complex source processes and the interaction of moderate earthquakes during the earthquake swarm in the hida mountains japan 1998 abstract this paper presents a set of detailed source models of 18 moderate m4 5 earthquakes that were part of the swarm activity beneath the hida mountains in central japan in 1998 to reveal the detailed rupture process this study uses a waveform inversion procedure with the empirical green function egf most of our models show a complicated rupture process that includes several separate rupture areas and that involves ruptures that in some events seem to repeat at almost the same location in our models we measured as characteristic values the maximums of slip slip rate stress drop and average rupture propagation velocity in comparison with the models for larger events determined by similar methods the maximum slip scales with the cubic squares of the seismic moment for large stress drop events only the slip rate has an upper limit of about 1 2 m s and the stress drop is distributed widely from 1 to 50 mpa the average stress drops estimated using the corner frequency are almost uniform at about 1 mpa the first event in each swarm cluster tends to have a large maximum stress drop which reflects the change in the stress field during swarm activity the average rupture velocity determined by the least squares method is relatively slow and shows a negative relationship with the maximum stress drop this finding cannot be explained by simple crack models and instead suggests a very complicated mechanism like the interactive fault system the interaction between the slip areas of neighbor events resulted in a non overlapping slip distribution this finding indicates that previous rupture areas restrict the size and location of later events even in such complex swarm activities

the state of stress on some faults of the san andreas system as inferred from near field strong motion data we present a simple method to calculate the stress produced on an earthquake fault during rupture this method allows the complete evaluation of the stress spatio temporal history over the fault we apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the san andreas fault system over the last 20 years the imperial valley earthquake of 1979 the morgan hill earthquake of 1984 the loma prieta earthquake of 1989 and the northridge earthquake of 1994 we use for this study the tomographic models of the fault rupture obtained from the inversion of the near field seismic data recorded during these earthquakes the results obtained show that the static and the dynamic stress drops vary greatly over the fault the peak values obtained for the four earthquakes studied range from about 20 to 100 mpa these high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault the regions of the fault which break with a high stress drop are also the regions where slip is large this suggests that most of the slip produced in a large earthquake takes place over the strong areas of the fault low slip regions tend to break with low stress drops after the earthquake the shear stress is increased over a significant portion of the fault which corresponds to low slip regions aftershock activity tends to be concentrated in these areas of stress increase the apparent strength of the fault before the earthquake that is the local shear stress increase which is required for rupture is also extremely heterogeneous the rupture velocity seems to be inversely correlated with this apparent fault strength the rupture accelerating over the weak areas of the fault and slowing down over the high strength areas

finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

the effect of bandwidth limitations on the inference of earthquake slip weakening distance from seismograms numerous researchers have obtained estimates of slip weakening distance dc and fracture energy for recent earthquakes dc is often observed to be a significant fraction of the total slip and tends to correlate with total slip although these observations may well be true of real earthquakes we show that low pass filtering of strong motion seismograms can also produce some of these effects in inverted rupture models we test the accuracy of dc estimates by calculating them in low pass filtered versions of models a and b of guatteri and spudich 2000 models a and b are two different rupture models for a hypothetical m 6 5 earthquake and they have nearly identical rupture time slip and stress drop distributions and nearly identical predicted seismograms but dc for model b is about twice that for model a by low pass filtering slip models a and b at 1 0 hz we simulate the blurring effects of band limited waveform inversions on these slip models at each point on a fault graphic 1 is defined to be the slip at the time of the peak slip speed at that point low pass filtering the slip models causes an upward bias in dc inferred from stress slip curves and it causes an artificial correlation between graphic 2 and the total slip low pass filtering might also bias fracture energy high and radiated energy low these biases should be considered when interpreting dc derived from band limited slip models of real earthquakes 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

self healing slip pulses in dynamic rupture models due to velocity dependent strength abstract seismological observations of short slip duration on faults short rise time on seismograms during earthquakes are not consistent with conventional crack models of dynamic rupture and fault slip in these models the leading edge of rupture stops only when a strong region is encountered and slip at an interior point ceases only when waves from the stopped edge of slip propagate back to that point in contrast some seismological evidence suggests that the duration of slip is too short for waves to propagate from the nearest edge of the ruptured surface perhaps even if the distance used is an asperity size instead of the entire rupture dimension what controls slip duration if not dimensions of the fault or of asperities in this study dynamic earthquake rupture and slip are represented by a propagating shear crack for all propagating shear cracks slip velocity is highest near the rupture front and at a small distance behind the rupture front the slip velocity decreases as pointed out by heaton 1990 if the crack obeys a negative slip rate dependent strength relation the lower slip velocity behind the rupture front will lead to strengthening that further reduces the velocity and under certain circumstances healing of slip can occur the boundary element method of hamano 1974 is used in a program adapted from andrews 1985 for numerical simulations of mode ii rupture with two different velocity dependent strength functions for the first function after a slip weakening displacement the crack follows an exponential velocity weakening relation the characteristic velocity v 0 of the exponential determines the magnitude of the velocity dependence at dynamic velocities the velocity dependence at high velocity is essentially zero when v 0 is small and the resulting slip velocity distribution is similar to slip weakening if v 0 is larger rupture propagation initially resembles slip weakening but spontaneous healing occurs behind the rupture front the rise time and rupture propagation velocity depend on the choice of constitutive parameters the second strength function is a natural log velocity dependent form similar to constitutive laws that fit experimental rock friction data at lower velocities slip pulses also arise with this function for a reasonable choice of constitutive parameters slip pulses with this function do not propagate at speeds greater than the raleighwave velocity the calculated slip pulses are similar in many aspects to seismic observations of short rise time in all cases of self healing slip pulses the residual stress increases with distance behind the trailing edge of the pulse so that the final stress drop is much less than the dynamic stress drop in agreement with the model of brune 1976 and some recent seismological observations of rupture

energy release and heat generation during the 1999 ms7 6 chi chi taiwan earthquake 1 on 20 september 1999 the ms7 6 chi chi earthquake ruptured the chelungpu fault in central taiwan integrating observed and inversed results of source parameters the fracture energy eg and frictional energy ef on the fault and its northern and southern segments are estimated together with given values of strain energy e and seismic radiation energy es the seismic efficiency i e es e and the radiation efficiency i e r es es eg are evaluated the average fracture energy per unit area g is also calculated from eg the frictional heat caused by dynamic frictional stress is calculated from ef results show a marked difference in source properties between the two segments the average frictional and ambient stress levels on the two segments are estimated the total energy budget of and heat generated by the earthquake are elucidated based on a two dimensional faulting model with frictional heat both observed and calculated results suggest the possible existence of fluids which produced suprahydrostatic gradients on the fault during faulting lubrication and thermal fluid pressurization might play a significant role on rupture

upgrading a synthetic seismicity model for more realistic fault ruptures a quasi static synthetic seismicity model for multiple interacting faults of any orientation is upgraded to include a more realistic rupture model in the original model entire ruptures covering many fault patches occur instantaneously and rupture propagation is ignored now time is introduced into the rupture process via a patch interaction delay in proportion to distance shear wave velocity also to try to mimic observed rise times for real earthquakes a simple time dependent friction law is used comparison of results for the old and new models for a simple case of two strike slip faults show that clustering of large events is somewhat reduced in the new model it remains to be seen if this conclusion will be verified in more complex fault networks the new model takes about twice as much computation time but the results may now be used to calculate synthetic seismograms

distribution of dynamic and static stress changes during 2000 tottori japan earthquake brief interpretation of the earthquake sequences foreshocks mainshock and aftershocks 1 the dynamic and static stress changes during the 2000 tottori earthquake have been recovered from the results of waveform inversion we use the dem to solve the elastodynamic equation specifying the slip along the fault obtained by a kinematic fault model the resulting shear stress distribution suggests an explanation of the foreshock and aftershock distributions we conclude that the fault zone heterogeneity is strong and most of the foreshocks and aftershocks were located in the zone of negative stress drop and mainly in the area surrounding the asperity this suggests that the asperity behaved as a barrier during the foreshocks and after the main shock the stress in the area surrounding the asperity increased and triggered most of the aftershocks the foreshock distribution was confined to a finite localized zone in the central part of the fault suggesting that this zone was bordered by barriers

ionospheric disturbances observed by dmsp at middle to low latitudes during the magnetic storm of june 4 6 1991 this paper extends a recent study of electric field penetration into the inner magnetosphere observed by the combined release and radiation effects crres satellite and the defense meteorological satellite program dmsp satellite f8 during the magnetic storm of june 4 6 1991 to consider its ionospheric consequences effects include the development of 1 km s subauroral ion drift said structures the formation of midlatitude density troughs and the vertical transport of equatorial plasma bubbles nearly simultaneous auroral electron and plasma drift measurements were acquired by three dmsp satellites with f8 and f9 in one hemisphere and flo in the other moderate to strong said structures were consistently detected for 10 hours during the early main phase of the storm weak saids were encountered during 8 hours of the early recovery phase dmsp data show that saids with similar characteristics developed at magnetically conjugate locations and extended for at least 3 hours in local time simultaneous measurements show that the saids spanned temporally grooving but latitudinally narrow plasma density troughs these observations suggest that the magnetospheric sources of saids act more like voltage than current generators energetic electron fluxes electric fields and plasma waves measured by crres indicate that during this storm the ring current shielding charges and said sources were located in regions of high plasma density characteristic of the plasmasphere the sequence in which dmsp detected equatorial plasma density irregularities is consistent with model predictions that stormtime electrodynamics at low latitudes operate on distinctive fast and slow timescales fejer and schcrliess 1997

