bayesian rupture imaging in a complex medium the 29 may 2012 emilia northern italy earthquake we develop a new approach to image earthquake rupture from strong motion data we use a large data set of aftershock waveforms interpolated over the seismic fault to obtain green s function approximations next we deploy a bayesian inversion method to characterize the slip distribution the rupture velocity the slip duration and their uncertainties induced by errors in the green s functions the method is applied to the 29 may 2012 mw 6 emilia earthquake which ruptured a fault buried below the po plain sediments northern italy despite the particularly complex wave propagation the near field strong motion observations are well reproduced with 15 rupture parameters the rupture and slip velocities were notably slow 0 5 vs and 0 5 m s respectively implying that the fault was difficult to break this method opens some perspectives for earthquake rupture studies in areas where numerical simulations suffer from imprecise knowledge of the velocity structure

nearly magnitude invariant stress drops in simulated crack like earthquake sequences on rate and state faults with thermal pressurization of pore fluids stress drops inferred to be magnitude invariant are a key characteristic used to describe natural earthquakes theoretical studies and laboratory experiments indicate that enhanced dynamic weakening such as thermal pressurization of pore fluids may be present on natural faults at first glance magnitude invariance of stress drops and enhanced dynamic weakening seem incompatible since larger events may experience greater weakening and should thus have lower final stresses and higher stress drops we hypothesize that enhanced dynamic weakening can be reconciled with magnitude invariant stress drops due to larger events having lower average prestress when compared to smaller events we conduct numerical simulations of long term earthquake sequences in fault models with rate and state friction and thermal pressurization and in the parameter regime that results mostly in crack like ruptures we find that such models can explain both the observationally inferred stress drop invariance and increasing breakdown energy with event magnitude smaller events indeed have larger average initial stresses than medium sized events and we find nearly constant stress drops for events spanning up to two orders of magnitude in average slip comparable to approximately six orders of magnitude in seismic moment segment spanning events have more complex behavior which depends on the properties of the arresting velocity strengthening region at the edges of the faults

spectral element simulations of long term fault slip effect of low rigidity layers on earthquake cycle dynamics we develop a spectral element method for the simulation of long term histories of spontaneous seismic and aseismic slip on faults subjected to tectonic loading our approach reproduces all stages of earthquake cycles nucleation and propagation of earthquake rupture postseismic slip and interseismic creep we apply the developed methodology to study the effects of low rigidity layers on the dynamics of the earthquake cycle in 2 d we consider two cases small m 1 earthquakes on a fault surrounded by a damaged fault zone and large m 7 earthquakes on a vertical strike slip fault that cuts through shallow low rigidity layers our results indicate how the source properties of repeating earthquakes are affected by the presence of a damaged fault zone with low rigidity compared to faults in homogeneous media we find 1 reduction in the earthquake nucleation size 2 amplification of slip rates during dynamic rupture propagation 3 larger recurrence interval and 4 smaller amount of aseismic slip based on linear stability analysis we derive a theoretical estimate of the nucleation size as a function of the width and rigidity reduction of the fault zone layer which is in good agreement with simulated nucleation sizes we further examine the effects of vertically stratified layers e g sedimentary basins on the nature of shallow coseismic slip deficit our results suggest that low rigidity shallow layers alone do not lead to coseismic slip deficit while the low rigidity layers result in lower interseismic stress accumulation they also cause dynamic amplification of slip rates with the net effect on slip being nearly zero

attenuation of radiated ground motion and stresses from three dimensional supershear ruptures 1 radiating shear and rayleigh waves from supershear ruptures form mach waves that transmit large amplitude ground motion and stresses to locations far from the fault we simulate bilateral ruptures on a finite width vertical strike slip fault of width w and half length l with l w breaking the surface of an elastic half space and focus on the wavefield out to distances comparable to l at distances much smaller than w two dimensional plane strain slip pulse models i e models in which the lateral extent of the slip zone is unbounded accurately predict the subsurface wavefield amplitudes in the shear mach wedge of those models are undiminished with distance from the fault when viewed from distances far greater than w rupture is accurately modeled as a moving point source that produces a shear mach cone and on the free surface rayleigh wave mach fronts geometrical spreading of the shear mach cone occurs radially and amplitudes there decrease with the inverse square root of distance the transition between these two asymptotic limits occurs at distances comparable to w similar considerations suggest that rayleigh mach waves suffer no attenuation in the ideally elastic medium studied here the rate at which fault strength weakens at the rupture front exerts a strong influence on the off fault fields only in the immediate vicinity of the fault for both sub rayleigh and supershear ruptures and at the mach fronts of supershear ruptures more rapid weakening generates larger amplitudes at the mach fronts

spatiotemporal variation of stress drop during the 2008 mogul nevada earthquake swarm we estimate stress drops for 148 shallow 6 km earthquakes in the complex 2008 mogul nevada swarm using empirical green s function derived spectral ratios near source temporary broadband seismometers deployed before the mw4 9 main shock provide high quality records of many foreshocks and aftershocks and an ideal opportunity to investigate uncertainties in corner frequency measurement as well as stress drop variation related to space time depth mechanism and magnitude we explore uncertainties related to source model measurement approach cross correlation limit and frequency bandwidth p s wave results range from 0 2 0 15 0 3 0 15 to 36 20 58 7 mpa a variation greater than the error range of each individual estimate although this variation is not explained simply by any one parameter spatiotemporal variation along the main shock fault plane is distinct coherent clusters of high and low earthquakes are seen and high foreshocks correlate with an area of reduced aftershock productivity these observations are best explained by a difference in rheology along the fault plane average s of 3 9 1 1 4 0 1 1 mpa using p s are similar to those found for earthquakes in a variety of settings implying that these shallow potentially fluid driven earthquakes do not have systematically lower than average tectonic earthquakes 4 mpa and therefore have similar or higher due to proximity to the surface expected ground motions compared to typical earthquakes the unprecedented detail achieved for these shallow small magnitude earthquakes confirms that when measured precisely is a valuable observation of physically meaningful fault zone properties and earthquake behavior

source parameter analysis of microearthquakes recorded around the underground gas storage in the montello collalto area southeastern alps italy abstract the study of seismic source parameters is crucial for understanding the origin of seismicity and retrieving information on the energy balance and the stress involved in earthquake rupture processes in active tectonic areas where underground industrial activities are carried out such parameters may help to understand whether earthquakes are induced triggered or natural the montello collalto area southeastern alps is located in an active tectonic environment and hosts a depleted natural reservoir used to store gas since 2012 a high quality seismic network monitors the microseismicity occurring around the underground gas storage reservoir to understand if the storage activity might induce seismicity in this paper we estimate the source parameters of low magnitude events representative of the seismicity occurring in the area surrounding the reservoir the analysis includes a preliminary removal of the site effects specifically computed within this study from all the records then using a parametric multistep inversion scheme we estimate the seismic moment the corner frequency and the static stress drop that can be set as reference for the microseismicity occurring in the study area all the investigated earthquakes show low seismic efficiency compatible with overshoot processes which is typical of natural i e tectonic earthquakes our procedure can be implemented in other tectonic regions hosting underground industrial activities to support the decisional processes related to real time monitoring

rupture process and stress drop of the great 1989 macquarie ridge earthquake on may 23 1989 a great mw 8 1 earthquake occurred in the macquarie ridge complex south of new zealand the earthquake is one of the largest in the macquarie ridge complex this century and represents the right lateral strike slip component of the motion between the pacific plate and the australian plate subduction initiation appears to be presently occurring in the macquarie ridge complex and it has been suggested that plate boundary strike slip earthquakes in a transitional tectonic environment have a high stress drop we have investigated the source rupture process of the 1989 earthquake using teleseismic p and sh waves the best point source depth is 12 km below the ocean bottom the deconvolved source time function is dominated by a single pulse with a large moment release 1 8 1021 nm and a short duration 20 s the moment rate increases slowly in the first 10 s of the rapture process and is suddenly truncated at 20 s there is no resolvable directivity to this sharp truncation which given the temporal resolution of the data set means that the spatial location of the sharp truncation is within approximately 50 km of the epicenter the stress drop and average displacement may be as high as 370 bars and 36 m these values are unusually high and strongly support the suggestion that high stress drop earthquakes are characteristic for transitional tectonic environments

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

corner frequency ratios of p and s waves and strain drops of earthquakes recorded by a tight network around the karadere segment of the north anatolian fault zone evidence for non classical source processes

variability of seismic source spectra estimated stress drop and radiated energy derived from cohesive zone models of symmetrical and asymmetrical circular and elliptical ruptures large variability of earthquake stress drops and scaled energy has been commonly reported in the literature but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties here we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry rupture directivity and rupture speeds we numerically simulate earthquake source scenarios using a cohesive zone model with the small scale yielding limit where the solution approaches a singular crack model with spontaneous healing of slip compared to symmetrical circular source models asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere the general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime although shear mach waves generated by the propagation of supershear rupture lead to much higher corner frequency and scaled energy estimates locally our results suggest that at least a factor of 2 difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops translating into a factor of 8 difference in estimated stress drops furthermore radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry

global catalog of earthquake rupture velocities shows anticorrelation between stress drop and rupture velocity abstract application of the scardec method provides the apparent source time functions together with seismic moment depth and focal mechanism for most of the recent earthquakes with magnitude larger than 5 6 6 using this large dataset we have developed a method to systematically invert for the rupture direction and average rupture velocity v r when unilateral rupture propagation dominates the approach is applied to all the shallow z

dynamics of the 2015 m7 8 nepal earthquake the 2015 m7 8 nepal earthquake ruptured part of the main himalayan thrust beneath kathmandu to study the dynamics of this event we compute p wave spectra of the main shock and of two large aftershocks to estimate stress drop and radiated energy we find that surface reflections depth phases of these shallow earthquakes produce interference that severely biases spectral measurements unless corrections are applied measures of earthquake dynamics for the main shock are within the range of estimates from global and regional earthquakes we explore the azimuthal and temporal variations of radiated energy and highlight unique aspects of the m7 8 rupture the beginning of the earthquake likely experienced a dynamic weakening mechanism immediately followed by an abrupt change in fault geometry correlation of backprojection results with frequency dependent variations in the radiated energy rate and with the suggested geometry of the main himalayan thrust yields new constraints on dynamic ruptures through geometrical barriers

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

parametrizing physics based earthquake simulations utilizing earthquake source parameter scaling relations we formulate an extensible slip weakening friction law for quasi static earthquake simulations this algorithm is based on the method used to generate fault strengths for a recent earthquake simulator comparison study of the california fault system here we focus on the application of this algorithm in the virtual quake earthquake simulator as a case study we probe the effects of the friction law s parameters on simulated earthquake rates for the ucerf3 california fault model and present the resulting conditional probabilities for california earthquake scenarios the new friction model significantly extends the moment magnitude range over which simulated earthquake rates match observed rates in california as well as substantially improving the agreement between simulated and observed scaling relations for mean slip and total rupture area

dynamic source model for the 2011 tohoku earthquake in a wide period range combining slip reactivation with the short period ground motion generation process this paper describes a validated dynamic rupture model of the 2011 tohoku earthquake that reproduces both long period 20 100 s and short period 3 20 s ground motions in order to reproduce the observed large slip area slip asperity we assign a large dc slip critical distance area following kinematic source inversion results sufficiently large slip is achieved through rupture reactivation by the double slip weakening friction model in order to reproduce the strong motion generation areas smgas we assign short dc and large stress drop areas following empirical green s function egf simulation results which indicate that although more distant from the hypocenter smga1 ruptured earlier than smga2 or smga3 which are closer to the hypocenter this observation is confirmed by the backprojection method in order to reproduce this important feature in dynamic simulation results we introduce a chain of small high stress drop patches between the hypocenter and smga1 by systematic adjustment of stress drops and dc the rupture reproduces the observed sequence and timing of smga ruptures and the final slip derived by kinematic models this model also reproduces the multiseismic wavefront observed from strong ground motion data recorded along the pacific coast of the tohoku region we compare the velocity waveforms recorded at rock sites along the coastline with one dimensional 1d synthetic seismograms for periods of 20 100 s the fit is very good at stations in the northern and central areas of tohoku we also perform finite difference method fdm simulations for periods of 3 20 s and confirm that our dynamic model also reproduces wave envelopes overall we are able to validate the rupture process of the tohoku earthquake

apparent break in earthquake scaling due to path and site effects on deep borehole recordings 1 we reexamine the scaling of stress drop and apparent stress rigidity times the ratio between seismically radiated energy to seismic moment with earthquake size for a set of microearthquakes recorded in a deep borehole in long valley california in the first set of calculations we assume a constant q and solve for the corner frequency and seismic moment in the second set of calculations we model the spectral ratio of nearby events to determine the same quantities we find that the spectral ratio technique which can account for path and site effects or nonconstant q yields higher stress drops particularly for the smaller events in the data set the measurements determined from spectral ratios indicate no departure from constant stress drop scaling down to the smallest events in our data set mw 0 8 our results indicate that propagation effects can contaminate measurements of source parameters even in the relatively clean recording environment of a deep borehole just as they do at the earth s surface the scaling of source properties of microearthquakes made from deep borehole recordings may need to be reevaluated

stress release and source scaling of the 2010 2011 canterbury new zealand earthquake sequence from spectral inversion of ground motion data the canterbury earthquake sequence beginning with the 2010 m w 7 2 darfield earthquake is one of the most notable and well recorded crustal earthquake sequences in a low strain rate region worldwide and as such provides a unique opportunity to better understand earthquake source physics and ground motion generation in such a tectonic setting ground motions during this sequence ranged up to extreme values of 2 2 g recorded during the february 2011 m w 6 2 event beneath the city of christchurch a better understanding of the seismic source signature of this sequence in particular the stress release and its scaling with earthquake size is crucial for future ground motion prediction and hazard assessment in canterbury but also of high interest for other low to moderate seismicity regions where high quality records of large earthquakes are lacking here we present a source parameter study of more than 200 events of the canterbury sequence covering the magnitude range m w 3 7 2 source spectra were derived using a generalized spectral inversion technique and found to be well characterized by the 2 source model we find that stress drops range between 1 and 20 mpa with a median value of 5 mpa which is a factor of 5 larger than the median stress drop previously estimated with the same method for crustal earthquakes in much more seismically active japan stress drop scaling with earthquake size is nearly self similar and we identify lateral variations throughout canterbury in particular high stress drops at the fault edges of the two major events the m w 7 2 darfield and m w 6 2 christchurch earthquakes

source parameters of the 1980 mammoth lakes california earthquake sequence from the more than 1500 mammoth lakes earthquakes recorded on three component digital seismographs spudich et al 1981 150 were used in an analysis of the locations mechanism and source parameters a composite fault plane solution of nine earthquakes 3 9 m 5 1 defines a right lateral strike slip mechanism on a steeply dipping nearly east west plane striking s75 e or left lateral strike slip on a nearly north south plane striking n10 e vertical cross sections of well located aftershocks indicate possibly three east west planes that coincide with the locations of the four largest earthquakes with ml 6 0 using the spectral analysis of s waves brune 1970 source parameters for 67 earthquakes were determined forty eight had magnitudes greater than or equal to 3 0 seismic moments ranged from 9 20 1018 dyn cm to 2 33 1024 dyn cm earthquakes with seismic moment greater than about 1 0 1021 dyn cm had nearly constant stress drops 50 bars earthquakes with seismic moment less than about 1 0 1021 dyn cm had stress drops that apparently decrease as seismic moment decreases

predominance of unilateral rupture for a global catalog of large earthquakes the manner in which an earthquake rupture propagates across a fault reflects both the initial properties of the fault and the dynamical stresses produced by the rupture we quantify the propagation of an earthquake rupture using the second moments of the earthquake space time distribution in particular the second moments provide a simple way to differentiate between approximately bilateral and predominantly unilateral ruptures we determined the second moments for a catalog of m w 7 earthquakes that have occurred since 1994 the results show that approximately 80 of large shallow ruptures are predominantly unilateral our result agrees well with strong motion inversions from previous studies on moderate and large earthquakes the predominance of unilateral propagation in large earthquakes is explained by a simple characteristic earthquake model in which large events rupture an entire structurally defined fault segment and nucleation points are uniformly distributed along the fault the predominance of unilateral rupture may also be enhanced by the dynamic stress field produced by contrasts in the elastic properties between the two sides of plate boundary faults manuscript received 16 november 2001

identifying the unique ground motion signatures of supershear earthquakes theory and experiments the near field ground motion signatures associated with sub rayleigh and supershear ruptures are investigated using the laboratory earthquake experiment originally developed by rosakis and co workers xia et al 2004 2005a lu et al 2007 rosakis et al 2007 heterodyne laser interferometers enable continuous high bandwidth measurements of fault normal fn fault parallel fp and vertical v particle velocity ground motion records at discrete locations on the surface of a homalite 100 test specimen as a sub rayleigh or a supershear rupture sweeps along the frictional fault photoelastic interference fringes acquired using high speed digital photography provide a synchronized spatially resolved whole field view of the advancing rupture tip and surrounding maximum shear stress field the first phase of experimental investigations examine and verify the ground motion signatures of supershear ruptures experimental results demonstrate that a shear mach front produced by a stable supershear rupture is characterized by a dominant fp velocity component the situation is shown to reverse in the sub rayleigh rupture speed regime whereby the fn particle velocity component dominates the ground motion record additional distinguishing particle velocity signatures consistent with theoretical and numerical predictions and repeatedly observed in experimental records are 1 a pronounced peak in the fp velocity record induced by the leading dilatational field which sweeps the measurement station in advance of the shear mach front and 2 a pronounced velocity swing in the fn record associated with the arrival of a trailing rayleigh sub rayleigh secondary rupture which follows the arrival of the shear mach front analysis of the particle velocity records also confirms 2d steady state theoretical predictions pertaining to the separation attenuation and radiation partitioning of the shear and dilatational portions of the rupture velocity field components the second phase of our experimental investigations re examine the 2002 mw7 9 denali fault earthquake and the remarkable set of near source ground motion records obtained at ps10 located approximately 85 km east of the epicenter and just 3 km north of the fault along the alaska pipeline motivated by the analysis and interpretation of these records by ellsworth et al 2004 dunham and archuleta 2004 2005 we attempt to mimic the denali strike slip rupture scenario and replicate the ps10 ground motion signatures using a laboratory earthquake experiment the experiments feature a left to right west to east propagating right lateral rupture within a homalite 100 test specimen with particle velocity data collected at a near field station situated just above north of the fault both sub rayleigh and supershear laboratory earthquake experiments are conducted using the denali ps10 configuration in order to compare and contrast the resulting particle velocity signatures supershear laboratory records capture all of the prominent features displayed within the ps10 ground motion records noted velocity signatures are correlated to the location of the rupture fronts and their noted arrival times in the synchronized photoelastic image sequence scaling relationships are also presented which transform the laboratory records through six orders of magnitude in time to match the scale of the ps10 ground motion records the strong correlation between the scaled experimental records and the actual ps10 ground motion records support the hypothesis that the denali strike slip fault exhibited a supershear burst finally we present a 2d steady state stress velocity formulation that relates the fp and fn particle velocity records measured close to the fault to the evolution of the stress tensor at the same location a locally steady state condition is assumed within a restricted time interval in order to invoke these relationships and estimate the dynamic stresses xx t and t at the near fault station dynamic stress measurements enable a new class of friction investigations using the laboratory earthquake configuration experimental findings are presented which capture the temporal and spatial distributions of xx and evolution of the dynamic friction coefficient and velocity weakening behavior of a supershear slip pulse

toward automated directivity estimates in earthquake moment tensor inversion rapid estimates of earthquake rupture properties are useful for both scientific characterization of earthquakes and emergency response to earthquake hazards rupture directivity is a particularly important property to constrain since seismic waves radiated in the direction of rupture can be greatly amplified and even moderate magnitude earthquakes can sometimes cause serious damage knowing the directivity of earthquakes is important for ground shaking prediction and hazard mitigation and is also useful for discriminating which nodal plane corresponds to the actual fault plane particularly when the event lacks aftershocks or outcropped fault traces here we propose a 3 d multiple time window directivity inversion method through direct waveform fitting with source time functions stretched for each station according to a given directivity by grid searching for the directivity vector in 3 d space this method determines not only horizontal but vertical directivity components provides uncertainty estimates and has the potential to be automated in real time synthetic tests show that the method is stable with respect to noise picking errors and site amplification and is less sensitive to station coverage than other methods horizontal directivity can be properly recovered with a minimum azimuthal station coverage of 180 whereas vertical directivity requires better coverage to resolve we apply the new method to the m w 6 0 nantou taiwan earthquake m w 7 0 kumamoto japan earthquake and m w 4 7 san jacinto fault trifurcation sjft earthquake in southern california for the nantou earthquake we corroborate previous findings that the earthquake occurred on a shallow east dipping fault plane rather than a west dipping one for the kumamoto and sjft earthquakes the directivity results show good agreement with previous studies and demonstrate that the method captures the general rupture characteristics of large earthquakes involving multiple fault ruptures and applies to earthquakes with magnitudes as small as m w 4 7

kīholo bay hawai i earthquake sequence of 2006 relationship of the main shock slip with locations and source parameters of aftershocks 1 we study the source process of the k holo bay earthquake mw 6 7 which occurred beneath the northwest part of the island of hawai i on 15 october 2006 and static stress drops of small earthquakes that occurred in 2006 and 2007 around the main shock including aftershocks we relocate the aftershocks to determine the fault plane from the two nodal planes the relocated aftershocks define an e w trending plane that dips to the south in good agreement with one of the nodal planes given by the global centroid moment tensor solution waveform inversion is performed with multiple time windows to investigate the rupture speed and the slip distribution of the main shock waveforms of an aftershock with mw 5 2 are used to calculate empirical green s functions our results indicate that the rupture propagated unilaterally to the west with a rupture speed greater than 3 0 km s 63 of the shear wave velocity this westward rupture is consistent with the fact that aftershocks are distributed predominantly to the west of the main shock epicenter most aftershocks are located on the edge of patches with a large slip or asperities and some also occur inside the patches we also estimate static stress drops of 39 earthquakes 2 5 ml 3 5 that occurred in 2006 and 2007 near the source region of the k holo bay earthquake static stress drops range from 0 12 to 8 6 mpa and aftershocks around large slip patches of the main shock likely to have larger stress drops

rupture directivity of fluid induced microseismic events observations from an enhanced geothermal system the rupture process of fluid induced microseismic events is still poorly understood mainly due to usually small magnitudes and sparse monitoring geometries the high quality recordings of the earthquake sequence 2006 2007 at the enhanced geothermal system at basel switzerland constitute a rare exception allowing a systematic directivity study of 195 events using the empirical green s function method we observe clear directivity signatures for about half the events which demonstrates that rupture directivity persists down to small magnitudes ml 1 the predominant rupture behavior is unilateral we further find evidence that directivity is magnitude dependent and varies systematically with distance from the injection source whereas pore pressure seems to play the dominant role close to the injection source and no preferred rupture direction is observable directivity aligns parallel to the event distribution with increasing distance 100 m and is preferably oriented away from the injection point the largest analyzed events ml 2 show a distinct behavior they rupture toward the injection source suggesting that they nucleate in the vicinity of the pressure front and propagate backward into the perturbed volume this finding is of particular relevance for seismic hazard assessment of georeservoirs since it implies that maximum event size is related to dimension of the fluid perturbed volume our study also resolves rupture complexities for a small group of events this shows that small fault heterogeneities exist down to a scale of a few tens of meters the observation of directivity and complexity in induced microseismic events suggest that future source studies account for these phenomena

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

aftershock zones of large shallow earthquakes fault dimensions aftershock area expansion and scaling relations summary we determine the aftershock areas from relocated hypocentres for 64 dip slip and eight strike slip earthquakes in the period 1977 1996 together with those for three recent earthquakes the 1998 antarctic plate earthquake the 1999 izmit turkey earthquake and the 2000 wharton basin earthquake we also include the data for 27 strike slip earthquakes from pegler das 1996 we find that the location of the hypocentre is essentially random along strike for both strike slip and dip slip earthquakes subduction zone earthquakes appear to initiate more frequently towards the down dip edge of the fault whereas the non subduction zone dip slip earthquakes do not have any preferred depth of initiation the aftershock zones of subduction zone earthquakes often expand substantially along strike and up dip but far less in the down dip direction whereas those for non subduction zone earthquakes do not expand significantly in either the up or the down dip direction subduction zone thrust earthquakes have larger and more numerous aftershocks than earthquakes in all other tectonic settings for strikeslip earthquakes we find that slip increases at least linearly with length for dip slip earthquakes we find that the ratio of length to width increases systematically with length for lengths 40 km indicating that there is some restriction on fault width slip is found to be proportional to length over the moment range 10 17 nm m0 3r10 21 nm

stress drop apparent stress and radiation efficiency of clustered earthquakes in the nucleation volume of the 6 april 2009 mw 6 1 l aquila earthquake

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

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

radiated seismic energy based on dynamic rupture models of faulting 1 by modeling spontaneous ruptures we study the mechanism dependence of radiated seismic energy from three hypothetical crustal events 30 dipping reverse fault 60 dipping normal fault and a vertical strike slip fault and the 1994 blind thrust northridge earthquake embedded in a homogeneous half space all three hypothetical faults have the same area and are subjected to the same shear and normal stress conditions and frictional parameters dynamic simulations produce apparent stress of 0 53 mpa 0 23 mpa and 0 34 mpa for the reverse normal and strike slip faults respectively the energy distribution on a distant surface shows that a large fraction of energy is concentrated in the forward direction of rupture propagation we use spontaneous rupture models to compute the radiated energy from the 1994 northridge earthquake the initial stress drop distribution is based on a kinematic slip distribution using a linear slip weakening friction law we modify both the initial stress and yield stress until the dynamic rupture produces near source synthetics that are consistent with the data the total radiated seismic energy from our model is 6 0 1014 j seismic moment 1 47 1019 nm apparent stress 1 5 mpa fracture energy 3 2 1014 j and slip weakening distance 0 25 m the energy flux distribution is heterogeneous with strong directivity effects these results suggest that correcting for directivity could be difficult but necessary for teleseismic and regional estimates of radiated energy dynamic source models constrained by ground motions can provide a stable and accurate energy estimate for large earthquakes

dynamic rupture and seismic radiation in a damage breakage rheology model we present simulations of dynamic ruptures in a continuum damage breakage rheological model and waves radiated by the ruptures and observed in the far field the propagating rupture produces rock damage and granulation in the process zone ahead of the rupture front an expansion compaction process in the process zone leads to an isotropic source term while shear motion that accumulates behind the propagating front produces a deviatoric source term and shear heating behind the rupture front the process zone dissipation due to the damage breakage mechanism and the isotropic source component significantly affect the s p energy partitioning and the radiation patterns of the waves the calculated s p seismic energy ratio can be significantly lower than results of standard models with no damage breakage mechanism and no source volume components the p radiation pattern becomes more isotropic compared with the classical deviatoric solution with increased lobes at 45 to the direction of rupture propagation the s radiation pattern is affected more strongly by the damage breakage process in the source volume mainly within the process zone and is significantly different from classical deviatoric results the s waves propagate from the rupture front through the process zone unlike the p waves experiencing stronger dissipation so the s radiation pattern is more affected than the p radiation pattern hence analysis of p waves can provide more reliable results on rupture directivity than s waves

moment energy stress drop and source spectra of western united states earthquakes from regional coda envelopes we present a new method to estimate stable seismic source parameters such as energy moment and orowan stress drop using regional coda envelopes from as few as one broadband station we use the method to compute path and site corrected seismic moment rate spectra for 117 recent western united states earthquakes empirical green s function corrections were applied to our surface and body wave coda envelope measurements to generate s wave source spectra these source spectra provide stable single station estimates of radiated seismic energy es and seismic moment mo that for common events are in excellent agreement with network averaged estimates obtained using local and regional data teleseismic moment estimates are compatible with our regional results but teleseismic energy estimates appear to be nearly an order of magnitude low we estimated the seismic moment of events ranging between mw 2 2 and 7 3 and energy estimates for which we had measured at least 70 of the total energy generally events above mw 3 3 we use these estimates to examine the behavior of derived parameters such as the orowan stress drop 2 es mo while the earthquakes we studied have a small range in orowan stress drop generally between 0 1 and 20 mpa they show a strong tendency for orowan stress drop to increase with moment approximately as mo0 25 we believe this is a source effect and is not due to inadequate bandwidth or attenuation correction and note that this trend appears to continue for microearthquakes as described in a recent deep borehole study in southern california many of the large high stress drop earthquakes show complexity in their moment rate spectra near the corner frequency and cannot be fit by a simple square model instead above the first corner frequency the spectral decay ranges between f 1 0 and f 1 5 this leads to larger estimates of radiated energy than predicted with a simple square model and has implications for seismic hazard estimation coda envelopes have three main advantages over direct arrivals for estimating seismic moment and energy 1 coda amplitudes vary little with geology and source radiation anisotropy and allow accurate single station applications 2 path corrected coda amplitude measurements can be applied to very large regions allowing a comparison of source parameters throughout the western united states using a common methodology and stations 3 because long period coda can last for hours for large local and regional events it allows the analysis of seismograms with clipped early arrivals

quasi dynamic models of simple earthquakes application to an aftershock of the 1975 oroville california earthquake a class of kinematic rupture models for simple earthquakes is proposed these models incorporate results from both theoretical and numerical solutions to the mixed boundary value problem of a three dimensional frictional rupture the analytic form for the slip velocity is divided into two phases the rupture growth during which the slip distribution is self similar kostrov 1964 and the healing during which the slip velocity decreases monotonically to zero the arrival of a p wave stopping phase propagating into the interior of the rupture area from the fault perimeter initiates the healing the duration of healing may be varied to produce a gradual or abrupt stopping of the rupture which permits modeling a wide range of high frequency spectral behavior these models are used to generate synthetics which are fit to the velocity and the velocity squared pulse shapes obtained from strong motion accelerograms of the 0103 aftershock of the 1975 oroville california earthquake the aftershock ml 4 6 has a moment of 1 8 1023 dyne cm and a total radiated energy of 2 7 1019 dyne cm the fitted source model propagates updip with a rupture velocity of 0 85 and a dynamic stress drop of 210 bars the arrival times of five stopping phases correlated across the suite of accelerograms are used to determine the relative locations of the hypocenter and five stopping events the description of rupture determined from these stopping events both corroborates the source model obtained from the waveform modeling and establishes an important counterpoint to the model by providing a dynamic interpretation of the unmodeled complexity of the waveforms

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

low frequency earthquakes observed in close vicinity of repeating earthquakes in the brittle upper crust of hakodate hokkaido northern japan we conducted a detailed investigation of an earthquake cluster distributed from the lower crust to the upper crust beneath hakodate hokkaido which included both low frequency earthquakes lfes a

variations of strain drops of aftershocks of the 1999 izmit and duzce earthquakes around the karadere duzce branch of the north anatolian fault summary we estimate the strain drops of 7498 aftershocks of the 1999 u and d uzce earthquakes using p waveforms recorded by a local seismic network along the karadere d uzce branch of the north anatolian fault in the 6 months following the u main shock and evaluate the site effects of the various recording stations the method is associated with separation of source traveltime and station spectral terms and stacking results at several stages to enhance the signal to noise ratio the strain drops are obtained by fitting iteratively the separated source spectra of 201 nearest neighbouring events in different amplitude bins to the 2 source spectral model the obtained strain drops generally increase with depth between 3 and 10 km and remain approximately constant for the deeper section a local shallow patch of seismicity north of the karadere segment has relatively low strain drop values along the relatively straight karadere segment the strain drops are lower than along the geometrically complex bounding regions in some sections the range of strain drop values decay with time from the u and d uzce main shocks whereas in others the values fluctuate within about constant ranges the observed spatial variations of strain drops can be explained by increasing normal stress with depth along with the degree of geometrical complexity of different fault sections and the expected slip deficit at different depth sections the seismic energy in the separated station spectra decreases overall with distance from the rupture zone and the spectra of various stations have three peaks at 6 14 and 25 hz the spectral peak at 6 hz is also observed in trapped waves studies and may be related to the damaged fault zone layer

spectral element modeling of spontaneous earthquake rupture on rate and state faults effect of velocity strengthening friction at shallow depths we develop a spectral element methodology sem for simulating dynamic rupture on rate and state faults and use it to study how the rupture is affected by a shallow fault region of steady state velocity strengthening friction our comparison of the developed sem and a spectral boundary integral method bim for an anti plane two dimensional test problem shows that the two methods produce virtually identical solutions for the finest resolution we use and that the convergence with grid reduction of the developed sem methodology is comparable to that of bim we also use the test problem to compare numerical resolution required for different state evolution laws and for linear slip weakening friction using our three dimensional implementation of the methodology we find that a shallow velocity strengthening fault region can significantly alter dynamic rupture and ground motion the velocity strengthening region suppresses supershear propagation at the free surface occurring in the absence of such region which could explain the lack of universally observed supershear rupture near the free surface in addition the velocity strengthening region promotes faster fall off of slip velocity behind the rupture front and decreases final slip throughout the entire fault causing a smaller average stress drop the slip decrease is largest in the shallow parts of the fault resulting in the depth profile of slip qualitatively consistent with observations of shallow co seismic slip deficit the shallow velocity strengthening region also reduces the amplification of strong ground motion due to a low velocity bulk structure