long range triggered earthquakes that continue after the wave train passes 1 large earthquakes can trigger distant earthquakes in geothermal areas some triggered earthquakes happen while the surface waves pass through a site but others occur hours or even days later does this prolonged seismicity require a special mechanism to store the stress from the seismic waves that differs from ordinary aftershock mechanisms these questions have driven studies of long range triggering since the phenomenon s discovery here i attempt to answer the questions by examining the statistics of triggered sequences two separate observations are consistent with the prolonged sequences being simply local aftershocks of earthquakes triggered early in the wave train first the sequences obey omori s law over both short 1 hour and longer 5 day time intervals secondly the number of observed triggered earthquakes in the first hour after the wave train can be predicted from the number of earthquakes triggered during the wave train even the very vigorous 10 day triggering at long valley from the 1992 landers mw 7 3 earthquakes can be interpreted as the aftershocks of either a local m 4 1 earthquake or an equivalent combination of several smaller mainshocks therefore long range triggering does not need to include a mechanism to produce sustained stresses other than the process that generates aftershocks of the earthquakes that occur while the wave train is passing

analysis of foreshock sequences in california and implications for earthquake triggering we analyze foreshock activity in california and compare observations with simulated catalogs based on a branching aftershock triggering model we first examine foreshock occurrence patterns for isolated m ge 5 earthquakes in southern california from 1981 to 2011 and in northern california from 1984 to 2009 among the 64 m ge 5 mainshocks excluding 3 swarms and 3 doubles 53 of the rest are preceded by at least one foreshock within 30 days and 5 km foreshock occurrence appears correlated with mainshock faulting type and depth foreshock area is correlated with the magnitude of the largest foreshock and the number of foreshocks however it is not correlated with mainshock magnitude we then examine the occurrence pattern of all seismicity clusters without a minimum magnitude requirement and the possibility that they are foreshocks of larger mainshocks only about 30 of the small clusters lead to a larger cluster about 66 of the larger clusters have foreshock activities and the spatial distribution pattern is similar to m ge 5 mainshocks with lower occurrence rates in the transverse range and central california and higher occurrence rates in the eastern california shear zone and the bay area these results suggest that foreshock occurrence is largely controlled by the regional tectonic stress field and fault zone properties in special cases foreshock occurrence may be useful for short term forecasting however foreshock properties are not reliably predictive of the magnitude of the eventual mainshock comparison with simulated catalogs suggest that the swarmy features and foreshock occurrence rate in the observed catalogs are not well reproduced from common statistical models of earthquake triggering

detailed observations of california foreshock sequences implications for the earthquake initiation process we find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes in this study we examine in detail the evolution of six california foreshock sequences the 1986 mount lewis ml 5 5 the 1986 chalfant ml 6 4 the 1986 stone canyon ml 4 7 the 1990 upland ml 5 2 the 1992 joshua tree mw 6 1 and the 1992 landers mw 7 3 sequence typically uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution however the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations we use these results to identify small scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock in general these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer instead the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process fault zone heterogeneity may also be important in controlling the number of foreshocks i e the stronger the heterogeneity the greater the number of foreshocks the size of the nucleation region as measured by the extent of the foreshock sequence appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase we also find evidence for slip localization as predicted by some models of earthquake nucleation

occurrence patterns of foreshocks to large earthquakes in the western united states observations of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake unfortunately the apparent randomness of foreshock occurrence they precede some events and not others has severely hampered their use in reliable earthquake prediction understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short term prediction will ever be possible1 here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western united states the incidence of foreshocks decreases with increasing depth of the mainshock and also depends on the mainshock slip orientation this pattern of occurrence may be explained by a decrease in small scale crustal heterogeneity with increasing depth and suggests that increasing normal stress both regional tectonic stress and lithostatic load inhibits the occurrence of foreshocks no relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process or that foreshocks are not part of this process

heterogeneity spectrum and scale anisotropy in the upper crust revealed by the german continental deep drilling ktb holes crustal heterogeneities and their statistical characteristics bear important information about the dynamic processes and evolution of the crust the velocity well log data from the german continental deep drilling project ktb offer a rare opportunity to measure directly the properties of crustal heterogeneities in this study we first estimated the power spectrum of crustal heterogeneities from the p velocity well logs of the two holes for the first time a power law spectrum of crustal heterogeneities was directly observed in the spatial wavelength range from a few meters to a few kilometers the slope of the vertical 1d power spectrum of the random velocity perturbations is about 1 1 corresponding to a flicker noise random medium the scale anisotropy ratio aspect ratio and the horizontal spectral slope of crustal heterogeneities were also estimated by cross correlation analysis between these two holes to be about 1 8 and 2 0 respectively

asymmetric lithospheric instability facilitated by shear modulus contrast implications for shear zones summary viscoplasticity has been considered to be a dominant element in causing the nucleation of shear instability leading to lithospheric weakening here we propose that a simple contrast in shear moduli may be sufficient for explaining the fast timescale asymmetric shear instability in a bimaterial setting not much attention has been paid to heterogeneous elasticity in geodynamical modelling because it is dominant only for short timescales up to now no studies have been made on asymmetric shear instability induced by elastic modulus contrast thermal mechanical numerical simulations based on high resolution from 0 4 km 0 4 km to 0 2 km 0 4 km meshes finite element methods were performed to understand the effects of shear modulus contrast on inducing asymmetric instabilities strain rate and stress dependent rheology are used with a wide range of activation energy 0 850 kj mol 1 for all models numerical results with enough shear modulus contrast show asymmetric shear instability which is generated around the interface and then propagates across the interface two parts of the lithosphere with different shear moduli stiff for higher and soft for lower shear modulus lithospheres which are simply connected to each other without a pre defined weak zone were compressed at a constant rate of 2 cm yr 1 having different shear modulus is justified by chemical heterogeneity of geological minerals and their pressure temperature dependence to explore the dynamical effects generated by the contrast in the elastic modulus the shear modulus of the soft lithosphere is fixed at 32 gpa whereas that of stiff lithosphere is increased systematically from 32 up to 640 gpa we also examined the role of activation energy 0 850 kj mol 1 on the geometrical pattern and the initiation time of asymmetric shear localization the shear modulus contrast has to be close to two for triggering asymmetric shear instability and is found to be by far a more important controlling factor in causing shear instability than activation energy of the creep law the instability develops rapidly between 250 000 and 500 000 yr after deformation begins and thermal weakening in the shear zone is greater when a stronger shear modulus contrast is prescribed our work suggests that initiation of lithosphere scale asymmetric instability would be faster than previous considerations our finding stresses that naturally occurring shear modulus contrast has also important impact on many geological problems related to bimaterial instability

the detection of low magnitude seismic events using array based waveform correlation summary it has long been accepted that occurrences of a known signal are most effectively detected by cross correlating the incoming data stream with a waveform template such matched signal detectors have received very little attention in the field of detection seismology because there are relatively few instances in which the form of an anticipated seismic signal is known a priori repeating events in highly confined geographical regions have been observed to produce very similar waveforms and good signals from events at a given site can be exploited to detect subsequent co located events at lower magnitudes than would be possible using traditional power detectors even greater improvement in signal detectability can be achieved using seismic arrays running correlation coefficients from single sensors can be stacked over an array or network to result in a network correlation coefficient displaying a significant array gain if two events are co located the time separating the corresponding patterns in the wave train as indicated by the cross correlation function is identical for all seismic stations and this property means that the correlation coefficient traces are coherent even when the waveforms are not we illustrate the power of array based waveform correlation using the 1997 august 16 kara sea event the weak event that occurred 4 hr after the main event was barely detected using an sta lta detector on the spits array but is readily detected by signal matching on a single channel the main event was also recorded by the far more distant norsar array but no conventional detection can be made for the second event a clear detection is however made when the correlation coefficient traces are beamformed over all sensors of the array we estimate the reduction in detection threshold of a test signal on a regional seismic array using waveform correlation by scaling down a master signal and immersing it into seismic noise we show that for this case waveform correlation using a single channel detects signals of approximately 0 7 orders of magnitude lower than is possible using an sta lta detector on the array beam waveform matching on the full array provides an additional improvement of approximately 0 4 magnitude units we describe a case study in which small seismic events at the barentsburg coal mine on spitsbergen were detected using the signals from a major rockburst as master waveforms many spurious triggers occurred in this study whereby short sections of signal exhibited coincidental similarity with unrelated incoming wave fronts we demonstrate how such false alarms can almost always be identified and screened out automatically by performing frequency wavenumber analysis upon the set of individual correlation coefficient traces

