effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

a kinematic model for deformation within brittle shear zones textural analysis of fault gouge indicates that displacement of particles within brittle shear zones occurs by three diaerent modes synthetic riedel shears antithetic riedel shears and shearing particulate flow a kinematic model is developed which incorporates these three slip modes and constrains the combined deformation to be simple shear parallel to the shear zone boundaries penetrative fabric p foliation and shape preferred orientation of survivor grains commonly observed in foliated fault gouge is assumed to be a result of the shearing particulate flow only the model which relates penetrative fabric orientation to the relative shear rates on riedel shears and shearing particulate flow reconciles inclined p foliation and high g 10 shear strain 1999 elsevier science ltd all rights reserved

mesoscale strike slip faults and damage zones at marsalforn gozo island malta well exposed strike slip faults in limestones in the north western part of gozo show damage zones that can be grouped into three categories based on their location along faults tip damage linking damage and distributed damage the predominant fracture types within damage zones include extension fractures and secondary faults tip damage zones usually show wedge shaped patterns formed by antithetic faults and extension fractures commonly accompanied by block rotation several fractures are combined at linking damage zones typically with the concentration of a high intensity of fractures structures in distributed damage zones are typically similar to the classical riedel shear pattern evolutionary and 3d models are proposed in terms of the geometries of damage zones for small displacement strike slip fault zones different evolutionary routes depend on fault tip modes and locations

influence of the state of stress on the brittle ductile transition in granitic rock evidence from fault steps in the sierra nevada california left lateral strike slip faults in the lake edison granodiorite central sierra nevada california are composed of en echelon segments relative displacements across the faults apparently are transferred between segments by ductile shearing at right steps and by extensional fracturing at left steps the granodiorite within right steps displays mylonitic foliation and thin sections show textures in quartz associated with dislocation glide recovery processes and dynamic recrystallization whereas textures in feldspar are related to fracturing only centimetres outside the right steps the rock fabric is approximately isotropic and deformation is accommodated by mineralized opening mode fractures the stress field calculated for the right step geometry when a boundary element model is used shows an increase in mean compressive stress of up to 25 mpa within the step relative to that outside this difference in stress apparently produced the contrasting behaviors of the granitic rock experimentally derived power law flow laws do not predict these behaviors

simultaneous pure and simple shear the unifying deformation matrix abstract any simultaneous combination of finite simple shear and finite pure shear is a linear transformation which can be expressed as a single transformation matrix for two dimensions the matrix is upper triangular with an off diagonal term called the effective shear strain is a simple function of the pure and simple shear components for three dimensions a simultaneous combination of thrusting in the x direction thrusting in the y direction and a wrench in the x direction in addition to 3 orthogonal components of coaxial strain can also be represented by a 3 3 upper triangular matrix here three off diagonal terms xy x y and y z occur xy is a simple function of the horizontal coaxial strain values and thrusting in the x direction yz depends on the coaxial strain components in the y and z directions and the thrusting in the y direction while xz is related to all six strain components the matrix also allows for volume change either homogeneously or preferentially in a single direction a method of decomposing the deformation matrix into a series of incremental deformation matrices where each incremental deformation records the same kinematic vorticity number as the finite deformation is shown the orientation and magnitude of the finite strain ellipsoid ellipse is easily and accurately found at any increment during the deformation

shear zone model for the origin of metamorphic core complexes core complex tectonite results from simple shear rotational strain within regional ductile shear zones that have accommodated a tertiary crustal stretching the emplacement of younger and or upper level nontectonite rocks on older and or deeper level tectonites is the ordinary and predictable result of normal slip translation along shear zones that cut upward through the local geologic column progressive deformation produces a structural stratigraphy marked from bottom to top by normal country rock protolith mylonitic gneiss tectonite carapace microbrecciated tectonite microbreccia decollement and upper plate detachment rocks as young as mid miocene the reconstructions suggest that local regional crustal stretching locally exceeded 100

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

influence of outcrop scale fractures on the effective stiffness of fault damage zone rocks we combine detailed mapping and microstructural analyses of small fault zones in granodiorite with numerical mechanical models to estimate the effect of mesoscopic outcrop scale damage zone fractures on the effective stiffness of the fault zone rocks the bear creek fault zones were active at depths between 4 and 15 km and localize mesoscopic off fault damage into tabular zones between two subparallel boundary faults producing a fracture induced material contrast across the boundary faults with softer rocks between the boundary faults and intact granodiorite outside the boundary faults using digitized fault zone fracture maps as the modeled fault geometries we conduct nonlinear uniaxial compression tests using a novel finite element method code as the experimental laboratory apparatus map measurements show that the fault zones have high nondimensional facture densities 1 and damage zone fractures anastamose and intersect making existing analytical effective medium models inadequate for estimation of the effective elastic properties numerical experiments show that the damage zone is strongly anisotropic and the bulk response of the fault zone is strain weakening normal strains as small as 2 can induce a reduction of the overall stiffness of up to 75 fracture induced effective stiffness changes are large enough to locally be greater than intact modulus changes across the fault due to juxtaposition of rocks of different lithologies therefore mesoscopic fracturing is as important as rock type when considering material or bimaterial effects on earthquake mechanics these results have important implications for earthquake rupture mechanics models because mesoscopic damage zone fractures can cause a material contrast across the faults as large as any lithology based material contrast at seismogenic depths and the effective moduli can be highly variable during a single rupture event

