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

constitutive relationships and physical basis of fault strength due to flash heating 1 we develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement laboratory faulting experiments conducted at high sliding velocities which show dramatic strength reduction below the threshold for bulk melting are well fit by the model the predicted slip speed for the onset of weakening is in the range of 0 05 to 2 m s qualitatively consistent with the limited published observations for this model earthquake stress drops and effective shear fracture energy should be linearly pressure dependent whereas the onset speed may be pressure independent or weakly pressure dependent on the basis of the theory flash weakening is expected to produce large dynamic stress drops small effective shear fracture energy and undershoot estimates of the threshold slip speed stress drop and fracture energy are uncertain due to poor knowledge of the average contact dimension shear zone thickness and gouge particle size at seismogenic depths

interannual variability of circulation under spring ice in a boreal lake a small range 1uc of under ice water temperature is shown to result in remarkably different circulation regimes under spring ice in a deep oligotrophic boreal lake with the water column at 4uc melting of snow led to deepening vertical convection before ice break and a final depth of convection inversely correlated with earlier deep water temperature we attribute that to the nonlinear dependence of water density on temperature albeit further affected by stochastic weather factors in four of nine study years convection led to complete under ice overturn of the lake indicating that this may not be uncommon in similar lakes with steep topography river inflow and more intensive warming of water in the littoral zone also created a horizontal density differential convection that involved flow down the sloping bottom and a lateral intrusion of this sinking water at a depth between the vertical convection and the quiescent deep water layers the vertical and horizontal convection together produced a profile of temperature slightly increasing from the surface to the bottom of the convection layer the contribution of horizontal convection to under ice mixing was interannually variable and in one of the study years it eventually dominated under ice mixing a thermal bar circulation regime developed occasionally and only in the open water between ice and shoreline we identified five different under ice mixing regimes that form an interannually variable continuum of behavior during the ice melting period the dependence on a narrow temperature range likely makes the circulation regime sensitive to a warming climate

production of nanoparticles during experimental deformation of smectite and implications for seismic slip abstract nanoparticles and amorphous materials are common constituents of the shallow sections of active faults understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets here we present the results of 59 rotary shear experiments conducted at room humidity conditions on gouge consisting of mixtures of smectite ca montmorillonite and quartz experiments with 60 50 25 0 wt ca montmorillonite were performed to investigate the influence of variable clay content on nanoparticle production and their influence on frictional processes all experiments were performed at a normal stress of 5 mpa slip rate of 0 0003 v 1 5 ms 1 and at a displacement of 3 m to monitor the development of fabric and the mineralogical changes during the experiments we investigated the deformed gouges using scanning and transmission electron microscopy combined with x ray powder diffraction quantitative phase analysis this integrated analytical approach reveals that at all slip rates and compositions the nanoparticles grain size of 10 50 nm are partly amorphous and result from cataclasis wear and mechanical solid state amorphization of smectite the maximum production of amorphous nanoparticle occurs in the intermediate slip rate range 0 0003 v 0 1 ms 1 at the highest frictional work and is associated to diffuse deformation and slip strengthening behavior instead the lowest production of amorphous nanoparticles occurs at co seismic slip rates v 1 3 ms 1 at the highest frictional power and is associated with strain and heat localization and slip weakening behavior our findings suggest that independently of the amount of smectite nanoparticles they produce fault weakening only when typical co seismic slip rates 0 1 ms 1 are achieved this implies that estimates of the fracture surface energy dissipated during earthquakes in natural faults might be extremely difficult to constrain

the thermal bar a simple theoretical model of the thermal bar is derived on the basis of heat budget equations for the following three zones of a wedge shaped water basin warmed from above i stably stratified shallow warm water zone ii vicinity of the bar iii convectively mixed deep cold zone in contrast to the traditional approach advective warming of the vicinity of the bar and associated facilitating of the thermal bar propagation are taken into account theoretical predictions are compared with the data of lacustrine and laboratory measurements taken from current literature new laboratory experiments have been carried out to examine the laminar regime of the thermal bar

weathering resistance of carbonate fault mirrors promotes rupture localization fractured rocks in fault zones regain their mechanical strength through a process called healing a central pathway for healing involves the dissolution and reprecipitation of minerals in the fault zone which cements the fractured rocks during interseismic periods however some faults contain highly polished surfaces coated in a thin nanoparticle layer along which slip is localized crucially these surfaces show little evidence of postseismic mineralization and healing here we use atomic force microscopy to show that naturally polished rocks from carbonate fault zones are resistant to dissolution in stark contrast to the reactive minerals that make up the fault breccia our results suggest that the low reactivity of the nanoparticle layer could retard healing helping to maintain the localization of the fault zone between seismic slip events as fault localization affects seismic motion the geochemical reactivity of fault mirrors could be an important control on seismicity along faults

experimental evidence for the simultaneous formation of pseudotachylyte and mylonite in the brittle regime the coexistence of pseudotachylyte a brittle fault rock representing seismic slip and mylonite a plastic or semiplastic fault rock associated with aseismic slip has been debated for its origin in earthquake generating faults deformation experiments on halite gouge at seismic slip rates show that mylonite forms simultaneously with frictional melt pseudotachylyte in the brittle regime equivalent to upper crustal conditions the plastic deformation of halite resulting in mylonite is facilitated by the conduction of heat from a slip concentrated molten layer formed by frictional heating at seismic slip rates thus mylonites which are 1 kinematically consistent and 2 just at the contact with pseudotachylyte might have formed together with the pseudotachylyte in the brittle regime during seismic slip

thermo mechanical pressurization of experimental faults in cohesive rocks during seismic slip earthquakes occur because fault friction weakens with increasing slip and slip rates since the slipping zones of faults are often fluid saturated thermo mechanical pressurization of pore fluids has been invoked as a mechanism responsible for frictional dynamic weakening but experimental evidence is lacking we performed friction experiments normal stress 25 mpa maximal slip rate 3 ms 1 on cohesive basalt and marble under 1 room humidity and 2 immersed in liquid water drained and undrained conditions in both rock types and independently of the presence of fluids up to 80 of frictional weakening was measured in the first 5 cm of slip modest pressurization related weakening appears only at later stages of slip thermo mechanical pressurization weakening of cohesive rocks can be negligible during earthquakes due to the triggering of more efficient fault lubrication mechanisms flash heating frictional melting etc

structures and high velocity frictional properties of the pingxi fault zone in the longmenshan fault system sichuan china activated during the 2008 wenchuan earthquake abstract this paper reports on the internal structures of the pingxi fault zone near the northeastern end of the coseismic surface ruptures associated with the 2008 wenchuan earthquake the fault zone consists of a 50 80 cm wide fault core and several meter wide fault breccia zones the fault core contains yellowish gouge yg gray blackish gouge gbg black gouge bg and gray blackish fault breccia a coseismic slip during the wenchuan earthquake occurred within gbg in a slip zone of 24 9 mm in width high velocity friction experiments were conducted at normal stresses n of 0 45 2 50 mpa and slip rates v of 0 0014 2 09 m s yg and gbg exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient p to steady state friction coefficient ss over a slip weakening distance dc specific fracture energy eg for yg and gbg was 0 37 2 02 mj m2 of the same order of magnitude as the fracture energy estimated for natural earthquakes empirical equations describing the changes in p ss and dc in terms of n and v indicate that eg decreases with increasing n so that higher normal stress experiments will lead to eg lower than seismic values thus our experiments probably reproduced deeper seismic fault motion under dry conditions with respect to eg frictional behavior of bg is characterized by a sharp initial weakening subsequent strengthening to the second peak friction and slip weakening towards the steady state friction the initial weakening and strengthening seem to be caused by the formation of a very sharp slip zone 5 m in width and subsequent formation of a widely deformed zone a comparison of our data with reported results for other outcrops indicates that the high velocity frictional properties of the longmenshan fault system may be quite uniform the observed dramatic high velocity weakening must have promoted dynamic rupture propagation during the wenchuan earthquake

water vaporization promotes coseismic fluid pressurization and buffers temperature rise we investigated the frictional properties of carbonate rich gouge layers at a slip rate of 1 3 m s under dry and water saturated conditions while monitoring temperature at different locations on one of the gouge host rock interfaces all experiments showed a peak frictional strength of 0 4 0 7 followed by strong slip weakening to steady state values of 0 1 0 3 experiments which used a pore fluid with a constant drainage path to the atmosphere showed the development of a temperature plateau beyond 100 c contemporaneous with the dynamic slip weakening and consistent with thermodynamic considerations of ongoing vaporization of pore water upon pore fluid vaporization the pore pressure increases while the temperature is buffered endothermically such that the pore water moves along the liquid vapor transition curve in a pressure temperature phase diagram pore fluid phase transitions of this kind are expected to occur in natural earthquakes at relatively shallow crustal levels enhancing fluid pressurization while impeding the achievement of high temperatures therefore the operation of vaporization may help explain the low downhole temperature anomalies obtained shortly after large earthquakes

geological observations of damage asymmetry in the structure of the san jacinto san andreas and punchbowl faults in southern california a possible indicator for preferred rupture propagation direction we present new in situ observations of systematic asymmetry in the pattern of damage expressed by fault zone rocks along sections of the san andreas san jacinto and punchbowl faults in southern california the observed structural asymmetry has consistent manifestations at a fault core scale of millimeters to meters a fault zone scale of meters to tens of meters and related geomorphologic features the observed asymmetric signals are in agreement with other geological and geophysical observations of structural asymmetry in a damage zone scale of tens to hundreds of meters in all of those scales more damage is found on the side of the fault with faster seismic velocities at seismogenic depths the observed correlation between the damage asymmetry and local seismic velocity structure is compatible with theoretical predictions associated with preferred propagation direction of earthquake ruptures along faults that separate different crustal blocks the data are consistent with a preferred northwestward propagation direction for ruptures on all three faults if our results are supported by additional observations asymmetry of structural properties determined in field studies can be utilized to infer preferred propagation direction of large earthquake ruptures along a given fault section the property of a preferred rupture direction can explain anomalous behavior of historic rupture events and may have profound implications for many aspects of earthquake physics on large faults

a model of the thermal bar circulation in a long basin a thermal bar is defined here as a zone of descending water at or near the fresh water temperature of maximum density in the present study we model physical properties such as velocity temperature turbulent fluxes time of onset and migration speed of the thermal bar in an idealized basin unlike models using prescribed eddy viscosity and diffusivity coefficients our model uses a second order turbulence closure scheme to model mixing due to turbulence the only inputs in the calculation are the surface wind the surface heat flux and the initial state of the fluid numerical simulations of the resulting circulation are presented for flow in a long rotating basin under prescribed surface heat and wind conditions the results show the influence of the relative magnitudes of mechanical and thermal forcing on the circulation a comparison with field observations shows reasonable agreement between computed and measured bar migration speeds the time necessary for the formation of the bar is computed and is found to depend on the surface forcing

a study of the thermal bar in lake ladoga using water surface temperature data from satellite images during spring and autumn a special thermohydrodynamic phenomenon called a thermal bar can be observed in dimictic lakes a dimictic lake is a lake where a complete turnover of the water occurs twice a year in spring and autumn the thermal bar is a zone of mixing that separates waters with temperatures above and below 4 c its practical importance is due to the possibility that it might isolate the nearshore region from the offshore region as seen from the thermal bar which may cause a serious pollution problem along the beach this article presents a study of the thermal bar and its temporal behavior in lake ladoga russia using noaa avhrr ch4 ch5 data as the thermal bar is associated with a rather characteristic temperature pattern that is relatively large temperature changes from the shore to the bar and weak gradients beyond implying almost isothermal conditions satellite data can provide a good means of detecting the existence of a thermal bar the satellite derived water surface temperature distributions calibrated with some in situ temperature data show that the thermal bar progresses from the shallow southern region to the deep northern part of the lake the thermal bar progression during spring obtained from sequences of satellite images is also compared with predictions from a simple theoretical model the model assumes that the heat entering through the surface of a water column remains within the column i e horizontal heat fluxes are neglected and that complete vertical mixing takes place by convective water movements the comparison shows a good agreement between satellite derived and model predicted thermal bar locations although there are uncertainties as to certain inputs to the model this indicates that advective and diffusive horizontal heat fluxes are not of primary importance for the thermal bar displacement and that the model is a good first approximation for predictions of the thermal bar location

shear localization in a mature mylonitic rock analog during fast slip highly localized slip zones developed within ductile shear zones such as pseudotachylite bands occurring within mylonitic fabric rocks are frequently interpreted as evidence for earthquake nucleation and or propagation within the ductile regime to understand brittle frictional shear localization processes in ductile shear zones and to relate these to earthquake nucleation and propagation we performed tests with large change in velocity on a brine saturated 80 20 wt mixture of halite and muscovite gouge after forming a mature mylonitic structure through frictional viscous flow the direct effect a on shear strength that occurs in response to an instantaneous upward velocity step is an important parameter in determining the nature of seismic rupture nucleation and propagation we obtained reproducible results regarding low velocity mechanical behavior compared with previous work but also obtained new insights into effects of sudden increases in slip velocity on localization and strength evolution at velocities above a critical velocity vc 20 m s we found that once a ductile mylonitic structure has developed in a shear zone subsequent cataclastic deformation is consistently localized in a narrow zone this switch to localized deformation is controlled by the imposed velocity and becomes most apparent at velocities above vc in addition the direct effect drops rapidly when the velocity exceeds vc this implies that slip can accelerate towards seismic velocities almost instantly and without much loss of fracture energy once vc is exceeded obtaining a measure for vc in natural faults is therefore of key importance for understanding earthquake nucleation and propagation in the brittle ductile transitional regime

the peak frequency of direct waves for microseismic events abstractdirect seismic waves p or s waves are used to locate and further characterize microseismic events the resolution of information obtained from direct waves depends on the peak frequencies of the waveforms the peak frequency results from combination of the source propagation and the receiver effects for frequencies below the corner frequency propagation effects control the peak frequency in observed seismograms of microseismic events the frequency dependence of direct body waves can be modeled by attenuation specifically the global attenuation factor this model is consistent with observed data along surface profiles explaining the difference between the peak frequencies of p and s waves in addition the model is consistent with the peak frequencies observed on downhole monitoring arrays this can be used to invert effective attenuation providing additional unique measurement from microseismic events the corner frequency can be estimated from the average stress drop and analytical sour

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

coseismic microstructures of experimental fault zones in carrara marble abstract experimental fault zones developed in carrara marble that were deformed at seismic slip rates 1 18 1 30 m s 1 using a high velocity rotary shear apparatus exhibit very low friction friction coefficient as low as 0 06 at steady state due to nanoparticle lubrication of the decomposition product lime the fault zones show a layered structure a central slip localization layer 5 60 m thick of lime nanograins mantled by gouge layers 5 150 m thick and a plastically deformed layer 45 500 m thick between the wall rock and gouge layer in the marginal portion of cylindrical specimens calcite grains of the wall rock adjacent to the slip zone deform by dislocation glide when subjected to frictional heating and a lower strain rate than that of the principal slip zone the very fine 2 5 m calcite grains in the gouge layer show a foam structure with relatively straight grain boundaries and 120 triple junctions this foam structure is presumed to develop by welding at high temperature and low strain once slip is localized along the central layer we suggest that a seismic event can be inferred from deformed marbles given i the presence of welded gouge with foam structure in a fault zone where wall rocks show no evidence of thermal metamorphism and ii a thin plastically deformed layer immediately adjacent to the principal slip zone of a cataclastic fault zone

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

microstructural evidence for seismic and aseismic slips along clay bearing carbonate faults in this multi methodological study microstructural observations of fault rocks are combined with micromechanical property analyses contact resonance atomic force microscopy cr afm and with rotary friction experiments shiva apparatus to find evidence of seismic to aseismic slip and understand the nanoscale rheology of clay bearing carbonate hosted faults fluidized structures truncated clasts pores and vesicles and phyllosilicate nano sized spherules and tubes suggest fast deformation events occurred during seismic slip whereas clay assisted pressure solution processes clumped clasts foliation surfaces and mantled clasts indicate slow deformation events occurred during postseismic interseismic periods cr afm measurements show that the occurrence of 5 wt of clay within the carbonate hosted gouges can significantly reduce the fault core stiffness at nanoscale in addition during high velocity friction experiments simulating seismic slip conditions the presence of ultra thin phyllosilicate bearing 3 wt layers within calcite gouges as those observed in the natural fault show faster dynamic weakening than that of pure calcite gouges the weak behavior of such layers could facilitate the upward propagation of seismic slip during earthquakes thus possibly enhancing surface faulting microstructural observations and experimental evidence fit some well known geophysical and geodetic observations on the short to long term mechanical behavior of faults such as post seismic interseismic aseismic creep interseismic fault locking and seismic slip propagation up to the earth s surface

the role of recrystallisation on the deformation behaviour of calcite rocks large strain torsion experiments on carrara marble abstract the rheological microstructural and textural evolution of carrara marble was studied during experimental deformation in torsion up to very large shear strains 0 4 50 experiments were performed at constant angular displacement rates corresponding to shear strain rates of 6 10 5 3 10 3 s 1 a confining pressure of 300 mpa and temperatures of 500 600 and 727 c microstructures and crystallographic preferred orientations cpo were analysed using light microscopy and electron backscatter diffraction ebsd at all conditions deformation occurred dominantly by dislocation creep even up to very large shear strains after peak stresses 2 recrystallisation mostly by subgrain rotation caused weakening 5 20 grain size reduction and a change in the cpo a monoclinic cpo that formed at small strains at all temperatures evolved into two different large strain cpos at different temperatures at large strains even after complete recrystallisation the cpo continued to strengthen and a secondary foliation was formed at a steep angle to the shear zone boundary this secondary foliation was continuously reset by both subgrain rotation and grain boundary migration recrystallisation and eventually overprinted the primary foliation misidentifying the primary and secondary foliations could result in misinterpretation of the applied strain field more generally information about the amount of strain is lacking for microstructures and cpos of highly deformed ultramylonites

earthquake rupture at focal depth part i structure and rupture of the pretorius fault tautona mine south africa we analyze the structure of the archaean pretorius fault in tautona mine south africa as well as the rupture zone that recently reactivated it the analysis is part of the natural earthquake laboratory in south african mines nelsam project that utilizes the access to 3 6 km depth provided by the mining operations the pretorius fault is a 10 km long oblique strike slip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in tautona mine we identify here three structural zones within the fault zone 1 an outer damage zone 100 m wide of brittle deformation manifested by multiple widely spaced fractures and faults with slip up to 3 m 2 an inner damage zone 25 30 m wide with high density of anastomosing conjugate sets of fault segments and fractures many of which carry cataclasite zones and 3 a dominant segment with a cataclasite zone up to 50 cm thick that accommodated most of the archaean slip of the pretorius fault and is regarded as the principal slip zone psz this fault zone structure indicates that during its archaean activity the pretorius fault entered the mature fault stage in which many slip events were localized along a single psz the mining operations continuously induce earthquakes including the 2004 m2 2 event that rejuvenated the pretorius fault in the nelsam project area our analysis of the m2 2 rupture zone shows that 1 slip occurred exclusively along four pre existing large quasi planer segments of the ancient fault zone 2 the slipping segments contain brittle cataclasite zones up to 0 5 m thick 3 these segments are not parallel to each other 4 gouge zones 1 5 mm thick composed of white rock flour formed almost exclusively along the cataclasite host rock contacts of the slipping segments 5 locally new fresh fractures branched from the slipping segments and propagated in mixed shear tensile mode 6 the maximum observed shear displacement is 25 mm in oblique normal slip the mechanical analysis of this rupture zone is presented in part ii heesakkers et al earthquake rupture at focal depth part ii mechanics of the 2004 m2 2 earthquake along the pretorius fault tautona mine south africa 2011 this volume

fault zone architecture and deformation processes within evaporitic rocks in the upper crust 1 recently in the northern apennines geophysical data have identified the triassic evaporites te anhydrites and dolomites as the source region of the major extensional earthquakes of the area m 6 in order to characterize fault zone architecture and deformation processes within the te we have studied exhumed evaporite bearing normal faults within the upper crust the structure of large displacement 100 m normal faults is given by 1 a zoned fault core with a wider portion of fault parallel foliated ca sulphates ductile deformation overprinted by an inner fault core ifc of localized brittle deformation and 2 wide dolostones to absent ca sulphates damage zones of fault fracture patterns fault rock assemblage within the ifc is characterized by fault breccia gouge and cataclasites of different grain size most of the deformation within the ifc is localized along thin and fault parallel principal slip surfaces pss made of dolomite rich fine grained cataclasite sem analyses show an evolution from ca to st to gypsum rich mineralization due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation the development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones progressive deformation within the fault core leads to the development of fault parallel dolomite rich cataclastic layers their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the ifc

deep water renewal in lake issyk kul driven by differential cooling during two field campaigns on lake issyk kul in march and august 2001 179 high resolution conductivitytemperature depth profiles were measured along with profiles of dissolved oxygen and light transmission on the basis of this extensive data set we investigated the large scale vertical advective processes responsible for deepwater renewal in lake issyk kul at some locations sampled in march variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions these intrusions have their origin in density plumes that propagate along the bottom of the channels in the eastern shelf region the cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter during summer vertical advective processes appear to be limited to the upper 200 m where upwelling in the open water leads to a doming structure the august data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water

can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation recent friction experiments carried out under upper crustal p t conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults these mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust upper mantle in this study displacement controlled experiments have been performed on carbonate gouges at seismic slip rates 1 m s 1 to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust at relatively low displacements 900 c grain boundary sliding gbs deformation mechanisms the observed micro textures are strikingly similar to those predicted by theoretical studies and those observed during experiments on metals and fine grained carbonates where superplastic behaviour has been inferred to a first approximation the measured drop in strength is in agreement with our flow stress calculations suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone rather than by frictional sliding along the main slip surfaces in the slip zone frictionally induced grainsize sensitive gbs deformation mechanisms can thus account for the self lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature

importance of borehole deviation surveys for monitoring of hydraulic fracturing treatments during seismic monitoring of hydraulic fracturing treatment it is very common to ignore the deviations of the monitoring or treatment wells from their assumed positions for example a well is assumed to be perfectly vertical but in fact it deviates from verticality this can lead to significant errors in the observed azimuth and other parameters of the monitored fracture system geometry derived from microseismic event locations for common hydraulic fracturing geometries a 2 deviation uncertainty on the positions of the monitoring or treatment well survey can cause a more than 20 uncertainty of the inverted fracture azimuths furthermore if the positions of both the injection point and the receiver array are not known accurately and the velocity model is adjusted to locate perforations on the assumed positions several millisecond discrepancies between measured and modeled sh p traveltime differences may appear along the receiver array these traveltime discrepancies may then be misinterpreted as an effect of anisotropy and the use of such anisotropic model may lead to the mislocation of the detected fracture system the uncertainty of the relative positions between the monitoring and treatment wells can have a cumulative nonlinear effect on inverted fracture parameters we show that incorporation of borehole deviation surveys allows reasonably accurate positioning of the microseismic events in this study we concentrate on the effects of horizontal uncertainties of receiver and perforation positions understanding them is sufficient for treatment of vertical wells and also necessary for horizontal wells

