anatomy of strong ground motion near source records and three dimensional physics based numerical simulations of the mw 6 0 2012 may 29 po plain earthquake italy

laboratory earthquakes the sub rayleigh to supershear rupture transition we report on the experimental observation of spontaneously nucleated supershear rupture and on the visualization of sub rayleigh to supershear rupture transitions in frictionally held interfaces the laboratory experiments mimic natural earthquakes the results suggest that under certain conditions supershear rupture propagation can be facilitated during large earthquake events

strong shaking in los angeles expected from southern san andreas earthquake 1 the southernmost san andreas fault has a high probability of rupturing in a large greater than magnitude 7 5 earthquake sometime during the next few decades new simulations show that the chain of sedimentary basins between san bernardino and downtown los angeles form an effective waveguide that channels love waves along the southern edge of the san bernardino and san gabriel mountains earthquake scenarios with northward rupture in which the guided wave is efficiently excited produce unusually high long period ground motions over much of the greater los angeles region including intense localized amplitude modulations arising from variations in waveguide cross section

three dimensional finite difference modeling of the san andreas fault source parameterization and ground motion levels olsen et al 1995 recently simulated an mw 7 75 earthquake on the san andreas fault predicting long period t 2 5 sec ground velocities of 140 cm sec in the los angeles basin about 60 km from the fault these motions are much larger than estimates derived from empirical relations or other numerical simulations standard area magnitude relations predict that the 170 16 km fault used in the simulations would produce an mw 7 5 earthquake giving a moment of 2 0 1027 dyne cm which is 2 4 times smaller than the moment used by olsen et al 1995 further self similar scaling predicts a rise time of 3 sec for an mw 7 75 event and 2 2 sec for an mw 7 5 event the filtered impulse slip function used by olsen et al 1995 has an effective rise time of 1 6 sec yielding a response that is about 2 times larger than expected for periods less than 5 sec this combination of high seismic moment and short rise time along with the use of a uniform slip distribution leads to the extreme ground motion levels predicted by olsen et al 1995 to quantify the sensitivity of the long period ground motion response to source parameterization we have performed 3d finite difference fd simulations using various combinations of seismic moment source rise time and slip heterogeneity these calculations incorporate the same grid dimensions fault size and bandwidth employed by olsen et al 1995 with a moment of 2 0 1027 dyne cm a rise time of 2 sec and a smoothly heterogeneous slip distribution we simulate peak long period ground velocities of 155 cm sec in the near fault region and 40 cm sec in the los angeles basin these values are much closer to although still higher than empirical predictions a uniform slip distribution produces the largest peak motions both in the near fault region and in the los angeles basin whereas a rough asperity slip distribution noticeably reduces the maximum near fault ground velocities our results indicate that the accurate simulation of long period ground motions requires a realistic source parameterization including appropriate choices of seismic moment and rise time as well as the use of spatial and temporal variations in slip distribution

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

shakeout d ground motion estimates using an ensemble of large earthquakes on the southern san andreas fault with spontaneous rupture propagation 1 we simulate ground motion in southern california from an ensemble of 7 spontaneous rupture models of large mw7 8 northwest propagating earthquakes on the southern san andreas fault shakeout d compared to long period spectral accelerations from the next generation attenuation nga empirical relations shakeout d predicts similar average rock site values i e within roughly their epistemic uncertainty but significantly larger values in los angeles and ventura basins due to wave guide focusing effects the shakeout d ground motion predictions differ from those of a kinematically parameterized geometrically similar scenario rupture 1 the kinematic rock site predictions depart significantly from the common distance attenuation trend of the nga and shakeout d results and 2 shakeout d predictions of long period spectral acceleration within the basins of the greater los angeles area are lower by factors of 2 3 than the corresponding kinematic predictions we attribute these differences to a less coherent wavefield excited by the complex rupture paths of the shakeout d sources

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

estimation of q for long period 2 sec waves in the los angeles basin we simulate 0 to 0 5 hz 3d wave propagation through the southern california earthquake center seismic velocity reference model version 2 for the 1994 northridge earthquake in order to examine the effects of anelastic attenuation and amplification within the near surface sediments we use a fourth order finite difference staggered grid method with the coarse grained frequency independent anelastic scheme of day and bradley 2001 and a variable slip distribution from kinematic inversion for the northridge earthquake we find that the near surface material with s wave velocity v s as low as 500 m sec significantly affects the long period peak ground velocities compared with simulations in which the s wave velocity is constrained to 1 km sec and greater anelastic attenuation also has a strong effect on ground motion amplitudes reducing the predicted peak velocity by a factor of up to 2 5 relative to lossless simulations our preferred q model is q s v s 0 02 v s in meters per second for v s less than 1 2 km sec and much larger q s v s 0 1 v s in meters per second for layers with higher velocities the simple model reduces the standard deviation of the residuals between synthetic and observed natural log of peak velocity from 1 13 to 0 26 relative to simulations for the lossless case the anelastic losses have their largest effect on short period surface waves propagating in the los angeles basin which are principally sensitive to q s in the low velocity near surface sediments of the basin the low frequency ground motion simulated here is relatively insensitive to q p as well as to the values of q s at depths greater than roughly that of the 2 km sec s wave velocity isosurface