evaluation of transtension and transpression within contractional fault steps comparing kinematic and mechanical models to field data abstract rock deformation often is investigated using kinematic and or mechanical models here we provide a direct comparison of these modeling techniques in the context of a deformed dike within a meter scale contractional fault step the kinematic models consider two possible shear plane orientations and various modes of deformation simple shear transtension transpression while the mechanical model uses the finite element method and assumes elastoplastic constitutive behavior the results for the kinematic and mechanical models are directly compared using the modeled maximum and minimum principal stretches the kinematic analysis indicates that the contractional step may be classified as either transtensional or transpressional depending on the modeled shear plane orientation suggesting that these terms may be inappropriate descriptors of step related deformation while the kinematic models do an acceptable job of depicting the change in dike shape and orientation they are restricted to a prescribed homogeneous deformation in contrast the mechanical model allows for heterogeneous deformation within the step to accurately represent the deformation the ability to characterize heterogeneous deformation and include fault slip not as a prescription but as a solution to the governing equations of motion represents a significant advantage of the mechanical model over the kinematic models

deformation around stepovers in strike slip fault zones abstract this stuck addresses the kinematics of stepovers in strike slip fault zones where left lateral faults step rightward or right lateral faults step leftward a stepover is regarded as containing an internal strike slip fault whose ends link to transform faults surrounded by uniformly distributed deformation the kinematics are governed by k the ratio of slip rates on the internal fault and transform faults and gq the angle between these taults the convergence direction across the deformation at angle to the internal fault is an azimuth along which vectors do not rotate around vertical axes the lebanon stopover on the dead sea fault zone and the big bend stepover on the san andreas fault zone provide case studies in lebanon the time averaged deformation since 15 ma involves k 0 06 32 and 34 it accounts for 30 of anticlockwise rotation of cretaceous magnetization vectors in the lebanon mountains and 50 rotation of jurassic vectors the present phase of deformation around the big bend which began at 5 7 ma and imolves k 0 89 20 and 98 has instead only caused small rotations both clockwise and anticlockwise much of this region s structure including the dramatic up to 80 clockwise rotation of magnetization vectors in the transverse ranges is shown to relate instead to an earlier deformation phase with much lower k

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

the dinevar transtensional pull apart basin nw zagros mountains iran a geological study and comparison to 2d finite element elastic models the dinevar transtensional pull apart basin is a nw se trending basin located in the northwest part of the zagros mountains sanandaj sirjan tectonic zone iran this basin was formed by oblique extension in a strike slip fault system which was controlled by the pre existing sahneh and the mianrahan faults in addition to a field study we constructed a 2d finite element elastic model based on the major fault segments according to the field study various geological structures are formed around the pre existing fault segments including pencil structures with 13 aspect ratio 19 tight to open asymmetric folds oblique normal faults synthetic and antithetic faults normal and reverse fault drags and negative and hybrid type flower structure fe model results show that around the fault step is a zone of displacement gradient that indicates extension transtension within the overlapping zone consequently subsidence is predicted over the releasing fault step the modelled stress distributions within the fault step and around the fault segment tips are asymmetric elastic modelling shows that the mean normal stress within the step is tensional which would impede plastic yield while the region outside the step is characterised by compressive mean normal stress the mean normal stress pattern is characterised by a regional minimum localised inside the extensional step transtension zone and two local maximum and minimum couples formed at the advancing and retreating sides respectively in the fault segment tips the results show that two main subsidence regions depocentres or sub basins form along and in the proximity of the fault segment tips the imposed relative motion is entirely accommodated by creep on the faults which causes plastic strain to accumulate outside the faults mostly localised within the fault step this localised strain between fault segments could initiate connecting faults or reactivate earlier faults generally at high angles relative to the master fault segments as suggested by the stress and strain patterns the rotation and magnitude variation in the stress and strain trajectories reflects heterogeneous deformation in general the dinevar transtensional pull apart basin is characterised by an elongate rhomboidal to trapezoidal shaped geometry the modelled stress distribution and strain localisations within the basin are in general agreement with the locations of fault segments within the dinevar transtensional pull apart basin

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

regional variations in bulk chemistry mineralogy and the compositions of mafic and accessory minerals in the batholiths of california we define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole rock geochemistry in the batholiths of california in so doing we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano plutonic arcs of active continental margins little studied metaluminous to strongly peraluminous granites containing fe rich biotite with log x mg x fe f oh and mn in biotite and amphibole increase on a regional scale from western i wc types to eastern i mc and i sc types parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons in contrast the belts of western i scr granites and eastern i wc quartz diorites and granodiorites disrupt the regional west to east systematics in both mineral and whole rock geochemistry spatial variations in the al content of amphibole are regional in scale and reflect pressures of pluton crystallization we conclude that significant previously unrecognized complexity exists in regional geochemical systematics in the california batholiths

static stress drop associated with brittle slip events on exhumed faults 1 we estimate the static stress drop on small exhumed strike slip faults in the lake edison granodiorite of the central sierra nevada california the subvertical strike slip faults were exhumed from 4 to 15 km depth and were chosen because they are exposed in outcrop along their entire tip to tip lengths of 8 12 m slip nucleated on joints and accumulated by crystal plastic shearing forming quartz mylonites from early quartz vein filling in joints and successive brittle faulting forming epidote bearing cataclasites the occurrence of thin 300 m wide pseudotachylytes along some small faults throughout the study area suggests that some if not all of the brittle slip on the study area faults may have been seismic we suggest that the contribution of brittle cataclastic slip to the total slip along the studied cataclasite bearing small faults may be estimated by the length of epidote filled rhombohedral dilatational jogs rhombochasms distributed quasi periodically along the length of the faults the interpretation that slip recorded by rhombochasms occurred in single events is based on evidence that 1 epidote crystals are randomly oriented and undeformed within the rhombochasm 2 cataclasite in principal slip zones does not include clasts of previous cataclasite and 3 rhombochasm lengths vary systematically along the length of the faults with slip maximum occurring near the fault center tapering to the fault tips we thereby constrain both the rupture length and slip on the basis of these measurements we calculate stress drops ranging over 90 250 mpa i e one to two orders of magnitude larger than typical seismological estimates for earthquakes but similar in magnitude to seismological estimates of small m2 earthquakes from the san andreas fault observatory at depth safod the slip events described in the present study occurred along small deep seated faults and given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz bearing mylonite we conclude that these were strong faults

