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

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

deformation structures from splay and decollement faults in the nankai accretionary prism sw japan iodp nantroseize expedition 316 evidence for slow and rapid slip in fault rocks

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

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

high resolution imaging of the bear valley section of the san andreas fault at seismogenic depths with fault zone head waves and relocated seismicity summary detailed imaging of fault zone fz material properties at seismogenic depths is a difficult seismological problem owing to the short length scales of the structural features seismic energy trapped within a low velocity damage zone has been utilized to image the fault core at shallow depths but these phases appear to lack sensitivity to structure in the depth range where earthquakes nucleate major faults that juxtapose rocks of significantly different elastic properties generate a related phase termed a fault zone head wave fzhw that spends the majority of its path refracting along the fault we utilize data from a dense temporary array of seismometers in the bear valley region of the san andreas fault to demonstrate that head waves have sensitivity to fz structure throughout the seismogenic zone measured differential arrival times between the head waves and direct p arrivals and waveform modelling of these phases provide high resolution information on the velocity contrast across the fault the obtained values document along strike fault normal and downdip variations in the strength of the velocity contrast ranging from 20 to 50 per cent depending on the regions being averaged by the ray paths the complexity of the fz waveforms increases dramatically in a region of the fault that has two active strands producing two separate bands of seismicity synthetic waveform calculations indicate that geological observations of the thickness and rock type of the layer between the two strands are valid also for the subsurface structure of the fault the results show that joint analysis of fzhws and direct p arrivals can resolve important smallscale elements of the fz structure at seismogenic depths detailed characterization of material contrasts across faults and their relation to earthquake ruptures is necessary for evaluating theoretical predictions of the effects that these structures have on rupture propagation

a shallow fault zone structure illuminated by trapped waves in the karadere duzce branch of the north anatolian fault western turkey summary we discuss the subsurface structure of the karadere duzce branch of the north anatolian fault based on analysis of a large seismic data set recorded by a local passcal network in the 6 months following the mw 7 4 1999 izmit earthquake seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long period oscillations in both p and s wave trains that do not exist in nearby off fault stations examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone the anomalous features in fault zone seismograms produced by events not necessarily in the fault may be referred to generally as fault zone related site effects the oscillatory shear wave trains after the direct s arrival in these seismograms are analysed as trapped waves propagating in a low velocity fault zone layer the time difference between the s arrival and trapped waves group does not grow systematically with increasing source receiver separation along the fault these observations imply that the trapping of seismic energy in the karadere duzce rupture zone is generated by a shallow fault zone layer traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3 4 km the synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m a velocity decrease relative to the surrounding rock of approximately 50 per cent and an s wave quality factor of 10 15 the results are supported by large 2 d and 3 d parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones the inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower type structure the motion amplification associated with fault zone related site effects increases the seismic shaking hazard near fault zone structures the effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large

internal structure of the median tectonic line fault zone sw japan revealed by borehole analysis abstract the internal structure of the median tectonic line mtl fault zone and the processes that prevailed at depths are described based on an analysis of a borehole the fault plane which defines the boundary between the ryoke and sanbagawa derived rocks dips at 56 to the north immediately beneath the boundary approximately 40 m thick fractured rocks form the major strand of the mtl fault zone the hanging wall above the boundary comprises variably deformed ryoke granitoids including several mylonite zones and cataclasite zones the fault zone has evolved through a series of faulting events under temperatures ranging from 400 to 200 c the mineral assemblages of the mylonites and cataclasites immediately above the boundary indicate that these fault rocks were formed at temperatures of about 300 c these mylonites and cataclasites represent therefore fault rocks that formed immediately below and above the brittle plastic transition respectively development of dissolution seams in these cataclasites suggests that the cataclasite has low strength the presence of pseudotachylytes in the cataclasite indicates the occurrence of seismicity immediately above the brittle plastic transition on the other hand the very fine grain size of recrystallised quartz in the mylonites indicates high differential stress immediately below the brittle plastic transition it is therefore likely that the differential stresses immediately below the brittle plastic transition are much higher than those immediately above the transition formation of laumontite in the major strand of the mtl fault zone occurred at temperatures of around 200 c the central slip zone of the major strand is about 30 cm thick and is surrounded by thick gouge zones this situation is favourable for thermal pressurisation during earthquake slips

temperature fields generated by the elastodynamic propagation of shear cracks in the earth w 1 the temperature maximum shifts toward the crack tip for faults having slip zone thickness of the order of centimeters or less the onset of thermally induced phenomena e g frictional melting thermal pressurization etc may occur at any point along the rupture depending on the degree of slip localization and rupture duration in the absence of significant increases in the pore fluid pressure localized fault slip may raise temperature by several hundred degrees sufficient to cause melting the onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area and high viscosity of silicate melts near solidus the inferred transient increases in the dynamic friction viscous braking are consistent with results of high speed rock sliding experiments and might explain field observations of the fault wall rip out structures associated with pseudotachylites possible effects of viscous braking on the earthquake rupture dynamics include 1 delocalization of slip and increases in the effective fracture energy 2 transition from a crack like to a pulse like rupture propagation or 3 ultimate rupture arrest assuming that the pulse like ruptures heal by incipient fusion the seismologic observations can be used to place a lower bound on the dynamic fault friction this bound is found to be of the order of several megapascals essentially independent of the earthquake size further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength index terms 3210 mathematical geophysics modeling 5104 physical properties of rocks fracture and flow 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 8010 structural geology fractures and faults

injection veins of crushing originated pseudotachylyte and fault gouge formed during seismic faulting this paper describes the injection veins of crushing originated pseudotachylyte and fault gouge formed along the iida matsukawa fault in granitic cataclasite southern nagano prefecture central japan the pseudotachylyte which shows a dense and aphanitic appearances and fault gouge occur as simple veins fault vein along the main fault plane and as complex network veins injection vein in the neighboring cataclasite locally the injection veins of pseudotachylyte and fault gouge can be traced back to the fault veins powder x ray diffraction patterns and petrological analysis indicate that both of the pseudotachylyte and fault gouge consist entirely of fine grained angular clasts and that the pseudotachylytes and fault gouges have a similar x ray diffraction patterns with that of the host granite the similarity of chemical compositions and distribution patterns of grain size also show that the injection veins of pseudotachylyte and fault gouge have the same source material as that of fault veins field occurrences and petrological characteristics strongly suggest that the injection veins of pseudotachylyte and fault gouge formed during seismic faulting by a rapid intrusion and fluidization of crushed fine grained materials generated in the shear zone in a gas solid fluid system

the relationship between slickenside surfaces in fine grained quartz and the seismic cycle abstract well polished or reflective fault surfaces with or without linear striae indicating the sliding direction are commonly referred to as slickensides this study examines the development of slickensides which are now exposed along the surface trace of a large seismically active normal fault zone in dixie valley nevada u s a geologic and mineralogic constraints indicate the slickenside surfaces formed at depths of less than 2 km and temperatures less than 270 c the slickenside surface material is composed of greater than 98 quartz with less than 2 kaolinite and iron oxide transmission electron microscope tem observations reveal that the slickenside surface material has an extremely fine yet variable grain size 0 01 1 m and an unusual nonequilibrium texture characterized by irregular grain boundaries and low dislocation density angular fragments in the cataclasite to either side of the slickensides provide clear evidence of cataclasis crystallographic preferred orientation in the slickenside surface material indicates that non brittle continuous deformation occurred within 0 1 10 mm of the fault surfaces in many areas non brittle continuous deformation must have alternated with cataclasis because some fragments in the cataclasite have strong preferred orientations we suggest that in the dixie valley slickensides continuous deformation and the development of crystallographic preferred orientations occurred at relatively low strain rates during the interseismic period while cataclasis occurred at higher strain rates associated with seismic events the wide range of strain rates occurring within a fault zone during the earthquake cycle may be an essential element in the formation of fine grained glassy slickensides both at dixie valley nevada and many other areas

observed source parameters for dynamic rupture with non uniform initial stress and relatively high fracture energy we have conducted dynamic rupture propagation experiments to establish the relations between insource stress drop fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 mpa to produce high fracture energy in the source we use a rough fault that has a large slip weakening distance an artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture making the initial stress non uniform shear stress particle velocity fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault normal distances stress drop increases weakly with normal stress average slip rate depends linearly on the fault strength loss and on static stress drop both with a nonzero intercept a minimum fracture energy of 1 8 j m 2 and a linear slip weakening distance of 33 mm are inferred from the intercept the large slip weakening distance also affects the average slip rate which is reduced by in source energy dissipation from on fault fracture energy because of the low normal stress and small per event slip w86 mm no thermal weakening such as melting or pore fluid pressurization occurs in these experiments despite the relatively high fracture energy and the very low heat production energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties the product of fracture energy and fault area is larger than the radiated energy seismic efficiency is low at w2 the ratio of apparent stress to static stress drop is w27 consistent with measured overshoot the fracture efficiency is w33 the static and dynamic stress drops when extrapolated to crustal stresses are 2e7 3 mpa and in the range of typical earthquake stress drops as the relatively high fracture energy reduces the slip velocities in these experiments the extrapolated average particle velocities for crustal stresses are 0 18e0 6 m s that these experiments are consistent with typical earthquake source properties suggests albeit indirectly that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops dominate earthquake source properties only for exceptional events unless crustal stresses are low published by elsevier ltd

earthquakes in the ductile regime pseudotachylytes from a crustal scale shear zone in central australia have developed in a cyclical manner once developed an individual pseudotachylyte is deformed in a ductile manner only to be overprinted at a later stage by a new generation of pseudotachylytes such cyclic generation and deformation of pseudotachylyte has been interpreted in the past as representing conditions at the brittleductile transition a different interpretation however is presented here it is proposed that psuedotachylytes and associated ultramylonites can develop entirely within the ductile regime as ductile instabilities such instabilities are different in nature to those previously discussed at length in the geophysical literature but are identical in principle with the instabilities that develop for velocity weakening frictional behavior in spring slider systems at a given strain rate a critical temperature tc is defined at which the transient work hardening equals the product of stress relaxation due to a thermal fluctuation and the heat generated by shearing a necessary condition for ductile instability at a given strain rate is that the temperature is belowtc then the rate of change of stress with respect to strain is negative an additional requirement is that this rate of change exceeds in magnitude the effective elastic stiffness of the loading system ductile instabilities are marginally possible at geological strain rates in quartzites but are possible at mid crustal temperatures in other rock types on the basis of these observations a new interpretation is presented for the base of the seismogenic zone in crustal regions

the thickness of subduction plate boundary faults from the seafloor into the seismogenic zone the thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior we have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges we describe a general geometric model for subduction thrust decollements which includes multiple simultaneously active anastomosing fault strands tens of meters thick the total thickness encompassing all simultaneously active strands increases to 100 350 m at 1 2 km below seafl oor and this thickness is maintained down to a depth of 15 km thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens ofmeters thick fault strands although fl attening primary inherited chaotic fabrics and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker to several kilometers this thickness does not describe the active fault at any depth these observations suggest that models should treat the subduction thrust plate boundary fault as 1 20 cm thick during earthquakes with a concentration of postseismic and interseismic creep in single to several strands 5 35 m thick with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands

the effect of water on strain localization in calcite fault gouge sheared at seismic slip rates abstract strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating the earthquake energy budget and the evolution of fault rock microstructure to assess the nature of strain localization during shearing of calcite fault gouges high velocity v max 1 m s rotary shear experiments at normal stresses of 3 20 mpa were conducted under room dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers when sheared at 1 m s the room dry gouges showed a prolonged strengthening phase prior to dynamic weakening whereas the wet gouges weakened nearly instantaneously microstructural analysis revealed that a thin

cataclasites along the saltville thrust u s a and their implications for thrust sheet emplacement abstract cataclasis and frictional wear are the primary bulk deformation mechanisms along steeply dipping portions of the saltville thrust in the southern appalachian foreland zone u s a fault character ranges from a single discrete sliding surface with negligible gouge to a zone of several discrete sliding surfaces or a zone up to 0 3 m thick of pervasive cataclasite marked fracturing occurs up to 20 m above the fault whereas minimal deformation is found in the footwall rocks hanging wall dolomites range from crush breccias less than 5 matrix to ultracataclasites with 90 matrix although cataclasites 50 70 matrix are predominant foliated cataclasites occur where dolomite is thrust over shale progressive development of cataclastic fabrics is due to comminution by fracturing and grinding along intersecting fractures continued frictional grinding results in complete disruption of the original fabric to produce cataclasite and minor ultracataclasite grain alignment occurs by rigid body rotation with subsequent local enhancement by pressure solution microstructural relations of the fault gouge suggest periodic fluctuations in fluid pressure where v ratio of fluid to overburden pressure probably ranged between 0 45 and 1 the saltville thrust sheet emplacement must have occurred in a caterpillar like fashion involving aseismic and seismic shear shear stresses accompanying fault motion as determined from dolomite twin lamellae are in the order of 65 mpa

concentrated slip zones with subsidiary shears their development on three scales in the cerro brass fault zone appalachian valley and ridge concentrated slip zones bound subsidiary shears at three scales within the cerro brass fault which cuts cambrian dolomites of the n any anticlinorium pennsylvania on the outcrop scale thrust faults along both cerro brass fault zone boundaries acted as concentrated slip zones bounding subsidiary shears developed along original bedding planes the outcrop scale subsidiary shears are zones of concentrated slip bounding hand specimen scale subsidiary shears in turn the hand specimen scale subsidiary shears are concentrated slip zones bounding even smaller scale subsidiary shears subsidiary shears in cerro brass fault zone are analogous to subsidiary shears i e ri and r2 commonly found in laboratory gouge friction experiments the orientations of subsidiary shears at the outcrop and hand specimen scales define a riedel within riedel geometry in which the original bedding played the role of r shears at the outcrop scale and at the same time operated as boundary faults at a smaller scale the presence of subsidiary shears on more than one scale suggests that the coulomb failure theory is not sufficient to explain their origin

extractable organic material in fault zones as a tool to investigate frictional stress abstract the detection of a frictional heating signature is an attractive but elusive way to measure frictional stress on exhumed faults with no evidence of melting we present a new paleothermometer for fault zones that utilizes the thermal maturity of extractable organic material to determine the maximum frictional heating experienced by the fault because there are no retrograde reactions in these organic systems maximum heating is preserved we investigate the punchbowl fault an ancient strand of the san andreas fault in southern california according to the methylphenanthrene index for thermal maturity there is no differential heating between the fault and off fault samples indicating that the fault did not get hotter than the surrounding rock during slip additionally the thermal maturity of the rocks falls within the range of heating expected from the bounds on burial depth and time and adds some constraint to the burial depth range simple heating models indicate that the fault must have had either 1 an extremely low shear stress during slip 2 an active slipping zone thicker than 1 mm or 3 earthquakes smaller than m6 to lack a differential heating signature after 44 km slip

occurrence of deep water corals on the mid atlantic ridge based on mar eco data occurrence of deep water corals on the mid atlantic ridge between the southern part of the reykjanes ridge and the azores has been examined based on video surveys using remotely operated vehicles rov and bycatch from longline and bottom trawl eight sites were surveyed with rovs and the bycatch material came from 16 trawl hauls and nine longline sets corals were observed at all sites surveyed with rovs at depths between 800 and 2400 m but most commonly shallower than 1400 m the species richness of corals was high with a total of 40 taxa recorded octocorals dominated the coral fauna with 27 taxa lophelia pertusa was one of the most frequently observed corals present at five of the eight surveyed sites it occurred on basaltic outcrops on the seamounts but always as relatively small colonies 0 5 m in diameter massive live reef structures were not observed the deepest record of lophelia was at 1340 m south of the charlie gibbs fracture zone accumulations of dead debris of coral skeletons could indicate a presence of former large lophelia reefs at several locations the number of megafaunal taxa was 1 6 times higher in areas where corals were present compared to areas without corals typical taxa that co occurred with lophelia were crinoids certain sponges the bivalve acesta excavata and squat lobsters signs of destructive fishing and lost gillnets were observed at several locations the impact of fishing on deep sea corals is discussed

dynamical weakening of faults by acoustic fluidization when a fault slips seismically some of the energy released may excite strong short wavelength vibrations near the fault core such vibrations can temporarily reduce the normal stress on the fault allowing it to slip at lower shear stresses than predicted by laboratory coefficients of friction this phenomenon of acoustic fluidization provides an alternative to theories that invoke pressurized fluids as an explanation for why some faults appear to be so weak

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

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

sidewall ripouts in strike slip faults abstract distinctive splay fault configurations in wallrocks associated with pseudotachylyte bearing strike slip fault structures at three different localities are recognized as sidewall ripouts sidewall ripout structures occur as asymmetric doubly tapered fault bounded lenses or slabs adjacent to a dominant planar fault surface scallop shaped lens ripouts develop in isotropic or weakly anisotropic host rocks and elongate slab ripouts develop in strongly anisotropic host rocks the tapered ends of these structures consist of leading contractional p type ramp structures at a low angle to the dominant planar fault surface and trailing extensional r or x type ramp structures at higher angles to the dominant fault surface the formation of ripout structures is attributed to transient variations in the coefficient of friction that cause adhesion between fault surfaces leading to rupture of the sidewalls ripouts are interpreted as large outcrop scale examples of adhesive wear during rupture of the sidewalls developed along planar fault surfaces during slip the asymmetry of the ripout structures in these exposures allows their use as a kinematic indicator where the narrow tapered leading end points in the direction of movement of the ripout lens or slab during adhesion to the opposing fault block wall