from slow to fast faulting recent challenges in earthquake fault mechanics faultsthin zones of highly localized shear deformation in the earthaccommodate strain on a momentous range of dimensions millimetres to hundreds of kilometres for major plate boundaries and of ti

frictional melt and seismic slip 1 frictional melt is implied in a variety of processes such as seismic slip ice skating and meteorite combustion a steady state can be reached when melt is continuously produced and extruded from the sliding interface as shown recently in a number of laboratory rock friction experiments a thin low viscosity high temperature melt layer is formed resulting in low shear resistance a theoretical solution describing the coupling of shear heating thermal diffusion and extrusion is obtained without imposing a priori the melt thickness the steady state shear traction can be approximated at high slip rates by the theoretical form ss n1 4 a under a normal stress n slip rate v radius of contact area r a is a dimensional normalizing factor and w is a characteristic rate although the model offers a rather simplified view of a complex process the predictions are compatible with experimental observations in particular we consider laboratory simulations of seismic slip on earthquake faults a series of high velocity rotary shear experiments on rocks performed for n in the range 1 20 mpa and slip rates in the range 0 5 2 m s 1 is confronted to the theoretical model the behavior is reasonably well reproduced though the effect of radiation loss taking place in the experiment somewhat alters the data the scaling of friction with n r and v in the presence of melt suggests that extrapolation of laboratory measures to real earth is a highly nonlinear nontrivial exercise

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

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

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

wavefield characterization of perforation shot signals in a shale gas reservoir abstract signals of perforation or string shots which are usually used for calibrating velocity models and estimating the orientation of 3 component down hole receivers during microseismic monitoring are occasionally hard to identify due to poor signal to noise ratios snr or confusion with the events induced during fracturing in adjacent wells a significant feature for distinguishing perforation signals from hydraulic fracturing events is the tube wave which is generated in the treatment well and received in the monitoring well we analyze seismic wavefields from perforations during a hydraulic fracturing operation on a pad well a group of horizontal wells with the wellheads at a same small surface area in a shale gas reservoir to understand the wave propagation phenomena including attenuation and the identification of tube waves guided wave in borehole and their conversions since they are dominated by high frequencies and lack energy at low frequencies the p and s wave arrivals of perforation shots decrease much more rapidly with propagation distance than that of induced events we identify six modes within the wavefields of the perforation or string shots recorded in a nearby well that are related to the tube waves the six modes include p s waves converted from tube waves in the shot treatment well at plugs or the well bottom an up going tube wave in the monitoring well generated by the tube wave in the nearby treatment well a down going tube wave from the treatment wellhead and the multiple a scatting body wave activated by tube waves in the treatment well these wave modes all originate from the waves radiated at the perforation point in the treatment well as tube waves and are then radiated into the formation and received by geophones in the nearby borehole as p s or tube waves if energy from the perforation shot is strong enough the tube wave will turn back from the surface and be reflected at the well bottom or plugs which would excite the further waves based on the features of the wavefield we identify the perforation events and facilitate better use of calibration from the perforation seismic recording for microseismic data processing we also evaluate attenuation of the signals with propagation distance and propose a procedure for developing a plan for monitoring of perforation and string shot signals

cohesive strengthening of fault zones during the interseismic period an experimental study 1 there is widespread evidence indicating that faults regain a portion of their strength during the interseismic period here we present experiments designed to understand and quantify the interseismic cohesive strengthening resulting from fluid rock reactions in fault zones the triaxial experiments consisted of fracturing cores of fontainebleau sandstone under dry conditions forming a localized shear failure zone stage 1 the specimens were then reacted hydrothermally under isostatic conditions allowing the fault damage zone to compact consolidate and strengthen stage 2 following reaction the specimens were then reloaded to failure under nominally dry conditions so that the increase in cohesive strength of the fault could be measured stage 3 experiments show that cohesion increase is positively correlated to temperature and pore pressure during reaction after 6 hours of reaction at the highest temperatures 927 c and pore pressures 200 mpa cohesion increases by as much as 35 mpa microstructural examination of the specimens showed that the gouge particles within the fault compacted and cemented together exhibiting textures typical of pressure solution and that fractures in the surrounding damage zone had healed a theoretical treatment of the data was conducted using these experiments in combination with results on time dependent changes in fault cohesion presented by tenthorey et al 2003 we find that the rate controlling process in our experiments has an activation energy q of approximately 70 kj mol 1 we use this information to develop a model for time dependent cohesive strengthening in fault zones within the continental seismogenic regime we conclude that significant cohesive strengthening of fault zones can occur during the interseismic period of medium to large earthquakes given the presence of reactive pore fluids

fracture compliance estimation using borehole tube waves abstractwe tested two models one for tube wave generation and the other for tube wave attenuation at a fracture intersecting a borehole that can be used to estimate fracture compliance fracture aperture and lateral extent in the tube wave generation model we consider tube wave excitation in the borehole when a p wave is incident on the fracture the amplitude ratio of the pressure due to the tube wave to that of the incident p wave is a function of fracture compliance aperture and length similarly the attenuation of a tube wave in the borehole as it crosses a fracture intersecting the borehole is also a function of fracture properties numerically solving the dispersion relation in the fracture we study tube wave generation and the attenuation coefficient as a function of frequency we observed that measuring amplitude ratios or attenuation near a transition frequency can help constrain the fracture properties the transition frequency corresponds to the regime in which the viscous skin depth in

seasonal mixing processes in an arctic fjord system abstract studies of water movement in the aafardlikavs qaumarujuk fjord system in west greenland have barn carried out in order to understand the movement of mine tailings discharged into agfardlikavs fjord observations are explained in terms of resonance of the internal tide deep convection associated with sea ice growth and basin water exchange occurring at the time of ice breakup in early summer mixing processes are dominated by these events illustrating the limitations of any seasonal description based on conventional diffusive processes

effect of water and rock composition on re strengthening of cohesive faults during the deceleration phase of seismic slip pulses abstract the elastic strain energy release rate and seismic waves emitted during earthquakes are controlled by the on fault temporal evolution of the shear stress during rupture propagation high velocity friction experiments highlighted that shear stress on the fault surface evolves rapidly during seismic slip pulses this temporal evolution of shear stress is controlled by both fault weakening at seismic slip initiation and re strengthening rate towards the end of slip while numerous studies focused on fault weakening less attention was given to co seismic re strengthening processes here we performed 53 friction experiments normal stress 30 mpa slip rate 6 5 m s 1 imposing constant slip acceleration and deceleration 7 8 m s 2 on cohesive carrara marble 99 calcite and micro gabbro silicate built rock under dry vacuum and water pressurized conditions microstructural observations showed that micro gabbro accommodated seismic slip by bulk melting of the sliding surfaces whereas carrara marble by coupled decarbonation and grain size dependent crystal plastic processes under room humidity conditions and low imposed power density i e product of normal stress per slip rate re strengthening rate during the deceleration stage was up to 17 times faster in marble than in micrograbbro in the latter the re strengthening rate increased slightly with the power density the presence of water enhanced further this trend on the contrary in marbles the re strengthening rate decreased drastically with power density and in the presence of water our experimental observations highlighted the first order importance of the mineralogy and rheology of the slip zone materials and to a second order of the presence of water in controlling co seismic re strengthening of faults during seismic slip deceleration

a two level model of a thermally forced ocean basin abstract some simple solutions mostly analytic are presented for the large scale baroclinic response to thermal forcing on a mid latitude beta plane surface heat flux is parameterized as ta tt tau with atmospheric temperature ta prescribed as a function of latitude varying ocean surface temperature tt and equilibration time for long times decades benthic forcing is included using a similar representation the model allows horizontal density variations at each level when there are no meridional boundaries there is only a local response to the forcing a geostrophic baroclinic zonal flow is driven by the north south temperature gradient but it has no associated advection or divergence effects this picture is greatly changed when east and or west coasts are added kelvin waves pass information rapidly about 200 km day 1 along coasts and rossby waves travel slowly offshore most effectively from the cut with speed c 1 km day 1 for spin up problems e g the response to a change in for

fabric transition with dislocation creep of a carbonate fault zone in the brittle regime abstract fabric transition by a switch in the dominant slip system of minerals in the plastic regime can be induced by changes in temperature strain rate or water content we propose here this fabric transition by frictional heating in seismogenic fault zones in the brittle regime the garam thrust in the taebaeksan basin of south korea has a hanging wall of cambrian dolostone juxtaposed against a footwall of ordovician limestone and records a minimum displacement of 120 m in a 10 cm thick plastically deformed layer adjacent to the principal slip layer of the fault zone the lattice preferred orientation of calcite grains suggests that the dominant slip system changes approaching the principal slip layer from r 02 21 and e twinning through r 02 21 and basal a to basal a this fabric transition requires a high temperature gradient of 40 c cm which we infer to result from frictional heating of the seismic fault zone we suggest that fabric transition within a thin plastically deformed layer adjacent to the principal slip layer of a fault zone indicates an unusually steep temperature gradient and provides strong evidence of seismic slip

a new interpretation for the nature and significance of mirror like surfaces in experimental carbonate hosted seismic faults highly reflective continuous smooth surfaces known as mirror like surfaces mss have been observed in experimental carbonate hosted faults which were sheared at both seismic and aseismic velocities mss produced during high velocity friction experiments 0 1 m s 1 are typically interpreted to be frictional principal slip surfaces where weakening mechanisms are activated by shear heating we re examined this model by performing friction experiments in a rotary shear apparatus on calcite gouge at seismic velocities up to v 1 4 m s 1 and normal stress n 25 mpa to analyze the evolution of microstructures as displacement increases after the onset of dynamic weakening when the friction coefficients are low 0 6 sheared gouges consistently develop a well defined porosity free principal slip zone psz of constant finite thickness a few tens of micrometers composed of nanometric material which displays polygonal grain shapes mss occur at both boundaries of the psz where they mark a sharp contrast in grain size with the sintered much coarser material on either side of the psz our observations suggest that with the onset of dynamic weakening mss partition the deformation by separating strong sintered wall rocks from a central weak actively deforming viscous psz therefore the mss do not correspond to frictional slip surfaces in the classical sense but constitute sharp rheological boundaries while in the psz shear is enhanced by thermal and grain size dependent mechanisms

in mine seismic delineation of mineralization and rock structure significant progress has been made towards the goal of generating detailed seismic images as an aid to mine planning and exploration at the kambalda nickel mines of western australia crosshole and vertical seismic profiling instrumentation including a slimline multi element hydrophone array three component geophone sensors and a multishot detonator sound source have been developed along with special seismic imaging software to map rock structure seismic trials at the hunt underground mine established that high frequency 1 khz signals can be propagated over distances of tens of meters tomographic as well as novel 3 d multicomponent reflection imaging procedures have been applied to the data to produce useful pictures of the ore stope geometry and host rock tomogram interpretation remains problematic because velocity changes not only relate to differing rock types and or the presence of mineralisation but can also be caused by alteration weathering and other rock condition variations ultrasoni

fast slip with inhibited temperature rise due to mineral dehydration evidence from experiments on gypsum anomalously low heat flow around active faults has been a recurrent subject of debate over past decades we present a series of high velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large scale endothermic dehydration reaction the tests were performed at 1 mpa normal stress and a velocity of 1 3 m s 1 while measuring the temperature close to the sliding surface and the relative humidity around the sample the temperature close to the sliding surface is remarkably stable at 100 c during the dehydration reaction of gypsum microstructural and x ray diffraction investigations show that dehydration occurs at the very beginning of the test and progresses into the bulk as slip increases in the hottest parts of the sample anhydrite crystal growth is observed the half thickness of the dehydrated layer ranges from 160 m at 2 m slip to 5 mm at 68 m slip thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10 to 50 of the total mechanical work input the temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam we suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake this is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature

review of major shale dominated detachment and thrust characteristics in the diagenetic zone part ii rock mechanics and microscopic scale abstract characterising large shale thrust zone behaviour down to the diagenetic metamorphic boundary is both simple and complex the task in critical taper analogue and numerical models has been successfully approximated using simple material parameters yet the weakness of shales thrusts on a case by case basis show great variability in key factors across a wide range of scales understanding of these variables and their relationships to different tectonic settings is patchy due to the different objectives of workers studying the various systems e g seismic hazards hydrocarbon exploration structural geology research and the types of available geological and geophysical data early research recognized that megathrusts feature creeping and locked patches which may be described using the rate and state formalism in addition to temperature differential stress and strain rate key controlling factors on thrust weakness are mineralogy amount of weak phase present structure localization mechanisms grain size and shapes porosity permeability and pore fluid pressure the smectite illite transition has been a focus of seismic hazard research due to its coincidence with the top of the seismogenic zone while there may be a relationship other factors such as pore fluid pressure variations or structure localization are also important the critical taper model is a simple means of determining whether basic rock mechanics data e g frictional strength pore fluid pressure is appropriate for natural wedges for example aseismic basal detachments of gravity driven systems smaller critical taper wedges appear to retain higher pore fluid pressures along the basal detachment than seismogenic basal detachments higher critical taper wedges in many continental and deltaic fold and thrust belts high organic carbon content is a very important factor in shale weakness due to 1 the overall more ductile and well cleaved nature of the shale when organic content is high 2 the presence of high overpressures due to maturation of organic material and 3 metamorphism of carbon to low friction graphite this multifaceted influence of organic carbon content is just one example of the diversity of potential influences on shale thrust zone weakness which enable shales to be weak despite considerable lateral deposition related and vertical burial related changes in composition some key variations between different types of fold and thrust belt gravity driven accretionary prism andean himalayan type lie in systematic variations in clay mineralogy magnitudes and origins of overpressures seismic versus aseismic detachments and different structural localization mechanisms further research is required to explore the viability of such distinctions and their impact on structural styles

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

nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation crystal plasticity at sub seismic slip rates at 18 150 c a central aim in fault mechanics is to understand the microphysical mechanisms controlling aseismic seismic transitions in fault gouges and to identify microstructural indicators for such transitions we present new data on the slip stability of calcite fault gouges and on microstructural development down to the nanometer scale our experiments consisted of direct shear tests performed dry at slip rates of 0 1 10 m s at a constant normal stress of 50 mpa at 18 150 c the results show a transition from stable to potentially unstable slip above 80 c all samples recovered showed an optical microstructure characterized by narrow 30 40 m wide riedel and boundary shear bands marked by extreme grain comminution and a crystallographic preferred orientation cpo boundary shear bands sectioned using fib sem focused ion beam scanning electron microscopy revealed angular grain fragments decreasing from 10 to 20 m at the outer margins to 0 3 m in the shear band core where dense aggregates of nanograins also occurred transmission electron microscopy applied to foils extracted from boundary shears using fib sem combined with the optical cpo showed that these aggregates consist of calcite nanocrystals 5 20 nm in size with the 104 201 dislocation glide system oriented parallel to the shear plane and direction our results suggest that the mechanisms controlling slip include cataclasis and localized crystal plasticity because crystal plasticity is strongly thermally activated we infer that the transition to velocity weakening slip is likely due to enhanced crystal plasticity at 80 c this implies that tectonically active limestone terrains will tend to be particularly prone to shallow focus seismicity

reduction in bet surface area of nojima fault gouge with seismic slip and its implication for the fracture energy of earthquakes abstract a common view concerning the energetics of seismogenic fault motion is that at least part of the fracture energy is consumed in grain crushing in the fault zone and that this part may be estimated by grain size analysis of fault rocks we address this problem by conducting room dry friction experiments on nojima fault gouge at subseismic to seismic slip rates 0 009 1 31 m s and at normal stresses up to 3 64 mpa and by measuring the bet surface area of the gouge before and after the experiments where it cuts granite the nojima fault zone has bet surface area of about 65 10 6 m 2 per unit fault area 1 m 2 clayey and granular fault gouges composed mainly of quartz plagioclase kaolinite and smectite were collected from a granitic fault zone at a new outcrop in funaki awaji island southwest japan both clayey and granular gouges exhibit dramatic weakening at high slip rates specific bet surface areas of clayey and granular gouges decreased with increasing slip rate from 46 0 and 15 4 m 2 g before deformation to about 20 and 5 m 2 g after deformation 55 70 reduction respectively microstructural observations revealed that grain welding within the slip zones at high slip rates reduced grain surface area the energetics of seismic fault motion should be examined with broader views taking into account grain crushing grain welding decomposition and frictional melting

investigation of the spring thermal regime in lake ladoga using field and satellite data we use a combination of field and satellite data to investigate the thermal regime in lake ladoga in spring 1992 the analysis concentrates mainly on the temperature structure in the convectively mixed region and the dynamics of a special thermo hydrodynamic phenomenon called thermal bar a zone of descending water with the temperature of maximum density equal to 4 c a theoretical model of the temperature distribution in the convectively mixed region temperatures 4 c and the thermal bar progression is also applied and compared with satellite data the study shows that the temperature offshore the 4 c isotherm in the convectively mixed region of the lake is almost depth constant because of strong convective mixing and that the horizontal heat flux seems to be of minor importance in this region the comparison between predictions from the simple theoretical model describing the temperature evolution and movement of the thermal bar with observations showed a fairly good agreement however calculations of the change of heat content in water columns along sections indicate heat transport from the nearshore stably stratified region toward the 4 c isotherm which will affect the movement of the thermal bar this effect has been incorporated in an improved model of the thermal bar progression a better agreement between predictions and observations was also obtained with this model

energy partition for grain crushing in quartz gouge during subseismic to seismic fault motion an experimental study abstract brunauer emmett teller bet surface area of quartz gouge was measured before and after shearing deformation at subseismic to seismic slip rates to determine how much of frictional work and fracture energy was consumed during grain crushing measurement of bet surface area provides more direct data on surface area change than grain size measurement does twenty four experiments on quartz gouge composed of ottawa sands or crushed quartz were conducted dry with room humidity or wet with added water at slip rates of 0 006 1 3 m s and normal stresses of 0 76 3 37 mpa using a high velocity friction apparatus and a low to high velocity friction apparatus about 1 mm thick gouge was placed between cylindrical specimens of belfast dolerite of 25 mm in diameter results show that grain crushing absorbed only 0 02 0 22 of frictional work and only 0 05 1 06 of fracture energy under dry and wet conditions thus grain comminution is unlikely to be an important energy sink at least for mature faults with well developed slip zone surface area of gouge decreases with deformation at seismic slip rates this and sem observations suggest partial sintering due to frictional heating thus a simple scenario of grain crushing as an energy sink does not hold for estimating the energy budget during earthquakes

on the evolution of elastic properties during laboratory stick slip experiments spanning the transition from slow slip to dynamic rupture the physical mechanisms governing slow earthquakes remain unknown as does the relationship between slow and regular earthquakes to investigate the mechanism s of slow earthquakes and related quasi dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration we reproduced the full spectrum of slip behavior from slow to fast stick slip by altering the elastic stiffness of the loading apparatus k to match the critical rheologic stiffness of fault gouge kc our experiments show an evolution from stable sliding when k kc to quasi dynamic transients when k kc to dynamic instabilities when k kc to evaluate the microphysical processes of fault weakening we monitored variations of elastic properties we find systematic changes in p wave velocity vp for laboratory seismic cycles during the coseismic stress drop seismic velocity drops abruptly consistent with observations on natural faults in the preparatory phase preceding failure we find that accelerated fault creep causes a vp reduction for the complete spectrum of slip behaviors our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults depending on frictional properties in agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by vp changes during the seismic cycle the observed reduction in vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors

reservoir characterization using surface microseismic monitoring microseismic monitoring of reservoir processes can be performed using surface or near surface arrays we review the published technical basis for the use of the arrays and the historical development of the method beginning with locating earthquakes through geothermal exploration to the growing field of hydraulic fracture monitoring practical considerations for the array deployment and data processing are presented the road ahead for the technology includes a move toward life of field buried arrays as well as opportunities for extended interpretation of the data particularly inversion for source mechanism estimation and measurement of anisotropy in the monitored subsurface

biomarker thermal maturity experiments at earthquake slip rates abstract evidence of temperature rise is useful for identifying where within a fault zone earthquake slip has occurred as well as some attributes of the earthquake such as energy output biomarker thermal maturity can elucidate structures within a fault zone that were hotter than surrounding rock and therefore likely to have hosted earthquakes we performed a series of friction experiments on thin gouge layers of woodford shale at slip rates of 1 m s our results show that biomarker thermal maturity can clearly delineate zones of slip that are as thin as 150 m we establish that the kinetics of methylphenanthrene reaction previously determined at longer durations and lower temperatures are applicable at seismic slip rates and timescales temperature estimates show that once the slipping zone reached 500 c biomarker reaction was significant as predicted by the reaction kinetics our results demonstrate that temperature rise in fault zones is a function of both the work density and power density during the earthquake biomarker reactions along with other arrhenius style reactions such as those that lead to dynamic shear weakening are both a function of slip speed and frictional work

catastrophic emplacement of giant landslides aided by thermal decomposition heart mountain wyoming the heart mountain landslide of northwest wyoming is the largest known sub aerial landslide on earth during its emplacement more than 2000 km32000 km3 of paleozoic sedimentary and eocene volcanic rocks slid 45 km 45 km on a basal detachment surface dipping 2 leading to 100 yr of debate regarding the emplacement mechanisms recently emplacement by catastrophic sliding has been favored but experimental evidence in support of this is lacking here we show in friction experiments on carbonate rocks taken from the landslide that at slip velocities of several meters per second co2 starts to degas due to thermal decomposition induced by flash heating after only a few hundred microns of slip this is associated with the formation of vesicular degassing rims in dolomite clasts and a crystalline calcite cement that closely resemble microstructures in the basal slip zone of the natural landslide our experimental results are consistent with an emplacement mechanism whereby catastrophic slip was aided by carbonate decomposition and release of co2 allowing the huge upper plate rock mass to slide over a cushion of pressurized material

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

fault lubrication and earthquake propagation in thermally unstable rocks experiments performed on dolomite or mg calcite gouges at seismic slip rates v 1 m s and displacements d 1 m show that the frictional coefficient decays exponentially from peak values m p 0 8 in the byerlee9s range to extremely low steady state values ss 0 1 attained over a weakening distance d w microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products periclase and lime or portlandite were produced in the slip zone during the transient stage d w these observations integrated with co 2 emissions data recorded during the experiments suggest that particle interaction in the slip zone produces flash temperatures that are large enough to activate chemical and physical processes e g decarbonation reactions t 550 c during steady state d d w shear strength is very low and not dependent upon normal stresses suggesting that pressurized fluids co 2 may have been temporarily trapped within the slip zone at this stage a continuous layer of nanoparticles is developed in the slip zone for d d w a slight but abrupt increase in shear strength is observed and interpreted as due to fluids escaping the slip zone at this stage dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone experimentally derived seismic source parameter w b i e breakdown work the energy that controls the dynamics of a propagating fracture 1 matches w b values obtained from seismological data of the a d 1997 m6 colfiorito italy earthquakes which nucleated in the same type of rocks tested in this study and 2 suggests similar earthquake scaling relationships as inferred from existing seismological data sets we conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms which are activated or inhibited by physicochemical reactions in the slip zone

gouge formation by dynamic pulverization during earthquake rupture it is proposed here that fault gouge forms by rock pulverization within the tip region of a fast propagating earthquake due to the intensity of stress and rates of strain in this zone to examine this mechanism we calculate the deformation conditions of a dynamic shear fracture propagating close to the limiting rayleigh wave velocity the mechanical conditions close to the fracture tip are extreme tensile stresses approach 5 gpa volumetric strain rates exceed 105 s 1 and volumetric expansion alternates with volumetric contraction it is expected that a localized zone a few millimeters wide will be pulverized under these conditions this mechanism of dynamic gouge formation provides new insight to recent observations of the texture of gouge from the san andreas fault zone and from rupture zones in south african mines further as such extreme conditions develop only during fast propagating earthquakes presence or absence of pulverized gouge may serve as an indicator of earthquake propagation velocity

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

flash weakening of serpentinite at near seismic slip rates 1 to investigate the processes responsible for dynamic frictional weakening in antigorite serpentinite we conducted single velocity and velocity stepping friction experiments at slip rates v from 0 01 to 0 4 m s 1 sliding displacements up to 40 mm and a normal stress of 5 mpa single velocity experiments demonstrate an approximately 1 v dependence of friction on velocity above a characteristic weakening velocity vw 0 1 m s 1 consistent with theoretical predictions for flash heating and subsequent weakening of asperities velocity stepping experiments impose stepwise increases in slip rate show stepwise weakening at slip rates above vw scanning electron microscopy of experimental fault surfaces reveals nanoparticle gouge textures at raised sites 10 m in diameter in tests that exhibit dramatic weakening furthermore x ray diffraction analyses of fault gouge from the sliding surface document the formation of significant talc in these tests indicating that weakening temperatures reached 500 c 700 c in contrast no talc is observed in samples for which v remained significantly below vw the observed value for vw is consistent with published microphysical models for flash weakening when independent constraints on the thermal stability and contact strength of antigorite are considered finally while serpentinite displays velocity strengthening behavior at plate tectonic slip rates our results indicate that seismic ruptures propagating into serpentinized regions in the shallow lithosphere may trigger seismicity or slow earthquakes after limited amounts of displacement

coseismic ultramylonites an investigation of nanoscale viscous flow and fault weakening during seismic slip abstract faults weaken during the propagation of earthquakes due to the onset of thermally activated mechanisms which vary depending on the rock type recent experimental work suggests that carbonate hosted faults are lubricated by viscous flow in nano granular aggregates having ultramylonitic textures however their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions strain rates as high as 104 which remain so far poorly investigated and understood we explore the formation evolution and deformation mechanisms of coseismic ultramylonites in carbonate hosted faults generated during high velocity 1 4 m s 1 displacement controlled shear experiments in a rotary apparatus microstructures were analysed using integrated sem and tem imaging while detailed crystallographic fabrics were investigated using the electron back scattered diffraction ebsd technique mechanical data show that the strength of the experimental fault decays dynamically with slip according to a characteristic four stage evolution each stage is associated with characteristic textures microstructural observations show that brittle processes dominate when the fault is strong friction coefficients 0 6 cataclasis aided by twinning and crystal plasticity operates forming an extremely comminuted shear band mean grain size 200 nm as the fault starts weakening shear localises within a well defined principal slip zone here thermally activated grain size sensitive gss and insensitive gsi creep mechanisms compete with brittle processes in controlling fault strength gsi mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage by the end of the transient weakening stage the slip zone has reached a steady state thickness 30 m and shows a nanogranular ultramylonitic texture the intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage due to grainsize sensitive creep mechanisms becoming gradually more dominant as the experimental fault re strengthens upon deceleration to arrest the ultramylonite may be partially reworked by brittle deformation our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow adjacent deactivated layers where mirror like surfaces are located this shows that ebsd techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and potentially natural faults

impact of lake level change on deep water renewal and oxic conditions in deep saline lake van turkey 1 changes in the hydrological regime of the saline closed basin lake van a large deep lake in eastern turkey resulted in a lake level increase by about 2 m between 1988 and 1995 followed by a 1 5 m decrease until 2003 and a relatively constant lake level thereafter based on measurements of transient tracers sulfur hexafluoride cfc 12 3h 3he 4he ne dissolved oxygen light transmission conductivity temperature depth profiles and thermistor data we investigate the implications associated with lake level fluctuations for deep water renewal and oxygenation our data suggest that deep water renewal was significantly reduced in lake van between 1990 and 2005 this change in mixing conditions resulted in the formation of a more than 100 m thick anoxic deep water body below 325 m depth apparently the freshwater inflows responsible for the lake level rise between 1988 and 1995 decreased the salinity of the surface water sufficiently that the generation of density plumes during winter cooling was substantially reduced compared to that in the years before the lake level rise significant renewal and oxygenation of the deep water did not occur until at least 2005 although by 2003 the lake level was back to almost the same level as in 1988 this study suggests that short term changes in the hydrological regime resulting in lake level changes of a couple of meters can lead to significant and long lasting changes in deep water renewal and oxic conditions in deep saline lakes

flash heating and local fluid pressurization lead to rapid weakening in water saturated fault gouges coseismic fault displacement is quite large at shallow depths in some earthquakes and it implies that fault gouges and sediments have extremely low dynamic friction during seismic slip however the dynamic weakening mechanisms of gouges under wet conditions are still not well constrained here we present direct evidence for the occurrence of flash heating and local fluid pressurization in water saturated gouges by performing low to high velocity v 10 m s to 1 m s friction experiments in a pressure vessel under conditions specially designed to suppress weakening effects of bulk thermal and compaction induced pressurization the tested gouges exhibit transition from velocity strengthening to drastic velocity weakening as slip rates increase strong dynamic weakening starts to occur at v 0 04 m s at the initiation of sliding 0 1 m which is much more efficient than previously reported in terms of the weakening velocity and distance furthermore the onset of weakening is always accompanied by an instantaneous dilatancy 10 25 m which is much larger than that observed in dry tests and in contrast with gradual changes displayed in the wet tests without dynamic weakening numerical modeling integrated with microstructural observation reveals that bulk thermal pressurization cannot explain the experimental results while flash weakening triggered by vaporization of water layers on around asperity contacts and the resultant local fluid pressurization may be responsible for the observed rapid weakening concomitant with instantaneous dilatancy given high efficiency of such weakening process fluid infiltrated faults could be weakened more rapidly than previously recognized during seismic slip

microphysical model predictions of fault restrengthening under room humidity and hydrothermal conditions from logarithmic to power law healing the maximum fault strength and rate of interseismic fault strengthening healing are of great interest to earthquake hazard assessment studies as they directly relate to event magnitude and recurrence time previous laboratory studies have revealed two distinct frictional healing behaviors referred to as dieterich type and non dieterich type healing these are characterized by respectively log linear and power law increase in the strength change with time to date there is no physical explanation for the frictional behavior of fault gouges that unifies these seemingly inconsistent observations using a microphysical friction model previously developed for granular fault gouges we investigate fault strengthening analytically and numerically under boundary conditions corresponding to laboratory slide hold slide tests we find that both types of healing can be explained by considering the difference in grain contact creep rheology at short and long time scales under hydrothermal conditions favorable for pressure solution creep healing exhibits a power law evolution with hold time with an exponent of 1 3 and an apparent cutoff time of hundreds of seconds under room humidity conditions where grain contact deformation exhibits only a weak strain rate dependence the predicted healing also exhibits a power law dependence on hold time but it can be approximated by a log linear relation with of a few seconds we derive analytical expressions for frictional healing parameters i e healing rate cutoff time and maximum healing of which the predictions are consistent with numerical implementation of the model finally we apply the microphysical model to small fault patches on a natural carbonate fault and interpret the restrengthening during seismic cycles

role of seismogenic processes in fault rock development an example from death valley california fault rocks developed along the mormon point turtleback of southern death valley suggest that a jog in the oblique slip death valley fault zone served as an ancient seismic barrier where dominantly strike slip ruptures were terminated at a dilatant jog dramatic spatial variations in fault rock thickness and type within the bend are interpreted as the products of 1 fault overshoot in which planar ruptures bypass the intersection of the two faults composing the bend and slice into the underlying footwall and 2 implosion brecciation in which coseismic ruptures arrested at a releasing bend in the fault lead to catastrophic collapse brecciation fluid influx and mineralization

tube wave to shear wave conversion at borehole plugs abstract in hydraulic fracturing experiments perforation shots excite body and tube waves that sample and thus can be used to characterize the surrounding medium while these waves are routinely employed in borehole operations their resolving power is limited by the experiment geometry the signal to noise ratio and their frequency content it is therefore useful to look for additional complementary signals that could increase this resolving power tube to body wave conversions scattering of tube to compressional or shear waves at borehole discontinuities are one such signal these waves are not frequently considered in hydraulic fracture settings yet they possess geometrical and spectral attributes that greatly complement the resolution afforded by body and tube waves alone here we analyze data from the jonah gas field wyoming usa to demonstrate that tube to shear wave conversions can be clearly observed in the context of hydraulic fracturing experiments these waves are identified primarily on the vertical and radial components of geophones installed in monitoring wells surrounding a treatment well they exhibit a significantly lower frequency content 10 100 hz than the primary compressional waves 100 1000 hz tapping into such lower frequencies could help to better constrain velocity in the formation thus allowing better estimates of fracture density porosity and permeability moreover the signals of tube to shear wave conversion observed in this particular study provide independent estimates of the shear wave velocity in the formation and of the tube wave velocity in the treatment well

petroleum reservoir characterization using downhole microseismic monitoring imaging of microseismic data is the process by which we use information about the source locations timing and mechanisms of the induced seismic events to make inferences about the structure of a petroleum reservoir or the changes that accompany injections into or production from the reservoir a few key projects were instrumental in the development of downhole microseismic imaging most recent microseismic projects involve imaging hydraulic fracture stimulations which has grown into a widespread fracture diagnostic technology this growth in the application of the technology is attributed to the success of imaging the fracture complexity of the barnett shale in the fort worth basin texas and the commercial value of the information obtained to improvecompletions and ultimately production in the field the use of commercial imaging in the barnett is traced back to earlier investigations to prove the technology with the cotton valley imaging project and earlier experiments at the m site in the piceance

detection of open fractures with vertical seismic profiling in vertical seismic profiling surveys tube waves are generated by compressional waves impinging on subsurface fractures or permeable zones the problem of generation of these waves by a nonnormal incident p wave for an inclined borehole intersecting a tilted parallel wall fracture is formulated theoretically the amplitude of tube waves depends on the permeability the length of the fracture and the frequency the relative effects of these parameters are studied individually the problem is also formulated for a thin oblate ellipsoidal penny shaped fracture the results for the two fracture models are compared and contrasted field data from tyngsboro massachusetts are shown for open fractures in granite from tube wave amplitudes normalized to p wave amplitudes calculated permeabilities are of the order of 100 mdarcy

circulation and turbulent exchange characteristics during the thermal bar in lake ontario we made observations from vertical moorings of thermistors and current meters in lake ontario to study the spring thermal bar circulation its effect in inhibiting the horizontal mixing and spatial and temporal variability of vertical mixing during this period thermal bar progression rates were compared with two analytical models both produced overestimates during the early part of the thermal bar and underestimates during the late spring in a zone between the shore and the thermal bar currents were influenced by the local wind stress and flowed in a counterclockwise direction the mean cross shore flows in the nearshore zone were reduced during the thermal bar period the alongshore horizontal exchange coefficients were higher than cross shore exchange coefficients during the thermal bar period the magnitude of both alongshore and cross shore exchange coefficients decreased when the bar was still within middepth 40 m when it was further offshore the thermal bar did not have any significant effect on alongshore exchange coefficients but cross shore exchanges decreased marginally the vertical exchange coefficients were high in the water column because of convective mixing during the thermal bar period during the evolution of spring thermal bar the vertical mixing decreased considerably in the nearshore regions under stable stratification whereas the high vertical mixing levels continued in the offshore region for a longer period

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths

fault zone structures and strain localization in clinoptilolite tuff nižný hrabovec slovak republic abstract clinoptilolite is a natural micro porous zeolite composed of silica and alumina tetrahedra that commonly forms as a devitrification product of glass in volcanic tuffs besides applications in the agriculture building and manufacturing industries clinoptilolite has been widely studied in veterinary and human medicine because of its positive effects on health based on field observations and microstructural investigations we show for the first time strain localization features along faults in natural zeolite tuffs in a clinoptilolite tuff open pit mine at ni n hrabovec slovak republic the faults are localized along pre existing joints with plumose structures that acted as a pathway for the infiltration of mainly mn and k rich fluids displacement along the faults created structures that are indicative of both velocity hardening accommodated by dissolution precipitation creep on sc scc foliation and velocity weakening accommodated by multi generations of ultracataclasites along principal slip surfaces behaviors rock fluid interaction was characterized by a high mobility of k rich fluids with k feldspar decorating the sc scc foliation infiltrating fractures in the damage zone of the faults and precipitating as idiomorphic crystals in open cavities and along the fault surface microstructures such as polished slickensides injection of fluidized cataclasites clast cortex grains in cataclasites and truncated grains along principal slip surfaces probably suggest that at least some of the faults formed during seismic slip

dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates to investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite gouge at subseismic to seismic slip rates we conducted single velocity step and multiple velocity step friction experiments on an antigorite rich and lizardite rich serpentinite at slip rates v from 0 003 m s to 6 5 m s sliding displacements up to 1 6 m and normal stresses n up to 22 mpa for gouge and 97 mpa for bare surfaces nominal steady state friction values nss in gouge at v 1 m s are larger than in bare surfaces for all n tested and demonstrate a strong n dependence nss decreased from 0 51 at 4 0 mpa to 0 39 at 22 4 mpa conversely nss values for bare surfaces remained 0 1 with increasing n and v additionally the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces extrapolation of the normal stress dependence for nss suggests that the behavior of antigorite gouge approaches that of bare surfaces at n 60 mpa x ray diffraction revealed dehydration reaction products in samples that frictionally weakened microstructural analysis revealed highly localized slip zones with melt like textures in some cases gouge experiments and in all bare surfaces experiments for v 1 m s one dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed

radiography of a normal fault system by 64 000 high precision earthquake locations the 2009 l aquila central italy case study 1 we studied the anatomy of the fault system where the 2009 l aquila earthquake mw 6 1 nucleated by means of 64 k high precision earthquake locations spanning 1 year data were analyzed by combining an automatic picking procedure for p and s waves together with cross correlation and double difference location methods reaching a completeness magnitude for the catalogue equal to 0 7 including 425 clusters of similar earthquakes the fault system is composed by two major faults the high angle l aquila fault and the listric campotosto fault both located in the first 10 km of the upper crust we detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops we observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays the width of the l aquila fault zone varies along strike from 0 3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution up to 1 5 km at the fault tips with an increase in the geometrical complexity these characteristics similar to damage zone properties of natural faults underline the key role of aftershocks in fault growth and co seismic rupture propagation processes additionally we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological i e creeping discontinuity explaining how normal faults detach at depth