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

evolution of olivine lattice preferred orientation during simple shear in the mantle article i nfo understanding the variation of olivine lattice preferred orientation lpo as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation we present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the josephine peridotite southwest oregon we find that the lpo in harzburgites re orients from a pre existing lpo outside the shear zone to a new lpo with the olivine 100 maximum aligned sub parallel to the shear direction between 168 and 258 shear strain the strain at which 100 aligns with the shear plane is slightly higher than that observed in experimental samples which do not have an initial lpo while our observations broadly agree with the experimental observations our results suggest that a pre existing lpo influences the strain necessary for lpo alignment with the shear direction in addition olivine re alignment appears to be dominated by slip on both 010 100 and 001 100 due to the orientation of the pre existing lpo fabric strengths quantified using both the j and m indices do not increase with increasing shear strain unlike experimental observations our natural samples do not have a secondary lpo peak the lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re alignment harzburgites exhibit girdle patterns among 010 and 001 axes while a dunite has point maxima combined with the observation that harzburgites are finer grained than dunites we speculate that additional phases i e pyroxenes limit olivine grain growth and promote grain boundary sliding grain boundary sliding may relax the requirement for slip on the hardest olivine system enhancing activation of the two easiest olivine slip systems resulting in the 010 and 001 girdle patterns overall our results provide an improved framework for calibration of lpo evolution models

biases of submerged bulk and freeze core samples freeze coring and bulk sampling are routine methods used to sample subsurface spawning gravel under shallow water both methods have limitations freeze coring is not believed to representatively sample coarse grain sizes and the sample volumes are relatively small conversely when bulk sampling even within an enclosure some fine sediment is suspended and washed away from the sample this paper assesses the biases in sampling performance between the two methods and determines whether the loss of fines that occurs when bulk sampling could be predicted and thus corrected for at six riffles the spawning substrate was sampled under approximately 50 cm of water with a bulk sample and three adjacent freeze cores for each riffle data from the two samples were combined using the method of fripp and diplas 1993 and the resultant composite sample was compared with the original freeze core and bulk samples to assess the relative precision and biases of the two techniques on average the d50 of the bulk samples was 4 mm larger and a one third loss of the 16 mm compared with composite samples based on six samples taken from six riffles the amount of sediment finer than 0 5 mm lost using our bulk sampling technique with an enclosure appears to be predictable and correctable copyright 2005 john wiley sons ltd

zanskar shear zone northeast southwest extension within the higher himalayas ladakh india a complete geologic section through the northwestern himalayas is well exposed in ladakh india the collision between the indian shield and the tibetan platform caused large scale intracrustal thrusting and piling up of the himalayan nappes this compressional event was accompanied by prograde regional metamorphism the metamorphism ranges from low grade anchimetamorphism in the indus suture zone in the northeast to upper amphibolite grade in the higher himalaya tectonic unit in the southwest in the zanskar area a well exposed and morphologically prominent shear zone 2 25 to 6 75 km wide can be followed from the northwest to the southeast for at least 80 km in contrast to the main compressional tectonics this shear zone indicates an extensional event within the higher himalaya tectonic unit the zanskar shear zone involves granitic rocks leucogranites of miocene age and various metasediments and in general separates the late precambrian early cambrian sedimentary sequence phe formation from the underlying crystalline basement zanskar crystalline unit a normal sense of shear has been determined by using asymmetric macroscopic and microscopic structures such as pegmatite boudins feldspar augen and crystallographic fabrics the metamorphic isograds are very close within this shear zone as a result of shear deformation a gradual transition from upper amphibolite to lower greenschist facies occurs within 200 m movement in this zone occurred late in the metamorphic history under greenschist or lower metamorphic conditions the minimum horizontal extension and vertical displacement are on the order of 16 and 19 km respectively

mechanical evolution of transpression zones affected by fault interactions insights from 3d elasto plastic finite element models abstract the mechanical evolution of transpression zones affected by fault interactions is investigated by a 3d elasto plastic mechanical model solved with the finite element method ductile transpression between non rigid walls implies an upward and lateral extrusion the model results demonstrate that a transpression zone evolves in a 3d strain field along non coaxial strain paths distributed plastic strain slip transfer and maximum plastic strain occur within the transpression zone outside the transpression zone fault slip is reduced because deformation is accommodated by distributed plastic shear with progressive deformation the 3 axis the minimum compressive stress rotates within the transpression zone to form an oblique angle to the regional transport direction 9 10 the magnitude of displacement increases faster within the transpression zone than outside it rotation of the displacement vectors of oblique convergence with time suggests that transpression zone evolves toward an overall non plane strain deformation the slip decreases along fault segments and with increasing depth this can be attributed to the accommodation of bulk shortening over adjacent fault segments the model result shows an almost symmetrical domal uplift due to off fault deformation generating a doubly plunging fold and a positive flower structure outside the overlap zone expanding asymmetric basins subside to negative flower structures on both sides of the transpression zone and are called transpressional basins deflection at fault segments causes the fault dip fall to less than 90 86 89 near the surface 1 5 km this results in a pure shear dominated triclinic and discontinuous heterogeneous flow of the transpression zone

geometry of co seismic surface ruptures and tectonic meaning of the 23 october 2011 mw 7 1 van earthquake east anatolian region turkey abstract the 23 october 2011 m w 7 1 usgs 2011a van earthquake was generated by the rupture of a main thrust fault plane striking n60 e and dipping 55 nw with a very small left lateral offset in the western part of the bardakci saray thrust fault zone located in the north of the bitlis zagros suture zone the incompatibly between the subsurface seismologic data and surface deformation data indicates that the fault that generated the earthquake could be blind the main thrust fault plane developed between the northern ercis and southern van continental blocks and indicates that the region is under the influence of a constant nw se pure compressional tectonic regime along with the primary thrust rupture a secondary back thrust fault a left lateral fault with ne strike and a right lateral fault with nw strike occurred in the contraction province additionally the tensional ground cracks that developed on the hanging wall block of the ruptured thrust fault plane due to intra continental collision are either transtensional that mainly fits to left lateral strike slip faulting or extensional deformations on the elevated block

relations between left lateral strike slip faults and right lateral monoclinal kink bands in granodiorite mt abbot quadrangle sierra nevada california small left lateral strike slip faults and right lateral monoclinal kink bands with subvertical fold axes may be related to the formation of a very large right lateral kink band bear creek kink band about 8 km wide and at least 15 km long trending n27w along bear creek valley in the mt abbot quadrangle sierra nevada california

variations in deformation fields during development of a large volume magmatic arc central sierra nevada california mid to late cretaceous plutons in the central sierra nevada magmatic arc show widely preserved magmatic foliation whereas regionally developed solid state foliation is absent relatively slow cooling of these plutons and expected strain rates 10 14 suggest that the plutons were emplaced in a neutral or weakly extensional deformation regime domains of solid state ductile shear of only slightly younger age than the plutons on the other hand indicate a contractional regime timing of pluton emplacement and movement on the shear zones have been constrained using pb u zircon and 40 ar 39 ar hornblende and biotite geochronology both plutons and ductile shear zones become younger toward the east the four more westerly shear zones which were active between ca 100 and 90 ma show steeply plunging stretching lineations whereas the most easterly and or youngest zones active between ca 88 and 78 ma show mostly oblique and or subhorizontal stretching lineations indicating a change in kinematics at ca 90 ma the above events define a complex deformation pattern in which strain regimes fluctuated in time and space between neutral or weak extensional and contractional we propose a tectonic model in which thenospheric mantle corner flow produced eddy pairs in the mantle corner that transmitted a neutral or weak extensional regime to the overlying crust and facilitated the movement of granitic magma to mid and upper levels probably as dikes via fractures slab flattening caused the neutral or weak extensional regime to move eastward away from the trench increased coupling between upper and lower plates induced by the slab flattening promoted contractional strain in the cooling plutons and domains of ductile shear formed in progressively younger plutons to the east the above events were accompanied by an oblique convergence vector between north america and farallon plates engebretson et al 1985 which imposed a relatively small component of right lateral shear onto the arc that increased with time we estimate that at ca 100 ma the convergence vector made an angle obl 20 to the arc normal and we suggest that around ca 90 ma obl passed through a critical value conceivably 20 oblcrit 30 at this juncture the component of right lateral shear became sufficiently large to induce significant arc parallel strike slip movement on the most easterly shear zones these kinematics continued as the dominant scheme possibly as late as ca 78 ma

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

high temperature fracturing and ductile deformation during cooling of a pluton the lake edison granodiorite sierra nevada batholith california abstract in the bear creek area of the sierra nevada batholith california the high temperature postmagmatic deformation structures of the lake edison granodiorite include steeply dipping orthogneiss foliations joints and ductile shear zones that nucleated on joints and leucocratic dykes exploitation of segmented joints resulted in sharply bounded thin shear zones and in large slip gradients near the shear zone tips causing the deformation of the host rock at contractional domains the orthogneiss foliation intensifies towards the contact with the younger mono creek granite and locally defines the dextral rosy finch shear zone rfsz a major kilometre wide zone crosscutting the pluton contacts joints predominantly strike at n70 90 e over most of the lake edison pluton and are exploited as sinistral shear zones both within and outside the rfsz in a narrow 250 m thick zone at the contact with the younger mono creek granite within the rfsz the lake edison granodiorite includes different sets of dextral and sinistral shear zones joints the latter corresponding to the set that dominates over the rest of the lake edison pluton these shear zones joints potentially fit with a composite y r r shear fracture pattern associated with the rfsz or with a pattern consisting of y r shear fractures and rotated t mode i extensional fractures the mineral assemblage of shear zones and the microstructure and texture of quartz mylonites indicate that ductile deformation occurred above 500 c joints and ductile shearing alternated and developed coevally the existing kinematic models do not fully capture the structural complexity of the area or the spatial distribution of the deformation and magmatic structures future models should account more completely for the character of ductile and brittle deformation as these plutons were emplaced and cooled

