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

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

dynamics of a velocity strengthening fault region implications for slow earthquakes and postseismic slip we consider the effect of permanent stress changes on a velocity strengthening rate and state fault through numerical simulations and analytical results on 1 d 2 d and 3 d models we show that slip transients can be triggered by perturbations of size roughly larger than lb gdc b where g is the shear modulus dc and b are the characteristic slip distance and the coefficient of the evolution effect of rate and state friction respectively and is the effective normal stress perturbations that increase the coulomb stress lead to the strongest transients but creep bursts can also be triggered by perturbations that decrease the coulomb stress in the latter case peak slip velocity is attained long after the perturbation so that it may be difficult in practice to identify their origin the evolution of slip in a creep transient shares many features with the nucleation process of a rate and state weakening fault slip initially localizes over a region of size not smaller than lb and then accelerates transiently and finally expands as a quasi static propagating crack the characteristic size lb implies a constraint on the grid resolution of numerical models even on strengthening faults that is more stringent than classical criteria in the transition zone between the velocity weakening and strengthening regions the peak slip velocity may be arbitrarily large and may approach seismic slip velocities postseismic slip may represent the response of the creeping parts of the fault to a stress perturbation of large scale comparable to the extent of the main shock rupture and high amplitude while slow earthquakes may represent the response of the creeping zones to a more localized stress perturbation our results indicate that superficial afterslip observed at usually seismogenic depths is governed by a rate strengthening rheology and is not likely to correspond to stable weakening zones the predictions of the full rate and state framework reduce to a pure velocity strengthening law on a timescale longer than the duration of the acceleration transient only when the triggering perturbation extends over length scales much larger than lb

effects of subducted seamounts on megathrust earthquake nucleation and rupture propagation 1 subducted seamounts have been linked to interplate earthquakes but their specific effects on earthquake mechanism remain controversial a key question is under what conditions a subducted seamount will generate or stop megathrust earthquakes here we show results from numerical experiments in the framework of rate and state dependent friction law in which a seamount is characterized as a patch of elevated effective normal stress on the thrust interface we find that whether subducted seamounts generate or impede megathrust earthquakes depends critically on their relative locations to the earthquake nucleation zone defined by depth variable friction parameters a seamount may act as a rupture barrier and such barrier effect is most prominent when the seamount sits at an intermediate range of the seamount to trench distances 20 100 of the nucleation zone to trench distance moreover we observe that seamount induced barriers can turn into asperities on which megathrust earthquakes can nucleate at shallow depths and rupture the entire seismogenic zone these results suggest that a strong barrier patch may not necessarily reduce the maximum size of earthquakes instead the barrier could experience large coseismic slip when it is ruptured

the evolution of illite to muscovite an example of the behaviour of isotopes in low grade metamorphic terrains abstract a review of the possibilities and limitations of the use of radiogenic isotopes in low grade terrains i e in the temperature range below the blocking temperature of potassic white mica is discussed using a background of a data base of over 500 isotope analyses on more than 250 illitic muscovitic samples published in various papers special stress is laid on the main problems 1 distinction between detrital and neoformed minerals 2 the slow evolution of minerals without pronounced breaks and 3 retention problems of radiogenic daughter products a brief description of low grade metamorphism is given and criteria enabling a subdivision into low to very low grade metamorphism are discussed comprising illite crystallinity mineralogy fluid inclusions and coal rank measurements the useful type of lithologies for this kind of work ranges from shales slates through volcaniclastic tuffs with high primary glass contents and limestone dolomites marbles to mylonites it can be concluded that carefully selected illitic muscovites yield geologically meaningful ages provided that more than one isotopic method is applied