how fast is rupture during an earthquake new insights from the 1999 turkey earthquakes we report that during the two devastating 1999 earthquakes in turkey rupture propagated over a large part of the nearly 200km long fault zone at supershear speed approaching 5km s we present observations and modeling which confirm the original inference of supershear rupture during the izmit earthquake and we show that supershear rupture also occurred during the duzce earthquake we show that the rupture velocity measured about 2 times the shear wave velocity is the value predicted by theoretical studies in fracture dynamics we look for clues to explain these observations

fault interactions and large complex earthquakes in the los angeles area faults in complex tectonic environments interact in various ways including triggered rupture of one fault by another that may increase seismic hazard in the surrounding region we model static and dynamic fault interactions between the strike slip and thrust fault systems in southern california we find that rupture of the sierra madre cucamonga thrust fault system is unlikely to trigger rupture of the san andreas or san jacinto strike slip faults however a large northern san jacinto fault earthquake could trigger a cascading rupture of the sierra madre cucamonga system potentially causing a moment magnitude 7 5 to 7 8 earthquake on the edge of the los angeles metropolitan region

terashake2 spontaneous rupture simulations of mw 7 7 earthquakes on the southern san andreas fault abstract previous numerical simulations terashake1 of large m w 7 7 southern san andreas fault earthquakes predicted localized areas of strong amplification in the los angeles area associated with directivity and wave guide effects from northwestward propagating rupture scenarios the terashake1 source was derived from inversions of the 2002 m w 7 9 denali alaska earthquake that source was relatively smooth in its slip distribution and rupture characteristics owing both to resolution limits of the inversions and simplifications imposed by the kinematic parameterization new simulations terashake2 with a more complex source derived from spontaneous rupture modeling with small scale stress drop heterogeneity predict a similar spatial pattern of peak ground velocity pgv but with the pgv extremes decreased by factors of 2 3 relative to terashake1 the terashake2 source excites a less coherent wave field with reduced along strike directivity accompanied by streaks of elevated ground motion extending away from the fault trace the source complexity entails abrupt changes in the direction and speed of rupture correlated to changes in slip velocity amplitude and waveform features that might prove challenging to capture in a purely kinematic parameterization despite the reduced pgv extremes northwest rupturing terashake2 simulations still predict entrainment by basin structure of a strong directivity pulse with pgvs in los angeles and san gabriel basins that are much higher than predicted by empirical methods significant areas of those basins have predicted pgv above the 2 probability of exceedance poe level relative to current attenuation relationships even when the latter includes a site term to account for local sediment depth and wave guide focusing produces localized areas with pgv at roughly 0 1 0 2 poe about a factor of 4 5 above the median in contrast at rock sites in the 0 100 km distance range the median terashake2 pgvs are in very close agreement with the median empirical prediction and extremes nowhere reach the 2 poe level the rock site agreement lends credibility to some of our source modeling assumptions including overall stress drop level and the manner in which we assigned dynamic parameters to represent the mechanical weakness of near surface material future efforts should focus on validating and refining these findings assessing their probabilities of occurrence relative to alternative rupture scenarios for the southern san andreas fault and incorporating them into seismic hazard estimation for southern california

supershear mach waves expose the fault breakdown slip abstract mikumo et al 2003 showed that it is possible to estimate the breakdown slip dc as the slip at the time of the peak slip rate for rupture propagation with subshear speeds cruz atienza et al 2009 later attempted to extend this method to extract information about dc as the displacement at the time of the peak particle velocity from seismic strong motion records however a reasonably accurate estimate of dc was only possible in a narrow zone adjacent to the fault typically on the order of hundreds of meters due to the fast decay with distance from the fault of the seismic energy related to the stress breakdown process when the rupture propagates with supershear speeds on the other hand this energy is carried much farther away from the fault by mach waves in particular rayleigh mach waves when rupture reaches the earth s surface dunham and bhat 2008 here we present a new approach to estimate dc from strong motion records containing mach waves first we show that the method by mikumo et al is valid for supershear rupture propagation this method is then used to estimate dc via an asymptotic approximation of the slip and slip rate time histories from the mach waves using spontaneous rupture simulations we demonstrate that for a visco elastic half space model dc can be estimated within an error of 40 from mach waves that have propagated a distance of at least 3 km from the fault the method is applied to estimate dc for the 2002 mw7 9 denali alaska earthquake 1 5 m and for the 1999 mw7 6 izmit turkey earthquake 1 7 m