four dimensional transform fault processes progressive evolution of step overs and bends many bends or step overs along strike slip faults may evolve by propagation of the strike slip fault on one side of the structure and progressive shut off of the strike slip fault on the other side in such a process new transverse structures form and the bend or step over region migrates with respect to materials that were once affected by it this process is the progressive asymmetric development of a strike slip duplex consequences of this type of step over evolution include 1 the amount of structural relief in the restraining step over or bend region is less than expected 2 pull apart basin deposits are left outside of the active basin and 3 local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes this type of evolution of step overs and bends may be common along the dextral san andreas fault system of california we present evidence at different scales for the evolution of bends and step overs along this fault system examples of pull apart basin deposits related to migrating releasing right bends or step overs are the pliopleistocene merced formation tens of km along strike the pleistocene olema creek formation several km along strike along the san andreas fault in the san francisco bay area and an inverted colluvial graben exposed in a paleoseismic trench across the miller creek fault meters to tens of meters along strike in the eastern san francisco bay area examples of migrating restraining bends or step overs include the transfer of slip from the calaveras to hayward fault and the greenville to the concord fault ten km or more along strike the offshore san gregorio fold and thrust belt 40 km along strike and the progressive transfer of slip from the eastern faults of the san andreas system to the migrating mendocino triple junction over 150 km along strike similar 4d evolution may characterize the evolution of other regions in the world including the dead sea pull apart the gulf of paria pull apart basin of northern venezuela and the hanmer and dagg basins of new zealand d 2004 elsevier b v all rights reserved

evidence from coseismic slip gradients for dynamic control on rupture propagation and arrest through stepovers received 30 july 2008 revised 14 november 2008 accepted 18 december 2008 published 27 february 2009 1 analysis of historic slip distributions from large magnitude continental strike slip earthquake ruptures reveals a pronounced correlation between the distance over which slip decreases as a rupture approaches a structural step in the fault and the ability of the rupture to propagate through the step our analysis of coseismic slip gradients near these stepovers indicates that in earthquakes in which slip decreases gradually toward a step rupture will not continue on the next segment conversely in earthquakes in which slip decreases abruptly rupture commonly renucleates on the next segment the fact that ruptures that stopped had low slip gradients near stepovers relative to those that continued indicates that rupture dynamics control the propagation of rupture through stepovers these results corroborate dynamic rupture models that show that ruptures in which slip decreases abruptly at a step generate strong seismic waves that serve to renucleate rupture on the opposite side of the structural step there are several potential causes for gradual decrease of displacement as rupture approaches a stepover including transference of deformation onto subsidiary structures rheological contrasts that could dictate favored rupture propagation directions the existence of stress shadows from previous earthquakes within the system or other material or frictional heterogeneities all of these are potentially observable and if mapped systematically could provide the basis for a strong constraint on the likely end points of future earthquakes inasmuch as earthquake magnitude is strongly dependent on the size of the rupture such predictions would be of great utility as a basic component of scenario based earthquake rupture forecasts

analog models of restraining stepovers in strike slip fault systems scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike slip fault systems in rigid basement models were run both with and without synkinematic sedimentation which was added incrementally to cover the growing antiformal structures vertical and horizontal sections of the completed models permit the full three dimensional 3 d structure of the pop ups to be analyzed in detail three representative end member experiments are described 30 underlapping restraining stepovers 90 neutral restraining stepovers and 150 overlapping restraining stepovers the experimental pop ups are typically sigmoidal to lozenge shaped antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults underlapping restraining stepovers typically form elongate lozenge shaped pop ups 90 neutral restraining stepovers produce shorter squat rhomboidal pop ups and overlapping restraining stepovers produce sigmoidal antiformal pop ups trans pop up cross fault systems are characteristic at large displacements on the basement fault system above the offset principal displacement zones the pop ups are commonly small narrow positive flower structures whereas in the stepover region they widen out and become markedly asymmetric this pop up asymmetry switches across the center of the stepover where the pop ups are largely symmetical maximum rotations measured within the central highly uplifted region of the pop ups increase from 7 counterclockwise for the underlapping 30 stepovers to 14 counterclockwise for the neutral 90 stepovers to 16 counterclockwise for the overlapping 150 stepovers in models where no synkinematic sediments were added during deformation the pop up structures are bound by convex flattening upward oblique slip reverse fault systems that link downward to the offsets in the basement fault system in contrast in the begin page 234 experiments where synkinematic sediments were added incrementally during deformation the pop ups are formed by oblique slip reverse faults that steepen upward into the synkinematic strata with the formation of fault propagation growth folds the analog models are compared with natural examples of pop up structures and show strong similarities in structural geometries and stratal architectures these models may provide structural templates for seismic interpretation of complex contractional structures in offset strike slip fault systems

viscous roots of active seismogenic faults revealed by geologic slip rate variations viscous flow in the deep crust and uppermost mantle can contribute to the accumulation of strain along seismogenic faults in the shallower crust1 it is difficult to evaluate this contribution to fault loading because it is unclear whether the viscous deformation occurs in localized shear zones or is more broadly distributed2 furthermore the rate of strain accumulation by viscous flow has a power law dependence on the stress applied yet there are few direct estimates of what the power law exponent is over the long term for active faults here we measure topography and the offset along fault surfaces created during successive episodes of slip on seismically active extensional faults in the italian apennines during the holocene epoch we show that these data can be used to derive a relationship between the stress driving deformation and the fault strain rate averaged over about 15 thousand years kyr we find that this relationship follows a well defined power law with an exponent in the range of 3 0 3 3 1 this exponent is consistent with nonlinear viscous deformation in the deep crust and crucially strain localization promoted by seismogenic faulting at shallower depths although we cannot rule out some distributed deformation we suggest that fault strain and thus earthquake recurrence in the apennines is largely controlled by viscous flow in deep localized shear zones over many earthquake cycles