the role of metasomatic alteration on frictional properties of subduction thrusts an example from a serpentinite body in the franciscan complex california abstract fluid rock interaction within accretionary prisms drastically changes the frictional strength and slip stability of the fault zone in order to understand the effect of ultramafic components on the degree of interplate coupling we present data on frictional properties of a reaction zone between serpentinite and argillite in a tectonic melange of the franciscan complex central california field and petrographic observations indicate that the argillite and serpentinite along the lithological boundary are metasomatized to tremolite and talc schists respectively forming a ductile shear zone simulated gouges made from fault and wall rock samples collected are sheared at effective normal stresses n eff of 60 180 mpa and temperatures t of 20 400 c using a hydrothermal ring shear machine the serpentinite is frictionally strong steady state friction coefficient ss 0 5 0 6 and exhibits both velocity weakening and velocity strengthening behavior the ss of the argillite and tremolite schist increases up to 0 7 and 0 6 respectively with increasing n eff and t while a b transitions from positive to negative with increasing t at a given experimental condition and a b values are lower and higher for the tremolite schist than for the argillite in contrast the talc schist exhibits very low friction ss 0 1 0 2 and velocity strengthening behavior at all experimental conditions tested our results demonstrate that si metasomatism along the argillite serpentinite contact results in reduced friction and stabilized slip forming frictionally weak velocity strengthening regions in the megathrust zone

a microphysical model for fault gouge friction applied to subduction megathrusts a microphysical model is developed for the steady state frictional behavior of illite quartz fault gouge and applied to subduction megathrust p t conditions the model assumes a foliated phyllosilicate supported microstructure which shears by rate independent frictional slip on the aligned phyllosilicates plus thermally activated deformation of the intervening quartz clasts at low slip rates or high temperatures the deformation of the clasts is easy accommodating slip on the foliation without dilatation with increasing velocity or decreasing temperature the shear of the clasts becomes more difficult increasing bulk shear strength until slip is activated on inclined portions of the phyllosilicate foliation where it anastomoses around the clasts slip at these sites leads to dilation involving clast matrix debonding balanced at steady state by compaction through thermally activated clast deformation model predictions taking pressure solution as the thermally activated mechanism show three regimes of velocity dependent frictional behavior at temperatures in the range of 200 500 c with velocity weakening occurring at 300 400 c in broad agreement with previous experiments on illite quartz gouge effects of slip rate normal stress and quartz fraction predicted by the model also resemble those seen experimentally extrapolation of the model to earthquake nucleation slip rates successfully predicts the onset of velocity weakening behavior at the updip seismogenic limit on subduction megathrusts the model further implies that the onset of seismogenesis is controlled by the thermally activated initiation of fault rock compaction through pressure solution of quartz which counteracts dilatation due to slip on the fault rock foliation

permo carboniferous paleoclimate of the congo basin evidence from lithostratigraphy clay mineralogy and stable isotope geochemistry abstract approximately 1000 m of strata in the upper paleozoic lukuga formation in the dekese core in the central congo basin provide lithostratigraphic mineralogical and isotopic evidence for substantial climatic variation within a long lived lacustrine basin lithostratigraphic indicators of cold climate include polymictic strata dropstone deposits and coupled laminations of fine clay size material and coarse silt glacial varves dropstones are concentrated in three stratigraphic zones in the lower 425 m of the lukuga formation and varved strata occur in two broad stratigraphic zones in the lower 700 m of the formation these sedimentological indicators suggest that the lower 2 3 of the lukuga formation was strongly influenced by frigid conditions and glacial like processes the clay size fraction of 97 samples is dominated by detrital minerals including quartz feldspar chlorite illite and poorly ordered expansible 2 1 phyllosilicates based on variation in the mineralogy of these samples the lukuga formation is divisible into three clay mineral zones cmzs numbered in ascending stratigraphic order cmz 1 and cmz 3 include several horizons of expansible 2 1 phyllosilicates that represent warmer wetter intervals intervening cmz 2 is a long 500 m zone of chlorite and illite with no expansible phyllosilicates and is interpreted as a continuous cold frigid interval there are numerous calcite cemented layers including spar filled veins that cross depositional bedding and represent postburial alteration radiaxial fibrous cements that displace detrital grains and micrite that crystallized near the time of deposition eighty two stable isotope analyses of micrite yield 13c values that range from 44 6 to 4 1 and 18o values that range from 20 0 to 5 0 vpdb the carbon isotope data likely reflect a range of local carbon sources derived from bacterial activity and are unrelated to paleoclimatic conditions in contrast stratigraphic patterns in the oxygen isotope data suggest five or six intervals of frigid conditions conducive to glacial processes