the shakeout earthquake scenario verification of three simulation sets summary this paper presents a verification of three simulations of the shakeout scenario an mw 7 8 earthquake on a portion of the san andreas fault in southern california conducted by three different groups at the southern california earthquake center using the scec community velocity model for this region we conducted two simulations using the finite difference method and one by the finite element method and performed qualitative and quantitative comparisons between the corresponding results the results are in good agreement with each other only small differences occur both in amplitude and phase between the various synthetics at ten observation points located near and away from the fault as far as 150 km away from the fault using an available goodness of fit criterion all the comparisons scored above 8 with most above 9 2 this score would be regarded as excellent if the measurements were between recorded and synthetic seismograms we also report results of comparisons based on time frequency misfit criteria results from these two criteria can be used for calibrating the two methods for comparing seismograms in those cases in which noticeable discrepancies occurred between the seismograms generated by the three groups we found that they were the product of inherent characteristics of the various numerical methods used and their implementations in particular we found that the major source of discrepancy lies in the difference between mesh and grid representations of the same material model overall however even the largest differences in the synthetic seismograms are small thus given the complexity of the simulations used in this verification it appears that the three schemes are consistent reliable and sufficiently accurate and robust for use in future large scale simulations

quantification of fault zone plasticity effects with spontaneous rupture simulations previous studies have shown that plastic yielding in crustal rocks in the fault zone may impose a physical limit to extreme ground motions we explore the effects of fault zone non linearity on peak ground velocities pgvs by simulating a suite of surface rupturing strike slip earthquakes in a medium governed by drucker prager plasticity using the awp odc finite difference code our simulations cover magnitudes ranging from 6 5 to 8 0 three different rock strength models and average stress drops of 3 5 and 7 0 mpa with a maximum frequency of 1 hz and a minimum shear wave velocity of 500 m s friction angles and cohesions in our rock models are based on strength criteria which are frequently used for fractured rock masses in civil and mining engineering for an average stress drop of 3 5 mpa plastic yielding reduces near fault pgvs by 15 30 in pre fractured low strength rock but less than 1 in massive high quality rock these reductions are almost insensitive to magnitude if the stress drop is doubled plasticity reduces near fault pgvs by 38 45 and 5 15 in rocks of low and high strength respectively because non linearity reduces slip rates and static slip near the surface plasticity acts in addition to and may partially be emulated by a shallow velocity strengthening layer the effects of plasticity are exacerbated if a fault damage zone with reduced shear wave velocities and reduced rock strength is present in the linear case fault zone trapped waves result in higher near surface peak slip rates and ground velocities compared to simulations without a low velocity zone these amplifications are balanced out by fault zone plasticity if rocks in the damage zone exhibit low to moderate strength throughout the depth extent of the low velocity zone sim 5 km we also perform dynamic non linear simulations of a high stress drop 8 mpa m 7 8 earthquake rupturing the southern san andreas fault along 250 km from indio to lake hughes non linearity in the fault damage zone and in near surface deposits would reduce peak ground velocities in the los angeles basin by 15 50 depending on the strength of crustal rocks and shallow sediments these results show that non linear effects may be relevant even at long periods in particular in earthquakes with high stress drop and in the presence of a low velocity fault damage zone

three dimensional simulation of a magnitude 7 75 earthquake on the san andreas fault simulation of 2 minutes of long period ground motion in the los angeles area with the use of a three dimensional finite difference method on a parallel supercomputer provides an estimate of the seismic hazard from a magnitude 7 75 earthquake along the 170 kilometer section of the san andreas fault between tejon pass and san bernardino maximum ground velocities are predicted to occur near the fault 2 5 meters per second and in the los angeles basin 1 4 meters per second where large amplitude surface waves prolong shaking for more than 60 seconds simulated spectral amplitudes for some regions within the los angeles basin are up to 10 times larger than those at sites outside the basin at similar distances from the san andreas fault

model validations and comparisons of the next generation attenuation of ground motions nga west project abstract the earthquake ground motion prediction equations developed for the next generation attenuation of ground motions nga west project in 2008 have established a new baseline for the estimation of ground motion parameters such as peak ground acceleration peak ground velocity and spectral acceleration for shallow crustal earthquakes in active tectonic regions we perform statistical goodness of fit analyses to quantitatively compare the predictive capabilities of the nga models and their predecessors using several testing subsets of the master database used in model development in addition we perform a blind comparison test using 1060 ground motion records from seven recent earthquakes recorded in california the 2003 m 6 5 san simeon event 2004 m 6 0 parkfield event 2005 m 5 2 anza event 2007 m 5 4 alum rock event 2008 m 5 4 chino hills event 2010 m 7 2 baja event and 2010 m 5 7 ocotillo event we assess how modeling decisions regarding the regression dataset functional forms input parameters and model complexity influence the models predictive capabilities by comparing the performance of each model we discuss various ground motion modeling strategies and offer recommendations for model development we find that increased model complexity does not necessarily lead to increased prediction accuracy that the inclusion of aftershocks in regression datasets may result in decreased predictive capabilities for mainshocks and that the use of measured site characteristics leads to greatly improved ground motion predictions a model validation framework is introduced to assess the prediction accuracy of ground motion prediction equations and to aid in their future development