the dynamics of strike slip step overs with linking dip slip faults fault step overs with linking dip slip faults are common features on long strike slip fault systems worldwide it has been noted by various researchers that under some circumstances earthquakes can jump across fault step overs to cascade into large events while under other circumstances rupture is arrested at step overs there is also evidence that fault step overs may be preferential locations for earthquake nucleation the present work uses the 3d finite element method to model the dynamics of strike slip fault systems with step overs and linking dip slip faults i find that the presence of a linking normal or thrust fault greatly increases the ability of earthquake rupture to propagate across the step over leading to a larger event additionally dilational step overs with linking normal faults are more prone to through going rupture than compressional step overs with linking thrust faults this difference is due to the sign of the normal stress increment on the dip slip fault caused by slip on the strike slip segments slip on the strike slip segments causes a negative unclamping normal stress increment on the linking normal fault in a dilational step over whereas the opposite effect occurs on the linking thrust fault in a compressional step over even in cases for which both dilational and compressional step overs can experience through going rupture dilational step overs typically experience higher slip particularly on the linking normal fault in the compressional case rupture nucleation on the linking thrust fault may increase the likelihood of through going rupture compared to nucleation on one of the strike slip segments near the intersections between the fault segments the stress interaction between the fault segments also causes a significant rotation of rake away from that which would be inferred from the regional stress field the results help to emphasize the importance of two way interactions between nearby fault segments during the earthquake rupture process the results also may have implications for the probability of large earthquakes along geometrically complex strike slip fault systems and may help explain why step overs sometimes act as barriers and other times as nucleation locations for large earthquakes

strain accommodation about strike slip fault discontinuities in granitic rock under brittle to ductile conditions abstract brittle and ductile structures adjacent to fault terminations and echelon fault steps of left lateral strike slip faults in the lake edison granodiorite of the central sierra nevada california are related to stress perturbations caused by fault slip the structures associated with fault terminations change from dilatant splay fractures only to splay fractures and ductile fabrics on opposing sides of the fault to ductile fabrics ahead of the fault with decreasing distance from a younger neighboring pluton and presumably with increased temperatures the distributions of deformation mechanisms correlate with the inferred local magnitudes and spatial distributions of the maximum tensile stress the mean stress and the maximum shear stress displacements along the faults are transferred across echelon stepovers by mineralized dilatant fractures in extensional steps to accommodate slip transfer across contractional steps rock apparently was squeezed vertically out of the step by ductile flow concentrations of mobile and immobile elements indicate that diffusive mass transfer was not significant increased mean stress in contractional steps apparently enhanced crystal plastic flow of quartz a flow law that includes a pressure effect through correlation with pressure dependent solidus temperatures is more successful than other experimentally derived flow laws in predicting the observed distribution of ductile fabrics

fault zone development and strain partitioning in an extensional strike slip duplex a case study from the mesozoic atacama fault system northern chile abstract upper crustal strike slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system detailed field mapping of the mesozoic atacama fault system in the coastal cordillera of northern chile documents the progressive development of second and third order faults forming a duplex at a dilational jog between two overstepping master faults the sinistral strike slip nnw striking jorgillo and bolfin faults these are constituted by a meter wide core of foliated s c ultracataclasite and cataclasite flanked by a damage zone of protocataclasite splay faults and veins lateral separation of markers along master faults is on the order of a few kilometers second order nw striking oblique slip subsidiary fault zones do not show foliated ultracataclasite lateral sinistral separations are in the range of 10 to 200 m with a relatively minor normal dip slip component in turn third order east west striking normal faults exhibit centimetric displacement oblique slip sinistral normal fault zones located at the southern termination of the bolfin fault form a well developed imbricate fan structure they exhibit a relatively simple architecture of extensional and extensional shear fractures bound by low displacement shear fractures kinematic analysis of fault slip data from mesoscopic faults within the duplex area document that the nw striking and the ew striking faults accommodate transtension and extension respectively examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure faults started from arrays of en echelon extensional extensional shear fractures that then coalesced into throughgoing strike slip faults further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement we interpret that the duplex formed by progressive linkage of horsetail like structures at the southern tip of the bolfin fault that joined splay faults coming from the jorgillo and coloso faults the geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike slip fault zones in the upper crust

integration of surface slip and aftershocks to constrain the 3d structure of faults involved in the m 7 3 landers earthquake southern california investigation of the mechanics of complex earthquakes requires insight into fault geometry we use linear elastic quasi static mechanical models of faults involved in the m 7 3 landers earthquake to test the influence of fault continuity dip and strike on fault slip and the perturbed stress field in regions of complex surface deformation and dense aftershock activity subsurface fault structure is constrained with both geological and geophysical data by comparing observed right lateral surface slip with slip along model faults and by comparing focal mechanisms with model coulomb planes at the locations of large aftershocks the refined structure of major faults at landers includes a southward extension of the johnson valley fault jvf at depth beyond the mapped fault trace a 75 w dip along the central johnson valley fault and a near surface discontinuity along the southern homestead valley fault hvf the final model better captures observed slip deficits along the johnson valley and homestead valley faults improves quantitative fit to the observed offset data and captures the local stress state in 9 of 12 locations studied relating aftershock orientations to local stresses resulting from mainshock fault slip provides a mechanical basis for deciphering between the two nodal planes provided by each focal mechanism slip along nonplanar faults with friction of 0 6 produces model failure planes in a variety of orientations indicating that heterogeneous aftershocks do not necessarily result from low fault friction mechanical selection of nodal planes suggests that aftershocks do not consistently reflect the orientation of their cluster or the orientation and slip direction of mainshock faults

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

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

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

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

mechanical aspects of pull apart basins and push up swells with applications to the dead sea transform abstract pull apart basins and push up swells are common features at steps along strike slip faults the structure of these features is examined here for two types of rheology perfect brittle and perfect ductile the stress fields derived for the step area predict that fault patterns mode of deformation and vertical displacement are different for the two rheologies these predictions are illustrated for basins and swells developed in laboratory experiments with modelling materials the structure of basins and swells along the dead sea transform is discussed here the pull apart basins display an intensively faulted southern end and a gently dipping flexure at the northern end a consistent dissimilarity appears between the structures of the eastern and western boundary faults of the basins according to the present interpretation these dissimilarities reflect a relatively ductile crust in arabia and a relatively brittle crust in sinai israel

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

geometric pattern rupture termination and fault segmentation of the dixie valley pleasant valley active normal fault system nevada u s a abstract active fault systems of both interplate and intraplate settings clearly show patterns of segmentation fault segmentation therefore is very important for understanding fault behavior and assessing seismic hazard the complexity of geometric patterns along fault systems has a strong influence on the propagation of earthquake ruptures because it can form structural heterogeneities that tend to inhibit rupture propagation this implies that there should be relations between fault segmentation and the complexity of fault geometry the active normal faults of the dixie valley pleasant valley system provide an excellent example to study such relations and to establish the geometric method of normal fault segmentation in the basin and range province studies of fault segmentation in the dixie valley pleasant valley fault system indicate that there are changes of both geometric pattern and geomorphic character in the area of the segment boundary the characteristics of segment boundaries on normal faults of the dixie valley pleasant valley system can be explained in terms of the nature of rupture propagation and termination these changes may be helpful criteria for distinguishing normal fault segmentation at least in the basin and range province

structure and mechanics of the hayward rodgers creek fault step over san francisco bay california a dilatational step over between the right lateral hayward and rodgers creek faults lies beneath san pablo bay in the san francisco bay area a key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step over enhancing its maximum possible magnitude if ruptures are terminated at the step over then another important issue is how strain transfers through the step we developed a combined seismic reflection and refraction cross section across south san pablo bay and found that the hayward and rodgers creek faults converge to within 4 km of one another near the surface about 2 km closer than previously thought interpretation of potential field data from san pablo bay indicated a low likelihood of strike slip transfer faults connecting the hayward and rodgers creek faults numerical simulations suggest that it is possible for a rupture to jump across a 4 km fault gap although special stressing conditions are probably required e g harris and day 1993 1999 slip on the hayward and rodgers creek faults is building an extensional pull apart basin that could contain hazardous normal faults we investigated strain in the pull apart using a finite element model and calculated a 0 02 mpa yr differential stressing rate in the step over on a least principal stress orientation nearly parallel to the strike slip faults where they overlap a 1 to 10 mpa stress drop extensional earthquake is expected on normal faults oriented per pendicular to the strike slip faults every 50 500 years the last such earthquake might have been the 1898 m 6 0 6 5 shock in san pablo bay that apparently pro duced a small tsunami historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step over possibly related to the earthquake we used a hydrodynamic model to show that a dip slip mechanism in north san pablo bay is the most likely 1898 rupture scenario to have caused the tsunami while we find no strike slip transfer fault between the hayward and rodgers creek faults a normal fault link could enable through going segmented rupture of both strike slip faults and may pose an independent hazard of m 6 earthquakes like the 1898 event

a mid crustal contractional stepover zone in a major strike slip system north cascades washington abstract the segmentation of strike slip faults at the earth s surface is well known but the geometry of these faults in the mid crust is poorly understood as are the intervening stepover zones a mid crustal example of a segmented strike slip fault is present within the 500 km long ross lake fault system where the nw striking twisp river fault zone and gabriel peak tectonic belt occur as overlapping left stepping dextral strike slip shear zones the contractional stepover between these structures is occupied by the black peak batholith which is riddled by ductile shear zones that are typically 0 5 1 5 m thick and are restricted to the stepover gently to moderately nw dipping reverse slip ductile shear zones in the center of the stepover strike at a high angle to the twisp river fault zone and gabriel peak tectonic belt and lie in an appropriate position to transfer most of the displacement between the strike slip segments the orientation of the reverse slip ductile shear zones relative to the strike slip segments is broadly similar to that of folds and thrusts in upper crustal stepovers the stepover width is relatively large however in comparison to many active stepovers emplacement of the black peak batholith across an originally narrower stepover or bend in the fault system may have created a rigid buttress and subsequent deformation jumped to the mechanically weak southwest margin of the body because plutons are commonly intruded into strike slip zones such partitioning of later displacements may be widespread

late cretaceous age of fractures in the sierra nevada batholith california regional sets of steeply dipping joints and faults are common throughout the sierra nevada batholith yet relatively little is known about how or when they formed within some east northeast striking left lateral fault zones in the mount abbot quadrangle of the central sierra nevada the host granodiorite is hydrothermally altered to a lower greenschist assemblage that contains muscovite the muscovite yields a mean k ar and sup 40 ar sup 39 ar age of 79 ma which provides a minimum age for the faulting field relations show that these faults developed from earlier formed mineralized joints so these ages also provide a minimum age for the jointing published ages of biotite hornblende and zircon from the host granodiorite of lake edison are 80 ma k ar 85 ma k ar and 90 ma u pb respectively the geochronology field relations and hydrothermal mineral assemblages together suggest that the mineralized joints and faults all formed between 85 and 79 ma soon after the host pluton was emplaced

subsurface structure and kinematics of the calaveras hayward fault stepover from three dimensional vp and seismicity san francisco bay region california the calaveras and hayward faults are major components of the san andreas fault system in the san francisco bay region dextral slip is presumed to transfer from the calaveras fault to the hayward fault in the mission hills region an area of uplift in the contractional stepover between the two faults here the estimated deep slip rates drop from 15 to 6 mm yr on the calaveras fault and slip begins on the hayward fault at an estimated 9 mm yr a lineament of microseismicity near the mission fault links the seismicity on the calaveras and hayward faults and is presumed to be related directly to this slip transfer however geologic and seismologic evidence suggest that the mission fault may not be the source of the seismicity and that the mission fault is not playing a major role in the slip transfer we perform a joint inversion for hypocenters and the 3d p wave velocity structure of the stepover region using 477 earthquakes we find strong velocity contrasts across the calaveras and hayward faults corroborated by geologic gravity and aeromagnetic data detailed examination of two seismic lineaments in conjunction with the velocity model and independent geologic and geophysical evidence suggests that they represent the southern extension of a northeasterly dipping hayward fault that splays off the calaveras fault directly accounting for the deep slip transfer the mission fault appears to be accommodating deformation within the block between the hayward and calaveras faults thus the calaveras and hayward faults need to be considered as a single system for developing rupture scenarios for seismic hazard assessments online material 3d interactive visualizations of the mission and alum rock hypocenters

impacts of off fault plasticity on fault slip and interaction at the base of the seismogenic zone direct observations of faults exhumed from mid crustal depths indicate that distributed inelastic deformation enhances fault slip and interaction across steps constrained by field measurements finite element models demonstrate that the slip distribution for a fault in a mises elastoplastic continuum differs significantly from that of a linear elastic model fault lobes of plastic shear strain align with fault tips and effectively lengthen the fault resulting in greater maximum slip and increased slip gradients near fault tips additionally distributed plastic shear strain facilitates slip transfer between echelon fault segments fault arrays separated by contractional steps which are subjected to greater mean normal stress and mises equivalent stress produce greater maximum slip than do those separated by extensional steps with no fractures these results provide insight into fault behavior at the base of the seismogenic zone with implications for rupture dynamics of discontinuous faults

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

deformation mechanics in inclined brittle ductile transpression zones insights from 3d finite element modelling abstract most natural examples of transpression zones developed at oblique convergence regime are inherently 3d and have inclined boundaries a 3d finite element model with an elasto plastic rheology is used to investigate the structural and mechanical evolution of inclined transpression zones in a rock sequence above a frictional basal detachment inelastic constitutive relationships allow permanent strains to develop in response to the applied loads fe modelling results show that oblique convergence is accommodated by discrete deformation at the main pre existing inclined faults 70 and by distributed brittle and ductile deformation at active blocks oblique contraction at the active blocks resulted mainly in layer parallel shortening orthogonal to the model outer boundaries whereas thickening in the horizontal and vertical directions was accommodated via layer parallel fault strike parallel extension and up dip extrusion i e inclined extrusion lateral extrusion should have compensated the rest and or volume loss took place folding and thickening of the mobile backstop produced a non cylindrical asymmetric bi vergent anticline where permanent strains developed principally in the steep forelimb secondary conjugate fault zones also accommodate oblique slip and contribute to uplift displacement vectors within the transpression zone are rotated counter clockwise ca 20 30 with respect to vectors in the fixed backstop areas with higher rotation values seem to correlate with those showing higher ellipticity values the presence of pre existing faults favored strain partitioning from the onset of deformation fe modelling results compared with analytical natural example and analogue modelling results show that our mechanical modelling can overall match inclined transpression zones geometry that present different modes of strain partitioning and localisation

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

thin pseudotachylytes in faults of the mt abbot quadrangle sierra nevada physical constraints for small seismic slip events abstract we document the occurrence of pseudotachylyte solidified melt produced during seismic slip along strike slip faults in the lake edison granodiorite of the mt abbot quadrangle sierra nevada california and provide constraints on ambient conditions during seismic faulting the pseudotachylytes are less than 0 3 mm thick and are found in faults typically up to 1 cm in thickness total measured left lateral offset along sampled faults is approximately 20 cm field and microstructural evidence indicate that the faults exploited pre existing mineralized joints and show the following overprinting structures with inferred ambient temperatures mylonites are more or less coeval with quartz veins 400 c cataclasites and pseudotachylytes 250 c more or less coeval with epidote veins and zeolite veins

testing constitutive equations for brittle ductile deformation associated with faulting in granitic rock uncertainty in constitutive equations for brittle ductile deformation limits our understanding of earthquake nucleation and propagation at the base of the seismogenic lithosphere to reduce this uncertainty we investigate exhumed strike slip faults and related deformation features in the lake edison granodiorite central sierra nevada ca that developed at 250 500 c and 250 mpa the seven gables outcrop contains a 10 cm wide contractional fault step separating two meter scale left lateral faults within the step a 4 cm thick leucocratic dike is stretched and rotated thus constraining the kinematics of deformation and the dike and surrounding granodiorite are strongly mylonitized petrographic and electron backscatter diffraction analyses reveal evidence for brittle and plastic deformation mechanisms including dislocation creep diffusion creep microfracturing and cataclasis we present a 2d finite element model of the seven gables outcrop that tests a series of candidate constitutive equations von mises elastoplasticity drucker prager elastoplasticity power law creep viscoelasticity two layer elastoviscoplasticity and coupled elastoviscoplasticity models based on von mises yielding most accurately match the outcrop deformation frictional plastic yield criteria i e drucker prager are incapable of reproducing the outcrop deformation due to the elevated mean compressive stress and reduced plastic yielding within the model fault step furthermore the power law creep viscoelastic model requires a high strain rate 10 4 s 1 to resolve slip on faults and fails to localize strain within the step region comparing model results and elastic stress fields with field observations suggests that deformation localizes in regions of elevated mean compressive stress and mises equivalent stress

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults