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

seismicity triggered by fluid injection induced aseismic slip anthropogenic fluid injections are known to induce earthquakes the mechanisms involved are poorly understood and our ability to assess the seismic hazard associated with geothermal energy or unconventional hydrocarbon production remains limited we directly measure fault slip and seismicity induced by fluid injection into a natural fault we observe highly dilatant and slow 4 micrometers per second m s aseismic slip associated with a 20 fold increase of permeability which transitions to faster slip 10 m s associated with reduced dilatancy and micro earthquakes most aseismic slip occurs within the fluid pressurized zone and obeys a rate strengthening friction law 0 67 0 045ln v v with v 0 1 m s fluid injection primarily triggers aseismic slip in this experiment with micro earthquakes being an indirect effect mediated by aseismic creep

observational constraints on the fracture energy of subduction zone earthquakes we relate seismologically observable parameters such as radiated energy seismic moment rupture area and rupture speed to the dynamics of faulting to achieve this objective we computed the radiated energy for 23 subduction zone earthquakes recorded between 1992 and 2001 most of these earthquakes have a magnitude m w 7 5 but we also included some smaller m w 6 7 well studied subduction zone earthquakes and six crustal earthquakes we compiled static stress drop estimates for these 29 earthquakes from literature and used a slip weakening model to determine the radiation efficiency of these earthquakes we also determined the rupture speed of these earthquakes from literature from fracture mechanics fracture energy and hence radiation efficiency can be related to the rupture speed the radiation efficiencies estimated from the partitioning of energy as given by the slip weakening model are consistent with the rupture speed estimated for these earthquakes most earthquakes have radiation efficiencies between 0 25 and 1 and are hence efficient in generating seismic waves but tsunami earthquakes and two deep earthquakes the 1994 bolivia and the 1999 russia china border earthquakes have very small radiation efficiencies 0 25 and hence dissipate a large amount of energy during faulting we suggest that differences in the radiation efficiencies of different types of earthquakes could be due to fundamental differences in their rupture mechanics in deep events the energy is probably dissipated in thermal processes in the fault zone while it is possible that the morphology of the trench causes large energy dissipation during the rupture process of tsunami earthquakes

ubiquitous weakening of faults due to thermal pressurization faults weaken during earthquakes analysis of the amount of energy released during earthquakes globally suggests that heat induced pressurization of pore fluids can weaken faults during earthquakes of all sizes laboratory simulations of earthquakes show that at high slip rates faults can weaken significantly aiding rupture1 2 3 various mechanisms such as thermal pressurization and flash heating have been proposed to cause this weakening during laboratory experiments1 4 5 6 yet the processes that aid fault slip in nature remain unknown measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening known as an event s fracture energy7 8 9 here we compile new and existing8 9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes we observe a distinct transition in how fracture energy scales with event size which implies that faults weaken differently during small and large earthquakes and earthquakes are not self similar we use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms we find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes over seven orders of fault slip magnitude we conclude that thermal pressurization is a widespread and prominent process for fault weakening

multifractal analysis of the pore space of real and simulated sedimentary rocks it is well known that sedimentary rocks having same porosity can have very different pore size distribution the pore distribution determines many characteristics of the rock among which its transport properties are often the most useful multifractal analysis is a powerful tool that is increasingly used to characterize the pore space in this study we performed multifractal analysis of pore distribution on simulated sedimentary rocks using the relaxed bidisperse ballistic deposition model rbbdm the rbbdm can generate a 3 d structure of sedimentary rocks of variable porosity by tuning the fraction p of particles of two different sizes we also performed multifractal analysis on two samples of real sedimentary rock to compare with the simulation studies one sample an oolitic limestone is of high porosity 40 per cent while the other is a reefal carbonate of low porosity around 7 per cent 2 d sections of x ray microtomographs of the real rocks were stacked sequentially to reconstruct the real rock specimens both samples show multifractal character the results from analysis of real rock agree quite well with the simulated structure of low porosity the simulated rock of high porosity showed a weak multifractal nature though the real rock sample of similar porosity was found to be strongly multifractal we propose a structure parameter which is a function of porosity and the generalized dimensions and controls the transport properties of the rock

