new constraints on the active tectonic deformation of the aegean 1 site velocities from six separate global positioning system gps networks comprising 374 stations have been referred to a single common eurasia fixed reference frame to map the velocity distribution over the entire aegean we use the gps velocity field to identify deforming regions rigid elements and potential microplate boundaries and build upon previous work by others to initially specify rigid elements in central greece the south aegean anatolia and the sea of marmara we apply an iterative approach tentatively defining microplate boundaries determining best fit rigid rotations examining misfit patterns and revising the boundaries to achieve a better match between model and data short term seismic cycle effects are minor contaminants of the data that we remove when necessary to isolate the long term kinematics we find that present day aegean deformation is due to the relative motions of four microplates and straining in several isolated zones internal to them the rms misfit of model to data is about 2 sigma very good when compared to the typical match between coseismic fault models and gps data the simplicity of the microplate description of the deformation and its good fit to the gps data are surprising and were not anticipated by previous work which had suggested either many rigid elements or broad deforming zones that comprise much of the aegean region the isolated deforming zones are also unexpected and cannot be explained by the kinematics of the microplate motions strain rates within internally deforming zones are extensional and range from 30 to 50 nanostrain year nstrain year 10 9 year 1 to 2 orders of magnitude lower than rates observed across the major microplate boundaries lower strain rates may exist elsewhere within the microplates but are only resolved in anatolia where extension of 13 4 nstrain year is required by the data our results suggest that despite the detailed complexity of active continental deformation revealed by seismicity active faulting fault geomorphology and earthquake fault plane solutions continental tectonics at least in the aegean is to first order very similar to global plate tectonics and obeys the same simple kinematic rules although the widespread distribution of aegean seismicity and active faulting might suggest a rather spatially homogeneous seismic hazard the focusing of deformation near microplate boundaries implies the highest hazard is comparably localized

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

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

late holocene slip rate of the san andreas fault and its accommodation by creep and moderate magnitude earthquakes at parkfield california investigation of a right laterally offset channel at the miller9s field paleoseismic site yields a late holocene slip rate of 26 2 6 4 4 3 mm yr 1 for the main trace of the san andreas fault at parkfield california this is the first well documented geologic slip rate between the carrizo and creeping sections of the san andreas fault this rate is lower than holocene measurements along the carrizo plain and rates implied by far field geodetic measurements 35 mm yr however the rate is consistent with historical slip rates measured to the northwest along the creeping section of the san andreas fault

observed source parameters for dynamic rupture with non uniform initial stress and relatively high fracture energy we have conducted dynamic rupture propagation experiments to establish the relations between insource stress drop fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 mpa to produce high fracture energy in the source we use a rough fault that has a large slip weakening distance an artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture making the initial stress non uniform shear stress particle velocity fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault normal distances stress drop increases weakly with normal stress average slip rate depends linearly on the fault strength loss and on static stress drop both with a nonzero intercept a minimum fracture energy of 1 8 j m 2 and a linear slip weakening distance of 33 mm are inferred from the intercept the large slip weakening distance also affects the average slip rate which is reduced by in source energy dissipation from on fault fracture energy because of the low normal stress and small per event slip w86 mm no thermal weakening such as melting or pore fluid pressurization occurs in these experiments despite the relatively high fracture energy and the very low heat production energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties the product of fracture energy and fault area is larger than the radiated energy seismic efficiency is low at w2 the ratio of apparent stress to static stress drop is w27 consistent with measured overshoot the fracture efficiency is w33 the static and dynamic stress drops when extrapolated to crustal stresses are 2e7 3 mpa and in the range of typical earthquake stress drops as the relatively high fracture energy reduces the slip velocities in these experiments the extrapolated average particle velocities for crustal stresses are 0 18e0 6 m s that these experiments are consistent with typical earthquake source properties suggests albeit indirectly that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops dominate earthquake source properties only for exceptional events unless crustal stresses are low published by elsevier ltd

variability of repeating earthquake behavior along the longitudinal valley fault zone of eastern taiwan 1 two major clusters of repeating earthquake sequences res are found on two fault patches near the longitudinal valley fault lvf in eastern taiwan the chihshang segment to the south and the hualien segment to the north the 25 m 2 3 4 6 res on the hualien segment are widely distributed in a nne direction over a distance of 45 km whereas on the chihshang segment the 30 m 2 2 3 4 res have a shorter along strike extent of about 15 km the longer spatial extent of res on the hualien segment may indicate a wider distributed creeping zone at depth m 3 8 res are observed on the hualien segment and are characterized by quasiperiodic recurrence statistical analysis of the size and number of earthquakes adjacent to the res indicates that the smaller number of large earthquakes in the vicinity may explain the quasiperiodic recurrence of res we argue that on the hualien segment m 4 or larger earthquakes are isolated enough in space to facilitate the occurrence of repeated ruptures

effect of clay content and mineralogy on frictional sliding behavior of simulated gouges binary and ternary mixtures of quartz illite and montmorillonite 1 we investigated the frictional sliding behavior of simulated quartz clay gouges under stress conditions relevant to seismogenic depths conventional triaxial compression tests were conducted at 40 mpa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz montmorillonite and illite in all cases frictional strengths of mixtures fall between the end members of pure quartz strongest and clay weakest the overall trend was a decrease in strength with increasing clay content in the illite quartz mixture the trend was nearly linear while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers two micromechanical models were used to analyze the critical clay fractions for the two regime transitions on the basis of clay porosity and packing of the quartz grains the transition from regime 1 high strength to 2 intermediate strength is associated with the shift from a stress supporting framework of quartz grains to a clay matrix embedded with disperse quartz grains manifested by the development of p foliation and reduction in riedel shear angle the transition from regime 2 intermediate strength to 3 low strength is attributed to the development of shear localization in the clay matrix occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects

an application of the time and magnitude predictable model for the long term prediction of strong shallow earthquakes in the japan area the country of japan and its surrounding area has been divided into 12 seismogenic sources on the basis of certain seismotectonic and geomorphological criteria in each of these sources interevent times between successive mainshocks with magnitudes equal to or larger than certain cutoff magnitudes were considered these interevent times as well as the magnitudes of the mainshocks have been used to determine the following relations log t t 0 30 m min 0 19 m p 0 18 log m 0 2 41 m f 0 88 m min 0 25 m p 0 46 log m 0 9 06 where tt is the interevent time measured in years m min is the surface wave magnitude of the smallest mainshock considered mp is the magnitude of the preceding mainshock mf is the magnitude of the following mainshock and m is the yearly released seismic moment in each source on the basis of these two relations and taking into account the time of occurrence and the magnitude of the last mainshock in each source time dependent conditional probabilities for the occurrence of the next large ms 7 5 shallow mainshocks in the 12 seismogenic sources during the next 10 yrs 1993 to 2002 and the magnitudes of the expected mainshocks are determined for seismogenic sources 5 fukushima 9 western honshu 7 southern hokkaido 1 ryukyu islands and 12 okushiri island relatively high probabilities p 10 0 5 were assigned it is of interest to note that in seismogenic source 12 okushiri for which an expected magnitude of 7 5 was calculated an earthquake with a magnitude of 7 6 national earthquake information center occurred on 12 july 1993

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

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

slip rate increase at parkfield in 1993 detected by high precision edm and borehole tensor strainmeters on two of the instrument networks at parkfield california the two color electronic distance meter edm network and borehole tensor strainmeter btsm network we have detected a rate change starting in 1993 that has persisted at least 5 years these and other instruments capable of measuring crustal deformation were installed at parkfield in anticipation of a moderate m6 earthquake on the san andreas fault many of these instruments have been in operation since the mid 1980s and have established an excellent baseline to judge changes in rate of deformation and the coherence of such changes between instruments the onset of the observed rate change corresponds in time to two other changes at parkfield from late 1992 through late 1994 the parkfield region had an increase in number of m4 to m5 earthquakes relative to the preceding 6 years the deformation rate change also coincides with the end of a 7 year period of sub normal rainfall both the spatial coherence of the rate change and hydrological modeling suggest a tectonic explanation for the rate change from these observations we infer that the rate of slip increased over the period 1993 1998

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

potential for geophysical experiments in large scale tests potential research applications for large specimen geophysical experiments include measurements of scale dependence of physical parameters and examination of interactions with heterogeneities especially flaws such as cracks in addition increased specimen size provides opportunities for improved recording resolution and greater control of experimental variables large scale experiments using a special purpose low stress 100 mpa

insights into active tectonics of eastern taiwan from analyses of geodetic and geologic data 1 about 50 mm yr of convergence between the philippine sea and eurasian plates is absorbed in eastern taiwan and it remains unclear how the convergence is partitioned among active faults the longitudinal valley fault lvf the most seismically active fault in eastern taiwan creeps at the surface in the south and not in the north however it is unclear how much of the fault is locked or creeping at depth to address these problems we model holocene and interseismic deformation of elastic lithospheric blocks moving over a viscoelastic asthenosphere in eastern taiwan through a fully probabilistic scheme we invert gps interferometric synthetic aperture radar creepmeter and holocene marine terrace data for block motions fault slip rates and distribution of interseismic creep the data are explained with four blocks separated by three faults central range fault lvf and an offshore fault the model explains the essential features of interseismic and holocene deformation we find that 35 55 mm yr of slip on the offshore fault is necessary to fit marine terrace uplift rates which is a larger fraction of the plate convergence than previously recognized the lvf has a holocene slip rate of 20 30 mm yr with approximately equal magnitudes of reverse slip and left lateral strike slip components only about half of the surface area of the longitudinal valley fault appears to be locked the southern segment of the lvf creeps at a rate of 5 28 mm yr down to a depth of 15 20 km while the northern segment is locked from the surface to a depth of 20 km

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

effects of physical fault properties on frictional instabilities produced on simulated faults laboratory studies of large scale simulated faults show that physical properties of the fault specifically normal stress and fault roughness significantly influence the unstable shear failure behavior of the fault in addition the experiments provide insights into important length scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension stick slip shear failures have been generated along a 2 m long simulated fault in a block of sierra white granite experiments were conducted at normal stresses between 0 6 and 4 mpa with two different prepared roughnesses of the simulated fault a smooth fault with a profilometer measured roughness of 0 2 m and a rough fault with a measured roughness of 80 m high speed records of shear strains and slip velocities clearly show both the propagation of the stick slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved both the dynamic stress drops of the stick slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress although critical slip weakening displacements are insensitive to normal stress they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault slip velocities in these experiments are influenced by roughness and are proportional to stress drop although the onset of slip is sharply defined the termination of slip is more gradual without a clear stopping phase measured rupture velocities on the rough fault are lower than those on the smooth fault when a length scale based on rupture nucleation dimensions directly proportional to critical displacements is introduced the data suggest that at fault lengths close to the nucleation lengths rupture velocity and slip velocity increase with increasing fault length

systematic variations in recurrence interval and moment of repeating aftershocks 1 the recurrence intervals for 194 repeating clusters on the calaveras fault follow a power law decay relation with elapsed time after the 1984 m6 2 morgan hill california mainshock the decay rates of repeating aftershocks in the immediate vicinity of a high slip patch that failed during the mainshock systematically exceed those that are farther away the trend between relative moment and recurrence interval which is a measure of the fault healing rate varies systematically with depth and changes from negative to positive value as the distance between the repeating aftershock and the mainshock slip patch increases we speculate that high strain rate in the early postseismic period may cause transient embrittlement and strengthening of the deep repeating clusters in the areas adjacent to the mainshock slip patch resulting in large moments that decrease with time as the strain rate diminishes our observations suggest that systematic behavior of repeating aftershocks reflect variations in the fault zone rheology

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

characteristic silent earthquakes in the eastern part of the boso peninsula central japan 1 analysis of global positioning system gps data shows transient crustal deformation in the boso peninsula central japan for about fifty days from october 2002 the southeastward horizontal displacements detected suggest the occurrence of interplate aseismic slip between the philippine sea plate and the north american plate six years after a similar event in 1996 we estimate an area of aseismic slip with moment magnitude mw 6 6 centered off the boso peninsula adjacent to associated seismic activity slip evolution of the 2002 aeismic slip started on october 4 and migrated from north to south for about ten days followed by gradual subsidence and restarting of slip which lasted until december 2 propagating more southward similarities in focal area and slip process between the 1996 and the 2002 events suggest the boso aseismic slips as characteristic silent earthquakes together with the repetitive occurrence of seismic swarm in a similar region every six to seven years a more tightly coupled southern area that did not release enough energy in the 1996 event may have caused the differences in magnitude central slip area and duration between the 1996 and the 2002 events if the hypothesized recurrence interval for strain accumulation is true based on swarm seismicity

tectonic stressing in california modeled from gps observations 1 what happens in the crust as a result of geodetically observed secular motions in this paper we find out by distorting a finite element model of california using gps derived displacements a complex model was constructed using spatially varying crustal thickness geothermal gradient topography and creeping faults gps velocity observations were interpolated and extrapolated across the model and boundary condition areas and the model was loaded according to 5 year displacements results map highest differential stressing rates in a 200 km wide band along the pacific north american plate boundary coinciding with regions of greatest seismic energy release away from the plate boundary gps derived crustal strain reduces modeled differential stress in some places suggesting that some crustal motions are related to topographic collapse calculated stressing rates can be resolved onto fault planes useful for addressing fault interactions and necessary for calculating earthquake advances or delays as an example i examine seismic quiescence on the garlock fault despite a calculated minimum 0 1 0 4 mpa static stress increase from the 1857 m 7 8 fort tejon earthquake results from finite element modeling show very low to negative secular coulomb stress growth on the garlock fault suggesting that the stress state may have been too low for large earthquake triggering thus the garlock fault may only be stressed by san andreas fault slip a loading pattern that could explain its erratic rupture history

empirical relationships between aftershock area dimensions and magnitude for earthquakes in the mediterranean sea region fault dimension estimates derived from the aftershock area extent of 36 shallow depth v31 km earthquakes that occurred in the mediterranean sea region have been used in order to establish empirical relationships between length width area and surface wave moment magnitude this dataset consists of events whose aftershock sequence was recorded by a dense local or regional network and the reported location errors did not exceed on average 3 5 km surface wave magnitudes for these events were obtained from the neic database and or published reports while moment magnitudes as well as focal mechanisms were available from the harvard usgs catalogues contrary to the results of some previously published studies we found no evidence in our dataset that faulting type may have an effect on the fault dimension estimates and therefore we derived relationships for the whole of the dataset comparisons by means of statistical f tests of our relationships with other previously published regional and global relationships were performed in order to check possible similarities or differences most such comparisons showed relatively low significance levels b95 since the differences in source dimension estimates were large mainly for magnitudes lower than 6 5 becoming smaller with increasing magnitude some degree of similarity however could be observed between our fault length relationship and the one derived from aftershock area lengths of events in greece while a difference was found between our regional and global fault length relationships a calculation of the ratio defined as the fault length derived from our relationships to the length estimated from regional empirical relationships involving surface ruptures showed that it can take a maximum value of about 7 for small magnitudes while it approaches unity at ms 7 2 when calculating the same ratio using instead global empirical relationships we see the maximum value not exceeding 1 8 while unity is reached at mw 7 8 indicating the existence of a strong regional variation in the fault lengths of earthquakes occurring in the mediterranean sea region also a relationship between the logarithms of the rupture area and seismic moment is established and it is inferred that there is some variation of stress drop as a function of seismic moment in particular it is observed that for magnitudes

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

slip on the san andreas fault at parkfield california over two earthquake cycles and the implications for seismic hazard parkfield california which experienced m 6 0 earthquakes in 1934 1966 and 2004 is one of the few locales for which geodetic observations span multiple earthquake cycles we undertake a comprehensive study of deformation over the most recent earthquake cycle and explore the results in the context of geodetic data collected prior to the 1966 event through joint inversion of the variety of parkfield geodetic measurements trilateration two color laser and global positioning system including previously unpublished two color data we estimate the spatial distribution of slip and slip rate along the san andreas using a fault geometry based on precisely relocated seismicity although the three most recent parkfield earthquakes appear complementary in their along strike distributions of slip they do not produce uniform strain release along strike over multiple seismic cycles since the 1934 earthquake more than 1 m of slip deficit has accumulated on portions of the fault that slipped in the 1966 and 2004 earthquakes and an average of 2 m of slip deficit exists on the 33 km of the fault southeast of gold hill to be released in a future perhaps larger earthquake it appears that the fault is capable of partially releasing stored strain in moderate earthquakes maintaining a disequilibrium through multiple earthquake cycles this complicates the application of simple earthquake recurrence models that assume only the strain accumulated since the most recent event is relevant to the size or timing of an upcoming earthquake our findings further emphasize that accumulated slip deficit is not sufficient for earthquake nucleation online material model fault geometry fit to the data for the inversions and model resolution

seismicity rate changes along the central california coast due to stress changes from the 2003 m 6 5 san simeon and 2004 m 6 0 parkfield earthquakes abstract we investigated the relationship between seismicity rate changes and modeled coulomb static stress changes from the 2003 m 6 5 san simeon and the 2004 m 6 0 parkfield earthquakes in central california coulomb stress modeling indicates that the san simeon mainshock loaded parts of the rinconada hosgri and san andreas strike slip faults along with the reverse faults of the southern los osos domain all of these loaded faults except for the san andreas experienced a seismicity rate increase at the time of the san simeon mainshock the parkfield earthquake occurred 9 months later on the loaded portion of the san andreas fault the parkfield earthquake unloaded the hosgri fault and the reverse faults of the southern los osos domain which both experienced seismicity rate decreases at the time of the parkfield event although the decreases may be related to the decay of san simeon triggered seismicity coulomb stress unloading from the parkfield earthquake appears to have altered the aftershock decay rate of the southern cluster of san simeon aftershocks which is deficient compared to the expected number of aftershocks from the omori decay parameters based on the pre parkfield aftershocks dynamic stress changes cannot explain the deficiency of aftershocks providing evidence that static stress changes affect earthquake occurrence however a burst of seismicity following the parkfield earthquake at ragged point where the static stress was decreased provides evidence for dynamic stress triggering it therefore appears that both coulomb static stress changes and dynamic stress changes affect the seismicity rate

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

fixed recurrence and slip models better predict earthquake behavior than the time and slip predictable models 1 repeating earthquakes 1 the behavior of individual events in repeating earthquake sequences in california taiwan and japan is better predicted by a model with fixed inter event time or fixed slip than it is by the time and slip predictable models for earthquake occurrence given that repeating earthquakes are highly regular in both inter event time and seismic moment the time and slip predictable models seem ideally suited to explain their behavior taken together with evidence from the companion manuscript that shows similar results for laboratory experiments we conclude that the short term predictions of the time and slip predictable models should be rejected in favor of earthquake models that assume either fixed slip or fixed recurrence interval this implies that the elastic rebound model underlying the time and slip predictable models offers no additional value in describing earthquake behavior in an event to event sense but its value in a long term sense cannot be determined these models likely fail because they rely on assumptions that oversimplify the earthquake cycle we note that the time and slip of these events is predicted quite well by fixed slip and fixed recurrence models so in some sense they are time and slip predictable while fixed recurrence and slip models better predict repeating earthquake behavior than the time and slip predictable models we observe a correlation between slip and the preceding recurrence time for many repeating earthquake sequences in parkfield california this correlation is not found in other regions and the sequences with the correlative slip predictable behavior are not distinguishable from nearby earthquake sequences that do not exhibit this behavior

three dimensional inversion of magnetotelluric mt resistivity data from reykjanes high temperature field in sw iceland abstract the resistivity structure of reykjanes is presented derived from a three dimensional 3d model developed in 2016 the model is the final results of a 3d inversion of mt data corrected for static shift by joint inversion with tem soundings at the same location as the mt soundings in the model presented here reysand a total of 136 sounding pairs are used for the inversion the model covers reykjanes area and sandvik potential geothermal area the resistivity structure of a high enthalpy system reflects the thermal alteration of the rock the reykjanes models reveal a conventional resistivity structure for a high enthalpy geothermal system i e a low resistivity cap underlain by a high resistivity core the low resistivity cap reaches surface in the hot spring area gunnuhver and dips down in all directions forming an elongated area of 2 5 km 3 km in strike direction down to 1 km depth bounded by the low resistivity the resistivity of the low resistivity cap is less 3 m and is underlain by a high resistivity core with resistivity of 10 30 m down to 3 km depth at greater depth high resistivity anomalies are prominent with a zone of lower resistivity in strike direction under the main field this zone coincides with an aseismic zone at depth and may indicate a fracture zone with higher temperature and or permeability within the geothermal system comparison to data from the iddp 2 drilling as well as seismic monitoring confirms that suggestion

tremor reveals stress shadowing deep postseismic creep and depth dependent slip recurrence on the lower crustal san andreas fault near parkfield 1 the 2003 magnitude 6 5 san simeon and the 2004 magnitude 6 0 parkfield earthquakes induced small but significant static stress changes in the lower crust on the central san andreas fault where recently detected tectonic tremor sources provide new constraints on deep fault creep processes we find that these earthquakes affect tremor rates very differently consistent with their differing transferred static shear stresses the san simeon event appears to have cast a stress shadow north of parkfield where tremor activity was stifled for 3 6 weeks in contrast the 2004 parkfield earthquake dramatically increased tremor activity rates both north and south of parkfield allowing us to track deep postseismic slip following this event rates initially increased by up to two orders of magnitude for the relatively shallow tremor sources closest to the rupture with activity in some sources persisting above background rates for more than a year we also observe strong depth dependence in tremor recurrence patterns with shallower sources generally exhibiting larger less frequent bursts possibly signaling a transition toward steady creep with increasing temperature and depth

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

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