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

modelling of short interval silent slip events in deeper subduction interfaces considering the frictional properties at the unstable stable transition regime summary recent high resolution observations of crustal movements have revealed silent slip events sses with propagation velocities of around 10 15 km d 1 and with intervals of 3 14 months along the deeper parts of the cascadia and nankai subduction zones this study develops 2 d and 3 d models of these short interval sses considering the frictional behaviour that was confirmed experimentally by shimamoto for the unstable stable transition regime to represent this frictional behaviour a small cut off velocity to an evolution effect is introduced in a rate and state dependent friction law when the cut off velocity to the evolution effect is significantly smaller than that to a direct effect steady state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities at the deeper cascadia and nankai subduction interfaces the pore pressure is inferred to be high because of the dehydration of materials in the descending plate under conditions where the pore fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small short interval sses with propagation velocities and slip velocities of 4 8 km d 1 and 2 4 10 7 m s 1 respectively can be reproduced the propagation velocity of short interval sses is in proportion to the slip velocity the results also show that during the nucleation process of large earthquakes the occurrence of short interval sses becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone our results suggest that monitoring of short interval sses might be useful for forecasting the main earthquakes

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

a microphysical interpretation of rate and state dependent friction for fault gouge the evolution of fault strength during the seismic cycle plays a key role in the mode of fault slip nature of earthquake stress drop and earthquake nucleation laboratory based rate and state dependent friction rsf laws can describe changes in fault strength during slip but the connections between fault strength and the mechanisms that dictate the mode of failure from aseismic creep to earthquake rupture remain poorly understood the empirical nature of rsf laws remains a drawback to their application in nature here we analyze an extensive data set of friction constitutive parameters with the goal of illuminating the microphysical processes controlling rsf we document robust relationships between 1 the initial value of sliding or kinetic friction 2 rsf parameters and 3 the time rates of frictional strengthening aging we derive a microphysical model based on asperity contact mechanics and show that these relationships are dictated by 1 an activation energy that controls the rate of asperity growth by plastic creep and 2 an inverse relationship between material hardness and the activation volume of plastic deformation collectively our results illuminate the physics expressed by the rsf parameters and which describe the absolute value of frictional strength and its dependence on time and slip rate moreover we demonstrate that seismogenic fault behavior may be dictated by the interplay between grain properties and ambient conditions controlling the local shear strength of grain scale asperity contacts

role of pore fluid pressure on transient strength changes and fabric development during serpentine dehydration at mantle conditions implications for subduction zone seismicity abstract to further investigate the dehydration embrittlement hypothesis and its possible link to subduction zone seismicity we conducted deformation experiments on antigorite serpentinite in a griggs type apparatus at conditions below and above antigorite stability temperature ramps crossing the antigorite thermal stability were used in conjunction with a new experimental method that allows fluid produced during dehydration reactions to be drained partially drained or undrained during temperature ramps weakening coupled with transient slip initiated at 650 c coincident with the predicted phase transition of antigorite to olivine and talc at 1 gpa the weakening rate and steady state strength were dependent on drainage conditions undrained samples weakened over a few minutes and supported the lowest shear stress 50 mpa while drained samples weakened over a few hours and supported the highest shear stress 210 mpa the coefficient of friction shear stress over normal stress in drained samples decreased from 0 4 to 0 16 after the temperature ramp the strengths of samples that were first annealed at 700 c for 12 h then deformed were similar to those observed in the temperature ramp experiments strain localization along fractures occurred in all samples during temperature ramping regardless of the drainage conditions however microstructural observations indicate deformation by ductile mechanisms at higher strain under both undrained and drained conditions the rheology and microstructures suggest dehydrating serpentinite deforms via semibrittle flow with grain scale ductile deformation more active at high pore fluid pressures our results suggest that earthquakes in serpentinized mantle do not nucleate as a direct result of unstable frictional sliding along fractures generated at the onset of dehydration reactions