surface rupturing and buried dynamic rupture models calibrated with statistical observations of past earthquakes in the context of the slip weakening friction model and simplified asper ity models for stress state we calibrate dynamic rupture models for buried and surface rupturing earthquakes constrained with statistical observations of past earth quakes these observations are the kinematic source models derived from source in versions of ground motion and empirical source models of seismic moment and rupture area the calibrated parameters are the stress drop distribution on the fault and average stress drop we develop a set of dynamic rupture models that consist of asperities and surrounding background areas the distribution of dynamic stress drop outside the asperity is characterized by a fraction of the stress drop on the as perity from this set of models we identify dynamic fault models with defined stress drop characteristics that satisfy the observations the selected dynamic fault models show that surface rupturing earthquakes are characterized by a large area of negative stress drop surrounding the asperities while buried earthquakes present positive or zero stress drop in addition the calibrated fault models that match the observations show that the average stress drop is independent of earthquake size for buried earth quakes but scale dependent for surface rupturing earthquakes this suggests that in the context of our parameterization buried earthquakes follow self similarity scaling and surface rupturing earthquakes break this self similarity we apply the calibrated dynamic models to simulate near source ground motion consistent with observations that suggest that buried earthquakes generate stronger ground motion than surface rupturing earthquakes at high frequency we propose possible mechanisms that satisfy this observation as follows buried rupture has a hypocenter location below the as perity this can produce strong directivity of the slip velocity function toward the free surface that effect in addition to a reduced fault area and low fracture energy during rupture may be significant in enhancing high frequency ground motion on the other hand surface rupturing earthquakes have a shallow hypocenter large fracture energy on the asperities and enhanced energy absorption due to large areas of negative stress drop in the background area these characteristics of large earthquakes inhibit severe directivity effects on the slip velocity function directly toward the free surface redu cing the high frequency ground motion

influence of an inhomogeneous stress field and fault zone thickness on the displacements and stresses induced by normal faulting we used the finite element method fem to simulate the temporal evolution of an inhomogeneous stress field with the aim of investigating the displacement and stress fields induced by slip upon a normal fault zone with a given thickness the modeling results reveal 1 the site of maximum slip is along the lower part of the fault not near the surface and the locations upon the fault of maximum shear stress drop and maximum slip do not coincide 2 the maximum horizontal and vertical displacements at the ground surface are located at some distance from the fault trace and 3 slip of the fault may result in two failure regions near the ground surface one close to the surface trace of the fault and the other within the hanging wall at some distance from the fault it is in the regions that a succession of normal faults is typically observed in geological examples these results differ from those obtained using seismic dislocation theory in semi infinite space where the effects of the inhomogeneous stress field and thickness of the fault zone are not considered

rupture process for micro earthquakes inferred from borehole seismic recordings we investigate the spatial extent of rupture and variability in fault slip for micro earthquakes by inverting seismic moment rate functions derived from empirical green s function deconvolution by using waveforms from an array of borehole seismometers we determine the spatial distributions of fault slip for m 3 earthquakes that occurred along the hayward fault in central california and identify a variety of slip behaviors including subevents directivity and high stress drop the 2013 m w 3 2 orinda earthquake exhibits a complex rupture process involving two subevents with northwest and up dip directivity the two subevents release 43 and 18 of the total seismic moment 6 7 1013 n m and their inferred peak stress drops are 18 and 8 mpa the 2011 m w 4 0 berkeley and 2012 m w 4 0 el cerrito earthquakes are marked by high stress drop the inferred peak and mean stress drops are about 100 130 and 40 mpa respectively which suggests that there are locally high levels of fault strength on the hayward fault our finite source modeling suggests that the radiation efficiency determined for these two earthquakes is very low 0 1 and implies that most energy is dissipated during the earthquake rupture process

characteristic features of local breakdown near a crack tip in the transition zone from nucleation to unstable rupture during stick slip shear failure abstract characteristic features of local breakdown near a propagating crack tip during slip failure nucleation and its transition process to unstable rupture under mode ii conditions are experimentally studied using a rock sample with a simulated fault the size of which is large compared with the size of the breakdown zone in addition distinctive features in the nucleation zone are compared with those in the zone of steady dynamic rupture propagation it is found that the local breakdown stress drop the local dynamic stress drop the local stress increase immediately before the slip weakening process the dimensionless parameter s defined by das and aki and the shear fracture energy are functions of crack length in the transition zone from nucleation to unstable rupture i e they all increase with crack growth in the nucleation zone this is contrasted with the observations in the zone of steady dynamic rupture propagation where these physical parameters do not depend on crack length an increase in the magnitudes of these parameters with crack growth in the nucleation zone results in increasing resistance to crack extension with crack growth in the nucleation zone the reason for the increase in crack growth resistance in this zone is that a slip failure nucleus is formed at a point or zone where the local strength and the shear fracture energy are at a minimum this basic physical information can provide the key to earthquake prediction since the nucleation itself is a precursory phenomenon the ratio of the size of the breakdown zone to the crack length decreases abruptly beyond the critical crack length rupture velocity v accelerates with crack growth and reaches the shear wave velocity or super shear and v is found to be expressed empirically as a function of crack length in the nucleation zone a positive correlation is found between s and v in the nucleation zone v increases with increasing s however no such correlation was observed in the zone of steady dynamic rupture propagation where v seems to have reached the shear wave velocity of super shear for s 0 5 this is consistent with das and aki s numerical result the interrelationships between shear stress slip displacement slip velocity and acceleration near the crack tip during the breakdown in the nucleation zone are revealed and they are compared with those in the zone of dynamic rupture propagation the present experiments suggest that the relationship between shear stress and slip displacement is more fundamental during the breakdown process than the relationship between shear stress and slip velocity a model is presented to describe the breakdown process in the nucleation zone

dynamic rupture and stress change in a normal faulting earthquake in the subducting cocos plate a large nearly vertical normal faulting earthquake mw 7 1 took place in 1997 in the cocos plate just beneath the ruptured fault zone of the great 1985 michoacan thrust event mw 8 1 dynamic rupture and resultant stress change during the 1997 earthquake have been investigated on the basis of near source strong motion records together with a 3 d dynamic model dynamically consistent waveform inversion reveals a highly heterogeneous distribution of stress drop including patch like asperities and negative stress drop zones zones of high stress drop are mainly confined to the deeper southeastern section of the vertical fault where the maximum dynamic stress drop reaches 280 bars 28 mpa the dynamically generated source time function varies with location on the fault and yields a short slip duration which is caused by a short scalelength of stress drop heterogeneities the synthetic seismograms calculated from the dynamic model are generally consistent with the strong motion velocity records in the frequency range lower than 0 5 hz the pattern of stress drop distribution appears in some sense to be consistent with that of coseismic changes in shear stress resulting from the 1985 thrust event this consistency suggests that the stress transfer from the 1985 event to the subducting plate could be one of the possible mechanisms that increased the chance of the occurrence of the 1997 earthquake

complexity of seismicity due to highly rate dependent friction we study a simple antiplane fault of finite length embedded in a homogeneous isotropic elastic solid in order to understand the origin of seismic source heterogeneity in the presence of nonlinear rate dependent friction all the mechanical properties of the medium and friction are assumed to be homogeneous starting from a heterogeneous initial stress distribution we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for more than 20 000 events in some cases the style of seismicity produced by this model is determined by a control parameter which measures the degree of rate dependence of friction for classical friction models with rate independent friction no complexity appears and seismicity is perfectly periodic for weakly rate dependent friction seismicity becomes slightly nonperiodic but most events are still characteristic earthquakes when friction is highly rate dependent seismicity is completely irregular and ruptures of all sizes occur inside the fault highly rate dependent friction destabilizes the healing process producing premature healing of slip and partial stress drop premature healing causes rupture to take the form of narrow propagating slip episodes the so called heaton s 1990 pulses partial stress drop produces large variations in the state of stress which in turn produce earthquakes of different sizes we make the conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity

diffusion creep of enstatite at high pressures under hydrous conditions mantle convection and large scale plate motion depend critically on the nature of the lithosphere asthenosphere boundary and thus on the viscosity structure of earth s upper mantle which is determined by the rheological properties of its constituent minerals to constrain the flow behavior of orthopyroxene the second most abundant constituent of the upper mantle deformation experiments were carried out in triaxial compressive creep on fine grained similar to 6m samples of enstatite at high pressures 3 8 6 3gpa and high temperatures 1323 1573k using a deformation dia apparatus based on results from this study the deformation behavior of enstatite is quantitatively presented in the form of a flow law that describes the dependence of deformation rate on differential stress water fugacity temperature and pressure specifically the creep rate depends approximately linearly on stress indicating deformation in the diffusion creep regime a least squares regression fit to our data yielded a flow law for diffusion creep with an activation energy of similar to 200kj mol and an activation volume of similar to 14x10 6 m 3 mol the magnitude of the water weakening effect is similar to that for olivine with a water fugacity exponent of r approximate to 0 7 this strong dependence of viscosity on water fugacity concentration indicates that the viscosity of an orthopyroxene bearing mantle varies from one geological setting to another depending on the large scale water distribution based on the rheology contrast between olivine and enstatite we conclude that olivine is weaker than enstatite throughout most of the upper mantle except in some shallow regions in the diffusion creep regime