a study of two mid crustal bright spots from southeast georgia usa summary two high amplitude reflections or bright spots occur on cocorp consortium for continental reflection profiling line georgia 16 at a record time of 5 9s corresponding to nearly 16 km depth they are significant because bright spots often mark great contrasts in subsurface physical properties the first is the surrency bright spot sbs an exceptionally bright antiformal reflection and the second is the reedy creek reflection rcr which lies 5 km to the south and resembles the south half of a diffraction the rcr is also imaged on a crossline georgia 18 as a relatively symmetric antiformal reflection these reflections coincide with the brunswick magnetic anomaly thought to mark the late palaeozoic alleghanian suture the sbs and the rcr are reflections from bodies that lie within the plane of georgia 16 migration and 2 d seismic modelling show the aniformal sbs is from a synformal reflector with a buried focus while the rcr is from a relatively small body the inferred geometry of the sbs reflector with its flat portion corresponding to the deepest part of the reflector argues that it does not represent fluids these results combined with recent evidence that the sbs is a reflection from a high velocity thin layer suggest that the sbs and the rcr might be reflections from ultramafic slices emplaced within the alleghanian suture zone during collision or from mafic sheets intruded into the suture zone during subsequent rifting the brightness of the sbs is attributed to a large impedance contrast constructive interference good data quality and a relatively large reflector size with respect to the fresnel zone and neighbouring reflectors

foreshock occurrence before large earthquakes rates of foreshock occurrence involving shallow m 6 and m 7 mainshocks and m 5 foreshocks were measured in two worldwide catalogs over 20 year intervals the overall rates observed are similar to ones measured in previous worldwide and regional studies when they are normalized for the ranges of magnitude difference they each span the observed worldwide rates were compared to a generic model of earthquake clustering based on patterns of small and moderate aftershocks in california the aftershock model was extended to the case of moderate foreshocks preceding large mainshocks overall the observed worldwide foreshock rates exceed the extended california generic model by a factor of 2 significant differences in foreshock rate were found among subsets of earthquakes defined by their focal mechanism and tectonic region with the rate before thrust events higher and the rate before strike slip events lower than the worldwide average among the thrust events a large majority composed of events located in shallow subduction zones had a high foreshock rate while a minority located in continental thrust belts had a low rate these differences may explain why previous surveys have found low foreshock rates among thrust events in california especially southern california while the worldwide observations suggests the opposite california lacking an active subduction zone in most of its territory and including a region of mountain building thrusts in the south reflects the low rate apparently typical for continental thrusts while the worldwide observations dominated by shallow subduction zone events are foreshock rich if this is so then the california generic model may significantly underestimate the conditional probability for a very large m 8 earthquake following a potential m 7 foreshock in cascadia the magnitude differences among the identified foreshock mainshock pairs in the harvard catalog are consistent with a uniform distribution over the range of observation

earthquake monitoring in southern california for seventy seven years 1932 2008 the southern california seismic network scsn has produced the scsn earthquake catalog from 1932 to the present a period of more than 77 yrs this catalog consists of phase picks hypocenters and magnitudes we present the history of the scsn and the evolution of the catalog to facilitate user understanding of its limitations and strengths hypocenters and magnitudes have improved in quality with time as the number of stations has increased gradually from 7 to 400 and the data acquisition and measuring procedures have become more sophisticated the magnitude of completeness m c of the network has improved from m c 3 25 in the early years to m c 1 8 at present or better in the most densely instrumented areas mainshock aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470 000 events the earthquake frequency size distribution has an average b value of 1 0 with m 6 0 events occurring approximately every 3 yrs the three largest earthquakes recorded were 1952 m w 7 5 kern county 1992 m w 7 3 landers and 1999 m w 7 1 hector mine sequences and the three most damaging earthquakes were the 1933 m w 6 4 long beach 1971 m w 6 7 san fernando and 1994 m w 6 7 northridge earthquakes all of these events ruptured slow slipping faults located away from the main plate boundary fault the san andreas fault their aftershock sequences constitute about a third of the events in the catalog the fast slipping southern san andreas fault is relatively quiet at the microseismic level and has not had an m 6 earthquake since 1932 in contrast the slower san jacinto fault has the highest level of seismicity including several m 6 events thus the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation

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

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

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

global ubiquity of dynamic earthquake triggering surface waves that were generated by 12 out of 15 earthquakes with magnitudes greater than 7 since 1990 led to a global increase in the number of small earthquakes this suggests that dynamic triggering of earthquakes is common and is independent of the tectonic environment earthquakes can be triggered by local changes in the stress field static triggering1 2 3 4 5 6 7 due to nearby earthquakes or by stresses caused by the passage of surface rayleigh and love waves from a remote large earthquake dynamic triggering8 9 10 11 12 13 14 15 16 17 18 however the mechanism frequency controlling factors and the global extent of dynamic triggering are yet to be fully understood because rayleigh waves involve compressional and dilatational particle motion volumetric changes as well as shearing whereas love waves only involve shearing triggering by either wave type implies fundamentally different physical mechanisms here we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes the low amplitude signals of such earthquakes would normally be masked by high amplitude surface waves our analysis reveals that out of 15 earthquakes studied of magnitude m greater than 7 0 that occurred after 1990 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the love and rayleigh waves we conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of the main earthquake or the triggered event

delayed triggering of the 1999 hector mine earthquake by viscoelastic stress transfer stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1 2 3 4 5 the 1999 hector mine earthquake in southern california magnitude 7 1 occurred only 20 km from and 7 years after the 1992 landers earthquake magnitude 7 3 this suggests that the hector mine earthquake was triggered in some fashion by the earlier event but uncertainties in the slip distribution and rock friction properties associated with the landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the hector mine hypocentre with estimates varying from 1 4 bar ref 6 to 0 5 bar ref 7 more importantly coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events here we present the results of a three dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the landers earthquake using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the landers earthquake our calculations suggest that lower crustal or upper mantle flow can lead to postseismic stress increases of up to 1 2 bar at the location of the hector mine hypocentre during this time period contributing to the eventual occurrence of the 1999 hector mine earthquake these results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8 9 10 11

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

three component seismogram envelope synthesis in randomly inhomogeneous semi infinite media based on the single scattering approximation abstract we present a method for synthesizing three component seismogram envelopes of local earthquake in a randomly inhomogeneous semi infinite medium the method extends the single scattering model of sato sato h 1984 attenuation and envelope formation of three component seismograms of small local earthquakes in randomly inhomogeneous lithosphere j geophys res 89 1221 1241 by incorporating the effects of a free surface frequency dependent non spherical radiation from a double couple point source and non isotropic scattering including wave type conversion we synthesize seismogram envelopes the root mean square rms amplitude of seismograms at a receiver located on the free surface by dividing an inhomogeneous medium into many small cells and summing the energy of scattered waves from the cells on the isochronal scattering shells of different scattering modes for a given lapse time our main focus is the free surface effect which alters the shape of isochronal scattering shells and the amplitudes of incident waves synthesizing three component seismogram envelopes for different source receiver configurations we find that these effects on seismogram envelopes are not negligible and are pronounced at early s coda p coda envelope shape is not very sensitive to free surface incorporation because of dominant sp scattered waves which come from the source direction

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

the long precursory phase of most large interplate earthquakes many earthquakes are preceded by foreshocks however the mechanisms that generate foreshocks and the reason why they occur before some earthquakes and not others are unknown here we use seismic catalogues from the best instrumented areas of the north pacific to analyse the foreshock sequences preceding all earthquakes there between 1999 and 2011 of magnitude larger than 6 5 and at depths shallower than 50 km the data set comprises 31 earthquakes at plate boundaries and 31 in plate interiors we find that there is a remarkable contrast between the foreshock sequences of interplate compared with intraplate earthquakes most large earthquakes at plate interfaces in the north pacific were preceded by accelerating seismic activity in the months to days leading up to the mainshock in contrast foreshocks are much less frequent in intraplate settings we suggest that at plate boundaries the interface between the two plates begins to slowly slip before the interface ruptures in a large earthquake this relatively long precursory phase could help mitigate earthquake risk at plate boundaries

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

evidence for mogi doughnut behavior in seismicity preceding small earthquakes in southern california 1 we examine the average space time behavior of seismicity preceding m 2 5 earthquakes in southern california from 1981 to 2005 using a high resolution catalog and identify regions of enhanced activity in a 1 day period preceding larger earthquakes at distances comparable to their predicted source radii the difference in precursory behavior between large and small earthquakes is subtle but statistically significant when averaged over many earthquakes and it has similarities to the mogi doughnut seismicity pattern observed to occur prior to some m 6 and larger earthquakes these results indicate that many standard earthquake triggering models do not account for all of the processes involved in earthquake occurrence

minimum magnitude of completeness in earthquake catalogs examples from alaska the western united states and japan we mapped the minimum magnitude of complete reporting m c for alaska the western united states and for the junec earthquake catalog of japan mc was estimated based on its departure from the linear frequency magnitude relation of the 250 closest earthquakes to grid nodes spaced 10 km apart in all catalogs studied mc was strongly heterogeneous in offshore areas the mc was typically one unit of magnitude higher than onshore on land also mc can vary by one order of magnitude over distance less than 50 km we recommend that seismicity studies that depend on complete sets of small earthquakes should be limited to areas with similar mc or the minimum magnitude for the analysis has to be raised to the highest com mon value of mc we believe that the data quality as reflected by the mc level should be used to define the spatial extent of seismicity studies where mc plays a role the method we use calculates the goodness of fit between a power law fit to the data and the observed frequency magnitude distribution as a function of a lower cutoff of the magnitude data mc is defined as the magnitude at which 90 of the data can be modeled by a power law fit mc in the 1990s is approximately 1 2 0 4 in most parts of california 1 8 0 4 in most of alaska aleutians and panhandle excluded and at a higher level in the junec catalog for japan various sources such as ex plosions and earthquake families beneath volcanoes can lead to distributions that cannot be fit well by power laws for the hokkaido region we demonstrate how neglecting the spatial variability of m c can lead to erroneous assumptions about deviations from self similarity of earthquake scaling

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

pervasive foreshock activity across southern california foreshocks have been documented as preceding less than half of all mainshock earthquakes these observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation which both suggest that foreshock activity should be nearly ubiquitous here we use a state of the art high resolution earthquake catalog to study foreshock sequences of magnitude m4 and greater mainshocks in southern california from 2008 2017 this highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity seventy two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate foreshock sequences vary in duration from several days to weeks with a median of 16 6 days the results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes

short wavelength crustal heterogeneities in the nikko area central japan revealed from the three component seismogram envelope analysis abstract the short wavelength crustal heterogeneity is investigated in the nikko area central japan using a three component seismogram envelope analysis applied to small local earthquakes we stochastically model fluctuations in crustal density and seismic wave velocities by using an exponential auto correlation function the model parameters are correlation length a and root mean square rms fractional fluctuation strength e free surface and non spherical source radiation effects are incorporated in the synthesis of three component seismogram envelopes we use 149 short period seismograms recorded by array stations from 10 local earthquakes with magnitudes 1 2 and epicentral distances less than 15 km a best fit minimum residual between observed and synthetic envelopes in the frequency band of 8 16 hz gives e 2 a 8 10 6 m 1 with the additional information on the strength of direct wave attenuation and the excitation of coda waves at low frequencies we estimate that a 300 800 m and e 5 8 this strong heterogeneity and short correlation length probably reflect the geological condition of the nikko area which is located in the vicinity of the volcanic front

small scale heterogeneities below the grafenberg array germany from seismic wavefield fluctuations of hindu kush events summary small scale elastic heterogeneities 5 km are found in the upper lithosphere under neath the grafenberg array southeast germany the results are based on the analysis of broadband recordings of 17 intermediate depth 201 272 km events from the hindu kush region the wavefront of the first p arrival and the following 40 s coda are separated into coherent and incoherent scattered parts in the frequency range from 0 05 to 5 hz the frequency dependent intensities of the mean and fluctuation wavefields are used to describe the scattering characteristics of the lithosphere underneath the receivers it is possible to discriminate a weak fluctuation regime of the wavefield in the frequency range below approximately 1 5 2 5 hz and a strong fluctuation regime starting at 2 0 2 5 hz and continuing to higher frequencies in order to explain the observed wavefield fluctuations an approach with seismic scattering at random media type structures is proposed the preferred model contains heterogeneities with 3 7 per cent perturbations in seismic velocity and correlation lengths of 0 6 4 8 km in the crust this is compatible with models from active seismic experiments scattering in the lithospheric mantle is not required but cannot be excluded at weak velocity contrasts 3 per cent

california foreshock sequences suggest aseismic triggering process 1 foreshocks are one of the few well documented precursors to large earthquakes therefore understanding their nature is very important for earthquake prediction and hazard mitigation however the triggering role of foreshocks is not yet clear it is possible that foreshocks are a self triggering cascade of events that simply happen to trigger an unusually large aftershock alternatively foreshocks might originate from an external aseismic process that ultimately triggers the mainshock in the former case the foreshocks will have limited utility for forecasting the latter case has been observed for several individual large earthquakes however it remains unclear how common it is and how to distinguish foreshock sequences from other seismicity clusters that do not lead to large earthquakes here we analyze foreshocks of three m 7 mainshocks in southern california these foreshock sequences appear similar to earthquake swarms in that they do not start with their largest events and they exhibit spatial migration of seismicity analysis of source spectra shows that all three foreshock sequences feature lower average stress drops and depletion of high frequency energy compared with the aftershocks of their corresponding mainshocks using a longer term stress drop catalog we find that the average stress drop of the landers and hector mine foreshock sequences is comparable to nearby swarms our observations suggest that these foreshock sequences are manifestations of aseismic transients occurring close to the mainshock hypocenters possibly related to localized fault zone complexity which have promoted the occurrence of both the foreshocks and the eventual mainshock

a comparative ground response study near los angeles using recordings of nevada nuclear tests and the 1971 san fernando earthquake a comparative ground response study at sites in the los angeles region is based on the extensive strong motion data set recorded in the 1971 san fernando earthquake and 159 three component recordings of distant nuclear tests in nevada amplitude spectral ratios computed for the nuclear test data over those frequency bands for which there is an adequate signal to noise ratio provide statistically stable estimates of the local ground response related to the type of local geologic conditions comparison of the strong motion data recorded from the 1971 san fernando earthquake with that recorded at 28 of the same sites for the nuclear tests show that the corresponding amplitude spectral ratios are statistically equivalent for most sites provided reference stations are chosen to minimize effects on the earthquake data of azimuthal source variations and crustal propagation path statistical equivalence of the spectral ratios for the two types of data sources suggests that amplitude spectral ratios computed with respect to the appropriate reference station provide a first order estimate of local ground response

monte carlo simulation of seismogram envelopes in scattering media the analysis of the seismogram coda envelopes of local and regional earthquakes is one of the most effective strategies for investigating the heterogeneous lithospheric structure characterized by the seismic scattering and attenuation in order to synthesize the coda envelope we introduce a numerical scheme called the direct simulation monte carlo method which has been used in the field of the kinetic theory of gases because of the simplicity of the algorithm the method has several advantages over previous methods in terms of the flexibility of the numerical calculation to incorporate various factors required to construct realistic seismogram envelopes on the basis of coda envelope simulations including multiple scattering we show that an increase of seismic velocity with depth severely affects the shape of the coda envelope the effects of ray bending due to the velocity increase at the moho and the reflection at the free surface are clearly found in the synthesized envelope for a shallow earthquake our simulation demonstrates that the amplitude of the envelope is magnified by stagnation of seismic energy at shallow depths due to the positive velocity gradient with depth because of this effect for an a priori assumption of a homogeneous velocity model the measurement of the scattering coefficient by conventional methods may be overestimated

stress strain changes and triggered seismicity at the geysers california the principal results of this study of remotely triggered seismicity in the geysers geothermal field are the demonstration that triggering initiation of earthquake failure depends on a critical strain threshold and that the threshold level increases with decreasing frequency or equivalently depends on strain rate this threshold function derives from 1 analyses of dynamic strains associated with surface waves of the triggering earthquakes 2 statistically measured aftershock zone dimensions and 3 analytic functional representations of strains associated with power production and tides the threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks the observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process assuming the physical processes underlying earthquake nucleation to be the same gomberg this issue discusses seismicity triggered by the mw 7 3 landers earthquake its constraints on the variability of triggering thresholds with site and the implications of time delays between triggering and triggered earthquakes our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a coulomb failure threshold sooner than they would have otherwise we interpret the rate dependent triggering threshold as evidence of several competing processes with different time constants the faster one s facilitating failure and the other s inhibiting it such competition is a common feature of theories of slip instability all these results not surprisingly imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities

deep seismic reflection characteristics of the continental crust four types of seismic reflection fabrics appear to characterize the continental crust 1 a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection moho 2 regions of thin skinned foreland deformation with abundant shallow but sparse deep reflections 3 a unidirectional fabric that dips toward the hinterland and may represent a crustal scale ramp zone and 4 zones of highly layered lower crust and high amplitude reflection moho the first three types are commonly observed in transects across single mountain belts but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere

a stochastic view of lower crustal fabric based on evidence from the ivrea zone despite its complicated history the ivrea zone is considered to be a representative surface exposure of extended continental crust we have digitized two standard 1 25 000 geological maps from this area and evaluated their structural statistics because of the subvertical orientation of the ivrea zone these maps can be considered as small scale cross sections through the lower continental crust the autocorrelation functions of the digitized lithologies measured from these maps show a clear self similar or fractal rather than a gaussian or deterministic trend we found that an anisotropic von karman correlation function with an aspect ratio around 4 and a hurst number of 0 3 corresponding to a fractal dimension of 2 7 matches the observed data our results represent an explicit confirmation of previous indirect evidence for the fractal nature of lithospheric heterogeneities and provide the means to construct realistic crustal scale seismic models of ivrea type lower continental crust

acoustic emissions document stress changes over many seismic cycles in stick slip experiments 1 the statistics of large earthquakes commonly involve large uncertainties due to the lack of long term robust earthquake recordings small scale seismic events are abundant and can be used to examine variations in fault structure and stress we report on the connection between stress and microseismic event statistics prior to the possibly smallest earthquakes those generated in the laboratory we investigate variations in seismic b value of acoustic emission events during the stress buildup and release on laboratory created fault zones we show that b values mirror periodic stress changes that occur during series of stick slip events and are correlated with stress over many seismic cycles moreover the amount of b value increase associated with slip events indicates the extent of the corresponding stress drop consequently b value variations can be used to approximate the stress state on a fault a possible tool for the advancement of time dependent seismic hazard assessment citation goebel t h w d schorlemmer t w becker g dresen and c g sammis 2013 acoustic emissions document stress changes over many seismic cycles in stick slip experiments geophys res lett 40 2049 2054 doi 10 1002 grl 50507

cocorp new perspectives on the deep crust summary relict sutures from colliding continents regions characterized by a young mono layering and faulting throughout the crust mid crustal magma traps and seismic bright spots which suggest deep crustal fluids are among recent cocorp findings in addition new studies of signal penetration noise mitigation recording geometry and coherency filtering have yielded better understanding of and substantial improvements in data quality amplitude anomalies or bright spots in the basin and range may be due to magma at mid crustal levels in one case a normal fault appears to link the deep magma with young surface volcanics another bright spot 15 km deep in southeastern georgia has a flat geometry that suggests a gas liquid interface perhaps within fluids underthrust along an appalachian suture the mono continues to appear relatively undisturbed in many regions of past deformation suggesting that its formation post dates these major tectonic episodes the diversity of reflection patterns from the u s cordillera casts further doubt on the generality of the common model of a reflective layered lower crust underlying a transparent upper crust