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

structure and composition of the plate boundary slip zone for the 2011 tohoku oki earthquake the mechanics of great subduction earthquakes are influenced by the frictional properties structure and composition of the plate boundary fault we present observations of the structure and composition of the shallow source fault of the 2011 tohoku oki earthquake and tsunami from boreholes drilled by the integrated ocean drilling program expedition 343 and 343t logging while drilling and core sample observations show a single major plate boundary fault accommodated the large slip of the tohoku oki earthquake rupture as well as nearly all the cumulative interplate motion at the drill site the localization of deformation onto a limited thickness less than 5 meters of pelagic clay is the defining characteristic of the shallow earthquake fault suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes

influence of grain boundary structural evolution on pressure solution creep rates intergranular pressure solution is a well known rock deformation mechanism in wet regions of the upper crust and has been widely studied especially in the framework of compaction of granular materials such as reservoir sandstones and fault rocks several analytical models exist that describe compaction creep by stress induced mass transport and the parameters involved are relatively well constrained by laboratory experiments while these models are capable of predicting compaction behavior observed at relatively high porosities they often overestimate compaction rates at porosities below 20 by up to several orders of magnitude this suggests that the microphysical processes operating at low porosities are different and are not captured well by existing models the implication is that available models cannot be extrapolated to describe compaction of sediments and fault rocks to the low porosities often reached under natural conditions to address this problem we propose a new thermodynamic model that describes the decline of pressure solution rates within individual grain contacts as a result of time averaged growth of asperities or islands and associated constriction of the grain boundary diffusion path here termed grain boundary evolution the resulting constitutive equations for single grain grain contacts are then combined and solved semianalytically the compaction rates predicted by the model are compared with those measured in high strain compaction experiments on wet granular halite a significant reduction in compaction rate is predicted when grain boundary evolution is considered which compares favorably with the experimental compaction data

strain localization and the onset of dynamic weakening in calcite fault gouge abstract to determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates single slide and slide hold slide experiments were conducted on 2 3 mm thick layers of calcite gouge at normal stresses up to 26 mpa and slip rates up to 1 m s 1 microstructures were analyzed from short displacement 35 cm experiments stopped prior to and during the transition to dynamic weakening in fresh calcite gouge layers dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness strain is initially distributed across the full thickness of the gouge layer but within a few millimeters displacement the strain becomes localized to a boundary parallel high strain shear band c 20 m wide during the strengthening phase which lasts between 3 and 30 cm under the investigated conditions the shear band broadens to become c 100 m wide at peak stress the transition to dynamic weakening in calcite gouges is associated with the nucleation of micro slip surfaces dispersed throughout the c 100 m wide shear band each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite interpreted to result from grain welding driven by local frictional heating in the shear band by the end of dynamic weakening strain is localized to a single 2 3 m wide principal slip surface flanked by layers of recrystallized gouge calcite gouge layers re sheared following a hold period weaken nearly instantaneously much like solid cylinders of calcite marble deformed under the same experimental conditions this is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide reducing the effective gouge layer thickness to a few microns our results suggest that formation of a high strain shear band is a critical precursor to dynamic weakening in calcite gouges microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate the delayed onset of dynamic weakening in fresh calcite gouge layers particularly at low normal stresses may inhibit large coseismic slip at shallow crustal levels in calcite bearing fault zones

progressive strain localization in a major strike slip fault exhumed from midseismogenic depths structural observations from the salzach ennstal mariazell puchberg fault system austria 1 analysis of a strike slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high strain fault core we focus on the ennstal segment of the 400 km long salzach ennstal mariazell puchberg semp strike slip fault system in the eastern alps which accommodated 60 km of left lateral displacement during oligo miocene time macroscopic and microscopic observations reveal a zoned fault featuring a high strain core at least 10 m wide within a fault zone that is at least 150 m wide grain size distribution analysis shows how the ennstal segment of the semp evolved our data reveal a 10 m wide high strain fault core characterized by a power law relationship of grain sizes d2 2 0 bordered by a 54 m wide transition zone where the largest and smallest grains are characterized by two power law relationships d2 2 0 and 1 6 respectively this zone is in turn bordered by a region with grain sizes that show a single low strain power law relationship of d2 1 6 we interpret these relationships to be the result of concentrated shear overprinting an initial low strain power law grain size distribution before strain localized to the core this is consistent with the theory that faults mature by smoothing geometrical complexities forming a highly localized high strain fault core the data do not support the idea that damage forms primarily in response to dynamic stresses during seismic rupture although they do suggest that this mechanism may operate within tens of meters of the fault once it has developed its zoned structure

ductility in fault gouge from a normal fault system death valley california a mechanism for fault zone strengthening and relevance to paleoseismicity fault gouge along the badwater turtleback fault system displays abundant evidence for mesoscopic ductility as foliation isolated and or rotated inclusions and isoclinal folds fault gouge thicknesses also vary dramatically at the outcrop scale gouge thicknesses can range from 0 to 1 m and at the 100 m scale they can range from 0 to 5 m because these variations are closely associated with other ductility related features they probably also formed because of ductile flow ductile flow appears to locally strengthen the fault zone by severe attenuation of the gouge and by transport of inclusions where gouge pinches out to zero thickness hanging wall rock rests directly on footwall rock a condition that produces an overall stronger fault zone as well as stick slip behavior in laboratory tests transport of inclusions within the gouge zone may create local inclusion jams which may also have a strengthening effect the presence of these stronger portions of the fault zone may give indirect evidence for ancient seismicity on the fault