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

the influence of ambient fault temperature on flash heating phenomena 1 recent high velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface i e flash heating here we explore the effects of elevated ambient fault temperature up to that at the base of the earth s seismogenic zone on the flash heating behavior of quartzite westerly granite and india gabbro the experiments demonstrate that elevated ambient fault temperature causes 1 an increase in the characteristic weakening velocity for flash heating and 2 an increase in the friction coefficient at a given velocity above this characteristic weakening velocity these unexpected results are reconciled with flash heating theory when temperature specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone

effect of water on the frictional behavior of cohesive rocks during earthquakes fluid rock interactions can control earthquake nucleation and the evolution of earthquake sequences experimental studies of fault frictional properties in the presence of fluid can provide unique insights into these interactions we report the first results from experiments performed on cohesive silicate bearing rocks microgabbro in the presence of pressurized pore fluids h2o drained conditions at realistic seismic deformation conditions the experimental data are compared with those recently obtained from carbonate bearing rocks carrara marble contrary to theoretical arguments and consistent with the interpretation of some field observations we show that frictional melting of a microgabbro develops in the presence of water in microgabbro the initial weakening mechanism flash melting of the asperities is delayed in the presence of water conversely in calcite marble the weakening mechanism brittle failure of the asperities is favored this opposite behavior highlights the importance of host rock composition in controlling dynamic frictional weakening in the presence of water cohesive carbonate bearing rocks are more prone to slip in the presence of water whereas the presence of water might delay or inhibit the rupture nucleation and propagation in cohesive silicate bearing rocks 2013 geological society of america

porosity and permeability changes in sedimentary rocks induced by injection of reactive fluid a simulation model summary numerical programs for simulating flow and reactive transport in porous media is essential for predicting reservoir properties related to co 2 sequestration performance subsurface storage and risk assessment in this paper we solve the navier stokes equation using finite difference method on a simulated porous rock structure to study the velocity distribution of fluid flowing through it under a constant pressure gradient a reactive solute carried through the fluid is allowed to interact with the minerals in the rock this chemical reaction dissolves the mineral which changes the rock structure thus affecting its flow properties these changes of flow properties are studied with variation in reactive solute concentration and pressure field the different mechanisms of dissolution responsible for the variation of flow properties for the different parameters is predicted before the onset of homogeneous dissolution variation in porosity follows a power law behaviour with change in permeability when the latter is scaled by the concentration of the reactive species the simulation results are compared with available experimental data and found to give a reasonable match

the fracture energy of ruptures driven by flash heating we present a model for dynamic weakening of faults based on local flash heating at microscopic asperity contacts coupled to bulk heating at macroscopic scale we estimate the fracture energy g associated with that rheology and find that for constant slip rate histories g scales with slip as math formula at small slip while math formula at large slip this prediction is quantitatively consistent with data from laboratory experiments conducted on dry rocks at constant slip rate we also estimate g for crack like ruptures propagating at constant speed and find that math formula in the large slip limit quantitative estimates of g in that regime tend to be several orders of magnitude lower than seismologically inferred values of g we conclude that while flash heating provides a consistent explanation for the observed dynamic weakening in laboratory experiments with kinematically imposed slip its contribution to the energy dissipation during earthquakes becomes negligible for large events when considering the elastodynamic coupling between strength and slip evolution

surface processes recorded by rocks and soils on meridiani planum mars microscopic imager observations during opportunity s first three extended missions the microscopic imager mi on the mars exploration rover opportunity has returned images of mars with higher resolution than any previous camera system allowing detailed petrographic and sedimentological studies of the rocks and soils at the meridiani planum landing site designed to simulate a geologist s hand lens the mi is mounted on opportunity s instrument arm and can resolve objects 0 1 mm across or larger this paper provides an overview of mi operations data calibration and analysis of mi data returned during the first 900 sols mars days of the opportunity landed mission analyses of opportunity mi data have helped to resolve major questions about the origin of observed textures and features these studies support eolian sediment transport rather than impact surge processes as the dominant depositional mechanism for burns formation strata mi stereo observations of a rock outcrop near the rim of erebus crater support the previous interpretation of similar sedimentary structures in eagle crater as being formed by surficial flow of liquid water well sorted spherules dominate ripple surfaces on the meridiani plains and the size of spherules between ripples decreases by about 1 mm from north to south along opportunity s traverse between endurance and erebus craters

triggering of earthquake aftershocks by dynamic stresses it is thought that small static stress changes due to permanent fault displacement can alter the likelihood of or trigger earthquakes on nearby faults many studies of triggering in the near field particularly of aftershocks rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault here we report a comparison of the aftershock pattern of the moment magnitude mw 7 3 landers earthquake not only with static stress changes but also with transient oscillatory stress changes transmitted as seismic waves that is dynamic stresses dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone these dynamically weakened faults may fail after the seismic waves have passed by and might even cause earthquakes that would not otherwise have occurred we find similar asymmetries in the aftershock and dynamic stress patterns the latter being due to rupture propagation whereas the static stress changes lack this asymmetry previous studies have shown that dynamic stresses can promote failure at remote distances but here we show that they can also do so nearby

microscopic pore structures of tight sandstone reservoirs and their diagenetic controls a case study of the upper triassic xujiahe formation of the western sichuan depression china abstract the study of microscopic pore structures is of central significance to the evaluation of the reservoir quality and well productivity of tight gas sandstone the second and fourth members abbreviated as t3x2 and t3x4 respectively of the upper triassic xujiahe formation of the western sichuan depression were used as cases to analyze microscopic pore structures and diagenetic controls in consideration of the disparities in reservoir quality and well productivity optical microscopy of thin sections micro ct scanning scanning electron microscopy sem high pressure mercury injection hpmi and apex methods based on the hpmi results were used to determine the pore and throat geometries and the types and pore size distributions in terms of the quantity and levels of pore connectivity tight sandstone reservoirs of the study area are characterized by poor physical properties with the t3x2 reservoirs presenting relatively higher levels of permeability and lower porosity levels than those of the t3x4 reservoirs the pore spacing configurations sizes and distributions and throat geometries reveal significant disparities primary pores and micro fractures were found to be more developed in the t3x2 reservoirs with bending and narrow pore throats measured as smaller than 0 5 m compared to those of the t3x2 reservoirs secondary pores including moldic and intracrystalline pores were more developed in the t3x4 reservoirs with relatively larger straighter and wider throats of 0 5 1 0 m an evaluation of the pore connectivity levels shows that only 26 86 and 28 90 pores are closely connected to the whole for t3x2 and t3x4 reservoirs respectively four major diagenetic processes were found to control reservoir quality and the microscopic pore structure mechanical compaction is the primary cause of the densification of reservoirs in the study area compared to that observed in the t3x4 reservoirs more intensive mechanical compaction was observed in the t3x2 reservoirs changing the configuration of the throats present reducing the pore throat radii and ultimately lowering the porosity of t3x2 reservoirs as a whole quartz overgrowth and late period carbonate cements were observed in partly filled pore spaces narrowing throats and reducing pore connectivity but eogenetic cementation and chlorite cements can increase the pressure resistance of sandstone and inhibit compaction quartz overgrowth thus t3x4 reservoirs are relatively more porous as a result of eogenetic calcite cementation and primary pores are more developed in t3x2 reservoirs with chlorite cements dissolution is a major factor that improves the tight sandstone reservoir quality because it generates a large volume of secondary pores the more intensive levels of dissolution in t3x4 reservoirs not only creates more intergranular intragranular moldic and intracrystalline pores but also further increases the porosity of t3x4 reservoirs fracturing effects have developed in the study area to improve pore connectivity levels and thus the permeability of tight sandstone reservoirs potentially explaining the pore connectivity and permeability of certain t3x2 reservoirs this work may be able to be used as a useful reference for the reservoir quality evaluation and exploration of tight gas sandstone reservoirs for similar oil fields worldwide

common evolution of mechanical and transport properties in thermally cracked westerly granite at elevated hydrostatic pressure increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties including mechanical strength elastic and mechanical anisotropy and the enhancement of transport properties using a non interactive crack effective medium nic theory as a fundamental tool we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density further we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness kic of rocks as measured via a suite of tensile failure experiments chevron cracked notch brazilian disk finally we include fluid flow in our analysis via the gueguen and dienes crack porosity permeability model using the crack density and aspect ratio recovered from the elastic wave velocity inversion we successfully compare permeability evolution with pressure with the laboratory measurements of permeability

fault valve behavior and the hydrostatic lithostatic fluid pressure interface abstract currently deforming regions of continental crust are characterized by seismic activity in a brittle carapace which presumably overlies regions undergoing local or distributed metamorphism one may note in this regard the global correlation between active seismic belts and the distribution of co2 rich springs barnes et al 1984 in regions of moderate to high heat flow 60 100 mw m 2 the seismogenic regime representing the zone of unstable frictional sliding typically extends to depths of 10 15 km marking the onset of greenschist metamorphic conditions sibson 1983 however in regions of rapid thrust convergence 1cm yr 1 seismic activity may extend to significantly greater depths through depression of isotherms as in the thrust front of the himalaya

elastohydrodynamic lubrication of faults the heat flow paradox provides evidence that a dynamic weakening mechanism may be important in understanding fault friction and rupture we present here a specific model for dynamic velocity weakening that uses the mechanics of well studied industrial bearings to explain fault zone processes an elevated fluid pressure is generated in a thin film of viscous fluid that is sheared between nearly parallel surface this lubrication pressure supports part of the load therefore reducing the normal stress and associated friction across the gap the pressure also elastically deforms the wall rock the model is parameterized using the sommerfeld number which is a measure of the lubrication pressure normalized by the lithostatic load for typical values of the material properties slip distance and velocity the sommerfeld number suggests that lubrication is an important process if the lubrication length scales as the slip distance in an earthquake the frictional stress during dynamically lubricated large earthquakes is 30 less than the friction with only hydrostatic pore pressure elastohydrodynamic lubrication also predicts a decrease in high frequency 1 hz radiation above a critical slip distance of a few meters this prediction is well matched by the strong motion data from the 1999 taiwan earthquake the observed 2 orders of magnitude variation in scaled radiated energy between small m w 6 is also predicted by the lubrication model

simulation of diagenesis and permeability variation in two dimensional rock structure summary a model is suggested to simulate the physical aspect of diagenesis in porous rocks a bidisperse ballistic deposition model with relaxation of deposited grains is used to generate the porous structure sedimentation and erosion are allowed to restructure the pore space as a fluid flows through the rock the effect of this restructuring of the pore space on permeability is studied the navier stokes equation is solved numerically by the finite difference method to determine the pressure and velocity distributions in the pore space we find that though deposition is the dominant process in our model of diagenesis reducing the porosity the permeability may increase dramatically in some cases these are when the erosion takes place at a single narrow constriction in the pore channel

frictional properties of a low angle normal fault under in situ conditions thermally activated velocity weakening the zuccale fault is a regional low angle normal fault exposed on the isle of elba in central italy that accommodated a total shear displacement of 6 8 km the fault zone structure and fault rocks formed at 8 km crustal depth the present day fault structure is the final product of several deformation processes superposed during the fault history in this study we report results from a series of rotary shear experiments performed on 1 mm thick powdered gouges made from several fault rock types obtained from the zuccale fault the tests were done under conditions ranging from room temperature to in situ conditions i e at temperatures up to 300 c applied normal stresses up to 150 mpa and fluid saturated the ratio of fluid pressure to normal stress was held constant at either 0 4 or 0 8 to simulate an overpressurized fault the samples were sheared at a constant sliding velocity of 10 m s for at least 5 mm after which a velocity stepping sequence from 1 to 300 m s was started to determine the velocity dependence of friction this can be represented by the rate and state parameter a b which was determined by an inversion of the data to the rate and state equations friction of the various fault rocks varies between 0 3 and 0 8 similar to values obtained in previous studies and decreases with increasing phyllosilicate content friction decreases mildly with temperature whereas normal stress and fluid pressure do not affect friction values systematically all samples exhibited velocity strengthening conditionally stable behavior under room temperature conditions and a b increased with increasing sliding velocity in contrast velocity weakening accompanied by stick slips was observed for the strongest samples at 300 c and sliding velocities below 10 m s an increase in fluid pressure under these conditions led to a further reduction in a b for all samples so that they exhibited unstable stick slip behavior at low sliding velocity the results suggest that phyllosilicate bearing fault rocks can deform by stable aseismic creep at low resolved shear stress and low shear rate conditions an increase in fluid pressure or loading of stronger portions could lead to a runaway instability the runaway instability might be limited in size because of 1 the fault heterogeneity 2 the observed strengthening at higher sliding velocities and 3 a co seismic drop in pore fluid pressure

fractal geometry of sedimentary rocks simulation in 3 d using a relaxed bidisperse ballistic deposition model several studies both theoretical and experimental show that sedimentary rocks have a fractal pore grain interface in this paper a computer simulated 3 d sedimentary rock structure generated by the relaxed ballistic bidisperse deposition model rbbdm is investigated to characterize the micro structure of its pores the pore volume and the rock pore interface show the same fractal dimension indicating that the pore volume is a fractal the two point density correlation is computed for the pore space and the results compare favourably with the range reported from experiments an array of 2 d x ray tomography micrograph sections of a real sedimentary rock an oolitic limestone pure calcite from the mondeville formation of middle jurassic age paris basin france was used to generate a 3 d bitmap the 3 d real rock sample generated in this manner was analysed for similar studies as the simulated structure the results were compared with those obtained from simulation the simulation results agree qualitatively with the real rock sample diffusion through the connected pore space of the simulated structure was studied using a random walk algorithm and the results compared with the similar simulation study done on the 3 d oolitic limestone specimen in both cases diffusion was found to be anomalous indicating that the sedimentary rock has a fractal geometry the favourable comparability of results between the simulated and real rock supports the usefulness of the model of sedimentary rock generation which can be applicable to transport phenomena

a constitutive scaling law and a unified comprehension for frictional slip failure shear fracture of intact rock and earthquake rupture 1 it is widely recognized that some of the physical quantities inherent in a rupture are scale dependent and the scale dependence is one of the most no facts and features of rupture phenomena the paper addresses how such scale dependent shear rupture of a broad range from laboratory scale frictional slip failure and shear fracture of intact rock to field scale rupture as an earthquake source can be unified by a single constitutive law noting that the earthquake rupture is a mixed process between frictional slip failure and the shear fracture of intact rock it is concluded that the constitutive law for the earthquake rupture be formulated as a unifying law that governs both frictional slip failure and shear fracture of intact rock it is demonstrated that the slip dependent constitutive law is such a unifying law and a constitutive scaling law is derived from laboratory data on both frictional slip failure and shear fracture of intact rock this constitutive scaling law enables one to provide a consistent and unified comprehension for scale dependent physical quantities inherent in the rupture over a broad range from small scale frictional slip failure and shear fracture in the laboratory to large scale earthquake rupture in the field

a simulation study of reactive flow in 2 d involving dissolution and precipitation in sedimentary rocks in this paper we solve the navier stokes equation using finite difference method on a simulated porous rock structure in 2 d to study the velocity distribution of fluid flowing through it under a constant pressure gradient a reactive solute carried through the fluid is allowed to interact with the minerals in the rock depending on the rock composition both dissolution and precipitation reactions may occur however precipitation occurs only through the cations that are released in the solution due to dissolution these combined dissolution precipitation reactions change the porosity permeability and pore geometry of the sedimentary rock we study the temporal changes of these properties as functions of peclet number concentration of the reactive solute and ratio of damkholer numbers of dissolution to precipitation the final flow property is decided by a combination of these parameters

accretion of asian dust to hawaiian soils isotopic elemental and mineral mass balances hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs including sea salt aerosols and asian dust we use soil mineralogy and radiogenic isotope geochemistry sr and nd to evaluate the impact of asian dust on a chronosequence of hawaiian soils dust becomes an important constituent of soils 20 ky and older near surface 50 cm depth horizons contain as much as 30 quartz a mineral absent from local parent material basaltic sr and nd isotope signatures in these horizons have been completely overprinted by asian dust signatures with 87sr 86sr ratios as high as 0 723 and end values as low as 7 ree patterns in these soils are indistinguishable from that of average upper continental crust quartz abundance and nd isotopes provide two independent tracers of long term dust additions to the chronosequence soils the two tracers indicate a minimum long term average dust accretion rate of 125 mg cm 2 ky 1 at the 150 ka chronosequence site roughly a factor of three higher than holocene dust accumulation rates estimated from north pacific sediment cores we find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older requiring a loss mechanism for accreted dust on the basis of the geomorphic stability of these sites observed preferential loss of dust derived mica relative to quartz and estimates of soil si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils dust has a profound effect on the budgets of elements that are susceptible to leaching losses dust becomes the dominant source of soil nutrients si and p in the oldest most intensely weathered soils we calculate a dust derived p input flux of 0 8 mg cm 2 ky 1 and a dust derived si input flux of 35 mg cm 2 ky 1 si leaching fluxes are high 1400 mg cm 2 ky 1 in the youngest 2 ka soils and drop systematically with soil age to a value that closely balances the dust derived si input flux by 4100 ka extremely refractory elements such as nb which are concentrated in soils by residual enrichment processes are much less readily impacted by dust addition although dust has had a pronounced impact on sr and nd isotopic budgets and on soil mineral composition at the 150 ka site dust cannot have contributed 4 of the total nb contained in this soil profile

pore fluid in experimental calcite bearing faults abrupt weakening and geochemical signature of co seismic processes while it is widely recognized that fluids influence fault strength and earthquake nucleation propagation and arrest their effects on co seismic sliding friction are only conjectured to shed light on these effects 55 high velocity 1ms 1 friction experiments were conducted at room temperature on carrara marble samples in the presence of pore fluid up to 15mpa pore pressure room humidity and vacuum 10 4mbar conditions in all the experiments the friction coefficient evolved from a peak value of 0 6 0 8 to a steady state value of 0 1 in about 1 1 5m of slip however experiments performed in the presence of pore fluid had a large and more abrupt decrease in friction at the initiation of sliding 65 after 20mm of slip whereas experiments performed under vacuum and room humidity conditions showed initial velocity strengthening behavior followed by a more gradual reduction in friction this indicates that calcite bearing rocks are more prone to slip in the presence of water under room humidity conditions co2 was detected during the entire duration of the experiment in the presence of pore fluid hco3 and ca2 were detected for slips 0 1m the lack of decarbonation products hco3 and ca2 in pore fluid experiments for slip 0 1m implies that the abrupt weakening is not related to decarbonation or that the abundance of the reaction products is below the resolution of the analytical methods given the modest thermal expansion of water the estimated thermal pressurization during the abrupt weakening appears to be negligible instead we suggest that abrupt weakening is due to subcritical crack growth hydrolytic weakening and brittle failure of the asperities on the sliding surfaces modeling shows that the occurrence in nature of co seismic water present decarbonation reactions similar to those triggered in the laboratory could yield sufficient reaction product to be detected in aquifers located in the proximity of active faults 2012 elsevier b v

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

transient triggering of near and distant earthquakes we demonstrate qualitatively that frictional instability theory provides a context for understanding how earthquakes may be triggered by transient loads associated with seismic waves from near and distance earthquakes we assume that earthquake triggering is a stick slip process and test two hypotheses about the effect of transients on the timing of instabilities using a simple spring slider model and a rate and state dependent friction constitutive law a critical triggering threshold is implicit in such a model formulation our first hypothesis is that transient loads lead to clock advances i e transients hasten the time of earthquakes that would have happened eventually due to constant background loading alone modeling results demonstrate that transient loads do lead to clock advances and that the triggered instabilities may occur after the transient has ceased i e triggering may be delayed these simple clock advance models predict complex relationships between the triggering delay the clock advance and the transient characteristics the triggering delay and the degree of clock advance both depend nonlinearly on when in the earthquake cycle the transient load is applied this implies that the stress required to bring about failure does not depend linearly on loading time even when the fault is loaded at a constant rate the timing of instability also depends nonlinearly on the transient loading rate faster rates more rapidly hastening instability this implies that higher frequency and or longer duration seismic waves should increase the amount of clock advance these modeling results and simple calculations suggest that near tens of kilometers small moderate earthquakes and remote thousands of kilometers earthquakes with magnitudes 2 to 3 units larger may be equally effective at triggering seismicity our second hypothesis is that some triggered seismicity represents earthquakes that would not have happened without the transient load i e accumulated strain energy would have been relieved via other mechanisms we test this using two new seismicity models that 1 are inherently unstable but slide at steady state conditions under the background load and 2 are conditionally stable such that instability occurs only for sufficiently large perturbations for the new seismicity models very small amplitude transients trigger instability relative to the clock advance models the unstable steady state models predict that the triggering delay depends inversely and nonlinearly on the transient amplitude as in the clock advance models we were unable to generate delayed triggering with conditionally stable models for both new seismicity models the potential for triggering is independent of when the transient load is applied or equivalently of the prestress unlike in the clock advance models in these models a critical triggering threshold appears to be inversely proportional to frequency further advancement of our understanding will require more sophisticated quantitative models and observations that distinguish between our qualitative yet distinctly different model predictions

stress drop variations of induced earthquakes at the basel geothermal site 1 we determine stress drops from p wave spectra of about 1000 earthquakes induced by hydraulic stimulation in crystalline rock for a deep heat mining project in basel switzerland we observe an increase in stress drop by about a factor of five with radial distance from 10 m to 300 m which suggests that stress drop correlates with pore pressure perturbations due to the injection we test this hypothesis by calculating the injection related pore pressure perturbation based on a simple linear pore pressure diffusion model and find a good correlation of the expected pore pressure perturbation with the estimated stress drops

frictional evolution acoustic emissions activity and off fault damage in simulated faults sheared at seismic slip rates we present a series of high velocity friction tests conducted on westerly granite using the slow to high velocity apparatus shiva installed at istituto nazionale di geofisica e vulcanologia roma with acoustic emissions aes monitored at high frequency 4 mhz both atmospheric humidity and pore fluid water pressure conditions were tested under effective normal stress sigma eff n in the range 5 20 mpa and at target sliding velocities v s in the range 0 003 3 m s under atmospheric humidity two consecutive friction drops were observed the first one is related to flash weakening and the second one to the formation and growth of a continuous layer of melt in the slip zone in the presence of fluid a single drop in friction was observed average values of fracture energy are independent of effective normal stress and sliding velocity however measurements of elastic wave velocities on the sheared samples suggested that larger damage was induced for 0 1 v s 0 3 m s this observation is supported by aes recorded during the test most of which were detected after the initiation of the second friction drop once the fault surface temperature was high some aes were detected up to a few seconds after the end of the experiments indicating thermal rather than mechanical cracking in addition the presence of pore water delayed the onset of aes by cooling effects and by reducing of the heat produced supporting the link between aes and the production and diffusion of heat during sliding using a thermoelastic crack model developed by fredrich and wong 1986 we confirm that damage may be induced by heat diffusion indeed our theoretical results predict accurately the amount of shortening and shortening rate supporting the idea that gouge production and gouge comminution are in fact largely controlled by thermal cracking finally we discuss the contribution of thermal cracking in the seismic energy balance in fact while a dichotomy exists in the literature regarding the partitioning between fracture and heat energy the experimental evidence reported here suggests that both contribute to fault weakening and off fault damage

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

earthquake fracture energy inferred from kinematic rupture models on extended faults 1 we estimate fracture energy on extended faults for several recent earthquakes by retrieving dynamic traction evolution at each point on the fault plane from slip history imaged by inverting ground motion waveforms we define the breakdown work wb as the excess of work over some minimum traction level achieved during slip wb is equivalent to seismological fracture energy g in previous investigations our numerical approach uses slip velocity as a boundary condition on the fault we employ a three dimensional finite difference algorithm to compute the dynamic traction evolution in the time domain during the earthquake rupture we estimate wb by calculating the scalar product between dynamic traction and slip velocity vectors this approach does not require specifying a constitutive law and assuming dynamic traction to be collinear with slip velocity if these vectors are not collinear the inferred breakdown work depends on the initial traction level we show that breakdown work depends on the square of slip the spatial distribution of breakdown work in a single earthquake is strongly correlated with the slip distribution breakdown work density and its integral over the fault breakdown energy scale with seismic moment according to a power law with exponent 0 59 and 1 18 respectively our estimates of breakdown work range between 4 105 and 2 107 j m2 for earthquakes having moment magnitudes between 5 6 and 7 2 we also compare our inferred values with geologic surface energies this comparison might suggest that breakdown work for large earthquakes goes primarily into heat production

microseismicity at the north anatolian fault in the sea of marmara offshore istanbul nw turkey 1 the north anatolian fault zone nafz below the sea of marmara forms a seismic gap where a major earthquake is expected to occur in the near future this segment of the fault lies between the 1912 ganos and 1999 izmit ruptures and is the only nafz segment that has not ruptured since 1766 to monitor the microseismic activity at the main fault branch offshore of istanbul below the cinarcik basin a permanent seismic array pires was installed on the two outermost prince islands yassiada and sivriada at a few kilometers distance to the fault in addition a temporary network of ocean bottom seismometers was deployed throughout the cinarcik basin slowness vectors are determined combining waveform cross correlation and p wave polarization we jointly invert azimuth and traveltime observations for hypocenter determination and apply a bootstrap resampling technique to quantify the location precision we observe seismicity rates of 20 events per month for m 2 5 along the basin the spatial distribution of hypocenters suggests that the two major fault branches bounding the depocenter below the cinarcik basin merge to one single master fault below 17 km depth on the basis of a cross correlation technique we group closely spaced earthquakes and determine composite focal mechanisms implementing recordings of surrounding permanent land stations fault plane solutions have a predominant right lateral strike slip mechanism indicating that normal faulting along this part of the nafz plays a minor role toward the west we observe increasing components of thrust faulting this supports the model of nw trending dextral strike slip motion along the northern and main branch of the nafz below the eastern sea of marmara