ultracataclasite structure and friction processes of the punchbowl fault san andreas system california abstract the punchbowl fault is an exhumed 40 km displacement fault of the san andreas system in the devil s punchbowl the fault contains a continuous ultracataclasite layer along which the punchbowl formation sandstone and an igneous and metamorphic basement complex are juxtaposed the fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer by analogy with nearby active faults we assume that the punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures we have mapped the ultracataclasite layer at 1 1 and 1 10 to determine the mode of failure and to constrain the processes of seismic slip on the basis of color cohesion fracture and vein fabric and porphyroclast lithology two main types of ultracataclasite are distinguished in the layer an olive black ultracataclasite in contact with the basement and a dark yellowish brown ultracataclasite in contact with the sandstone the two are juxtaposed along a continuous contact that is often coincident with a single continuous nearly planar prominent fracture surface pfs that extends the length of the ultracataclasite layer in all exposures no significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other the ultracataclasites are cohesive throughout except for thin accumulations of less cohesive reworked ultracataclasite along the pfs structural relations suggest that 1 the black and brown ultracataclasite were derived from the basement and sandstone respectively 2 the black and brown ultracataclasites were juxtaposed along the pfs 3 the subsequent final several kilometers of slip on the punchbowl fault occurred along the pfs and 4 earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite by comparison with rock friction experiments the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values and thus relatively small nucleation and breakdown dimensions for earthquake ruptures of the various mechanisms proposed to explain the low strength of the san andreas and to produce dynamic weakening of faults those that require or assume extreme localization of slip are most compatible with our observations

the effect of rupture complexity on estimates of source size a general technique is proposed for estimating rupture geometry from measurements of body wave durations wherein measurements of the overall signal duration or the duration of a single pulse identified in all of the arrivals are inverted to estimate the geometry of the overall rupture or of a specific subevent respectively the complexity of a waveform is determined by comparing the overall signal duration to the duration of the most prominent pulse in the waveform this complexity is used to estimate the ratio of the average rupture velocity to the subevent rupture velocity the shear waves radiated by eight moderate 3 6 ml 4 9 multiply recorded aftershocks of the 1975 oroville california earthquake are analyzed using measurements of the corner frequency the characteristic frequency the pulse duration and the signal duration comparison of these four different estimates of source size indicates that the corner frequency and the characteristic frequency are more sensitive to the dominant subevent size than to the overall event size the difference between the estimates of source size determined from the signal duration measurements and the spectral measurements increases as the rupture complexity increases

stress triggering of large earthquakes complicated by transient aseismic slip episodes 1 we investigate how a static stress change caused by a nearby earthquake perturbs sliding processes on a fault and the time to the next earthquake for this purpose we model numerically a reverse fault in a two dimensional semi infinite and elastic medium partly stable and partly unstable under a rate and state dependent friction law when no stress change intervenes steady sliding spreads out of the deep zone of frictional stability and keeps expanding updip during the interseismic period aseismic slip episodes ases develop spontaneously within the region of steady sliding and repeat quasi periodically when steady sliding has spread beyond a certain critical length the nucleation length above the zone of frictional stability one of the ases undergoes rapid buildup and avalanches into high speed seismic sliding when a step like stress change intervenes ases of larger amplitudes arise and evolve similarly to their spontaneous counterparts until one of them avalanches into an earthquake if the stress change arrives not too early nor too late in the seismic cycle the basic picture is as follows delaying the stress step be it positive or negative delays the timing of the ase sequence and thereby postpones the next earthquake when the stress step is delayed to more than a certain extent however an earlier ase gets to evolve into an earthquake advancing its time discontinuously only in the final stage of a seismic cycle does a positive stress step systematically advance the next earthquake and a negative stress step systematically delay it

containment of stick slip failures on a simulated fault by pore fluid injection unstable stick slip events have been confined to a portion of a finely ground 40 by 200 cm simulated fault in a granite sample by selectively injecting water onto the ends of the stressed fault source parameters of eight confined events generated at normal stresses between 0 7 and 4 1 mpa have been measured dynamic stress drops increased from 0 06 to 0 38 mpa with increasing normal stress such that the ratio of stress drop to normal stress is about 0 09 fractional shear stress drops of these events are less than 0 2 total fault slip measured at the center of the slipped portion ranged from 8 5 to 38 9 mu m maximum relative slip velocities at the same location ranged from 2 5 to 34 7 cm s slip velocity increased with increasing dynamic stress drop

stressing of locked patches along a creeping fault an analysis is presented of the stressing of locked patches along a fault zone which is creeping elsewhere the model consists of strike slip faulted elastic lithospheric plates loaded in a manner equivalent to imposition of a remotely uniform tectonic shear stress the fault zone has an inhomogeneous strength distribution both depth wise and along strike it is modeled as being composed of locked patches and freely slipping parts treated as cracks the solution is given with the use of the line spring model which analyzes the problem by thickness averaged plane stress theory for lithospheric plates which slip along a discontinuity cut at the plate boundary as a boundary condition thickness averaged stress and slip at each local section along the cut was related to one another by the result of an antiplane strain analysis of slip for the crack or crack pair which describes the slipping and locked depth ranges at that section the analysis indicates that the slip distribution along creeping parts of the fault as well as the stress distribution along locked patches depends strongly on the geometry of these zones the model is used to examine stress concentrations associated with a slip deficient seismic gap along strike and to study the effect of local irregularities in the margin of a locked region it is also used to simulate slip and stressing processes associated with the creeping portion of the san andreas fault in central california between the presently locked zones of the great 1906 and 1857 ruptures and to constrain the nature of an apparently locked zone at the southeastern end which ruptures in characteristic parkfield earthquakes near fault creep and broadscale displacement data along the fault since the 1966 parkfield earthquake and inferences from seismicity distributions are used limitations of the modeling procedure at short spatial wavelengths prohibit an accurate description of the parkfield locked patch but results suggest that it may be localized and occupy a small fraction of area of the normal seismogenic zone an effective remote stressing rate of order 0 3 10 6 shear modulus yr is inferred together with a less well constrained 30 to 40 km lithospheric thickness for consistency with the displacement data results enable estimates of stress accumulation along the locked 1857 rupture zone and the build up of fracture energy release capability of order 107 j m2 in 150 yr at its lower margin

effects of loading velocity stiffness and inertia on the dynamics of a single degree of freedom spring slider system numerical simulation of the nonlinear dynamics of a single degree of freedom spring slider system can provide useful insight into the effects of loading velocity friction environment elastic stiffness and inertia on the stress drop and dynamic behavior of earthquakes in our numerical simulations using a rate and state dependent friction law we varied the loading velocity by 7 orders of magnitude the stiffness by 2 orders of magnitude the mass by 5 orders of magnitude and the velocity weakening parameter b a by a factor of 160 four stability regimes were identified in the load point velocity stiffness space when inertia was taken into account for cyclic stick slip instabilities the stress drop amplitude increases with decreasing load point velocity with decreasing stiffness with increasing velocity weakening parameter b a and with decreasing mass simple scaling relations for the static stress drop dynamic stress drop static friction and shear fracture energy with the natural logarithm of load point velocity were observed using the scaling relation for static stress drop we infer the normalized velocity weakening parameter b a with respect to normal stress to range from 0 0011 to 0 0039 for triaxial experiments on saw cut westerly granite specimens sandwiched with a simulated gouge layer of ultrafine quartz at pressures from 10 mpa to 100 mpa unless we take b a throughout the seismogenic layer to be much higher than experimental measurements the effect of loading velocity by itself is not sufficient to account for the increase of stress drop with increasing earthquake recurrence time inferred from seismological data to the extent that our simple system can be treated as an analog for earthquakes our study shows that the velocity weakening parameter b a controls the magnitude of the static stress drop far more than the loading velocity if velocity weakening increases systematically with increasing healing time then the coupled effect of loading velocity variation and change in friction environment can account for the seismological observations

shallow slip deficit due to large strike slip earthquakes in dynamic rupture simulations with elasto plastic off fault response summary slip inversions of geodetic data from several large magnitude 7 strike slip earthquakes point to coseismic slip deficit at shallow depths 3 4 km that is coseismic slip appears to decrease towards the earth surface while the inferred slip distribution may be consistent with laboratory derived rate and state friction laws suggesting that the uppermost brittle crust may be velocity strengthening there remains a question of how the coseismic slip deficit is accommodated throughout the earthquake cycle the consequence of velocity strengthening fault friction at shallow depths is that the deficit of coseismic slip is relieved by post seismic afterslip and interseismic creep however many seismic events with inferred shallow slip deficit were not associated with either resolvable shallow interseismic creep or robust shallow afterslip hence the origin of shallow slip deficit remains uncertain in this study we investigate whether inelastic failure in the shallow crust due to dynamic earthquake rupture can explain the inferred deficit of shallow slip evidence for such failure is emerging from geologic seismic and geodetic observations we find that the amount of shallow slip deficit is proportional to the amount of inelastic deformation near the earth surface such deformation occurs under a wide range of parameters that characterize rock strength in the upper crust however the largest magnitude of slip deficit in models accounting for off fault yielding is 2 4 times smaller than that inferred from kinematic inversions of geodetic data to explain this discrepancy we further explore to what extent assumptions in the kinematic inversions may bias the inferred slip distributions inelastic deformation in the shallow crust reduces coseismic strain near the fault introducing an additional artificial deficit of up to 10 per cent of the maximum slip in inversions of geodetic data that are based on purely elastic models the largest magnitude of slip deficit in our models combined with the bias in inversions accounts for up to 25 per cent of shallow slip deficit which is comparable but still smaller than 30 60 per cent deficit inferred from kinematic inversions we discuss potential mechanisms that may account for the remaining discrepancy between slip deficit predicted by elasto plastic rupture models and that inferred from inversions of space geodetic data

ubiquitous weakening of faults due to thermal pressurization faults weaken during earthquakes analysis of the amount of energy released during earthquakes globally suggests that heat induced pressurization of pore fluids can weaken faults during earthquakes of all sizes laboratory simulations of earthquakes show that at high slip rates faults can weaken significantly aiding rupture1 2 3 various mechanisms such as thermal pressurization and flash heating have been proposed to cause this weakening during laboratory experiments1 4 5 6 yet the processes that aid fault slip in nature remain unknown measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening known as an event s fracture energy7 8 9 here we compile new and existing8 9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes we observe a distinct transition in how fracture energy scales with event size which implies that faults weaken differently during small and large earthquakes and earthquakes are not self similar we use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms we find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes over seven orders of fault slip magnitude we conclude that thermal pressurization is a widespread and prominent process for fault weakening

role of fluid injection on earthquake size in dynamic rupture simulations on rough faults an outstanding question for induced seismicity is whether the volume of injected fluid and or the spatial extent of the resulting pore pressure and stress perturbations limit rupture size we simul

earthquake ruptures with strongly rate weakening friction and off fault plasticity part 1 planar faults abstract observations demonstrate that faults are fractal surfaces with deviations from planarity at all scales we study dynamic rupture propagation on self similar faults having root mean square rms height fluctuations of order 10 3 to 10 2 times the profile length our 2d plane strain models feature strongly rate weakening fault friction and off fault drucker prager viscoplasticity the latter bounds otherwise unreasonably large stress concentrations in the vicinity of bends our choice of a cohesionless yield function prevents tensile stress states and thus fault opening a consequence of strongly rate weakening friction is the existence of a critical background stress level above which self sustaining rupture propagation in the form of self healing slip pulses first becomes possible around this level at which natural faults are expected to operate ruptures become extremely sensitive to fault roughness and exhibit substantial fluctuations in rupture velocity except for shallow inclinations of the maximum compressive stress to the fault less than about 20 the fluctuations are anticorrelated with the local fault slope these accelerations and decelerations of the rupture together with naturally emerging slip heterogeneity excite waves of all wavelengths and result in ground acceleration spectra that are flat at high frequency consistent with observed strong motion records

constructing constitutive relationships for seismic and aseismic fault slip for the purpose of modeling natural fault slip a useful result from an experimental fault mechanics study would be a physically based constitutive relation that well characterizes all the relevant observations this report describes an approach for constructing such equations where possible the construction intends to identify or at least attribute physical processes and contact scale physics to the observations such that the resulting relations can be extrapolated in conditions and scale between the laboratory and the earth the approach is developed as an alternative but is based on ruina 1983 and is illustrated initially by constructing a couple of relations from that study in addition two example constitutive relationships are constructed these describe laboratory observations not well modeled by ruina s equations the unexpected shear induced weakening of silica rich rocks at high slip speed goldsby and tullis 2002 and fault strength in the brittle ductile transition zone shimamoto 1986 the examples provided as illustration may also be useful for quantitative modeling

constitutive relations between dynamic physical parameters near a tip of the propagating slip zone during stick slip shear failure abstract constitutive relations between physical parameters in the cohesive zone during stick slip shear failure are experimentally investigated stick slip was generated along a 40 cm long precut fault in tsukuba granite samples using a servocontrolled biaxial loading apparatus dynamic behavior during local breakdown processes near a tip of the slipping zone is revealed the slip velocity and acceleration are given as a function of the slip displacement and the cohesive or breakdown shear stress as a function of the slip velocity a cycle of the breakdown and restrengthening process of stick slip is composed of five phases characterized in terms of the cohesive strength and the slip velocity the cohesive strength can degrade regardless of the slip velocity during slip instabilities the maximum slip acceleration u max and the maximum slip velocity u max are obtained experimentally as u max 2 u c u max 2 and u max b g v where u c is the critical displacement b the breakdown stress drop g the rigidity and v the rupture velocity these relations are consistent with ida s theoretical estimation based on the cohesive zone model the above formula gives good estimates for the maximum slip acceleration of actual earthquakes the cutoff frequency max of the power spectral density of the slip acceleration increases with increasing normal stress in particular max is found to be directly proportional to the normal stress n within the normal stress range less than 17 mpa as max khz 4 0 n mpa n max increase with an increase in u max or u max all these results lead to the conclusion that u max u max and max increase with increasing normal stress this is consistent with a previous observation that b increases with increasing normal stress the above empirical linear relation between max and n can be explained by a linear dependence of b on n the size scale dependence of physical parameters is discussed and such size scale dependent parameters as u max and apparent fracture energy are scaled to the breakdown zone size regarded as a characteristic length to extend the results obtained in the laboratory to an earthquake failure in the earth

precursory surface deformation in great plate boundary earthquake sequences abstract we present an analysis of time dependent precursory source processes and associated ground surface deformation prior to the great earthquake rupture of a long seismic gap zone along a transform plate boundary this work is based on a theoretical model proposed by li and rice 1983 and reviewed briefly here in the model thickness averaged stress transmission in the lithosphere is analyzed by a generalized elsasser plane stress model which includes coupling to a viscoelastic asthenosphere upward progression of preseismic rupture at each section along strike is analyzed as quasi static extension in local antiplane strain of an elastic brittle crack this provides the necessary boundary condition in the context of the line spring procedure along strike for the elsasser plate model the approach to instability is simulated using what are thought to be typical material and tectonic parameters it is found that prior to a large earthquake the shear strain increases rapidly near the seismic gap zone but tends to diminish at moderate distances from the plate boundary such a strain reversal phenomenon although difficult to measure at present due to its small magnitude may be helpful in the interpretation of geodetic measurements and in the planning of geodetic network locations for the purpose of earthquake prediction the acceleration of slip with consequent acceleration of stress increase in the upper brittle crust predicted by the model may provide a plausible explanation for nonlinear features associated with seismicity preceding some major events see e g raleigh et al 1982 a precursor time t prec during which anomalous strain rate at the seismic gap exceeds twice its background level is found to be in the range of a couple of months to 5 yr depending on the tectonic loading rate t prec varies approximately as r 1 3 where r is a dimensionless loading rate parameter this precursor time may be associated with physical anomalities related to rapid stress increase such as ground tilting water table variation and radon emission which precede some large earthquakes consistent with observations the analysis predicts longer precursor times for larger earthquakes

fault model for preseismic deformation at parkfield california we construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate m 5 5 6 earthquake on the san andreas fault near parkfield california the quasi static model simulates fault slip fault shear stress and ground deformation for all stages of repeated earthquake cycles unstable slip which is the analog of a moderate parkfield mainshock is caused by overall failure of a postulated strong area or patch of the fault zone the brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at parkfield starting just after the 1966 mainshock in two ways in the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations moments and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock in the second way we assign values of most parameters from laboratory friction experiments and a temperature depth profile inferred from heat flow data overall agreement between model results and observations is comparable for the two methods in all simulations preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements however for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to detect

a mechanical model for deformation and earthquakes on strike slip faults a two dimensional model for stress accumulation and earthquake instability associated with strike slip faults is considered the model consists of an elastic lithosphere overlying a viscous asthenosphere and a fault of finite width with an upper brittle zone having an elastoplastic response and a lower ductile zone having an elastoviscoplastic response for the brittle or seismic zone the behavior of the fault material is assumed to be governed by a relation which involves strain hardening followed by a softening regime with strength increasing with depth for the fault material in the ductile or aseismic section the viscous effect is included through use of a nonlinear creep law and the strength is assumed to decrease with depth hence because of the lesser strength and the viscous effect continuous flow occurs at great depths causing stress accumulation at the upper portion of the fault and leading to failure at the bottom of the brittle zone the failure is initially due to localized strain softening but with further flow the material above the softened zone reaches its maximum strength and begins to soften this process accelerates and may result in an unstable upward rupture propagation

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

instability model for recurring large and great earthquakes in southern california the locked section of the san andreas fault in southern california has experienced a number of large and great earthquakes in the past and thus is expected to have more in the future to estimate the location time and slip of the next few earthquakes an earthquake instability model is formulated the model is similar to one recently developed for moderate earthquakes on the san andreas fault near parkfield california in both models unstable faulting the earthquake analog is caused by failure of all or part of a patch of brittle strain softening fault zone in the present model the patch extends downward from the ground surface to about 12 km depth and extends 500 km along strike from parkfield to the salton sea the variation of patch strength along strike is adjusted by trial until the computed sequence of instabilities matches the sequence of large and great earthquakes sincea d 1080 reported by sieh and others the last earthquake was them 8 3 ft tejon event in 1857 the resulting strength variation has five contiguous sections of alternately low and high strength from north to south the approximate locations of the sections are 1 parkfield to bitterwater valley 2 bitterwater valley to lake hughes 3 lake hughes to san bernardino 4 san bernardino to palm springs and 5 palm springs to the salton sea sections 1 3 and 5 have strengths between 53 and 88 bars sections 2 and 4 have strengths between 164 and 193 bars patch section ends and unstable rupture ends usually coincide although one or more adjacent patch sections may fail unstably at once the model predicts that the next sections of the fault to slip unstably will be 1 3 and 5 the order and dates depend on the assumed length of an earthquake rupture in about 1700

unloading induced rock fracture activation and maximum seismic moment prediction abstract hundreds of anthropogenic earthquakes have recently occurred worldwide due to underground space creation and energy extraction the mechanism behind the human induced geohazards is most likely associated with the reduction of normal stress on pre existing fractures and faults this study reports a series of laboratory experiments to investigate the mechanism of unloading induced fracture activation and proposes a simple approach to predict the maximum seismic moment for a critically stressed fracture the unloading driven shear test results exhibit that the unloading process induces the stress states of the sawcut and natural fractures to approach the mohr coulomb failure envelope and the normal stress unloading rate influences the peak slip rate the fracture instability is dependent on the relationship between the stiffness of the system and the slip weakening rate of the fracture and the shear dilation mainly occurs after the fracture activation the test results also show that the critical shear stress of the sawcut fracture during the unloading driven shear test is approximately equal to the residual shear strength after the displacement driven fracture slip this relationship inspires us to develop a new approach to estimate the maximum seismic moment our data demonstrate that the maximum seismic moments for both the fractures obtained from the unloading driven shear tests are all below the upper limit lines indicating that the proposed approach is reasonable the uncertainty analysis shows that the accurate estimation of fault size can improve the maximum seismic moment prediction

nucleation and triggering of earthquake slip effect of periodic stresses abstract results of stability analyses for spring and slider systems with state variable constitutive properties are applied to slip on embedded fault patches unstable slip may nucleate only if the slipping patch exceeds some minimum size subsequent to the onset of instability the earthquake slip may propagate well beyond the patch it is proposed that the seismicity of a volume of the earth s crust is determined by the distribution of initial conditions on the population of fault patches that nucleate earthquake slip and the loading history acting upon the volume patches with constitutive properties inferred from laboratory experiments are characterized by an interval of self driven accelerating slip prior to instability if initial stress exceeds a minimum threshold this delayed instability of the patches provides an explanation for the occurrence of aftershocks and foreshocks including decay of earthquake rates by time 1 a population of patches subjected to loading with a periodic component results in periodic variation of the rate of occurrence of instabilities the change of the rate of seismicity for a sinusoidal load is proportional to the amplitude of the periodic stress component and inversely proportional to both the normal stress acting on the fault patches and the constitutive parameter a 1 that controls the direct velocity dependence of fault slip values of a 1 representative of laboratory experiments indicate that in a homogeneous crust correlation of earthquake rates with earth tides should not be detectable at normal stresses in excess of about 8 mpa correlation of earthquakes with tides at higher normal stresses can be explained if there exist inhomogeneities that locally amplify the magnitude of the tidal stresses such amplification might occur near magma chambers or other soft inclusions in the crust and possibly near the ends of creeping fault segments if the creep or afterslip rates vary in response to tides observations of seismicity rate variations associated with seasonal fluctuations of reservoir levels appear to be consistent with the model

the origin of large or great thrust type earthquakes along subducting plate boundaries abstract seismicity deformation state of stress and abundance of fluids along subducting plate boundaries are reviewed and the origin of large or great thrust type earthquakes is discussed based on the recent experimental results on the slip behavior of halite and serpentine gouges shallow subducting plate boundaries above 20 25 km in depth are characterized by low seismicity low tectonic stress inter plate decoupling ductile deformation associated with the formation of metamorphic schistosity except at very shallow depths metamorphism suggesting solution processes on massive scale and presence of abundant h 2 o it is argued that these unique features are due to pressure solution processes to high fluid pressure to low strength and stable behavior of clayey sediments under wet environments and or to the deformation of soft unconsolidated sediments at very shallow depths the low seismicity in this zone is in marked contrast with major strike slip faults along which large earthquakes occur at depths shallower than 15 20 km it is emphasized that these unique features are expected only for restricted regions where there is constant supply of h 2 o due to progressive metamorphism or where fluids in the rocks are trapped and cannot escape to the surface large or great thrust type earthquakes in subduction zones initiate at depths of 30 50 km below the shallow decoupled zone in this focal depth range the supply of h 2 o during progressive metamorphism perhaps diminishes downwards the overriding and subducting plates are coupled and stick to each other during much of the inter seismic period and the resistance to slip or shear stress is presumably high it is suggested that these earthquakes begin to occur at a depth where the plate boundary zone becomes fairly dry deformation at these depths appears to be predominantly ductile so that the earthquakes cannot be regarded simply as a brittle phenomenon 1 creep instability i e instability associated with plastic deformation and 2 dehydration induced instability are the most likely mechanisms for initiating the earthquakes and both have some experimental support stick slip of halite gouge while undergoing ductile deformation primarily by intracrystalline gliding is described and discussed as a supporting evidence for 1 shear resistance of halite gouge increases with increasing confining pressure in stick slip regimes hence the observed stick slip may be a semi brittle phenomenon with respect to the pressure dependence of the shear resistance although the deformation texture cannot be distinguished from that formed by pressure insensitive flow serpentine gouge exhibits violent stick slip upon its decomposition under dry not wet environments supporting the mechanism 2 above exact mechanisms which lead to the unstable fault motion are poorly understood as yet but stick slip of both halite and serpentine gouges is recognized only when the slip rate dependence of friction is negative i e lower friction at faster slip rate consistent with the theoretical prediction of rice and ruina 1983 there is a possibility that the thrust type earthquakes can be explained essentially within the framework of fault constitutive laws developed by dieterich 1979 and ruina 1983

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

constitutive behavior and stability of frictional sliding of granite an understanding of the frictional sliding on faults that can lead to earthquakes requires a knowledge of both constitutive behavior of the sliding surfaces and its mechanical interaction with the loading system we have determined the constitutive parameters for frictional sliding of initially bare surfaces of westerly granite using a recently developed high pressure rotary shear apparatus that allows long distances of sliding and therefore a greater assurance of attaining steady state behavior from experiments conducted at room temperature and normal stresses of 27 84 mpa several important results have been found 1 a gouge layer 100 to 200 m thick was developed from the initially bare rock surfaces after 18 to 70 mm of sliding 2 the steady state frictional resistance attained after about 10 mm of sliding is proportional to the negative of the logarithm of the sliding velocity 3 abrup changes in the velocity of sliding result in initial changes in the frictional resistance which have the same sign as the velocity change and are followed by a gradual decay to a new steady state value over a characteristic distance of sliding this velocity weakening behavior is essentially identical with that found by several previous workers on the same material at lower normal stress 4 our results are well described by a two state variable constitutive law the values of the constitutive parameters are quite similar to those found previously at low normal stress but the characteristic distance is about an order of magnitude smaller than that found at 10 mpa normal stress with thicker layers of coarser gouge 5 we have approximated our results with a one state variable constitutive law and compared the results with the predictions of existing nonlinear stability analysis in addition we have extended the stability analysis to systems possessing two state variables with such formulations good agreement is found between the experimentally observed and theoretically predicted transitions between stable and unstable sliding these results allow a better understanding of the instabilities that lead to earthquakes

spacing of faults at the scale of the lithosphere and localization instability 2 application to the central indian basin 1 tectonic deformation in the central indian basin cib is organized at two spatial scales long wavelength 200 km undulations of the basement and regularly spaced faults the fault spacing of order 7 11 km is too short to be explained by lithospheric buckling we show that the localization instability derived by montesi and zuber 2003 provides an explanation for the fault spacing in the cib localization describes how deformation focuses on narrow zones analogous to faults the localization instability predicts that localized shear zones form a regular pattern with a characteristic spacing as they develop the theoretical fault spacing is proportional to the depth to which localization occurs it also depends on the strength profile and on the effective stress exponent ne which is a measure of localization efficiency in the brittle crust and upper mantle the fault spacing in the cib can be matched by ne 300 if the faults reach the depth of the brittle ductile transition bdt around 40 km or ne 100 if the faults do not penetrate below 10 km these values of ne are compatible with laboratory data on frictional velocity weakening many faults in the cib were formed during seafloor spreading the preexisting faults near target locations separated by the wavelength of the localization instability were preferentially reactivated during the current episode of compressive tectonics the long wavelength undulations may result from the interaction between buckling and localization index terms 8010 structural geology fractures and faults 8020 structural geology mechanics 8159 tectonophysics evolution of the earth rheology crust and lithosphere keywords faults central indian basin fault spacing folds strength envelope diffuse plate boundary

earthquake cycle simulations with rate and state friction and power law viscoelasticity abstract we simulate earthquake cycles with rate and state fault friction and off fault power law viscoelasticity for the classic 2d antiplane shear problem of a vertical strike slip plate boundary fault we investigate the interaction between fault slip and bulk viscous flow with experimentally based flow laws for quartz diorite and olivine for the crust and mantle respectively simulations using three linear geotherms dt dz 20 25 and 30 k km produce different deformation styles at depth ranging from significant interseismic fault creep to purely bulk viscous flow however they have almost identical earthquake recurrence interval nucleation depth and down dip coseismic slip limit despite these similarities variations in the predicted surface deformation might permit discrimination of the deformation mechanism using geodetic observations additionally in the 25 and 30 k km simulations the crust drags the mantle the 20 k km simulation also predicts this except within 10 km of the fault where the reverse occurs however basal tractions play a minor role in the overall force balance of the lithosphere at least for the flow laws used in our study therefore the depth integrated stress on the fault is balanced primarily by shear stress on vertical fault parallel planes because strain rates are higher directly below the fault than far from it stresses are also higher thus the upper crust far from the fault bears a substantial part of the tectonic load resulting in unrealistically high stresses in the real earth this might lead to distributed plastic deformation or formation of subparallel faults alternatively fault pore pressures in excess of hydrostatic and or weakening mechanisms such as grain size reduction and thermo mechanical coupling could lower the strength of the ductile fault root in the lower crust and concomitantly off fault upper crustal stresses

surface displacements and energy release rates for constant stress drop slip zones in joined elastic quarter spaces a three dimensional quasi static model of faulting in an elastic half space with a horizontal change of material properties i e joined elastic quarter spaces is considered a boundary element method is used with a stress drop slip zone approach so that the fault surface relative displacements as well as the free surface displacements are approximated in elements over their respective domains stress intensity factors and free surface displacements are calculated for a variety of cases to show the phenomenological behavior of faulting in such a medium these calculations showed that the behavior could be distinguished from a uniform half space slip in a stiffer material increases while slip in a softer material decreases the energy release rate and the free surface displacements also the 1989 kalapana earthquake was located on the basis of a series of forward searches using this method and leveling data the located depth is 8 km which is the closer to the seismically inferred depth than that determined from other models finally the energy release rate which can be used as a fracture criterion for fracture at this depth is calculated to be 11 1 106 j m 2

dynamic modelling of induced seismicity by using seismic efficiency constraints and a new scaling law for slip weakening distance in the numerical simulation of induced seismicity much attention is generally paid to the calibration of the frictional resistance of the causative fault to obtain a seismic moment consistent with that of the actual event whereas sufficient investigation is not made in the estimation of the slip weakening distance dc as well as in the calibration of seismically radiated energy the present study addresses this problem by numerically and analytically investigating the relation between dc and seismic source parameters first this study performs the dynamic simulation of an induced seismic event caused by a decrease in the effective normal stress the analysis demonstrated that seismic efficiency can be used to improve the accuracy of estimating the critical slip weakening distance and the coefficient of kinetic friction d whilst considering not only seismic moment but also radiated energy in the calibration this gave insight into the development of the new calibration method for induced seismicity that considers energy related seismic source parameters furthermore a new scaling law of the slip weakening distance was derived from the theoretical expression of seismic efficiency considering seismic moment mo and scaled energy hat e the proposed scaling law can yield the relation between dc and mo which is shown to be similar to that obtained from a previous study but additionally considers the relation between seismically radiated energy and dc the dependency of dc on seismically radiated energy implied from the proposed scaling law has been verified from the dynamic analyses where 0 06 was used to place a constraint on dc for seismic events with different magnitudes the developed numerical simulation methodology of induced seismicity as well as the scaling law considering the energy indices significantly contributes to improving the accuracy of back analysis thus leading to a more accurate estimation of the mechanical properties of faults and or shear zones in seismically active regions of deep underground mines or reservoirs composed of discontinuous hard rock masses

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

bulk friction modeling of afterslip and the modified omori law abstract afterslip data from the superstition hills fault in southern california a creep event on the same fault the modified omori law and cumulative moments from aftershocks of the 1957 aleutian islands earthquake all indicate that the original formulation by dieterich 1981 constitutive properties of faults with simulated gouge agu geophys monogr 24 103 120 for friction evolution is more appropriate for systems far from instability than the commonly used approximation developed by ruina 1983 slip instability and state variable friction laws j geophys res 88 10359 10370 to study instability the mathematical framework we use to test the friction models is a one dimensional massless spring slider under the simplifying assumption proposed by scholz 1990 the mechanics of earthquakes and faulting cambridge university press and used by marone et al 1991 on the mechanics of earthquake afterslip j geophys res 96 8441 8452 that the state variable takes on its velocity dependent steady state value throughout motion in response to a step in stress this assumption removes explicit state variable dependence from the model obviating the need to consider state variable evolution equations anti derivatives of the modified omori law fit our data very well and are very good approximate solutions to our model equations a plausible friction model with omori law solutions used by wesson 1988 dynamics of fault creep j geophys res 93 8929 8951 to model fault creep and generalized by rice 1983 constitutive relations for fault slip and earthquake instabilities pure appl geophys 121 443 475 to a rate and state variable friction model yields exactly omori s law with exponents greater than 1 but yields unstable solutions for omori exponents less than 1 we estimate from the dieterich formulation the dimensionless parameter a which is equal to the product of the nominal coefficient of friction and the more commonly reported friction parameter a we find that a is typically positive qualitatively consistent with laboratory observations although our observations are considerably larger than laboratory values however we also find good model fits for a when data correspond to omori exponents less than 1 a modification of the stability analysis by rice and ruina 1983 stability of steady frictional slipping j appl mech 50 343 349 indicates that a is not a consequence of our assumption regarding state variable evolution a consistent interpretation of a in terms of laboratory models appears to be that the data are from later portions of processes better characterized by two state variable friction models a is explained by assuming that our data cannot resolve the co seismic evolution of a short length scale state variable to a velocity weakening state our parameterization leads to an apparent negative instantaneous viscosity we estimate the largest critical slip distance associated with afterslip to be 1 10 cm consistent with other estimates for near surface materials we assume that our observed large values for a reflect the fact that our model ignores the geometrical complexities of three dimensional stresses in fractured crustal materials around a fault zone with frictional stresses that vary on a fault surface our one dimensional model parameters reflect spatially averaged bulk stress and frictional properties of a fault zone where we clearly cannot specify the details of the averaging process our analysis of omori s law suggests that bulk frictional properties of a fault zone are well described by our simple laboratory based models but they would need to change during the seismic cycle for a mainshock instability to recur unless a mainshock aftershock sequence were characterized by a process similar to the arrested instabilities possible in two state variable systems

scaling of ground motion parameters state of stress and focal depth the ground motion parametersrv and ra where r is hypocentral distance v is peak velocity is density and a is peak acceleration are found observationally to be strong functions of stress state and linearly related to focal depth z if the stress state and focal depth are properly taken into account then the ground motion parameters depend on the seismic moment m0 as expected from basic scaling principles that is rv m01 3 and ra is independent of earthquake size regression lines fitted to observations covering broad ranges in seismic moment and focal depth indicate that for extensional and compressional tectonic regimes corresponding to normal and thrust or reverse faulting earthquakes respectively peak acceleration is given by ra 1 08 mpa 3 06 mpa km z normal and ra 5 65 mpa 8 76 mpa km z thrust and for peak velocity rv m01 3 10 4 m2 s n m 1 3 3 00 0 69 km 1 z normal and rv m01 3 10 4 m2 s n m 1 3 4 63 1 82 km 1 z thrust for strike slip earthquakes the data currently are insufficient to define regression fits but such lines would definitely lie between those for the normal and thrust stress regimes these equations are appropriate for ground motion at small hypocentral distance as recorded in a whole space it follows that for comparable hypocentral depth peak acceleration in a compressional tectonic regime for example much of the eastern united states is a factor of about 3 greater than in an extensional regime such as nevada and for peak velocity a similar comparison yields a factor of 2 difference the similarity in behavior of the peak acceleration parameter to crustal strength estimated from byerlee s law of friction with respect to depth and stress state indicates that crustal strength is probably the factor governing the seismic source processes that give rise to the high frequency peak ground motion analysis of the peak acceleration data in the context of crustal strength suggests bounds on this ground motion parameter as recorded at a typical surface site that depend on stress state specifically a normal 0 5 g and a thrust 1 9 g thus the state of stress as well as the focal depth clearly is an important factor to be taken into account in the prediction of seismic ground motion

multi event simulations of slow slip events for a cascadia like subduction zone 1 we employ the earthquake simulator rsqsim which incorporates rate and state dependent friction to investigate characteristics of slow slip events sses along a cascadia like mega thrust the simulations consist of 100 000 sses with equivalent moment magnitudes mw4 0 7 0 the largest simulated sses mw6 4 7 0 have inter event times of 19 months durations of 10 40 days mean slips of 2 2 4 1 cm and along strike propagation speeds of 7 20 km day which are comparable to observations from cascadia the simulations show quiescence after mw 6 4 sses followed by a progressive increase in both the magnitude and frequency of sses prior to the next large event small sses below mw5 6 develop in an incoherent manner and have irregular geometries while larger sses show highly coherent growth of the slip region and spontaneous but transient event to event segmentation the change in event characteristics at mw5 6 corresponds to a break in the scaling of seismic moment with slip fault area and duration

constitutive relations for fault slip and earthquake instabilities constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults the constitutive relations discussed include simple rate independent slip weakening models in which shear strength degrades with ongoing slip to a residual frictional strength and also more realistic but as yet less extensively applied slip rate and surface state dependent relations for the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip seeking values consistent with the current slip rate models of crustal faults range from simple single degree of freedom spring slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere

probability of earthquake recurrence with nonuniform stress rates and time dependent failure current methods for calculation of long term probabilities for the recurrence of large earthquakes on specific fault segments are based upon models of the faulting process that implicitly assume constant stress rates during the interval separating earthquakes and instantaneous failure at a critical stress threshold however observations indicate that the process of stress recovery following an earthquake involves rate variations at all time scales in addition to stress steps caused by nearby earthquakes additionally the existence of foreshocks aftershocks and possible precursory processes suggest that there may be significant time dependence of the earthquake nucleation process a method for determining the conditional probabilities for earthquake occurrence under conditions of irregular stressing is developed that could be useful at all time scales including those pertinent to short and intermediate term prediction used with models for earthquake occurrence at a stress threshold the addition of variable stressing introduces a simple scaling of the conditional probabilities by stress level and stress rate a model for the time dependent nucleation of earthquake slip has been proposed recently that is based upon laboratory observations of fault strength this failure criterion results in large but relatively short duration changes in the probability of earthquake recurrence particularly following stress steps applied to populations of earthquakes the models predicts a 1 t decay of seismicity following stress steps as observed for aftershocks and for frequency of foreshock mainshock pairs the model suggests that variations of seismicity rates of small earthquakes in the nucleation zone of the expected earthquake directly indicate variations in probability of recurrence of the large earthquake

effects of temperature and sliding rate on frictional strength of granite layers of artificial granite gouge have been deformed on saw cut granite surfaces inclined 30 to the sample axes samples were deformed at a constant confining pressure of 250 mpa and temperatures of 22 to 845 c the velocity dependence of the steady state coefficient of friction ss was determined by comparing sliding strengths at different sliding rates the results of these measurements are consistent with those reported bysolberg andbyerlee 1984 at room temperature andstesky 1975 between 300 and 400 c stesky found that the slip rate dependence of ss increased above 400 c in the present study however the velocity dependence of ss was nearly independent of temperature

a physical model for interseismic erosion of locked fault asperities the conventional asperity model posits that faults are partitioned into fixed velocity weakening vw patches asperities that are locked interseismically and velocity strengthening vs regions that creep stably without accumulating stress however studies of gps derived deformation in northern japan have shown that interseismic strain in the tohoku region did not accumulate at a constant rate as expected but gradually decreased from 1996 to 2011 this change in strain rate is consistent with locked asperities shrinking by 75 in area during this period here we consider a modification to the conventional asperity model such that thermal pressurization tp is active over an area that encompasses a vw region and part of the surrounding vs region in our quasi dynamic simulations tp causes shear stress during rapid slip to decrease to very low levels during the interseismic period stress gradually recovers to steady state friction at the plate rate at which point stable creep initiates the creep front propagates inward effectively eroding the locked asperity for uniform properties the locked area shrinks roughly linearly in time through the vs region locked asperities shrink more slowly with higher nominal friction coefficient or background effective normal stress in the vs region lower hydraulic diffusivity and larger tp zones lateral heterogeneity in properties can give rise to nonlinear erosion predictions from this model can be compared against gps data to test whether the model can explain the observed changes in interseismic strain rate in tohoku

physical models of earthquake instability and precursory processes this paper selectively reviews physical models of earthquake instability in these models instability arises as a result of interaction of a fault constitutive relation with deformation of the surrounding material that occurs in response to remote tectonic loading in contrast to kinematic models in which the fault slip is imposed it is calculated in physical models and consequently these models are essential for understanding precursory processes some kind of weakening behavior for the fault constitutive relation is required to produce an instability analogous to an earthquake two commonly employed idealizations discussed here are rate independent slip weakening and rate state dependent friction when these constitutive models are employed on surfaces embedded in elastic half spaces or layers possibly coupled to a viscoelastic substrate the results are capable of simulating realistically some aspects of earthquake occurrence common to all models is the prediction that earthquake instability is preceded by precursory slip which produces a departure of surface strain rate from the background level near the epicenter of a moderate to large earthquake the magnitude of this departure appears to be well within the range of current geodetic measurement accuracy and its duration is of the order of months to years however details depend on a variety of factors including the modelling of the constitutive relation near peak stress coupling of elastic crust to the asthenosphere and coupling of deformation with pore fluid diffusion

slow slip slow earthquakes period two cycles full and partial ruptures and deterministic chaos in a single asperity fault abstract the assimilation of geological geophysical and laboratory data in physics based models of fault dynamics promises increasingly realistic simulations of the seismic cycle to assist this effort i explore the dynamics of a single velocity weakening asperity to delineate the relationship between a fault physical properties and the style of faulting during the seismic cycle based on a micro physical model of rate and state friction from a dimensional analysis four non dimensional parameters control the characteristics of slip events and the behavior during the interseismic period including the dieterich ruina rice number ru which is proportional to the ratio of asperity to critical nucleation size w h and rb b a b where a and b control the direct and local evolutionary effects respectively for sufficiently large rb the ru number controls the spectrum of fault slip from periodic slow slip events to aperiodic fast ruptures in finite faults but not in semi infinite faults the transition between these end members includes a bifurcation that involves period two period four period six cycles and deterministic chaos with increasing ru macroscopic fault slip is stabilized as rb vanishes even as ru 1 for finite faults but not for semi infinite faults the transition between slow and fast ruptures for increasing rb includes another type of bifurcation with period two and period four cycles of slow and fast ruptures for 0

a mechanical model of precursory source processes for some large earthquakes a mechanical model is presented of precursory source processes for some large earthquakes along plate boundaries it is assumed that the pre seismic period consists of the upward progression of a zone of slip from lower portions of the lithosphere towards the earth s surface the slip front is blocked by local asperities of different size and strength these asperities may be zones of real alteration of inherent strength or instead may be zones which are currently stronger due to a local slowdown of a basically rate dependent frictional response such blocking by a single large asperity or array of asperities produces quiescence over a segment of plate boundary until gradual increase of the stress concentration forces the slip zone through the blocked region at one end of the gap thus nucleating a seismic rupture that propagates upwards and towards the other end this model is proposed to explain certain distinctive seismicity patterns that have been observed before large earthquakes notably quiescence over the gap zone followed by clustering at its ends prior to the main event a discussion of mechanical factors influencing the process is presented and some introductory modelling performed with the use of a generalized elsasser model for lithospheric plates and the line spring model for part through flaws slip zones at plate boundaries is outlined briefly

observations models and mechanisms of failure of surface rocks surrounding planetary surface loads geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with increased distance from the center of the load of radial thrust faults to strike slip faults to concentric normal faults these model predictions are in conflict with the absence of annular zones of strike slip faults around prominent loads such as lunar maria martian volcanoes and the martian tharsis rise we suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the stress models indications that model stresses are inappropriate for use in fault type prediction include 1 tensile principal stresses larger than realistic values of rock tensile strength and or 2 stress differences significantly larger than those allowed by rock strength criteria predictions of surface faulting that are consistent with observations can be obtained instead by using tensile and shear failure criteria along with calculated stress differences and trajectories with model stress states not greatly in excess of the maximum allowed by rock fracture criteria

why blind thrust faults do not propagate to the earth s surface numerical modeling of coseismic deformation associated with thrust related anticlines high fault tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the earth s surface seismic profiles of blind thrust related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation such as flexural slip or extensional fracturing the complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems the influence of bedding plane slip on fault propagation is simulated with numerical models using the boundary element method we use two parameters to estimate the tendency for thrust fault propagation 1 the mode ii stress intensity factor and 2 the maximum coulomb stress near the fault tip calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the earth s surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation

modeling gravity variations caused by dilatancies abstract one postulated earthquake mechanism is the dilatancy model the dilatations due to deformation microfracture fracture and stress relaxation either accompanied by fluid invasion or not change the density the geometrical configuration in space and the porosity and the contents of the stressed volume of a tectonically active region observed gravity therefore is expected to respond to the sequence of events in the deformation cycle of dilatation in this paper a formulation of the gravity variations associated with the dilatation processes based on a combined dilatancy model is given the combined dilatancy model consists of the dilatancy diffusion the dilatancy instability and the fault zone dilatancy models for numerical calculations the total tectonically active region involved is represented by a vertical finite right circular cylinder within which the fault zone volume of the potential hypocentral region represented by a disk is embedded the cylinder has a radius of 40 km and thickness of 15 km with the bottom of the cylinder coinciding with the depth of a tectonically detached zone the disk is 2 km thick and 11 km in radius with the bottom of the disk at a depth of 15 km this representation is compatible with that of earthquakes of magnitude 4 5 that frequently occurred through the last decade or so in the beijing tianjin tangshan zhangjiakou bttz region in china the results of the calculations show that the variation forms of gravity changes are a function of space and time depending upon the size and shape of both the total tectonically active region and the fault zone volume therefore the calculation of theoretical gravity variations for a specific region comparable to that of the bttz region may be made on the basis of a thorough prior knowledge of the geological setting and the tectonic stress distribution of that region

effects of frictional heating on the thermal hydrologic and mechanical response of a fault the mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating in this study we formulate a fault model which incorporates the effects of frictional heating on the thermal hydrologic and mechanical response of a small patch of the failure surface this model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant the problem has two time scales a characteristic slip duration and a characteristic time for thermal pressurization the slip duration is set by the fault geometry the characteristic time for thermal pressurization is set by the slip rate the friction coefficient and the thermal and hydraulic characteristics of the medium the response of the fault depends on the relative magnitude of these two times results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response for earthquakes occurring across zones of low porous medium compressibility 10 9 pa 1 and permeability 10 18 m2 the characteristic time for thermal pressurization is small in this case frictional heating can cause fluid pressures to approach lithostatic values the shear strength to approach zero and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault moderate slip events where shear strains exceed two cause substantial strain weakening and consequently large stress drops accelerations and displacements thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones both the dynamic stress drop and total displacement decrease for zones with larger compressibility permeability or width if the compressibility or permeability exceeds 10 8 pa 1 or 10 14 m2 or the shear strain is less than one then the effects of frictional heating may be negligible and the fault will exhibit no strain weakening characteristics consequently the patch acts as barrier that halts or resists further fault motion extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface explaining many of the features of active faulting e g barriers nonuniform slip rupture stoppage random ground accelerations strong motions and frequency magnitude relations

imaging shear strength along subduction faults subduction faults accumulate stress during long periods of time and release this stress suddenly during earthquakes when it reaches a threshold this threshold the shear strength controls the occurrence and magnitude of earthquakes we consider a 3 d model to derive an analytical expression for how the shear strength depends on the fault geometry the convergence obliquity frictional properties and the stress field orientation we then use estimates of these different parameters in japan to infer the distribution of shear strength along a subduction fault we show that the 2011 mw9 0 tohoku earthquake ruptured a fault portion characterized by unusually small variations in static shear strength this observation is consistent with the hypothesis that large earthquakes preferentially rupture regions with relatively homogeneous shear strength with increasing constraints on the different parameters at play our approach could in the future help identify favorable locations for large earthquakes

characterisation of a fractured reservoir using microearthquakes induced by hydraulic injections abstract microseismic activity is induced by a number of engineering activities which involve fluid injection or extraction in naturally fractured rock masses evidence collected during the last 15 years of hot dry rock hdr geothermal energy research has clearly indicated the very strong positive correlation between induced microearthquake locations and fluid flow paths within the fractured rock mass microseismic monitoring is now used regularly in hdr research as the only effective means of mapping hydraulic stimulations and monitoring hydromechanical processes during operation of the fractured reservoir many of the techniques that have been developed through hdr research are directly applicable to the hydrocarbon industry for the monitoring of hydraulic fracturing treatments waterflood operations and reservoir depletion this paper provides a brief introduction to the hydromechanical interactions that result in induced microseismicity and presents case studies from three european hdr sites where the monitoring of microseismic activity has proved vital in targeting production wells mapping the progress of stimulation and waterfloods and understanding the hydromechanical behaviour of the fractured system

earthquakes and rock deformation in crustal fault zones the physical origin of earthquakes lies ultimately in the geological structure of fault zones and the deformation processes that occur therein in response to tectonic stress although the possibility now exists that the nucleation regions for large earthquakes at depths of 10 km may shortly become directly accessible by deep drilling our present knowledge of fault structure and the shallow earthquake source is derived largely from a variety of indirect sources these include seismological studies surface studies of fault zones and earthquake ruptures geodetic information on modes of fault slip geophysical constraints on fault zone structure and rheology and information garnered from materials science and experi mental rock deformation studies of fault zone structure and the rock products of faulting provide complementary information on deformation processes at depth in fault zones although descriptions of fault rocks are widespread in the geological literature e g spry 1969 higgins 1971 it is only in the past decade that they have begun to be interpreted in the context of the physical conditions and processes prevalent in seismically active fault zones at different crustal depths sibs on 1977 watts williams 1979 anderson et a11983 wise et ai 1984 such interpretations are still at an early stage but the deformation textures and structural associations of fault rocks have already been shown to have the potential to yield information on such diverse topics as shear stress levels power dissipation and seismic efficiency during earthquake faulting on fluid pressure levels and episodic fluid flow accompanying

dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters abstract mikumo t 1992 dynamic fault rupture and stress recovery processes in continental crust under depth dependent shear strength and frictional parameters in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 201 222 the dynamic rupture process of large earthquakes in continental crust and the post seismic stress recovery process due to visco plastic behaviour are investigated by three and two dimensional numerical modelling taking into account the possible depth variations of the shear strength tectonic stress expected stress drop and of the critical weakening slip which are suggested by recent laboratory experiments on frictional parameters it was found that dynamic rupture can initiate at any depth in a brittle zone if high stress is concentrated with a dimensionless stress ratio s less than a certain limit but that the rupture cannot nucleate in a semi brittle zone with negative stress drop this suggests that the boundary between the fully brittle and semi brittle zones delineates the base of the seismogenic zone of background seismicity and aftershock distribution however the dynamic rupture of large earthquakes originating from the brittle zone can propagate into the semi brittle zone after some temporary arrest or with decelerated velocities which could generate low frequency seismic waves the depth extent of dynamic faulting and the maximum fault displacement are affected by the depth to the brittle semi brittle boundary and hence by the geothermal state within the crust the stress recovery due to visco plastic behaviour in the semi brittle and plastic regimes suggests that the rupture for eventual earthquakes tends to be nucleated near the upper boundary of the semi brittle zone

modulation of fault strength during the seismic cycle by grain size evolution around contact junctions abstract earthquake prediction is fundamentally hampered by insufficient knowledge of the constitutive behavior of faults laboratory and theoretical work by dieterich 1978 and ruina 1983 formed the basis of rate and state friction a constitutive framework that captures the healing and weakening of frictional contacts the rate and state law successfully explains the frictional behavior of a wide range of rocks and many phenomena associated with fault slip the physics underlying the rate and state friction law is not fully understood but it is generally accepted that several micro mechanisms operate depending on the texture maturity fluid content and other physical and kinematic conditions of the fault here i formulate a constitutive framework that unifies a number of laboratory experiments on silicate rocks and the gouges derived from these rocks that describe the effect of grain size temperature real area of contact gouge thickness and fault roughness fundamental to the model is that fault strength is controlled by the area of the interfacial contact junctions that support the shear and normal loads the curvature of asperities weakly controls the area of their contact junction and the resulting fault strength as a result the dynamics of grain size evolution around micro asperities enables the seismic cycle the model quantitatively explains the correlation between the characteristic weakening distance and the gouge thickness marone and kilgore 1993 the correlation of the static friction coefficient with the rate dependence parameters ikari et al 2011 the dependence of frictional resistance to sub solidus temperatures chester 1994 and predicts a range of grain sizes that is compatible with observations at exposed fault zones however the dynamics of grain size evolution is still challenging to verify experimentally the model features a regularization at vanishing slip speeds that implies distinct dynamics at low strength for friction coefficients lower than 0 1 seismic cycle simulations predict the emergence of frictional instabilities with complex source time functions this is a significant departure from the predictions of rate and state friction that is most relevant to the dynamics of decollements serpentinized shear zones and other low strength faults

preseismic rupture progression and great earthquake instabilities at plate boundaries we present a procedure for modeling the initially quasi static upward progression of a zone of slip from some depth in the lithosphere toward the earth s surface along a transform plate margin culminating in a great crustal earthquake stress transmission in the lithosphere is analyzed with a generalized elsasser model in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a maxwellian viscoelastic asthenosphere upward progression of rupture over a finite length of plate boundary corresponding to a seismic gap along strike is analyzed by a method based on the line spring concept whereby a two dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness averaged stress and slip used as a boundary condition in the generalized elsasser plate model the formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike a simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip softening slope at the boundary falling below the elastic unloading stiffness of the surroundings the results also indicate a delay of ultimate seismic instability due to the stiffer short versus long time asthenospheric response and predict a final period of self driven creep toward instability the procedures for prediction of rupture progression and instability are illustrated in detail for an elastic brittle crack model of slip zone advance and parameters of the model are chosen consistently with great earthquake slips and stress drops for example an effective crack fracture energy of the order 4 106 j m2 at the peak 7 to 10 km below surface of a gaussian bell shaped distribution of fracture energy with depth with variance of the order 5 km simulating strength build up in a seismogenic layer leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike precursory surface straining in the self driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake this interval being greater for longer distances along strike over which the preseismic upward rupture progression takes place

fracture energy of stick slip events in a large scale biaxial experiment the concept of apparent fracture energy for the shear failure process is employed by many authors in modeling earthquake sources as dynamically extending shear cracks using records of shear strain and relative displacement from stick slip events generated along a simulated prepared fault surface in a large 1 5m x 1 5m x 0 4m granite block and a slip weakening model for the fault direct estimates of the apparent shear fracture energy of the stick slip events have been obtained for events generated on a finely ground fault surface apparent fracture energy ranges from 0 06 j m sup 2 at a normal stress of 1 1 mpa to 0 8 j m sup 2 at a normal stress of 4 6 mpa in contrast to estimates for tensile crack formation we find that the apparent fracture energy of stick slip events increases linearly with normal stress the results for the slip weakening model for the stick slip events are generally consistent with constitutive fault models suggested by observations of stable sliding in smaller scale experiments

computational framework for simulating rock burst in shear and compression abstract this paper presents results from a series of numerical modeling tests conducted with the objective of developing a computational framework for studying rock burst events a commercially available distinct element code with its explicit time stepping scheme is adopted for modeling the initial quasi static and the subsequent dynamic response of rock burst in compression and shear ground reaction curves are introduced to discuss criteria for stability of a failure through stress analyses energy balance equations are analyzed for assessing the rock burst intensity by estimating radiated seismic energy once conditions of instability emerge two methods are discussed for estimating radiated seismic energy tracking of the energy conversion during failure recording the kinetic energy that is damped by mechanical damping schemes modeling approach and methodologies for studying compressive type rock burst are developed and calibrated against available analytical solutions with respect to radiated seismic energy and excavation convergence factors participating in the intensity of rock burst in compression are examined by modeling a rectangular tabular excavation supported by a single pillar with ductile semi brittle and brittle failure responses under different compressive loading a shear type rock burst along a strike slip fault model is simulated and modeling approach is calibrated by checking the resulting slip distribution radiated seismic energy and seismic moment against analytical solutions factors contributing to the occurrence and intensity of shear type rock burst are explored by simulating different slip scenarios under different loading conditions with presented analyses this paper provides a computational framework calibrated for studying rock burst events and as a reference for treatment of more complex cases shows how young s modulus of the rock failure stress drop rock brittleness and slip weakening behaviors of faults govern the rock burst occurrence and intensity

experimental and modeling study of the effect of fault roughness on dynamic frictional sliding abstract friction governs many processes in nature from the motion of glaciers and landslides to tremors and earthquakes the roughness of the sliding interfaces presumably affects their dynamics in particular the condition for stable or unstable sliding we investigate this issue by modeling numerically tri axial stick slip experiments on westerly granite samples with different levels of roughness the model accounts for surface roughness at the millimeter scale and assumes rate and state friction with enhanced dynamic weakening it reproduces key characteristics observed in the experiments such as sequences of fast stick slip events for smooth surface and sequence of slow slip events with small stress drop followed by a large stress drop unstable slip event for the rough interface the increase in loading stiffness due to the additional resistance from roughness can explain the initial slow slip the fast unstable event may be the result of the higher critical stiffness due to the increase of local normal tractions with slip as well as of the reduction of the loading stiffness due to contacts losses the study allows examining numerically the relationship between the frictional behavior on the interface and the apparent friction estimated from external loads we find that because of dynamic overshoot the reduction in apparent friction is larger than that of the actual friction coefficient on the interface

the effects of entrained debris on the basal sliding stability of a glacier 1 new laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick slip motion of debris laden ice over rock with implications for glacier behavior friction between a rock substrate and clasts in ice generates heat which melts nearby ice to produce lubricating water an increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase low initial lubrication can result from cold or drained conditions whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads strengthening of the interface healing during stick intervals between slip events occurs primarily through meltwater refreezing when healing and unstable sliding are taken together the experiments reported here suggest that stick slip behavior is common from motion of debris laden glacier ice over bedrock

a benthic foraminiferal proxy of pulsed organic matter paleofluxes in the temperate open ocean of the modern northeast atlantic the spring bloom of phytoplankton leads to a seasonal pulse of detrital organic material phytodetritus to the ocean floor opportunistic benthic foraminifera rapidly colonise this food resource producing large numbers of individuals whose tests are ultimately added to the sediment one of these taxa epistominella exigua shows periodic peaks in abundance in the fossil record at many open ocean sites previously such peaks have been commonly interpreted to result from changes in physicochemical properties of bottom water mass and thus deep sea circulation the main purpose of this paper is to propose thate exigua may be used as a proxy of pulsed organic matter inputs to the deep ocean and therefore as an indicator of relative changes in productivity

injection driven fracture instability in granite mechanism and implications abstract an unprecedented increase in earthquakes associated with hydraulic stimulation has been recognized as a major barrier to the development of enhanced geothermal systems however the mechanism of injection induced seismicity remains enigmatic here we simulate the critical stress states of sawcut and natural fractures and examine the fracture instabilities due to fluid injection we shed light on fluid pressure heterogeneity injection induced fracture instability acoustic emission characteristics and fracture surface evolution we find that injection induced fracture instability is strongly dependent on fluid pressure heterogeneity and fracture surface heterogeneity high injection rates can induce a high fluid overpressure ratio in the sawcut fracture promoting the rupture of fracture extending into the unpressurized area the instability of the natural fracture is due to the near uniform reduction in effective normal stress associated with a low fluid overpressure ratio our data indicate that maintaining a low fluid overpressure ratio during injection activities could confine the rupture of fracture within the pressurized area a turning point of the total moment release may be used as a reference to optimize the fluid injection strategy the seismic response of rock fracture could be modified by changing fluid pathways between the injection borehole and the fracture and by evolving surface roughness of the fracture within the stimulated volume

earthquake nucleation on dip slip faults 1 the nucleation of unstable slip on a fault is of key importance in our understanding of the seismic cycle we investigate how the asymmetric geometry of dip slip faults affects the nucleation of unstable slip on such faults previous researchers have devoted much effort to understanding this nucleation process on geometrically simple faults using a variety of frictional parameterizations however there are many reasons to believe that earthquake nucleation may be affected by fault complexity the breakdown of symmetry on a nonvertical dip slip fault means that normal stress is not constant during the slip process and that the hanging wall and footwall may not necessarily move equally using a slip strengthening and weakening friction law in a two dimensional quasi static model based on a variational boundary integral method we show that nucleation on dip slip faults is affected by both the dip angle and the direction of slip normal versus thrust reverse under otherwise identical conditions in a homogeneous half space i e neglecting depth dependent frictional or material properties thrust faults nucleate closer to the earth s surface than normal faults and take less time to do so these differences decrease as the dip angle of the fault increases to 90 the amount of preseismic surface slip is a much more complicated function of dip angle the results show that fault geometry may have an important role in the nucleation process as well as the process of dynamic rupture further research is needed to combine rigorous nucleation models with full rupture dynamics

seismic estimates of stress release five seismic estimates of stress release are compared for eight multiply recorded aftershocks of the 1975 oroville california earthquake the dynamic stress drop the arms stress drop and the apparent stress provide stable estimates which are strongly correlated across the data set the uncertainties of these estimates the standard error of the mean divided by the mean are approximately 15 estimates of the brune stress drop are uncorrelated with estimates of the average static stress drop calculated from the seismic moments and the rupture geometries of the aftershocks determined from the signal durations of the recorded s waves the brune stress drops are most strongly correlated with the arms stress drops implying that the brune stress drop provides an estimate of the average dynamic stress drop as brune originally proposed

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

forecast model for great earthquakes at the nankai trough subduction zone an earthquake instability model is formulated for recurring great earthquakes at the nankai trough subduction zone in southwest japan the model is quasistatic two dimensional and has a displacement and velocity dependent constitutive law applied at the fault plane a constant rate of fault slip at depth represents forcing due to relative motion of the philippine sea and eurasian plates the model simulates fault slip and stress for all parts of repeated earthquake cycles including post inter pre and coseismic stages calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the m 8 1 1946 nankaido earthquake in model simulations accelerating fault slip has two time scales the first time scale is several years long and is interpreted as an intermediate term precursor the second time scale is a few days long and is interpreted as a short term precursor accelerating fault slip on both time scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace

contemporary plate motion and crustal deformation the measurement of relative plate velocities during the past few years is a signal accomplishment in earth science leading to refinement of the precepts of steady motion of plate interiors and cyclic deformation along plate margins regrettably deformation premonitory to an earthquake has yet to be detected with confidence delineation of the spatial and temporal buildup of strain between earthquakes however has put limits on models of the earthquake cycle a diverse set of fault structures and crustal rheologic conditions can explain the pattern of surface strain accumulation and release along strike slip faults in contrast the geometry of thrusts and normal faults revealed by earthquake deformation has been found to differ markedly from expectations geodetic observations have proved vital to monitor the ascent of magma through the earth s crust and to predict volcanic eruptions at the earth s surface episodic vertical and horizontal deformation in southern california remains a subject of dispute if anything it is less certain than it once seemed

earthquake aftereffects and triggered seismic phenomena the physical basis for delayed aftereffects of large crustal earthquakes is analyzed these effects apparently range in scale from typical aftershocks near the source region of a prior earthquake to the triggering of groundwater and deformation related anomalies possibly including other large earthquakes at locations as much as a few hundred kilometers from the source of a great earthquake observational reports of such aftereffects are reviewed and possible mechanisms for their occurrence are discussed

symmetry and the critical slip distance in rate and state friction laws 1 we performed laboratory experiments to investigate the processes responsible for rate and state friction rsf behavior in fault rocks we focused on the symmetry of the frictional constitutive response to velocity changes and the mechanics of the critical friction slip distance dc experiments were conducted in double direct shear at 1 and 25 mpa normal stress at room temperature and for shearing velocity from 1 to 300 m s we studied three granular materials and bare surfaces of westerly granite ruina s law which predicts frictional symmetry between velocity increases and decreases better matches our data than dieterich s law which predicts that velocity decreases should evolve to steady state at a smaller displacement however for granular shear in some cases dc is smaller for velocity increases than for velocity decreases contrary to expectations from either law on bare granite surfaces the frictional response is symmetric for velocity increases decreases two distinct length scales for dc and two state variables are required for granular shear in some cases we hypothesize that asymmetry and two state behavior are caused by shear localization and changes in shear fabric in fault gouge our measurements show that during steady state frictional shear dilation after a velocity increase is smaller than compaction after a decrease normal stress oscillations cause a marked decrease in dc reduction of dc reduces frictional stability enhancing the possibility of seismic slip our experiments show that shear localization and fabric within the fault gouge can influence the rsf parameters that dictate earthquake nucleation and dynamic rupture

an experimental study of triggered stick slip stick slips have been studied in the laboratory on granite labrodorite and sandstone samples of two different sizes different roughness was achieved on the sawcut surfaces by finishing them with different grinding compounds ranging from grit 40 to grit 1000 stick slips occurred as a result of 1 slowly increasing the shear and normal stresses 2 superimposing a sinusoidal stress modulation 0 1 and 10 hz on the slowly increasing stresses 3 triggering by a stress impulse when the shear stress was well below the levels where stick slips occurred without the impulse and 4 foreshocks stick slips triggered by impulses or foreshocks occurred long after the beginning of the triggering events i e long in comparison with elastic wave travel times through the sample all triggered events were very rich in high frequencies corner frequency of 100 khz the untriggered stick slips did not contain much energy at the high frequencies corner frequency of 10 khz the dynamic friction coefficients for the triggered stick slips were smaller than for the untriggered events the long delay between the onset of the trigger and the stick slip and the high frequencies may be a consequence of corrosion of asperities the ultimate triggering and the rate of corrosion are likely related to the interplay of the normal and shear stresses as they load and unload the fault surface the consistent shape of the high frequency spectra is probably due to sample resonances which are excited rather than being characteristic of the details of the stick slips if these laboratory observations are directly applicable to earthquake seismology the spectra of earthquakes which were triggered by other earthquakes should be anomalously rich in high frequencies

a regional soil and sediment geochemical study in northern california abstract regional scale variations in soil geochemistry were investigated in a 20 000 km 2 study area in northern california that includes the western slope of the sierra nevada the southern sacramento valley and the northern coast ranges over 1300 archival soil samples collected from the late 1970s to 1980 in el dorado placer sutter sacramento yolo and solano counties were analyzed for 42 elements by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry following a near total dissolution these data were supplemented by analysis of more than 500 stream sediment samples from higher elevations in the sierra nevada from the same study site the relatively high density data 1 sample per 15 km 2 for much of the study area allows the delineation of regional geochemical patterns and the identification of processes that produced these patterns the geochemical results segregate broadly into distinct element groupings whose distribution reflects the interplay of geologic hydrologic geomorphic and anthropogenic factors one such group includes elements associated with mafic and ultramafic rocks including cr ni v co cu and mg using cr as an example elevated concentrations occur in soils overlying ultramafic rocks in the foothills of the sierra nevada median cr 160 mg kg as well as in the northern coast ranges low concentrations of these elements occur in soils located further upslope in the sierra nevada overlying tertiary volcanic metasedimentary and plutonic rocks granodiorite and diorite eastern sacramento valley soil samples defined as those located east of the sacramento river are lower in cr median cr 84 mg kg and are systematically lower in this suite compared to soils from the west side of the sacramento valley median cr 130 mg kg a second group of elements showing a coherent pattern including ca k sr and ree is derived from relatively silicic rocks types this group occurs at elevated concentrations in soils overlying volcanic and plutonic rocks at higher elevations in the sierras e g median la 28 mg kg and the east side of the sacramento valley median 20 mg kg compared to soils overlying ultramafic rocks in the sierra nevada foothills median 15 mg kg and the western sacramento valley median 14 mg kg the segregation of soil geochemistry into distinctive groupings across the sacramento river arises from the former presence of a natural levee now replaced by an artificial one along the banks of the river this levee has been a barrier to sediment transport sediment transport to the valley by glacial outwash from higher elevations in the sierra nevada and more recently debris from placer au mining has dominated sediment transport to the eastern valley high content of mafic elements and low content of silicic elements in surface soil in the west side of the valley is due to a combination of lack of silicic source rocks transport of ultramafic rock material from the coast ranges and input of sediment from the late mesozoic great valley group which is itself enriched in mafic elements a third group of elements zn cd as and cu reflect the impact of mining activity soil with elevated content of these elements occurs along the sacramento river in both levee and adjacent flood basin settings it is interpreted that transport of sediment down the sacramento river from massive sulfide mines in the klamath mountains to the north has caused this pattern the pb and to some extent zn distribution patterns are strongly impacted by anthropogenic inputs elevated pb content is localized in major cites and along major highways due to inputs from leaded gasoline zinc has a similar distribution pattern but the source is tire wear

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

large earthquakes and creeping faults faults are ubiquitous throughout the earth s crust the majority are silent for decades to centuries until they suddenly rupture and produce earthquakes with a focus on shallow continental active tectonic regions this paper reviews a subset of faults that have a different behavior these unusual faults slowly creep for long periods of time and produce many small earthquakes the presence of fault creep and the related microseismicity helps illuminate faults that might not otherwise be located in fine detail but there is also the question of how creeping faults contribute to seismic hazard it appears that well recorded creeping fault earthquakes of up to magnitude 6 6 that have occurred in shallow continental regions produce similar fault surface rupture areas and similar peak ground shaking as their locked fault counterparts of the same earthquake magnitude the behavior of much larger earthquakes on shallow creeping continental faults is less well known because there is a dearth of comprehensive observations computational simulations provide an opportunity to fill the gaps in our understanding particularly of the dynamic processes that occur during large earthquake rupture and arrest

grain size evolution and the rheology of ductile shear zones from laboratory experiments to postseismic creep abstract geologic experimental and geodetic studies indicate that ductile shear zones do not deform in steady state we present a theoretical model for an elastically loaded ductile shear zone deforming by combined dislocation and diffusion creep in which grain size is allowed to evolve toward its equilibrium recrystallized value several grain size evolution laws are considered simulations of laboratory experiments show that grain size evolution can lead to changes in the dominant deformation mechanism that are accompanied by transient strain weakening or strain strengthening episodes reminiscent of laboratory observations ductile shear zones can also produce postseismic creep as observed in geodetic data following large earthquakes non steady state ductile creep is difficult to differentiate from frictional sliding with velocity strengthening friction hence ductile creep may play a role in the transition from velocity weakening to velocity strengthening friction with increasing temperature

modeling of crustal layering using three dimensional analysis of stress drop slip zones in an elastic layer half space system abstract a numerical scheme for calculating the elastic fields in a layer over a half space medium due to embedded slip zones of prescribed stress drop was developed to examine how the layer influences fault mechanics although the method is general for simplicity only penny shaped slip zones were considered in the current analysis for slip zones that are embedded in the half space and near the interface it was found that the layer causes the slip to depart from that for a uniform medium softer layers tend to increase but stiffer layers tend to decrease the magnitude and gradient of the slip near the upper end of a slip zone consequently the layer causes an increase if the layer is softer or a decrease if the layer is stiffer in the energy release rate of the slip zone the layer was also seen to have a strong effect on the surface displacements to the extent that the distance change predicted between a pair of stations across the slip zone could be biased by a uniform half space model the results also showed that the layer can cause a significant change in the uniform normal stress which can alter the frictional resistance

role of drainage conditions in deformation and fracture of porous rocks under triaxial compression in the laboratory 1 in order to investigate the role of drainage conditions in deformation and fracture behaviors of porous rocks the authors carried out a series of rock fracture tests under triaxial compression in the laboratory the detailed space time distribution of acoustic emission due to microcracking was used to examine pre failure damage and failure behavior in berea sandstone which has a porosity of 20 and a permeability of 100 md the pore pressures or flow rates at the ends of the test sample were precisely controlled to simulate different drainage conditions experimental results indicate that drainage conditions play a governing role in deformation and fracture the well established dilatancy hardening effect can be greatly suppressed by dilatancy driven fluid flowing under good drainage conditions fast diffusion of pore pressure leads to a significant reduction in rock strength and stabilization of the dynamic rupture process furthermore good drainage conditions have the potential to enlarge the nucleation dimension and duration thereby improving the predictability of the final catastrophic failure in addition compaction bands which were observed in porous rocks under higher confining pressure were also observed at low confining pressure corresponding to a depth of 1 km in undrained tests these results are particularly important for research fields in which fluid migration or pore pressure diffusion is expected to play a role such as hydrocarbon reservoirs enhanced geothermal systems geological storage of co2

earthquake recurrence in simulated fault systems we employ a computationally efficient fault system earthquake simulator rsqsim to explore effects of earthquake nucleation and fault system geometry on earthquake occurrence the simulations incorporate rate and state dependent friction high resolution representations of fault systems and quasi dynamic rupture propagation faults are represented as continuous planar surfaces surfaces with a random fractal roughness and discontinuous fractally segmented faults simulated earthquake catalogs have up to 106 earthquakes that span a magnitude range from m4 5 to m8 the seismicity has strong temporal and spatial clustering in the form of foreshocks and aftershocks and occasional large earthquake pairs fault system geometry plays the primary role in establishing the characteristics of stress evolution that control earthquake recurrence statistics empirical density distributions of earthquake recurrence times at a specific point on a fault depend strongly on magnitude and take a variety of complex forms that change with position within the fault system because fault system geometry is an observable that greatly impacts recurrence statistics we propose using fault system earthquake simulators to define the empirical probability density distributions for use in regional assessments of earthquake probabilities

plastic instabilities implications for the origin of intermediate and deep focus earthquakes adiabatic or catastrophic plastic shear has been reported in metals polymers and metallic glasses the phenomenon is associated with rapid stress drops and audible pings or clicks as the material deforms in a plastic manner the driving force for the plastic instability is the stored elastic strain energy of the loading system and in many respects the behavior is reminiscent of the shear stress response arising from stick slip events during unstable frictional sliding although the precise mechanism is different it is shown here that adiabatic plastic shear is capable of explaining the detailed distribution of intermediate and deep focus earthquakes within subduction zones the earthquake events being the result of instabilities in material undergoing plastic flow it is argued that for a particular strain rate there exists a critical temperature tc which is depth dependent for temperatures below tc the material is strain rate softening and for a soft enough loading system may undergo catastrophic plastic shear for temperatures above tc the material is strain rate hardening and is always stable during plastic shear the cutoff depth for deep focus earthquakes then corresponds to the transition from strain rate softening to strain rate hardening material and for commonly accepted geothermal gradients within the slab corresponds to approximately 800 km the precise distribution of earthquakes within the slab is a function of the subtle interplay between the geothermal gradient and the tc gradient in particular a decrease in seismic activity is to be expected below about 300 km in the slab with total stress drops decreasing from a maximum of 700 mpa above 300 km to a maximum of 50 mpa below 300 km the differences in foci distribution between subduction zones such as tonga new hebrides and peru result from minor differences in the geothermal gradients within the slabs the model predicts the development of triple seismic zones high in the slab double seismic zones down to approximately 300 km and single seismic zones down to 800 km such a distribution is to be expected of relatively young cool slabs as the slab heats up the seismic activity retreats up the slab the paper only proposes a deformation mechanism for earthquake generation it does not address the stress field within the slab but only the distribution of strength thus the distribution of focal plane mechanisms is not considered only the locations where earthquakes due to plastic instabilities are possible the absence of earthquakes does not necessarily mean that the slab does not exist it only means that the slab is too hot to undergo plastic instability this means that aseismic subduction is a distinct possibility in many regions of high geothermal gradient within the slab i e circa 3 c km 1

the effect of shear load on frictional healing in simulated fault gouge we report on frictional strengthening healing in granular quartz gouge as a function of time of true stationary contact to distinguish between the slip dependent ruina 1983 and time dependent dieterich 1979 friction constitutive laws we designed tests similar to conventional slide hold slide shs tests except that shear load was completely removed prior to holds we find large healing values 0 033 0 054 for holds of 101 102 s compared with quasi static shs experiments and our data indicate time dependent weakening in contrast with strengthening observed from shs tests gouge layer compaction increases with increasing hold time comparable to observations from conventional shs tests our data indicate that purely time dependent processes have a minor influence on healing under the conditions studied and or that such effects are efficiently erased by particle rearrangement during removal reapplication of shear load the data are not adequately described by either the time dieterich or slip ruina dependent state evolution laws

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust