particles for tracking motion of volcanic ash clouds and eventually for computing airborne ash concentrations during the last decade the increasing demand for model accuracy and forecast reliability has pushed on two fronts first the original gap between these different families of models has been filled with the emergence of multi scale and multi purpose models second new modeling strategies including for example ensemble and probabilistic forecast or model data assimilation are being investigated for future implementation in models and or modeling strategies this paper reviews the evolution of tephra transport and dispersal models during the last two decades presents the status and limitations of the current modeling strategies and discusses some emergent perspectives expected to be implemented at operational level during the next few years improvements in both real time forecasting and long term hazard assessment are necessary to loss prevention programs on a local regional national and international level

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

retardations in fault creep rates before local moderate earthquakes along the san andreas fault system central california records of shallow aseismic slip fault creep obtained along parts of the san andreas and calaveras faults in central california demonstrate that significant changes in creep rates often have been associated with local moderate earthquakes an immediate postearthquake increase followed by gradual long term decay back to a previous background rate is generally the most obvious earthquake effect on fault creep this phenomenon identified as aseismic afterslip usually is characterized by above average creep rates for several months to a few years in several cases minor step like movements called coseismic slip events have occurred at or near the times of mainshocks one extreme case of coseismic slip recorded at cienega winery on the san andreas fault 17 5 km southeast of san juan bautista consisted of 11 mm of sudden displacement coincident with earthquakes ofm l 5 3 andm l 5 2 that occurred 2 5 minutes apart on 9 april 1961 at least one of these shocks originated on the main fault beneath the winery creep activity subsequently stopped at the winery for 19 months then gradually returned to a nearly steady rate slightly below the previous long term average

on the mechanics of earthquake afterslip we propose a model for earthquake afterslip based on rate and state variable friction laws in the model afterslip is attributed to the interaction of a velocity weakening region at depth within which earthquakes nucleate with an upper region of velocity strengthening frictional behavior the existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments in our model afterslip is the result of relaxation of a stress perturbation within the velocity strengthening region which arises when an earthquake propagates into that region from below we derive the stress perturbation and its decay from the friction constitutive law using a simple 1 degree of freedom approximation for the elastic interaction between the fault and its surroundings this approximation is based on thickness averaged displacements and slip velocities within the velocity strengthening region which is assumed to slip as a rigid block coseismic and postseismic slip are coupled through the thickness averaged stiffness k of the velocity strengthening region we assume k to be inversely proportional to the thickness of this region which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip we model the afterslip time histories of the 1966 parkfield and 1987 superstition hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults in accord with field observations our model predicts 1 that afterslip on some faults scales with the thickness of the unconsolidated sedimentary cover and 2 that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth velocity strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones about 500 m within their shallow regions 3 5 km combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip

variability of fault slip behavior along the san andreas fault in the san juan bautista region an improved understanding of the time history of fault slip at depth is an essential step toward understanding the underlying mechanics of the faulting process using a waveform cross correlation approach we document spatially and temporally varying fault slip along the northernmost creeping section of the san andreas fault near san juan bautista sjb california by systematically examining spatiotemporal behaviors of characteristically repeating earthquakes cres the spatial distribution of pre 1998 sjb earthquake 1984 1998 fault slip rate inferred from the cres reveals a 15 km long low creep or partially locked section located near the 1998 mw 5 1 sjb earthquake rupture a finite fault slip inversion reveals that the rupture of the 1998 sjb earthquake is characterized by the failure of a compact 4 km2 asperity with a maximum slip of about 90 cm and corresponding peak stress drop of up to 50 mpa whereas the mean stress drop is about 15 mpa following the 1998 earthquake the cre activity was significantly increased in a 5 10 km deep zone extending 2 7 km northwest of the main shock which indicates triggering of substantial aseismic slip the postseismic slip inferred from the cre activity primarily propagated to the northwest and released a maximum slip of 9 cm in this 5 10 km depth range the estimated postseismic moment release is 8 6 1016 n m which is equivalent to mw 5 22 the aseismic slip distribution following the 1998 earthquake is not consistent with coseismic stress driven afterslip but represents a triggered long lasting slow earthquake

postseismic relaxation driven by brittle creep a possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks application to the chi chi earthquake taiwan we evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth assuming a velocity strengthening brittle creep rheology we show that this model can help reconcile the time evolution of afterslip as measured from geodesy with aftershocks decay we propose an analytical expression for slip of the brittle creeping fault zone bcfz that applies to any dynamic or static stress perturbation including shear stress and normal stress changes the model predicts an initial logarithmic increase of slip with time postseismic slip rate decays over a characteristic time t r a that does not depend on the amplitude of the stress perturbation and it asymptotically joins the long term creep imposed by interseismic stress buildup given that the seismicity rate might be considered proportional to the sliding velocity of the bcfz the model predicts a decay rate of aftershocks that follows omori s law with a mathematical formalism identical to that of dieterich 1994 although based on a different mechanical rationale our model also differs from dieterich s model in that it requires that aftershock sequences and deep afterslip as constrained from geodetic measurements should follow the same temporal evolution we test this for the 1999 chi chi earthquake m w 7 6 and find that both sets of data are consistent with a model of afterslip due to the response of the bcfz the inferred relaxation time t r 8 5 years requires a value for a log v being the coefficient of friction in the range between 1 3 10 3 and 10 2

san andreas fault zone california m 5 5 earthquake history the san andreas fault zone has been a very significant source of major california earthquakes from 1812 to 1906 it generated four major earthquakes of m 7 or larger in two pairs on two major portions of the fault a pair of major earthquakes occurred on the central to southern region where the 1857 faulting overlapped the 1812 earthquake faulting a pair of major earthquakes occurred on the northern region where the 1906 faulting overlapped the 1838 earthquake faulting also earthquakes of m 7 occurred in the san francisco bay area on the hayward fault in 1868 and the santa cruz mountains near loma prieta in 1989 and on the imperial fault near the border with mexico in 1940 the 1838 earthquake s damage effects throughout the bay area from san francisco to santa clara valley and monterey were unequalled by any historical earthquake other than the 1906 event this and numerous strong possible aftershocks during the following 3 years in the san juan bautista vicinity suggest 1838 faulting from san francisco to san juan bautista cycles of seismicity and quiescence were associated with the bay area earthquakes of 1868 1906 and 1989 the 1868 earthquake on the hayward fault was preceded by 12 earthquakes of m 5 5 from 1855 to 1866 within 60 km of the hayward fault and was followed by 13 quiet years the 1906 san andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of m 5 5 and was followed by quiescence with only three earthquakes of m 5 5 until 1954 the bay area has been seismically quiet at the m 5 5 level since the 1989 loma prieta earthquake and its 1990 aftershocks which contrasts with the 10 years before 1989 when five m 5 5 6 2 events occurred the loma prieta earthquake is of similar magnitude to the 1868 hayward event and could be followed by a similarly short quiet period the 1857 earthquake had immediate foreshocks in the lonoak bitterwater region 50 km northwest of parkfield in the northern end zone of the 1857 rupture extending southeast from bitterwater 70 km to parkfield the rate of seismic moment release has decreased with time since 1857 this may reflect the decay with time of the stress loading due to the 9 m 1857 fault displacements 80 km southeast of parkfield and explain why the predicted earthquake which was based on the assumption of regular recurrence of parkfield earthquakes has not yet occurred the extent of the 1812 earthquake fault rupture is not well defined jacoby et al 1988 estimated that it extended 170 km from cajon pass to tejon pass based on this estimate we present the hypothesis that the rupture occurred in two segments in december 1812 the eastern segment generated the 8 december earthquake that damaged san juan capistrano san gabriel san fernando and san buenaventura thirteen days later the western segment ruptured generating the earthquake that damaged san fernando and san buenaventura again as well as santa barbara santa ynez and purisima concepcion

earthquake potential along the northern hayward fault california the hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace dislocation models of the surface deformation adjacent to the hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at 7 millimeters per year to the bottom of the seismogenic zone along a 20 kilometer long northern fault segment microearthquakes with the same waveform repeatedly occur at 4 to 10 kilometer depths and indicate deep creep at 5 to 7 millimeters per year the difference between current creep rates and the long term slip rate of 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern hayward fault adjacent to the locked 1868 earthquake rupture which broke the southern 40 to 50 kilometers of the fault the potential for a major independent earthquake of the northern hayward fault might be less than previously thought

seismological evidence for differential rotation of the earth s inner core the travel times of seismic waves that traverse the earth s inner core show a small but systematic variation over the past three decades this variation is best explained by a rotation of the inner core that moves the symmetry axis of its known seismic anisotropy the inferred rotation rate is on the order of 1 per year faster than the daily rotation of the mantle and crust

migration of seismic scatterers associated with the 1993 parkfield aseismic transient event the time varying deformation field within a fault zone particularly at depths where earthquakes occur is important for understanding fault behaviour and its relation to earthquake occurrence1 2 3 but detection of this temporal variation has been extremely difficult although laboratory studies have long suggested that certain structural changes such as the properties of crustal fractures should be seismically detectable4 here we present evidence that such structural changes are indeed observable in particular we find a systematic temporal variation in the seismograms of repeat microearthquakes that occurred on the parkfield segment of the san andreas fault over the decade 1987 97 our analysis reveals a change of the order of 10 m in the location of scatterers which plausibly lie within the fault zone at a depth of 3 km the motion of the scatterers is coincident in space and time with the onset of a well documented aseismic transient deformation event we speculate that this structural change is the result of a stress induced redistribution of fluids in fluid filled fractures caused by the transient event

earthquake source scaling relationships from 1 to 5 ml using seismograms recorded at 2 5 km depth the scaling relationships of earthquake sources less than about magnitude 3 have been the subject of considerable controversy over the last two decades studies of such events have shown a tendency for the constant stress drop self similarity of larger earthquakes to breakdown at small magnitudes and an apparent minimum source dimension of about 100 m has been observed other studies showed that this apparent breakdown in scaling could be an artifact of severe near surface attenuation limiting the spatial resolution of surface data in this study source parameters are determined for over 100 nearby tectonic earthquakes from recordings at a depth of 2 5 km in granite in the cajon pass scientific drill hole southern california comparison of the seismograms recorded at this depth with those at the wellhead clearly demonstrates the effect of the severe attenuation in the upper kilometers of the earth s crust source parameters are calculated by spectral modeling of three component p and s waves assuming four source models based on the brune 2 n 2 model in model l n 2 is fixed and q of p and s waves is determined to be 912 581 1433 and 1078 879 1323 respectively the numbers in parentheses are 1 standard deviation in model 2 qp qs 1000 is assumed and n is allowed to vary the 2 model is a good average for the data set but there is some real scatter supported by the data in model 3 qp qs 1000 is also assumed and 2 is constrained and in model 4 attenuation is ignored and n is allowed to vary source dimensions of less than 10m are observed for all four models 10 times smaller than the proposed minimum no breakdown in constant stress drop scaling is seen in the downhole data approximately ml 1 to 5 5 m0 109 1016 nm the ratio between radiated seismic energy estimated by integrating the velocity squared spectra with adequate signal bandwidth and seismic moment appears to decrease gradually with decreasing moment in the magnitude range 1 to 7 this is not incompatible with a constant stress drop but could result from errors in calculating energy the ratio of the s wave energy to that radiated by the p waves is about 14 after correction for attenuation this low value is consistent with the corner frequency shift of about 1 3 this corner frequency shift is observed for all four source models and therefore is interpreted as being source controlled

earthquake stress drops and inferred fault strength on the hayward fault east san francisco bay california we study variations in earthquake stress drop with respect to depth faulting regime creeping versus locked fault behavior and wall rock geology we use the p wave displacement spectra from borehole seismic recordings of m 1 0 4 2 earthquakes in the east san francisco bay to estimate stress drop using a stack and invert empirical green s function method the median stress drop is 8 7 mpa and most stress drops are in the range between 0 4 and 130 mpa an apparent correlation between stress drop and magnitude is entirely an artifact of the limited frequency band of 4 55 hz there is a trend of increasing stress drop with depth with a median stress drop of 5 mpa for 1 7 km depth 10 mpa for 7 13 km depth and 50 mpa deeper than 13 km we use s p amplitude ratios measured from the borehole records to better constrain the first motion focal mechanisms high stress drops are observed for a deep cluster of thrust faulting earthquakes the correlation of stress drops with depth and faulting regime implies that stress drop is related to the applied shear stress we compare the spatial distribution of stress drops on the hayward fault to a model of creeping versus locked behavior of the fault and find that high stress drops are concentrated around the major locked patch near oakland this also suggests a connection between stress drop and applied shear stress as the locked patch may experience higher applied shear stress as a result of the difference in cumulative slip or the presence of higher strength material the stress drops do not directly correlate with the strength of the proposed wall rock geology at depth suggesting that the relationship between fault strength and the strength of the wall rock is complex

distribution of postseismic slip on the calaveras fault california following the 1984 m6 2 morgan hill earthquake abstract repeating earthquakes res are sequences of events that have virtually identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth to investigate the postseismic deformation after the 1984 m6 2 morgan hill earthquake we identify re sequences occurring on the central calaveras fault between 1984 and 2005 using a combination of cross correlation and spectral coherence techniques both the accelerated slip transients due to the earthquake as well as the return to interseismic background creep rates can be imaged from our dataset a comparison between the regions of the fault that ruptured coseismically and the locations of the res show that res preferentially occur in areas adjacent to the coseismic rupture using calculated re derived subsurface slip distributions at 6 months and 18 months after the mainshock we predict surface electronic distance meter edm line length changes between stations near the morgan hill rupture area the re derived slip model underpredicts a subset of the observed line length changes inclusion of transient aseismic slip below the seismogenic zone is needed to better match the measured surface deformation

repeating earthquakes and seismic potential along the northern longitudinal valley fault of eastern taiwan 1 the northern longitudinal valley fault in eastern taiwan creeps at the surface with a small rate of 1 cm yr but slips in large earthquakes to improve seismic hazard assessment it is important to comprehend the slip deficit rate distribution at depth we discovered 25 ml 2 1 4 6 repeating earthquakes in this area and inverted gps measurements for producing an image of the along strike spatial distribution of deep fault slip rates the repeating events are located at the depths of 10 22 km with 24 9 77 5 mm yr slip rates which are comparable with the gps derived slip rates of 47 5 5 8 mm yr at similar depth ranges based on distribution of gps derived slip deficits since 1951 the northern longitudinal valley fault has become capable of releasing stored strain in a future mw 7 3 earthquake

small repeating earthquake activity interplate quasi static slip and interplate coupling in the hyuga nada southwestern japan subduction zone 1 small repeating earthquake re analysis is a useful method for estimating interplate quasi static slip which is a good indicator of interplate coupling we detected 170 continual type interplate re groups and then estimated the spatial variation in quasi static slip in the hyuga nada over the past 17 years the re activity in this region has different characteristics compared with that in the northeast japan subduction zone presumably reflecting differences in the subduction properties our results revealed that interplate coupling spatially changes along the trench axis and dip direction a phenomenon that cannot be resolved by land based global positioning system gps analysis by comparing seismicity the low slip rate areas correspond with the location of hypocenters and asperities for large and moderate sized interplate earthquakes suggesting strong interplate coupling at these sites these results indicate that the slip rate distribution estimated from re activity is reliable and useful for assessing the potential of future large earthquakes

periodic pulsing of characteristic microearthquakes on the san andreas fault deep fault slip information from characteristically repeating microearthquakes reveals previously unrecognized patterns of extensive large amplitude long duration quasiperiodic repetition of aseismic events along much of a 175 kilometer segment of the central san andreas fault pulsing occurs both in conjunction with and independent of transient slip from larger earthquakes it extends to depths of 10 to 11 kilometers but may be deeper and it may be related to similar phenomena occurring in subduction zones over much of the study area pulse onset periods also show a higher probability of larger earthquakes which may provide useful information for earthquake forecasting

seismic evidence for rock damage and healing on the san andreas fault associated with the 2004 m 6 0 parkfield earthquake we deployed a dense linear array of 45 seismometers across and along the san andreas fault near parkfield a week after the m 6 0 parkfield earthquake on 28 september 2004 to record fault zone seismic waves generated by aftershocks and explosions seismic stations and explosions were co sited with our previous exper iment conducted in 2002 the data from repeated shots detonated in the fall of 2002 and 3 months after the 2004 m 6 0 mainshock show 1 0 1 5 decreases in seismic wave velocity within an 200 m wide zone along the fault strike and smaller changes 0 2 0 5 beyond this zone most likely due to the coseismic damage of rocks during dynamic rupture in the 2004 m 6 0 earthquake the width of the damage zone characterized by larger velocity changes is consistent with the low velocity waveguide model on the san andreas fault near parkfield that we derived from fault zone trapped waves li et al 2004 the damage zone is not symmetric but extends farther on the southwest side of the main fault trace waveform cross correlations for repeated aftershocks in 21 clusters with a total of 130 events located at different depths and distances from the array site show 0 7 1 1 increases in s wave velocity within the fault zone in 3 months starting a week after the earthquake the velocity recovery indicates that the damaged rock has been healing and regaining the strength through rigidity recovery with time most likely due to the closure of cracks opened during the mainshock we estimate that the net decrease in seismic velocities within the fault zone was at least 2 5 caused by the 2004 m 6 0 parkfield earthquake the healing rate was largest in the earlier stage of the postmainshock healing process the magnitude of fault healing varies along the rupture zone being slightly larger for the healing beneath middle mountain correlating well with an area of large mapped slip the fault healing is most promi nent at depths above 7 km

repeating earthquake finite source models strong asperities revealed on the san andreas fault 1 we investigate the rupture process of a sequence of repeating mw 2 1 earthquakes on the san andreas fault in parkfield spanning the occurrence of the september 28 2004 mainshock by inverting seismic moment rate functions obtained from empirical green s function deconvolution the results show that these events have extremely concentrated slip patches with radii on the order of 10 20 m with peak slip between 8 4 and 11 4 cm the rupture speed and rise time are consistent with values of larger earthquakes the spatial distribution of stress drop for the events shows low average values 2 5 5 6 mpa and very large peak values of 66 7 93 9 mpa the results show that strong asperities can exist at small scales on an otherwise weak fault and helps reconcile differences between traditional spectra based and tectonic loading methods for determining the stress drop of small repeating earthquakes

source inversion of the 1988 upland california earthquake determination of a fault plane for a small event abstract we examined short period p waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small m l 4 6 earthquake near upland california we removed path and site complications by choosing a small aftershock m l 2 7 as an empirical green function the main shock p waves were deconvolved by using the empirical green function to produce simple far field displacement pulses we used a least squares method to invert these pulses for the slip distribution on a finite fault both nodal planes strike 125 dip 85 and strike 221 dip 40 of the first motion focal mechanism were tested at various rupture velocities the southwest trending fault plane consistently gave better fitting solutions than the southeast trending plane we determined a moment of 4 2 10 22 dyne cm the rupture velocity and thus the source area could not be well resolved but if we assume a reasonable rupture velocity of 0 87 times the shear wave velocity we obtain a source area of 0 97 km 2 and a stress drop of 38 bars choice of a southwest trending fault plane is consistent with the trend of the nearby portion of the transverse ranges frontal fault zone and indicates left lateral motion this method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations

observations of changes in waveform character induced by the 1999 m w7 6 chi chi earthquake 1 we observe changes in the waveforms of repeating earthquakes in eastern taiwan following the 1999 mw7 6 chi chi earthquake while their recurrence intervals appear to be unaffected there is a clear reduction in waveform similarity and velocity changes indicated by delayed phases at the time of the chi chi event these changes are limited to stations in and paths that cross the 70 100 km region surrounding the chi chi source area the area where seismic intensity and co seismic surface displacements were largest this suggests that damage at the near surface is responsible for the observed waveform changes delays are largest in the late s wave coda reaching approximately 120 ms this corresponds to a path averagedswave velocity reduction of approximately 1 there is also evidence that damage in the fault zone caused changes in waveform character at sites in the footwall where source receiver paths propagate either along or across the rupture the reduction in waveform similarity persists through the most recent repeating event in our study november 15 2007 indicating that the subsurface damage induced by the chi chi earthquake did not fully heal within the first 8 years following the chi chi earthquake

implications of recent asperity failures and aseismic creep for time dependent earthquake hazard on the hayward fault abstract the probability of large seismic events on a particular fault segment may vary due to external stress changes imparted by nearby deformation events including other earthquakes and aseismic processes such as fault creep and postseismic relaxation the hayward fault hf undergoing both seismic and aseismic fault slip provides a unique opportunity to study the mutual relation of seismic and aseismic processes on a fault system we use surface deformation data obtained from insar interferometric synthetic aperture radar creepmeters and alinement arrays together with constraints provided by repeating earthquakes to investigate the kinematics of fault creep on the northern hf and its relation to two seismic clusters m w 4 1 in october 2011 and march 2012 and an m w 4 2 event in july 2007 recurrences of nearby repeating earthquakes show that these episodes involved both seismic and aseismic slip we model the stress changes due to fault creep and the recent seismic activity on the locked central asperity of the hf which is believed to be the rupture zone of past and future m 7 earthquakes the results show that the shallow fault creep stresses the major locked central patch at an average rate of 0 001 0 003 mpa yr in addition to background stressing at 0 01 0 015 mpa yr given the time dependent nature of the creep occasional deviations from this stressing rate occur we find that the 2011 seismic cluster occurred in areas on the fault that are stressed up to 0 01 mpa yr due to aseismic slip on the surrounding segments suggesting that the occurrence of these events was encouraged by the fault creep changes in the probability of major earthquakes can be estimated from the imparted stress from the recent earthquakes and associated fault creep transients we estimate that the 1 day probability of a large event on the hf only increased by up to 0 18 and 0 05 due to the static stress increase and stressing rate change by the 2011 and 2012 clusters for the july 2007 south oakland event m w 4 2 the estimated increase of short term probabilities is 50 highlighting the importance of short term probability changes due to transient stress changes

remote triggering of fault strength changes on the san andreas fault at parkfield a 20 year seismological record of from the parkfield area of the san andreas fault has been used to identify two occasions when long term changes in fault strength seem to have been induced remotely by large seismic events the 2004 sumatra andaman and the 1992 landers earthquakes in both cases two changes occurred that are consistent with variations in fault strength variation in the properties of seismic scatterers with time reflecting stress induced migration of fluids and systematic variation in the characteristics of repeating earthquake sequences these findings suggest that the largest earthquakes have a global influence on the strength of the earth s fault systems in support of that hypothesis an unusually large number of m 8 earthquakes occurred in the three years following the 2004 sumatra andaman earthquake changes in fault strength over time can increase or decrease the likelihood of failure and the ultimate triggering of seismic events observation of the parkfield area at the san andreas fault over the years 1987 2008 now reveals two occasions the 2004 sumatra andaman earthquake and the 1992 landers earthquake where long term changes in fault strength have probably been induced remotely by large seismic events fault strength is a fundamental property of seismogenic zones and its temporal changes can increase or decrease the likelihood of failure and the ultimate triggering of seismic events although changes in fault strength have been suggested to explain various phenomena such as the remote triggering of seismicity1 there has been no means of actually monitoring this important property in situ here we argue that 20 years of observation 1987 2008 of the parkfield area at the san andreas fault have revealed a means of monitoring fault strength we have identified two occasions where long term changes in fault strength have been most probably induced remotely by large seismic events namely the 2004 magnitude m 9 1 sumatra andaman earthquake and the earlier 1992 m 7 3 landers earthquake in both cases the change possessed two manifestations temporal variations in the properties of seismic scatterers probably reflecting the stress induced migration of fluids and systematic temporal variations in the characteristics of repeating earthquake sequences that are most consistent with changes in fault strength in the case of the 1992 landers earthquake a period of reduced strength probably triggered the 1993 parkfield aseismic transient2 3 4 5 as well as the accompanying cluster of four m 4 earthquakes at parkfield the fault strength changes produced by the distant 2004 sumatra andaman earthquake are especially important as they suggest that the very largest earthquakes may have a global influence on the strength of the earth s fault systems as such a perturbation would bring many fault zones closer to failure it should lead to temporal clustering of global seismicity this hypothesis seems to be supported by the unusually high number of m 8 earthquakes occurring in the few years following the 2004 sumatra andaman earthquake

aseismic slip and fault normal strain along the central creeping section of the san andreas fault 1 we use gps data to measure the aseismic slip along the central san andreas fault csaf and the deformation across adjacent faults comparison of edm and gps data sets implies that except for small scale transients the fault motion has been steady over the last 40 years we add 42 new gps velocities along the csaf to constrain the regional strain distribution shear strain rates are less than 0 083 0 010 strain yr adjacent to the creeping saf with 1 4 5 mm yr of contraction across the coast ranges dislocation modeling of the data gives a deep long term slip rate of 31 35 mm yr and a shallow 0 12 km creep rate of 28 mm yr along the central portion of the csaf consistent with surface creep measurements the lower shallow slip rate may be due to the effect of partial locking along the csaf or reflect reduced creep rates late in the earthquake cycle of the adjoining saf rupture zones

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

depth extent of damage zones around the central calaveras fault from waveform analysis of repeating earthquakes summary we systematically investigate spatial variations of temporal changes and depth extent of damage zones along the calaveras fault that ruptured during the 1984 morgan hill earthquake by the waveform analysis of 333 sets of repeating earthquakes we use a sliding window waveform cross correlation technique to measure traveltime changes in waveforms generated by each repeating cluster we find clear traveltime delays in the s and early s coda waves for events immediately after the morgan hill main shock the amplitudes of the time delays decrease logarithmically with time since the main shock indicating a time dependent recovery healing process following the abrupt coseismic temporal changes the largest temporal changes are observed at station cco that is the closest to the rupture zone of the morgan hill main shock the time delays at this station are larger for clusters in the top 6 km and decrease systematically at larger depth in comparison the time delays observed at other five stations are much smaller and do not show clear relationship with hypocentral depth we suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near surface layers while the temporal changes at station cco are likely associated with the damage zone around the calaveras fault that is well developed in the top few kilometres of the upper crust our results are consistent with the inference of a widespread damage and non linearity in the near surface layers associated with strong ground motions of nearby large earthquakes and localized damages and flower type structures around active faults based on previous studies of fault zone structures and recent 3 d numerical simulations

spatial and temporal stress drop variations in small earthquakes near parkfield california 1 we estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the parkfield segment of the san andreas fault in central california we use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed p wave source spectra with a spatially varying empirical green s function egf our brune type stress drop estimates vary from 0 1 to over 100 mpa with a median value of 6 75 mpa which is nearly constant with moment implying self similarity over the ml 0 5 to 3 0 range of our data the corner frequency decreases for earthquakes at shallower depths consistent with slower rupture velocities and reduced shear wave velocities in local velocity models the estimated median stress drops show significant lateral variations we find lower stress drops in the middle mountain asperity and along the creeping fault section and higher stress drops in the hypocentral region of the 2004 m6 0 parkfield earthquake the main shock did not alter the overall pattern of high and low stress drop regions however a statistical test reveals areas with significant changes in computed stress drops after the main shock which we compare to estimated absolute shear stress changes from a main shock slip model by calculating t from the spectral egf ratio we also identify areas with increased attenuation after the main shock and we are able to distinguish source effects and near source attenuation effects in the spectral analysis these results are confirmed independently from spectral ratios of repeating microearthquake clusters

plate boundary slip associated with the 2003 off tokachi earthquake based on small repeating earthquake data 1 plate boundary slip associated with the 2003 off tokachi earthquake northern japan is determined by analysis of repeating earthquakes in the source region a total of 251 repeating earthquake groups consisting of 666 events are identified in and around the aftershock region from october 1 2000 to may 31 2004 although a number of the repeating earthquakes in this region occurred before the main shock there are many events that occurred after the main event the repeating earthquakes are located in the major afterslip region and distributed to the southeast based on this analysis it is estimated that the areas to the east and southeast of the coseismic area undergone approximately 50 cm of slip over three months following the earthquake and less than 5 cm over the three years prior to the earthquake repeating earthquakes can provide afterslip data in offshore areas where resolution from land based gps is poor

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

nonlinear strong ground motion in the ml 5 4 chittenden earthquake evidence that preexisting damage increases susceptibility to further damage 1 we use 55 repeating earthquake sequences located near the 1989 mw 6 9 loma prieta earthquake to identify time dependent velocity changes in the shallow crust in addition to large delays caused by the loma prieta mainshock the ml 5 4 chittenden earthquake an aftershock of loma prieta caused direct s wave delays of up to 6ms and s coda delays exceeding 15ms we attribute the delays to cracks formed or opened during the strong shaking of the chittenden earthquake the same mechanism believed responsible for the delays observed following loma prieta the magnitude of the delays caused by chittenden strongly correlate with those caused by loma prieta this suggests that rocks recently damaged by nonlinear strong ground motion are particularly susceptible to further damage until they are completely healed therefore we expect that the onset of nonlinearity will occur at substantially lower ground motion for large aftershocks than would otherwise be anticipated

aseismic slip and fault interaction from repeating earthquakes in the loma prieta aftershock zone 1 along creeping sections of the san andreas and other faults small asperities in the fault zone load and fail in characteristic repeating earthquake sequences which can be used as subsurface creepmeters here we use these virtual creepmeters to examine and compare deep slip rates on both the northwestern end of the creeping section of the san andreas fault near san juan bautista and on the nearby subparallel sargent fault while creep on the san andreas increased dramatically due to static stress changes in response to the 1989 loma prieta earthquake the sargent showed very little immediate response consistent with loma prieta finite slip models that put this section of the fault in a region of less than 3 bar coulomb stress increase after about 10 years the san andreas creep rate fell back close to the interseismic rate and variations in creep became coherent in time with the sargent indicating a mutual driving force in the system

fault slip rates at depth from recurrence intervals of repeating microearthquakes unique attributes in sequences of recurring similar microearthquakes at parkfield california provide a means for inferring slip rate at depth throughout the active fault surface from the time intervals between sequence events application of the method using an 11 year microseismicity record revealed systematic spatial and temporal changes in the slip rate that were synchronous with earthquake activity and other independent measures of fault zone slip if this phenomenon is found to be generally common behavior in active faults it forms the basis for a method to monitor the changing strain field throughout a seismogenic fault zone

using repeated sources to quantitatively determine temporal change of medium properties theory and an example 1 we develop a theory of using difference wavefields of repeated sources to locate and quantify temporal medium change and apply the theory to locate temporal change of seismic properties beneath the japan subduction zone using repeated earthquakes our theory states the difference wavefields of two repeated sources in a temporally changed medium can be equivalently treated as wavefields propagating from conceptual sources with their location at the place of temporal change and their strengths equal to the product of magnitude of medium property change and magnitude of the initial wavefields from the repeated sources when the medium change extends to a finite region the conceptual sources become volumetric sources distributed over the region of the medium change and propagating in the direction of the initial wave the conceptualization establishes a theoretical framework for possible applications of using difference wavefields to locate and quantify temporal medium changes in geological sciences ultrasonic experiments civil engineering and medical imaging we search repeating earthquakes occurring in the japan subduction zone formulate an empirical procedure to extract the difference wavefields between repeating earthquakes and determine temporal change of seismic properties using a back projection method we locate the temporal change of seismic properties beneath the japan subduction zone to be at 37 2 n 142 e and estimate the magnitude of the conceptual body force associated with the temporal change to be 1 15 1010n or as a reference a 0 87 density change for an assumed volume of temporal change of 103 km3

complexity in earthquake sequences controlled by multiscale heterogeneity in fault fracture energy 1 a series of dynamic rupture events under constant tectonic loading is simulated on a fault with multiscale heterogeneity and a stochastic rupture initiation process the fracture energy of the fault plane is assumed to have multiscale heterogeneous distribution using fractal circular patches the stochastic rupture initiation process as a function of the accumulated stress is introduced in order to take account of unknown smaller scale heterogeneity and variability five realizations of a statistical spatial distribution of fracture energy fault heterogeneity maps are tested for the simulations of earthquake sequences during a few seismic cycles the diversity of earthquake sequences is principally controlled by the spatial distribution of the patches the effect of dynamic rupture appears in the residual stress after the characteristic events because of their directivity and this localizes the subsequent sequences although the characteristic earthquakes occur rather regularly in time and similarly in different seismic cycles some irregular behavior is found based on the heterogeneity maps and the randomness of the preceding earthquake sequence leading to a visible anomaly in the seismicity such anomalies are not predicable but understandable through the analysis of the considered earthquakes during the cycle the similarity and the diversity simulated in this study governed by the structure of an inherent distribution of multiscale heterogeneity suggest the importance of the pre existing heterogeneity field along the fault for the appearance of earthquake sequences including those that are characteristic

postseismic response of repeating aftershocks the recurrence intervals of repeating earthquakes on the san andreas fault in the loma prieta aftershock zone follow the characteristic 1 t decay of omori s law a model in which these earthquakes occur on isolated patches of the fault that fail in stick slip with creep around them can explain this observation in this model the recurrence interval is inversely proportional to the loading rate due to creep logarithmic velocity strengthening friction predicts 1 t decay in creep rate following the mainshock the time dependence of recurrence is inconsistent with a simple viscous constitutive relationship which predicts an exponential decay of loading rate thus our observations imply postseismic slip at seismogenic depth under a power law rheology the time dependence of postseismic deformation measured geodetically may be diagnostic of whether postseismic deformation is caused by creep or possible viscoelastic deformation at greater depths

a unified source model for the 1906 san francisco earthquake abstract we reconcile two previously discordant source models of the 1906 san francisco earthquake and obtain a model that satisfies both triangulation and seismic data by allowing the rupture velocity to exceed the shear wave velocity employing a projection method to remove the dependence on initial station positions allowed us to make use of a more stable triangulation network including nonrepeated angle observations along the northern san andreas fault this strengthens the case for significant slip over the entire northern segment of the san andreas fault from san juan bautista to cape mendocino during the 1906 earthquake we also found that the teleseismic body wave data can be reconciled with the geodetically derived slip model by allowing supershear rupture this resolves a longstanding conflict between the two previous slip models geodetic and seismic of this earthquake supershear rupture has long been recognized as a theoretical possibility for strike slip faulting and it has been observed in several recent large strike slip earthquakes which raises the prospect that it might be typical for such events supershear rupture leads to substantially different strong ground motion and as a result may need to be taken into account when developing ground motion prediction relations for large strike slip earthquakes our final slip model has a seismic moment of 7 9 10 20 n m which corresponds to a moment magnitude of m w 7 9

orientation of the stress field surrounding the creeping section of the san andreas fault evidence for a narrow mechanically weak fault zone we determine how the orientations of the principal stresses change with distance from the creeping section of the san andreas fault saf our data are 2392 earthquake focal mechanisms which are grouped according to distance from the saf into 33 bins parallel to fault strike the width of the bins are dictated by the spatial organization and density of the seismicity only events with m 1 5 depth 2 km and well constrained focal mechanisms are used the focal mechanisms are inverted simultaneously for the orientations and relative amplitudes of the principal stresses using two different inversion programs a bootstrap resampling method provides confidence limits on the stress orientations our general result is that the maximum horizontal compression lies mostly at high angles to the saf except in a narrow zone on the fault no wider than 1 3 km in which it appears to lie at a smaller angle 45 from the fault strike in particular the maximum horizontal compression lies at a high angle to the saf trend immediately adjacent to the fault this stress state is most readily interpreted as due to a narrow mechanically weak saf in this region the narrowness of this zone contrasts with the region wider than 20 30 km of rotated stresses found surrounding the saf in southern california hardebeck and hauksson 1999 suggestive of different mechanical behaviors of the saf in these two regions this approach may provide a way to define the width of the mechanically weak part of faults

inferring the coseismic and postseismic stress changes caused by the 2004 mw 6 parkfield earthquake from variations of recurrence times of microearthquakes 1 kinematic models of coseismic stress inverted from ground motion data do not usually find good correlation between the location of aftershocks and high stress patches in particular numerous earthquakes are recorded in areas of the fault where the stress decreases however most of coseismic slip distributions have limited spatial resolution typically not better than 1 km here we investigate the stress changes produced by the 2004 mw 6 parkfield earthquake on and near its rupture zone at the scale of magnitude 2 earthquake asperities approximately tens of meters after relocating earthquakes in this zone between 1984 and 2007 we form repeating highly similar earthquake sequences and study how the quasiperiodicity of occurrence at each sequence observed during the 20 years preceding the 2004 main shock is perturbed by this event we apply a simple model of the seismic cycle to infer the coseismic and postseismic stresses experienced by the repeatedly failing asperities despite being spatially sparse these stress distributions have resolutions only limited by the typical scale of an asperity we propose that the high spatial variability of the seismicity patterns following the mw 6 earthquake results from an heterogeneous coseismic stress field the emergence of the omori utsu law observed at large scale greater than kilometers at parkfield is simply the outcome of averaging such quasi deterministic patterns over many sequences the fact that the coseismic stress can significantly change over distances of the order of 100 m adds credence to the hypothesis that earthquake rupture is intrinsically very heterogeneous

migration of early aftershocks following the 2004 parkfield earthquake a significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues an analysis of the waveforms of 3 647 relocated earthquakes along the parkfield section of the san andreas fault reveals 11 times more aftershocks within three days of the 2004 parkfield earthquake than listed in a standard catalogue large shallow earthquakes are immediately followed by numerous aftershocks a significant portion of these events is missing in existing earthquake catalogues mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks1 2 3 4 however recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering2 3 4 and for tracking postseismic deformation around the rupture zone associated with the mainshock5 6 7 here we use the waveforms of 3 647 relocated earthquakes8 along the parkfield section of the san andreas fault as templates9 10 to detect missing aftershocks within three days of the 2004 magnitude 6 0 parkfield earthquake we identify 11 times more aftershocks than listed in the standard catalogue of the northern california seismic network we find that the newly detected aftershocks migrate in both along strike and down dip directions with logarithmic time since the mainshock consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip11 12 the cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days supporting the view that aftershocks are driven primarily by afterslip13 14

behavior of repeating earthquake sequences in central california and the implications for subsurface fault creep repeating earthquakes res are sequences of events that have nearly identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth we investigate the occurrence of these res along faults in central california to determine which faults exhibit creep and the spatiotemporal distribution of this creep at the juncture of the san andreas and southern calaveras paicines faults both faults as well as a smaller secondary fault the quien sabe fault are observed to produce res over the observation period of march 1984 through may 2005 res in this area reflect a heterogeneous creep distribution along the fault plane with significant variations in time cumulative slip over the observation period at individual sequence locations is determined to range from 5 5 58 2 cm on the san andreas fault from 4 8 14 1 cm on the southern calaveras paicines fault and from 4 9 24 8 cm on the quien sabe fault creep at depth appears to mimic the behaviors seen for creep on the surface in that evidence of steady slip triggered slip and episodic slip phenomena are also observed in the re sequences for comparison we investigate the occurrence of res west of the san andreas fault within the southern coast range events within these re sequences occurred only minutes to weeks apart from each other and then did not repeat again over the observation period suggesting that res in this area are not produced by steady aseismic creep of the surrounding fault surface

deep slip rates along the longmen shan fault zone estimated from repeating microearthquakes 1 we investigated deep slip rates along the longmen shan fault zone where the devastating 2008 m7 9 wenchuan earthquake occurred using repeating earthquake data recorded by two digital seismic networks near the fault using cross correlation analysis of waveform data we identified a total of 231 doublets and 224 multiplets that have highly similar waveforms most of these sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days for each multiplet we first constructed a reference seismogram at each station and used it to measure the delay time of the p and s waves of each event within the cluster we used the delay times to determine the relative distances between each event and the cluster centroid the relative distances were used in screening repeating events in each multiplet we identified a total of 12 repeating earthquake clusters in the source region of the m7 9 wenchuan earthquake most of the clusters are located at the edge of locked areas where large coseismic slips were observed during the 2008 wenchuan earthquake suggesting a close spatial relationship between microearthquakes and impending large devastating earthquakes the measured in situ deep slip rates appeared to increase with depth and varied from 3 5 to 9 6 mm yr at depth range of 4 18 km they are approximately twice as large as those inferred from surface gps and geological data our results here have significant implications to the understanding of faulting as well as risk assessment of seismic hazards

rupture directivity of moderate earthquakes in northern california we invert peak ground velocity and acceleration pgv and pga to estimate rupture direction and rupture velocity for 47 moderate earthquakes 3 5 m 5 4 in northern california we correct sets of pgas and pgvs recorded at stations less than 55 125 km depending on source depth for site amplification and source receiver distance then fit the residual peak motions to the unilateral directivity function of ben menahem 1961 we independently invert pga and pgv the rupture direction can be determined using as few as seven peak motions if the station distribution is sufficient the rupture velocity is unstable however if there are no takeoff angles within 30 of the rupture direction rupture velocities are generally subsonic 0 5 0 9 for stability we limit the rupture velocity at v 0 92 the rayleigh wave speed for 73 of 94 inversions the rupture direction clearly identifies one of the nodal planes as the fault plane the 35 strike slip earthquakes have rupture directions that range from nearly horizontal 6 events to directly updip 5 events the other 24 rupture partly along strike and partly updip two strike slip earthquakes rupture updip in one inversion and downdip in the other all but 1 of the 11 thrust earthquakes rupture predominantly updip we compare the rupture directions for 10 m 4 0 earthquakes to the relative location of the mainshock and the first two weeks of aftershocks spatial distributions of 8 of 10 aftershock sequences agree well with the rupture directivity calculated for the mainshock

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

the shuffling rotation of the earth s inner core revealed by earthquake doublets earth s inner core rotates at a different rate than the mantle and discrepancies exist between rotation rates derived from geophysical observations and geodynamical simulations an inverse analysis of seismic data from repeating earthquakes over the past 50 years suggests that the rotation rate of the inner core fluctuates on decadal timescales