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

slow slip predictions based on granite and gabbro friction data compared to gps measurements in northern cascadia received 7 october 2008 revised 17 april 2009 accepted 12 june 2009 published 29 september 2009 1 for episodic slow slip transients in subduction zones a large uncertainty in comparing surface deformations predicted by forward modeling based on rate and state friction to gps measurements lies in our limited knowledge of the frictional properties and fluid pore pressure along shallow subduction faults in this study we apply the laboratory rate and state friction data of granite and gabbro gouges under hydrothermal conditions to a cascadia like 2 d model to produce spontaneous aseismic transients and we compare the resulting intertransient and transient surface deformations to gps observations along the northern cascadia margin an inferred region along dip of elevated fluid pressure is constrained by seismological observations where available and by thermal and petrological models for the cascadia and sw japan subduction zones for the assumed

sliding behavior and deformation textures of heated illite gouge abstract the run products of a series of triaxial friction experiments on an illite rich gouge have been examined petrographically to study the relationship between textural development and sliding mode the samples show a complete range of textures from ones in which the entire gouge layer is deformed to ones in which the deformation is concentrated along narrow subsidiary shears and the rest of the gouge layer is massive the samples with a pervasively developed deformation fabric slide stably whereas the samples containing shear bands show stick slip motion if the intersection angles between boundary parallel and cross cutting riedel shears are also relatively high these textural differences suggest that localization of shear combined with higher angle riedel shears are somehow involved in stick slip motion the orientation of riedel type shears in natural fault zones may also have potential as a paleoseismological tool

frictional properties of megathrust fault gouges at low sliding velocities new data on effects of normal stress and temperature abstract friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions we investigated the influence of in situ effective stresses and temperatures on the frictional properties of simulated fault gouges prepared either from nankai odp material or illite shale at sliding velocities approaching those relevant for earthquake nucleation and sses biaxial double direct shear experiments were performed at room temperature normal stresses of 5 30 mpa and sliding velocities of 0 16 18 m s all materials exhibited velocity strengthening under these conditions along with an increase in the friction coefficient and slip hardening rate with increasing normal stress illite gouge showed increased velocity strengthening towards higher normal stresses the effect of temperature was investigated by means of ring shear experiments on illite gouge at 200 300 c an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa and sliding velocities of 1 100 m s these experiments showed a transition from velocity strengthening to velocity weakening at 250 c our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and sses at low effective normal stresses

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

modeling short and long term slow slip events in the seismic cycles of large subduction earthquakes 1 slow slip events sses occur in the deeper extents of areas where large interplate earthquakes are expected in subduction zones such as the nankai region of japan and the cascadia region of north america in the nankai region sses are divided into long and short term sses depending on their duration and recurrence interval we modeled and examined the occurrence of long and short term sses and changes in their behavior during the seismic cycles of large interplate earthquakes in these numerical simulations we adopted a rate and state dependent friction law with cutoff velocities and assumed that the distribution of pore fluid controls the recurrence interval of both long and short term sses the recurrence intervals of reproduced short term sses decrease during a long term sse as observed in western shikoku in the nankai region the recurrence intervals of both types of sses become shorter in the later stages of interseismic periods large interplate earthquakes nucleate between the region where sses occur and the locked region of the large earthquakes as suggested from observations of the 1944 tonankai earthquake our numerical results suggest that the stress buildup process in a seismic cycle affects the recurrence behavior of sses

interseismic re strengthening and stabilization of carbonate faults by non dieterich healing under hydrothermal conditions abstract friction experiments consisting of a velocity stepping stage a slide hold slide shs stage and a second velocity stepping stage have been performed to study the frictional healing behavior of carbonate fault gouge and the effects of healing on the velocity dependence of friction at 20 140 c and at 50 mpa effective normal stress dry experiments show classical dieterich healing characterized by a transient peak in friction after each hold period with no effects of shs testing on steady state friction or velocity dependence by contrast the wet tests show 1 an increase in apparent steady state friction upon re sliding after a hold period and 2 a pronounced increase in the velocity dependence parameter a b after the shs stage while the first of these non dieterich type healing effects has been observed in previous hydrothermal experiments on simulated quartz gouges it has not been reported previously for carbonate gouges the observed effect of shs testing on a b has never been reported our findings suggest that under in situ hydrothermal conditions interseismic fluid assisted deformation processes such as pressure solution can significantly promote fault re strengthening in carbonates and can cause slip stabilization if the results are applicable to active faults in carbonate terrains they have important implications for understanding how the extent of the seismogenic zone and how earthquake magnitude and aftershock distributions may evolve with repeated cycles of natural seismicity and interseismic healing

determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones implications for strength of asperities and andean type mountain building 1 we construct the differential stress profile across the fore arc in a subduction zone from the force balance between the shear stress at seismogenic megathrust and the lithostatic pressure we assume that is written by 1 n where is the pore fluid pressure ratio is the coefficient of static friction and n is the normal stress given a density structure of the fore arc wedge we determine by comparing calculated fore arc stresses with observed ones as 0 95 0 98 in shikoku miyagi peru north chile and south chile and 0 90 0 93 in south vancouver island and washington the parameter averaged over the seismogenic megathrust is of the order of 10 mpa stress drops of great earthquakes in these zones occupy 14 87 and not a constant fraction of they on the other hand increase linearly with 1 we propose a simple fault model in which the area of asperities as a fraction of the total fault area is proportional to 1 variation of fractional area of asperities thus may explain the observed correlation and the regional variation of assuming that the differential stress at summit of the andean mountains is zero not at the coast as observed at present we determine to be 0 84 in north chile in the mountain building stage such a smaller value of along with 0 9 in subduction zones would suggest that variation of controls the tectonic style of the earth

experimental investigation into the microstructural and mechanical evolution of phyllosilicate bearing fault rock under conditions favouring pressure solution abstract mature crustal fault zones are known to be zones of persistent weakness this weakness is believed to result from microstructural modifications during deformation such as grain size reduction and foliation development around the brittle ductile transition phyllosilicates are expected to have a significant effect on fault strength in particular under conditions favouring pressure solution to study such effects we performed rotary shear experiments on brine saturated halite kaolinite mixtures aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate the results show significant strain weakening and a transition with progressive strain towards more rate sensitive and less normal stress sensitive behaviour this was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate asymmetric clasts in a fine grained foliated matrix the results demonstrate that strain weakening and the development of a typical mylonitic microstructure can occur as a consequence of grain size reduction by cataclasis and a transition to pressure solution accommodated deformation even in the absence of dislocation creep the data raise questions regarding the reliability of microstructures as rheology indicators as well as on the use of low strain monomineralic flow laws for modelling crustal dynamics

thermal and metamorphic environment of subduction zone episodic tremor and slip 1 episodic tremor and slip ets have been detected in the cascadia and southwest japan subduction zones where the subducting crust is relatively warm because of the young incoming lithosphere 20 ma and modest plate convergence rates 40 60 mm a in the southwest japan subduction zone low frequency earthquakes occur on the plate interface at depths of 30 35 km beneath shikoku where finite element thermal models predict temperatures of 425 c in the subducting oceanic crust and at depths of 30 40 km beneath the kii peninsula where predicted temperatures are 325 c warmer temperatures of 575 c are predicted at ets depths beneath southern vancouver island in the cascadia subduction zone but here tremor also occurs within the overlying fore arc crust where temperatures are lower in the southwest japan and cascadia subduction zones subducting oceanic crust passes through the blueschist greenschist and amphibolite metamorphic facies where mineral dehydration reactions are complex the different temperatures predicted for the two subduction zones suggest that ets does not coincide with a specific temperature or metamorphic reaction several lines of evidence indicate that a free h2o rich fluid is present at least transiently in subducting oceanic crust and fluids released by prograde metamorphic dehydration reactions may help trigger or enable ets within the subducting plate less clear is the role h2o may play in tremor observed in the cascadia fore arc crust where free h2o may exist locally in faults and fractures but retrograde hydration reactions are expected to consume h2o

hydrogeology and mechanics of subduction zone forearcs fluid flow and pore pressure at subduction zones fluid flow pore pressure and tectonic processes are tightly interconnected excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration and it has profound effects on fault and earthquake mechanics through its control on effective stress the egress of these overpressured fluids which is in part governed by the presence of permeable fault zones is a primary mechanism of volatile and solute transport to the oceans recent field measurements new constraints gained from laboratory studies and numerical modeling efforts have led to a greatly improved understanding of these coupled processes here we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs and focus on recent advances that have quantified permeability architecture fluxes the nature and timing of transience and pressure distribution thus providing new insights into the connections between fluid metamorphic mechanical and fault slip proc

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

influence of subduction zone conditions and gouge composition on frictional slip stability of megathrust faults to understand the temperature depth distribution of destructive earthquakes in subduction megathrusts and the mechanisms of nucleation of these events data on the frictional behaviour of phyllosilicate quartz rich megathrust fault gouges under in situ conditions are needed we performed rotary shear friction experiments at effective normal stresses of 25 200 mpa pore fluid pressures of 50 200 mpa at 140 600 c and sliding velocities of 1 100 m s using gouge mixtures with an illite quartz ratio between 65 35 and zero experiments on 65 35 mixtures deformed at an effective normal stress neff of 170 mpa a pore fluid pressure pf of 100 mpa and 150 500 c provided a reference dataset this showed three temperature dependent slip stability regimes with potentially unstable velocity weakening behaviour at 250 400 c and velocity strengthening at lower and higher temperatures the velocity weakening regime was found to shift towards higher temperatures with decreasing neff being located at 350 600 c at 50 mpa increasing quartz content and decreasing sliding velocity also displaced the velocity weakening regime towards lower temperatures increasing pf increased a b at all temperatures narrowing the temperature extent of the velocity weakening regime we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and a b with velocity and temperature are brought about by changes in the relative importance of deformation of the clast phase by thermally activated stress corrosion cracking and pressure solution versus athermal granular flow of the mixture accompanied by dilatation our results imply that the depth extent of the seismogenic zone on subduction megathrusts depends not only on temperature and that to predict its location it is essential to have well constrained depth profiles for pore fluid pressure and effective normal stress

the instantaneous rate dependence in low temperature laboratory rock friction and rock deformation experiments 1 earthquake occurrence probabilities that account for stress transfer and time dependent failure depend on the product of the effective normal stress and a lab derived dimensionless coefficient a this coefficient describes the instantaneous dependence of fault strength on deformation rate and determines the duration of precursory slip although an instantaneous rate dependence is observed for fracture friction crack growth and low temperature plasticity in laboratory experiments the physical origin of this effect during earthquake faulting is obscure we examine this rate dependence in laboratory experiments on different rock types using a normalization scheme modified from one proposed by tullis and weeks 1987 we compare the instantaneous rate dependence in rock friction with rate dependence measurements from higher temperature dislocation glide experiments the same normalization scheme is used to compare rate dependence in friction to rock fracture and to low temperature crack growth tests for particular weak phyllosilicate minerals the instantaneous friction rate dependence is consistent with dislocation glide in intact rock failure tests for each rock type considered the instantaneous rate dependence is the same size as for friction suggesting a common physical origin during subcritical crack growth in strong quartzofeldspathic and carbonate rock where glide is not possible the instantaneous rate dependence measured during failure or creep tests at high stress has long been thought to be due to crack growth however direct comparison between crack growth and friction tests shows poor agreement the crack growth rate dependence appears to be higher than the rate dependence of friction and fracture by a factor of two to three for all rock types considered

