a scaling law for slow earthquakes with the availability of the global positioning system and other technical advances a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized they include deep episodic tremor low frequency earthquakes slow slip events and silent earthquakes based on data chiefly from western japan ide et al report that these slow seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes a diverse array of unusual earthquake phenomena occurring at relatively long periods slow events follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes indicating that they comprise a new earthquake category recently a series of unusual earthquake phenomena have been discovered including deep episodic tremor1 low frequency earthquakes2 very low frequency earthquakes3 slow slip events4 and silent earthquakes5 6 7 8 9 each of these has been demonstrated to arise from shear slip just as do regular earthquakes but with longer characteristic durations and radiating much less seismic energy here we show that these slow events follow a simple unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes we find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant with a spectral high frequency decay of f 1 this scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category the observed scale dependence of rupture velocity for these events can be explained by either a constant low stress drop model or a diffusional constant slip model this new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation

some comparisons between mining induced and laboratory earthquakes although laboratory stick slip friction experiments have long been regarded as analogs to natural crustal earthquakes the potential use of laboratory results for understanding the earthquake source mechanism has not been fully exploited because of essential difficulties in relating seismographic data to measurements made in the controlled laboratory environment mining induced earthquakes however provide a means of calibrating the seismic data in terms of laboratory results because in contrast to natural earthquakes the causative forces as well as the hypocentral conditions are known a comparison of stick slip friction events in a large granite sample with mining induced earthquakes in south africa and canada indicates both similarities and differences between the two phenomena the physics of unstable fault slip appears to be largely the same for both types of events for example both laboratory and mining induced earthquakes have very low seismic efficiencies eta tau a bar tau where a is the apparent stress and bar tau is the average stress acting on the fault plane to cause slip nearly all of the energy released by faulting is consumed in overcoming friction in more detail the mining induced earthquakes differ from the laboratory events in the behavior of as a function of seismic momentm0 whereas for the laboratory events 0 06 independent ofm0 depends quite strongly onm0 for each set of induced earthquakes with 0 06 serving apparently as an upper bound it seems most likely that this observed scaling difference is due to variations in slip distribution over the fault plane in the laboratory a stick slip event entails homogeneous slip over a fault of fixed area for each set of induced earthquakes the fault area appears to be approximately fixed but the slip is inhomogeneous due presumably to barriers zones of no slip distributed over the fault plane at constant bar tau larger events correspond to larger a as a consequence of fewer barriers to slip if the inequality a bar tau 0 06 has general validity then measurements of a ea m0 where is the modulus of rigidity andea is the seismically radiated energy can be used to infer the absolute level of deviatoric stress at the hypocenter

the frictional behavior of lizardite and antigorite serpentinites experiments constitutive models and implications for natural faults laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults we have conducted friction experiments on antigorite and lizardite serpentinites rocks common to both continental and oceanic crustal faults we conducted both velocity step tests and timed hold tests on bare surfaces and gouge layers of serpentinite at room temperature we find that the coefficient of friction of lizardite serpentinite is quite low 0 15 0 35 and could explain the apparent low stresses observed on crustal transform faults while that of antigorite serpentinite is comparable to other crustal rocks 0 50 0 85 the frictional behavior of both types of serpentinite is well described by a two mechanism model combining state variable dominated behavior at high slip velocities and flow dominated behavior at low velocities the two mechanism model is supported by data from velocity step tests and timed hold tests the low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite

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

constructing constitutive relationships for seismic and aseismic fault slip for the purpose of modeling natural fault slip a useful result from an experimental fault mechanics study would be a physically based constitutive relation that well characterizes all the relevant observations this report describes an approach for constructing such equations where possible the construction intends to identify or at least attribute physical processes and contact scale physics to the observations such that the resulting relations can be extrapolated in conditions and scale between the laboratory and the earth the approach is developed as an alternative but is based on ruina 1983 and is illustrated initially by constructing a couple of relations from that study in addition two example constitutive relationships are constructed these describe laboratory observations not well modeled by ruina s equations the unexpected shear induced weakening of silica rich rocks at high slip speed goldsby and tullis 2002 and fault strength in the brittle ductile transition zone shimamoto 1986 the examples provided as illustration may also be useful for quantitative modeling

maximum slip in earthquake fault zones apparent stress and stick slip friction the maximum slip observed or inferred for a small patch within the larger fault zone of an earthquake is a remarkably well constrained function of the seismic moment a large set of maximum slips mostly derived from slip models of major earthquakes indicate that this parameter increases according to the cube root of the seismic moment consistent with this finding neither the average slip rate for the patches of maximum slip nor the apparent stresses of earthquakes show any systematic dependence on seismic moment maximum average slip rates are several meters per second independent of moment and for earthquakes in continental crustal settings the apparent stress is limited to about 10 mpa results from stick slip friction experiments in the laboratory combined with information about the state of stress in the crust can be used to predict quite closely the maximum slips and maximum average slip rates within the fault zones of major earthquakes as well as their apparent stresses these findings suggest that stick slip friction events observed in the laboratory and earthquakes in continental settings even with large magnitudes have similar rupture mechanisms manuscript received 24 february 2003

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting 1 to understand how frictional melting affects fault instability we performed a series of high velocity friction experiments on gabbro at slip rates of 0 85 1 49 m s 1 at normal stresses of 1 2 2 4 mpa and with displacements up to 124 m experiments have revealed two stages of slip weakening one following the initial slip and the other immediately after the second peak friction the first weakening is associated with flash heating and the second weakening is due to the formation and growth of a molten layer along a simulated fault the two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin continuous molten layer at the second peak friction the frictional coefficient decays exponentially from 0 8 1 1 to 0 6 during the second weakening the host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer melt viscosity increases notably soon after a molten layer forms however a fault weakens despite the increase in melt viscosity and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip weakening distance effect of frictional melting on fault motion can be predicted by solving a stefan problem dealing with moving host rock molten zone boundaries

attempting to bridge the gap between laboratory and seismic estimates of fracture energy 1 to investigate the behavior of the fracture energy associated with expanding the rupture zone of an earthquake we have used the results of a large scale biaxial stick slip friction experiment to set the parameters of an equivalent dynamic rupture model this model is determined by matching the fault slip the static stress drop and the apparent stress after confirming that the fracture energy associated with this model earthquake is in reasonable agreement with corresponding laboratory values we can use it to determine fracture energies for earthquakes as functions of stress drop rupture velocity and fault slip if we take account of the state of stress at seismogenic depths the model extrapolation to larger fault slips yields fracture energies that agree with independent estimates by others based on dynamic rupture models for large earthquakes for fixed stress drop and rupture speed the fracture energy scales linearly with fault slip

unusual early morning development of the equatorial anomaly in the brazilian sector during the halloween magnetic storm 1 the solar events that occurred at the end of october 2003 gave rise to very strong geomagnetic disturbances that peaked twice with dst values reaching 345 nt around 0000 ut on 30 october and 400 nt around 2300 ut on the same day disturbances in several ionospheric parameters were observed over brazil this work will focus on the ionospheric response to the initial westward prompt penetration electric field and on the strong intensification of the equatorial ionization anomaly that occurred because of the electric field polarity reversal that followed in the early morning hours of 29 october the f layer peak height over the equator first decreased under the strong prompt penetration westward electric field which was followed by significant height increase under eastward electric field we have used sheffield university plasmasphere ionosphere model supim with an intensified westward disturbed electric field in the presunrise hours presumably due to prompt penetration from the magnetosphere in order to study the effect of such a field in the ionosphere the simulation results showed that prompt penetration of magnetospheric electric fields of westward polarity to the nightside equatorial region seems to be the most probable cause of the initial f layer height decreases the intensification of the equatorial ionization anomaly and the unusual enhancement on f layer peak density which was not modeled by the supim are explained as caused by the strong eastward electric field that followed the initial phase in combination with a highly variable disturbed meridional transequatorial wind system as inferred from the f2 layer peak height variations the highly dynamic wind pattern with a short term response 2 4 hours is compatible with the predictions of some previous theoretical model calculations reported in the literature

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

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

terrestrial originated submarine groundwater discharge through deep multilayered aquifer systems beneath the seafloor a large quantity of submarine groundwater discharge sgd of about 1000 m3 day 1 m 1 of the 600 km long shoreline of south atlantic bight has been estimated by moore global biogeochemical cycles 2010b 24 gb4005 doi 10 1029 2009gb003747 however there is great uncertainty in estimating the percentage of net land originated groundwater recharge of sgd moreover most previous studies considered the homogeneous case for the coastal superficial aquifers here we investigated the terrestrial originated sgd through a multilayered submarine aquifer system which comprises two confined aquifers and two semi permeable layers the inland recharge includes a constant part representing the annual average and a periodical part representing its seasonal variation an analytical solution was derived and used to analyse the distributions of the terrestrial originated sgd from the multilayered aquifers along the winyah bay transect south atlantic bight it is found that the width of the zone of sgd from the upper aquifer ranges from 0 8 to 8 0 km depending on the leakance of the seabed semi permeable layer a head of the upper aquifer at a coastline 1 0 m higher than the mean sea level will cause a sgd of 1 82 18 3 m3 day 1 m 1 from that aquifer as the seabed semi permeable layer s leakance varies from 0 001 to 0 1 day 1 providing considerable possibility for considerable land originated sgd seasonal terrestrial originated sgd variations predicted by the analytical model provide consistent explanation of the seasonal variation of 226ra observed by moore journal of geophysics 2007 112 c10013 doi 10 1029 2007jc004199 the contribution of the lower aquifer to sgd is only 1 2 12 of that of the upper aquifer copyright 2014 john wiley sons ltd

evolved granitic systems as a source of rare element deposits the ponte segade case galicia nw spain abstract in the ponte segade area galicia nw spain strongly differentiated granites often associated with rare element mineralization sn ta nb li be cs that could be of economic interest have recently been discovered these granites appear in the northern sector of the ollo de sapo antiform central iberian zone variscan orogen three different muscovite rich synkinematic and peraluminous types of leucogranite leucogranites s s albite rich leucogranites i and albite rich leucogranites ii and two pegmatite types associated with the albite rich leucogranites zoned and banded have been identified in the studied area the geochemistry of whole rock leucogranites indicates an enrichment in incompatible elements lithium rubidium beryllium cesium and hafnium al2o3 and na2o and an impoverishment in barium strontium zirconium cerium yttrium and sio2 geochemical studies of zircon muscovite k feldspar and tourmaline in the different types of granites and pegmatites indicate the grade of evolution of the granitic system with differentiation of the system the zircon is enriched in hafnium and uranium and is impoverished in zirconium in muscovite and k feldspar there is an increase in cesium and rubidium the opposite behavior is observed with regards to the mg fe and ti contents in the case of tourmaline the increase in li is the best indicator of the grade of evolution by contrast fe and mg decrease the sequence of evolution of the granitic system obtained from the geochemical studies indicates that the leucogranites s s are the least differentiated evolving gradually in accordance with field relationships to albite rich leucogranites i the albite rich leucogranites ii are the most evolved but no direct relationship between them and leucogranites s s has been found the banded pegmatites associated with the albite rich leucogranites ii are more differentiated than the zoned pegmatites associated with the albite rich leucogranites i but are the poorest in sn ta and nb

aseismic slip transients emerge spontaneously in three dimensional rate and state modeling of subduction earthquake sequences 1 to investigate the possible physical mechanisms of recently observed aseismic slip events in the cascadia japan and mexico subduction zones we apply a dieterich ruina rate and state friction law to a three dimensional model of a shallow subduction fault that is loaded by imposed steady plate slip rate far downdip along the thrust interface friction properties are temperature and hence depth dependent such that sliding is stable at depths below about 30 km the system is perturbed into a significantly nonuniform slip mode along strike by introducing small along strike variations in the constitutive parameters a and a b in addition to large heterogeneous earthquake slip at seismogenic depths and associated postseismic transients we found that slip events which have clearly aseismic slip rates emerge spontaneously around the downdip end of the seismogenic zone both transients which start well after a seismic event and those which are triggered by other transients are observed from the simulations the slip velocity depth range and sometimes along strike migration speed of simulated transients are similar to the observations from natural subduction zones unstable stable transitional friction properties near the downdip end of the seismogenic zone are suggested to be an ingredient allowing such transients simulated transients can weaken the locking intensity of the updip seismogenic zone while enhancing that of the transition zone spatial temporal correlation of aseismic transients and nearby seismicity in the guerrero mexico area suggests that transients may signal a period of increased probability for nucleating their high frequency counterparts as damaging subduction thrust events

strengths of serpentinite gouges at elevated temperatures serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones to test these hypotheses we have measured the strength of chrysotile lizardite and antigorite rich serpentinite gouges under hydrothermal conditions with emphasis on chrysotile which has thus far received little attention at 25 c the coefficient of friction of chrysotile gouge is roughly 0 2 whereas the lizardite and antigorite rich gouges are at least twice as strong the very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content when the adsorbed water is removed chrysotile is as strong as pure antigorite gouge at room temperature heating to 200 c causes the frictional strengths of all three gouges to increase limited data suggest that different polytypes of a given serpentine mineral have similar strengths thus deformation induced changes in polytype should not affect fault strength at 25 c the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities consistent with previous studies at temperatures up to 200 c however chrysotile strength is essentially independent of velocity at low velocities overall chrysotile has a restricted range of velocity strengthening behavior that migrates to higher velocities with increasing temperature less information on velocity dependence is available for the lizardite and antigorite gouges but their behavior is consistent with that outlined for chrysotile the marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth the velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip but in the presence of a hydrostatic fluid pressure gradient all varieties of serpentine are too strong to explain the apparent weakness of faults such as the san andreas

slow earthquakes coincident with episodic tremors and slow slip events we report on the very low frequency earthquakes occurring in the transition zone of the subducting plate interface along the nankai subduction zone in southwest japan seismic waves generated by very low frequency earthquakes with seismic moment magnitudes of 3 1 to 3 5 predominantly show a long period of about 20 seconds the seismicity of very low frequency earthquakes accompanies and migrates with the activity of deep low frequency tremors and slow slip events the coincidence of these three phenomena improves the detection and characterization of slow earthquakes which are thought to increase the stress on updip megathrust earthquake rupture zones

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

fault stability inferred from granite sliding experiments at hydrothermal conditions seismicity on crustal faults is concentrated in the depth interval 1 3 to 12 15km tse and rice 1986 suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening destabilizing to velocity strengthening stabilizing friction they inferred this transition from friction data for dry granite however pore fluids exist at elevated temperatures throughout the crust and may strongly influence strength and sliding behavior we present new data from sliding experiments on granite at elevated t 23 to 600 plus elevated ph2o 100 mpa our results show velocity strengthening at room temperature but velocity weakening from 100 to 350 c except at 250 from 350 to 600 there are systematic trends from velocity weakening to strong velocity strengthening and from high to low friction neither trend was seen in tests on dry granite the velocity dependence data imply the potential for unstable slip in the interval 100 to 350 using a geotherm to map temperature to depth this interval closely matches the observed earthquake distribution

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

moisture related weakening and strengthening of a fault activated at seismic slip rates 1 from high velocity friction experiments on gabbro rock at a rate of 85 mm s we found that a fault with a thin gouge layer experienced slip weakening with a drop of about 80 in the friction coefficient at room humidity conditions the fault subsequently recovered its complete strength over an interval of several minutes after the slip terminated the strength recovery did not occur under dry n2 gas saturated condition this result can be interpreted as the interaction of moisture with gouge particles moisture drained weakening due to frictional heating and moisture absorbed strengthening due to cooling this new mechanism can cause dynamic weakening of faults which controls earthquake rupture propagation

fracture energy of stick slip events in a large scale biaxial experiment the concept of apparent fracture energy for the shear failure process is employed by many authors in modeling earthquake sources as dynamically extending shear cracks using records of shear strain and relative displacement from stick slip events generated along a simulated prepared fault surface in a large 1 5m x 1 5m x 0 4m granite block and a slip weakening model for the fault direct estimates of the apparent shear fracture energy of the stick slip events have been obtained for events generated on a finely ground fault surface apparent fracture energy ranges from 0 06 j m sup 2 at a normal stress of 1 1 mpa to 0 8 j m sup 2 at a normal stress of 4 6 mpa in contrast to estimates for tensile crack formation we find that the apparent fracture energy of stick slip events increases linearly with normal stress the results for the slip weakening model for the stick slip events are generally consistent with constitutive fault models suggested by observations of stable sliding in smaller scale experiments

on relating apparent stress to the stress causing earthquake fault slip apparent stress a is defined as where is the average shear stress loading the fault plane to cause slip and is the seismic efficiency defined as ea w where ea is the energy radiated seismically and w is the total energy released by the earthquake the results of a recent study in which apparent stresses of mining induced earthquakes were compared to those measured for laboratory stick slip friction events led to the hypothesis that this hypothesis is tested here against a substantially augmented data set of earthquakes for which can be estimated mostly from in situ stress measurements for comparison with a the expanded data set which includes earthquakes artificially triggered at a depth of 9 km in the german kontinentales tiefbohrprogramm der bundesrepublik deutschland ktb borehole and natural tectonic earthquakes covers a broad range of hypocentral depths rock types pore pressures and tectonic settings nonetheless over 14 orders of magnitude in seismic moment apparent stresses exhibit distinct upper bounds defined by a maximum seismic efficiency of 0 06 consistent with the hypothesis proposed before this behavior of a and can be expressed in terms of two parameters measured for stick slip friction events in the laboratory the ratio of the static to the dynamic coefficient of friction and the fault slip overshoot typical values for these two parameters yield seismic efficiencies of 0 06 in contrast to efficiencies for laboratory events for which is always near 0 06 those for earthquakes tend to be less than this bounding value because ea for earthquakes is usually underestimated due to factors such as band limited recording thus upper bounds on appear to be controlled by just a few fundamental aspects of frictional stick slip behavior that are common to shallow earthquakes everywhere estimates of from measurements of a for suites of earthquakes using are found to be comparable in magnitude to estimates of shear stress on the basis of extrapolating in situ stress data to seismogenic depths

symmetry and the critical slip distance in rate and state friction laws 1 we performed laboratory experiments to investigate the processes responsible for rate and state friction rsf behavior in fault rocks we focused on the symmetry of the frictional constitutive response to velocity changes and the mechanics of the critical friction slip distance dc experiments were conducted in double direct shear at 1 and 25 mpa normal stress at room temperature and for shearing velocity from 1 to 300 m s we studied three granular materials and bare surfaces of westerly granite ruina s law which predicts frictional symmetry between velocity increases and decreases better matches our data than dieterich s law which predicts that velocity decreases should evolve to steady state at a smaller displacement however for granular shear in some cases dc is smaller for velocity increases than for velocity decreases contrary to expectations from either law on bare granite surfaces the frictional response is symmetric for velocity increases decreases two distinct length scales for dc and two state variables are required for granular shear in some cases we hypothesize that asymmetry and two state behavior are caused by shear localization and changes in shear fabric in fault gouge our measurements show that during steady state frictional shear dilation after a velocity increase is smaller than compaction after a decrease normal stress oscillations cause a marked decrease in dc reduction of dc reduces frictional stability enhancing the possibility of seismic slip our experiments show that shear localization and fabric within the fault gouge can influence the rsf parameters that dictate earthquake nucleation and dynamic rupture

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

olivine friction at the base of oceanic seismogenic zones 1 we investigate the strength and frictional behavior of olivine aggregates at temperatures and effective confining pressures similar to those at the base of the seismogenic zone on a typical ridge transform fault triaxial compression tests were conducted on dry olivine powder grain size 60 m at effective confining pressures between 50 and 300 mpa using argon as a pore fluid temperatures between 600 c and 1000 c and axial displacement rates from 0 06 to 60 m s axial strain rates from 3 10 6 to 3 10 3 s 1 yielding shows a negative pressure dependence consistent with predictions for shear enhanced compaction and with the observation that samples exhibit compaction during the initial stages of the experiments a combination of mechanical data and microstructural observations demonstrate that deformation was accommodated by frictional processes sample strengths were pressure dependent and nearly independent of temperature localized shear zones formed in initially homogeneous aggregates early in the experiments the frictional response to changes in loading rate is well described by rate and state constitutive laws with a transition from velocity weakening to velocity strengthening at 1000 c microstructural observations and physical models indicate that plastic yielding of asperities at high temperatures and low axial strain rates stabilizes frictional sliding extrapolation of our experimental data to geologic strain rates indicates that a transition from velocity weakening to velocity strengthening occurs at approximately 600 c consistent with the focal depths of earthquakes in the oceanic lithosphere

on the physical mechanism of silent slip events along the deeper part of the seismogenic zone 1 to investigate the mechanism of recently observed silent slip events we simulated earthquake preparation processes using the dieterich ruina rate and state dependent friction law to ensure realistic modeling of the unstable stable transition we considered small cut off velocity to an evolution effect in the friction law for the transition zone when the cut off velocity to the evolution effect is significantly smaller than that of a direct effect steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity this frictional behavior was experimentally and theoretically confirmed for the unstable stable transition zone the results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion eventually propagates horizontally along the unstable stable transition over a period of several years silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust