locally and remotely triggered aseismic slip on the central san jacinto fault near anza ca from joint inversion of seismicity and strainmeter data we study deep aseismic slip along the central section of the san jacinto fault near the anza seismic gap in southern california elevated strain rates following the remote m w7 2 4 april 2010 el mayor cucapah and the local m w5 4 7 july 2010 collins valley earthquakes were recorded by plate boundary observatory borehole strainmeters near anza and were accompanied by vigorous aftershock sequences we introduce a method to infer the distribution of triggered aseismic slip from combined seismicity and geodetic data based on a rate and state friction model that maps observed changes in seismicity rates into stress changes we invert for the cumulative slip in the 10 day period following each main shock synthetic tests show that the effect of aftershock interactions on the inferred slip distribution is negligible the joint data set is more consistent with a model in which aseismic slip on a principal fault triggers seismicity on adjacent faults than with one in which aseismic slip and seismicity are coplanar our results indicate that aseismic slip primarily occurs along the rim of two seismicity clusters adjacent to anza gap as well as beneath the anza gap itself at depths larger than 10 km the triggered aseismic slip generated by the two main shocks has little overlap a pattern also found in sequences of large earthquakes occurring on a same fault stresses inferred from seismic activity leading to the collins valley main shock suggest that this earthquake was triggered by stresses imposed by the el mayor cucapah remote triggered aseismic slip which persisted for more than 2 months

source parameters and time dependent slip distributions of slow slip events on the cascadia subduction zone from 1998 to 2008 1 we invert for the time dependent slip history of slow slip events on the cascadia subduction zone using gps data from 1998 to 2008 the 16 slip transients have sufficient station coverage to solve for the slip distribution on the plate interface gps time series are inverted for fault slip using the extended network inversion filter limited station coverage south of portland 45 5 n latitude restricts our analysis to events on the northern half of the subduction zone slip is resolved at the base of the seismogenic zone and the slip distributions suggest a potential segment boundary near seattle 47 6 n that correlates roughly with geologic and tectonic boundaries events that initiate to the north and south tend to overlap at about this latitude we compile statistics on source parameters such as propagation rate recurrence interval and stress drop which can be used to constrain proposed models of the source mechanics over a 10 year period total strain release from slow slip events is nonuniform along strike with the greatest cumulative slip 27 cm centered beneath port angeles 48 1 n this slip patch also exhibits the most regular recurrence of mw 6 events relative to other locations along strike the spatial extent of the slip patch beneath port angeles correlates with the along strike bend of the cascadia subduction zone in northwestern washington suggesting that plate geometry plays an important role in controlling the along strike characteristics of slow slip

interseismic strain localization in the san jacinto fault zone we investigate interseismic deformation across the san jacinto fault at anza california where previous geodetic observations have indicated an anomalously high shear strain rate we present an updated set of secular velocities from gps and insar observations that reveal a 2 3 km wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two gps occupations of an alignment array installed in 1990 across the fault trace at anza allow us to rule out shallow creep as a possible contributor to the observed strain rate using a dislocation model in a heterogeneous elastic half space we show that a reduction in shear modulus within the fault zone by a factor of 1 2 1 6 as imaged tomographically by allam and ben zion geophys j int 190 1181 1196 2012 can explain about 50 of the observed anomalous strain rate however the best fitting locking depth in this case 10 4 1 3 km is significantly less than the local depth extent of seismicity 14 18 km we show that a deep fault zone with a shear modulus reduction of at least a factor of 2 4 would be required to explain fully the geodetic strain rate assuming the locking depth is 15 km two alternative possibilities include fault creep at a substantial fraction of the long term slip rate within the region of deep microseismicity or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period

dynamics of a velocity strengthening fault region implications for slow earthquakes and postseismic slip we consider the effect of permanent stress changes on a velocity strengthening rate and state fault through numerical simulations and analytical results on 1 d 2 d and 3 d models we show that slip transients can be triggered by perturbations of size roughly larger than lb gdc b where g is the shear modulus dc and b are the characteristic slip distance and the coefficient of the evolution effect of rate and state friction respectively and is the effective normal stress perturbations that increase the coulomb stress lead to the strongest transients but creep bursts can also be triggered by perturbations that decrease the coulomb stress in the latter case peak slip velocity is attained long after the perturbation so that it may be difficult in practice to identify their origin the evolution of slip in a creep transient shares many features with the nucleation process of a rate and state weakening fault slip initially localizes over a region of size not smaller than lb and then accelerates transiently and finally expands as a quasi static propagating crack the characteristic size lb implies a constraint on the grid resolution of numerical models even on strengthening faults that is more stringent than classical criteria in the transition zone between the velocity weakening and strengthening regions the peak slip velocity may be arbitrarily large and may approach seismic slip velocities postseismic slip may represent the response of the creeping parts of the fault to a stress perturbation of large scale comparable to the extent of the main shock rupture and high amplitude while slow earthquakes may represent the response of the creeping zones to a more localized stress perturbation our results indicate that superficial afterslip observed at usually seismogenic depths is governed by a rate strengthening rheology and is not likely to correspond to stable weakening zones the predictions of the full rate and state framework reduce to a pure velocity strengthening law on a timescale longer than the duration of the acceleration transient only when the triggering perturbation extends over length scales much larger than lb

along strike variations in short term slow slip events in the southwest japan subduction zone 1 short term slow slip events accompanied by nonvolcanic deep low frequency tremors and deep very low frequency earthquakes in southwest japan were investigated systematically by means of ground tilting studies the change in tilt usually lasts for several days by using a genetic algorithm and a least squares method we inverted the data for tilt steps that were caused by slow slip events and were detected by at least four stations situated near the source of the tremor fault parameters were estimated for 54 slow slip episodes that occurred mainly in the western shikoku northeastern kii and tokai regions from 2001 to 2008 in eastern shikoku two slow slip episodes were detected quantitatively for the first time the fault geometries of all the slow slip events were located within the belt like distribution of tremors in the transition zone between the locked and aseismic slip zones at the plate interface of the subducting philippine sea plate the spatial extent of the fault geometry corresponds roughly to the distribution of clusters of nonvolcanic tremors and very low frequency earthquakes the moment magnitudes ranged from 5 4 to 6 2 and the slip was 1 cm for each slow slip event the rate of moment release by the detected slow slip events was 40 60 of the moment accumulation expected from the relative plate motion and it showed regional differences they may reflect the along strike variations in plate convergence and or the characteristic size of the slow slip fault plane on the plate interface

postseismic deformation induced by brittle rock damage of aftershocks 1 large earthquakes are commonly followed by abundant aftershocks that are densely located around the coseismic rupture zone laboratory experiments indicate that microscopic brittle rock failures acoustic emission are associated collectively with a macroscopic damage related inelastic relaxation utilizing basic relations between local brittle failures and gradual inelastic strain in a viscoelastic damage rheology model we develop connections between aftershock decay rates and the aftershocks induced component of geodetic deformation the discussed mechanism is relevant for postseismic relaxation produced by sources located within the seismogenic zone and especially in regions that overlap locations of high aftershocks activity assuming the omori utsu decay rate for aftershocks we find that the temporal decay of the damage related postseismic relaxation follows a generalized power law relation with the standard omori utsu law as a limit case the results provide a way for estimating the separate contributions to observed postseismic displacements that stem from brittle failures in the seismogenic zone aftershocks and other aseismic processes using the obtained theoretical expectations we analyze postseismic displacements measured by gps stations around the north anatolian fault 3 months following the 1999 m7 4 izmit earthquake we find that the observed postseismic displacements decay slower than the aftershock seismicity based on our theoretical results we conclude that up to 50 of the measured surface displacements at near fault sites can be attributed to aftershock induced inelastic deformation in the seismogenic zone the remainder postseismic deformation can generally be explained by relaxation in the deeper ductile substrate

aftershocks of microearthquakes as probes of the mechanics of rupture 1 using a waveform cross correlation technique rubin and gillard 2000 obtained precise relative locations for 4300 0 5 m 3 5 earthquakes occurring along 50 km of the san andreas fault this study adds to that another 5000 earthquakes distributed along 10 km of the san andreas fault and 20 km of the calaveras fault errors in relative location are typically tens of meters for earthquakes separated by hundreds of meters and after correcting for time dependent station delays meters for earthquakes separated by tens of meters along both faults the minimum separation between consecutive earthquakes scales with magnitude in a manner consistent with a magnitude independent stress drop by treating each earthquake on the san andreas as if it were a main shock scaling the distances to all subsequent earthquakes by main shock size and stacking the results a composite aftershock sequence is produced that has many of the characteristics predicted by rate and state friction models projected onto the fault surface these aftershocks outline a quasi elliptical roughly 4 mpa stress drop main shock elongate in the slip parallel direction by 40 at the ends of the major axes of this ellipse over twice as many aftershocks occur to the nw than to the se an asymmetry attributed to the contrast in material properties across the fault unlike the san andreas the calaveras fault exhibits little p wave velocity contrast and no discernible aftershock asymmetry however the earliest part of the aftershock sequence on the calaveras might be truncated by the 30 s blind time of the network following a triggering event

a 21 event 4 000 year history of surface ruptures in the anza seismic gap san jacinto fault and implications for long term earthquake production on a major plate boundary fault paleoseismic work completed at hog lake on the san jacinto fault sjf near anza california indicates that at least 21 surface ruptures have occurred in the anza seismic gap over the past 4 000 years the ages of the ruptures are constrained by 111 radiocarbon dates 97 of which fall in stratigraphic order the average recurrence interval for all ruptures for this period is about 185 105 years although some ruptures such as occurred in the april 1918 earthquake caused only minor displacement we rate the expression of each interpreted event in each of the twelve developed field exposures presented in this work by assigning numeric values for the presence of different criteria that indicate rupture to a paleo ground surface weakly expressed ruptures for example the deformation we interpret to be the result of the historical 1918 earthquake received low scores and are interpreted as smaller earthquakes from this analysis we infer that at least fifteen of the identified ruptures are indicative of large earthquakes similar to the penultimate earthquake inferred to be the m w 7 3 22 november 1800 earthquake the adjusted recurrence interval for large earthquakes lengthens to approximately 254 years comparison with the rupture history at the mystic lake paleoseismic site on the claremont strand indicates that it is plausible that several of the large ruptures identified at hog lake could have jumped the hemet step over at mystic lake and continued on the claremont strand or vice versa but most of the event ages do not match between the two sites indicating that most ruptures do not jump the step finally comparison with san andreas fault ruptures both to the north and south of its juncture with the sjf suggest that some northern sjf ruptures identified at mystic lake may correlate with events identified at wrightwood but that these northern ruptures have no match at hog lake and can not indicate rupture of the entire sjf onto the saf

time dependent dike propagation from joint inversion of seismicity and deformation data dike intrusions both deform earth s surface and induce propagating earthquake swarms we develop methods to utilize both deformation and seismicity from brittle volcano tectonic earthquakes to image time dependent dike propagation dieterich s 1994 seismicity rate theory is used to relate dike induced stress changes to seismicity rate and is combined with elastic green s functions relating dike opening to deformation different space time patterns of seismicity develop if earthquakes occur at the same depth as the dike compared to above below the dike in the former seismicity initiates near the dike s leading edges but shuts off as the dike tips pass and seismogenic volumes fall into stress shadows in the latter seismicity continues at a decaying rate after the tips pass we focus on lateral propagation and develop a nonlinear inversion method that estimates dike length and pressure as a function of time the method is applied to the 2007 father s day intrusion in kilauea volcano seismicity is concentrated at 3 km depth comparable to geodetic estimates of dike depth and decays rapidly in time with lateral propagation only and a vertical dike tip line it is difficult to fit both gps data and the rapid down rift jump in seismicity suggesting significant vertical propagation for the events to have occurred below the dike requires a very short aftershock decay time hence unreasonably high background stressing rate the rapid decay is better explained if the dike extends somewhat below the seismicity we suggest that joint inversion is useful for studying the diking process and may allow for improved short term eruption forecasts

time dependent modeling of slow slip events and associated seismicity and tremor at the hikurangi subduction zone new zealand we present a time dependent slip model of 12 slow slip events sses occurring in the hikurangi margin of new zealand during 2010 and 2011 this model is obtained by inverting daily gps solutions from geonet s continuous gps network on the north island and northern south island we compare the properties of these sses to observations in japan cascadia and mexico and find that hikurangi sses have comparatively large amounts of slip up to 27 cm high slip rates up to 1 4 cm d and a large range of depths 10 40 km durations 7 270 days and sizes mw 5 9 6 9 we further investigate the relationship between the cape turnagain sse and an associated seismic swarm and find that observations are consistent with stress triggering outside the slowly slipping region however other explanations cannot be ruled out we also compare slip during the long term manawatu sse with the tremor epicenters found by ide 2012 and note that tremor locations are offset in the downdip direction relative to the slipping region similar to observations in the bungo channel of japan and guerrero mexico

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

earthquakes as beacons of stress change aftershocks occurring on faults in the far field of a large earthquake rupture can generally be accounted for by changes in static stress on these faults caused by the rupture1 2 this implies that faults interact and that the timing of an earthquake can be affected by previous nearby ruptures3 4 5 6 here we explore the potential of small earthquakes to act as beacons for the mechanical state of the crust we investigate the static stress changes resulting from the 1992 landers earthquake in southern california which occurred in an area of high seismic activity stemming from many faults we first gauge the response of the regional seismicity to the landers event with a new technique and then apply the same method to the inverse problem of determining the slip distribution on the main rupture from the seismicity assuming justifiable parameters we derive credible matches to slip profiles obtained directly from the landers mainshock7 8 our results provide a way to monitor mechanical conditions in the upper crust and to investigate processes leading to fault failure

seismicity of the adriatic microplate abstract the adriatic microplate was previously considered to be a unique block tectonically active only along its margins the seismic sequences that took place in the basin from 1986 to 1990 give new information about the geodynamics of this area three subsets of well recorded events were relocated by the joint hypocentre determination technique on the whole this seismic activity was concentrated in a belt crossing the southern adriatic sea around latitude 42 in connection with regional e w fault systems some features of this seismicity similar to those observed in other well known active margins of the adriatic plate support a model of a southern adriatic lithospheric block detached from the northern one other geophysical information provides evidence of a transitional zone at the same latitude

borehole strainmeter measurements spanning the 2014 mw6 0 south napa earthquake california the effect from instrument calibration the 24 august 2014 mw6 0 south napa california earthquake produced significant offsets on 12 borehole strainmeters in the san francisco bay area these strainmeters are located between 24 and 80 km from the source and the observed offsets ranged up to 400 parts per billion ppb which exceeds their nominal precision by a factor of 100 however the observed offsets of tidally calibrated strains differ by up to 130 ppb from predictions based on a moment tensor derived from seismic data the large misfit can be attributed to a combination of poor instrument calibration and better modeling of the strain field from the earthquake borehole strainmeters require in situ calibration which historically has been accomplished by comparing their measurements of earth tides with the strain tides predicted by a model although the borehole strainmeter accurately measures the deformation within the borehole the long wavelength strain signals from tides or other tectonic processes recorded in the borehole are modified by the presence of the borehole and the elastic properties of the grout and the instrument previous analyses of surface mounted strainmeter data and their relationship with the predicted tides suggest that tidal models could be in error by 30 the poor fit of the borehole strainmeter data from this earthquake can be improved by simultaneously varying the components of the model tides up to 30 and making small adjustments to the point source model of the earthquake which reduces the rms misfit from 130 ppb to 18 ppb this suggests that relying on tidal models to calibrate borehole strainmeters significantly reduces their accuracy

a case study of two m 5 mainshocks in anza california is the footprint of an aftershock sequence larger than we think it has been traditionally held that aftershocks occur within one to two fault lengths of the mainshock here we demonstrate that this perception has been shaped by the sensitivity of seismic networks the 31 october 2001 mw 5 0 and 12 june 2005 mw 5 2 anza mainshocks in southern california occurred in the middle of the densely instrumented anza seismic network and thus were unusually well recorded for the june 2005 event aftershocks as small as m 0 0 could be observed stretching for at least 50 km along the san jacinto fault even though the mainshock fault was only 4 5 km long it was hypothesized that an observed aseismic slipping patch produced a spatially extended aftershock triggering source presumably slowing the decay of aftershock density with distance and leading to a broader aftershock zone we find however the decay of aftershock density with distance for both anza se quences to be similar to that observed elsewhere in california this indicates there is no need for an additional triggering mechanism and suggests that given widespread dense instrumentation aftershock sequences would routinely have footprints much larger than currently expected despite the large 2005 aftershock zone we find that the probability that the 2005 anza mainshock triggered the m 4 9 yucaipa mainshock which occurred 4 2 days later and 72 km away to be only 14 1 this probability is a strong function of the time delay had the earthquakes been separated by only an hour the probability of triggering would have been 89 online material movies exploring the spatial extent of aftershocks from the 2001 and 2005 anza sequences

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

triggering of seismicity at short timescales following californian earthquakes 1 a method for measuring seismicity rate changes caused by the occurrence of major earthquakes in the surrounding crust is proposed it is based on a nonstationary poisson modeling of earthquake activity an estimate of the seismicity rate change and a probability measure of seismicity triggering following a large shock are derived and the removal of the influence of independent aftershock sequences of other nearby previous earthquakes is attempted without using more traditional declustering techniques the seismicity rate change estimator is corrected for its natural bias in favor of positive changes at short timescales that has so far strongly hindered measures of seismicity quiescences three californian earthquakes are examined using this method the 1989 m7 1 loma prieta the 1992 m7 3 landers and the 1994 m6 7 northridge earthquakes positive triggering is very commonly observed as expected but quiescence is however more seldomly obtained for timescales up to 100 days after the main shock this relative absence of quiescence as compared to typical predictions of coulomb modeling for example is also independently observed through direct correlation analyses of californian seismicity the possibility that the stresses caused by the main shock are spatially highly variable may explain these observations

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

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

geodetic slip rates in the southern san andreas fault system effects of elastic heterogeneity and fault geometry 1 we use high resolution interferometric synthetic aperture radar and gps measurements of crustal motion across the southern san andreas fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern san andreas and san jacinto faults two possibilities have been proposed to explain this observation large contrasts in crustal rigidity across the faults or an alternate fault geometry such as a dipping san andreas fault or a blind segment of the san jacinto fault we evaluate these possibilities using a two dimensional elastic model accounting for heterogeneous structure computed from the southern california earthquake center crustal velocity model cvm h 6 3 the results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates however we find that small changes in the location of faults at depth can strongly impact the results our preferred model includes a san andreas fault dipping northeast at 60 and two active branches of the san jacinto fault zone in this case we infer nearly equal slip rates of 18 1 and 19 2mm yr for the san andreas and san jacinto fault zones respectively these values are in good agreement with geologic measurements representing average slip rates over the last 10 10 years implying steady long term motion on these faults

deep creep as a cause for the excess seismicity along the san jacinto fault earthquakes are far more common along the san jacinto fault in california than the nearby southern san andreas fault an analysis of seismic and geodetic data suggests that the excess seismicity along the san jacinto fault is due to a creeping motion along the deeper parts of the fault

seismicity rate changes in the salton sea geothermal field and the san jacinto fault zone after the 2010 mw 7 2 el mayor cucapah earthquake

estimating tectonic stress rate and state with landers aftershocks a new method of analyzing the location and timing of aftershocks that makes possible estimates of the rate at which faults in the area are being loaded with stress has been developed this new method is an application of a theory of aftershocks proposed by dieterich 1994 the stress step from the mainshock triggers the aftershocks which release energy stored by tectonic loading and the mainshock and the tectonic stressing rate is responsible for the background activity thus a comparison of the number of aftershocks with the rate of background activity as a function of stress step can provide an estimate of tectonic stress rate this method requires analysis of a catalog which includes background activity as well as aftershocks the method has been applied to the aftershocks of the 1992 landers california earthquake the tectonic stress rate estimated for the mojave block is 1 2 pa d which corresponds to a repeat time for large events of 14 000 years and suggests that much of the observed strain in the mojave is anelastic the southern san andreas fault system is found to have a stress fate of 100 pa d which corresponds to a repeat time of 270 years and is consistent with primarily elastic strain accommodation inversions for the best fitting background stress state are consistent with fits of focal mechanisms and support an effective friction coefficient near 0 6 fits to the aftershock decay provide estimates of the dieterich friction parameter ad ranging from 0 02 if lithostatic pore fluids are present to 0 0003 if they are not

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

interactions and triggering in a 3 d rate and state asperity model we present a 3 d continuous quasi dynamic rate and state model of multiple seismic asperities forced by surrounding aseismic creep and motivated by observations of coplanar multiplets our model allows to study the physics of interactions among a set of asperities first we show that the amount of interactions and clustering characterized by the omori law and interevent time distribution depends on how far the system is from a critical density of asperities which is related to the friction properties of the barriers separating the sources this threshold controls the ability of a population of asperities to destabilize the creeping barriers between them and therefore determines whether dynamic sequences including several asperities in the same event might occur in agreement with what is expected from observed magnitude frequency distributions therefore the concept of critical density of asperity provides a mechanical interpretation of statistical properties of seismicity as an illustration we used our numerical results in the specific case of parkfield in the period preceding the mw 6 2004 earthquake in order to infer the steady state friction parameter a b characterizing the creep of this part of the san andreas fault we estimate a value of a b that locally exceeds 0 001 which is in the upper range of what has already been proposed for the postseimic period of the mw 6 2004 parkfield earthquake

heat transfer measurements of the 1983 kilauea lava flow convective heat flow measurements of a basaltic lava flow were made during the 1983 eruption of kilauea volcano in hawaii eight field measurements of induced natural convection were made giving heat flux values that ranged from 1 78 to 8 09 kilowatts per square meter at lava temperatures of 1088 and 1128 degrees celsius respectively these field measurements of convective heat flux at subliquidus temperatures agree with previous laboratory measurements in furnace melted samples of molten lava and are useful for predicting heat transfer in magma bodies and for estimating heat extraction rates for magma energy

complementary slip distributions of the august 4 2003 mw 7 6 and november 17 2013 mw 7 8 south scotia ridge earthquakes abstract the south scotia ridge transform ssrt plate boundary between the scotia and antarctic plates experienced large strike slip earthquakes on august 4 2003 m w 7 6 and november 17 2013 m w 7 8 these events have overlapping aftershock zones which is unusual a 36 45 southward dipping fault zone ruptured with left lateral displacements in each event along the northern margin of the south orkney micro continent near 60 s slip distributions for the two events are determined using teleseismic body and surface wave recordings along with constraints from gps ground motion recordings at station borc on laurie island south orkney islands just south of the ssrt the aftershock distributions high frequency back projections and unconstrained body wave finite fault inversions permit significant overlap of the 2003 and 2013 slip zones however the gps static displacements resolve differences in the large slip regions of the two ruptures the 2013 earthquake sequence along the ssrt initiated with m w 6 1 november 13 and m w 6 8 november 16 foreshocks located 50 km west of the mainshock hypocenter and had aftershocks extending 250 km eastward the rupture spread primarily eastward at 2 5 km s with a total rupture duration of about 120 s with two distinct patches of large slip located northwest and northeast of the south orkney islands the rupture swept past borc with high rate gps hrgps ground motion recordings capturing the time varying slip history of the faulting traditional gps data require that the largest slip region of the shorter rupture in 2003 is located in the gap nne of borc between the two patches that ruptured in 2013 there appears to be some overlap of lower slip regions the complementary slip distributions comprise a relatively uniform offset along this portion of the ssrt which is one of the most seismically active regions of the entire antarctic plate boundary

the use of earthquake rate changes as a stress meter at kilauea volcano stress changes in the earth s crust are generally estimated from model calculations that use near surface deformation as an observational constraint but the widespread correlation of changes of earthquake activity with stress has led to suggestions that stress changes might be calculated from earthquake occurrence rates obtained from seismicity catalogues although this possibility has considerable appeal because seismicity data are routinely collected and have good spatial and temporal resolution the method has not yet proven successful owing to the non linearity of earthquake rate changes with respect to both stress and time here however we present two methods for inverting earthquake rate data to infer stress changes using a formulation for the stress and time dependence of earthquake rates application of these methods at kilauea volcano in hawaii yields good agreement with independent estimates indicating that earthquake rates can provide a practical remote sensing stress meter

coseismic slip and afterslip of the great mw 9 15 sumatra andaman earthquake of 2004 we determine coseismic and the first month postseismic deformation associated with the sumatra andaman earthquake of 26 december 2004 from near field global positioning system gps surveys in northwestern sumatra and along the nicobar andaman islands continuous and campaign gps measurements from thailand and malaysia and in situ and remotely sensed observations of the vertical motion of coral reefs the coseismic model shows that the sunda subduction mega thrust ruptured over a distance of about 1500 km and a width of less than 150 km releasing a total moment of 6 7 7 0 10 22 n m equivalent to a magnitude m w 9 15 the latitudinal distribution of released moment in our model has three distinct peaks at about 4 n 7 n and 9 n which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated t waves our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very long period surface waves the tsunami predicted from this model fits rela tively well the altimetric measurements made by the jason and topex satellites neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave the near field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500 sec long seismic rupture the postseismic geodetic moment of about 2 4 10 22 nm mw 8 8 is equal to about 30 5 of the coseismic moment release evolution of postseismic deformation is consistent with rate strengthening frictional afterslip online material summary of geodetic data used in this study

slip on faults in the imperial valley triggered by the 4 april 2010 mw 7 2 el mayor cucapah earthquake revealed by insar 1 radar interferometry insar field measurements and creepmeters reveal surface slip on multiple faults in the imperial valley triggered by the main shock of the 4 april 2010 el mayor cucapah mw 7 2 earthquake co seismic offsets occurred on the san andreas superstition hills imperial elmore ranch wienert coyote creek elsinore yuha and several minor faults near the town of ocotillo at the northern end of the mainshock rupture we documented right lateral slip 40 mm on northwest striking faults and left lateral slip 40 mm on southwest striking faults slip occurred on 15 km and 20 km long segments of the san andreas fault in the mecca hills 50 mm and durmid hill 10 mm respectively and on 25 km of the superstition hills fault 37 mm field measurements of slip on the superstition hills fault agree with insar and creepmeter measurements to within a few millimeters dislocation models of the insar data from the superstition hills fault confirm that creep in this sequence as in previous slip events is confined to shallow depths 3 km

an electrical resistivity measurement in molten basalt during the 1983 kilauea eruption during the recent 1983 eruption of kilauea volcano in hawaii a field measurement of electrical resistivity was made in an 1118 c molten lava flow a probe containing a four electrode wenner aray was inserted into the molten basaltic lava flow at a point about 100 m from the erupting vent the probe indicated an electrical resistivity of approximately 40 ohm m for the freshly erupted molten lava this value is consistent with existing but sparce laboratory data for molten rock in this temperature range

coseismic and initial postseismic deformation from the 2004 parkfield california earthquake observed by global positioning system electronic distance meter creepmeters and borehole strainmeters global positioning system gps electronic distance meter creepmeter and strainmeter measurements spanning the m 6 0 parkfield california earthquake are examined using these data from 100 sec through 9 months following the mainshock the omori s law with rate inversely related to time 1 t p and p ranging between 0 7 and 1 3 characterizes the time dependent deformation during the postseismic period these results are consistent with creep models for elastic solids with an accurate function of postseismic response the coseismic displacements can be estimated from the high rate 1 min sampling gps and the coseismic displacements are approximately 75 of those estimated from the daily solutions consequently fault slip models using daily solutions overestimate coseismic slip in addition at 2 months and at 8 months following the mainshock postseismic displacements are modeled as slip on the san andreas fault with a lower bound on the moment exceeding that of the coseismic moment online material data description and supplementary figures tables and data used in models and time series analysis

modeling afterslip and aftershocks following the 1992 landers earthquake one way to probe the rheology of the lithosphere and fault zones is to analyze the temporal evolution of deformation following a large earthquake in such a case the lithosphere responds to a known stress change that can be assessed from earthquake slip models constrained from seismology and geodesy here we model the postseismic response of a fault zone that is assumed to obey a rate strengthening rheology where the frictional stress varies as a ln e e being the deformation rate and a 0 a rheological parameter the model is simple enough that these parameters can be estimated by inversion of postseismic geodetic data we apply this approach to the analysis of geodetic displacements following the m w 7 3 1992 landers earthquake the model adjusts well the measured displacements and implies a 0 47 0 53 mpa in addition we show that aftershocks and afterslip follow the same temporal evolution and that the spatiotemporal distribution of aftershocks is consistent with the idea that they are driven by reloading of the seismogenic zone resulting from frictional afterslip

surface wave potential for triggering tectonic nonvolcanic tremor corrected abstract source processes commonly posed to explain instances of remote dynamic triggering of tectonic nonvolcanic tremor by surface waves include frictional failure and various modes of fluid activation the relative potential for love and rayleigh wave dynamic stresses to trigger tectonic tremor through failure on critically stressed thrust and vertical strike slip faults under the coulomb griffith failure criteria as a function of incidence angle are anticorrelated over the 15 to 30 km depth range that hosts tectonic tremor love wave potential is high for strike parallel incidence on low angle reverse faults and null for strike normal incidence the opposite holds for rayleigh waves love wave potential is high for both strike parallel and strike normal incidence on vertical strike slip faults and minimal for 45 incidence angles the opposite holds for rayleigh waves this pattern is consistent with documented instances of tremor triggered by love waves incident on the cascadia megathrust and the san andreas fault saf in central california resulting from shear failure on weak faults apparent friction is 0 2 documented instances of tremor triggered by surface waves with strike parallel incidence along the nankai megathrust beneath shikoku japan however are associated primarily with rayleigh waves this is consistent with the tremor bursts resulting from mixed mode failure crack opening and shear failure facilitated by near lithostatic ambient pore pressure low differential stress with a moderate friction coefficient 0 6 on the nankai subduction interface rayleigh wave dilatational stress is relatively weak at tectonic tremor source depths and seems unlikely to contribute significantly to the triggering process except perhaps for an indirect role on the saf in sustaining tremor into the rayleigh wave coda that was initially triggered by love waves

tidal calibration of plate boundary observatory borehole strainmeters 1 the plate boundary observatory the geodetic component of the earthscope program includes 74 borehole strainmeters installed in the western united states and on vancouver island canada in this study we calibrate 45 of the instruments by comparing the observed m2 and o1 earth tides with those predicted using earth tide models for each strainmeter we invert for a coupling matrix that relates the gauge measurements to the regional strain field assuming only that the measured strains are linear combinations of the regional areal and shear strains we compare these matrices to those found when constraints are imposed which require the coupling coefficients to lie within expected ranges for this strainmeter design similar unconstrained and constrained coupling matrices suggest the instrument is functioning as expected as no other coupling matrix can be found that better reduces the misfit between observed and predicted tides when the inversion is unconstrained differences imply a coupling matrix with coefficients outside typical ranges gives a better fit between the observed and predicted tides we find that 22 of the strainmeters examined have coupling matrices for which there is little difference between the constrained and unconstrained inversions if we allow a greater divergence in the shear coupling coefficients and consider the possibility that one gauge may not function as expected the discrepancies between the unconstrained and constrained coupling matrices are resolved for a subset of the remaining strainmeters our results also indicate that most of the strainmeters are less sensitive to areal strain than expected from theory

frictional afterslip following the 2005 nias simeulue earthquake sumatra continuously recording global positioning system stations near the 28 march 2005 rupture of the sunda megathrust moment magnitude m w 8 7 show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust with a major fraction of this slip in the up dip direction from the main rupture eleven months after the main shock afterslip continues at rates several times the average interseismic rate resulting in deformation equivalent to at least a m w 8 2 earthquake in general along strike variations in frictional behavior appear to persist over multiple earthquake cycles aftershocks cluster along the boundary between the region of coseismic slip and the up dip creeping zone we observe that the cumulative number of aftershocks increases linearly with postseismic displacements this finding suggests that the temporal evolution of aftershocks is governed by afterslip

a new estimation of the decay of aftershock density with distance to the mainshock we investigate how aftershocks are spatially distributed relative to the mainshock compared to previous studies ours focuses on earthquakes causally related to the mainshock rather than on aftershocks of previous aftershocks we show that this distinction can be made objectively but becomes uncertain at long time scales and large distances analyzing a regional earthquake data set it is found that at time t following a mainshock of magnitude m the probability of finding an aftershock at distance r relative to the mainshock fault decays as r where is typically between 1 7 and 2 1 for 3 m 6 and is independent of m for r less than 10 to 20 km and t less than 1 day uncertainties on this probability at larger r and t do not allow for a correct estimation of this spatial decay we further show that a static stress model coupled with a rate and state friction model predicts a similar decay with an exponent 2 2 in the same space and time intervals this suggests that static stress changes could explain the repartition of aftershocks around the mainshock even at distances larger than 10 times the rupture length

what controls the spatial distribution of remote aftershocks motivated by recent studies showing that stress modifications due to small quakes are important in earthquake triggering we use a quasi dynamic spatially discrete model of a rate and state dependent fault to examine the effect of multiple elastic stress transfers on the spatial extent of aftershock activity we show that multiple stress transfers may significantly alter the spatial distribution of aftershock sequences and give rise to stress seismicity relations that are inconsistent with dieterich s aftershock model specifically we present an example in which owing to the effect of multiple elastic stress transfers the area experiencing seismicity rate change is much larger than that subjected to a stress change we define a parameter that quantifies the proximity to the failure in the model and show that differences in the magnitude of aftershock rates are due to differences in the proximity to failure that prevailed prior to the mainshock furthermore we show that accelerating and decelerating cumulative benioff strains are indicative of a region approaching or moving away from failure respectively finally we compare the cumulative benioff strains in remote sites triggered either by the landers or the hector mine earthquake and infer that differences in the state of stress that prevailed prior to these mainshocks may explain the different spatial distribution of their remote aftershocks

distribution of seismicity across strike slip faults in california 1 the distribution of seismicity about strike slip faults provides measurements of fault roughness and damage zone width in california seismicity decays with distance from strike slip faults according to a power law 1 x2 d2 2 this scaling relation holds out to a fault normal distance x of 3 6 km and is compatible with a rough fault loading model in which the inner scale d measures the half width of a volumetric damage zone and the roll off rate is governed by stress variations due to fault roughness according to dieterich and smith s 2 d simulations approximates the fractal dimension of along strike roughness near fault seismicity is more localized on faults in northern california nocal d 60 20 m 1 65 05 than in southern california socal d 220 40 m 1 16 05 the parkfield region has a damage zone half width d 120 30 m consistent with the safod drilling estimate its high roll off rate 2 30 25 indicates a relatively flat roughness spectrum k 1 versus k 2 for nocal k 3 for socal our damage zone widths the first direct estimates averaged over the seismogenic layer can be interpreted in terms of an across strike fault core multiplicity that is 1 in nocal 2 at parkfield and 3 in socal the localization of seismicity near individual faults correlates with cumulative offset seismic productivity and aseismic slip consistent with a model in which faults originate as branched networks with broad multicore damage zones and evolve toward more localized lineated features with low fault core multiplicity thinner damage zones and less seismic coupling our results suggest how earthquake triggering statistics might be modified by the presence of faults

decay and expansion of the early aftershock activity following the 2011 mw9 0 tohoku earthquake the 2011 m w9 0 tohoku earthquake was followed by an abundant amount of seismicity providing a unique opportunity to analyze the triggering mechanism of great earthquakes although the tohoku earthquake occurred close to a dense seismic network many aftershocks that occurred in the first few hours after the mainshock are not recorded in the earthquake catalogs here we use a template waveform approach to recover as many as possible missing events in the first 12 hours following the tohoku mainshock our analysis is able to detect about 1 4 times more events than those listed in the high sensitivity seismograph hi net earthquake catalog combining our new dataset with earthquakes that occurred at latter times we are able to observe a continuous decay of the aftershock rate and along strike expansion of aftershock area we relate the latter observation to the occurrence of post seismic slip over the deep interface

scaling of small repeating earthquakes explained by interaction of seismic and aseismic slip in a rate and state fault model because of short recurrence times and known locations small repeating earthquakes present a rare predictable opportunity for detailed field observations they are used to study fault creeping velocities earthquake nucleation stress drops and other aspects of tectonophysics earthquake mechanics and seismology an intriguing observation about repeating earthquakes is their scaling of recurrence time with seismic moment which is significantly different from the scaling based on a simple conceptual model of circular ruptures with stress drop independent of seismic moment and no aseismic slip here we show that a model of repeating earthquakes based on laboratory derived rate and state friction laws reproduces the observed scaling in the model a small fault patch governed by steady state velocity weakening friction is surrounded by a much larger velocity strengthening region long term slip behavior of the fault is simulated using a methodology that fully accounts for both aseismic slip and inertial effects of occasional seismic events the model results in repeating earthquakes with typical stress drops and sizes comparable with observations for a fixed set of friction parameters the observed scaling is reproduced by varying the size of the velocity weakening patch in simulations a significant part of slip on the velocity weakening patches is accumulated aseismically even though the patches also produce seismic events the proposed model supplies a laboratory based framework for interpreting the wealth of observations about repeating earthquakes provides indirect evidence that rate and state friction acts on natural faults and has important implications for possible scenarios of slip partition into seismic and aseismic parts

spatiotemporal analyses of earthquake productivity and size distribution observations and simulations we use relocated catalogs of microearthquakes to investigate earthquake interaction along sections of the sargent calaveras and san andreas faults in cali fornia we examine the stress dependence of seismicity rate change along the three fault segments and find that the seismicity rate following a mainshock decays ap proximately as 1 time the duration of the aftershock activity seems to be independent of distance from the mainshock and the seismicity rate at lag times of up to about 100 sec is nearly constant in the san andreas and the calaveras catalogs where the return of the seismicity rate to the background level is well resolved we find that the return to the background in the distance range of 1 2 rupture radii from a previous earthquake is preceded by a period during which the seismicity rate falls about 30 below the background rate we also examine the effect of a stress step on earthquake size distribution along these faults and find that the exponent of the power law dis tribution of earthquake magnitudes within 10 4 sec of a previous earthquake is sig nificantly lower than that of the long term while the 1 time decay of seismicity rate the independence of aftershock duration from distance from the mainshock and the constant seismicity rate at short lag times are predicted by dieterich s 1994 model the decrease of seismicity rate below the background level and the changes in earth quake size distribution are not for comparison with our observations we simulate earthquake activity on an in herently discrete fault model that is governed by an approximate constitutive friction law similar to the one used by dieterich 1994 we find that the observed response of earthquake productivity and size distribution to a stress step are produced by these simulations the effect of the mainshock is not only to raise the local seismicity rate but also to systematically modify the earthquake size distribution this is because fault patches that are near failure at the time of the stress step are strengthened whereas fault patches that at the same time were far from failure are weakened as a result similar to what is observed for the calaveras and the san andreas segments late during the aftershock sequence the seismicity rate may decrease below the back ground rate we suggest that the time dependent modification of the earthquake size distribution by a stress step can explain observations of lower b values immediately following a stress step

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

quasi periodic slow slip events in the afterslip area of the 1996 hyuga nada earthquakes japan 1 the time evolution of afterslip on a plate boundary experiencing interplate earthquakes is expected to show logarithmic decay the global positioning system network in japan has been monitoring transient deformation since the occurrence of two large interplate earthquakes with moment magnitudes of 6 8 and 6 7 in the hyuga nada area southwest japan in 1996 the spatial and temporal evolution of aseismic interplate slip based on crustal deformation data indicates that afterslip followed the two earthquakes and gradually declined to background rates by around 2004 with total moment magnitude of 7 3 however quasi periodic slow slip events suddenly began within the afterslip area in 2005 with approximately one year duration and two year recurrence interval the moment magnitudes of the three slow slip events since january 2005 range from 6 7 to 6 8 this differs greatly from the expected behavior of logarithmic decay over time both velocity strengthening and velocity weakening rate and state modes have been implicated as the cause of afterslip whose location is complementary to the main shock area of velocity weakening while a slow slip event occurs in the velocity weakening area with different frictional properties from those of an afterslip area in light of the seemingly different frictional properties the coexistence of afterslip and slow slip events in the same area would provide additional information about precisely how the plate interface is behaving the monitoring of these slow slip events should give the clues to understanding the coexistence of long term afterslip and slow slip events and the increasing risk of earthquakes in neighboring areas

earthquakes triggered by silent slip events on kīlauea volcano hawaii silent earthquakes or slow slip events happen so slowly that they cannot be detected by conventional seismographs but now the availability of gps data means that they can be followed in detail segall et al report on a series of small earthquakes on k lauea volcano hawaii the earthquakes at depths of 7 to 8 km appear to be triggered by otherwise silent slow slips though small triggered earthquakes of this type can be used to help quantify the increased hazard due to slow slip events slow slip events or silent earthquakes have recently been discovered in a number of subduction zones including the nankai trough1 2 3 in japan cascadia4 5 and guerrero6 in mexico but the depths of these events have been difficult to determine from surface deformation measurements although it is assumed that these silent earthquakes are located along the plate megathrust this has not been proved slow slip in some subduction zones is associated with non volcanic tremor7 8 but tremor is difficult to locate and may be distributed over a broad depth range9 except for some events on the san andreas fault10 slow slip events have not yet been associated with high frequency earthquakes which are easily located here we report on swarms of high frequency earthquakes that accompany otherwise silent slips on k lauea volcano hawaii for the most energetic event in january 2005 the slow slip began before the increase in seismicity the temporal evolution of earthquakes is well explained by increased stressing caused by slow slip implying that the earthquakes are triggered the earthquakes located at depths of 7 8 km constrain the slow slip to be at comparable depths because they must fall in zones of positive coulomb stress change triggered earthquakes accompanying slow slip events elsewhere might go undetected if background seismicity rates are low detection of such events would help constrain the depth of slow slip and could lead to a method for quantifying the increased hazard during slow slip events because triggered events have the potential to grow into destructive earthquakes

aftershock distribution as a constraint on the geodetic model of coseismic slip for the 2004 parkfield earthquake several studies of the 2004 parkfield earthquake have linked the spatial distribution of the event s aftershocks to the mainshock slip distribution on the fault using geodetic data we find a model of coseismic slip for the 2004 parkfield earthquake with the constraint that the edges of coseismic slip patches align with aftershocks the constraint is applied by encouraging the curvature of coseismic slip in each model cell to be equal to the negative of the curvature of seismicity density the large patch of peak slip about 15 km northwest of the 2004 hypocenter found in the curvature constrained model is in good agreement in location and amplitude with previous geodetic studies and the majority of strong motion studies the curvature constrained solution shows slip primarily between aftershock streaks with the continuation of moderate levels of slip to the southeast these observations are in good agreement with strong motion studies but inconsistent with the majority of published geodetic slip models southeast of the 2004 hypocenter a patch of peak slip observed in strong motion studies is absent from our curvature constrained model but the available gps data do not resolve slip in this region we conclude that the geodetic slip model constrained by the aftershock distribution fits the geodetic data quite well and that inconsistencies between models derived from seismic and geodetic data can be attributed largely to resolution issues

locking depths estimated from geodesy and seismology along the san andreas fault system implications for seismic moment release 1 the depth of the seismogenic zone is a critical parameter for earthquake hazard models independent observations from seismology and geodesy can provide insight into the depths of faulting but these depths do not always agree here we inspect variations in fault depths of 12 segments of the southern san andreas fault system derived from over 1000 gps velocities and 66 000 relocated earthquake hypocenters geodetically determined locking depths range from 6 to 22 km while seismogenic thicknesses are largely limited to depths of 11 20km theseseismogenicdepthsbestmatchthegeodeticlockingdepthswhenestimated at the 95 cutoff depth in seismicity and most fault segment depths agree to within 2 km however the imperial coyote creek and borrego segments have significant discrepancies in these cases the geodetically inferred locking depths are much shallower than the seismogenic depths we also examine variations in seismic moment accumulation rate per unit fault length as suggested by seismicity and geodesy and find that both approaches yield high rates 1 5 1 8 1 0 13 nm yr km along the mojave and carrizo segments and low rates 0 2 10 13 nm yr km along several san jacinto segments the largest difference in seismic moment between models is calculated for the imperial segment where the moment rate from seismic depths is a factor of 2 5 larger than that from geodetic depths such variability has important implications for the accuracy to which future major earthquake magnitudes can be estimated

preceding coseismic and postseismic slips of the 2011 tohoku earthquake japan 1 we estimated the spatial and temporal evolution of the preceding aseismic slip from january 2003 to january 2011 the coseismic slip of the tohoku earthquake and the postseismic slip after the earthquake based on global positioning system gps data time dependent analysis indicates aseismic slip offshore of miyagi and fukushima prefectures from 2004 associated with a series of subduction earthquakes that overlap the aseismic slip area these preceding aseismic and coseismic slip areas are centered between the centers of the coseismic and afterslip areas of the tohoku earthquake offshore of miyagi prefecture while they overlap the coseismic and afterslip areas of the tohoku earthquake off fukushima prefecture the timing of moment magnitude nine mw9 class earthquakes appears to be controlled by multiple preceding slip events smaller earthquakes and their afterslip the preceding aseismic and coseismic slip decreased the coupling rate off the tohoku coast suggesting the possibility that the preceding slip represented a precursive stage of the tohoku earthquake the afterslip of the tohoku earthquake occurred in an area where the coseismic slip was not large complementing the large coseismic slip zone the afterslip along iwate miyagi extends up to 80 km in depth and is currently the sole mechanism of strain release in this depth range the source region of the anticipated miyagi oki earthquake shows small postseismic slip after the tohoku earthquake reflecting the energy release at the time of the earthquake aftershock activity is roughly governed by an afterslip process

low velocity zones along the san jacinto fault southern california from body waves recorded in dense linear arrays we derive high resolution information on low velocity fault zone fz structures along the san jacinto fault zone sjfz southern california using waveforms of local earthquakes that are recorded at multiple linear cross fault arrays we observe clear across fault delays of direct p and s waves indicating damage zones at different segments of the sjfz we then compute synthetic traveltimes and waveforms using generalized ray theory and perform forward modeling to constrain the fz parameters at the southern section near the trifurcation area the low velocity zone lvz of the clark branch has a width of 200 m 30 45 reduction in vp and 50 reduction in vs from array data across the anza seismic gap we find a lvz with 200 m width and 50 reduction in both vp and vs nearly as prominent as that on the southern section we only find prominent lvzs beneath three out of the five arrays indicating along strike variations of the fault damage fz reflected phases are considerably less clear than those observed above the rupture zone of the 1992 landers earthquake shortly after the event this may reflect partially healed lvzs with less sharp boundaries at the sjfz given the relatively long lapse time from the last large surface rupturing event alternatively the lack of observed fz reflected phases could be partially due to the relatively small aperture of the arrays nevertheless the clear signatures of damage zones at anza and other locations indicate very slow healing process at least in the top few kilometers of the crust

time dependent model of creep on the hayward fault from joint inversion of 18 years of insar and surface creep data 1 spatial and temporal variations of aseismic fault creep influence the size and timing of large earthquakes along partially coupled faults to solve for a time dependent model of creep on the hayward fault we invert 18 years of surface deformation data 1992 2010 obtained by interferometric processing of 52 and 50 sar images acquired by the ers1 2 and envisat satellites respectively and surface creep data obtained at 19 alinement and 4 creepmeter stations for multi temporal analysis of the sar data we developed a method for identifying stable pixels using wavelet multi resolution analysis we also implement a variety of wavelet based filters for reducing the effects of environmental artifacts using a reweighted least squares approach we inverted the interferometric data to generate a time series of surface deformation over the san francisco bay area with a precision of better than a few millimeters to jointly invert the insar displacement time series and the surface creep data for a time dependent model of fault creep we use a robust inversion approach combined with a kalman filter the time dependent model constrains a zone of high slip deficit that may represent the locked rupture asperity of past and future m 7 earthquakes we identify several additional temporal variations in creep rate along the hayward fault the most important one being a zone of accelerating slip just northwest of the major locked zone we estimate that a slip rate deficit equivalent to mw 6 3 6 8 has accumulated on the fault since the last event in 1868

comprehensive analysis of earthquake source spectra in southern california we compute and analyze p wave spectra from earthquakes in southern california between 1989 and 2001 using a method that isolates source receiver and path dependent terms we correct observed source spectra for attenuation using both fixed and spatially varying empirical green s function methods estimated brune type stress drops for over 60 000 m l 1 5 to 3 1 earthquakes range from 0 2 to 20 mpa with no dependence on moment or local b value median computed stress drop increases with depth in the upper crust from about 0 6 mpa at the surface to about 2 2 mpa at 8 km where it levels off and remains nearly constant in the midcrust down to about 20 km however the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop spatially coherent variations in median stress drop are observed with generally low values for the imperial valley and northridge aftershocks and higher values for the eastern transverse ranges and the north end of the san jacinto fault we find no correlation between observed stress drop and distance from the san andreas and other major faults significant along strike variations in stress drop exist for aftershocks of the 1992 landers earthquake which may correlate with differences in main shock slip

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

seismicity analysis through point process modeling a review the occurrence times of earthquakes can be considered to be a point process and suitable modeling of the conditional intensity function of a point process is useful for the investigation of various statistical features of seismic activity this manuscript summarizes likelihood based methods of analysis of point processes and reviews useful models for particular analyses of seismicity most of the analyses can be implemented by the computer programs published by the author and collaborators

dynamic triggering of creep events in the salton trough southern california by regional m 5 4 earthquakes constrained by geodetic observations and numerical simulations abstract since a regional earthquake in 1951 shallow creep events on strike slip faults within the salton trough southern california have been triggered at least 10 times by m 5 4 earthquakes within 200 km the high earthquake and creep activity and the long history of digital recording within the salton trough region provide a unique opportunity to study the mechanism of creep event triggering by nearby earthquakes here we document the history of fault creep events on the superstition hills fault based on data from creepmeters insar and field surveys since 1988 we focus on a subset of these creep events that were triggered by significant nearby earthquakes we model these events by adding realistic static and dynamic perturbations to a theoretical fault model based on rate and state dependent friction we find that the static stress changes from the causal earthquakes are less than 0 1 mpa and too small to instantaneously trigger creep events in contrast we can reproduce the characteristics of triggered slip with dynamic perturbations alone the instantaneous triggering of creep events depends on the peak and the time integrated amplitudes of the dynamic coulomb stress change based on observations and simulations the stress change amplitude required to trigger a creep event of a 0 01 mm surface slip is about 0 6 mpa this threshold is at least an order of magnitude larger than the reported triggering threshold of non volcanic tremors 2 60 kpa and earthquakes in geothermal fields 5 kpa and near shale gas production sites 0 2 0 4 kpa which may result from differences in effective normal stress fault friction the density of nucleation sites in these systems or triggering mechanisms we conclude that shallow frictional heterogeneity can explain both the spontaneous and dynamically triggered creep events on the superstition hills fault

afterslip and aftershocks in the rate and state friction law istituto nazionale di geofisica e vulcanologia italy ingv centre national de la recherche scientifique cnrs exxonmobil upstream research company

waveform relocated earthquake catalog for southern california 1981 to june 2011 we determine a new relocated catalog hys catalog 2011 for southern california from 1981 through june 2011 about 75 3 of the hypocenters are calculated with absolute and differential travel time picks and 24 7 could be relocated only by using absolute travel time picks with 3d or 1d velocity models the total catalog consists of more than 502 000 earthquakes in the region extending from baja california in the south to coalinga and owens valley in the north the catalog consists of three m 7 1 m 7 2 and m 7 3 mainshocks their foreshocks and aftershocks and background seismicity caused by tectonic and other processes in the southern california crust hypocenters in the new relocated catalog exhibit tighter spatial clustering of seismicity than does the routinely generated catalog and the depth distribution is tighter and reflects the thickness of the seismogenic zone more accurately compared to the standard catalog the relocated hypocenters are more easily related to other data sets such as mapped late quaternary faults

lidar and field observations of slip distribution for the most recent surface ruptures along the central san jacinto fault we measured offsets on tectonically displaced geomorphic features along 80 km of the clark strand of the san jacinto fault sjf to estimate slip per event for the past several surface ruptures we identify 168 offset features from which we make over 490 measurements using b4 light detection and ranging lidar imagery and field observations our results suggest that lidar technology is an exemplary supplement to traditional field methods in slip per event studies displacement estimates indicate that the most recent surface rupturing event mre produced an average of 2 5 2 9 m of right lateral slip with maximum slip of nearly 4 m at anza a m w 7 2 7 5 earthquake average multiple event offsets for the same 80 kms are 5 5 m with maximum values of 3 m at anza for the penultimate event cumulative displacements of 9 10 m through anza suggest the third event was also similar in size paleoseismic work at hog lake dates the most recent surface rupture event at ca 1790 a poorly located large earthquake occurred in southern california on 22 november 1800 we relocate this event to the clark fault based on the mre at hog lake we also recognize the occurrence of a younger rupture along 15 20 km of the fault in blackburn canyon with 1 25 m of average displacement we attribute these offsets to the 21 april 1918 m w 6 9 event these data argue that much or all of the clark fault and possibly also the casa loma fault fail together in large earthquakes but that shorter sections may fail in smaller events online material topographic contour maps and hillshades generated from b4 lidar data corresponding field photographs and data tables comparing lidar based and field based slip measurements for individual geomorphic features along the clark fault

a real time procedure for progressive multiplet relative relocation at the hawaiian volcano observatory a real time procedure has been set up to select and relocate relatively similar events recorded by the u s geological survey hawaiian volcano observatory hvo hawaii seismic network at the observatory it allows the relocation of such similar events with an uncertainty whose order of magnitude is 50 m horizontally and 100 m vertically as a first step a systematic exploration of the volcano to find similar events within the seismicity recorded between years 1988 and 2000 has been undertaken and allows events to be gathered into large multiplets a generalized relative relocation of these similar events has been performed and a database has been built from the result of this relocation incoming earthquakes are routinely compared to this database and relocated and the database is updated preliminary results are presented and discussed manuscript received 21 may 2001

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

complementary slip distributions of the largest earthquakes in the 2012 brawley swarm imperial valley california we investigate the finite rupture processes of two m 5 earthquakes in the 2012 brawley swarm by joint inversion of nearby strong motion and high rate gps data waveform inversions up to 3 hz were made possible by using a small event m w3 9 for path calibration of the velocity structure our results indicate that the first m w5 3 event ruptured a strong concentrated asperity with offsets of 20 cm centered at a depth of 5 km the subsequent m w5 4 event occurred 1 5 h later with a shallower slip distribution that surrounds and is complementary to that of the earlier event the second event has a longer rise time and weaker high frequency energy release compared to the m w5 3 event both events display strong rupture directivity toward the southwest and lack of very shallow 2 km coseismic slip the hypocenters for these events appear to be near or in the bedrock but most of the slip is distributed at shallower depths 6 km and can explain a large part of the gps offsets for the swarm the complementary slip distributions of the two events suggest a triggering relationship between them with no significant creep needed to explain the various data sets

on the role of multiple interactions in remote aftershock triggering the landers and the hector mine case studies the observed seismicity rate increase after large earthquakes in sites that are located several source lengths away from the mainshock centroid poses a major problem this is because the static stress change induced by a mainshock in that region seems to be insignificant and the dynamic stress changes can only en hance the seismicity during the passage of the seismic waves but not at later times in quest for a physically viable triggering mechanism for delayed aftershocks in remote sites we examine earthquake activities in remote sites that were triggered by two california earthquakes the magnitude 7 1 hector mine earthquake and the mag nitude 7 3 landers earthquake we introduce a new method for quantifying the de gree to which the triggering effect of each aftershock is locally more important than the triggering effect of the mainshock we apply this method to the landers and the hector mine remote aftershock sequences we show that multiple stress transfers from early aftershocks to later aftershocks played an important role in the enhance ment of both the landers and the hector mine aftershock activities in remote sites we present a time space diagram of the hector mine remote aftershock sequence in the imperial valley which shows that this sequence is made up of several subse quences and that the onset of activity migrated southward

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

stress and aftershock constrained joint inversions for coseismic and postseismic slip applied to the 2004 m6 0 parkfield earthquake 1 both aftershocks and geodetically measured postseismic displacements are important markers of the stress relaxation process following large earthquakes postseismic displacements can be related to creep like relaxation in the vicinity of the coseismic rupture by means of inversion methods however the results of slip inversions are typically non unique and subject to large uncertainties therefore we explore the possibility to improve inversions by mechanical constraints in particular we take into account the physical understanding that postseismic deformation is stress driven and occurs in the coseismically stressed zone we do joint inversions for coseismic and postseismic slip in a bayesian framework in the case of the 2004 m6 0 parkfield earthquake we perform a number of inversions with different constraints and calculate their statistical significance according to information criteria the best result is preferably related to a physically reasonable model constrained by the stress condition namely postseismic creep is driven by coseismic stress and the condition that coseismic slip and large aftershocks are disjunct this model explains 97 of the coseismic displacements and 91 of the postseismic displacements during day 1 5 following the parkfield event respectively it indicates that the major postseismic deformation can be generally explained by a stress relaxation process for the parkfield case this result also indicates that the data to constrain the coseismic slip model could be enriched postseismically for the 2004 parkfield event we additionally observe asymmetric relaxation process at the two sides of the fault which can be explained by material contrast ratio across the fault of 1 15 in seismic velocity

evidence for postseismic deformation of the lower crust following the 2004 mw6 0 parkfield earthquake previous studies have shown that postseismic relaxation following the 2004 mw6 0 parkfield ca earthquake is dominated by afterslip however we show that some fraction of the afterslip inferred from kinematic inversion to have occurred immediately below the seismically ruptured area may in fact be a substitute for viscous postseismic deformation of the lower crust using continuous gps and synthetic aperture radar interferometry we estimate the relative contribution of shallow afterslip at depth less than 20km and deeper seated deformation required to account for observed postseismic surface displacements exploiting the possible separation in space and time of the time series of displacements predicted from viscoelastic relaxation we devise a linear inversion scheme that allows inverting jointly for the contribution of afterslip and viscoelastic flow as a function of time we find that a wide range of models involving variable amounts of viscoelastic deformation can fit the observations equally well provided that they allow some fraction of deep seated deformation at depth larger than 20 km these models require that the moment released by postseismic relaxation over 5 years following the earthquake reached nearly as much as 200 of the coseismic moment all the models show a remarkable complementarity of coseismic and shallow afterslip distributions some significant deformation at lower crustal depth 20 26 km is required to fit the geodetic data the condition that postseismic deformation cannot exceed complete relaxation places a constraint on the amount of deep seated deformation the analysis requires an effective viscosity of at least 10 18 pa s of the lower crust assuming a semi infinite homogeneous viscous domain this deep seated deformation is consistent with the depth range of tremors which also show a transient postseismic response and could explain as much as 50 of the total postseismic geodetic moment the remaining fraction being due to afterslip at depth shallower than 20 km lower crustal postseismic deformation could reflect a combination of localized ductile deformation and aseismic frictional sliding

tidal modulation of slow slip in cascadia 1 several studies have shown that the seismic tremor in episodic tremor and slip is tidally modulated suggesting a sensitivity to the rather small tidal stresses we address whether the slip rate in slow slip events is also tidally modulated by examining data from six borehole strainmeters in northwest washington and southern vancouver island we simultaneously fit data from multiple stations and from slow slip events occurring over a 3 year interval from january 2007 to june 2009 as we are unable to extract a meaningful signal from a single record we find modulation of the strain rate with a 12 4 h period that of the tide with the largest amplitude that is significant at the 99 level the amplitude of this modulation suggests that the slip rate during slow slip events oscillates on average 25 above and below its mean value during a tidal cycle tidal modulation estimates at three other periods are significant with more than 70 probability the phase of maximum strain rate in the 12 4 h m2 period coincides with the phase of the maximum tremor rate taken from a catalog in an overlapping region comparison with a simple tidal loading model shows that the phase of maximum strain rate in the m2 period may occur at the maximum shear stress or up to 90 before it depending on the location of slip in the subduction zone

quantifying focal mechanism heterogeneity for fault zones in central and southern california summary we present a statistical analysis of focal mechanism orientations for nine california fault zones with the goal of quantifying variations of fault zone heterogeneity at seismogenic depths the focal mechanism data are generated from first motion polarities for earthquakes in the time period 1983 2004 magnitude range 0 5 and depth range 0 15 km only mechanisms with good quality solutions are used we define fault zones using 20 km wide rectangles and use summations of normalized potency tensors to describe the distribution of double couple orientations for each fault zone focal mechanism heterogeneity is quantified using two measures computed from the tensors that relate to the scatter in orientations and rotational asymmetry or skewness of the distribution we illustrate the use of these quantities by showing relative differences in the focal mechanism heterogeneity characteristics for different fault zones these differences are shown to relate to properties of the fault zone surface traces such that increased scatter correlates with fault trace complexity and rotational asymmetry correlates with the dominant fault trace azimuth these correlations indicate a link between the long term evolution of a fault zone over many earthquake cycles and its seismic behaviour over a 20 yr time period analysis of the partitioning of san jacinto fault zone focal mechanisms into different faulting styles further indicates that heterogeneity is dominantly controlled by structural properties of the fault zone rather than time or magnitude related properties of the seismicity

spatiotemporal evolution of a transient slip event on the san andreas fault near parkfield california 1 in 1993 several baselines of the two color electronic distance meter edm network at parkfield california deviated from their long term rates coincident with anomalous observations from nearby strain meters and a creep meter as well as an increase in microseismicity between october 1992 and december 1994 three m 4 5 earthquakes occurred beneath middle mountain near the hypocenter of the 1934 and 1966 parkfield m6 events we analyzed the two color edm data using a kalman filtering based technique to image the spatiotemporal evolution of slip on the fault at parkfield between the mid 1980s and 2003 this method accounts for localized random walk motion of the geodetic monuments and a prominent seasonal signal that affects many baselines we find that a slip rate increase occurred between january 1993 and july 1996 on the upper 8 km of the fault near middle mountain the peak estimated slip rate during this time was 49 mm yr which exceeds the long term geologic rate of 35 mm yr the slip rate evolution appears episodic with an initial modest increase after the m4 3 earthquake and a much larger jump following the shallower m4 7 event in december 1994 this temporal correlation between inferred slip and seismicity suggests that the moderate earthquakes triggered the aseismic fault slip the edm data cannot resolve whether transient slip propagated across the nucleation zone of the 1934 and 1966 m6 parkfield earthquakes however transient slip and its associated stress release in the hypocentral area of previous parkfield events is consistent with the nucleation of the 2004 m6 parkfield earthquake elsewhere on the fault