stable creeping fault segments can become destructive as a result of dynamic weakening faults in earth s crust accommodate slow relative motion between tectonic plates through either similarly slow slip or fast seismic wave producing rupture events perceived as earthquakes these types of behaviour are often assumed to be separated in space and to occur on two different types of fault segment one with stable rate strengthening friction and the other with rate weakening friction that leads to stick slip the 2011 tohoku oki earthquake with moment magnitude m w 9 0 challenged such assumptions by accumulating its largest seismic slip in the area that had been assumed to be creeping here we propose a model in which stable rate strengthening behaviour at low slip rates is combined with coseismic weakening due to rapid shear heating of pore fluids allowing unstable slip to occur in segments that can creep between events the model parameters are based on laboratory measurements on samples from the fault of the m w 7 6 1999 chi chi earthquake the long term slip behaviour of the model which we examine using a unique numerical approach that includes all wave effects reproduces and explains a number of both long term and coseismic observations some of them seemingly contradictory about the faults at which the tohoku oki and chi chi earthquakes occurred including there being more high frequency radiation from areas of lower slip the largest seismic slip in the tohoku oki earthquake having occurred in a potentially creeping segment the overall pattern of previous events in the area and the complexity of the tohoku oki rupture the implication that earthquake rupture may break through large portions of creeping segments which are at present considered to be barriers requires a re evaluation of seismic hazard in many areas

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

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

temperature and gas brine content affect seismogenic potential of simulated fault gouges derived from groningen gas field caprock we investigated the rate and state frictional properties of simulated anhydrite carbonate fault gouge derived from the basal zechstein caprock overlying the seismogenic groningen gas reservoir in the ne netherlands direct shear experiments were performed at in situ conditions of 50 150 c and 40 mpa effective normal stress using sliding velocities of 0 1 10 m s reservoir pore fluid compositions were simulated using 4 4 molar nacl brine as well as methane air and brine gas mixtures brine saturated samples showed friction coefficients of 0 60 0 69 with little dependence on temperature along with velocity strengthening at 50 100 c transitioning to velocity weakening at 120 c and above by contrast gas filled evacuated and partially brine saturated samples showed values of 0 72 0 02 plus strongly velocity weakening behavior accompanied by stick slip at 100 c the only temperature investigated for gas bearing and dry samples a microphysical model for gouge friction assuming competition between dilatant granular flow and thermally activated compaction creep captures the main trends seen in our brine saturated samples but offers only a qualitative explanation for our gas bearing and dry samples since the reservoir temperature is 100 c our results imply high potential for seismogenic slip nucleation on faults that cross cut and juxtapose the basal zechstein anhydrite carbonate caprock against the groningen reservoir sandstone specifically in the gas filled upper portion of the reservoir system

nucleation of frictional instability caused by fluid pressurization in subducted blueschist pore pressure is an important factor in controlling the slip instability of faults and thus the generation of earthquakes particularly slow earthquakes are widespread in subduction zones and usually linked to the occurrence of high pore pressure yet the influence of fluid pressure and effective stress on the mechanics of earthquakes is poorly understood therefore we performed shear experiments on blueschist fault rocks which likely exist at depth in cold and old subduction zones to investigate the influence of effective stress on frictional behavior our results show potentially unstable behavior at temperatures characterizing the seismogenic zone as well as a transition from stable to unstable behavior with decreasing effective normal stress which is mechanically equivalent to increasing fluid pressure this transition is a prerequisite for generating slow earthquakes our results imply that high pore pressures are a key factor for nucleating slip leading to both megathrust and slow earthquakes

abyssal peridotite mylonites implications for grain size sensitive flow and strain localization in the oceanic lithosphere abstract microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation fabrics associated with deformation processes at slow spreading mid ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations the most abundant class are medium to coarse grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere these tectonites give geothermometric temperatures of 755 c or higher interpreted to represent lower temperature limits for diffusive exchange in coarse grained abyssal peridotites during cooling this conclusion is consistent with flow laws for olivine at these temperatures the second class of abyssal peridotites previously largely undescribed for the mid ocean ridge environment include fine grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle plastic transition extends into mantle rocks these mylonites give temperatures of 600 c which we suggest represent a lower temperature limit for plastic deformation reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse grained tectonites based on flow laws for olivine we suggest that deformation in some fine grained mylonites occurred by diffusion creep down to 600 c rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature we conclude that a reduction in grain size by cataclasis or dynamic recrystallization resulted in a transition in deformation mechanisms from dislocation to diffusion creep during uplift and or cooling the observation of these fine grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep weak faults may promote the development of long lived detachments in the upper mantle this inference supports mid ocean ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge axis discontinuities are exhumed along long lived detachment faults

a microphysical model for strong velocity weakening in phyllosilicate bearing fault gouges 1 previous rotary shear experiments performed on a halite muscovite fault gouge analogue system have shown that the presence of phyllosilicates under conditions favoring the operation of cataclasis and pressure solution in the matrix phase can have major effects on the frictional behavior of gouges while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behavior the strength of mixtures containing 10 30 muscovite is both normal stress and sliding velocity dependent at high sliding velocities 1 m s 1 such mixtures show unusually marked velocity weakening along with the development of a structureless cataclastic microstructure in the present paper a micromechanical model is developed in an attempt to explain this behavior the model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution the predictions of the model agree favorably with the experimental results extension of the model to quartz mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote unstable velocity weakening behavior at rapid slip rates on natural faults under midcrustal conditions static stress drop predictions based on the model agree reasonably well with estimates from seismic observations our results may help explain the discrepancy between laboratory derived rate and state friction parameter values obtained for dry low strain and or single phase rock systems and the values for natural fault rocks inferred from seismological data

spontaneous and triggered aseismic deformation transients in a subduction fault model 1 aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault sliding experiments when that constitutive formulation is applied to model subduction earthquake sequences transients can arise spontaneously for certain effective stress variations with depth we show that if interstitial fluids are present and pore pressure is near lithostatic around and downdip from the frictional stability transition transients with recurrence intervals of 1 year are predicted on the basis of laboratory friction parameters and their temperature hence depth variations the recurrence interval decreases with and reaches 14 months when is 2 3 mpa dimensional analysis and numerical studies show that the fault response primarily depends on a parameter w h here the high pore pressure zone extends distance w updip from the stability transition and h is the stable patch size for steady sliding evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed transient sequences can also be triggered by a modest one time step like interseismic stress perturbation on the subduction fault due to nearby earthquakes or to pore pressure changes e g during episodes of metamorphic fluid release properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced its relative location along the subduction fault and its magnitude

the effects of sliding velocity on the frictional and physical properties of heated fault gouge the frictional properties of a crushed granite gouge and of gouges rich in montmorillonite illite and serpentine minerals have been investigated at temperatures as high as 600 c confining pressures as high as 2 5 kbar a pore pressure of 30 bar and sliding velocities of 4 8 and 4 8 10 2 m sec the gouges showed nearly identical strength behaviors at the two sliding velocities all four gouges however showed a greater tendency to stick slip movement and somewhat higher stress drops in the experiments at 4 8 10 2 m sec varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges the principal mineralogical difference was that at 400 c and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4 8 10 2 m sec but not in those at 4 8 m sec the textures developed in the gouge layers were in part functions of the gouge type and the temperature but changes in the sliding velocity affected among other features the degree of mineral deformation and the orientation of some fractures

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

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

new constraints on megathrust slip stability under subduction zone p t conditions abstract to understand and model subduction zone seismogenesis data are needed on the frictional properties of meta pelitic subduction zone materials under in situ megathrust conditions here we report the results of rotary shear friction experiments on simulated illite quartz fault gouge at an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa at 150 500 c and sliding velocities of 1 100 m s the results show three temperature dependent regimes characterized by velocity strengthening at 150 250 c velocity weakening at 250 400 c and velocity strengthening at 400 500 c the regimes are defined by a decrease in the rate and state friction rsf parameter a b at 150 300 c followed by an increase at 300 500 c these trends correlate with systematic changes in other rsf parameters as well as an increase in friction coefficient above 300 c we explain the effects of increasing temperature in terms of a transition from frictional granular flow 150 250 c through granular flow whereby dilatation is balanced by thermally activated compaction involving stress corrosion cracking scc of quartz clasts 250 400 c to non dilatant slip on the phyllosilicates with accommodation at quartz clasts by scc 400 500 c taking into account the effects of sliding velocity the observed velocity weakening regime broadly explains the extent of the seismogenic zone within subduction megathrusts