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

expected seismic shaking in los angeles reduced by san andreas fault zone plasticity computer simulations of large m 7 8 earthquakes rupturing the southern san andreas fault from se to nw e g shakeout widely used for earthquake drills have predicted strong long period ground motions in the densely populated los angeles basin due to channeling of waves through a series of interconnected sedimentary basins recently the importance of this waveguide amplification effect for seismic shaking in the los angeles basin has also been confirmed from observations of the ambient seismic field by simulating the shakeout earthquake scenario based on a kinematic source description for a medium governed by drucker prager plasticity we show that nonlinear material behavior could reduce the earlier predictions of large long period ground motions in the los angeles basin by up to 70 as compared to viscoelastic solutions these reductions are primarily due to yielding near the fault although yielding may also occur in the shallow low velocity deposits of the los angeles basin if cohesions are close to zero fault zone plasticity remains important even for conservative values of cohesions suggesting that current simulations assuming a linear response of rocks are overpredicting ground motions during future large earthquakes on the southern san andreas fault

near source ground motion from steady state dynamic rupture pulses 1 ground motion from two dimensional steady state dynamic ruptures is examined for both subshear and supershear rupture velocities synthetic seismograms demonstrate that coherent high frequency information about the source process rapidly attenuates with distance from the fault for subshear ruptures such records provide almost no resolution of the spatial extent of the stress breakdown zone at supershear speeds s waves radiate away from the fault preserving the full source spectrum and carrying an exact history of the slip velocity on both the fault parallel and fault normal components of motion whose amplitudes are given by a function of rupture speed that vanishes at times the s wave speed the energy liberated from the strain field by the passage of a supershear rupture is partitioned into fracture energy dissipated within the fault zone and far field s wave radiation the partition depends on both the rupture velocity and the size of the breakdown zone

scale model and numerical simulations of near fault seismic directivity foam rubber experiments simulating unilaterally propagating strike slip earthquakes provide a means to explore the sensitivity of near fault ground motions to rupture geometry subsurface accelerometers on the model fault plane show rupture propagation that approaches a limiting velocity close to the rayleigh velocity the slip velocity waveform at depth is cracklike slip duration of the order of narrower fault dimension w divided by s wave speed surface accelerometers record near fault ground motion enhanced along strike by rupture induced directivity most experimental features initiation time shape duration and absolute amplitude of acceleration pulses are successfully reproduced by a 3d spontaneous rupture numerical model of the experiments numerical and experimental model acceleration pulses show similar decay with distance away from the fault and fault normal components in both models show similar large amplitude growth with distance along fault strike this forward directivity effect is also evident in response spectra the fault normal spectral response peak at period w 3 increases approximately sixfold along strike on average in the experiments with similar increase about fivefold in the corresponding numerical simulation the experimental and numerical model response spectra agree with an empirical directivity model for natural earthquakes at long periods near w and both overpredict shorter period empirical directivity effects with the amount of overprediction increasing systematically with diminishing period we attribute this difference to rupture and wavefront incoherence in natural earthquakes due to fault zone heterogeneities in stress frictional resistance and elastic properties present in the earth but absent or minimal in the experimental and numerical models rupture front incoherence is an important component of source models for ground motion prediction but finding an effective kinematic parameterization may be challenging

seismic shaking in the north china basin expected from ruptures of a possible seismic gap a 160 km long seismic gap which has not been ruptured over 8000 year was identified recently in north china in this study we use a dynamic source model and a newly available high resolution 3 d velocity structure to simulate long period ground motion up to 0 5 hz from possibly worst case rupture scenarios of the seismic gap we find that the characteristics of the earthquake source and the local geologic structure play a critical role in controlling the amplitude and distribution of the simulated strong ground shaking rupture directivity and slip asperities can result in large amplitude i e 1 m s ground shaking near the fault whereas long duration shaking may occur within sedimentary basins in particular a deep and closed quaternary basin between beijing and tianjin can lead to ground shaking of several tens of cm s for more than 1 minute these results may provide a sound basis for seismic mitigation in one of the most populated regions in the world

a physical model of the effect of a shallow weak layer on strong ground motion for strike slip ruptures we report results of foam rubber modeling of the effect of a shallow weak layer on ground motion from strike slip ruptures computer modeling of strong ground motion from strike slip earthquakes has involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault e g fixing the slip to be zero along the upper 2 kilometers of the fault plane in order to match certain strong motion accelerograms most modeling studies of earthquake strong ground motion have used what is termed kinematic dislocation modeling in kinematic modeling the time function for slip on the fault is prescribed and the response of the layered medium is calculated unfortunately there is no guarantee that the model and the prescribed slip are physically reasonable unless the true nature of the medium and its motions are known ahead of time there is good reason to believe that in many cases faults are weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes physical models of faulting as distinct from numerical or mathematical models are guaranteed to obey static and dynamic mechanical laws foam rubber modeling studies have been reported in a number of publications the object of this paper is to present results of physical modeling using a shallow weak layer in order to verify the physical basis for assuming a long rise time and a reduced high frequency pulse for the slip on the shallow part of faults it appears a 2 kilometer deep weak zone along strike slip faults could indeed reduce the high frequency energy radiated from shallow slip and that this effect can best be represented by superimposing a small amplitude short rise time pulse at the onset of a much longer rise time slip a weak zone was modeled by inserting weak plastic layers of a few inches in thickness into the foam rubber model for the 15 cm weak zone the average pulse is reduced by a factor of 0 46 the factor for the 20 cm case reduction is 0 11 for the 30 cm case it is 0 045 from these results we can see that the thicker the weak layer the more difficult it is for a short rise time acceleration pulse to push its way through the weak layer to the surface this is thus an approximate justification for reducing the high frequency radiation from shallower parts of strike slip faults if it is known that the shallow part of the fault is weak or has not stored up shear stress

full 3d tomography for the crustal structure of the los angeles region we apply full 3d tomography f3dt to a regional dataset in the los angeles area in f3dt the starting model as well as the model perturbation is three dimensional and the sensitivity frechet kernels are computed using numerical schemes that account for the full physics of 3d wave propagation f3dt can account for the nonlinearity of structural inverse problem through iteration thus providing the most efficient means for assimilating seismic observations into dynamic ground motion models we have successfully applied a scattering integral si formulation of f3dt to improve a 3d elastic structure model the southern california earthquake center scec community velocity model version 3 0 cvm3 0 with lowest shear velocity clamped at 1 km sec in the los angeles region our data are time and frequency localized measurements of phase delay anomalies relative to synthetics computed from the 3d elastic starting model the frechet kernels for our measurements were computed by convolving the earthquake wave fields generated by point earthquake sources with the receiver green tensors rgts which are the spatial temporal fields produced by three orthogonal unit impulsive point forces acting at the receiver locations we inverted 7364 phase delay measurements of p and s body waves by using the lsqr method the revised 3d model laf3d provides substantially better fit to the observed waveform data than the 3d starting model to our knowledge this study is the first successful application of f3dt using real data in structural seismology

supershear transition due to a free surface in 3 d simulations of spontaneous dynamic rupture on vertical strike slip faults supershear rupture propagation has been inferred from seismic observations for natural faults and observed in laboratory experiments we study the effect of the free surface on the transition of earthquake rupture from subshear to supershear speeds using simulations of spontaneous dynamic rupture on vertical strike slip faults we find that locally supershear rupture near the free surface can occur due to i the generalized burridge andrews mechanism that is a supershear loading field between p and sv wave arrivals generated by the main rupture front at depths and ii the phase conversion of sv to p diffracted waves at the free surface weaker supershear slip due to the generalized burridge andrews mechanism is caused by the low strength at shallow portions of the fault relative to deeper ones dominant supershear rupture is supported by the additional supershear loading field produced by phase conversion locally supershear propagation at the free surface occurs regardless of the level of prestress and can cause transition to supershear propagation over the entire seismogenic depth such global supershear transition which depends on prestress can occur under prestress levels lower than the theoretical estimates for models with no free surface although the effectiveness of supershear transition due to the free surface can be diminished by several potentially important factors it may play an important role on natural faults at least in those strike slip earthquakes that accumulate significant surface slip

dynamic simulation of the 1999 chi chi taiwan earthquake 1 we apply a modified three dimensional finite difference method to investigate the dynamics of the 1999 chi chi earthquake we determine the dynamic source rupture process and simulate the strong ground motions over a frequency range of 0 05 0 5 hz our dynamic source model reveals that the rupture process of the chi chi earthquake is much more complex than that described in the classical propagation rupture models the chi chi earthquake rupture process includes the jumping phenomenon the northern parts on the fault plane have a longer slip duration than the southern parts the motions on the hanging wall are larger than those on the footwall side which is a consequence of the asymmetry of the fault with respect to the free surface in general the synthetics calculated by the dynamic source model agree well with the observed data our dynamic source model reproduces distinctive velocity pulses at stations located in the forward rupture direction and at stations close to the fault surface breaks the apparent rupture directivity propagating from south to north then produces noticeably larger peak ground velocity to the north these results demonstrate that the dynamic rupture model reproduces the main features of long period ground motions these results may lead to improved understanding of rupture processes accompanying large earthquakes the dynamic modeling scheme may produce more reasonable and reliable strong ground motion predictions for assessment of seismic hazard

estimation of radiated energy of finite source earthquake models we estimated seismically radiated energy for kinematic earthquake rupture models instead of integrating far field waveforms we used an expression for radiated energy with slip and stress on the fault plane using an analytic crack model of sato and hirasawa 1973 we show that the finite difference method can estimate the radiated energy fairly well however if the rupture propagation velocity is near the s wave velocity and the grid interval is coarse we may underestimate the radiated energy substantially we analyzed five models of three earthquakes of m w 5 7 for the 1995 kobe earthquake we used three different kinematic models and obtained consistent values of radiated energy of 2 4 3 6 1014 j there are common features among the three models 1 more efficient energy radiation from the deeper regions of the fault and 2 little energy radiation beneath the city of kobe the models of the kobe earthquake and the other two earthquakes show that energy radiation is concentrated near the hypocenter and the initiation point of asperities we also find the area of negative energy radiation energy absorption near the edge of large slipped areas although our estimates tend to be smaller by about a factor of 3 than previous estimates the difference is reasonable because the frequency range is not complete and we cannot deal with detailed rupture behavior such as rapid rupture acceleration and deceleration manuscript received 18 january 2002

p wave seismic velocity structure derived from sonic logs and industry reflection data in the los angeles basin california 1 we present a new three dimensional seismic p wave velocity model of the los angeles basin that is based on more than 150 sonic logs and 7000 stacking velocities from industry reflection profiles the model was developed using kriging techniques implemented in gocad a commercial application specializing in the construction of surfaces the analysis of spatially distributed data and the modeling or simulation of properties and is characterized by a heterogeneous spatial varying velocity gradient with maximum velocities of 5000 m s in the sedimentary section variability of the observed p wave velocities increases with depth reflecting complex structural and sedimentological controls the model contains a new representation of the basin shape as defined by the sediment basement interface which was mapped using well control and seismic reflection profiles our model differs substantially in approach and outcome from previous representations of crustal velocity structure in southern california

case studies of damage to tall steel moment frame buildings in southern california during large san andreas earthquakes on 9 january 1857 a large earthquake of magnitude 7 9 occurred on the san andreas fault with rupture initiating at parkfield in central california and propagating in a southeasterly direction over a distance of more than 360 km such a unilateral rupture produces significant directivity toward the san fernando and los angeles basins indeed newspaper reports of sloshing observed in the los angeles river point to long duration 1 2 min and long period 2 8 sec shaking if such an earthquake were to happen today it could impose significant seismic demand on present day tall buildings using state of the art computational tools in seismology and structural engineering validated using data from the 17 january 1994 magnitude 6 7 northridge earthquake we determine the damage to an existing and a new 18 story steel moment frame building in southern california due to ground motion from two hypothetical magnitude 7 9 earthquakes on the san andreas fault our study indicates that serious damage occurs in these buildings at many locations in the region in one of the two scenarios for a north to south rupture scenario the peak velocity is of the order of 1 m sec 1 in the los angeles basin including downtown los angeles and 2 m sec 1 in the san fernando valley while the peak displacements are of the order of 1 m and 2 m in the los angeles basin and san fernando valley respectively for a south to north rupture scenario the peak velocities and displacements are reduced by a factor of roughly 2

a fault constitutive relation accounting for thermal pressurization of pore fluid 1 the heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip the amount of fluid pressure rise depends on the associated fluid flow the heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time if the slip velocity function is crack like there is a prompt fluid pressure rise at the onset of slip followed by a slower increase the stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance the threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity if hydraulic diffusivity is 0 02 m2 s estimated from borehole samples of fault zone material the threshold propagation distance is 300 m the stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity both of which will lessen the fluid pressure effect nevertheless if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value then stress drop is complete in large earthquakes

the scec southern california reference three dimensional seismic velocity model version 2 we describe version 2 of the three dimensional 3d seismic velocity model of southern california developed by the southern california earthquake center and designed to serve as a reference model for multidisciplinary research activities in the area the model consists of detailed rule based representations of the major southern california basins los angeles basin ventura basin san gabriel valley san fernando valley chino basin san bernardino valley and the salton trough embedded in a 3d crust over a variable depth moho outside of the basins the model crust is based on regional tomographic results the model moho is represented by a surface with the depths determined by the receiver function technique shallow basin sediment velocities are constrained by geotechnical data the model is implemented in a computer code that generates any specified 3d mesh of seismic velocity and density values this parameterization is convenient to store transfer and update as new information and verification results become available

constraints on stress and friction from dynamic rupture models of the 1994 northridge california earthquake we have simulated several scenarios of dynamic rupture propagation for the 1994 northridge california earthquake using a three dimensional finite difference method the simulations use a rate and slip weakening friction law starting from a range of initial conditions of stress and frictional parameters a critical balance between initial conditions and friction parameters must be met in order to obtain a moment as well as a final slip distribution in agreement with kinematic slip inversion results we find that the rupture process is strongly controlled by the average stress and connectivity of high stress patches on the fault in particular a strong connectivity of the high stress patches is required in order to promote the rupture propagation from the initial nucleation to the remaining part of the fault moreover we find that a small amount of rate weakening is needed in order to obtain a level of inhomogeneity in the final slip similar to that obtained in the kinematic inversion results however when the amount of rate weakening is increased the overall moment drops dramatically unless the average prestress is raised to unrealistic levels a velocity weakening parameter on the order of 10 cm per second is found to be adequate for an average prestress of about a hundred bars the presence of the free surface and of the uppermost low impedance layers in the model are found to have negligible influence on the rupture dynamics itself because the top of the fault is at a depth of several kilometers the 0 1 0 5 hz radiated waves from the dynamic simulation provides a good fit to strong motion data at sites nwh and ssa underprediction of the recorded peak amplitude at jfp is likely due to omission of near surface low velocity and 3 d basin effects in the simulations

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

earthquake fracture energy inferred from kinematic rupture models on extended faults 1 we estimate fracture energy on extended faults for several recent earthquakes by retrieving dynamic traction evolution at each point on the fault plane from slip history imaged by inverting ground motion waveforms we define the breakdown work wb as the excess of work over some minimum traction level achieved during slip wb is equivalent to seismological fracture energy g in previous investigations our numerical approach uses slip velocity as a boundary condition on the fault we employ a three dimensional finite difference algorithm to compute the dynamic traction evolution in the time domain during the earthquake rupture we estimate wb by calculating the scalar product between dynamic traction and slip velocity vectors this approach does not require specifying a constitutive law and assuming dynamic traction to be collinear with slip velocity if these vectors are not collinear the inferred breakdown work depends on the initial traction level we show that breakdown work depends on the square of slip the spatial distribution of breakdown work in a single earthquake is strongly correlated with the slip distribution breakdown work density and its integral over the fault breakdown energy scale with seismic moment according to a power law with exponent 0 59 and 1 18 respectively our estimates of breakdown work range between 4 105 and 2 107 j m2 for earthquakes having moment magnitudes between 5 6 and 7 2 we also compare our inferred values with geologic surface energies this comparison might suggest that breakdown work for large earthquakes goes primarily into heat production

rupture dynamics and ground motion from 3 d rough fault simulations 1 we perform three dimensional 3 d numerical calculations of dynamic rupture along non planar faults to study the effects of fault roughness on rupture propagation and resultant ground motion the fault roughness model follows a self similar fractal distribution over length scales spanning three orders of magnitude from 102 to 105 m the fault is governed by a strongly rate weakening friction and the bulk material is subject to drucker prager viscoplasticity fault roughness promotes the development of self healing rupture pulses and a heterogeneous distribution of fault slip at the free surface and at depth the inelastic deformation generated by the large dynamic stress near rupture fronts occurs in a narrow volume around the fault with heterogeneous thickness correlated to local roughness slopes inelastic deformation near the free surface however is induced by the stress waves originated from dynamic rupture at depth and spreads to large distances 10 km away from the fault the present simulations model seismic wave excitation up to 10 hz with rupture lengths of 100 km permitting comparisons with empirical studies of ground motion intensity measures of engineering interest characteristics of site averaged synthetic response spectra including the distance and period dependence of the median values absolute level and intra event standard deviation are comparable to appropriate empirical estimates throughout the period range 0 1 3 0 s this class of model may provide a viable representation of the ground motion excitation process over a wide frequency range in a large spatial domain with potential applications to the numerical prediction of source and path specific effects on earthquake ground motion

morphometric comparison of weathering features on side by side carbonate rock surfaces with different exposure ages a case from the croatian coast this study presents the comparison of morphometry of karren features developed on two rock surfaces located side by side having different exposure times cosmogenic 36cl analyses were used to determine the age of a rock surface exposed after a rockslide that occurred 2400 years ago whereas the rock surface not affected by the slide was exposed to weathering for a longer time period clint and grikes trittkarren rillenkarren solution pits and kamenitzas were measured comparatively less density of grikes on the surface affected by the slide made it having a fresh appearance most of the grikes were inherited from previous weathering stage and exposure time since the rockslide event has not been sufficient to homogenize these features between both rock surfaces tritkarren and rillenkarren were preferentially found on the rockslide surface indicating that their genesis is likely to occur at decadal to centennial timescale solution pits and kamenitzas were found on both surfaces and some of their size parameters were indistinguishable between the populations of both rock surfaces area and depth parameters of both forms were clearly correlated and we define a morphometric index a d ratio as the slope between these two parameters within each population after an initial stage preferential weathering occurs along the vertical axis in solution pits and laterally in kamenitzas thus we suggest that the a d ratio is related to the evolution relative age of the karren features thus under rock surfaces affected by the same weathering conditions lower a d ratios of solution pits and higher a d ratios of kamenitzas indicate longer time exposed to weathering thus although solution pits and kamenitzas are likely to be developed in decadal to centennial timescales their morphology changes through time at millennial timescale and it can be tracked by morphometric analysis

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

3d finite difference dynamic rupture modeling along nonplanar faults properunderstandingofseismicemissionsassociatedwiththe growth of complexly shaped microearthquake networks and larger scale nonplanar fault ruptures both in arbitrarily heterogeneousmedia requiresaccuratemodelingoftheunderlyingdynamic processes we present a new 3d dynamic rupture finitedifference model called the finite difference fault element fdfe method it simulates the dynamic rupture of nonplanar faults subjected to regional loads in complex media fdfe is based on a 3d methodology for applying dynamic rupture boundary conditions along the fault surface the fault is discretizedbyasetofparallelepipedfaultelementsinwhichspecific boundary conditions are applied these conditions are applied to the stress tensor once transformed into a local fault reference frame numerically determined weight functions multiplying particle velocities around each element allow accurate estimates offaultkinematicparametersi e slipandsliprateindependent of faulting mechanism assuming a coulomb like slip weakeningfrictionlaw aparametricstudysuggeststhatthefdfemethod converges toward a unique solution provided that the cohesive zone behind the rupture front is well resolved i e four or more elements inside this zone solutions are free of relevant numerical artifacts for grid sizes smaller than approximately 70 m results yielded by the fdfe approach are in good quantitative agreement with those obtained by a semianalytical boundaryintegralmethodalongplanarandnonplanarparabola shaped faults thefdfemethodthusprovidesquantitative accurateresults for spontaneous rupture simulations on intricate fault geometries

gravity wave characteristics in the lower atmosphere at south pole a 4 year 1993 1996 temperature and wind data set obtained from over 2000 high resolution balloon soundings at south pole is used to study gravity wave characteristics in the troposphere and lower stratosphere extensive analyses of energy density spectra and static stability are performed to present a comprehensive view of the gravity wave characteristics in the lower atmosphere at south pole our results show that the gravity waves are ubiquitous and often fairly strong at south pole even though the generation mechanisms are not clear gravity wave characteristics are in general similar to those obtained at other high latitude southern hemisphere stations potential energies vary between about 0 5 j kg and 5 j kg with season and altitude variations in kinetic energies are not well correlated with potential energy variations and range from 1 j kg to 11 j kg we observe significant seasonal variations of the slope and magnitude of the vertical wavenumber spectrum of temperature fluctuations especially in the stratosphere in general the gravity waves in the stratosphere are stronger weaker in austral spring fall wave activity in the troposphere has little seasonal dependence stability analysis shows that instabilities are more likely to occur in the troposphere than in the stratosphere the probability of wave instability is 13 7 in the troposphere and 5 4 in the stratosphere this is due to the less stable stratification in the troposphere where the buoyancy period averages 8 3 min compared to 4 9 min in the stratosphere dynamic shear instability is more likely to occur than convective instability 11 versus 2 6 in the troposphere and 4 7 versus 0 7 in the stratosphere due to the prevailing strong wind shear the instability probabilities vary seasonally with the austral winter exhibiting the highest probability of instabilities dynamic and convective instabilities combined in both the troposphere and stratosphere

broadband 0 4 hz ground motions for a magnitude 7 0 hayward fault earthquake with three dimensional structure and topography author s rodgers aj pitarka a petersson na sjogreen b mccallen db abstract 2018 the authors we performed fully deterministic broadband 0 4 hz high performance computing ground motion simulations of a magnitude 7 0 scenario earthquake on the hayward fault hf in the san francisco bay area of northern california simulations consider average one dimensional 1 d and three dimensional 3 d anelastic structure with flat and topographic free surfaces ground motion intensity measures gmims for the 3 d model display dramatic differences across the hf due to geologic heterogeneity with low wave speeds east of the hf amplifying motions the median gmims agree well with ground motion prediction equations gmpes however the 3 d model generates more scatter than the 1 d model ratios of 3 d 1 d gmims from the same source allow isolation of path and site effects for the 3 d model these ratios show remarkably similar trends as site specific factors for the gmpe predictions suggesting that wave propagation effects in our 3 d simulations are on average consistent with empirical data