structural and fluid chemical properties of seismogenic normal faults abstract structures in extensional fault zones can be classified using the configuration of fault branch lines and tip lines important classes include the segment bend segment termination segment branch cross fault intersection and segment offset the effect of these structures on rupture history is not necessarily consistent neither between individual earthquakes nor between different fault zones rupture behavior is dependent on several other factors including loading conditions regional and localized rupture tip stress fields fluid mechanical processes and chemical processes fracture toughness is partly controlled by the angular discordance between slip directions on adjacent fault segments greater discordance between slip directions and intersection lines of fault segments results in greater strain incompatibility an internal fracture network generally evolves within a segment boundary to maintain compatibility and transfer slip between the segments the dimensions and structure of this fracture network may also partly control rupture propagation presumably activation of a fault network with large angular discordances between slip directions and intersection lines will generate numerous asperities as the subsidiary faults mutually interact and offset each other the geometry of a segment boundary may change with depth and the three dimensional nature of the structure may be important in controlling rupture history fluids influence rupturing via fluid pressure effects and time dependent chemical processes fracture propagation by stress corrosion may favor instability and chemical alteration may produce minerals of lower strengths allowing time dependent creep sealing and healing of fractures however may remove damage and increase strength elementary computations indicate that representative times for sealing and chemical alteration are between 1 and 1000 years for reasonable physical conditions well within the recurrence intervals of most large earthquakes time to failure for stress corrosion cracking is more highly variable and strongly sensitive to applied stress and fluid pressure

spatial variability in microscopic deformation and composition of the punchbowl fault southern california implications for mechanisms fluid rock interaction and fault morphology abstract we examine the distribution and nature of microstructures geochemistry and mineralogy along two traverses across the punchbowl fault southern california to determine the morphology and deformation mechanisms of the fault zone in schistose rocks the punchbowl fault is an exhumed fault that has two main strands of slip localization and has a total of 44 km of right slip protolith in the study area consists of the pelona schist which is primarily a quartz albite muscovite actinolite schist with thin to medium banding and rare metabasalts the traverses are 1 3 km apart and were conducted at a site with a single fault strand and a site where both principal strands of the fault are exposed the fault zone thickness is a function of the type of measurement that is used to define it for the single strand site analysis of the distribution of microfractures shows that the fault zone consists of a roughly 40 m thick damaged zone adjacent to the fault core the damaged zone is marked by an increase in veins thin cataclasite bands inter and intragranular fracturing and alteration relative to the country rock brittle grain size reduction occurs in a zone 10 m thick as measured from the fault core which consists of a continuous 10 cm thick very fine grained cataclasite that experienced repeated alteration vein injection and grain size reduction whole rock geochemical analyses of the fault related rocks suggest that the geochemically defined fault zone is less than 10 m thick volume loss at the site with one fault strand appears to have been small the dominant alteration reactions associated with the fault core are the hydration of hornblende and actinolite accompanied by the alteration of muscovite to produce a quartz chlorite albite epidote ultracataclasite the composition of the fault core is variable and locally influenced by one of the adjacent protoliths the examination of the two fault strand sites shows that two damaged zone fault core structures are present the region between the two strands experienced a greater degree of deformation than the protolith but the total deformation is much less than immediately adjacent to the fault cores the total thickness of the damaged zone around the two strands is less than 200 m the fault core enveloped by a damaged zone morphology as well as the textures of the fault core rocks are similar to rocks associated with the north branch san gabriel fault which formed in crystalline rocks with a total displacement of 22 km fault thickness is less in the pelona schist than in the crystalline rocks perhaps owing to more efficient strain localization in the schists thus the faults of the san andreas system may be thinner in regions where schists or franciscan rocks are the protolith but the main fault core may be a constant feature of the fault zone transformation induced weakening is less important in this part of the punchbowl fault since the protolith has a large amount of mica the structure of the fault zone with two principal slip surfaces is marked by both chemical changes and microstructures and indicates that some parts of the san andreas fault system may consist of multiple slip surfaces each with a damaged zone that together may create a fault zone 100 m thick and in which slip is localized to zones meters to decimeters thick

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

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

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

structural analysis of a previously unknown active fault that triggered the 2013 mw 5 8 awajishima earthquake southwest japan the m w 5 8 awajishima earthquake occurred on 13 april 2013 in southwest awaji island 25 km southwest of the epicenter of the 1996 m w 6 8 kobe earthquake southwest japan analyses of aerial photographs and 3d perspective images field investigations and structural analysis of fault rocks reveal that 1 a previously undocumented fault here called the yamada fault strikes northwest southeast and dips southwest at 86 along a topographic lineament at the geological boundary between mesozoic granitic rocks and the late pliocene quaternary osaka group composed of interbedded sandstone and mudstone 2 the main shear zone of the yamada fault consists of a fault core with a 10 cm wide zone of fault gouge generally 1 5 cm a fault breccia zone of 100 cm wide and a damage zone of 10 50 m wide composed of cataclastic rocks and fractures 3 foliations characterized by s c fabrics that have developed in the shear zone indicate a dominantly thrust fault sense consistent with that revealed by the focal mechanism and 4 coseismic surface ruptures occurred locally along the main trace of the yamada fault consisting of numerous short fissures ranging in length from centimeters to several meters and concentrated in a zone 5 m wide our findings show that the newly identified yamada fault is an active fault and that it is probably the fault on which the 2013 m w 5 8 awajishima earthquake occurred therefore it is necessary to construct a fault model to better understand the deformation characteristics of the seismogenic source fault and for reassessing the seismic hazards on the densely populated awaji island of southwest japan

shear zones and rock history abstract shear zones that have widened with time can display margin to interior microstructural profiles unlike the real microstructural history of the interior rocks shear zones that have not widened with time can provide more reliable views of microstructural history and therefore of deformation processes even the latter zones can mislead however where there has been an annealing overprint during or following the shearing interval these sources of uncertainty need to be acknowledged more regularly and if possible eliminated by workers interpreting deformation processes from shear zone strain gradients

fault architecture and deformation mechanisms in exhumed analogues of seismogenic carbonate bearing thrusts abstract faults in carbonates are well known sources of upper crustal seismicity throughout the world in the outer sector of the northern apennines ancient carbonate bearing thrusts are exposed at the surface and represent analogues of structures generating seismicity at depth we describe the geometry internal structure and deformation mechanisms of three large displacement thrusts from the km scale to the microscale fault architecture and deformation mechanisms are all influenced by the lithology of faulted rocks where thrusts cut across bedded or marly limestones fault zones are thick tens of metres and display foliated rocks s cc tectonites and or ypr cataclasites characterized by intense pressure solution deformation in massive limestones faulting occurs in localized narrow zones that exhibit abundant brittle deformation a general model for a heterogeneous carbonate bearing thrust is proposed and discussed fault structure affected by stratigraphic heterogeneity and inherited structures influences the location of geometrical asperities and fault strain rates the presence of clay minerals and the strain rate experienced by fault rocks modulate the shifting from cataclasis dominated towards pressure solution dominated deformation resulting structural heterogeneity of these faults may mirror their mechanical and seismic behaviour we suggest that seismic asperities are located at the boundaries of massive limestones in narrow zones of localized slip whereas weak shear zones constitute slowly slipping portions of the fault reflecting other types of aseismic behaviour

seismicity on a fault controlled by rate and state dependent friction with spatial variations of the critical slip distance 1 we perform systematic simulations of slip using a quasi dynamic continuum model of a two dimensional 2 d strike slip fault governed by rate and state dependent friction the depth dependence of the a b and l frictional parameters are treated in an innovative way that is consistent with available laboratory data and multidisciplinary field observations various realizations of heterogeneous l distributions are used to study effects of structural variations of fault zones on spatiotemporal evolution of slip we demonstrate that such realizations can produce within the continuum class of models realistic features of seismicity and slip distributions on a fault we explore effects of three types of variable l distributions 1 a depth dependent l profile accounting for the variable width of fault zones with depth 2 uncorrelated 2 d random distributions of l with different degrees of heterogeneity and 3 a hybrid distribution combining the depth dependent l profile with the 2 d random l distributions the first type of l distribution with relatively small l over the depth range corresponding to the seismogenic zone and larger l elsewhere generates stick slip events in the seismogenic zone and ongoing creep above and below that region the 2 d heterogeneous parameterizations generate frequency size statistics with event sizes spanning 4 orders of magnitude our results indicate that different degrees of heterogeneity of l distributions control 1 the number of simulated events and 2 the overall stress level and fluctuations other observable trends are 3 the dependency of hypocenter location on l and 4 different nucleation phases for small and large events in heterogeneous distributions

healing behavior of simulated fault gouges from the groningen gas field and implications for induced fault reactivation we investigated the frictional strength recovery healing and subsequent reactivation and slip weakening behavior of simulated fault gouges derived from key stratigraphic units in the seismogenic groningen gas field n e netherlands direct shear slide hold slide shs experiments were performed at in situ conditions of 100 c 40 mpa effective normal stress and 10 15 mpa pore fluid pressure synthetic formation brine sheared gouges were allowed to heal for periods up to 100 days before subsequent reshearing the initial coefficient of steady sliding friction was highest in the basal zechstein caprock 0 65 0 02 and slochteren sandstone reservoir 0 61 0 02 gouges and the lowest in the ten boer claystone at the reservoir top 0 38 0 01 and in the carboniferous shale substrate 0 45 healing and subsequent reactivation led to a marked increase in static friction coefficient of up to 0 16 in basal zechstein and 0 07 in slochteren sandstone gouges for the longest hold periods investigated followed by a sharp strength drop up to 25 and slip weakening trajectory by contrast the ten boer and carboniferous gouges showed virtually no healing or strength drop healing rates in the basal zechstein and slochteren sandstone gouges were significantly affected by the stiffness of different machines used in line with the ruina slip law and with a microphysical model for gouge healing our results point to marked stratigraphic variation in healed frictional strength and healing rate of faults in the groningen system and high seismogenic potential of healed faults cutting the reservoir and basal zechstein caprock units upon reactivation

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

stability and localization of rapid shear in fluid saturated fault gouge 1 linearized stability analysis field observations of major earthquake fault zones show that shear deformation is often confined to principal slipping zones that may be of order 1 100 m wide located within a broader gouge layer of order 10 100 mm wide this paper examines the possibility that the extreme strain localization observed may be due to the coupling of shear heating thermal pressurization and diffusion in the absence of a stabilizing mechanism shear deformation in a continuum analysis will collapse to an infinitesimally thin zone two possible stabilizing mechanisms studied in this paper are rate strengthening friction and dilatancy for rate strengthening friction alone a linear stability analysis shows that uniform shear of a gouge layer is unstable for perturbations exceeding a critical wavelength using this critical wavelength we predict a width for the localized zone as a function of the gouge properties taking representative parameters for fault gouge at typical centroidal depths of crustal seismogenic zones we predict localized zones of order 5 40 m wide roughly consistent with field and experimental observations for dilatancy alone linearized strain rate perturbations with a sufficiently large wavelength will undergo transient exponential growth before decaying back to uniform shear the total perturbation strain accumulated during this transient strain rate localization is shown to be largely controlled by a single dimensionless parameter e which is a measure of the dilatancy of the gouge material due to an increase in strain rate

extensional duplexing in the york cliffs strike slip fault system southern coastal maine abstract a 150 m long sinistral strike slip fault system consisting of left stepping en echelon fault segments cuts deformed paleozoic metasediments and early mesozoic dike intrusions in coastal terrace outcrops in southern maine the longer fault segments follow the vertical bedding in the metasediments and accommodate approximately 1 m of displacement detailed mapping at scales down to 1 6 indicates that overlap zones between offset layer parallel fault segments consist of high dilation angular breccias rounded attrition breccias and extensional duplexes where the offset segments straddle upright quartzite layers in one 10 m long duplex r and r shears cut the layering in the quartzite at 28 and 69 respectively relative to the dominant layer parallel fault segments creating a doubly tapered imbricate r shear lens segmentation structure individual imbricate lenses accommodate additional layer parallel extension through conjugate x x shear fractures with average orientations of 109 and 60 respectively relative to the dominant fault segments extensional fractures oblique to the dominant fault segments are rare within this duplex but commonly form distinctive horsetail arrays at fault terminations that average 44 to the layer parallel faults strain analysis shows that approximately 60 80 cm of measured strike slip displacement is accommodated internally within this duplex as layer parallel extensional strain using the r shear and x x shear fracture mechanisms the strain distribution and observed cross cutting relations suggest an outward symmetric growth of the duplex structure from left stepping sinistral fault segments that developed sequences of tip to plane r shear overlap linkages during continued displacements cyclic r r to x x shear fracturing may relate to stress reorientation following r shear ruptures during episodic growth of the duplex and incremental slip of the fault system

co seismic frictional melting along an out of sequence thrust in the shimanto accretionary complex implications on the tsunamigenic potential of splay faults in modern subduction zones abstract out of sequence thrusts ost play an important role in the thickening of accretionary prisms formation of forearc basins and tsunami generation in subduction zones by detailed investigations of geological and paleogeothermal structures an ancient seismogenic ost is identified in the shimanto accretionary complex southwest japan this ost consists of a mappable en echelon fault system developed at a late stage of the accretionary process the estimated accumulated displacement of 2 5 8 5 km and formation depth of 2 5 5 5 km based on a thermal analysis indicates much higher fault activities sense of slip with a sinistral slip and the large displacement was distributed among multiple small faults each with a small offset most of the faults in the en echelon fault system recorded repeated brittle failure and one fault preserves pseudotachylyte a fault rock indicative of seismic faulting this is the first report of pseudotachylyte along an ost in an accretionary prism repeated fracturing in the same narrow shear zones each only a few millimeters thick suggests the osts perform as major shear localized zone in the accretionary prism shallow estimated formation depth and consequent low normal stress also support the hypothesis by a weak coupling along this fault zone these findings are in accord with a thrust activity in the shallow portion of the accretionary prism being associated with rupture propagation from the deep seismogenic region this ost showed high activity and repeated faulting in the shallow portion of the accretionary prism comparable to that of the submarine tsunamigenic ost in the present nankai trough

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

low to high velocity frictional properties of the clay rich gouges from the slipping zone of the 1963 vaiont slide northern italy the final slip of about 450 m at about 30 m s of the 1963 vaiont landslide italy was preceded by 3 year long creeping phase which was localized in centimeter thick clay rich layers 60 70 smectites 20 30 calcite and quartz here we investigate the frictional properties of the clay rich layers under similar deformation conditions as during the landslide 1 5 mpa normal stress 2 10 7 to 1 31 m s slip rate and displacements up to 34 m experiments were performed at room humidity and wet conditions with biaxial torsion and rotary shear apparatus the clay rich gouge was velocity independent to velocity weakening in both room humidity and wet conditions in room humidity experiments the coefficient of friction decreased from 0 47 at v 0 70 m s full lubrication results from the formation of a continuous water film in the gouge the vaiont landslide occurred under wet to saturated conditions the unstable behavior of the landslide is explained by the velocity weakening behavior of the vaiont clay rich gouges the formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero even without invoking the activation of thermal pressurization this explains the extraordinary high velocity achieved by the slide during the final collapse

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

shear zone broadening controlled by thermal pressurization and poroelastic effects during model earthquakes as a result of the comminution that takes place over numerous earthquake cycles mature faults are characterized by thick layers of pulverized gouge with finite porosity that is saturated with water at seismogenic depths the heat generated during earthquakes raises the gouge temperature and thermal expansion of the pore fluid and surrounding solids produce elevated pore pressures that cause fault strength to decrease in the process known as thermal pressurization building upon this framework we describe a model that imposes a plane strain configuration and shows that the stress variations caused by porothermoelasticity promote the mohr coulomb failure of previously undeformed regions except in special cases where the friction is rate strengthening we find that the frictional strength must vary throughout the post failure region which we identify in our model with the shear zone we introduce a strain rate function that describes the overall influence of distributed slip on energy dissipation and fault strength as the shear zone thickness expands using typical fault parameters at 7 km depth the shear zone reaches several millimeters of thickness after 1 s sliding at an overall rate of 1 m s the expansion of the shear zone limits the temperature rise to several hundred degrees celsius and the average fault strength falls to about a tenth of the static frictional strength

fault rocks within the blueschist metabasalts of the diamante terranova unit southern italy potential fossil record of intermediate depth subduction earthquakess vitale et al fault rocks within blueschist metabasalts

geologic evidence for multiple slip weakening mechanisms during seismic slip in crystalline rock we examine exhumed seismogenic faults to investigate the mechanisms that may have achieved dynamic fault weakening during ancient ruptures field and microscope observations imply more than one weakening mechanism must have been active during slip events on the faults pseudotachylytes that are continuous over the scale of field exposures are indicative of melt lubrication a fault breccia crosscutting earlier formed cataclasites was mobilized during faulting and possibly represents a pressurized fault rock that resulted from thermal pressurization or elastohydrodynamic lubrication in some faults pseudotachylytes are developed in patches several meters long cataclasites in which there is no evidence for melting are present immediately along strike from the pseudotachylyte patches by considering the energy required for melting we show that the pseudotachylytes must have formed during ruptures larger than the patches implying that the cataclasites also accommodated seismic slip the distribution of fault rock types shows that the frictional response to slip during a single event was spatially variable reworked pseudotachylytes also indicate that coseismic processes change over time at a point on a fault these observations emphasize that macroscopic dynamic fault weakening is a function of multiple coeval processes at microscales to mesoscales detailed observations of the discontinuous pseudotachylytes show that slip zone thickness is the critical parameter that controls active coseismic processes the frictional response of a fault to slip is therefore dependent on the internal structure of faults given the along strike heterogeneity of most mapped fault zones the coexistence of multiple slip weakening mechanisms in a single earthquake will be common

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

lithification facilitates frictional instability in argillaceous subduction zone sediments abstract previous work suggests that in subduction zones the onset of large earthquake nucleation at depths 5 10 km is likely driven by a combination of factors associated with the process of lithification at these depths lithification processes affect the entire fault system by modifying the mechanical properties of both the plate boundary fault zone and the wall rock to test the hypothesis that lithification of subduction zone sediments produces rocks capable of earthquake nucleation via diagenesis and low grade metamorphism we conducted friction experiments on fossil subduction zone sediments recovered from exposures in the shimanto belt in sw japan these meta sediments represent accreted and subducted material which has experienced maximum temperatures of 125 to 225 c which are representative of seismogenic depths along the active nankai subduction megathrust in the foreland of the shimanto belt we find that intact shimanto rock samples which preserve the influence of diagenetic and metamorphic processes exhibit the potential for unstable slip under in situ pressure conditions powdered versions of the same samples tested under the same conditions exhibit only velocity strengthening friction thus demonstrating that destroying the lithification state also removes the potential for unstable slip using advanced porosity loss to quantify the lithification process we demonstrate that increased velocity weakening correlates with increasingly advanced lithification in combination with documented frictionally stable behavior of subduction zone sediments from shallower depths our results provide evidence that the sediment lithification hypothesis can explain the depth dependent onset of large earthquake nucleation along subduction zone megathrusts

statistics of earthquake stress drops on a heterogeneous fault in an elastic half space we investigate properties of earthquake stress drops in simulations of evolving seismicity and stress field on a heterogeneous fault the model consists of an inherently discrete strike slip fault surrounded by a 3d elastic half space we consider various spatial distributions of frictional properties and analyze results generated by 150 300 model years in all cases the self organized heterogeneous initial stress distributions at the times of earthquake failure lead to stress drops that are systematically lower than those predicted for a homogeneous process in particular the large system sized events have stress drops that are consistently 25 of predictions based on the average fault strength the type and amount of assumed spatial heterogeneity on the fault affect the stress drop statistics of small earthquakes m l 5 more than those of the larger events this produces a decrease in the range of stress drops as the earthquake magnitudes increase the results can resolve the discrepancy between traditional estimates of stress drops and seismological observations the general tendency for low stress drops of large events provides a rationale for reducing the statistical estimates of potential ground motion associated with large earthquakes

high resolution structures of the landers fault zone inferred from aftershock waveform data summary high frequency body waves recorded by a temporary seismic array across the surface rupture trace of the 1992 landers california earthquake were used to determine fault zone structures down to the seismogenic depth we first developed a technique to use generalized ray theory to compute synthetic seismograms for arbitrarily oriented tabular low velocity fault zone models we then generated synthetic waveform record sections of a linear array across a vertical fault zone they show that both arrival times and waveforms of p and s waves vary systematically across the fault due to transmissions and reflections from boundaries of the low velocity fault zone the waveform characteristics and arrival time patterns in the record sections allow us to locate the boundaries of the fault zone and to determine its p and s wave velocities independently as well as its depth extent therefore the trade off between the fault zone width and velocities can be avoided applying the method to the landers waveform data reveals a low velocity zone with a width of 270 360 m and a 35 60 per cent reduction in p and s velocities relative to the host rock the analysis suggests that the low velocity zone extends to a depth of 7 km the western boundary of the low velocity zone coincides with the observed main surface rupture trace

hot on the trail coseismic heating on a localized structure along the muddy mountain fault nevada abstract recent advances in the use of thermal proxies to identify past earthquake slip in exhumed faults have created an extraordinary opportunity to map the geometry of past ruptures in unprecedented detail this approach can reveal along strike differences in the structure and speed of earthquake slip here we present organic thermal maturity data collected along the muddy mountain thrust in nevada to investigate where it was within the fault that earthquake slip occurred and whether this is consistent along strike we observe large changes in thermal maturity which represent temperature rise variations along the fault modeling of thermal maturity measurements yield peak temperatures of 760 1090 c where the principal slip zone psz is narrowest 0 8 mm nearby where the psz is thicker 1 5 mm these temperatures did not exceed 500 c based upon estimates of temperature rise within a submillimeter psz mean frictional work during earthquake slip was 6 3 7 1 mjm 2 these results show that thickness of the active slipping zone can vary at small spatial scales and development of a continuously narrow

fabric induced weakness of tectonic faults 1 mature fault zones appear to be weaker than predicted by both theory and experiment one explanation involves the presence of weak minerals such as talc however talc is only a minor constituent of most fault zones and thus the question arises what proportion of a weak mineral is needed to satisfy weak fault models existing studies of fault gouges indicate that 30 of the weak phase is necessary to weaken faults a proportion not supported by observations here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt talc provided the talc forms a critically aligned through going layer observations of foliated fault rocks in mature large offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults

al to oxide and ti to organic linkages in biogenic sediment relationships to paleo export production and bulk al ti abstract to increase our understanding of the mechanisms that control the distribution of al and ti within marine sediment we performed sequential extractions targeting the chemical signatures of the loosely bound exchangeable carbonate oxide organic opal and residual fraction of sediment from a carbonate dominated regime equatorial pacific and from a mixed opal terrigenous regime west antarctic peninsula we observe a systematic partitioning of al and ti between sediment phases that is related to bulk al ti we show that where we can quantify an alexcess component the dissolved al is preferentially affiliated with the oxide fraction resulting in al ti molar ratios of 500 3000 this is interpreted as the result of surface complexation in the water column of dissolved al onto oxyhydroxides we also observe a previously undetected tiexcess with as much as 80 of the total ti in the organic fraction which is most likely a function of metal organic colloidal removal from the water column in samples where the excess metals are obscured by the detrital load the al and ti are almost exclusively found in the residual phase this argues for the paired removal of al preferentially by the oxide component and ti preferentially by the organic component from the water column by settling particulate matter this research builds upon earlier work that shows changes in the bulk ratio of al to ti in carbonate sediment from the central equatorial pacific that coincide with changes in the sedimentary bulk accumulation rate bar the ratios that are observed are as much as three times higher than typical shale values and were interpreted as the result of scavenging of dissolved al onto particles settling in the water column because this non terrigenous alexcess accounts for up to 50 of the total sedimentary al inventory and correlates best with bar the bulk al ti may be a sensitive tracer of particle flux and therefore export production because we show that the excess metals are the result of scavenging processes the bulk al ti may be considered a sensitive proxy for this region