earthquake rupture complexity due to dynamic nucleation and interaction of subsidiary faults we numerically study the dynamic interaction of propagating cracks it is assumed that propagating cracks can nucleate and drive subsidiary cracks because of shear strain enhancement near the propagating crack tips the critical strain fracture criterion is assumed in the analysis intense interaction is expected to occur among the cracks all the cracks are assumed to be parallel and antiplane strain deformation is assumed in the computation

effects of prestress state and rupture velocity on dynamic fault branching 1 we consider a mode ii rupture which propagates along a planar main fault and encounters an intersection with a branching fault using an elastodynamic boundary integral equation formulation allowing the failure path to be dynamically self chosen we study the following questions does the rupture initiate along the branch does it continue is the extensional or compressional side most favored for branching does rupture continue on the main fault too failure is described by a slip weakening law for which the strength at any amount of slip is proportional to normal stress our results show that dynamic stresses around the rupture tip which increase with rupture velocity at locations off the main fault plane relative to those on it could initiate rupture on a branching fault as suggested by prior work whether branched rupture can be continued to a larger scale depends on principal stress directions in the prestress state and on rupture velocity the most favored side for branching rupture switches from the extensional to the compressional side as we consider progressively shallower angles of the direction of maximum compressive prestress with the main fault simultaneous rupturing on both faults can be activated when the branching angle is wide but is usually difficult for a narrow branching angle due to strong stress interactions between faults however it can be also be activated by enhanced dynamic stressing when the rupture velocity is very near the rayleigh velocity natural examples seem consistent with the simulations that we present

control of rupture by fault geometry during the 1966 parkfield earthquake a reanalysis of the available data for the 1966 parkfleld california earthquake mr 5 suggests that although the ground breakage and aftershocks extended about 40 km along the san andreas fault the initial dynamic rupture was only 20 to 25 km in length the foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault detailed analysis of the p wave first motions from these events at the gold hill station 20 km southeast indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at parkfield analysis of the strong motion records suggests that the rupture stopped at another discontinuity in the fault plane an en echeion offset near gold hill that lies at the boundary on the san andreas fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred foreshocks to the 1857 earthquake occurred in this area sieh 1978 and the epicenter of the main shock may have coincided with the offset zone if it did a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do

fractal structure of spatial distribution of microfracturing in rock summary a constant stress fracture experiment of oshima granite was carried out at the confining pressure of 40mpa hypocentres of 2064 acoustic emissions were located during the experiment using the correlation integral we found that the spatial distribution of hypocentres of acoustic emission is a fractal and that the fractal dimension decreases with the evolution of rock fracturing the spatial distribution of earthquake s hypocentres reveals fractals ranging from regional to worldwide distribution if we extrapolate from laboratory measurements it is possible to predict the occurrence of large earthquakes by the decrease in the fractal dimension

dynamic nucleation process of shallow earthquake faulting in a fault zone two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes low frequency and low amplitude waves following the onset of a p wave p1 are interrupted by the arrival of the second impulsive phase p2 enriched with high frequency components this observation suggests that a large shallow earthquake involves two qualitatively different stages of rupture at its nucleation we propose a theoretical model that can naturally explain the above nucleation behaviour the model is 2 d and the deformation is assumed to be anti plane a key clement in our model is the assumption of a zone in which numbers of pre existing cracks are densely distributed this cracked zone is a model for the fault zone dynamic crack growth nucleated in such a zone is intensely affected by the crack interactions which exert two conflicting effects one tends to accelerate the crack growth and the other tends to decelerate it the accelerating and decelerating effects are generally ascribable to coplanar and non coplanar crack interactions respectively we rigorously treat the multiple interactions among the cracks using the boundary integral equation method biem and assume the critical stress fracture criterion for the analysis of spontaneous crack propagation our analysis shows that a dynamic rupture nucleated in the cracked zone begins to grow slowly due to the relative predominance of non coplanar interactions this process radiates the p1 phase if the crack continues to grow coalescence with adjacent coplanar cracks occurs after a short time then coplanar interactions suddenly begin to prevail and crack growth is accelerated the p2 phase is emitted in this process it is interpreted that the two distinct phases appear in the process of the transition from non coplanar to coplanar interaction predominance

role of fault branches in earthquake rupture dynamics 1 we analyze earthquake ruptures propagating along a straight main fault and encountering a finite length branch fault such intersections are often observed in natural fault systems the predicted effects of the interaction with the branch that we report can be remarkable they can strongly perturb the propagation velocity on the main fault and in some cases even arrest that propagation earlier work kame et al 2003 bhat et al 2004 emphasized the role of the fault pre stress state branch geometry i e branching angle and the incoming rupture velocity at the branching junction in determining whether the rupture would follow the branch or continue on the main fault or both through simulations which did not let a rupture on the branch encounter a barrier or a fault end called infinite branch cases henceforth in this study we look at finite branch cases and study the effect also of branch length with rupture being blocked from propagation beyond the branch end it is known that sudden stoppage of a dynamic rupture front leads to the propagation of large dynamic stress perturbations in the medium these have been known to nucleate or terminate ruptures on adjacent fault segments harris et al 1991 harris and day 1993 1999 harris et al 2002 fliss et al 2005 among others we thus anticipate interaction between the rupture on the main fault and the branched one at two stages when the rupture is propagating on the branch and when it is suddenly blocked at the branch end we show that in general rupture termination on a compressional branch little affects propagation on the main fault compared to the infinite branch cases for branches on the extensional side we show in some cases that whereas an infinite branch would have allowed or stopped rupture propagation on the main fault a finite branch stops or allows propagation on the main fault such results have a dependence on branch length that we document we also illustrate branch related complexities in rupture velocity evolution which could be one of the sources of the high frequency content of strong ground motion record complexities in the slip distribution often associated with a presumed heterogeneous strength distribution along the fault can also be observed when rupture is terminated on a branch

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

source characteristics of the 1985 michoacan mexico earthquake at periods of 1 to 30 seconds source characteristics of the sept 19 1985 michoacan mexico earthquake and its aftershock on sept 21 were inferred from broadband and short period teleseismic gdsn records we fourier transformed the p waves corrected for instrument response attenuation geometrical spreading and radiation pattern including the depth phases and then averaged to obtain the teleseismic source spectrum from 1 to 30 s the michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events source spectra for the sept 21 aftershock 1981 playa azul 1979 petatlan and 1978 oaxaca events follow a trend similar to that of the 1985 michoacan event this spectral trend may characterize the mexican subduction zone a station by station least squares inversion of the michoacan earthquake records for the source time function yielded three source pulses which we interpreted as events on the fault plane the first two are similar in moment and the third contains only 20 of the moment of the first directivity is evident in the timing at each station we measured the time differences between the pulses and performed a least squares nonlinear estimation of the strike distance and time separation between the events to locate them relative to one another the second event occurred 26 s after the first and 82 km southeast of it indicating southeastward rupture along the trench the third event occurred 21 s after the second and about 40 km seaward of it the two large events are also seen in the near field strong motions the mainshock records spectrum and time functions contain less high frequency radiation than those of the 1985 valparaiso chile earthquake apparently the michoacan earthquake ruptured two relatively smooth strong patches which generated large 30 s waves but small 1 to 10 s waves such behavior contrasts with the valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy this difference is consistent with the higher near field accelerations recorded for the valparaiso event

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

heterogeneous fault stresses from previous earthquakes and the effect on dynamics of parallel strike slip faults 1 we combine a viscoelastic model for the interseismic process and an elastodynamic model for the coseismic process to explore the dynamics over multiple earthquake cycles of two parallel strike slip faults embedded in a two dimensional full space the step over fault geometry results in a buildup of heterogeneous fault stress near the step over this heterogeneous stress accumulates at the early stage of the evolution of the fault system and finally stabilizes after a number of earthquake cycles the heterogeneity in fault stress varies with the geometrical parameters e g width and along strike overlap gap of the step over as well as the rupture history of the fault system this heterogeneous fault stress from previous earthquakes has significant effects on earthquake rupture initiation propagation and termination the locations with a low normal stress level near a step over are favorable points for earthquake initiation rupture can jump a 4 km wide compressional step over and a 8 km or wider dilational step over if the fault system has historically experienced many earthquakes a young step over with less induced heterogeneity allows rupture to jump only smaller step over widths these results may have important implications for seismic hazard analysis in areas where segmented strikeslip faults predominate particularly for estimating maximum earthquake potential citation duan b and d d oglesby 2006 heterogeneous fault stresses from previous earthquakes and the effect on dynamics of parallel strike slip faults j geophys res 111 b05309 doi 10 1029 2005jb004138

mapping of the high frequency source radiation for the loma prieta earthquake california waveform inversion has been successfully applied to map the spatial distribution of earthquake fault slip from strong motion records but its application has been restricted to a relatively lower frequency range of the data 5 hz generation is incoherent between neighboring patches on the fault we formulated an analytical model for calculating the envelope time history of the squared high frequency displacement seismogram this envelope time history is found through the line integration of the squared energy radiation intensity over the isochron using a recursive stochastic inversion scheme we determined both the lower frequency slip distributions from the low pass filtered strong motion displacement data and the higher frequency energy radiation intensity from the envelope of the squared high frequency displacement data on an extended fault for the 1989 loma prieta earthquake respectively our resulting lower frequency slip model shows that there are two main slip patches and little slip directly updip of the hypocenter one major slip zone located 10 km northwest of the hypocenter with predominant dip slip and the other located 12 km southeast of the hypocenter with predominant oblique slip our preliminary results of the high frequency energy radiation intensity inversion indicate three large high frequency sources and few smaller ones located on the outer periphery of the large slip zones this suggests that the high frequency energy sources are located along or near the boundaries of large slip zones which is consistent with the theoretical consideration that high frequencies are primarily generated from the rupture stopping areas or places with large slip variation

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

rupture termination and jump on parallel offset faults proper evaluation of seismic hazard depends on accurate estimates of potential earthquake size in areas with complex multisegment fault systems such an estimate in turn depends on an ability to predict the circumstances under which rupture may jump between fault segments many observational and numerical studies have analyzed the phenomenon of jumping rupture but none has focused on how the process of rupture termination on the primary nucleating fault segment affects the ability of rupture to jump to a secondary fault segment in the current study i model the dynamics of a simple 2d strike slip fault system with two parallel segments arranged with either a compressional or extensional stepover i vary the suddenness with which the initial shear stress tapers to zero on the primary section if the initial shear stress goes to zero over a very small 100 m distance rupture readily jumps both compressional and extensional stepovers of 1 km if the initial shear stress tapers to zero over 1 km rupture can jump the compressional stepover but not the extensional stepover if the initial shear stress tapers to zero over 2 5 km rupture cannot jump either the compressional or the extensional stepover the results illustrate the importance of the slip gradient and the resultant static stress field and the acceleration of the rupture front and the resultant generation of stopping phases in determining the probability of jumping rupture

rupture propagation beyond fault discontinuities significance of fault strike and location summary earthquake rupture sometimes occurs on several faults and often decelerates or terminates at fault stepovers factors that control the rupture processes across stepovers are important for an understanding of earthquake growth and termination in this study we investigate such factors by calculating the spontaneous rupture processes of two non coplanar faults in a 3 d model dealing with two extreme models in which two strike slip faults are either parallel or perpendicular we show that the rupture processes beyond fault discontinuities are drastically different for the two models we find three factors influencing rupture processes beyond fault discontinuities depth of the upper edge of the two faults location of the edge of the first fault and geometry of the two faults these factors determine the time and location of rupture jumps to the second fault for rupture propagation to the second fault it is essential for rupture on the first fault to arrive at the edge of the fault in particular whether rupture on the first fault reaches the earth s surface or not controls the difficulty of rupture jumps and the locations where the rupture is triggered which is also related to the step direction of the two faults this is because the stress perturbation at the fault edge is affected by the earth s free surface

models of aftershock occurrence summary the omori formula for aftershock activity can be shown to be a consequence of each of two models of a complex earthquake in the first case aftershocks are assumed to be caused by subsequent slip on asperities of a fault which are locked during the fracture in the main shock in the second aftershocks are caused by catastrophic coalescence of nearby small fractures with the fracture surface of the main shock hence the first source model describes aftershock activity in the interior of a main shock rupture zone while the second describes aftershock activity near the outer boundary of a main shock rupture zone stress corrosion cracking is assumed to be the physical cause of the quasistatic growth of the main fracture zone at the expense of the area of the locked patches in the first model and is also the cause of the growth of the small satellite fractures in the second

numerical modelling of space and time variations of seismic activity before major earthquakes summary the spatial and temporal variations of seismic activity before large earthquakes including precursory swarm earthquakes clustering pre seismic quiescence in the epicentral area doughnut patterns and foreshocks are numerically modelled by quasi static and dynamical rupture processes on three dimensional heterogeneous faults with various types of non uniform strength distributions the main physical parameters that could yield these precursory changes of seismicity patterns are the form of distributions of static and sliding frictional strengths and their spatial variations over the fault and also the rate of increase of tectonic stress for weakly heterogeneous faults a large scale main shock rupturing the entire fault surface occurs without any precursory indications if the fault has moderately to heavily non uniform strengths on a small scale gradually increasing activity tends to occur over a long period prior to a main event with no swarms or quiescence for extremely heterogeneous faults including a number of small to moderate size asperities with medium to high strengths a typical sequence appears with swarms quiescence foreshocks and a main shock event the temporal behaviour of these events seems to depend primarily on the form of distribution of the strengths if the fault includes a large size asperity with medium to high strengths in its central part a doughnut pattern of seismicity repeatedly appears with high activity and quiescence in the surrounding and central asperity regions if a belt like barrier region with extremely high strengths lies in the central part of the fault dividing the entire region into two sections large scale shocks take place successively in the adjacent unruptured regions at slightly different times and a main shock event rupturing the entire fault occurs after a long period of quiescence these results from numerical simulations could provide possible explanations of observed seismicity patterns prior to major earthquakes

the effect of thermal pressurization on dynamic fault branching s u m m a r y we numerically investigate the effect of thermal pressurization tp on fault branch behaviour during dynamic rupture propagation a situation likely to occur during large earthquakes at subduction interfaces we consider a 2 d mode ii in plane rupture in an infinite medium that propagates spontaneously along a planar main fault and encounters an intersection with a pre existing branching fault the fault system is subjected to uniform external stresses we adopt the values used by kame et al we use the 2 d boundary integral equation method and the slip weakening friction law with a coulomb failure criterion allowing the effective normal stress to vary as pore pressure changes due to tp we reveal that tp can alter rupture propagation paths when the dip angle of the main fault is small the rupture propagation paths depend on the branching angle when tp is not in effect on either of the faults as described by kame et al however the dynamic rupture processes are controlled more by tp than by the branching angle when tp is in effect on the main fault only the rupture propagates along the main fault it propagates along the branch when tp is in effect on both faults finally we considered the case where there is a free surface above the branch fault system in this case the rupture can propagate along both faults because of interaction between the free surface and the branch fault in addition to tp on the main fault

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

aftershocks of the coyote lake california earthquake of august 6 1979 a detailed study aftershock hypocenters and focal mechanism solutions for the coyote lake california earthquake reveal a geometrically complex fault structure consisting of multiple slip surfaces the faulting surface principally consists of two right stepping en echelon northwest trending partially overlapping nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 parkfield california earthquake the overlap occurs near a prominent bend in the surface trace of the calaveras fault at san felipe lake slip during the main rupture as inferred from the distribution of early aftershocks appears to have been confined to a 14 km portion of the northeastern sheet between 4 and 10 km depth focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex with left stepping imbricate structure seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock aftershocks in this zone define a single vertical plane 8 km long between 3 and 7 km depth within the overlap zone between the two main slip surfaces the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone in close agreement with the stress perturbations predicted by crack interaction models aftershock activity in the overlap zone is not associated with a simple dislocation surface space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone this alternation is consistent with local stress perturbations predicted by crack interaction models we conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which in turn control the displacements table 5 is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j82 006 1 00 payment must accompany order

application of fracture mechanics to the simulation of seismicity and recurrence of characteristic earthquakes on a fault we present a fault model consisting of a large number of collinear cracks and intervening asperities for the simulation of seismicity and recurrence of large earthquakes on a fault we apply fracture mechanics to the analysis of the interaction of cracks and to the consideration of fracture condition our fault model gives rise to a complex frequency distribution of event sizes even if no externally imposed spatial inhomogeneity is assumed for the distribution of cracks and asperities and a homogeneous statistical distribution is assumed for fracture strengths the frequency distribution of event sizes is found to be bimodal with no intermediate size event in all the calculations for the smaller events the frequency distribution shows a power law behavior whereas the larger size events denote ones that rupture the whole fault statistically the same frequency distributions are obtained for inhomogeneous distributions of crack lengths and fracture strengths the occurrence of events that rupture the whole fault is shown to be periodic when the fault is isolated the magnitudes of these large events generally increase with increasing recurrence period for given fault length and rate of remote loading the events having a longer recurrence period have a shorter fault length for given magnitude and rate of remote loading if there are fault interactions the periodicity in the recurrence of the large events that would be observed for the isolated fault disappears and the recurrence of the large events becomes complex

possible mechanisms of dynamic nucleation and arresting of shallow earthquake faulting abstract the nucleation and arresting mechanisms of large shallow earthquakes are investigated from both observational and theoretical viewpoints we show that two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes the first phase denotes the onset of the p wave which shows a very gradual increase in amplitude with time this gradual change is interrupted by the arrival of the second phase which causes an abrupt change in the amplitude our seismological observation shows that the time interval between the onset of the two phases is strongly correlated with the magnitude of the earthquake it is probable that the above two phases are related to some aspect of inhomogeneities in the earth s crust one of the important sources of such inhomogeneities is known to be preexisting cracks and their interactions we theoretically show in this paper that a rupture occurring in a zone of densely distributed cracks radiates elastic waves that can simulate the features of the above two phases note that such a rupture is considerably affected by crack interactions theoretical calculation is also carried out to investigate the effect of crack interactions on the arresting of rupture propagation a propagating crack can excite subsidiary cracks ahead of its crack tip which gives rise to crack interactions we show that these interactions sometimes facilitate the arresting of rupture propagation

model for intermediate term precursory clustering of earthquakes a causal sequence of an earthquake swarm extended quiescence foreshocks and main shock can be understood in terms of an aggregation model of crack growth and ultimate fusion on a multiplanar earthquake fault system the rheological model is one of accelerated crack growth under conditions of slip weakening coupled with a slow recovery of static friction after fracture once a slip feature has been generated that spans a large part of the available geometry a stress shadow is cast on other neighboring cracks that strongly retards and inhibits crack growth thus initiating the quiescent phase abruptly the success of the model depends on the assumption that nonlinear rheology regulates the rate of slip and that earthquake faults are not simply connected surfaces but instead have many strands

dynamic fault interaction during a fluid injection induced earthquake the 2017 mw 5 5 pohang event the november 15th 2017 mw 5 5 pohang earthquake south korea has been linked to hydraulic stimulation and fluid injections making this the largest induced seismic event associated with an enhanced geothermal system egs to understand its source dynamics and fault interactions we conduct the first 3d high resolution spontaneous dynamic rupture simulations of an induced earthquake we account for topography off fault plastic deformation under depth dependent bulk cohesion rapid velocity weakening friction and 1d subsurface structure a guided fault reconstruction approach that clusters spatio temporal aftershock locations including their uncertainties is used to identify a main and a secondary fault plane that intersects under a shallow angle of 15 based on simple mohr coulomb failure analysis and 180 dynamic rupture experiments in which we vary local stress loading conditions fluid pressure and relative fault strength we identify preferred two fault plane scenarios that well reproduce observations we find that the regional far field tectonic stress regime promotes pure strike slip faulting while local stress conditions constrained by borehole logging generate the observed thrust faulting component our preferred model is characterized by overpressurized pore fluids non optimally oriented but dynamically weak faults and a close to critical local stress state in our model earthquake rupture jumps to the secondary fault by dynamic triggering generating a measurable non double couple component our simulations suggest that complex dynamic fault interaction may occur during fluid injection induced earthquakes and that local stress perturbations dominate over the regional stress conditions these findings therefore have important implications for seismic hazard in active geo reservoir

numerical study of splay faults in subduction zones the effects of bimaterial interface and free surface 1 splay faults are a characteristic branching fault system in some subduction zones we model a megathrust earthquake rupture with a branching fault inhomogeneous media and a free surface using a spontaneously propagating mode ii crack on a bimaterial interface for this purpose we develop an explicit finite element code to solve the elastodynamic equations and a slip weakening friction law on the fault plane in a homogeneous prestress condition the rupture on the branching fault is enhanced when the upper medium is more compliant even when the upper medium is less compliant a fast rupture propagating at a velocity close to the generalized rayleigh velocity produces the strong dynamic stress around the rupture front and activates the branching fault when prestress linearly increases with depth from zero at the free surface only a slight change of normal stress that is due to seismic waves causes an initiation of rupture on the branching fault at the free surface and this rupture propagates downward on the branching fault the arrival of this descending rupture at the branching point greatly changes the stress field in the surrounding area it affects the slip distribution on the main fault and even terminates the main rupture propagation if it occurs before the arrival of the main rupture on the branching point

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

numerical simulation of spontaneous rupture processes on twonon coplanar faults the effect of geometry on fault interaction summary analyses of earthquake sources have revealed that the earthquake rupture process is complex and that the rupture does not occur on a single plane earthquake faults are often composed of several subfaults and rupture propagation tends to decelerate or terminate at places where the fault strike changes these observations imply that fault geometry including fault steps and fault strike change plays an important role in earthquake rupture complexity in this paper we calculate the spontaneous rupture processes of two non coplanar faults in 2 d in plane problems attempting to clarify the eiect of fault geometry we consider two simple modelsmodels in which two faults are either parallel or perpendicular to each other we calculate spontaneous rupture propagation on the faults bya nite diierence method andwe then compare the results in our simulations rupture initially grows on the main fault and stress perturbation from the main rupture then triggers rupture on the secondary fault propagation of the main fault rupture controls a spatio temporal pattern ofstress diierence in the uniform elastic medium which determines the rupture process of the secondary fault the rupture propagation and termination of the secondary fault are signi cantly diierent between the two models the diierence is obvious when rupture of the main fault is arrested and the secondary fault is located near the arrested end of the main fault when the secondary fault is parallel to the main fault rupture can propagate ahead on the secondary fault however when the secondary fault is perpendicular to the main fault rupture is either not triggered on the secondary fault or soon terminates if triggered this variation of the rupture process implies that fault interaction depending on geometry can explain the termination and change of rupture at places where the fault strike varies this shows the importance of the fault geometry in studying spontaneous dynamic rupture processes

a cellular automata slider block model for earthquakes ii demonstration of self organized criticality for a 2 d system summary it has been suggested that distributed seismicity is an example of self organized criticality if this is the case the earth s crust in an active tectonic zone is in a near critical state and faults can interact over large distances observed seismicity and earthquake statistics are consequences of the dynamical interaction of seismic faulting over a wide range of scales we address this problem by considering a two dimensional array of slider blocks with static dynamic friction the twodimensional system is treated as a cellular automaton such that only one slider block is allowed to slip at a given time and interacts only with its nearest neighbours through connecting springs because of this treatment the amount of slip for each failed block can be obtained analytically and the system is deterministic with no stochastic inputs or spatial heterogeneities in many cases the slip of one block induces the slip of adjacent blocks the size of an event is specified by the number of blocks that participate in the event the number of small events with a specified size has a power law dependence on the size with a power close to 1 36 the distributions of normalized recurrence times for most events are close to a poisson process and gradually deviate towards periodicity for large events the recurrence time statistics are generally insensitive to parameter variations large events may occur at stress levels considerably lower than the failure strength of an individual block and the stress drops associated with large events are generally small this may provide an explanation for observed low stress levels in tectonically active

the bright spot of an earthquake abstract umeda y 1992 the bright spot of an earthquake in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 13 22 field surveys in the source region for some large shallow earthquakes reveal that there is a special spot characterized by high acceleration aftershock gap and parallel faults system this spot is called an earthquake bright spot which occupies a relatively small region compared to the final rupture region at least two kinds of phases p1 and p2 which are responsible for the rupture initiation and the bright spot formation can be identified on the broad band seismograms for large earthquakes duration times of preliminary waves p1 p2 are proportional to the earthquake magnitudes by using this evidence from the field and seismograms a model having three rupture stages is proposed for a growth process of large earthquakes the 1st stage is an initiation and propagation of rupture the 2nd stage is formation of the bright spot which produces the high frequency seismic waves in the 3rd stage smooth dislocations again propagate in bi laterally and radiate the long period seismic waves

fault branching and rupture directivity 1 could the directivity of a complex earthquake be inferred from the ruptured fault branches it created typically branches develop in forward orientation making acute angles relative to the propagation direction direct backward branching of the same style as the main rupture e g both right lateral is disallowed by the stress field at the rupture front here we propose another mechanism of backward branching in that mechanism rupture stops along one fault strand radiates stress to a neighboring strand nucleates there and develops bilaterally generating a backward branch such makes diagnosing directivity of a past earthquake difficult without detailed knowledge of the branching process as a field example in the landers 1992 earthquake rupture stopped at the northern end of the kickapoo fault jumped onto the homestead valley fault and developed bilaterally there nnw to continue the main rupture but also sse for 4 km forming a backward branch we develop theoretical principles underlying such rupture transitions partly from elastostatic stress analysis and then simulate the landers example numerically using a two dimensional elastodynamic boundary integral equation formulation incorporating slip weakening rupture this reproduces the proposed backward branching mechanism based on realistic if simplified fault geometries prestress orientation corresponding to the region standard lab friction values for peak strength and fracture energies characteristic of the landers event we also show that the seismic s ratio controls the jumpable distance and that curving of a fault toward its compressional side like locally along the southeastern homestead valley fault induces near tip increase of compressive normal stress that slows rupture propagation

a model of foreshock occurrence summary we have constructed a model for the occurrence of foreshocks and foreshock sequences the foreshocks as well as the main shock are assumed to occur because of the abrupt coalescence of pre existing cracks coalescence occurs if the stress intensity factors at the crack tips exceed a certain critical value in the sub critical state each crack undergoes accelerated extension quasi statically due to stress corrosion until it reaches the critical state at which point growth becomes catastrophic the rate of foreshock occurrence increases as the time of the main shock approaches and is described by a power law function of time the exponent in the power law is independent of the distributions of crack sizes spacings and fracture strengths this universality implies that foreshock patterns should be useful identifiers for the prediction of the occurrence times of large earthquakes however we find that there is considerable diversity in the patterns of energy release nevertheless there is some hope that the magnitudes of large earthquakes can be predicted from the magnitudes of the events in the foreshock sequences if one has a priori information describing the distributions of barrier strengths and geometries

gross structure of a fault during its formation process in westerly granite abstract we investigated the gross structure of a fault in three dimensions during its formation process the observed sample was westerly granite recovered from compression after peak stress but before dynamic rupture the sample was infused with a fluorescent resin mechanically stripped then observed with a stereo microscope under uv light the main fault plane was found to be macroscopically elliptical and the fault trace on each polished surface was identical to that detected by x ray ct scanning evidence suggests that the fault plane widened its cracked zone as it grew the gross structures of the fault system such as a step over structure and sub faults parallel to the main fault were also detected we conclude that the step over structure was formed during the fault formation process

comparison of conventional and garnet aluminosilicate quartz o isotope thermometry insights for mineral equilibration in metamorphic rocks oxygen isotope thermometry based on fractionations among quartz garnet kyanite sillimanite and potassium feldspar in amphibolite to granulite facies schists and gneisses was compared to thermometry based on mineral equilibria conventional thermometry the former approach takes advantage of the exchange between quartz and more refractory minerals such as garnet and the aluminosilicates and their potential for preserving peak oxygen isotopic fractionations although isotopic thermometry using these phases is potentially an approach for circumventing the problem in slowly cooled rocks of retrograde oxygen isotopic exchange with micas and feldspars and of fe and mg exchange between garnet and biotite the results are accurate or geologically meaningful in only certain settings in most cases the results of isotopic thermometry must be evaluated with the aid of quantitative forward models for calculating the amount of retrograde diffusive exchange among micas feldspars and quartz two models used here predict quartz isotopic compositions that differ by only 0 1 per mil and in most cases reproduce to within 0 2 per mil the measured isotopic composition of quartz excellent agreement between isotopic and conventional thermometry is obtained for mid to upper amphibolite facies rocks that have experienced one period of recrystallization and mineral growth and if they are coarse grained or cooled in the absence of a hydrous fluid phase isotopic temperatures of 850 degrees c are in excellent agreement with conventional thermometry in migmatitic granulite facies metapelites if cooling occurred under anhydrous conditions in other cases isotopic temperatures in granulite facies rocks are higher than conventional temperatures a reasonable result in view of the tendency for retrograde cation exchange between garnet and biotite in samples that have undergone multiple periods of recrystallization or metamorphism isotopic temperatures from coexisting quartz garnet and aluminosilicates may be modified by diffusional exchange of quartz with micas and feldspars aided by recrystallization by retrograde reactions involving the latter phases or by open system interaction with externally derived fluids external fluid infiltration combined with recrystallization of quartz and aluminosilicate may result in isotopic reversals between garnet and matrix phases conventional thermobarometry in such complex rocks based in large part on mineral equilibria involving garnet may be erroneously assigned geological significance in the absence of insights provided by oxygen isotope analysis

dynamic propagation and interaction of a rupture front on a planar fault we investigate the propagation and interaction of a rupture front that propagates on a planar fault using a boundary integral equation method we show first that the rupture velocity is controlled by a delicate balance between consumed fracture energy and supplied elastic strain energy a very sharp boundary in parameter space separates models in which ruptures stop spontaneously from those in which rupture propagates at super shear speeds the transition zone or bifurcation is shown to be stable with reference to small scale heterogeneities of the stress field using the relations derived from this analysis we examined the mechanism to generate high slip rate when two rupture fronts collide we found that collision at slow rupture velocities causes abrupt stress drop and generates high slip rates however these features tend to be moderated by large slip weakening distances finally we simulated rupture front focusing at the initial stages of an earthquake a phenomenon that may cause high slip rate pulses and therefore generate high frequency seismic waves we assume a pre slip region in which stress has decreased quasi statically to the dynamic friction level due to this pre slip strong stress concentration has developed around the pre slip area and a dynamic rupture starts at a certain point on the rim of the pre slip region we observe rupture front focusing that generates high slip rate pulses we also studied a double pre slip model in which two pre slip regions exist close to each other before the earthquake and found that multiple pre slips enhance the focusing effects

the morphology of strike slip faults examples from the san andreas fault california fault zones are seldom straight nor are fractures within them continuous permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip these strain fields may be manifest as vertical deformation near the fault strain fields that increase area result in subsidence and strain fields that reduce area result in uplift although these effects are modified by erosion and deposition the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike graphical views of strain fields associated with simple vertical strike slip faults are derived using boundary element methods if the fault is straight and parallel to the applied shear strain the hills and valleys resulting from finite fault rupture are separated by the fault trace this is rarely observed in nature instead strike slip faults cut through hills and valleys apparently of their own making we show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments we test our theory with examples from parts of the san andreas fault system in california the morphologies of the parkfield region and the southern san andreas fault in the coachella valley are shown to be consistent with plausible distributions of slip on mapped surface faults the location of the salton sea appears to be a consequence of a reduction of slip on the san andreas fault toward the brawley seismic zone

high amplitude seismic waves radiated from the bright spot of an earthquake abstract in the source region of a large shallow earthquake there is likely to be a small spot characterized by the production of high accelerations and a gap in the aftershocks on the seismograms of most large shallow earthquakes and on some seismograms of moderate main shocks two types of p waves may be detected the relatively low amplitude waves following phase p 1 and the predominant high frequency waves following phase p 2 the duration of the seismic waves following phase p 1 the p 1 p 2 time increases proportionately with the earthquake magnitude this fact strongly suggests that the preliminary rupture period is closely related to the radiation of the predominant high frequency seismic waves to explain the above observation the shear strain waves radiated from a moving rupture are calculated because the seismic wave velocity is generally higher than the rupture propagation velocity maximum shear strains by seismic waves usually precede the rupture front if the total shear strain which is equal to the static pre earthquake shear strain plus dynamic shear strain excited by seismic waves exceeds the critical shear strain 10 4 the secondary ruptures break out in the forward direction from the initial rupture the rupture processes of large or moderate earthquakes occur in two stages one is the initiation of rupture corresponding to phase p 1 and the other is the development of secondary ruptures triggered by seismic shear strain waves the latter process generates an earthquake bright spot which radiates high amplitude seismic waves

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments

is there any evidence of mega lake manly in the eastern mojave desert during oxygen isotope stage 5e 6 a comment on hooke r l 1999 lake manly shorelines in the eastern mojave desert california quaternary research52 328 336