importance of small earthquakes for stress transfers and earthquake triggering 1 we estimate the relative importance of small and large earthquakes for static stress changes and for earthquake triggering assuming that earthquakes are triggered by static stress changes and that earthquakes are located on a fractal network of dimension d this model predicts that both the number of events triggered by an earthquake of magnitude m and the stress change induced by this earthquake at the location of other earthquakes increase with m as similar to 10 dm 2 the stronger the spatial clustering the larger the influence of small earthquakes on stress changes at the location of a future event as well as earthquake triggering if earthquake magnitudes follow the gutenberg richter law with b d 2 small earthquakes collectively dominate stress transfer and earthquake triggering because their greater frequency overcomes their smaller individual triggering potential using a southern california catalog we observe that the rate of seismicity triggered by an earthquake of magnitude m increases with m as 10 alpha m where alpha 1 05 0 05 we also find that the magnitude distribution of triggered earthquakes is independent of the triggering earthquake s magnitude m when alpha approximate to b small earthquakes are roughly as important to earthquake triggering as larger ones we evaluate the fractal correlation dimension d of hypocenters using two relocated catalogs for southern california the value of d measured for distances 0 1 r 5 km is d 1 54 for the shearer et al catalog and d 1 73 for the hauksson et al catalog the value of d reflects both the structure of the fault network and the nature of earthquake interactions by considering only those earthquake pairs with interevent times larger than 1000 days we can largely remove the effects of short term clustering then d approximate to 2 close to the value d 2 alpha 2 1 predicted by assuming that earthquake triggering is due to static stress the value d approximate to 2b implies that small earthquakes are as important as larger ones for stress transfers between earthquakes and that considering stress changes induced by small earthquakes should improve models of earthquake interactions

nontrivial decay of aftershock density with distance in southern california the decay of the aftershock density with distance plays an important role in the discussion of the dominant underlying cause of earthquake triggering here we provide evidence that its form is more complicated than typically assumed and that in particular a transition in the power law decay occurs at length scales comparable to the thickness of the crust this is supported by an analysis of a very recent high resolution catalog for southern california sc and surrogate catalogs generated by the epidemic type aftershock sequence etas model which take into account inhomogeneous background activity short term aftershock incompleteness anisotropic triggering and variations in the observational magnitude threshold our findings indicate specifically that the asymptotic decay in the aftershock density with distance is characterized by an exponent larger than 2 which is much bigger than the observed exponent of approximately 1 35 observed for shorter distances ranging from the main shock rupture length up to a length scale comparable to the thickness of the crust this has also important consequences for time dependent seismic hazard assessment based on the etas model

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

larger aftershocks happen farther away nonseparability of magnitude and spatial distributions of aftershocks aftershocks may be driven by stress concentrations left by the main shock rupture or by elastic stress transfer to adjacent fault sections or strands aftershocks that occur within the initial rupture may be limited in size because the scale of the stress concentrations should be smaller than the primary rupture itself on the other hand aftershocks that occur on adjacent fault segments outside the primary rupture may have no such size limitation here we use high precision double difference relocated earthquake catalogs to demonstrate that larger aftershocks occur farther away than smaller aftershocks when measured from the centroid of early aftershock activity a proxy for the initial rupture aftershocks as large as or larger than the initiating event nucleate almost exclusively in the outer regions of the aftershock zone this observation is interpreted as a signature of elastic rebound in the earthquake catalog and can be used to improve forecasting of large aftershocks

earthquake clusters in southern california i identification and stability 1 we use recent results on statistical analysis of seismicity to present a robust method for comprehensive detection and analysis of earthquake clusters the method is based on nearest neighbor distances of events in space time energy domain the method is applied to a 1981 2011 relocated seismicity catalog of southern california having 111 981 events with magnitudes m 2 and corresponding synthetic catalogs produced by the epidemic type aftershock sequence etas model analysis of the etas model demonstrates that the cluster detection results are accurate and stable with respect to 1 three numerical parameters of the method 2 variations of the minimal reported magnitude 3 catalog incompleteness and 4 location errors application of the method to the observed catalog separates the 111 981 examined earthquakes into 41 393 statistically significant clusters comprised of foreshocks mainshocks and aftershocks the results reproduce the essential known statistical properties of earthquake clusters which provide overall support for the proposed technique in addition systematic analysis with our method allows us to detect several new features of seismicity that include 1 existence of a significant population of single event clusters 2 existence of foreshock activity in natural seismicity that exceeds expectation based on the etas model and 3 dependence of all cluster properties except area on the magnitude difference of events from mainshocks but not on their absolute values the classification of detected clusters into several major types generally corresponding to singles burst like and swarm like sequences and correlations between different cluster types and geographic locations is addressed in a companion paper

foreshock activity related to enhanced aftershock production foreshock activity sometimes precedes the occurrence of large earthquakes but the nature of this seismicity is still debated and whether it marks transient deformation and or slip nucleation is still unclear we here study at the worldwide scale how foreshock occurrence affects the postseismic phase and find a significant positive correlation between foreshock and aftershock activities earthquakes preceded by accelerating seismicity rates produce 40 more aftershocks on average and the length of the aftershock zone after 20 days is 20 larger these observations cannot be reproduced by standard earthquake clustering models that predict the accelerating pattern of foreshock occurrence but not its impact on aftershock activity this strongly suggests that slow deformation transients possibly related to episodic creep could initiate prior to the main shock and extend past the coseismic phase resulting in compound ruptures that include a very long period up to tens of days component

a common origin for aftershocks foreshocks and multiplets we demonstrate that the statistics of earthquake data in the global cen troid moment tensor cmt and national earthquake information center neic catalogs and local california council of the national seismic system cnss catalog are consistent with the idea that a single physical triggering mechanism is responsible for the occurrence of aftershocks foreshocks and multiplets specifically we test the hypothesis that tectonic earthquakes usually show clustering only as a result of an initial earthquake triggering subsequent ones and that the magnitude of each trig gered earthquake is entirely independent of the magnitude of the triggering earth quake therefore a certain percentage of the time as determined by the gutenberg richter magnitude frequency relationship an earthquake should by chance be larger than or comparable in size to the earthquake that triggered it this hypothesis predicts that the number of times foreshocks or multiplets are observed should be a fixed fraction of the number of aftershock observations we find that this is indeed the case in the global cmt and neic catalogs the average ratios between foreshock aftershock and multiplet rates are consistent with what would be predicted by the gutenberg richter relationship with b 1 we give special attention to the solomon islands where it has been claimed that unique fault structures lead to unusually high numbers of multiplets we use monte carlo trials to demonstrate that the solomon islands multiplets may be explained simply by a high regional aftershock rate and earthquake density we also verify our foreshock results from the more complete recordings of small earthquakes available in the california catalog and find that foreshock rates for a wide range of foreshock and mainshock magnitudes can be projected from aftershock rates using the gutenberg richter relationship with b 1 and the relationship that the number of earthquakes triggered varies with triggering earthquake magnitude m as c10m where c is a productivity constant and is equal to 1 finally we test an alternative model that proposes that foreshocks do not trigger their mainshocks but are instead triggered by the mainshock nucleation phase in this model the nucleation phase varies with mainshock magnitude so we would expect mainshock magnitude to be correlated with the magnitude number or spatial extent of the foreshocks we find no evidence for any of these correlations

crustal earthquake bursts in california and japan their patterns and relation to volcanoes 1 we analyze 153 bursts of earthquakes in southern california and japan the burst patterns are similar in southern california and japan they fill a spectrum between swarm like sequences without obvious mainshocks and mainshocks with omori law abiding aftershocks in agreement with our previous work the swarm like sequences in japan have more events are more voluminous and tend to expand with time when compared to mainshock aftershock type sequences in both regions we find that the sequences starting with their largest events tend to be much shorter in duration bursts within 50 km of volcanoes are similar in character to those elsewhere except they tend to have longer duration we hypothesize that swarminess is a proxy for fluid pressure redistribution and or aseismic slip driving the seismicity bursts and conversely the mainshock aftershock style sequences have end member behavior that results solely from a cascade of elastic failures the complexity of the spatial seismicity distribution does not correlate with the style of swarm observed indicating that fluid conditions and composition are likely more influential than geometry in determining the patterns we observe

adaptive spatiotemporal smoothing of seismicity for long term earthquake forecasts in california we present new methods for time independent earthquake forecasting that employ space time kernels to smooth seismicity the major advantage of the methods is that they do not require prior declustering of the catalog circumventing the relatively subjective choice of a declustering algorithm past earthquakes are smoothed in space and time using adaptive gaussian kernels the bandwidths in space and time associated with each event are a decreasing function of the seismicity rate at the time and location of each earthquake this yields a better resolution in space time volumes of intense seismicity and a smoother density in volumes of sparse seismicity the long term rate in each spatial cell is then defined as the median value of the temporal history of the smoothed seismicity rate in this cell to calibrate the model the earthquake catalog is divided into two parts the early part the learning catalog is used to estimate the model and the latter one the target catalog is used to compute the likelihood of the model s forecast we optimize the model s parameters by maximizing the likelihood of the target catalog to estimate the kernel bandwidths in space and time we compared two approaches a coupled near neighbor method and an iterative method based on a pilot density we applied these methods to californian seismicity and compared the resulting forecasts with our previous method based on spatially smoothing a declustered catalog werner et al 2011 all models use small m 2 earthquakes to forecast the rate of larger earthquakes and use the same learning catalog our new preferred model slightly outperforms our previous forecast providing a probability gain per earthquake of about 5 relative to a spatially uniform forecast

automatic reconstruction of fault networks from seismicity catalogs three dimensional optimal anisotropic dynamic clustering 1 we propose a new pattern recognition method that is able to reconstruct the three dimensional structure of the active part of a fault network using the spatial location of earthquakes the method is a generalization of the so called dynamic clustering or k means method that partitions a set of data points into clusters using a global minimization criterion of the variance of the hypocenters locations about their center of mass the new method improves on the original k means method by taking into account the full spatial covariance tensor of each cluster in order to partition the data set into fault like anisotropic clusters given a catalog of seismic events the output is the optimal set of plane segments that fits the spatial structure of the data each plane segment is fully characterized by its location size and orientation the main tunable parameter is the accuracy of the earthquake locations which fixes the resolution i e the residual variance of the fit the resolution determines the number of fault segments needed to describe the earthquake catalog the better the resolution the finer the structure of the reconstructed fault segments the algorithm successfully reconstructs the fault segments of synthetic earthquake catalogs applied to the real catalog constituted of a subset of the aftershock sequence of the 28 june 1992 landers earthquake in southern california the reconstructed plane segments fully agree with faults already known on geological maps or with blind faults that appear quite obvious in longer term catalogs future improvements of the method are discussed as well as its potential use in the multiscale study of the inner structure of fault zones

average stress drops of southern california earthquakes in the context of crustal geophysics implications for fault zone healing to understand how fault healing processes affect earthquake stress drops we search for a possible dependency of stress drops on crustal conditions and geophysical parameters we reanalyze the stress drop values of 60 000 earthquakes in southern california which were originally determined by sheareret al j geophys res 111 b06303 2006 using a spectral method we modify the dataset to include only stress drops that are derived from at least 10 spectra and with corner frequencies between 3 and 30 hz and correct the rupture velocity for increasing s wave speed with depth we see no dependence of stress drop on moment magnitude or depth except for a small poorly determined increase from 15 to 25 km we use six crustal geophysics parameters to search for obvious correlations that may explain changes in the mean values of the stress drops 1 crustal thickness 2 isostatic gravity 3 heat flow 4 shear strain rate 5 crustal stress regime and 6 style of faulting none of the variables reduce the scatter but most can explain up to 10 20 variations in the mean stress drops the geographical distribution of the grouped mean stress drops includes very high stress drops near ridgecrest eastern california as well as near fault jogs within the san andreas fault system low stress drops dominate in trans tensional regions heat flow and gps based shear strain rate estimates have the largest influence on stress drop variations in the range of low to medium heat flow the stress drops increase with increasing heat flow in contrast at high heat flow in thin crust the stress drops decrease systematically with increasing heat flow increasing shear strain rate systematically correlates with decreasing stress drops the crustal stress regime and style of faulting also influence the stress drops as demonstrated by lower stress drops for north northeast trending principal horizontal stress and in areas of dip slip faulting the mean variations in stress drops with heat flow stress regime crustal thickness and density can be explained in the context of fault healing grain boundary growth and corresponding increase in fault zone strength on time scales modulated by the tectonic shear strain rate

properties of aftershock sequences in southern california the temporal behavior of 39 aftershock sequences in southern california 1933 1988 was modeled by the modified omori relation minimum magnitudes for completeness of each sequence catalog were determined and the maximum likelihood estimates of the parameters k p and c with the standard errors on each were determined by the ogata algorithm the b value of each sequence was also calculated many of the active faults in the region both strike slip and thrust were sampled the p values were graded in terms of the size of the standard error relative to the p value itself most of the sequences were modeled well by the omori relation many of the sequences had p values close to the mean of the whole data set 1 11 0 25 but values significantly different from the mean as low as 0 7 and as high as 1 8 exist no correlation of p with either the b value of the sequence or the mainshock magnitude was found the results suggest a direct correlation of p values is with surface heat flow with high values in the salton trough high heat flow and one low value in the san bernardino mountains and on the edge of the ventura basin both low heat flow the large fraction of the sequences with p values near the mean are at locations where the heat flow is near the regional mean 74 mw m2 if the hypothesis that aftershock decay rate is controlled by temperature at depth is valid the effects of other factors such as heterogeneity of the fault zone properties are superimposed on the background rate determined by temperature surface heat flow is taken as an indicator of crustal temperature at hypocentral depths but the effects on heat flow of convective heat transport and variations in near surface thermal conductivity invalidate any simple association of local variations in heat flow with details of the subsurface temperature distribution the interpretation is that higher temperatures in the aftershock source volume caused shortened stress relaxation times in the fault zone materials leading to a faster decay rate higher p value in 1967 mogi proposed such a relation on the basis of japanese data seismic velocity distributions published for southern california generally support the hypothesis with low velocities higher temperatures corresponding to high p values and vice versa

static stress triggering explains the empirical aftershock distance decay the shape of the spatial aftershock decay is sensitive to the triggering mechanism and thus particularly useful for discriminating between static and dynamic stress triggering for california seismicity it has been recently recognized that its form is more complicated than typically assumed consisting of three different regimes with transitions at the scale of the rupture length and the thickness of the crust the intermediate distance range is characterized by a relative small decay exponent of 1 35 previously declared to relate to dynamic stress triggering we perform comprehensive simulations of a simple clock advance model in which the number of aftershocks is just proportional to the coulomb stress change to test whether the empirical result can be explained by static stress triggering similarly to the observations the results show three scaling regimes for simulations adapted to the depths and focal mechanisms observed in california we find a remarkable agreement with the observation over the whole distance range for a fault distribution with fractal dimension of 1 8 which is shown to be in good agreement with an independent analysis of california seismicity

parameterization and simulation of near bed orbital velocities under irregular waves in shallow water a set of empirical formulations is derived that describe important wave properties in shallow water as functions of commonly used parameters such as wave height wave period local water depth and local bed slope these wave properties include time varying near bed orbital velocities and statistical properties such as the distribution of wave height and wave period empirical expressions of characteristic wave parameters are derived on the basis of extensive analysis of field data using recently developed evolutionary algorithms the field data covered a wide range of wave conditions though there were few conditions with wave periods greater than 15 s comparison with field measurements showed good agreement both on a time scale of a single wave period as well as time averaged velocity moments

fluid induced earthquakes with variable stress drop the static stress drop of an earthquake which quantifies the ratio of seismic slip to the size of the rupture is almost constant over several orders of magnitudes although variations are often observed it is difficult however to attribute these variations either to a well defined phenomenon or simply to measurement uncertainty in this study we analyze the static stress drop of earthquakes that occurred during a water circulation test in the soultz sous forets france geothermal reservoir in 2010 during this circulation test 411 earthquakes were recorded the largest event having a magnitude md2 3 we show that several earthquakes in the reservoir can be combined into groups of closely located similar repeating waveforms we infer that the amplitudes and hence magnitudes vary between the repeaters although the waveforms and spectra are both similar in shape we measure similar corner frequencies for these events despite their different magnitudes suggesting a similar rupture size our results imply that events at the same location may exhibit stress drop variations by as much as a factor of 300 we interpret that this variation in stress drop is caused by fluid pressure at the interface reducing the normal stress we also hypothesize that the observed variations reflect a transition from stable to unstable slip on the imaged asperities

detection and characterization of transient forcing episodes affecting earthquake activity in the aleutian arc system abstract crustal slow deformation transients can be caused by fluid or magmatic intrusions and by slow slip on faults they can affect earthquake dynamics if they occur close to or within seismically active zones we here further develop and test a statistical method for detecting and characterizing seismicity anomalies that is only based on earthquake occurrence times and locations we make use of this method to analyze the 2004 2013 seismicity at m c 3 5 in the aleutian subduction system to find six statistically significant anomalies with typical 1 day duration and 30 to 50 km size that are likely related to slow deformation transients they tend to be located in zones characterized by intermediate seismic coupling and to mark the termination of past large to mega thrust earthquakes these anomalies account for a non negligible 9 part of the total activity proving that non stationary aseismic loading plays an important role in the dynamics of crustal deformation

high resolution long term and short term earthquake forecasts for california we present two models for estimating the probabilities of future earth quakes in california to be tested in the collaboratory for the study of earthquake predictability csep the first is a time independent model of adaptively smoothed seismicity that we modified from helmstetter et al 2007 the model provides five year forecasts for earthquakes with magnitudes m 4 95 we show that large earthquakes tend to occur near the locations of small m 2 events so that a high resolution estimate of the spatial distribution of future large quakes is obtained from the locations of the numerous small events we further assume a universal gutenberg richter magnitude distribution in retrospective tests we show that a poisson distri bution does not fit the observed rate variability in contrast to assumptions in current earthquake predictability experiments we therefore issued forecasts using a better fitting negative binomial distribution for the number of events the second model is a time dependent epidemic type aftershock sequence etas model that we modified from helmstetter et al 2006 and that provides next day forecasts for m 3 95 in this model the forecasted rate is the sum of a background rate propor tional to the time independent model rate and of the expected rate of triggered events due to all prior earthquakes each earthquake triggers events with a rate that increases exponentially with its magnitude and decays in time according to the omori utsu law an isotropic kernel models the spatial density of aftershocks for small m 5 5 events while for larger quakes we smooth early aftershocks to forecast later events we estimate parameter values by optimizing retrospective forecasts and find that the short term model realizes a probability gain of about 6 0 per earthquake over the time independent model online material identification of explosions and etas parameters

on the characteristics of earthquake stress release variations in japan abstract the amount of stress released during earthquakes is a fundamental characteristic of the earthquake rupture process as such it represents a key parameter for improving our understanding of earthquake source physics and for reliable ground motion prediction large earthquake populations usually show variations in stress release as large as three orders of magnitude but the underlying mechanisms have remained largely elusive and particularly the dependence of stress release on earthquake size is still a matter of debate here i use a unique dataset from japan encompassing earthquakes of a wide magnitude range and spatial coverage to show that stress release variations of crustal earthquakes are strongly correlated with heat flow variations indicating that they are thermally controlled in contrast subcrustal events depict highest stress release in regions of strong subduction interface coupling and overall less pronounced variability as compared with crustal earthquakes stress release is overall only weakly dependent on earthquake size but at local scales i e within individual earthquake sequences the dependence can be very strong and apparently varies with stress regime accounting for these systematic variations reduces the stress release variability on local scale by a factor of two to three as compared with the full earthquake population a finding that is of key significance in the endeavor to reduce the uncertainties in future ground motion predictions

mechanisms controlling the complete accretionary beach state sequence accretionary down state beach sequence is a key element of observed nearshore morphological variability along sandy coasts we present and analyze the first numerical simulation of such a sequence using a process based morphodynamic model that solves the coupling between waves depth integrated currents and sediment transport the simulation evolves from an alongshore uniform barred beach storm profile to an almost featureless shore welded terrace summer profile through the highly alongshore variable detached crescentic bar and transverse bar rip system states a global analysis of the full sequence allows determining the varying contributions of the different hydro sedimentary processes sediment transport driven by orbital velocity skewness is critical to the overall onshore sandbar migration while gravitational down slope sediment transport acts as a damping term inhibiting further channel growth enforced by rip flow circulation accurate morphological diffusivity and inclusion of orbital velocity skewness opens new perspectives in terms of morphodynamic modeling of real beaches

spatial variability of seismicity parameters in aftershock zones the spatial variability of the b value of the frequency magnitude relationship and the decay rate of aftershocks as described by the p value of the modified omori law is investigated by using dense spatial grids we map out the distribution of b and p values within the landers northridge morgan hill and kobe aftershock sequences considerable spatial variability is found with b values of independent subvolumes ranging from 0 6 to 1 4 and p values ranging from 0 6 to 1 8 these systematic and statistically highly significant differences argue that it is an oversimplification to assign one single p and b value to an aftershock sequence that extends up to 100 km the spatial distribution of these two parameters is compared with the slip distribution during the mainshock suggesting that the areas of largest slip release correlate with high b value regions we hypothesize that the frictional heat created during the event may influence the p value distribution within an aftershock zone while applied shear stress crack density and pore pressure govern the frequency magnitude distribution by investigating the frequency magnitude distribution separately for preseismic and postseismic periods for the morgan hill mainshock we find that only the volume in the vicinity of the highest slip release shows a significant increase in the b value which decays to premainshock values within a year surrounding areas of the aftershock zone show an approximately constant b value with time because the aftershock hazard after a mainshock depends strongly on both the b and p value we propose that aftershock hazard assessment can be improved by taking into account the spatial distribution of the parameters

earthquake scaling characteristics and the scale in dependence of seismic energy to moment ratio insights from kik net data in japan 1 we investigate earthquake source characteristics and scaling properties using the results of a spectral inversion of more than 29 000 accelerometric borehole recordings from 1 826 earthquakes mjma 2 7 8 throughout japan we find that the calculated source spectra can be well characterized by the omega square model and show on average self similar scaling over the entire magnitude range with median stress drops of 1 1 and 9 2 mpa for crustal and subcrustal events respectively the seismic energy to moment ratio as theoretically expected if the omega square model is valid shows a strong dependency on stress drop only which in conjunction with data selection practice in some studies to cope with limited recording bandwidth can explain the often observed apparent scale dependence our observations suggest that there is no significant deviation from similarity of the energy radiation in the investigated magnitude range and that the observed scatter is mainly related to the scatter in stress drop

variability in earthquake stress drop and apparent stress 1 we apply empirical green s function coda based analysis to four earthquake sequences in japan that span a magnitude range of 1 8 to 6 9 to measure radiated energy corner frequency and stress drop we find no systematic dependence of apparent stress or stress drop on seismic moment for these sequences and find they both are log normally distributed however we identify several anomalous events both energetic and enervated that show sharply different spectral signatures from the rest of the population these events indicate that much of the variation in apparent stress and stress drop is statistically significant which may have important implications for seismic hazard analysis

comprehensive analysis of earthquake source spectra and swarms in the salton trough california 1 we study earthquakes within california s salton trough from 1981 to 2009 from a precisely relocated catalog we process the seismic waveforms to isolate source spectra station spectra and travel time dependent spectra the results suggest an average p wave q of 340 agreeing with previous results indicating relatively high attenuation in the salton trough stress drops estimated from the source spectra using an empirical green s function egf method reveal large scatter among individual events but a low median stress drop of 0 56 mpa for the region the distribution of stress drop after applying a spatial median filter indicates lower stress drops near geothermal sites we explore the relationships between seismicity stress drops and geothermal injection activities seismicity within the salton trough shows strong spatial clustering with 20 distinct earthquake swarms with at least 50 events they can be separated into early mmax and late mmax groups based on the normalized occurrence time of their largest event these swarms generally have a low skew value of moment release history ranging from 9 to 3 0 the major temporal difference between the two groups is the excess of seismicity and an inverse power law increase of seismicity before the largest event for the late mmax group all swarms exhibit spatial migration of seismicity at a statistical significance greater than 85 a weighted l1 norm inversion of linear migration parameters yields migration velocities from 0 008 to 0 8 km hour to explore the influence of fluid injection in geothermal sites we also model the migration behavior with the diffusion equation and obtain a hydraulic diffusion coefficient of approximately 0 25 m2 s for the salton sea geothermal site which is within the range of expected values for a typical geothermal reservoir the swarms with migration velocities over 0 1 km hour cannot be explained by the diffusion curve rather their velocity is consistent with the propagation velocity of creep and slow slip events these variations in migration behavior allow us to distinguish among different driving processes

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

infragravity wave modulation of short wave celerity in the surf zone the cross shore evolution of individual wave celerity is investigated using two high resolution laboratory experiments on bichromatic waves individual waves are tracked during their onshore propagation and their characteristics including celerity are estimated the intrawave variability in celerity is low in the shoaling zone but increases strongly after breaking it is maximum when the infragravity wave height to water depth ratio is the largest that is to say close to the shoreline there the observed range of individual wave celerity can be as large as the mean celerity value this variability can be largely explained by the variations in water depth and velocity induced by the infragravity waves the differences in celerity are such that they lead to the merging of the waves in the inner surf zone for most of the wave conditions considered again the location at which the first waves start merging strongly correlates with the infragravity wave height to water depth ratio the consequences of these findings for celerity based depth inversion techniques are finally discussed surprisingly accounting for the infragravity wave modulation of the velocity field in the celerity estimate does not significantly improve depth estimation in the surf zone however it is shown that the occurrence of bore merging decreases significantly the coherence of the wavefield in the surf zone this loss of coherence could hamper celerity estimation from pixel intensity time series and explain at least partly the relatively poor performance of depth inversion techniques in the inner surf zone

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

improving the parameterization of wave nonlinearities the importance of wave steepness spectral bandwidth and beach slope abstract wave velocity nonlinearities are among the main drivers of sediment transport for practical engineering purposes they can be described by simple parameterizations that allow their easier inclusion in nearshore morphodynamic models most existing parameterizations propose the estimation of velocity nonlinearities only from local wave parameters such as the ursell number herein it is demonstrated that this provides inaccurate estimations of the wave nonlinearities furthermore the effect of offshore wave steepness offshore spectral bandwidth and beach slope on the velocity nonlinearities is shown to be sufficiently important to merit its inclusion in the existing parameterizations ruessink et al 2012 28 parameterization is modified in order to include both offshore spectral bandwidth and a new parameter np 0 which takes into account the beach slope and the squared offshore wave steepness the new parameterization results in a reduction of the wave nonlinearities estimation error of more than 50 particularly for the maximum values of nonlinearity near breaking that contribute the most for sediment transport

aftershock patterns of ms 7 earthquakes in the india asia collision belt anomalous results from the muzaffarabad earthquake sequence kashmir 2005 the seismicity rate triggered by the muzaffarabad kashmir 2005 m w 7 6 m s 7 7 earthquake is above the average when analyzing the aftershock rate of the 18 m s 7 0 events in the india asia collision belt between 1973 and 2008 all 17 m s 7 0 events occurred within 10 latitude and 20 longitude distance from the 2005 muzaffarabad earthquake epicenter we determined the huge aftershock productivity for this earthquake is not driven by a specific omori law parameter this anomaly in aftershock rate value corresponds to a relatively higher rate decay as measured by the p value and a larger density value in a higher background seismicity rate setting prestress conditions than the other 17 sequences respectively after normalizing by the mainshock size and by the magnitude range of observation we resolved the anomalies with a 95 confidence level for five aftershock sequences for either the duration the productivity the density or the background seismicity rate when we did not include the local processes that drive these specific anomalies two other global results emerged for the averaged data from the 18 m s 7 0 aftershock sequences first for all sequences background seismicity and duration are anticorrelated graphic 1 within the framework of rate and state friction law this result supports the assertion that the observed background seismicity rate can be used as a proxy for the stressing rate dieterich 1994 second we robustly resolve the dependence of aftershock productivity on the mainshock faulting styles for example the aftershock productivity for strike slip mainshocks is on average four times smaller than the productivity of thrust faulting mainshocks 1 embed inline graphic 1 gif

the aftershock signature of supershear earthquakes recent studies show that earthquake faults may rupture at speeds exceeding the shear wave velocity of rocks this supershear rupture produces in the ground a seismic shock wave similar to the sonic boom produced by a supersonic airplane this shock wave may increase the destruction caused by the earthquake we report that supershear earthquakes are characterized by a specific pattern of aftershocks the fault plane itself is remarkably quiet whereas aftershocks cluster off the fault on secondary structures that are activated by the supershear rupture the post earthquake quiescence of the fault shows that friction is relatively uniform over supershear segments whereas the activation of off fault structures is explained by the shock wave radiation which produces high stresses over a wide zone surrounding the fault

video based depth inversion techniques a method comparison with synthetic cases abstract applications of video based depth inversion in the near shore coastal environment are growing in numbers video based capabilities in nearshore monitoring are improving and coastal monitoring programs are expanding due to greater availability and reduced costs video derived beach state indicators such as beach width bar position and wave and current parameters are supplemented by accurate depth estimations through inversion video based depth inversion knows two main approaches a spectral and temporal method of celerity estimation the two methods have so far never been compared as video systems are often tailored for the chosen celerity estimation method here a spectral and temporal method are compared using controlled synthetic datasets obtained using the serre1d boussinesq model to estimate celerity and invert depth the assessment is carried out on a set of wave boundary conditions with over linear and barred bottom profile both methods invert depth with a similar accuracy for the most realistic jonswap cases an evident correlation is found between wave skewness non linearity and depth estimation error linked to the limits of the linear dispersion relation a residual sensing error is linked to method based parameters and a changing wave shape as incident waves propagate inshore the inversion error can be reduced significantly including a wave height dependent non linear correction importantly a method based error is introduced for the temporal method to increase the suitability for data assimilation likewise the spectral method has its own existing depth error estimation to feed into the kalman filter however these method based error estimates show very weakly or no relation to the observed error between estimated cross shore profile and bottom profile used for the model input

on the use of the radon transform in studying nearshore wave dynamics in the nearshore describing the complex individual wave dynamics remains a key challenge in this paper we test the ability of the radon transform to produce estimates of individual wave celerities and to separate incoming and outgoing waves conserving the temporal characteristics the radon transform is a projection of a two dimensional field into polar space oblique features such as propagating crests in a spatio temporal space are identified with density peaks in the polar space in this paper the radon transform is applied to synthetic test cases including a wide range of beach slopes and wave conditions the radon transform shows good skills at estimating individual celerity and separating incoming and outgoing components with a relative rms error lower than 10 even a standing wave node the accuracy is fairly insensitive to wave characteristics whereas the main limitations rise from the sampling scheme and are the number and density of wave gauges the distance between gauges should be less than one third of the shortest wavelength while the set of gauges should cover more than one third of the longest wavelength

earthquake scaling relations for mid ocean ridge transform faults 1 a mid ocean ridge transform fault rtf of length l slip rate v and moment release rate can be characterized by a seismic coupling coefficient ae at where ae v is an effective seismic area and at l3 2v 1 2 is the area above an isotherm tref a global set of 65 rtfs with a combined length of 16 410 km is well described by a linear scaling relation 1 ae at which yields 0 15 0 05 for tref 600 c therefore about 85 of the slip above the 600 c isotherm must be accommodated by subseismic mechanisms and this slip partitioning does not depend systematically on either v or l rtf seismicity can be fit by a truncated gutenberg richter distribution with a slope 2 3 in which the cumulative number of events n0 and the upper cutoff moment mc dcac depend on at data for the largest events are consistent with a self similar slip scaling dc ac1 2 and a square root areal scaling 2 ac at1 2 if relations 1 and 2 apply then moment balance requires that the dimensionless seismic productivity 0 0 atv should scale as 0 at 1 4 which we confirm using small events hence the frequencies of both small and large earthquakes adjust with at to maintain constant coupling rtf scaling relations appear to violate the single mode hypothesis which states that a fault patch is either fully seismic or fully aseismic and thus implies ac ae the heterogeneities in the stress distribution and fault structure responsible for relation 2 may arise from a thermally regulated dynamic balance between the growth and coalescence of fault segments within a rapidly evolving fault zone

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

how variable is the number of triggered aftershocks aftershock activity depends at first order on the mainshock magnitude but also shows important fluctuations between shocks of equal magnitude we here investigate these fluctuations by quantifying them and by relating them to the mainshock stress drop and other variables for southern california earthquakes a method is proposed in order to only count directly triggered aftershocks rather than secondary aftershocks i e triggered by previous aftershocks and to only quantify fluctuations going beyond the natural poisson variability testing of the method subjected to various model errors allows to quantify its robustness it is found that these fluctuations follow a distribution that is well fitted by a log normal distribution with a coefficient of variation of about 1 0 to 1 1 a simple model is proposed to relate this observed dependence to mainshock stress drop variability

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

mainshocks are aftershocks of conditional foreshocks how do foreshock statistical properties emerge from aftershock laws 1 the inverse omori law for foreshocks discovered in the 1970s states that the rate of earthquakes prior to a mainshock increases on average as a power law 1 tc t p of the time to the mainshock occurring at tc here we show that this law results from the direct omori law for aftershocks describing the power law decay 1 t tc p of seismicity after an earthquake provided that any earthquake can trigger its suit of aftershocks in this picture the seismic activity at any time is the sum of the spontaneous tectonic loading and of the activity triggered by all preceding events weighted by their corresponding omori law the inverse omori law then emerges as the expected in a statistical sense trajectory of seismicity conditioned on the fact that it leads to the burst of seismic activity accompanying the mainshock in particular we predict and verify by numerical simulations on the epidemic type aftershock sequence etas model that p is always smaller than or equal to p and a function of p of the b value of the gutenberg richter law gr and of a parameter quantifying the number of direct aftershocks as a function of the magnitude of the mainshock the often documented apparent decrease of the b value of the gr law at the approach to the mainshock results straightforwardly from the conditioning of the path of seismic activity culminating at the mainshock however we predict that the gr law is not modified simply by a change of b value but that a more accurate statement is that the gr law gets an additive or deviatoric power law contribution with exponent smaller than b and with an amplitude growing as a power law of the time to the mainshock in the space domain we predict that the phenomenon of aftershock diffusion must have its mirror process reflected into an inward migration of foreshocks toward the mainshock in this model foreshock sequences are special aftershock sequences which are modified by the condition to end up in a burst of seismicity associated with the mainshock foreshocks are not just statistical creatures but are genuine forerunners of large shocks as shown by the large prediction gains obtained using several of their qualifiers

non hydrostatic modelling of infragravity waves under laboratory conditions the non hydrostatic wave model swash is compared to flume observations of infragravity waves propagating over a plane slope and barred beach the experiments cover a range of infragravity wave conditions including forcing by bichromatic and irregular waves varying from strongly dissipative to strongly reflective so that model performance can be assessed for a wide range of conditions the predicted bulk wave parameters such as wave height and mean wave period are found to be in good agreement with the observations moreover the model captures the observed breaking of infragravity waves these results demonstrate that swash can be used to model the nearshore evolution of infragravity waves including nonlinear interactions dissipation and shoreline reflections

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

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

impact of earthquake rupture extensions on parameter estimations of point process models abstract stochastic point processes are widely applied to model spatiotemporal earthquake occurrence in particular the epidemic type aftershock sequence etas model has been shown to successfully reproduce the short term clustering of earthquakes an important parameter of the model is the value describing the scaling of the aftershock productivity with magnitude of the triggering earthquake according to 10 m fitting of the space dependent etas model to empirical data yields values that are typically much smaller than the scaling inverted from more simple stacking of aftershock sequences we show by means of synthetic simulations that this is likely to result from assuming spatial isotropy of aftershock occurrence that in fact aligns along the mainshock rupture we fit the space dependent and space independent etas models to simulations where each earthquake is a line source with an empirical magnitude length relation although the space time model describes past activity quite well it overestimates the forecasted earthquake rate on the other hand the application of the space independent etas model predicts future seismicity well and can therefore be applied for forecasting purposes our test for the observed aftershock sequence following the 1992 m 7 3 landers earthquake supports these results

onshore sandbar migration in the surf zone new insights into the wave induced sediment transport mechanisms we present a novel process based morphodynamic model which includes transport processes due to both velocity and acceleration skewness and a new formulation for intrawave motions that successfully simulates observations of onshore sandbar migration results confirm findings of previous studies in which each process was considered separately and in which sediment transport was computed from the observed water motion however our results indicate that accounting for the joint action of both velocity and acceleration skewnesses causes major improvement of the modeled onshore bar migration and is essential to accurately model the evolution of the entire cross shore bottom profile when compared with observations we also demonstrate that the morphodynamics in the shoaling zone are dominated by velocity skewness bed shear stresses while sediment transport induced by acceleration skewness pressure gradients controls the morphodynamics in the inner surf zone

common dependence on stress for the two fundamental laws of statistical seismology the gutenberg richter relation and the omori utsu law both power laws are two of the long standing relationships of statistical seismology here aftershock sequences are described according to the faulting style of their main shocks showing that the time delay before the onset of the power law aftershock decay rate is on average shorter for thrust main shocks than for normal fault earthquakes these similar dependences on the faulting style indicate that both the fundamental power laws are governed by the state of stress

multifractal analysis of earthquakes multifractal properties of the epicenter and hypocenter distribution and also of the energy distribution of earthquakes are studied for california japan and greece the calculateddq q curves the generalized dimension indicate that the earthquake process is multifractal or heterogeneous in the fractal dimension japanese earthquakes are the most heterogeneous and californian earthquakes are the least since the earthquake process is multifractal a single value of the so called fractal dimension is not sufficient to characterize the earthquake process studies of multifractal models of earthquakes are recommended temporal changes of thedq q curve are also obtained for californian and japanese earthquakes thedq q curve shows two distinctly different types in each region the gentle type and the steep type the steeptype corresponds to a strongly heterogeneous multifractal which appears during seismically active periods when large earthquakes occur dq for smallq or negativeq is considerably more sensitive to the change in fractal structure of earthquakes thandq forq 2 we recommend use ofdq at smallq to detect the seismicity change in a local area

comparison of short term and time independent earthquake forecast models for southern california we have initially developed a time independent forecast for southern california by smoothing the locations of magnitude 2 and larger earthquakes we show that using small m 2 earthquakes gives a reasonably good prediction of m 5 earthquakes our forecast outperforms other time independent models kagan and jackson 1994 frankel et al 1997 mostly because it has higher spatial resolution we have then developed a method to estimate daily earthquake probabilities in south ern california by using the epidemic type earthquake sequence model kagan and knopoff 1987 ogata 1988 kagan and jackson 2000 the forecasted seismicity rate is the sum of a constant background seismicity proportional to our time independent model and of the aftershocks of all past earthquakes each earthquake triggers aftershocks with a rate that increases exponentially with its magnitude and decreases with time following omori s law we use an isotropic kernel to model the spatial distribution of aftershocks for small m 5 5 mainshocks for larger events we smooth the density of early aftershocks to model the density of future aftershocks the model also assumes that all earthquake magnitudes follow the gutenberg richter law with a uniform b value we use a maximum likelihood method to estimate the model parameters and test the short term and time independent forecasts a retro spective test using a daily update of the forecasts between 1 january 1985 and 10 march 2004 shows that the short term model increases the average probability of an earthquake occurrence by a factor 11 5 compared with the time independent forecast

eulerian measurements of horizontal accelerations in shoaling gravity waves laboratory and field measurements of suspended sediment in the nearshore suggest that fluid accelerations are an important factor in sediment transport by oscillatory waves here eulerian accelerations of the cross shore velocity are calculated from measurements of velocity obtained by an array of bottom mounted electromagnetic flow meters spanning a natural surf zone large shoreward accelerations of brief duration are associated with the steep front faces of both near breaking and breaking waves weaker offshore accelerations of longer duration occur during passage of the more gently sloped rear faces the acceleration field is thus strongly skewed in the shoreward direction power spectra and bispectra indicate as expected that statistics of the acceleration field are significantly influenced by high frequency motions but are rather insensitive to surf beat

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

extending earthquakes reach through cascading earthquakes whatever their size can trigger other earthquakes mainshocks cause aftershocks to occur which in turn activate their own local aftershock sequences resulting in a cascade of triggering that extends the reach of the initial mainshock a long lasting difficulty is to determine which earthquakes are connected either directly or indirectly here we show that this causal structure can be found probabilistically with no a priori model nor parameterization large regional earthquakes are found to have a short direct influence in comparison to the overall aftershock sequence duration relative to these large mainshocks small earthquakes collectively have a greater effect on triggering hence cascade triggering is a key component in earthquake interactions

microseismicity but no tremor accompanying slow slip in the hikurangi subduction zone new zealand abstract geodetically detected episodes of slow slip appear in several subduction zones to be accompanied by bursts of low frequency coherent noise known as seismic tremor but whether a single physical process governs this association or even whether slow slip is invariably accompanied by tremor remains unresolved detailed analysis of broadband seismic data spanning a slow slip episode in the hikurangi subduction zone new zealand reveals that slow slip was accompanied by distinct reverse faulting microearthquakes rather than tremor the timing location and faulting style of these earthquakes are consistent with stress triggering down dip of the slow slip patch either on the subduction interface or just below it these results indicate that tremor is not ubiquitous during subduction zone slow slip and that slow slip in subduction zone environments is capable of triggering high frequency earthquakes near the base of the locked subduction thrust in this and other locations hawaii boso peninsula where slow slip is accompanied by triggered microseismicity the estimated upper extent of the slow slip is shallower less than 20 km than in those locations from which tremor has been reported this suggests that ambient temperature or pressure dependent factors govern the character of the seismic response to slow slip on subduction thrusts and other large faults with rheological or lithological conditions at shallow depths triggering high frequency microearthquakes and those at greater depths triggering seismic tremor