source spectral scaling and stress release estimates using strong motion records in greece a data set of 50 horizontal strong motion accelerograms from 18 main events from moderate and large earthquakes 5 2 mw 6 9 and distances 9 km r 116 km occurred at three different tectonic settings normal strike slip and thrust faulting were used to estimate stress parameters three different definitions of stress release were adopted the classical brune the dynamic rms root mean square and the apparent stress corner frequencies f0 based on the andrews 1986 methodology were computed and using these calculations seismic moments were estimated from independent sources teleseismic data the calculated corner fre quencies seem to be consistent based on seismic moments with previous estimates for the same area based on those source parameters a theoretical x square spectrum was derived from describing source radiation in terms of earthquake size frequency dependent site amplifications were included in the simulations separating the re cordings in three different categories b c and d corresponding to nehrp na tional earthquake hazards reduction program site classification the theoretical and observational spectra are in good agreement in a frequency range from 0 05 to 25 hz except for the cases where steep topography affects the recorded data a poor fit is also observed in cases where the distance of the recorded data is greater than 100 km and the attenuation model seems to be inadequate to describe the theoretical model the calculated parameters present a mean value of 55 16 bars for brune stress 72 26 bars for dynamic rms stress and 17 10 bars for apparent stress for all determined stresses the thrust faulting earthquakes exhibited higher values than the calculated normal and strike slip fault seismic events

dynamic rupture analysis inversion for the source process of the 1990 izu oshima japan earthquake m 6 5 a waveform inversion has been applied to strong motion data using a dynamic shear crack model we studied the 1990 izu oshima earthquake mjma 6 5 which has vertical strike slip faulting with unilateral rupture propagation the inversion has two steps a waveform inversion and a crack inversion that are applied iteratively a waveform inversion is used to determine the distribution of rupture starting times and slip dislocations using the slip functions calculated by the initial crack model or by previous crack inversion a crack inversion is used to calculate dynamic crack propagation that explains the results of the above inversion in this step we use the estimated rupture times as a locking fracture criterion the maximum shear stress attained before a fault segment breaks gives a lower bound estimate of the peak shear strength at each fault segment then the dynamic stress drop distribution is estimated from the slip distribution obtained from waveform inversion assuming a dynamic crack model from the results we determine the rise time distribution and the distribution of a dimensionless stress ratio s defined as strength excess stress drop our analysis gives the following picture of the rupture process of the 1990 izu oshima earthquake 1 an asperity type faulting having large slip and high stress drop was detected in the region around the initiation point of rupture 2 south of the asperity zone barrier type faulting characterized by incoherent propagating rupture small slip long rise time and high strength excess was detected this zone corresponds to the intersection of the fault with the 1978 earthquake mjma 7 0

constraint of fault parameters inferred from nonplanar fault modeling 1 we study the distribution of initial stress and frictional parameters for the 28 june 1992 landers california earthquake through dynamic rupture simulation along a nonplanar fault system we find that observational evidence of large slip distribution near the ground surface requires large nonzero cohesive forces in the depth dependent friction law this is the only way that stress can accumulate and be released at shallow depths we then study the variation of frictional parameters along the strike of the fault for this purpose we mapped into our segmented fault model the initial stress heterogeneity inverted by peyrat et al 2001 using a planar fault model simulations with this initial stress field improved the overall fit of the rupture process to that inferred from kinematic inversions and also improved the fit to the ground motion observed in southern california in order to obtain this fit we had to introduce an additional variations of frictional parameters along the fault the most important is a weak kickapoo fault and a strong johnson valley fault

generation mechanism of slow earthquakes numerical analysis based on a dynamic model with brittle ductile mixed fault heterogeneity 1 various characteristics have been discovered for small slow earthquakes occurring along subduction zones which are deep nonvolcanic tremor low frequency earthquakes lfes and very low frequency earthquakes vlfs in this study we model these slow earthquakes using a dynamic model consisting of a cluster of frictionally unstable patches on a stable background the controlling parameters in our model are related to the patch distribution and the viscosity of both the patches and the background by decreasing patch density or increasing viscosity we observed the transition in rupture propagation mechanism that is from fast elastodynamic interactions characterized by an elastic wave propagation to slow diffusion limited by viscous relaxation times of traction on fault patches and or background some sets of these geometrical and frictional parameters collectively explain the moment rate functions source spectra and scaled energy of observed slow earthquakes in addition we successfully explain both parabolic and constant velocity migrations in the case of the diffusion limited rupture therefore the observed various characteristics of tremor lfes vlfs and potentially slow slip events may be essentially explained by our simple model with a few parameters describing source structures and frictional properties of brittle ductile transition zones along plate boundaries

compressive failure of mudstone samples containing quartz veins using rapid ae monitoring the role of asperities this paper presents the results of an ongoing experimental investigation of compressive failure of homogeneous and heterogeneous rocks we used a rapid data acquisition system to monitor the spatio temporal distribution of acoustic emission ae during fault nucleation under conditions of either constant rate loading or static loading in order to examine the effect of asperities on faulting we conducted a series of experiments on mudstone with quartz veins the bedding planes of the mudstone were oriented at an angle of 30 with the maximum compressive stress and were expected as the fracture plane the quartz veins are much stronger than the bedding plane of mudstone and thus the samples model faults having strong asperities experimental results show that 1 ae activity initiated close to the peak stress and almost all ae hypocenters appeared at the intersections of the veins and the fault planes suggesting that the vein asperities control faulting 2 the b value changed with time and shows multiple large and short term fluctuations 3 the change of the b value correlated closely with the spatio temporal hierarchy of the fracture process and 4 fault segments along the bedding plane show behavior of slip having large compressive deformation before the peak stress while the vein asperities show large precursor dilatancy prior to dynamic rupture these experimental results are helpful for the understanding of seismic precursor phenomena associated with strain localization dilatancy change in level of ground water as well as the b value

scaling relationships of source parameters for slow slip events to better understand the physical mechanisms of slow slip events sses detected worldwide we explore the scaling relationships of various source parameters and compare them with similar scaling laws for earthquakes these scaling relationships highlight differences and similarities between slow slip events and earthquakes and hold implications for the degree of heterogeneity and fault healing characteristics the static stress drop remains constant for different sized events as is observed for earthquakes however the static stress drop of slow slip events is within a range of 0 01 1 0 mpa 1 2 orders of magnitude lower than that found for earthquakes which could be related to the low stress state on the fault the average rupture velocity ranging from kilometers per second to kilometers per day decreases linearly with increasing seismic moment in log log space unlike earthquakes that are nearly constant this inverse relationship of rupture velocity with seismic moment could be related to the heterogeneity of fault properties slow slip events typically have ratios of event duration over dislocation rise time less than 3 while earthquakes have ratios greater than 3 this indicates that slow slip events are less pulselike than earthquakes in their mode of propagation and suggests that the healing behind the rupture front is delayed the recurrence statistics of slow slip events on the northern cascadia subduction zone are weakly time predictable and moderately antislip predictable that is the event size and preevent recurrence interval are anticorrelated which may indicate that healing between events strengthens the fault with time online material table of source parameters and data sources

a grid search inversion method looking for the best classification of polyphase fault slip data abstract a novel grid search based stress inversion method is developed in this paper to find the global minima in the solution region for the classification of fault slip data into many single phase subsets exhaustively repetitious grid searches are taken to deal with possible local minima in a departure from existing grid search based inversion methods two stopping rules to stop at the abrupt change of the objective function or at the least change of the classification are adopted in the method to look for the best classification much calculation time is saved by using a modified version of conventional grid search the feasibility of this proposed method is demonstrated by applying it to two artificial examples and two real examples however enormous time in calculation is still needed in the case of a data set either with a large number of data or for a large number of assigned subsets

dynamic faulting under rate dependent friction we discuss the effects of rate dependent friction on the propagation of seismic rupture on active faults several physicists using burridge and knopoff s box and spring model of faulting have proposed that fault complexity may arise from the spontaneous development of a self similar stress distribution on the fault plane if this model proves to be correct it has important consequences for the origin of the complexity of seismic sources in order to test these ideas on a more realistic earthquake model we developed a new boundary integral equation method for studying rupture propagation along an antiplane fault in the presence of nonlinear rate dependent friction we study rupture dynamics of models with single and twin asperities in our models asperities are places on the fault with a higher value of prestress othewise all fault parameters are homogeneous we show that for models with such asperities a slip velocity weakening friction leads to the propagation of supersonic healing phases and to the spontaneous arrest of fracture if the prestress outside the asperities is low enough for models with asperities we can also observe narrow slip velocity pulses qualitatively similar to the so called heaton pulses observed in some earthquake accelerograms we also observe a complex distribution of stress after the rupture that depends on details of the initial distribution of asperities and on the details of the friction law

conditions for earthquake surface rupture along the san andreas fault system california earthquake ruptures that nucleate along faults at depth can only propagate to the earth s surface if the shear stresses during faulting exceed the yield strength of the uppermost crust where fluid pressures are usually hydrostatic suprahydrostatic fluid pressures may be required at depth for the seismic reactivation of misoriented faults such as the san andreas fault system based on fracture criteria for the wall and fault rock slip events that can cause ground breakage are estimated to occur at seismogenic depth along fault planes that form angles of less than 65 on average with the maximum principal stress direction such slip events require minimum shear stresses of about 30 mpa at hypocentral depth for faults with long interseismic periods and thus inferred high cohesive strength the predicted reactivation angle is 55 suggesting that several segments of the san andreas fault system in southern california including the san bernardino area parts of the elsinore fault zone and the san jacinto fault provide the most probable hypocentral sites for future large earthquakes that disrupt the surface to provide further constraints on locations for such earthquakes we urgently need to investigate frictional properties of fault rocks and the state of stress at depth by employing laboratory experiments focal mechanism studies and by drilling into seismically active faults such as the san andreas fault system

numerical simulations of large earthquakes dynamic rupture propagation on heterogeneous faults our current conceptions of earthquake rupture dynamics especially for large earthquakes require knowledge of the geometry of the faults involved in the rupture the material properties of the rocks surrounding the faults the initial state of stress on the faults and a constitutive formulation that determines when the faults can slip in numerical simulations each of these factors appears to play a significant role in rupture propagation at the kilometer length scale observational evidence of the earth indicates that at least the first three of the elements geometry material and stress can vary over many scale dimensions future research on earthquake rupture dynamics needs to consider at which length scales these features are significant in affecting rupture propagation

dynamic 3d simulations of earthquakes on en echelon faults one of the mysteries of earthquake mechanics is why earthquakes stop this process determines the difference between small and devastating ruptures one possibility is that fault geometry controls earthquake size we test this hypothesis using a numerical algorithm that simulates spontaneous rupture propagation in a three dimensional medium and apply our knowledge to two california fault zones we find that the size difference between the 1934 and 1966 parkfield california earthquakes may be the product of a stepover at the southern end of the 1934 earthquake and show how the 1992 landers california earthquake followed physically reasonable expectations when it jumped across en echelon faults to become a large event if there are no linking structures such as transfer faults then strike slip earthquakes are unlikely to propagate through stepover s 5 km wide

pulse like and crack like ruptures in experiments mimicking crustal earthquakes theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws stress conditions on faults energy partition and heat generation during earthquakes scaling laws and spatiotemporal complexity of fault slip early theoretical models treated earthquakes as crack like ruptures but seismic inversions indicate that earthquake ruptures may propagate in a self healing pulse like mode a number of explanations for the existence of slip pulses have been proposed and continue to be vigorously debated this study presents experimental observations of spontaneous pulse like ruptures in a homogeneous linear elastic setting that mimics crustal earthquakes reveals how different rupture modes are selected based on the level of fault prestress demonstrates that both rupture modes can transition to supershear speeds and advocates based on comparison with theoretical studies the importance of velocity weakening friction for earthquake dynamics

linearized inversion for fault rupture behavior application to the 1984 morgan hill california earthquake we present a technique to infer the rupture history of an earthquake from near source records of ground motion unlike most previous studies each point on the fault is assumed to slip only once when the rupture front passes with a spatially variable slip intensity in this parameterization the data are linearly related to slip intensity but nonlinearly related to rupture time we perform a linearized inversion for slip intensity and rupture time by iteratively perturbing an assumed starting model the inverse problem for the model perturbation is solved using a tomographic back projection technique smoothing and inequality constraints are applied to ensure that the resulting solution is stable and feasible asymptotic ray theory is used to calculate theoretical seismograms and partial derivatives with respect to model parameters in test cases with noisy synthetic data sets we found that it is possible to distinguish between rupture models having variable slip amplitude and models having variable rupture velocity provided the station coverage is adequate we apply the technique to recordings of the april 24 1984 morgan hill earthquake the results indicate that slip amplitude on the fault plane was extremely variable and that the rupture front did not propagate uniformly away from the hypocenter in particular rupture was delayed on a 12 km2 section of the fault approximately 14 km to the southeast of the hypocenter the rupture front surrounded the region which subsequently failed with a component of rupture propagation back toward the hypocenter similar behavior has been observed in dynamic rupture models with stress or strength inhomogeneities this segment of the fault ruptured with a large slip amplitude releasing 12 of the total seismic moment from 4 of the total area of the aftershock zone the surface trace of this section of the fault is characterized by a complex left step that could act to increase the normal stress acting across the fault however the distribution of aftershocks suggests that the fault at depth is simpler and that it may bend to the right in either case our rupture model suggests that this segment of the fault represents an asperity which initially resisted rupture but eventually ruptured massively we estimate the shear fracture energy for this earthquake to be 2 106 j m2

dynamic source modeling of the 1978 and 2005 miyagi oki earthquakes interpretation of fracture energy 1 we constructed spontaneous dynamic rupture models of the 1978 and 2005 miyagi oki japan earthquakes which occurred on the same plate boundary repeatedly with different magnitudes to reproduce the slip and the rupture velocity predicted by kinematic source models comparison of the dynamic source parameters of these events enables us to consider the behavior of rupture dynamics during different source processes on the same fault in particular we compared the stress drop and the fracture energy which can be evaluated stably between two events the maximum and average values of the stress drop over the whole fault plane and on an asperity that ruptured during both events are almost identical in the two models on the other hand the 1978 event whose magnitude was larger than the 2005 event has a fracture energy value larger than that of the 2005 event not only over the whole fault but also on the asperity that ruptured repeatedly these results are consistent with previous studies of the scaling of fracture energy with the seismic moment we also compared the fracture energy values with those of other inland events estimated for previous studies our values for the miyagi oki earthquakes are smaller differences in tectonic setting between shallow inland events and deep subduction zone events might cause this discrepancy

the 1994 northridge california earthquake investigation of rupture velocity risetime and high frequency radiation a hybrid global search algorithm is used to solve the nonlinear problem of calculating slip amplitude rake risetime and rupture time on a finite fault thirty five strong motion velocity records are inverted by this method over the frequency band from 0 1 to 1 0 hz for the northridge earthquake four regions of larger amplitude slip are identified one near the hypocenter at a depth of 17 km a second west of the hypocenter at about the same depth a third updip from the hypocenter at a depth of 10 km and a fourth updip from the hypocenter and to the northwest the results further show an initial fast rupture with a velocity of 2 8 to 3 0 km s followed by a slow termination of the rupture with velocities of 2 0 to 2 5 km s the initial energetic rupture phase lasts for 3 s extending out 10 km from the hypocenter slip near the hypocenter has a short risetime of 0 5 s which increases to 1 5 s for the major slip areas removed from the hypocentral region the energetic rupture phase is also shown to be the primary source of high frequency radiation 1 15 hz by an inversion of acceleration envelopes the same global search algorithm is used in the envelope inversion to calculate high frequency radiation intensity on the fault and rupture time the rupture timing from the low and high frequency inversions is similar indicating that the high frequencies are produced primarily at the mainshock rupture front two major sources of high frequency radiation are identified within the energetic rupture phase one at the hypocenter and another deep source to the west of the hypocenter the source at the hypocenter is associated with the initiation of rupture and the breaking of a high stress drop asperity and the second is associated with stopping of the rupture in a westerly direction

attempting to bridge the gap between laboratory and seismic estimates of fracture energy 1 to investigate the behavior of the fracture energy associated with expanding the rupture zone of an earthquake we have used the results of a large scale biaxial stick slip friction experiment to set the parameters of an equivalent dynamic rupture model this model is determined by matching the fault slip the static stress drop and the apparent stress after confirming that the fracture energy associated with this model earthquake is in reasonable agreement with corresponding laboratory values we can use it to determine fracture energies for earthquakes as functions of stress drop rupture velocity and fault slip if we take account of the state of stress at seismogenic depths the model extrapolation to larger fault slips yields fracture energies that agree with independent estimates by others based on dynamic rupture models for large earthquakes for fixed stress drop and rupture speed the fracture energy scales linearly with fault slip

one dimensional dynamical modeling of slip pulses abstract propagating slip pulses in earthquake ruptures heaton 1990 are studied based on a one dimensional n degree of freedom dynamical spring slider system with a constant ratio of static to dynamic frictional forces results show that for almost all cases in the study the rise time of ruptures at a particular site is much smaller than the duration of ruptures along the whole fault a propagating slip pulse is generated along the fault hence simulation results are in agreement with heaton s propagating slip pulse model

rupture modes in laboratory earthquakes effect of fault prestress and nucleation conditions seismic inversions show that earthquake risetimes may be much shorter than the overall rupture duration indicating that earthquakes may propagate as self healing pulse like ruptures several mechanisms for producing pulse like ruptures have been proposed including velocity weakening friction interaction of dynamic rupture with fault geometry and local heterogeneity and effect of bimaterial contrast we present experimental results on rupture mode selection in laboratory earthquakes occurring on frictional interfaces which were prestressed both in compression and in shear our experiments demonstrate that pulse like ruptures can exist in the absence of a bimaterial effect or of local heterogeneities we find a systematic variation from crack like to pulse like rupture modes with both 1 decreasing nondimensional shear prestress and 2 decreasing absolute levels of shear and normal prestress for the same value of nondimensional shear prestress both pulse like and crack like ruptures can propagate with either sub rayleigh or supershear rupture speeds our experimental results are consistent with theories of ruptures on velocity weakening interfaces implying that velocity weakening friction plays an important role in governing the dynamic behavior of earthquake ruptures we show that there is no measurable fault normal stress decrease on the fault plane due to the nucleation procedure employed in experiments and hence this is not a factor in the rupture mode selection we find that pulse like ruptures correspond to the levels of nondimensional shear prestress significantly lower than the static friction coefficient suggesting that faults hosting pulse like ruptures may operate at low levels of prestress compared to their static strength

dynamics of fault interaction parallel strike slip faults we use a two dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures our results indicate that a strike slip earthquake is unlikely to jump a fault step wider than 5 km in correlation with field observations of moderate to great sized earthquakes we also find that dynamically propagating ruptures can jump both compressional and dilational fault steps although wider dilational fault steps can be jumped dilational steps tend to delay the rupture for a longer time than compressional steps do this delay leads to a slower apparent rupture velocity in the vicinity of dilational steps these dry cases assumed hydrostatic or greater pore pressures but did not include the effects of changing pore pressures in an additional study we simulated the dynamic effects of a fault rupture on undrained pore fluids to test sibson s 1985 1986 suggestion that wet dilational steps are a barrier to rupture propagation our numerical results validate sibson s hypothesis by demonstrating that the effect of the rupture on the undrained pore fluids is to inhibit the rupture from jumping dilational stepovers the basis of our result differs from sibson s hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments

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

regions of reduced static stress drop near fault tips for large strike slip earthquakes abstract we present a case study of slip distributions for the 1992 landers m w 7 3 and the 1999 hector mine m w 7 1 earthquakes in california s mojave desert slip distributions as determined from geophysical inversion of geodetic strong ground motion and teleseismic data are complex heterogeneous and often exhibit linearly tapering or concave upward patterns toward segment tips distinctly different from the elliptical slip distributions characteristic of uniform stress drop mechanical interaction of discontinuous fault segments fully explains the reduced slip near the southern termination of the camp rock emerson fault segment of the landers rupture however numerical models demonstrate that such interactions are insufficient to explain slip distributions observed at other segment terminations numerical models demonstrate that long 5 25 km zones of reduced stress drop in the vicinity of some rupture segment terminations can explain the slip distributions for these large earthquakes zones of reduced stress drop are implemented as regions of increased coulomb strength slip distributions are improved 30 70 relative to models with uniform stress drop regions of reduced stress drop appear to play a relatively greater role near segment tips at which rupture terminates than near segment tips at which rupture jumps to a nearby fault segment similar results are obtained implementing discrete stepwise and spatially linear reductions of stress drop plausible mechanical explanations for such zones of reduced stress drop include heterogeneous fault strength or friction spatial or temporal changes in pore pressure geometric complexity of the fault surface heterogeneity of normal tractions resolved on the fault surface inelastic deformation and dynamic rupture effects

how strong is an asperity a recent study of repeating earthquakes on the san andreas fault in central california by nadeau and johnson 1998 found that the smallest events occurred on patches having a linear dimension of the order of 0 5 m displacements of about 2 cm and stress drops of the order of 2000 mpa roughly 10 times larger than rock strengths measured in the laboratory the stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 mpa at about magnitude 6 these large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed a hierarchical fractal asperity model is presented which is based on recent laboratory observations of contact distributions in sliding friction experiments this cantor dust model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size the spatial distribution of hypocenters in the parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of d 1 and discrete rescaling factor of about 20

influence of measurement errors on stress estimated from single phase fault slip data abstract the influence of measurement errors on estimated stress is investigated in reduced sigma space on the basis of numerous sets of homogeneous fault slip data generated under different extensional stress and then modified by varying measurement errors for the striation pitch for a given measurement error error propagation to a datum vector depends upon the controlling stress and fault orientation with respect to the controlling stress for a given stress ratio both the dispersion of datum vectors i e root mean square cosine and the misfit between estimated and controlling stresses i e similarity and stress difference increase with the increase in measurement error the two measures appear generally smaller at small measurement error than at large error and have a strong tendency to increase with stress ratio for a given measurement error the measurement error has an apparent effect in estimated stress only if it is greater than 10 implying a limited effect on homogeneous fault slip data that is carefully measured at outcrop in which a smaller error can be guaranteed in a majority of cases

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

a slip pulse model with fault heterogeneity for low frequency earthquakes and tremor along plate interfaces 1 deep low frequency earthquakes lfes and nonvolcanic tremor have distinctive characteristics unlike those of regular earthquakes including strong anisotropy in their migration velocity and source spectra displaying 1 f decay we show that a physical model can explain these features in a simple framework with slip pulses originating on fault heterogeneity and triggered by slow slip events lfe tremor source areas in the model consist of unstable patches sparsely and heterogeneously distributed following a gaussian distribution the difference in their migration speeds along dip and along strike was reproduced without anisotropic rheological properties by introducing alignments of their sources similar to observed streaks of lfes tremor the key to reproducing inverse linear spectral decay is that the slip pulse has a constant mean moment rate this model provides new insights into the physical source process of lfes and tremor and should find practical use in assessing properties of deep plate interfaces

a constitutive scaling law and a unified comprehension for frictional slip failure shear fracture of intact rock and earthquake rupture 1 it is widely recognized that some of the physical quantities inherent in a rupture are scale dependent and the scale dependence is one of the most no facts and features of rupture phenomena the paper addresses how such scale dependent shear rupture of a broad range from laboratory scale frictional slip failure and shear fracture of intact rock to field scale rupture as an earthquake source can be unified by a single constitutive law noting that the earthquake rupture is a mixed process between frictional slip failure and the shear fracture of intact rock it is concluded that the constitutive law for the earthquake rupture be formulated as a unifying law that governs both frictional slip failure and shear fracture of intact rock it is demonstrated that the slip dependent constitutive law is such a unifying law and a constitutive scaling law is derived from laboratory data on both frictional slip failure and shear fracture of intact rock this constitutive scaling law enables one to provide a consistent and unified comprehension for scale dependent physical quantities inherent in the rupture over a broad range from small scale frictional slip failure and shear fracture in the laboratory to large scale earthquake rupture in the field

short slip duration in dynamic rupture in the presence of heterogeneous fault properties recent studies of strong motion data consistently show that the risetime duration of slip at particular locations on the fault is significantly shorter than the overall rupture duration the physical explanation for this observation and its implications have become central issues in earthquake source studies two classes of mechanisms have been proposed to explain short risetimes one explanation is that velocity weakening frictional behavior on the fault surface causes the fault to self heal this possibility is suggested by rate dependent friction observed in laboratory experiments and by some two dimensional dynamic numerical simulations of earthquake rupture it has recently been demonstrated however that the velocity dependence of friction observed in the laboratory is too weak to cause faults to self heal an alternative explanation for short risetimes is that spatially heterogeneous fault strength e g barriers limit the slip duration in this paper we investigate this second explanation for short risetimes by constructing a three dimensional dynamic rupture model for the 1984 morgan hill california earthquake mw 6 2 using a kinematic model previously obtained from waveform inversion of strong motion data we assume velocity independent friction and a critical stress fracture criterion and derive a dynamic model specified by the spatial distribution of dynamic stress drop and strength excess that reproduces the slip and rupture time of the kinematic model the slip velocity time functions calculated from this dynamic model are then used in a subsequent inversion to fit the strong motion data by alternating between dynamic and kinematic modeling we obtain a dynamic model that provides an acceptable fit to the recorded waveforms in this dynamic model the risetime is short over most of the fault which is attributable entirely to the short scale length slip stress drop heterogeneity required by the strong motion data a self healing mechanism such as strongly velocity dependent friction is not required to explain the short risetimes observed in this earthquake

constraining fault constitutive behavior with slip and stress heterogeneity 1 we study how enforcing self consistency in the statistical properties of the preshear and postshear stress on a fault can be used to constrain fault constitutive behavior beyond that required to produce a desired spatial and temporal evolution of slip in a single event we explore features of rupture dynamics that 1 lead to slip heterogeneity in earthquake ruptures and 2 maintain these conditions following rupture so that the stress field is compatible with the generation of aftershocks and facilitates heterogeneous slip in subsequent events our three dimensional finite element simulations of magnitude 7 events on a vertical planar strike slip fault show that the conditions that lead to slip heterogeneity remain in place after large events when the dynamic stress drop initial shear stress and breakdown work fracture energy are spatially heterogeneous in these models the breakdown work is on the order of mj m 2 which is comparable to the radiated energy these conditions producing slip heterogeneity also tend to produce narrower slip pulses independent of a slip rate dependence in the fault constitutive model an alternative mechanism for generating these confined slip pulses appears to be fault constitutive models that have a stronger rate dependence which also makes them difficult to implement in numerical models we hypothesize that self consistent ruptures could also be produced by very narrow slip pulses propagating in a self sustaining heterogeneous stress field with breakdown work comparable to fracture energy estimates of kj m 2

dynamic modeling of the 1992 landers earthquake we have used observed band pass filtered accelerograms and a previously determined slip distribution to invert for the dynamic rupture propagation of the 1992 landers earthquake in our simulations dynamic rupture grows under the simultaneous control of initial stress and rupture resistance by friction which we modeled using a simple slip weakening law we used a simplified landers fault model where the fault segments were combined into a single vertical planar fault by trial and error we modified an initial stress field inferred from the kinematic slip distribution proposed by wald and heaton 1994 until dynamic rupture generated a rupture history and final slip distribution that approximately matched those determined by the kinematic inversion we found that rupture propagation was extremely sensitive to small changes in the distribution of prestress and that a delicate balance with energy release rate controls the average rupture speed for the inversion we generated synthetic 0 5 hz ground displacements using an efficient green s function propagator method axitra this method enables us to propagate the radiation generated by the dynamic rupture to distances greater than those feasible using the finite difference method the dynamic model built by trial and error inversion provides a very satisfactory fit between synthetics and strong motion data we validated this model using records from stations used in the slip inversion as well as some which were not included we also inverted for a complementary model that fits the data just as well but in which the initial stress was perfectly uniform while rupture resistance was heterogeneous this demonstrates that inversion of ground motion is nonunique

dynamic stress changes during earthquake rupture we assess two competing dynamic interpretations that have been pro posed for the short slip durations characteristic of kinematic earthquake models de rived by inversion of earthquake waveform and geodetic data the first interpretation would require a fault constitutive relationship in which rapid dynamic restrengthen ing of the fault surface occurs after passage of the rupture front a hypothesized mechanical behavior that has been referred to as self healing the second inter pretation would require sufficient spatial heterogeneity of stress drop to permit rapid equilibration of elastic stresses with the residual dynamic friction level a condition we refer to as geometrical constraint these interpretations imply contrasting pre dictions for the time dependence of the fault plane shear stresses we compare these predictions with dynamic shear stress changes for the 1992 landers m 7 3 1994 northridge m 6 7 and 1995 kobe m 6 9 earthquakes stress changes are com puted from kinematic slip models of these earthquakes using a finite difference method for each event static stress drop is highly variable spatially with high stress drop patches embedded in a background of low and largely negative stress drop the time histories of stress change show predominantly monotonic stress change after passage of the rupture front settling to a residual level without significant evidence for dynamic restrengthening the stress change at the rupture front is usu ally gradual rather than abrupt probably reflecting the limited resolution inherent in the underlying kinematic inversions on the basis of this analysis as well as recent similar results obtained independently for the kobe and morgan hill earthquakes we conclude that at the present time the self healing hypothesis is unnecessary to explain earthquake kinematics

a new nonlinear finite fault inversion with three dimensional green s functions application to the 1989 loma prieta california earthquake 1 we present a new procedure to invert for kinematic source parameters on a finite fault on the basis of the reciprocity relation of the green s functions we use a newly developed fourth order viscoelastic finite difference algorithm to calculate three dimensional 3 d green s functions actually the tractions on the fault we invert the data for the unknown source parameters at the nodes or corners of the subfaults the source parameters within a subfault area are allowed to vary this variation is calculated through bilinear interpolation of the four nodal quantities we have developed a global nonlinear inversion algorithm that is based on simulated annealing methods to solve efficiently for the nodal parameters we apply this method to the 1989 loma prieta california m 6 9 earthquake for both a 1 d and 3 d velocity structure we show 1 the bilinear interpolation technique reduces the dependence of inversion results on the subfault size by naturally including the effects of nearby subfaults 2 while the number of synthetic seismograms that must be computed is greatly increased by the bilinear interpolation the structure of the inversion method minimizes the actual numbers of computations 3 as expected complexity in the velocity structure is mapped into the source parameters that describe the rupture process there are significant differences between faulting models derived from 1 d and 3 d structural models

stress breakdown time and slip weakening distance inferred from slip velocity functions on earthquake faults we estimate the critical slip weakening distance on earthquake faults by using a new approach which is independent of the estimate of fracture energy or radiated seismic energy the approach is to find a physically based relation between the breakdown time of shear stress tb the time of peak slip velocity tpv and the slip weakening distance dc from the time histories of shear stress slip and slip velocity at each point on the fault which can be obtained from dynamic rupture calculations using a simple slip weakening friction law numerical calculations are carried out for a dynamic shear crack propagating either spontaneously or at a fixed rupture velocity on a vertical fault located in a 3d half space and a more realistic horizontally layered structure with finite difference schemes the results show that tpv is well correlated with tb for faults even with a heterogeneous stress drop dis tribution except at locations near strong barriers and the fault edges we also inves tigate this relation for different types of slip weakening behavior we have applied the method to two recent strike slip earthquakes in western japan the 2000 tottori and the 1995 kobe events we integrated the slip velocity functions on the vertical fault obtained from kinematic waveform inversion of strong motion and teleseismic records from the arrival time of rupture tr to the time of the peak slip velocity tpv and we then corrected the slip obtained at tpv for the errors expected from the dynamic calculations it was found that the slip weakening dis tance d c estimated in the frequency window between 0 05 and 0 5 hz ranges between 40 and 90 cm on the two earthquake faults however if we consider the limited frequency resolution of the observed waveforms probable time errors in the slip velocity functions obtained from kinematic inversion and the uncertainty of the slip weakening behavior the above estimates may be those located between the minimum resolvable limit and the upper bound of their real values the estimated dc values do not necessarily seem to indicate larger values in the shallower part and smaller values in the deeper part of the fault but rather a spatially heterogeneous distribution that appears to be dependent on the local maximum slip this possible dependence might be interpreted by the frictional properties of the fault such as the degree of roughness or the thickness of gouge layers in addition to stress heterogeneities

stochastic fault stress implications for fault dynamics and ground motion up to the present time ground motion calculations for future earthquakes have almost exclusively been made from kinematic models however dynamic faulting models offer many benefits over kinematic models including the assurance the faulting models obey at least the general rules of elastodynamics and friction and contain a natural relationship between stress drop slip rise time and rupture velocity dynamic models also offer insight into the physics of the rupture and slip processes and can show how these processes lead to patterns of ground motion in the current work we use the 3d finite difference method and a suite of stochastic stress patterns with variable assumptions on strength and stress inhomogeneity to investigate two issues 1 the effect of assumptions about stress pattern on the evolution of rupture and slip on the fault and 2 the effect of these assumptions on the resultant ground motion we find that stress drop has a complicated relationship with slip rise time and rupture velocity especially in faults with strongly heterogeneous strength we also find that these inhomogeneous strength faults can produce highly inhomogeneous slip even without any form of frictional restrengthening at healing time finally we find that smoother strength models produce a better fit than the coarse models to the directivity pulse often observed on the surface the results help to shed light on the transition of the faulting system between locally controlled and more globally controlled rupture and also show how dynamic models may be used to generate ground motion estimates for seismic hazard calculations

three dimensional finite difference simulation of fault dynamics rectangular faults with fixed rupture velocity we analyze three dimensional finite difference solutions for a simple shear crack model of faulting to determine the effects of fault length and width on the earthquake slip function the fault model is dynamic with only rupture velocity fault dimensions and dynamic stress drop prescribed the numerical solutions are accurate for frequencies up to 5 hz and are combined with asymptotic results for shear cracks in order to characterize the slip function at higher frequencies near the hypocenter the slip velocity exhibits a square root singularity whose intensity increases with hypocentral distance at distances greater than the fault width w growth of the velocity intensity ceases and the slip function becomes nearly invariant with distance along the fault length closed form expressions are developed for the dependence of static slip s slip rise time tr and slip velocity intensity v on fault geometry along the center line of a long narrow fault at hypocentral distances exceeding w these expressions reduce to s w tr 0 5 w vr and v w 2 vr where is the dynamic stress drop the shear modulus and vr the rupture velocity the numerical results imply that uniform dislocation kinematic earthquake models in which slip is represented by a ramp time function will underpredict high frequency ground motion relative to low frequency ground motion a further implication of the numerical solutions is that the nature of inelastic processes at the advancing edge of a long fault will depend on fault width but will be independent of rupture length

fault steps and the dynamic rupture process 2 d numerical simulations of a spontaneously propagating shear fracture fault steps may have controlled the sizes of the 1966 parkfield 1968 borrego mountain 1979 imperial valley 1979 coyote lake and the 1987 superstition hills earthquakes this project investigates the effect of fault steps of various geometries on the dynamic rupture process we have used a finite difference code to simulate spontaneous rupture propagation in two dimensions we employ a slip weakening fracture criterion as the condition for rupture propagation and examine how rupture on one plane initiates rupture on parallel fault planes the geometry of the two parallel fault planes allows for stepover widths of 0 5 to 10 0 km and overlaps of 5 to 5 km our results demonstrate that the spontaneous rupture on the first fault segment continues to propagate onto the second fault segment for a range of geometries for both compressional and dilational fault steps a major difference between the compressional and dilational cases is that a dilational step requires a longer time delay between the rupture front reaching the end of the first fault segment and initiating rupture on the second segment therefore our dynamic study implies that a compressional step will be jumped quickly whereas a dilational step will cause a time delay leading to a lower apparent rupture velocity we also find that the rupture is capable of jumping a wider dilational step than compressional step

use of fault striations and dislocation models to infer tectonic shear stress during the 1995 hyogo ken nanbu kobe earthquake dislocation models of the 1995 hyogo ken nanbu kobe earthquake derived by yoshida et al 1996 show substantial changes in direction of slip with time at specific points on the nojima and rokko fault systems as do striations we observed on exposures of the nojima fault surface on awaji island spudich 1992 showed that the initial stress that is the shear traction on the fault before the earthquake origin time can be derived at points on the fault where the slip rake rotates with time if slip velocity and stress change are known at these points from yoshida s slip model we calculated dynamic stress changes on the ruptured fault surfaces to estimate errors we compared the slip velocities and dynamic stress changes of several published models of the earthquake the differences between these models had an exponential distribution not gaussian we developed a bayesian method to estimate the probability density function pdf of initial stress from the striations and from yoshida s slip model striations near toshima and hirabayashi give initial stresses of about 13 and 7 mpa respectively we obtained initial stresses of about 7 to 17 mpa at depths of 2 to 10 km on a subset of points on the nojima and rokko fault systems our initial stresses and coseismic stress changes agree well with postearthquake stresses measured by hydrofracturing in deep boreholes near hirabayashi and ogura on awaji island our results indicate that the nojima fault slipped at very low shear stress and fractional stress drop was complete near the surface and about 32 below depths of 2 km our results at depth depend on the accuracy of the rake rotations in yoshida s model which are probably correct on the nojima fault but debatable on the rokko fault our results imply that curved or cross cutting fault striations can be formed in a single earthquake contradicting a common assumption of structural geology

distribution of stress drop stiffness and fracture energy over earthquake rupture zones 1 using information provided by slip models and the methodology of mcgarr and fletcher 2002 we map static stress drop stiffness k u where is static stress drop and u is slip and fracture energy over the slip surface to investigate the earthquake rupture process and energy budget for the 1994 m6 7 northridge 1992 m7 3 landers and 1995 m6 9 kobe earthquakes the distributions of static stress drop show strong heterogeneity emphasizing the importance of asperities in the rupture process average values of static stress drop are 17 11 and 4 mpa for northridge landers and kobe respectively these values are substantially higher than estimates based on simple crack models suggesting that the failure process involves the rupture of asperities within the larger fault zone stress drop as a function of depth for the northridge and landers earthquakes suggests that stress drops are limited by crustal strength for these two earthquakes regions of high slip are surrounded by high values of stiffness particularly for the northridge earthquake the prominent patch of high slip in the central part of the fault is bordered by a ring of high stiffness and is consistent with expectations based on the failure of an asperity loaded at its edge due to exterior slip stiffness within an asperity is inversely related to its dimensions estimates of fracture energy based on static stress drop slip and rupture speed were used to investigate the nature of slip weakening at four locations near the hypocenter of the kobe earthquake for comparison with independent results based on a dynamic model of this earthquake one subfault updip and to the ne of the hypocenter has a fracture energy of 1 1 mj m2 and a slip weakening distance dc of 0 66 m right triangles whose base and height are dc and the dynamic stress drop respectively approximately overlie the slip dependent stress given by ide and takeo 1997 for the same locations near the hypocenter the total fracture energy for the kobe earthquake 3 7 1014 j is about the same as the seismic energy ea 3 2 1014 j

cyclic ductile and brittle deformation related to coseismic thrust fault propagation structural record at the base of a basement nappe preveli crete 1 the structural record at the base of a basement nappe preveli nappe crete greece thrust upon sedimentary rocks is investigated aimed on understanding mechanisms which result in decoupling of the thrust sheet from its original substratum we identify several superimposed deformation stages each with characteristic structural style and indications of episodic deformation at initially high differential stress the final stage involves formation of a matrix supported breccia transected by pseudotachylytes comprising the lowermost 30 m of the nappe brecciation and pseudotachylyte formation occurred in a single event and structures were not modified afterward complete induration of breccia and composition of phengite crystallized during devitrification of pseudotachylytes place the sequence of events into the middle crust we propose a model relating episodic deformation and cyclic stress history to propagation of a thrust fault in a limited number of seismic events terminal brecciation and frictional fusion record passage of the fault front beneath the site of observation and decoupling of the thrust sheet absence of discernible further deformation is consistent with negligible basal friction during transport as a nappe brecciation and pseudotachylyte formation mark the switch from a history of repeated coseismic loading and postseismic stress relaxation in the plastosphere driven by seismic events on the approaching thrust fault to passive transport with deformation localized in a weak thrust plane for a sequence of superimposed ductile to brittle structures our model provides an alternative to progressive cooling and exhumation concomitant with deformation over millions of years

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

effects of a low velocity zone on a dynamic rupture dynamic crack earthquake simulations generally assume that the crustal material surrounding faults is laterally homogeneous tomographic and near fault seismic studies indicate that the crust near faults is instead comprised of rocks of varying material velocities we have tested the effects of adding material velocity variation to simulations of spontaneously propagating earthquakes we used two dimensional plane strain conditions coupled with a slip weakening fracture criterion and examined earthquakes on faults that bisect finite width low velocity zones embedded in country rock and earthquakes on faults that bound two different velocity materials when a fault bisects a low velocity zone the normal stress remains unchanged but both the rupture velocity and slip velocity pulse shape are perturbed the presence of the low velocity zone induces high frequency oscillations in the slip function near the rupture front when the fault is on the edge of the low velocity zone the oscillations are more pronounced and repeated sticking and slipping can occur near the rupture front for the slip weakening velocity independent friction model however the temporary sticking does not lead to permanent arrest of slip and slip duration is still controlled by the overall rupture dimension when an earthquake ruptures a fault juxtaposing a lower velocity material against a higher velocity material the normal stress across the fault near the crack tip is perturbed the sign of the normal stress perturbation depends on the direction of rupture leading in some cases to a directional dependence of rupture velocity when slip is accompanied by stress reduction a positive feedback develops between the normal and shear stress changes as previously noted by andrews and ben zion 1997 resulting in an apparently unavoidable grid size dependence in computation of stress change near the rupture front numerical experiments indicate however that the rupture velocity is insensitive to this zone size dependence which is highly localized immediately behind the crack tip the factors controlling the rupture velocity in the simulations including directional dependence are further elucidated by a new analytical solution for rupture of an asperity on a frictionless interface

dynamic rupture modelling of the 1999 duzce turkey earthquake the dynamic rupture process and near source ground motion of the 1999 mw 7 1 duzce earthquake are simulated the fault rupture is governed by the slip weakening friction model coupled to a three dimensional viscoelastic wave equation the problem is solved numerically by a 3 d dynamic rupture code that uses a generalized finite difference method initial parameterization of stress drop delta tau and strength excess s text e for dynamic rupture calculations is obtained from the slip velocity distribution of a kinematic waveform inversion ki model by solving the elastodynamic equation with the kinematic slip as a boundary condition using the kinematic slip distribution and observed ground motion as constraints a trial and error procedure was followed to define the stress parameterization preferred model describes the source in terms of stress with three asperities located respectively at the deep middle and shallow and strong barriers between asperities s text e is as high as 19 mpa at barriers between the three asperities and delta tau is maximum about 40 mpa at the deepest asperity this heterogeneity in stress distribution produces abrupt jumps in rupture velocity exhibiting locally apparent rupture speed exceeding the p wave velocity at the borders between barriers and asperities due to sharp changes of fault strength and stress drop at those areas overall consistent with other studies the rupture propagation is dominated by supershear speed toward the eastern asperities and at shallow surface simulated surface rupture at the eastern fault is consistent with other studies nevertheless the western shallower parts did not rupture during the simulation suggesting that those regions may have already broken during the 1999 kocaeli event which occurred three months earlier ground motion simulation catches the major characteristics of the observed waveforms distribution of simulated peak ground velocity pgv in low frequency 0 1 0 5 hz inside the study area reveals the propagation pattern on the field with pgv reaching to 1 2 and 2 2 m s in the ns and ew components respectively

the transition of dynamic rupture styles in elastic media under velocity weakening friction although kinematic earthquake source inversions show dominantly pulse like subshear rupture behavior seismological observations laboratory experiments and theoretical models indicate that earthquakes can operate with different rupture styles either as pulses or cracks that propagate at subshear or supershear speeds the determination of rupture style and speed has important implications for ground motions and may inform about the state of stress and strength of active fault zones we conduct 2d in plane dynamic rupture simulations with a spectral element method to investigate the diversity of rupture styles on faults governed by velocity and state dependent friction with dramatic velocity weakening at high slip rate our rupture models are governed by uniform initial stresses and are artificially initiated we identify the conditions that lead to different rupture styles by investigating the transitions between decaying steady state and growing pulses cracks sub shear and super shear ruptures as a function of background stress nucleation size and characteristic velocity at the onset of severe weakening our models show that small changes of background stress or nucleation size may lead to dramatic changes of rupture style we characterize the asymptotic properties of steady state and self similar pulses as a function of background stress we show that an earthquake may not be restricted to a single rupture style but that complex rupture patterns may emerge that consist of multiple rupture fronts possibly involving different styles and back propagating fronts we also demonstrate the possibility of a super shear transition for pulse like ruptures finally we draw connections between our findings and recent seismological observations

dynamic stress field of a kinematic earthquake source model with k squared slip distribution summary source models such as the k squared stochastic source model with k dependent rise time are able to reproduce source complexity commonly observed in earthquake slip inversions an analysis of the dynamic stress field associated with the slip history prescribed in these kinematic models can indicate possible inconsistencies with physics of faulting the static stress drop the strength excess the breakdown stress drop and critical slip weakening distance dc distributions are determined in this study for the kinematic k squared source model with k dependent rise time several studied k squared models are found to be consistent with the slip weakening friction law along a substantial part of the fault a new quantity the stress delay is introduced to map areas where the yielding criterion of the slip weakening friction is violated hisada s slip velocity function is found to be more consistent with the source dynamics than boxcar brune s and dirac s slip velocity functions constant rupture velocities close to the rayleigh velocity are inconsistent with the k squared model because they break the yielding criterion of the slip weakening friction law the bimodal character of dc dtot frequency magnitude distribution was found dc approaches the final slip dtot near the edge of both the fault and asperity we emphasize that both filtering and smoothing routinely applied in slip inversions may have a strong effect on the space time pattern of the inferred stress field leading potentially to an oversimplified view of earthquake source dynamics

the slip history of the 1994 northridge california earthquake determined from strong motion teleseismic gps and leveling data we present a rupture model of the northridge earthquake determined from the joint inversion of near source strong ground motion recordings p and sh teleseismic body waves global positioning system gps displacement vectors and permanent uplift measured along leveling lines the fault is defined to strike 122 and dip 40 to the south southwest the average rake vector is determined to be 101 and average slip is 1 3 m the peak slip reaches about 3 m our estimate of the seismic moment is 1 3 0 2 10 26 dyne cm potency of 0 4 km3 the rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike nearly 20 km in the dip direction and there is no indication of slip shallower than about 5 to 6 km the up dip strong motion velocity waveforms are dominated by large s wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals a few seconds apart stations at southern azimuths indicate two main s wave arrivals separated longer in time about 4 to 5 sec these observations are best modeled with a complex distribution of subevents the initial s wave arrival comes from an asperity that begins at the hypocenter and extends up dip and to the north where a second larger subevent is centered about 12 km away the secondary s arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km 8 km west of the hypocenter the resolving power of the individual data sets is examined by predicting the geodetic gps and leveling displacements with the dislocation model determined from the waveform data and vice versa and also by analyzing how well the teleseismic solution predicts the recorded strong motions the general features of the geodetic displacements are not well predicted from the model determined independently from the strong motion data likewise the slip model determined from geodetic data does not adequately reproduce the strong motion characteristics whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data the strong motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong motion velocity recordings it does a poor job of predicting the geodetic data consequently a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets

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

heterogeneous distribution of dynamic stress drop and relative fault strength recovered from the results of waveform inversion the 1984 morgan hill california earthquake the dynamic rupture process of the 1984 morgan hill california earthquake m 6 2 has been investigated on the basis of a three dimensional 3d dynamic shear crack model from previous waveform inversion results for this purpose a locking fracture criterion is introduced for rupture propagation from which a lower bound of the peak shear stress just before rupturing and hence of the relative fault strength has been estimated at each fault segment the distribution of static and dynamic stress drops has also been independently evaluated from that of fault slips by linear and nonlinear inversion procedures respectively the results show that large slips located from the hypocenter to about 10 km and 14 to 17 km along the strike at depths between 8 and 12 km result from local dynamic stress drops exceeding 40 and 140 bars respectively negative stress drops down to 15 bars are required to explain very low slips over a shallow fault section if a non negative stress drop condition is imposed somewhat larger slips are obtained there than those from the waveform inversion the above results suggest that there could be a zone of velocity strengthening frictional behavior in the shallow crust which may have arrested slip motion during rupture propagation the strength excess is found to be generally small but somewhat larger at a small zone that has delayed rupture propagation the dynamic rupture initiated from a small nucleus zone with a low stress drop propagated southeastward breaking the deeper fault section with high stress drop and then broke a relatively high strength zone after a short time of arrest with highest stress drop

frequency domain inversion of strong motions application to the 1992 landers earthquake we present a frequency domain inversion in which the observed earthquake strong ground motions are used to constrain the space time dependence of slip on a fault green s functions are numerically evaluated and the parameters describing the rupture are the local slip rupture time and rise time these parameters are simultaneously evaluated without additional constraints this procedure allows for large variations in the local rupture velocity the june 28 1992 landers earthquake mw 7 3 offers an exceptional opportunity to apply this technique to a major strike slip event we model the rupture evolution including local differences in slip durations and variations in rupture velocity our final results are in good agreement with other inversion studies geodetic and surface observations the main discrepancies occurred at depth and at the end of the johnson valley fault we show that a relatively low resolution could be an explanation for these differences rupture velocity and slip are extremely heterogeneous both along strike and with depth a moment of 0 90 1020 n m was found the slip distribution shows that this event consists of a series of regions of high slip subevents separated by regions of relative low slip approximately 50 of the moment was released on the homestead valley fault in this region of large slip the rupture velocity inferred by our inversion is well constrained and is equal to 3 0 km s at depth and 2 5 km s near the surface our inversion favors the hypothesis that the duration of the slip at each point of the fault is of the order of the duration of rupture of each subevent

determination of constitutive relations of fault slip based on seismic wave analysis constitutive laws define the boundary conditions on fault plane and govern many aspects of earthquake failure although several constitutive laws have been formulated based on laboratory rock experiments and applied to theoretical studies in various fields no actual relation during a natural earthquake has been determined the 1995 kobe earthquake is suitable for detailed kinematic analysis and this enables the first evaluation of constitutive relations for a natural earthquake in this study we determine spatiotemporal slip distribution on an assumed fault plane of the 1995 kobe earthquake by waveform inversion and then solve elastodynamic equations using a finite difference method to determine the stress distribution and constitutive relations on the fault plane an inversion method based on bayes theorem is employed to obtain a spatiotemporal slip distribution and enables us to ensure the objective uniqueness of the solution with numerous parameters and smoothing constraints this slip distribution is then used as part of the boundary condition in the finite difference calculation the time histories of slip and shear stress obtained then provide a constitutive relation at each point on the fault plane they show slip weakening relations almost everywhere on the fault plane while slip rate dependency is not clear the slip weakening behavior has a clear depth dependency indicating that the slip weakening rate d du is smaller in the shallow crust than that in the deep crust this may be associated with the paucity of shallow seismicity observed in the source region of this earthquake as reported for many mature fault systems

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

a composite source model of the 1994 northridge earthquake using genetic algorithms abstract the 17 january 1994 northridge earthquake m w 6 7 occurred on a buried thrust fault in the northwest los angeles metropolitan area we investigate the source process of this earthquake using the csmip strong motion records and a composite source model developed by zeng et al 1994a for realistic earthquake strong ground motion prediction our previous studies demonstrated the realism of the synthetic strong motions generated from the composite source model by comparing them with observed records from earthquakes in many areas of the world this article addresses an inverse study of the problem to find a specific composite source model for the northridge earthquake this is done by adjusting the location of a suite of composite subevents using genetic algorithms holland 1975 to best match the observed waveforms a test run of the genetic algorithm on synthetic data sets finds a very good convergence of the approach we reduce largely the intensive computation time by identifying subevents with major contribution to the waveform fit our result for the 1994 northridge earthquake indicates a complex earthquake rupture process with three large slip zones one at the hypocenter and the other two to the west of the hypocenter we then use this model to compute the high frequency strong motion velocity and acceleration the results show that the composite source model provides a very realistic broadband source description for the northridge earthquake

fault interactions and subduction tectonics a re examination of the weber new zealand earthquake sequence of 1990 summary two moderate magnitude mw 6 2 and 6 4 earthquakes in the hikurangi subduction margin north island new zealand occurred 3 months apart in 1990 the epicentres are nearly coincident but the first weber 1 primarily normal faulting occurred within the subducting pacific plate depth about 28 km and the second weber 2 a mix of thrusting and right lateral motion occurred within the overlying australian plate depth about 13 km the plate interface in between the plate interface is interpreted to be locked trenchward se from about the position of these events with a transition to aseismic slip further down dip to the nw several stress interaction questions are examined first to see whether weber 1 triggered weber 2 a range of possible mainshock parameters are used to calculate induced changes in the static coulomb failure stress cfs in most cases the results are consistent with triggering secondly previous work showed that the rate of aftershock occurrence for weber 1 decreased markedly about 35 days before weber 2 recovering afterwards to see whether aseismic pre slip on the weber 2 fault as predicted by rate and state friction could be the cause of the decrease the same fault parameters have been used in reverse the results are ambiguous some fault parameters giving results consistent with the hypothesis and others not the amount of pre slip required for significant inhibition however is about equal to that in the mainshock and distributed over the entire fault plane thirdly observations of episodic aseismic slip down dip from locked sections of other plate interfaces are becoming more common could such slip have triggered both weber events the induced changes in cfs for such slip are uniformly positive on the weber 1 fault plane and mostly positive on the weber 2 fault plane so the answer is yes although there is no independent evidence for aseismic slip prior to the weber sequence this case shows that such slip may trigger events on other nearby faults besides loading the locked section of the plate interface static stress triggering considerations are thus likely to be important in subduction environments

the 1999 izmit turkey earthquake a 3d dynamic stress transfer model of intraearthquake triggering before the august 1999 izmit kocaeli turkey earthquake theoretical studies of earthquake ruptures and geological observations had provided estimates of how far an earthquake might jump to get to a neighboring fault both numerical simulations and geological observations suggested that 5 km might be the upper limit if there were no transfer faults the izmit earthquake appears to have followed these expectations it did not jump across any step over wider than 5 km and was instead stopped by a narrower step over at its eastern end and possibly by a stress shadow caused by a historic large earthquake at its western end our 3d spontaneous rupture simulations of the 1999 izmit earthquake provide two new insights 1 the west to east striking fault segments of this part of the north anatolian fault are oriented so as to be low stress faults and 2 the easternmost segment involved in the august 1999 rupture may be dipping an interesting feature of the izmit earthquake is that a 5 km long gap in surface rupture and an adjacent 25 restraining bend in the fault zone did not stop the earthquake the latter observation is a warning that significant fault bends in strike slip faults may not arrest future earthquakes manuscript received 30 august 2000