seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

origin and significance of clay coated fractures in mudrock fragments of the safod borehole parkfield california 1 the clay mineralogy and texture of rock fragments from the safod borehole at 3067 m and 3436 m measured depth md was investigated by electron microscopy sem tem and x ray diffraction xrd the washed and ultrasonically cleaned samples show slickenfiber striations and thin films of ca k bearing smectite that are formed on polished fault surfaces along freshly opened fractures and within adjacent mineralized veins the cation composition and hydration behavior of these films differ from the na montmorillonite of the fresh bentonite drilling mud although there is more similarity with circulated mud recovered from 3479 m md we propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the san andreas fault

seismically invisible fault zones laboratory insights into imaging faults in anisotropic rocks phyllosilicate rich rocks which commonly occur within fault zones cause seismic velocity anisotropy however anisotropy is not always taken into account in seismic imaging and the extent of the anisotropy is often unknown laboratory measurements of the velocity anisotropy of fault zone rocks and gouge from the carboneras fault zone in se spain indicate 10 15 velocity anisotropy in the gouge and 35 50 anisotropy in the mica schist protolith greater differences in velocity are observed between the fast and slow directions in the mica schist rock than between the gouge and the slow direction of the rock this implies that the orientation of the anisotropy with respect to the fault is key in imaging the fault seismically for example for fault parallel anisotropy a significantly greater velocity contrast between fault gouge and rock will occur along the fault than across it highlighting the importance of considering the foliation orientation in design of seismic experiments

elasticity of marine sediments rock physics modeling we offer an effective medium model for the elastic moduli of high porosity ocean bottom sediments the elastic constants of the dry sediment frame depend on porosity elastic moduli of the solid phase and effective pressure the model connects two end points in the elastic modulus porosity plane the hertz mindlin modulus of a dense elastic sphere pack at critical porosity and zero at 100 porosity the elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation unlike the suspension model our model assigns non zero elastic constants to the dry sediment frame and can predict the shear wave velocity unlike various modifications of the travel time average equation it is first principle based and contains only physical parameters we justify this model by matching sonic data in shallow marine sediments and in an odp well

release of mineral bound water prior to subduction tied to shallow seismogenic slip off sumatra plate boundary fault rupture during the 2004 sumatra andaman subduction earthquake extended closer to the trench than expected increasing earthquake and tsunami size international ocean discovery program expedition 362 sampled incoming sediments offshore northern sumatra revealing recent release of fresh water within the deep sediments thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial induced temperature increases in the past 9 million years complete dehydration of silicates is expected before plate subduction contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction shallow slip offshore sumatra appears driven by diagenetic strengthening of deeply buried fault forming sediments contrasting with weakening proposed for the shallow tohoku oki 2011 rupture but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records

effect of clay content and mineralogy on frictional sliding behavior of simulated gouges binary and ternary mixtures of quartz illite and montmorillonite 1 we investigated the frictional sliding behavior of simulated quartz clay gouges under stress conditions relevant to seismogenic depths conventional triaxial compression tests were conducted at 40 mpa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz montmorillonite and illite in all cases frictional strengths of mixtures fall between the end members of pure quartz strongest and clay weakest the overall trend was a decrease in strength with increasing clay content in the illite quartz mixture the trend was nearly linear while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers two micromechanical models were used to analyze the critical clay fractions for the two regime transitions on the basis of clay porosity and packing of the quartz grains the transition from regime 1 high strength to 2 intermediate strength is associated with the shift from a stress supporting framework of quartz grains to a clay matrix embedded with disperse quartz grains manifested by the development of p foliation and reduction in riedel shear angle the transition from regime 2 intermediate strength to 3 low strength is attributed to the development of shear localization in the clay matrix occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects

subsurface valleys and geoarcheology of the eastern sahara revealed by shuttle radar the shuttle imaging radar sir a carried on the space shuttle columbia in november 1981 penetrated the extremely dry selima sand sheet dunes and drift sand of the eastern sahara revealing previously unknown buried valleys geologic structures and possible stone age occupation sites radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand and alluvium filled valleys some nearly as wide as the nile valley and perhaps as old as middle tertiary the now vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat hyperarid region underfit and incised dry wadis many superimposed on the large valleys represent erosion by intermittent running water probably during quaternary pluvials stone age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation the presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present day oases which have been centers of episodic human habitation radar penetration of dry sand and soils varies with the wavelength of the incident signals 24 centimeters for the sir a system incidence angle and the electrical properties of the materials which are largely determined by moisture content the calculated depth of radar penetration of dry sand and granules based on laboratory measurements of the electrical properties of samples from the selima sand sheet is at least 5 meters recent september 1982 field studies in egypt verified sir a signal penetration depths of at least 1 meter in the selima sand sheet and in drift sand and 2 or more meters in sand dunes

evolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode laboratory and theoretical studies provide insight into the mechanisms that control earthquake nucleation when fault slip velocity is slow 0 001 cm s and dynamic rupture when fault slip rates exceed centimeters per second the application of these results to tectonic faults requires information about fabric evolution with shear and its impact on the mode of faulting here we report on laboratory experiments that illuminate the evolution of shear fabric and its role in controlling the transition from stable sliding v 0 001 cm s to dynamic stick slip v 1 cm s the full range of fault slip modes was achieved by controlling the ratio k k k c where k is the elastic loading stiffness and k c is the fault zone critical rheologic stiffness we show that k controls the transition from slow and silent slip k 0 9 to fast and audible k v 3 cm s slip duration 0 003 s slip events microstructural observations show that with accumulated strain deformation concentrates in shear zones containing sharp shear planes made of nanoscale grains which favor the development of frictional instabilities once this fabric is established fault fabric does not change significantly with slip velocity and fault slip behavior is mainly controlled by the interplay between the rheological properties of the slipping planes and fault zone stiffness

links between clay transformation and earthquakes along the costa rican subduction margin we investigate the depth distribution of smectite clay transformation and its along strike variability at the middle america trench offshore costa rica we take advantage of recent well constrained thermal models that refine our understanding of the margin s thermal structure and which capture significant along strike variability using these thermal models together with sediment compositions defined by drilling we compute the distribution of smectite transformation and associated fluid production we show that the hypocenters of large m 6 9 well located megathrust earthquakes lie consistently down dip of peak fluid production we suggest that silica cementation associated with smectite transformation promotes lithification and slip weakening behavior that in combination with declining fluid pressures facilitate the initiation of unstable slip the earthquake ruptures extend up dip into the region of peak reaction possibly due to excess pore pressures that facilitate their propagation these results are consistent with the hypothesis that smectite transformation contributes to the onset of stick slip behavior and acts as an important control on earthquake nucleation and propagation

shuttle imaging radar physical controls on signal penetration and subsurface scattenng in the eastern sahara sir a signal penetration and subsurface backscatter within the upper meter or so of the sediment blanket in the eastern sahara of southern egypt and northern sudan are enhanced both by radar sensor parameters and by the physical and chemical characteristics of eolian and alluvial materials interpretation of sir a images by mccauley et al 1 2 dramatically changed previous concepts of the role that fluvial processes have played over the past 10000 to 30000000 years in shaping this now extremely flat featureless and hyperarid landscape in the present paper we summarize the near surface stratigraphy the electrical properties of materials and the types of radar interfaces found to be responsible for different classes of sir a tonal response the dominant factors related to efficient microwave signal penetration into the sediment blanket include a favorable distribution of particle sizes b extremely low moisture content and c reduced geometric scattering at the sir a frequency 1 3 ghz the depth of signal penetration that results in a recorded backscatter here called radar imaging depth was documented in the field to be a maximum of 1 5 m or 0 25 of the calculated skin depth for the sediment blanket radar imaging depth is estimated to be between 2 and 3 m for active sand dune materials diverse permittivity interfaces and volume scatterers within the shallow subsurface are responsible for most of the observed backscatter not directly attributable to grazing outcrops

healing of the shallow fault zone from 1994 1998 after the 1992 m7 5 landers california earthquake we conducted seismic surveys at the johnson valley fault in 1994 1996 and 1998 we found that the shear velocity of the fault zone rock increased by 1 2 between 1994 and 1996 and increased further by 0 7 between 1996 and 1998 this trend indicates the landers rupture zone has been healing by strengthening after the mainshock most likely due to the closure of cracks that opened during the 1992 earthquake the observed fault zone strength recovery is consistent with a decrease of 0 03 in the apparent crack density within the fault zone the ratio of decrease in travel time for p to s waves changed from 0 75 in the earlier two years to 0 65 in the later two years between 1994 and 1998 suggesting that cracks near the fault zone are partially fluid filled and have became more fluid saturated with time

regional dust deposits on mars physical properties age and history major dust storms on mars play an important role in the deposition and removal of fine dust material thermal radar and visual remote sensing observations provide important constraints on the martian regolith which have been used to determine the location and physical properties of regional dust deposits these deposits are located in three northern equatorial regions tharsis 20 s to 50 n 60 to 190 w arabia 5 s to 30 n 300 to 360 w and elysium 10 to 30 n 210 to 225 w they are covered by fine 2 40 m bright albedo 0 27 particles with fewer exposed rocks and coarse deposits than found elsewhere dust is currently deposited uniformly throughout the equatorial region at a rate of 40 m global storm over geologic time the rate of accumulation may vary from 0 to 250 m yr due to changes in atmospheric conditions produced by orbital variations dust deposited during global storms is subsequently removed only from dark regions resulting in a net accumulation in the low inertia bright regions the thickness of these current dust deposits is 0 1 2 m the thermal inertia places a lower limit of 0 1 m on the thickness of these deposits while the sparse but ubiquitous presence of exposed rocks and the degree of visible mantling indicate that the thickness is less than 5 m dual polarization radar observations of a very rough texture in tharsis are consistent with this model with a 2 m thick dust layer burying most of the surface rocks but permitting radar sampling of the rough subsurface based on their thickness and rate of accumulation the age of these deposits is 105 106 years suggesting a cyclic process of deposition and removal one possible cause may be cyclic variations in the magnitude and location of maximum wind velocities related to variations in mars orbit at present perihelion and maximum wind velocities occur in the south whereas regional dust deposits occur in the north suggesting net transport from south to north orbital parameters oscillate with periods ranging from 5 104 to 106 years the agreement between these periods and the dust deposit age suggests a possible link at different stages in orbit evolution maximum wind velocities will occur in the north with subsequent erosion and redistribution of the accumulated fines based on this model much of the uppermost martian surface is very young and is being continually reworked

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

the nature and importance of phyllonite development in crustal scale fault cores an example from the median tectonic line japan abstract like many large crustal scale faults the median tectonic line mtl in sw japan has a long history of movement having been active predominantly as a strike slip fault since the mid cretaceous fault rock exposures in the core of the mtl preserve a history of deformation at a range of mid to shallow crustal depths ryoke mylonites 1 4 km north of the main contact record deeper level cretaceous top to the south sinistral movements the remainder of the fault zone core is surprisingly narrow exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn tectonic fluid influx over a range of deformation conditions exposures within 50 m of the central slip zone display a progressive sequence in fault rock evolution from ultramylonite cataclasite foliated cataclasite phyllonite breccia gouge this sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone this leads to the replacement of load bearing phases such as feldspar by fine grained foliated aggregates of intrinsically weaker phyllosilicates such as white mica and chlorite the grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid assisted stress induced diffusive mass transfer mechanisms comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the mtl will lead to long term weakening provided the network of phyllosilicate rich fault rocks are able to form an interconnected thin layer of weak material on kilometre to tens of kilometre length scales

dust streaks on mars global mapping and photometry of selected areas on mars are used to investigate the nature of bright and dark wind streaks that extend from topographic obstacles occurrence of both bright and dark streaks is strongly latitude dependent and is only weakly correlated with surface properties such as albedo and thermal inertia data on the colors albedos and phase behavior of streaks are consistent with models of bright streaks as mosaics of plains material and brighter redder dust less than 20 of the ground need be covered by the optically thick dust in the brightest parts of the streaks the amount of dust in optically thick layers could be as little as 10 3 g cm2 dark streaks can be interpreted as erosional windows in a patchy dust cover our model of dust deposition in optically thick patches is sedimentologically different from scenarios involving the deposition of ubiquitous optically thin layers it has the advantage that large amounts of dust can be deposited without affecting regional albedos

multi scale compressional wave velocity structure of the san gregorio fault zone 1 understanding fault architecture at multiple scales is crucial to delineate in situ fault zone physical properties and rupture dynamics through modeling and geophysical imaging monitoring an exposure of the active large offset strike slip san gregorio fault at moss beach ca provides a unique field site to relate the well mapped fault zone architecture with compressional wave velocity vp structure measured at centimeter to meter scales laboratory ultrasonic velocities of fault zone samples adjusted for fluid related frequency and structural dispersion indicate that i a seismic velocity reduction of 30 characterizes the central smectite rich clay gouge relative to the rocks 100 m away in the relatively undeformed host rocks and ii the across fault velocity profile trends for the seismic to ultrasonic bandwidth correlate almost exactly to the previously mapped macroscale fault zone structure these results highlight the value of conducting multiscaled investigations when measuring fault zone properties defined by physical elements at multiple scale lengths

microstructure of apatite replaceing carbonate in synthesized and natural samples abstract the structure of the apatite replacing carbonate shells and oolites in experimental apatite synthesis by bacterial activity has been shown to be microspherical size a fraction of one micron to several microns and type of the microspheres spherical or ovoid ovoid or cauliflowerlike isolated siamese twinlike or in chains fit both the size and type of monocellular organisms very well the pseudomorphic replacement of carbonates by apatite is well known in natural phosphorites however the detailed structure of the replacing apatite has not yet caught the eye of petrographers bacterial structures in phosphorites have been reported recently but these structures were found isolated rather than accurately located within phosphatized shells our results show that while the initial aragonitic structure of phosphatized shells is prismatic the pseudomorphic apatite is microspherical and phosphatization appears to spread beyond the shell walls towards the center of the chambers all stages between the almost empty and the totally infilled apatite phosphatized shells could be observed in one sample from morocco the similarity of the apatite microstructures in experimental and natural samples lends further support to the genesis model we proposed earlier that combination of phosphorus from rna released by bacteria and calcium from carbonate is one of the possible genesis models of apatite in nature

climbing zibars of the algodones abstract zibars are coarse grained low relief slipfaceless eolian bedforms that occur on sand sheets and within interdune corridors of many eolian sand seas zibar surface features consist exclusively of wind ripples and local shadow and shrub coppice dunes associated with vegetation a study of zibars located on the southwestern margin of the algodones dune field california shows that zibars and their associated interzibar areas are migrating features oriented transverse to oblique to the resultant sand transport direction zibars leave deposits formed by deposition on the zibar lee face the zibar shape modifies the near surface airflow to produce a subtle grain size change across the zibar interzibar surface flow expansion not separation leeward of the zibar crest causes the lee face deposits those preserved upon bedform migration to have the finest grain size on the zibar interzibar surface sediment coarser than that deposited on the zibar lee face is transported more slowly and less frequently and thus becomes concentrated on the zibar stoss slope and ultimately becomes interzibar deposits migration of zibars and interzibar areas can produce two characteristic depositional sequences depending upon the angle of climb for low angles of climb a sequence of stacked amalgamated interzibar deposits is preserved zibar deposits are absent amalgamated interzibar deposits consist of low angle usually less than 5 wind ripple laminae of medium to coarse sand given higher angles of climb deposits of both the entire interzibar and zibar lower lee face are preserved low angle interzibar deposits of medium to coarse sand are overlain tangentially by finer grained fine to medium sand wind ripple laminae dipping less than 15 of the zibar lee face lee face wind ripple laminae are sharply truncated and overlain by the succeeding set of interzibar wind ripple deposits this zibar interzibar sequence is distinctive and allows for the detailed interpretation of some ancient sand sheet deposits as left by migrating zibars a possible example of ancient zibar deposits is present in the entrada sandstone jurassic of southwestern colorado

chemical composition of martian fines of the 21 samples acquired for the viking x ray fluorescence spectrometer 17 were analyzed to high precision compared to typical terrestrial continental soils and lunar mare fines the martian fines are lower in al higher in fe and much higher in s and cl concentrations protected fines at the two lander sites are almost indistinguishable but concentration of the element s is somewhat higher at utopia duricrust fragments successfully acquired only at the chryse site invariably contained about 50 higher s than fines no elements correlate positively with s except cl and possibly mg a sympathetic variation is found among the triad si al ca positive correlation occurs between ti and fe sample variabilities are as great within a few meters as between lander locations 4500 km apart implying the existence of a universal martian regolith component of constant average composition the nature of the source materials for the regolith fines must be mafic to ultramafic

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

effect of hydration state on the frictional properties of montmorillonite based fault gouge 1 we report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge we tested four mixtures of ca montmorillonite and quartz 100 70 50 and 30 montmorillonite at four hydration states dry 16 0 wt water we controlled the hydration state using either oven drying for 13 wt h2o under conditions of controlled relative humidity for each clay quartz mixture and hydration state we measured frictional properties over a range of normal stresses 5 100 mpa and sliding velocities 1 300 m s we observe a systematic decrease in the coefficient of friction with increasing water content normal stress and clay content values of for 50 50 mixtures range from 0 57 to 0 64 dry and decrease to 0 21 0 55 for the most hydrated cases wet for layers of 100 montmorillonite ranges from 0 41 0 62 dry to 0 03 0 29 wet as water content is increased from 0 to 20 0 wt the friction rate parameter a b becomes increasingly positive variation in a b values decreases dramatically as normal stress increases if our experimental results can be applied to natural fault gouge the combination of stress state hydration state and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite

low velocity damaged structure of the san andreas fault at parkfield from fault zone trapped waves 1 we used dense linear seismic arrays across and along the san andreas fault saf at parkfield california to record fault zone trapped waves generated by explosions and microearthquakes in 2002 prominent trapped waves appeared at stations close to the saf main fault trace while some energy was trapped in the north strand at the array site observations and 3 d finite difference simulations of trapped waves at 2 5 hz show evidence of a damaged core zone on the main saf the zone from the surface to seismogenic depths is marked by a low velocity waveguide 150 m wide in which q is 10 50 and shear velocities are reduced by 30 40 from wall rock velocities with the greatest velocity reduction at shallow depth we interpret that this distinct low velocity zone on the main saf is a remanent of damage due to past large earthquakes on the principal fault plane at parkfield a less developed low velocity zone may be evident on the north strand that experienced minor breaks in the 1966 m6 event

shear zones in clay rich fault gouge a laboratory study of fabric development and evolution abstract clay rich fault rocks have long been recognized to host distinctive fabric elements and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime although the geometries of fabric elements in fault gouges have been described for almost a century the genesis and evolution of these elements during shear and their links to bulk mechanical properties remain poorly understood we characterize the development and evolution of fabric elements with increasing shear in a variety of clay rich experimental gouges over shear strains of 20 and at normal stresses of 2 150 mpa in the double direct shear configuration in addition to sem observations of experiment products at a variety of shear strains we quantified clay fabric intensity and the degree of grain size reduction using x ray texture goniometry xtg and particle size distribution psd measurements we also measured p and s wave velocities during shear to further probe the evolution of shear fabric and gouge properties we find that clay fabric elements develop in a systematic manner regardless of the gouge material riedel shears in the r1 orientation and boundary parallel shears are the dominant fabric elements riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress clay particles rotate into the p orientation shortly after riedels propagate through the layer the riedel shears are through going but are 10 thinner than similar zones observed in coarser granular materials our results suggest that the weakness of clay rich fault gouge may be less a function of anisotropic crystal structure as has been suggested previously and more a consequence of very thin shear surfaces permitting deformation in clay rich materials with minimal dilation or cataclasis the very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal psd s

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

frictional properties and sliding stability of the san andreas fault from deep drill core the strength of tectonic faults and the processes that control earthquake rupture remain central questions in fault mechanics and earthquake science we report on the frictional strength and constitutive properties of intact samples across the main creeping strand of the san andreas fault saf california united states recovered by deep drilling we find that the fault is extremely weak friction coefficient 0 10 and exhibits both velocity strengthening frictional behavior and anomalously low rates of frictional healing consistent with aseismic creep in contrast wall rock to the northeast shows velocity weakening frictional behavior and positive healing rates consistent with observed repeating earthquakes on nearby fault strands we also document a sharp increase in strength to values of 0 40 over 1 m distance at the boundary between the fault and adjacent wall rock the friction values for the saf are sufficiently low to explain its apparent weakness as inferred from heat flow and stress orientation data our results may also indicate that the shear strength of the saf should remain approximately constant at 10 mpa in the upper 5 8 km rather than increasing linearly with depth as is commonly assumed taken together our data explain why the main strand of the saf in central california is weak extremely localized and exhibits aseismic creep while nearby fault strands host repeating earthquakes

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

frictional heterogeneities on carbonate bearing normal faults insights from the monte maggio fault italy observations of heterogeneous and complex fault slip are often attributed to the complexity of fault structure and or spatial heterogeneity of fault frictional behavior such complex slip patterns have been observed for earthquakes on normal faults throughout central italy where many of the mw 6 to 7 earthquakes in the apennines nucleate at depths where the lithology is dominated by carbonate rocks to explore the relationship between fault structure and heterogeneous frictional properties we studied the exhumed monte maggio fault located in the northern apennines we collected intact specimens of the fault zone including the principal slip surface and hanging wall cataclasite and performed experiments at a normal stress of 10 mpa under saturated conditions experiments designed to reactivate slip between the cemented principal slip surface and cataclasite show a 3 mpa stress drop as the fault surface fails then velocity neutral frictional behavior and significant frictional healing overall our results suggest that 1 earthquakes may readily nucleate in areas of the fault where the slip surface separates massive limestone and are likely to propagate in areas where fault gouge is in contact with the slip surface 2 postseismic slip is more likely to occur in areas of the fault where gouge is present and 3 high rates of frictional healing and low creep relaxation observed between solid fault surfaces could lead to significant aftershocks in areas of low stress drop

conditions favourable for the formation of warm climate aeolian sand sheets aeolian sand sheets are areas of aeolian sand where dunes with slipfaces are generally absent sand sheets are ubiquitous to modern warm climate sand seas generally occurring marginal to dune fields although they may exist within the interior of a sand sea or independent of a dune field sand sheet deposits are recognized in ancient aeolian sequences where they may account for significant accumulations of low angle aeolian stratification we suggest that the occurrence of sand sheets instead of dunes indicates that conditions are outside the range within which dunes form or that one or more factors interfere with dune development while also favouring the accumulation of sand sheets a study of six modern sand sheets in north america located at great sand dunes gran desierto dumont algodones padre island and colorado river delta indicates that the factors favourable for sand sheet development are 1 a high water table 2 surface cementation or binding 3 periodic flooding 4 a significant coarse grained sediment population and 5 vegetation these factors are reflected in the nature of stratification and the accessory features of sand sheet accumulations within the areas of modern sand sheets as well as in their ancient counterparts in the triassic dolores and pennsylvanian permian rico formations

experimental investigation of incipient shear failure in foliated rock abstract it has long been known that rock fabric plays a key role in dictating rock strength and rheology throughout earth s crust however the processes and conditions under which rock fabric impacts brittle failure and frictional strength are still under investigation here we report on laboratory experiments designed to analyze the effect of foliation orientation on the mechanical behavior and associated microstructures of simulated fault rock sheared at constant normal stress of 50 mpa intact samples of pennsylvania slate were sheared with a range of initial fabric orientations from 0 to 165 with respect to the imposed shear direction foliation orientations of 0 30 produce initial failure at the lowest shear stress while samples oriented at higher angles are less favorable resulting in higher peak failure strength in all samples post peak development of through going deformation zones in the r1 riedel orientation results in a residual strength that is lower than that observed for powdered gouge from the same material as shear strain increases all samples approach a residual apparent friction defined by the ratio of shear strength to normal stress of 0 4 however the final deformation microstructure depends strongly on the orientation of the pre existing foliation when fabric is oriented at low angles to the shear direction

seismic velocities and poisson s ratio of shallow unconsolidated sands we determined p and s wave velocity depth profiles in shallow unconsolidated beach sand by analyzing three component surface seismic data p and s wave velocity profiles were calculated from traveltime measurements of vertical and tangential component seismograms respectively the results reveal two discrepancies between theory and data whereas both velocities were found to be proportional to the pressure raised to the power of 1 6 as predicted by the hertz mindlin contact theory the actual values of the velocities are less than half of those calculated from this theory we attribute this discrepancy to the angularity of the sand grains assuming that the average radii of curvature at the grain contacts are smaller than the average radii of the grains we modify the hertz mindlin theory accordingly we found that the ratio of the contact radius to the grain radius is about 0 086 the second disparity is between the observed poisson s ratio of 0 15 and the theoretical value 0 008 for random pack of quartz spheres this discrepancy can be reconciled by assuming slip at the grain contacts because slip decreases the shearing between grains poisson s ratio increases

deflection of sand movement on a sinuous longitudinal seif dune use of fluorescent dye as tracer abstract sand was marked by fluorescent dye in order to trace sand movement and deposition on a longitudinal seif sand dune in the sinai desert the wind regime was monitored simultaneously tracing the dyed sand was possible after light to moderate sand storms and was graphically represented on maps the dune was subjected to a seasonally bidirectional wind regime with the wind hitting the dune obliquely on either side on the windward flank the sand was transported parallel to the wind direction on the lee flank sand movement was deflected towards parallelism with the crest line sand movement was deflected if the dune had a sharp profile which favored separation of wind flow on the lee flank the deflection depended on the angle of incidence between the wind and the crest line when the angle of incidence was

experimental constraints on the relationship between clay abundance clay fabric and frictional behavior for the central deforming zone of the san andreas fault the presence of smectite saponite in fault gouge from the central deforming zone of the san andreas fault at parkfield ca has been linked to low mechanical strength and aseismic slip however the precise relationship between clay mineral structure fabric development fault strength and the stability of frictional sliding is not well understood we address these questions through the integration of laboratory friction tests and fib sem analysis of fault rock recovered from the san andreas fault observatory at depth safod borehole intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or safod clasts extracted from the sample nano textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges such slip planes have multiples of random distribution mrd values of 3 0 4 9 the mrd values measured are higher than previous estimates mrd 1 5 that show lower degrees of shear localization and clay alignment averaged over larger volumes the intact fault rock exhibits less well developed nano clay fabrics than the experimentally sheared materials and mrd values decrease with smectite content we show that the abundance strength and shape of clasts all influence fabric evolution via strain localization quartz clasts yield more strongly developed clay fabrics than serpentine dominated safod clasts our results suggest that 1 both clay abundance and the development of nano scale fabrics play a role in fault zone weakening and 2 aseismic creep is promoted by slip along clay shears with 20 wt smectite content and mrd values 2 7

compositional and fluid pressure controls on the state of stress on the nankai subduction thrust a weak plate boundary abstract we show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the nankai subduction plate boundary off southwest japan ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients r at stress levels between 1 0 and 40 mpa the direct shear r values for purified smectite are 0 14 0 02 for illite 0 25 0 01 and for chlorite 0 26 0 02 for point load velocities of 0 0001 mm s these clay minerals dominate the nankai subduction decollement zone illite plus quartz is mechanically important in the altered incoming muroto section and the predicted decollement r should lie between 0 2 and 0 32 this low residual strength together with elevated fluid pressure limits shear stresses to below 4 mpa within the frontal 50 km of the subduction system consistent with the low wedge taper in this region a higher wedge taper off the ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in r are predicted to occur as a result of variations in smectite and total clay content these variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite however our data suggest the low r values of the clay rich decollement still limit shear stresses to between 17 and 29 mpa within the frontal 50 km of the wedge consistent with other estimates of plate boundary weakness indeed we propose that it should be expected that subduction plate boundaries like nankai will be weak because of the intrinsic presence of clay rich faults and moderate fluid overpressures our data do not support the hypothesis that the smectite to illite reaction directly controls the onset of seismogenic behavior deep in the nankai system because there is already a mechanical dominance of illite rather than smectite in the shallow decollement zone and we find all the clay phases tend to velocity strengthen however temperature activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up dip limit of the seismogenic zone

friction of sheared granular layers role of particle dimensionality surface roughness and material properties 1 we report on laboratory experiments designed to investigate three fundamental deformation mechanisms for frictional shear of granular fault gouge sliding rolling and dilation mechanisms were isolated by shearing layers composed of rods in geometric configurations that resulted in one dimensional two dimensional and rolling only particle interactions results of digital video are presented with measurements of friction and strain to illuminate the distribution of shear and the relationship between particle motions and friction the double direct shear configuration was used with boundary conditions of constant layer normal stress 1 mpa and controlled shear loading rate 10 m s with initial layer thickness of 6 mm layers were sheared in a servo hydraulic testing machine at room temperature 22 c and relative humidity 5 to 10 three materials were studied alloy 260 brass dried semolina pasta and hardwood dowels with particle diameters of 1 59 mm 1 86 mm and 2 06 mm respectively pasta layers had mean sliding friction coefficients of 0 24 0 11 and 0 02 in 2 d 1 d and rolling configurations respectively layers of brass rods had average friction coefficients of 0 23 0 15 and 0 01 respectively in 2 d 1 d and rolling configurations and the wood samples exhibited friction values of 0 18 0 19 and 0 09 respectively evolution of strength during shear correlated strongly with the displacement derivative of layer thickness sem images document the role of surface finish on frictional properties rapid reorientations of particles correspond to stick slip stress drops and may be related to the collapse and reformation of granular force chains we find a systematic relationship between the strength of granular layers and 1 the surface roughness of particles and 2 the number of particle contact dimensions our data provide important insights on the mechanics of granular fault gouge and constraints on the fundamental parameters used in numerical models of tectonic faulting

a method for determining absolute ultrasonic velocities and elastic properties of experimental shear zones abstract laboratory experiments are a vital tool for assessing elastic properties of rock and determining the underlying geomechanical processes that inform large scale predictive models in some cases relative values of elastic properties are sufficient but absolute values are necessary when comparing between locations and to address upscaling from lab to field settings however determining absolute values of ultrasonic velocity and elastic parameters in laboratory experiments is often complex and hampered by apparatus design moreover measuring the evolution of elastic properties with shear deformation has proven especially difficult here we describe a method that allows measurements of p and s wave velocity as a function of shear deformation under stresses of 10 s of mpa the approach includes rigorous calibration experiments and accounts for the evolution of impedance contrasts at sample interfaces as a function of strain we describe our method by applying it to sheared layers that represent simulated fault zones composed of clay quartz mixtures and fault rocks recovered from drilling p wave arrival times range from 10 to 20 s and wave speeds are 2 4 km s during shear of layers a few mm in thickness subject to normal stress of 25 mpa and shear strains 30 travel time data for apparatus calibration are fit with rational functions and root mean square error is used to assess uncertainty wave speed varies systematically with shear stress and increases with shear strain due to comminution compaction internal strain localization and shear fabric development

surface geology from viking landers on mars a second look aside from reddish dust products of martian weathering are not abundantly obvious on viking lander images surface stones appear sound spalls and disintegration fragments are largely lacking pitting of stones is probably not due to differential weathering and reddish stone colors more likely result from adhered dust than from patination judging from wind scoured drifts deflated ground rippled bed forms lee side tails basal scours fretted rocks and ubiquitous dust wind is currently the most effective transportive and erosive agent the dogma that martian eolian material is almost exclusively of 10 100 size is challenged by granule ripples and the possibility that wind drifted deposits consist largely of sand sized particles fretting is more common than faceting on martian ventifacts crusts on martian surface fines are more akin to case hardened rinds than to true duricrust crusting a possibly continuing process may be the product of atmospheric breathing of the regolith pits on surfaces of many martian stones are probably vesicles modified by eolian erosion pits on nonvesicular rocks may be solely the product of wind blasting vesicular lava dense lava and breccias are the most likely rocks among surface stones around the landers the bedrock outcrops near vl 1 may be large boulders embedded in a breccia substrate pyroclastics seem a likely constituent of surface deposits