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

analysis of fault damage zones using three dimensional seismic coherence in the anadarko basin oklahoma fault damage zones may significantly affect subsurface fluid migration and the development of unconventional resources most analyses of fault damage zones are based on direct field observations and we expand these analyses to the subsurface by investigating the damage zone structure of an approximately 32 km 105 ft long right lateral strike slip fault in oklahoma we used the three dimensional 3 d seismic attribute of coherence to first define its regional and background levels and then we evaluated the damage zone dimensions at multiple sites we found damage zone thickness of approximately 1600 m 5300 ft at a segment that is dominated by subsidiary faults and it is slightly thicker at a segment with a pull apart basin the damage zone intensity decays exponentially with distance from the fault core in agreement with field observations and distribution of seismic events the coherence map displays a strong asymmetry of the damage zone between the two sides of the 3 d fault which is related to the subsidiary structures of the fault zone we discuss the effects of heterogeneous stress field on damage zone evolution through the detected subsidiary structures it appears that seismic coherence is an effective tool for subsurface characterization of fault damage zones

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

off fault damage and acoustic emission distributions during the evolution of structurally complex faults over series of stick slip events variations in fault structure for example surface roughness and deformation zone width influence the location and dynamics of large earthquakes as well as the distribution of small seismic events in nature changes in fault roughness and seismicity characteristics can rarely be studied simultaneously so that little is known about their interaction and evolution here we investigate the connection between fault structure and near fault distributions of seismic events over series of stick slip cycles in the laboratory we conducted a set of experiments on rough faults that developed from incipient fracture surfaces we monitored stress and seismic activity which occurred in the form of acoustic emissions aes we determined ae density distributions as a function of fault normal distance based on high accuracy hypocentre locations during subsequent interslip periods the characteristics of these distributions were closely connected to different structural units of the faults that is the fault core off fault and background damage zone the core deformation zone was characterized by consistently high seismic activity whereas the off fault damage zone displayed a power law decay of seismic activity with increasing distance from the fault core the exponents of the power law distributed off fault activity increased with successive stick slip events so that later interslip periods showed a more rapid spatial decay of seismic activity from the fault the increase in exponents was strongest during the first one to three interslip periods and reached approximately constant values thereafter the relatively rapid spatial decay of ae events during later interslip periods is likely an expression of decreasing fault zone complexity and roughness our results indicate a close relationship between fault structure stress and seismic off fault activity a more extensive mapping of seismic off fault activity decay has the potential to significantly advance the understanding of fault zone properties including variations in fault roughness and stress

roughness of fault surfaces over nine decades of length scales 1 we report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales from 50 m to 50 km we used three scanner devices lidar laser profilometer white light interferometer spanning complementary scale ranges from 50 m to 10 m to measure the 3 d topography of the same objects i e five exhumed slip surfaces vuache sillingy bolu corona heights dixie valley magnola a consistent geometrical property i e self affinity emerges as the morphology of the slip surfaces shows at first order a linear behavior on a log log plot where axes are fault roughness and spatial length scale covering five decades of length scales the observed fault roughness is scale dependent with an anisotropic self affine behavior described by four parameters two power law exponents h constant among all the faults studied but slightly anisotropic h 0 58 0 07 in the slip direction and h 0 81 0 04 perpendicular to it and two pre factors showing variability over the faults studied for larger scales between 200 m and 50 km we have analyzed the 2 d roughness of the surface rupture of thirteen major continental earthquakes these ruptures show geometrical properties consistent with the slip perpendicular behavior of the smaller scale measurements our analysis suggests that the inherent non alignment between the exposed traces and the along or normal slip direction results in sampling the slip perpendicular geometry although a data gap exists between the scanned fault scarps and rupture traces the measurements are consistent within the error bars with a single geometrical description i e consistent dimensionality over nine decades of length scales

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

competition between slow slip and damage on and off faults revealed in 4d synchrotron imaging experiments abstract in the continental crust faults may accommodate deformation through aseismic creep slow slip events or seismic slips that produce dynamic damage or a combination of these endmembers a variety of parameters controls the occurrence of these mechanical behaviors in a series of laboratory experiments we image centimeter scale faults during sliding under in situ conditions we perform four experiments of slip on centimeter scale crystalline rock samples prepared with a saw cut interface at 45 from the direction to the maximum compressive stress and at stress conditions of 2 3 km depth we image fault slip and off fault fracture development using 4d synchrotron x ray microtomography three faults have an initial rough interface and deformation occurs with increasing differential stress by a combination of slow slip events and off fault damage until catastrophic failure and the formation of new faults conversely the pre cut fault with a smooth initial surface deforms mainly by slow slip develops numerous striations along its slip plane and no microfractures are detected in the wall rock our experiments reproduce aseismic and seismic faulting behavior and demonstrate that the roughness of the fault plane is one of the parameters that control the transition between these two behaviors a fault with a rougher interface may tend to develop more off fault damage and seismic behavior for the rough fault experiments the secondary faulting occurs along a network of faults oriented at high angles from the pre existing saw cut plane a behavior similar to several earthquake sequences that occurred along orthogonal faults

the influence of fault geometry on small strike slip fault mechanics abstract meter scale subvertical strike slip fault traces in the central californian sierra nevada exhibit geometric complexities that significantly contribute to their mechanical behavior sections of faults that opened at depth channelized fluid flow as evidenced by hydrothermal mineral infillings and alteration haloes thin sections show a variation in the style of ductile deformation of infill along the fault with greater intensities of deformation along restraining bends orthorectified photomosaics of outcrops provide model geometries and parameter constraints used in a two dimensional displacement discontinuity model incorporating a complementarity algorithm model results show that fault shape influences the distribution of opening and consequently the spatial distribution of fluid conduits geometric irregularities are present at many scales and sections of opening occur along both releasing and restraining bends model sensitivity tests focus on boundary conditions along the fault frictional properties on closed sections and fluid pressure within sections of opening the influence of the remote stress state varies along a non planar fault complicating the relationships between remote stresses frictional properties slip and opening discontinuous sections of opening along model faults are similar in spatial distribution and aperture to the epidote infill assemblages observed in the field

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

evolution of slip surface roughness through shear a significant part of displacement in fault zones occurs along discrete shear surfaces the evolution of fault surface topography is studied here in direct shear laboratory experiments matching tensile fracture surfaces were sheared under imposed constant normal stress and sliding velocity the roughness evolution was analyzed using measurements of surface topography before and after slip we show that shearing reduces the initial surface roughness at all measurement scales at all wavelengths the roughness ratio between initial and final roughness increases as a function of the slip distance for a given test the roughness ratio increases with wavelength up to a few millimeters beyond which the ratio becomes wavelength independent at this region the roughness measured after slip follows a power law similar to that of the initial tensile fracture surface we interpret this geometrical evolution as a consequence of the deformation stage of interlocked asperities which is followed by shear induced dilation

identifying fault heterogeneity through mapping spatial anomalies in acoustic emission statistics 1 seismicity clusters within fault zones can be connected to the structure geometric complexity and size of asperities which perturb and intensify the stress field in their periphery to gain further insight into fault mechanical processes we study stick slip sequences in an analog laboratory setting analysis of small scale fracture processes expressed by acoustic emissions aes provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events the present work connects x ray computer tomography ct scans of faulted rock samples with spatial maps of b values slope of the frequency magnitude distribution seismic moments and event densities our current experimental setup facilitates the creation of a series of stick slips on one fault plane thus allowing us to document how individual stick slips can change the characteristics of ae event populations in connection to the evolution of the fault structure we found that geometric asperities identified in ct scan images were connected to regions of low b values increased event densities and moment release over multiple stick slip cycles our experiments underline several parallels between laboratory findings and studies of crustal seismicity for example that asperity regions in lab and field are connected to spatial b value anomalies these regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes

the equivalent strength of geometrical barriers to earthquakes we present a quantitative framework for evaluating the influence of non planar fault geometry on repeated seismic ruptures we model quasistatic ruptures on a non planar fault trace imbedded in a two dimensional elastic medium under in plane strain because of the presence of fault segments that are not parallel to the regional shear stress i e bends the apparent strength at a given point on the fault is not fixed but fluctuates with normal stress compressional features behave as increasingly strong barriers to fracture unless the stored normal stress is released in order to unlock the fault since slip on the fault itself cannot get rid of the normal stress this is achieved through the action of off fault morphological features such as secondary faulting folding and vertical motions that we introduce parametrically in the form of an aseismic relaxation the apparent strength of a fault bend will stabilize in a narrow interval of values after repeated ruptures characterized by a non dimensional hardness parameter whereby the relaxation rate is scaled by the tectonic loading rate on a fault structure having several small widely separated bends three families of events can be identified whose frequency and magnitude depend on the hardness relaxation parameter and the geometry small events that cluster in the tension zones of the bends intermediate size ruptures involving a single interbend segment and large ruptures that break through bends and link on or more interbend segments large multi segment events are most likely to occur for low values of the hardness i e fast relaxation and slow loading rate regions with compressional features act as barriers that stop most ruptures stress is stored at these sites until they themselves break and initiate motion on the smoother long reaches of the fault

steady activity of microfractures on geological faults loaded by mining stress abstract acoustic emissions ae down to m w 4 were recorded at a site 1 km beneath the surface in the cooke 4 mine south africa several planar ae clusters with lateral extent of 10 100 m were identified most of them were located several tens of meters away from the mining front and exhibited steady activity during the analysis period of about two months some of the clusters coincided with mapped faults the planar cluster aes were sharply aggregated within a thickness of several decimeters likely delineating the fracture interface of the fault and its higher order morphology such as branches bends and stepovers the composite focal mechanism evaluated for each cluster was consistent with slip events on the fracture interface these results imply that numerous shear microfractures occur steadily on a natural fault surface subjected to a mining related stress increase the planar clusters consist of very small aes 99 7 were smaller than m w 2 exhibiting high b values much exceeding unity this contrasts with the more usual b values of the stope cluster aes which were aggregated within 20 m of the mining front and exhibited a more scattered distribution the size distribution of microfractures on a fracture interface may directly reflect fine scale irregularities of the interface on the other hand many other mapped faults near the planar ae clusters were not accompanied by ae activities despite the fact that these quiet faults were subjected to a similar stress history the presence or absence of ae activities on a fault may reflect different states of the fault including stress and strength

slickenline orientations as a record of fault rock rheology abstract contact of geometric asperities across rough faults causes perturbations to the shear traction resolved on a fault surface that could potentially deflect the local slip direction slickenlines which record the relative displacement of opposing sides of a fault may therefore be sensitive to fault surface geometry to investigate the relationship between fault geometry shear traction and slip we use ground based lidar to measure the orientations of slickenlines defined by centimeter amplitude corrugations on three fault surfaces slickenline rakes measured in the mean plane of each fault rarely vary by more than a few degrees deviations from the mean rake do not correlate with the fault surface topography at scales up to a few meters but a weak correlation may be present at larger scales slip directions are therefore insensitive to any shear traction perturbations from the contact of geometric asperities at small scales this observation is consistent with scale dependent deformation we show that the roughness of fault surfaces implies that short wavelength asperities fail inelastically because of flattening during fault slip a crossover to elastic deformation occurs at lengths of 10 2 to 10 0 m which defines the minimum dimension of a strength asperity that influences the displacement field the roughness at the crossover length scale corresponds to the typical thickness of fault rocks in the fault core suggesting that fault rock thickness is related to the crossover length scale the data requires that multiple processes combine to produce the fault surface geometry we reconcile the variety of processes with the consistent surface roughness scaling by noting that all of the processes are governed in different ways by the elastic limit of rock

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

rupture dynamics with energy loss outside the slip zone 1 energy loss in a fault damage zone outside the slip zone contributes to the fracture energy that determines rupture velocity of an earthquake a nonelastic two dimensional dynamic calculation is done in which the slip zone is modeled as a fault plane and material off the fault is subject to a coulomb yield condition in a mode 2 crack like solution in which an abrupt uniform drop of shear traction on the fault spreads from a point coulomb yielding occurs on the extensional side of the fault plastic strain is distributed with uniform magnitude along the fault and it has a thickness normal to the fault proportional to propagation distance energy loss off the fault is also proportional to propagation distance and it can become much larger than energy loss on the fault specified by the fault constitutive relation the slip velocity function could be produced in an equivalent elastic problem by a slip weakening friction law with breakdown slip dc increasing with distance fracture energy g and equivalent dc will be different in ruptures with different initiation points and stress drops so they are not constitutive properties they are determined by the dynamic solution that arrives at a particular point peak slip velocity is however a property of a fault location nonelastic response can be mimicked by imposing a limit on slip velocity on a fault in an elastic medium

crack to pulse transition and magnitude statistics during earthquake cycles on a self similar rough fault abstract faults in nature demonstrate fluctuations from planarity at most length scales that are relevant for earthquake dynamics these fluctuations may influence all stages of the seismic cycle earthquake nucleation propagation arrest and inter seismic behavior here i show quasi dynamic plane strain simulations of earthquake cycles on a self similar and finite 10 km long rough fault with amplitude to wavelength ratio 0 01 the minimum roughness wavelength m i n and nucleation length scales are well resolved and much smaller than the fault length stress relaxation and fault loading is implemented using a variation of the backslip approach which allows for efficient simulations of multiple cycles without stresses becoming unrealistically large i explore varying m i n for the same stochastically generated realization of a rough fractal fault decreasing m i n causes the minimum and maximum earthquakes sizes to decrease thus the fault seismicity is characterized by smaller and more numerous earthquakes on the other hand increasing the m i n results in fewer and larger events however in all cases the inferred b value is constant and the same as for a reference no roughness simulation 0 i identify a new mechanism for generating pulse like ruptures seismic events are initially crack like but at a critical length scale they continue to propagate as pulses locking in an approximately fixed amount of slip i investigate this transition using simple arguments and derive a characteristic pulse length l c m i n 4 4 2 and slip distance c based on roughness drag i hypothesize that the ratio m i n 2 can be roughly estimated from kinematic rupture models furthermore i suggest that when the fault size is much larger than l c then most space time characteristics of slip differ between a rough fault and a corresponding planar fault

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

slip localization on the southern alpine fault new zealand 1 results of a detailed field study of the southern onshore portion of new zealand s alpine fault reveal that for 75 km along strike dextral normal slip on this long lived structure is highly localized in phyllosilicate rich fault core gouges and along their contact with more competent rocks at three localities martyr river mckenzie creek and hokuri creek we document complete cross sections through the fault new 40ar 39ar dates on mylonites combined with microstructural and mechanical data on phyllosilicate rich fault core gouges show that modern slip is localized onto a single steeply dipping 1 to 12 m thick fault core composed of impermeable k 10 20 to 10 22 m2 frictionally weak s 0 12 0 37 velocity strengthening illite chlorite and saponite chlorite lizardite fault gouges fault core materials are 1 comparable to those of other major weak cored faults e g san andreas fault and 2 most compatible with fault creep despite paleoseismic evidence of quasiperiodic large magnitude earthquakes mw 7 on this portion of the alpine fault we conclude that frictional properties of gouges at the surface do not characterize the overall seismogenic behavior of the southern alpine fault

constraints from fault roughness on the scale dependent strength of rocks principal slip surfaces in faults have measurable roughness generated during slip the roughness both records previous events and poses the boundary conditions for future rupture digital high precision roughness data are now available at the field scale tens of centimeters to tens of meters for at least 22 faults and at the laboratory scale millimeters to tens of centimeters for a subset of these we quantify the slip surface roughness by measuring the aspect ratio which is the average asperity height divided by the profile length higher aspect ratios indicate rougher surfaces from the field studies two major trends have emerged 1 fault surface roughness lies in a restricted range with aspect ratios in the slip parallel direction of 0 07 0 5 for profiles of 1 m length and 2 fault surfaces are rougher at small scales than large ones these features can both be interpreted as fingerprints of scale dependent strength which sets a limit to the aspect ratio of the surface the measurements imply that shear strength scales with the observation scale l as l 0 4 the new understanding of the physical controls on roughness allows generalization of the extant measurements of a wide array of faults

the slip behavior and source parameters for spontaneous slip events on rough faults subjected to slow tectonic loading we study the response to slow tectonic loading of rough faults governed by velocity weakening rate and state friction using a 2 d plane strain model our numerical approach accounts for all stages in the seismic cycle and in each simulation we model a sequence of two earthquakes or more we focus on the global behavior of the faults and find that as the roughness amplitude br increases and the minimum wavelength of roughness decreases there is a transition from seismic slip to aseismic slip in which the load on the fault is released by more slip events but with lower slip rate lower seismic moment per unit length m0 1d and lower average static stress drop on the fault t even larger decreases with roughness are observed when these source parameters are estimated only for the dynamic stage of the rupture for br 0 002 the source parameters m0 1d and t decrease mutually and the relationship between t and the average fault strain is similar to that of a smooth fault for faults with larger values of br that are completely ruptured during the slip events the average fault strain generally decreases more rapidly with roughness than t

a source model for induced earthquakes at the geysers geothermal reservoir the results of a previous study on source mechanisms of small earthquakes at the geysers geothermal reservoir in northern california are used to investigate an extended crack model for seismic events the seismic events are characterized by their first degree moment tensors and interpreted in terms of a model that is a combination of a shear crack and wing cracks solutions to both forward and inverse problems are obtained that can be used with either dynamic or static moment tensors the model contains failure criteria explains isotropic parts that are commonly observed in induced earthquakes and produces estimates of crack dimensions and maximum amount of slip effects of fluid pressure are easily incorporated into the model as an effective stress the model is applied to static moment tensors of 20 earthquakes that occurred during a controlled injection project in the northwest geysers for earthquakes in the moment magnitude range of 0 9 2 8 the model predicts shear crack radii in the range of 10 150 m wing crack lengths in the range of 2 25 m and maximum slips in the range of 0 3 1 1 cm only limited results are obtained for the time dependence of the earthquake process but the model is consistent with corner frequencies of the isotropic part of the moment tensor being greater than the deviatoric part and waveforms of direct p waves that become more emergent for larger events

earthquake source properties from pseudotachylite abstract earthquake radiated motions contain information that can be interpreted as source displacement and therefore related to stress drop except in a few notable cases these displacements cannot be easily related to the absolute stress level or the fault strength or attributed to a particular physical mechanism in contrast paleoearthquakes recorded by exhumed pseudotachylite have a known dynamic mechanism whose properties constrain the coseismic fault strength pseudotachylite can be used to directly address a discrepancy between seismologically measured stress drops which are typically a few mpa and much larger dynamic stress drops expected from thermal weakening during slip at seismic speeds in crystalline rock mckenzie and brune 1972 sibson 1973 lachenbruch 1980 mase and smith 1987 rice 2006 and as have been observed in laboratory experiments at high slip rates di toro hirose nielsen pennacchioni et al 2006 this places pseudotachylite derived estimates of fault strength and inferred crustal stress within the context and bounds of naturally observed earthquake source parameters apparent stress stress drop and overshoot including consideration of fault surface roughness off fault damage fracture energy and the strength excess the analysis which assumes stress drop is related to corner frequency as in the madariaga 1976 source model is restricted to earthquakes of the gole larghe fault zone in the italian alps where the dynamic shear strength is well constrained by field and laboratory measurements we find that radiated energy is similar to or exceeds the shear generated heat and that the maximum strength excess is 16 mpa these events have inferred earthquake source parameters that are rare for instance a low percentage of the global earthquake population has stress drops as large unless fracture energy is routinely greater than in existing models pseudotachylite is not representative of the shear strength during the earthquake that generated it or the strength excess is larger than we have allowed

revisiting the 1992 landers earthquake a bayesian exploration of co seismic slip and off fault damage existing models for the distribution of subsurface fault slip associated with the 1992 landers ca earthquake m w 7 3 show significant dissimilarities in particular they exhibit different amounts of slip at shallow depths 5 km these discrepancies can be primarily attributed to the ill posed nature of the slip inversion problem and to the use of physically unjustifiable smoothing or regularization constraints in this study we propose a new coseismic model obtained from the joint inversion of multiple observations in a relatively unregularized and fully bayesian framework we use a comprehensive data set including gps terrestrial geodesy multiple sar interferograms and co seismic offsets from correlation of aerial images these observations provide dense coverage of both near and far field deformation to limit the impact of modelling uncertainties we develop a 3 d fault geometry designed from field observations co seismic offsets and the distribution of aftershocks in addition we account for uncertainty in the assumed elastic structure used to compute the green s functions our solution includes the ensemble of all plausible models that are consistent with our prior information and fit the available observations within data and prediction uncertainties using near fault high resolution ground deformation measurements and the density of aftershocks we investigate the properties of the damage zone and its impact on the inferred slip at depth we attribute a part of the inferred slip deficit at shallow depth to our models not including the impact of a damage zone associated with a reduction of shear modulus in the vicinity of the fault

stress near geometrically complex strike slip faults application to the san andreas fault at cajon pass southern california slip on an undulatory strike slip fault induces predictable residual stresses in the adjacent crust elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries across fault parallel planes domains of reverse normal right lateral and left lateral residual stresses are induced near the bends in the fault these residual stresses increase in magnitude from one slip event to the next typically reaching failure level after several events a two dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip induced stresses and fourier synthesis allows for the solution with arbitrary but small fault distortions maximum fault normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault reduced to 2 e times the maximum value at a distance equal to 1 wavenumber this fault parallel residual shear stress oscillates between dextral and sinistral along the fault with maximum magnitude adjacent to the maximum excursions in the fault trace fault parallel residual shear stress is maximum at a distance from the fault of 1 wavenumber where its magnitude is 1 e times the maximum normal stress on the fault two and three dimensional finite element analyses extend the analytic model they account for depth decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations application of the model to the san andreas fault in the cajon pass region produces the complex distribution of fault parallel normal reverse right lateral and left lateral structures recognized near the main trace left lateral stresses on planes subparallel to the san andreas fault at the cajon pass well are predicted by this model

fault slip rates and interseismic deformation in the western transverse ranges california to better constrain fault slip rates and patterns of interseismic deformation in the western transverse ranges of southern california we present results from analysis of gps and interferometric synthetic aperture radar insar data and three dimensional mechanical and kinematic models of active faulting anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous gps site locations gps measures contraction rates across the ventura basin of 7 mm yr oriented west northwest with rates decreasing to the west and east the santa barbara channel is accommodating 6 5 mm yr in the east and 2 5 mm yr in the western portions of n s contraction inversion of horizontal gps velocities highlights a zone of localized fast contraction rates following the ventura basin using a mechanical model driven by geodetically calculated strain rates we show that there are no significant discrepancies between short term slip rates captured by geodesy and longer term slip rates measured by geology mechanical models reproduce the first order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the ventura basin due to the inadequate homogeneous elastic properties of the model existing two dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the gps data mechanical models predict zones of fast contraction in the santa barbara channel and offshore near malibu suggesting that offshore faults represent a significant seismic hazard to the region furthermore many active faults throughout the region may produce little to no interseismic deformation making accurate seismic hazard assessment challenging 2013 american geophysical union all rights reserved

thrust fault nucleation due to heterogeneous bedding plane slip evidence from an ohio coal mine abstract shales often form the roof and floor rocks of coal mines in the appalachian basin from a geologic perspective these mines offer outstanding 3d exposures of fresh rocks otherwise only accessible via boreholes or heavily weathered surface outcrops and present an excellent opportunity to directly observe pristine in situ natural fractures in shale in carroll county ohio small thrust faults that predate mining operations are well exposed within the roof rocks and develop near areas where gradients of coal shale contacts are steepest causing ground instability we hypothesize that bedding contact surfaces served as natural displacement discontinuities that perturbed the local stress state resulting in localized thrust fault development using borehole and in mine survey data we digitally constrain bedding contact geometry to model slip along this interface and resolve related stress perturbations our modeling results suggest that inherited non planarity of contact surfaces influenced the nucleation of secondary faults simulation results as constrained by the integrated field and modeling approach taken in this study help us to better understand the relationship between subsurface stress perturbations and the formation of secondary fractures for mining purposes recognizing subsurface stress heterogeneities due to variable bedding geometry within a regional stress field can improve our predictive capabilities of underground structural hazards

distributed damage faulting and friction we present a formulation for mechanical modeling of geological processes in the seismogenic crust using damage rheology the seismogenic layer is treated as an elastic medium where distributed damage modifying the elastic stiffness evolves as a function of the deformation history the model damage rheology is based on thermodynamic principles and fundamental observations of rock deformation the theoretical analysis leads to a kinetic equation for damage evolution having two principal coefficients the first is a criterion for the transition between strength degradation and recovering healing and is related to friction the second is a rate coefficient of damage evolution which can have different values or functional forms for positive degradation and negative healing evolution we constrain these coefficients by fitting model predictions to laboratory data including coefficient of friction in sawcut setting intact strength in fracture experiments first yielding in faulting experiments under three dimensional strain onset and evolution of acoustic emission and dynamic instability the model damage rheology accounts for many realistic features of three dimensional deformation fields associated with an earthquake cycle these include aseismic deformation gradual strength degradation development of process zones and branching faults around high damage areas strain localization brittle failure and state dependent friction some properties of the model damage rheology e g cyclic stick slip behavior with possible accompanying creep are illustrated with simplified analytical results the developments of the paper provide an internally consistent framework for simulating long histories of crustal deformation and studying the coupled evolution of regional earthquakes and faults this is done in a follow up work

distribution of seismicity across strike slip faults in california 1 the distribution of seismicity about strike slip faults provides measurements of fault roughness and damage zone width in california seismicity decays with distance from strike slip faults according to a power law 1 x2 d2 2 this scaling relation holds out to a fault normal distance x of 3 6 km and is compatible with a rough fault loading model in which the inner scale d measures the half width of a volumetric damage zone and the roll off rate is governed by stress variations due to fault roughness according to dieterich and smith s 2 d simulations approximates the fractal dimension of along strike roughness near fault seismicity is more localized on faults in northern california nocal d 60 20 m 1 65 05 than in southern california socal d 220 40 m 1 16 05 the parkfield region has a damage zone half width d 120 30 m consistent with the safod drilling estimate its high roll off rate 2 30 25 indicates a relatively flat roughness spectrum k 1 versus k 2 for nocal k 3 for socal our damage zone widths the first direct estimates averaged over the seismogenic layer can be interpreted in terms of an across strike fault core multiplicity that is 1 in nocal 2 at parkfield and 3 in socal the localization of seismicity near individual faults correlates with cumulative offset seismic productivity and aseismic slip consistent with a model in which faults originate as branched networks with broad multicore damage zones and evolve toward more localized lineated features with low fault core multiplicity thinner damage zones and less seismic coupling our results suggest how earthquake triggering statistics might be modified by the presence of faults

dynamic rupture activation of backthrust fault branching we perform dynamic rupture simulations to investigate the possible reactivation of backthrust branches triggered by ruptures along a main thrust fault simulations with slip weakening fault friction and uniform initial stress show that fast propagation speed or long propagation distance of the main rupture promotes reactivation of backthrust over a range of branch angles the latter condition may occur separately from the former if rupture speed is limited by an increasing slip weakening distance towards the junction direction the results suggest a trade off between the amplitude and duration of the dynamic stress near the main rupture front for backthrust reactivation termination of the main rupture by a barrier can provide enhanced loading amplitude and duration along a backthrust rooted near the barrier facilitating its reactivation especially with a high frictional resistance the free surface and depth dependent initial stress can have several additional effects the sign of the triggered motion along the backthrust can be reversed from thrust to normal if a deeply nucleated main rupture breaks the free surface while it is preserved as thrust if the main rupture is terminated by a barrier at depth the numerical results are discussed in relation to several recent megathrust earthquakes in sumatra chile and japan and related topics such as branch feedbacks to the main fault the dynamic view on backthrust fault branching provided by the study fills a gap not covered by quasi static models or observations a specific examined case of antithetic fault branching may be useful for indicating a barrier like behavior along the main fault

multicycle dynamics of nonplanar strike slip faults 1 we perform two dimensional dynamic models of strike slip faults with a change in strike a bend over multiple earthquake cycles to examine the long term effects of nonplanar fault geometry a viscoelastic model a proxy for off fault deformation and tectonic loading is introduced for the interseismic process to avoid pathological stress buildup around the bend a finite element method with an elastodynamic model is used to simulate dynamic earthquake ruptures we find that stresses near the bend differ strongly from the regional stress field and that the fault develops a relatively steady state in which the stress level and the event pattern on the fault are stable reduced normal stress on the dilatational side and increased normal stress on the compressive side of the bend during dynamic ruptures result in the bend serving as an initiation and or a termination point s for rupture typical events on such a fault consist of two classes unilateral events that rupture only the favorable segment and bilateral events that rupture the favorable segment and part of or the entire unfavorable segment in the latter class of events a time delay in rupture around the bend results from a high yield stress on the compressive side of the bend other effects of the bent fault geometry include higher displacement on the inward wall than on the outward wall higher slip on the more favorable segment than on the less favorable segment and a large slip velocity on the compressive side of the bend

coseismic and postseismic subsurface displacements and strains for a vertical strike slip fault in a three layer elastic medium a three layer elastic gravitational fault displacement model using dislocation theory has been developed and used to examine the effect of layering of earth elastic moduli on surface and subsurface displacement fields for a vertical strike slip fault the model has been used to examine the effect of depth variation of elastic properties at coseismic and postseismic time scales for pure strike slip motion the effect of gravity on coseismic and postseismic horizontal deformation is negligible for coseismic deformation the model predicts that for constant poisson s ratio an increase in elastic moduli with depth attenuates the displacements within the upper layers with respect to displacement distribution for a uniform half space while an inclusion of a soft layer between the top layer and lower half space amplifies upper layer displacements the effect of variation in poisson s ratio on surface and subsurface displacements has also been examined

euclidean and fractal models for the description of rock surface roughness accurate description of the topography of rock surfaces is essential because surface roughness affects frictional strength the flow of fluids in joints and fractures the seismic behavior of faults and the formation of gouge and breccia in fault zones real rock surfaces can be described using self similar and self affine fractal models of surface roughness if a surface is self similar a small portion of the surface when magnified isotropically will appear statistically identical to the entire surface if a surface is self affine a magnified portion of the surface will only appear statistically identical to the entire surface if different magnifications are used for the directions parallel and perpendicular to the surface at least two parameters are required to describe a fractal model one parameter typically describes how roughness changes with scale while the other specifies the variance or surface slope at a reference scale the divider method and the spectral method are in common use to determine the best fit fractal model from surface profile data power spectra from self similar surfaces have slopes of 3 on log log plots of power spectral density versus spatial frequency while spectra from self affine surfaces have slopes other than 3 power spectra can be interpreted with greater facility if dimensionless amplitude to wavelength ratios are contoured on plots of power spectral density versus frequency the topography of many natural rock surfaces including both fractures and faults is approximately self similar within the 6 5 order of magnitude wide wavelength band of 10 m to 40 m within smaller wavelength bands natural rock surfaces may exhibit self affine behavior

predicting reservoir scale faults with area balance application to growth stratigraphy a reservoir that appears to lack faults at one scale of resolution or at one sampling density may nevertheless contain faults that are below the resolution of the observations the area depth relationship from a balanced cross section is shown to contain the necessary information for predicting the sub resolution fault heave existing area depth theory is extended to include growth units allowing structural length and thickness changes to be separated from the depositional changes the technique is validated with a numerical model of a growth full graben and a sand box model of a half graben then field tested in the gilbertown graben a growth structure within the regional peripheral fault trend along the northern margin of the gulf of mexico a cross section developed from wells alone is used to infer the abundance of sub resolution faults by the area balance technique a small but significant amount of sub resolution extension is predicted and then confirmed with a highresolution seismic line q 2003 elsevier science ltd all rights reserved

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

quantifying near field and off fault deformation patterns of the 1992 mw 7 3 landers earthquake coseismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods e g insar lidar differencing and gps current methods however either fail to capture patterns of near field coseismic surface deformation or lack preevent data consequently the characteristics of off fault deformation and the parameters that control it remain poorly understood we develop a standardized method to fully measure the surface near field coseismic deformation patterns at high resolution using the cosi corr program by correlating pairs of aerial photographs taken before and after the 1992 m w 7 3 landers earthquake cosi corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists for the landers earthquake our measured displacements are systematically larger than the field measurements indicating the presence of off fault deformation we show that 46 of the total surface displacement occurred as off fault deformation over a mean deformation width of 154 m the magnitude and width of off fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system with a weak correlation with the type of near surface materials through which the rupture propagated both the magnitude and width of distributed deformation are largest in stepovers bends and at the southern termination of the surface rupture we find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self affine fractal with dimension of 1 56

spatial separation of large earthquakes aftershocks and background seismicity analysis of interseismic and coseismic seismicity patterns in southern california we associate waveform relocated background seismicity and aftershocks with the 3 d shapes of late quaternary fault zones in southern california major earthquakes that can slip more than several meters aftershocks and near fault background seismicity mostly rupture different surfaces within these fault zones major earthquakes rupture along the mapped traces of the late quaternary faults called the principal slip zones pszs aftershocks occur either on or in the immediate vicinity of the pszs typically within zones that are 2 km wide in contrast the near fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces and forms spatially decaying distributions extending out to distances of 10 km from the pszs we call the regions where the enhanced rate of background seismicity occurs the seismic damage zones one possible explanation for the presence of the seismic damage zones and associated seismicity is that the damage develops as faults accommodate bends and geometrical irregularities in the pszs the seismic damage zones mature and reach their finite width early in the history of a fault during the first few kilometers of cumulative offset alternatively the similarity in width of seismic damage zones suggests that most fault zones are of almost equal strength although the amount of cumulative offset varies widely it may also depend on the strength of the fault zone the time since the last major earthquake as well as other parameters in addition the seismic productivity appears to be influenced by the crustal structure and heat flow with more extensive fault networks in regions of thin crust and high heat flow

characterization of fault roughness at various scales implications of three dimensional high resolution topography measurements accurate description of the topography of active fault surfaces represents an important geophysical issue because this topography is strongly related to the stress distribution along fault planes and therefore to processes implicated in earthquake nucleation propagation and arrest to date due to technical limitations studies of natural fault roughness either performed using laboratory of field profilometers were obtained mainly from 1 d profiles with the recent development of light detection and ranging lidar apparatus it is now possible to measure accurately the 3 d topography of rough surfaces with a comparable resolution in all directions both at field and laboratory scales in the present study we have investigated the scaling properties including possible anisotropy properties of several outcrops of two natural fault surfaces vuache strike slip fault france and magnola normal fault italy in limestones at the field scale digital elevation models of the fault roughness were obtained over surfaces of 0 25 m2 to 600 m2 with a height resolution ranging from 0 5 mm to 20 mm at the laboratory scale the 3 d geometry was measured on two slip planes using a laser profilometer with a spatial resolution of 20 m and a height resolution less than 1 m

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

influence of fault roughness on surface displacement from numerical simulations to coseismic slip distributions field studies have characterized natural faults as rough nonplanar surfaces at all scales fault roughness induces local stress perturbations during slip which dramatically affect rupture behavior resulting in slip heterogeneity however the relation between fault roughness and slip heterogeneity remains a key knowledge gap between current numerical and field studies in this study we analyze numerical simulations of earthquake rupture to determine how roughness influences final slip using a rupture catalog containing thousands of dynamic rupture simulations on band limited self similar fractal fault profiles with varying roughness and background shear stress levels we quantify how fault roughness affects the spectral characteristics of the resulting slip distribution we find that slip distributions become increasingly more self affine that is containing more short wavelength fluctuations as compared to the self similar fault profiles as roughness increases we also find that at very short wavelengths 1km the fractal dimension of the slip distributions dramatically changes with increasing roughness background shear stress

deterministic model of earthquake clustering shows reduced stress drops for nearby aftershocks while a number of viable physical mechanisms have been offered to explain the temporal clustering of aftershocks the spatial clustering of aftershocks in particular the concentrated productivity of aftershocks very near the mainshock rupture area has been difficult to reproduce with physical models here we present a new deterministic physical model capable of reproducing both the spatial and temporal clustering we apply this new model to a longstanding puzzling question raised by ground motion observations which suggest that nearby aftershocks show reduced ground motions relative to similar magnitude mainshocks in the model the physical basis for these observations is reduced stress drops for nearby aftershocks compared to similar magnitude mainshocks these reduced stress drops are due to nearby aftershocks rerupturing incompletely healed parts of the fault which ruptured in the mainshock

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

stress and deformation along wavy frictional faults locally inhomogeneous stress states are expected along faults owing to slip on geometrically irregular fault surfaces we use an analytical model of elastic deformation along a wavy frictional fault to evaluate the variation in local stress state as a function of surface roughness elastic modulus slip coefficient of friction and far field stress the total stress state along the fault may be described by the sum of a basic stress component resulting from frictional slip on a planar fault surface and a perturbed stress component resulting from the presence of roughness roughness produces a variation in normal stress across the fault surface and assuming roughness and modulus appropriate to crustal faults the normal stress should be reduced to a near zero magnitude locally such that separation of fault walls is likely the large variation in normal stress along the fault surface resulting from fault roughness may be responsible in part for complexity in moment release during large earthquakes and for lateral variation in seismic coupling along faults the variation in principal stress orientations and magnitudes along a fault increases with a decrease in the coefficient of friction of the fault the location and size of regions with a high likelihood for brittle failure depend on the orientation of the far field principal stress and fault friction the average orientation of the principal stresses in the region of likely failure is not the same as the far field principal stress orientation although inversion of earthquake and fabric data for stress orientation along a fault may be possible the model results suggest that inversion results are insufficient to determine far field stress states and fault friction without additional independent data

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

speculations on the geometry of the initiation and termination processes of earthquake rupture and its relation to morphology and geological structure earthquake initiation and termination processes are commonly described in terms of barriers and asperities barriers fall into two classes geometric barriers are associated with places where the orientation of a failure surface changes and relaxation barriers where stress is low because aseismic creep processes outpace tectonic loading geometric barriers fall into conservative and nonconservative subgroups according to whether finite fault motion can proceed without the creation of new structures or whether it demands the creation of new faulting or void space the multiple faulting or fragmentation associated with some nonconservative barriers can disrupt fault planes and form asperities by means of selected examples it is shown that a description in terms of these barriers can help one to visualise the processes of earthquake rupture and its relation to the geological environment

the accommodation of large strains in the upper lithosphere of the earth and other solids by self similar fault systems the geometrical origin of b value plate tectonics has provided a method of visualizing the geometry of the deformation of the earth s lithosphere on a large scale the description is so concise that for many purposes it provides an explanation of geological processes that overshadows the need to understand the driving processes the mechanics of the zones between the plates are less well understood particularly in continental regions where large areas are subject to deformation both continuous and discontinuous models have been tried but both have obvious drawbacks

inferring earthquake physics and chemistry using an integrated field and laboratory approach abstract earthquakes are the result of a combination of 1 physico chemical processes operating in fault zones which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate and 2 of the geometrical complexity of fault zones in this review paper we summarize recent experimental findings from high velocity conducted at about 1 m s slip rate or typical seismic slip rates rock friction experiments with an emphasis on potential dynamic weakening mechanisms melt lubrication nanopowder lubrication etc and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones we discuss how earthquake source parameters coseismic fault strength weakening distances energy budgets etc might be derived from the field and laboratory experiments additionally we discuss what needs to be considered in terms of fault zone geometry and morphology focusing on fault surface roughness in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates with an emphasis on the case of melt lubrication all experimental data and in the case of melt lubrication microphysical models indicate that faults must be very weak

slip on wavy frictional faults is the 3rd dimension a sticking point abstract the formulation for the 3d triangular displacement discontinuity boundary element method with frictional constraints is described in detail its accuracy in comparison to analytical solutions is then quantified we show how this can be used to approximate stress intensity factors at the crack tips using this method we go on to quantify how slip is reduced on fault surfaces with topography where the asperities are approximated as a sinusoidal waveform i e corrugations we use stress boundary conditions compressive orientated such that frictional contacts shear we show that slip reductions relative to planar faults for 2d line and 3d penny shaped crack models are comparable within 10 when slip is perpendicular to the corrugations using the 3d model we then show how slip is reduced more when corrugation wavelengths are doubled compared to the reduction due to corrugation misalignment with the slip direction when slip is parallel with the corrugation alignment we show that reducing the out of plane stress from the normal traction acting on the fault when planar to that resolved on a perpendicular plane has the same effect as halving the length of the corrugation waveform in terms of slip reduction for a given amplitude

mechanics slip behavior and seismic potential of corrugated dip slip faults 1 to better understand the mechanics and seismic potential of nonplanar fault surfaces we present results from a suite of numerical models of faults with sinusoidal corrugations in the downdip direction systematic variations in corrugation wavelength amplitude and loading angle are introduced to determine the effects on slip behavior and seismic moment release we find that corrugated faults in general slip less than planar faults changes in slip behavior are nearly scale independent and are dominantly controlled by the amplitude wavelength of corrugations model results suggest that obliquely loaded corrugated faults accumulate less strike slip than a planar fault with the same tip line dimensions and average orientation this result implies that slip direction is not a reliable indicator of regional stress direction and may at least partially explain repeated nearly pure dip slip coseismic events at oblique plate boundaries though the scalar seismic moment release is always less for corrugated fault surfaces due to a greater reduction in slip compared to increased surface area for geologically reasonable corrugation geometries changes in total scalar moment release are not significantly different than planar faults techniques that utilize highly simplified fault geometries may therefore accurately reproduce scalar moment release but will nonetheless incorrectly predict coseismic slip magnitudes and distributions as well as regional stress orientations

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

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

location of largest earthquake slip and fast rupture controlled by along strike change in fault structural maturity due to fault growth earthquake slip distributions are asymmetric along strike but the reasons for the asymmetry are unknown we address this question by establishing empirical relations between earthquake slip profiles and fault properties we analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults in particular on the directions of their long term lengthening we find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long term fault propagating tips i e on the most mature half of the broken fault sections meanwhile slip decreased linearly over most of the rupture length in the direction of long term fault propagation i e of decreasing structural maturity along strike we suggest that this earthquake slip asymmetry is governed by along strike changes in fault properties including fault zone compliance and fault strength induced by the evolution of off fault damage fault segmentation and fault planarity with increasing structural maturity we also find higher rupture speeds in more mature rupture sections consistent with predicted effects of low velocity damage zones on rupture dynamics since the direction s of long term fault propagation can be determined from geological evidence it might be possible to anticipate in which direction earthquake slip once nucleated may increase accelerate and possibly lead to a large earthquake our results could thus contribute to earthquake hazard assessment and earthquake early warning

rough faults distributed weakening and off fault deformation 1 we report systematic spatial variations in fault rocks along nonplanar strike slip faults cross cutting the lake edison granodiorite sierra nevada california sierran wavy fault and lobbia outcrops of the adamello batholith in the italian alps lobbia wavy fault in the case of the sierran fault pseudotachylyte formed at contractional fault bends where it is found as thin 1 2 mm fault parallel veins epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends we argue that the presence of fluids as illustrated by this example does not necessarily preclude the development of frictional melt in the case of the lobbia fault pseudotachylyte thickness varies along the length of the fault but the pseudotachylyte veins thicken and pool in extensional bends we conduct a quantitative analysis of fault roughness microcrack distribution stress and friction along the lobbia fault numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness resulting in an overall weaker fault than suggested by the corresponding static friction coefficient the models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip rather than being confined to the region close to the dynamically propagating crack tip

quantifying focal mechanism heterogeneity for fault zones in central and southern california summary we present a statistical analysis of focal mechanism orientations for nine california fault zones with the goal of quantifying variations of fault zone heterogeneity at seismogenic depths the focal mechanism data are generated from first motion polarities for earthquakes in the time period 1983 2004 magnitude range 0 5 and depth range 0 15 km only mechanisms with good quality solutions are used we define fault zones using 20 km wide rectangles and use summations of normalized potency tensors to describe the distribution of double couple orientations for each fault zone focal mechanism heterogeneity is quantified using two measures computed from the tensors that relate to the scatter in orientations and rotational asymmetry or skewness of the distribution we illustrate the use of these quantities by showing relative differences in the focal mechanism heterogeneity characteristics for different fault zones these differences are shown to relate to properties of the fault zone surface traces such that increased scatter correlates with fault trace complexity and rotational asymmetry correlates with the dominant fault trace azimuth these correlations indicate a link between the long term evolution of a fault zone over many earthquake cycles and its seismic behaviour over a 20 yr time period analysis of the partitioning of san jacinto fault zone focal mechanisms into different faulting styles further indicates that heterogeneity is dominantly controlled by structural properties of the fault zone rather than time or magnitude related properties of the seismicity

nonplanar faults mechanics of slip and off fault damage stress interactions and sliding characteristics of faults with random fractal waviness in a purely elastic medium differ both qualitatively and quantitatively from those of faults with planar surfaces with nonplanar fault models solutions for slip diverge as resolution of the fractal features increases and the scaling of fault slip with fault rupture dimension becomes nonlinear we show that the nonlinear scaling of slip and divergence of solutions arise because stresses from geometric interactions at irregularities along nonplanar faults grow with increasing slip and produce backstresses that progressively impede slip however in real materials with finite strength yielding will halt the growth of the interaction stresses which will profoundly affect slip of nonplanar faults we infer that in the brittle seismogenic portion of the earth s crust off fault yielding occurs on pervasive secondary faults predicted rates of stress relaxation with distance from major faults with random fractal roughness follow a power law relationship that is consistent with reported clustering of background seismicity up to 15 kilometers from faults

the work budget of rough faults abstract faults in nature have measurable roughness at many scales and are not planar as generally idealized we utilize the boundary element method to model the geomechanical response of synthetic rough faults in an isotropic linear elastic continuum to external tectonic loading in terms of the work budget faults are generated with known fractal roughness parameters including the root mean square slope a measure of roughness amplitude and the hurst exponent h a measure of geometric self similarity energy within the fault models is partitioned into external work w ext internal elastic strain energy w int gravitational work w grav frictional work w fric and seismic energy w seis results confirm that w ext or work done on the external model boundaries is smallest for a perfectly planar fault and steadily increases with increasing this pattern is also observed in w int the energy expended in deforming the host rock the opposite is true for gravitational work or work done against gravity in uplifting host rock as well as with frictional work or energy dissipated with frictional slip on the fault and w seis or seismic energy released during slip events effects of variation in h are not as large as for but w grav w fric and w seis increase with increasing h with w int and w ext decreasing across the same range remarkably however for a narrow range of roughness amplitudes which are commonly observed along natural faults the total work of the system remains approximately constant while slightly larger than the total work of a planar fault faults evolve toward the most mechanically efficient configuration therefore we argue that this range of roughness amplitudes may represent an energy barrier preventing faults from removing asperities and evolving to smooth planar discontinuities a similar conclusion is drawn from simulations at relatively shallow depths with results showing that shallower faults have larger energy barriers and can be mechanically efficient at higher roughness amplitudes

corrugated megathrust revealed offshore from costa rica exhumed faults are rough often exhibiting topographic corrugations oriented in the direction of slip such features are fundamental to mechanical processes that drive earthquakes and fault evolution however our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth especially at subducting plate boundaries here we present three dimensional seismic reflection data of the costa rica subduction zone that image a shallow megathrust fault characterized by corrugated and chaotic and weakly corrugated topographies the corrugated surfaces extend from near the trench to several kilometres down dip exhibit high reflection amplitudes consistent with high fluid content pressure and trend 11 18 oblique to subduction suggesting 15 to 25 mm yr 1 of trench parallel slip partitioning across the plate boundary the corrugations form along portions of the megathrust with greater cumulative slip and may act as fluid conduits in contrast weakly corrugated areas occur adjacent to active plate bending faults where the megathrust has migrated up section forming a nascent fault surface the variations in megathrust roughness imaged here suggest that abandonment and then reestablishment of the megathrust up section transiently increases fault roughness analogous corrugations may exist along significant portions of subduction megathrusts globally mature parts of the shallow megathrust beneath costa rica are characterized by striking corrugations that may channel fluids according to seismic images nascent sections of the subduction zone plate boundary appear only weakly corrugated