stuck in the mud earthquake nucleation and propagation through accretionary forearcs 1 subduction zone earthquakes can propagate to the surface causing large seafloor displacements resulting in tsunamis this requires the earthquake to rupture through clay rich sediments of the accretionary wedge which are largely aseismic as found previously the frictional properties of a range of wet clays at low slip velocity are velocity strengthening thus inhibiting earthquake nucleation however at high slip velocity the same materials weaken almost immediately resulting in a negligible critical slip weakening distance and fracture energy we interpret this behaviour as rapid thermal pressurization of the pore fluid within the clay gouge the lack of fracture energy can explain how a large rupture propagating from depth might not be arrested by clay rich velocity strengthening sediments as is commonly seen the results suggest that generally earthquakes may be difficult to nucleate on mature faults dominated by clay but the propagation of earthquakes through these zones is energetically very favourable

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

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

frictional properties of natural gouges from longmenshan fault zone ruptured during the wenchuan mw7 9 earthquake abstract we investigated the properties of frictional sliding of natural gouges collected from longmenshan fault zone which ruptured during the wenchuan mw7 9 earthquake based on xrd analysis five gouge samples of different mineralogical assemblies and mineral contents are chosen for frictional sliding experiments with initial confining pressure of 50 mpa and at different temperatures from 25 c to 150 c to obtain the rate dependence of friction shearing rates are stepped up and down in the range of 0 0488 to 6 1 m s while the friction coefficient of most gouge samples decreases with the clay content following a general trend consistent with previous studies the organic matter bearing sample px 3 shows a much lower value than the prediction by the general trend indicating further strength reduction due to the 9 organic matter included we also found a significant effect of temperature on the sliding behavior of organic matter bearing and carbonate rich gouges when temperature increases to 150 c the px 3 sample with organic matter exhibits unstable oscillation that indicates velocity weakening frictional sliding behavior in contrast to the velocity strengthening behavior at lower temperatures the velocity dependence of carbonate rich samples px 1 px 2 with more than 50 wt carbonates is also sensitive to temperature and shearing rates and both samples have smaller velocity dependence of friction a b at higher temperatures the a b value of the px 2 sample with 80 carbonates even decreases to the velocity neutral point at 150 c and slow shearing rates based on the rate and state friction framework fast slip nucleation is possible for faults associated with the organic matter bearing gouge at 150 c whereas triggered seismic slips are possible in faults that include the carbonate rich samples which have a small a b value at 150 c the temperature dependent sliding behavior of carbonate dominant and organic matter bearing gouges probably provides a new clue to the problem of transition from shallow aseismic zone to seismic zone in subduction interfaces of the oceanic plate

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

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

evidence of thermal pressurization in high velocity friction experiments on smectite rich gouges terra nova 22 347 353 2010 abstract thermal pressurization of pore fluid is one of the possible mechanisms responsible for dynamic weakening in landslides and earthquakes but to date has not been reproduced in the laboratory here we report high velocity experiments performed in a rotary shear friction apparatus on smectite rich gouges from the 1963 vaiont landslide italy the gouges were slid under 1 mpa normal stress for displacements up to 30 m and a slip rate of 1 31 m s 1 under room humidity and water saturated conditions sample dilatancy was observed in room humidity runs after 3 4 m of slip concomitant with an increase in normal stress and a decrease in shear stress mineralogical and microstructural investigations suggest that dilatancy resulted from expansion of the h2o released by the collapse of the smectite structure due to frictional heating of the slipping zone at t 200 c we conclude that sample dilatancy is due to thermal pressurization of the clay rich gouge

holocene paleoearthquake history on the qingchuan fault in the northeastern segment of the longmenshan thrust zone and its implications abstract although much work has been performed for faults with high slip rates little attention has been paid to low slip rate faults such as the longmenshan thrust zone ltz on which the wenchuan earthquake occurred the ltz is a long and matured fault that evolved during the mesozoic as a structural boundary but its quaternary activity had been considered insignificant the wenchuan earthquake on the central segment of the ltz and the following lushan earthquake on the southwestern segment illustrate the necessity of assessing the regional seismic potential around the northeastern extension of the ltz however little is known about its activity and paleoearthquake history to solve these problems we conducted paleoseismological trench excavations at two sites on the qingchuan fault to establish its paleoearthquake history only one and the latest event that occurred in the holocene is identified based on the radiocarbon dating the faulting event is constrained to occur between 4115 and 3820 b c and a long recurrence interval of greater than 6000 years is thus estimated judging from the matured fault structure of the qingchuan fault the latest event was likely to have ruptured the full length of the fault according to the empirical scaling laws between magnitude and rupture length the magnitude of the event is estimated to be mw 7 6 7 9 on the basis of the slip rate from gps measurements and the elapsed time since the last event it is estimated that a seismic moment equivalent to mw 7 5 has been accumulated on the qingchuan fault considering the increased coulomb failure stress and the shortened time of earthquake recurrence triggered by the wenchuan earthquake it is suggested a high seismic risk along the qingchuan fault and its neighboring area furthermore the slow strain buildup unadapted geometry and matured fault structure of the ltz may be the reason why it produces rare but large intraplate earthquakes

high velocity frictional properties of alpine fault rocks mechanical data microstructural analysis and implications for rupture propagation abstract the alpine fault in new zealand is a major plate bounding structure that typically slips in m8 earthquakes every c 330 years to investigate the near surface high velocity frictional behavior of surface and borehole derived alpine fault gouges and cataclasites twenty one rotary shear experiments were conducted at 1 mpa normal stress and 1 m s equivalent slip velocity under both room dry and water saturated wet conditions in the room dry experiments the peak friction coefficient p p n of alpine fault cataclasites and fault gouges was consistently high mean p 0 67 0 07 in the wet experiments the fault gouge peak friction coefficients were lower mean p 0 20 0 12 than the cataclasite peak friction coefficients mean p 0 64 0 04 all fault rocks exhibited very low steady state friction coefficients ss room dry experiments mean ss 0 16 0 05 wet experiments mean ss 0 09 0 04 of all the experiments performed six experiments conducted on wet smectite bearing principal slip zone psz fault gouges yielded the lowest peak friction coefficients p 0 10 0 20 the lowest steady state friction coefficients ss 0 03 0 09 and commonly the lowest specific fracture energy values e g 0 01 0 69 mj m 2 microstructures produced during room dry and wet experiments on a smectite bearing psz fault gouge were compared with microstructures in the same material recovered from the deep fault drilling project dfdp 1 drill cores the near absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments mechanical data and microstructural observations suggest that alpine fault ruptures propagate preferentially through water saturated smectite bearing fault gouges that exhibit low peak and steady state friction coefficients

high velocity frictional strength of longmenshan fault gouge and its comparison with an estimate of friction from the temperature anomaly in wfsd 1 drill hole this paper addresses the issue of whether high velocity friction experiments yield frictional strengths of fault materials that are consistent with the level of friction estimated from the postseismic temperature anomaly measured across a coseismic fault zone experiments were conducted on gouge composed of quartz dolomite illite albite and other clay minerals collected from a large outcrop of the yingxiu beichuan fault zone and from wenchuan earthquake fault scientific drilling project hole 1 drill cores in hongkou sichuan province china this fault is a major fault of the longmenshan fault system that caused the 2008 mw7 9 wenchuan earthquake all experiments revealed dramatic weakening at high velocities with peak friction coefficient p of 0 07 0 35 and steady state friction coefficient ss of 0 02 0 15 for wet gouge as compared with p 0 49 0 86 and ss 0 12 0 21 for dry gouge the average friction coefficients over a displacement of 5 5 m coseismic displacement extrapolated to the normal stress in the fault zones are 0 1 0 05 and 0 06 0 03 for dry and wet gouges respectively the average friction coefficients of dry and wet gouges are within the upper and lower bounds of friction coefficients estimated from the temperature profiles and there are overall agreements between the two sets of data that are completely independent dry and wet gouges are characterized by highly sheared slip zones often forming overlapping slip zone structures and by broad shear zones respectively wet gouge textures are perhaps closer to those of natural fault zone but repeated slip experiments are needed to determine the textural evolution of wet gouge

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

co seismic ruptures of the 12 may 2008 ms 8 0 wenchuan earthquake sichuan east west crustal shortening on oblique parallel thrusts along the eastern edge of tibet abstract the m s 8 0 wenchuan earthquake which devastated the mountainous western rim of the sichuan basin in central china produced a surface rupture over 200 km long with oblique thrust dextral slip and maximum scarp heights of 10 m it thus ranks as one of the world s largest continental mega thrust events in the last 150 yrs field investigation shows clear surface breaks along two of the main branches of the ne trending longmen shan thrust fault system the principal rupture on the nw dipping beichuan fault displays nearly equal amounts of thrust and right lateral slip basin ward of this rupture another continuous surface break is observed for over 70 km on the parallel more shallowly nw dipping pengguan fault slip on this latter fault was pure thrusting with a maximum scarp height of 3 5 m this is one of the very few reported instances of crustal scale co seismic slip partitioning on parallel thrusts this out of sequence event with distributed surface breaks on crustal mega thrusts highlights regional ew directed present day crustal shortening oblique to the longmen shan margin of tibet the long rupture and large offsets with strong horizontal shortening that characterize the wenchuan earthquake herald a re evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau and require a re assessment of seismic hazard along potentially under rated active faults across the densely populated western sichuan basin and mountains

vaporization of fault water during seismic slip laboratory and numerical studies as well as field observations indicate that phase transitions of pore water might be an important process in large earthquakes we present a model of the thermo hydro chemo mechanical processes including a two phase mixture model to incorporate the phase transitions of pore water occurring during fast slip i e a natural earthquake in order to investigate the effects of vaporization on the coseismic slip using parameters from typical natural faults our modeling shows that vaporization can indeed occur at the shallow depths of an earthquake irrespective of the wide variability of the parameters involved sliding velocity friction coefficient gouge permeability and porosity and shear induced dilatancy due to the fast kinetics water vaporization can cause a rapid slip weakening even when the hydrological conditions of the fault zone are not favorable for thermal pressurization e g when permeability is high at the same time the latent heat associated with the phase transition causes the temperature rise in the slip zone to be buffered our parametric analyses reveal that the amount of frictional work is the principal factor controlling the onset and activity of vaporization and that it can easily be achieved in earthquakes our study shows that coseismic pore fluid vaporization might have played important roles at shallow depths of large earthquakes by enhancing slip weakening and buffering the temperature rise the combined effects may provide an alternative explanation for the fact that low temperature anomalies were measured in the slip zones at shallow depths of large earthquakes

initial 182hf 180hf in meteoritic zircons zircons from the simmern h5 chondrite and pomozdino eucrite have been analyzed for their hf w isotope systematics zircons have high intrinsic hf contents and coupled with low w make them ideal hf w chronometers however measurements of 182hf 180hf 0 are far from straightforward with low signals of radiogenic 182w and difficulties in calibrating hf w ratios zircons were analyzed from the simmern chondrite and the pomozdino eucrite the simmern zircon has an ultrarefractory enriched trace element pattern a feature commonly associated with refractory inclusions analyses of simmern zircon show variable hf w that is likely due to surface contamination simmern chondrite zircon yields a 182hf 180hf 0 of 7 2 4 5 10 5 pomozdino eucrite zircon analyses show very high hf w values indicating 182hf 180hf 0 of 1 7 1 1 10 5 the simmern value is in good agreement with the initial value of the solar system indicated from hf w systematics of chondrites the pomozdino value is lower than expected if eucrites formed within several million years of refractory inclusions as suggested from al mg systematics of eucrites

shear induced graphitization of carbonaceous materials during seismic fault motion experiments and possible implications for fault mechanics abstract carbonaceous materials often concentrate in fault zones developed in pelitic rocks among carbonaceous minerals graphite is known as a lubricant and possibly plays a key role in frictional properties of the fault graphite reported from slip localized zones suggests that graphitization can occur during seismogenic fault motion thus we performed friction experiments on amorphous carbon and graphite to investigate how graphite forms in association with fault motion and how these carbonaceous minerals affect frictional properties of faults experiments were done at normal stresses of 0 5 2 8 mpa and slip rates of 50 m s to 1 3 m s in atmospheres of air and n 2 gas using rotary shear apparatuses xrd and tem analyses revealed that graphitization can indeed occur during seismogenic fault motion perhaps due to large shear strain short lived flash heating and stress concentration at asperity contacts even at low temperatures and pressures under anoxic environments we found large differences in steady state friction coefficient ss between graphite ss 0 1 and amorphous carbon ss 0 54 at low slip rate but amorphous carbon exhibits marked velocity weakening at slip rate above 10 mm s and its steady state friction reduces to the same level as that of graphite at a slip rate of 1 3 m s faults with amorphous carbon are not weak at low slip rates but they can become dynamically weak to foster fault motion during the generation of large earthquakes enriched graphite in fault zones can lubricate at all slip rates even at great depths and should receive more attention

high velocity frictional properties of clay rich fault gouge in a megasplay fault zone nankai subduction zone 1 we conducted high velocity friction experiments on clay rich fault gouge taken from the megasplay fault zone in the nankai subduction zone under dry and wet conditions in the dry tests dehydration of clay minerals occurred by frictional heating and slip weakening is related to thermal pressurization associated with water vaporization resulting in a random distribution of clay clast aggregates in the gouge matrix in the wet tests slip weakening is caused by pore fluid pressurization via shear enhanced compaction and frictional heating and there is a very weak dependence of the steady state shear stress on the normal stress the resulting microstructure reflects the grain size segregation in a granular fluid shear flow at high shear rates these results suggest that earthquake rupture propagates easily through clay rich fault gouge by high velocity weakening potentially leaving the microstructures resulting from the frictional heating or the flow sorting at high slip rates

experimental investigation of flash weakening in limestone abstract flash heating and weakening could operate during earthquake nucleation and propagation we performed 27 friction experiments in a compression torsion apparatus on ring shaped limestone samples at sub seismic to seismic slip rates 340 mm s centimetric displacements and normal stresses of 8 mpa friction decreases dramatically at slip rates of 50 150 mm s flash weakening was contemporaneous with a peripheral temperature rise of 90 c measured with an infrared camera the peripheral temperature yields a lower limit to the slipping zone temperature the decrease in friction may result from weakening of the asperity contacts due to decarbonation of calcite induced by 1 flash heating or 2 mechanically activated reactions however x ray powder diffraction and raman spectroscopy analyses do not reveal the presence of decarbonation products in the slipping zone instead white light interferometry and field emission scanning electron microscope observations reveal the presence of a smooth sliding surface made of nanometric particles the mechanical data can be fit by the rate and state dependent friction model or by a quadratic model the latter proposed for powder lubrication we conclude that flash heating and weakening and powder lubrication may operate together to decrease dynamic friction in limestone in experiments and for the conditions investigated here in nature

the 2011 tohoku oki earthquake displacement reaching the trench axis we detected and measured coseismic displacement caused by the 11 march 2011 tohoku oki earthquake moment magnitude mw 9 0 by using multibeam bathymetric surveys the difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the japan trench suggesting that the fault rupture reached the trench axis the sea floor on the outermost landward area moved about 50 meters horizontally east southeast and 10 meters upward the large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement

dynamic weakening of gouge layers in high speed shear experiments assessment of temperature dependent friction thermal pressurization and flash heating 1 the rotary shear of gouge demonstrates dramatic weakening at coseismic slip rates but the inherent variation in shear rate with radius of the rotary shear configuration prohibits determining friction constitutive properties from measures of whole sample response here representative results of rotary shear high speed experiments including both constant velocity tests 0 1 1 3 m s and constant acceleration tests 0 1 3 m s 0 05 and 0 1 m s2 are analyzed using a thermal mechanical and hydrologic finite element method fem model to constrain friction constitutive properties and test hypotheses of dynamic weakening by thermal pressurization and flash heating the observed frictional behavior of room dry gouge can be explained by using a state variable friction constitutive relation in which the friction coefficient is inversely proportional to temperature and by employing a two mechanism constitutive formulation in which the friction coefficient increases with temperature temperature strengthening at low temperatures and decreases with temperature temperature weakening at higher temperatures water dampened gouge displays a transient weakening during the early stages of constant acceleration tests fem analysis indicates the weakening could reflect thermal pressurization of pore water provided both the permeability of the gouge layer and the sealing capacity of the teflon sleeve used to contain the gouge during shear contribute to restricting fluid flow microstructural observations indicate that the dynamic weakening coincides with slip localization and temperature increase by frictional heating which are conditions that favor weakening by flash heating at steady state the relationship between slip rate and coefficient of friction by the fem model analysis is consistent with predictions of micromechanical models for weakening by flash heating

thermal pressurization and slip weakening distance of a fault an example of the hanaore fault southwest japan a controversy over the last decade is the large difference in the slip weakening distance d c between laboratory measured and seismically estimated values our numerical analyses of the thermal pressurization process based on measured transport properties of the hanaore fault zone in japan reveal that the process is effective for this fault and that d c becomes of the order of several decimeters to about 1 m when the width of the deformation zone is less than 10 20 mm this d c value is of exactly the same order as that determined seismically so thermal pressurization can be a mechanism for solving the d c paradox the thermal pressurization process effectively reduces the heat production rate and the temperature rise expected for this fault zone is at most about 600 c which is not high enough a temperature to melt rocks

strain localization and the onset of dynamic weakening in calcite fault gouge abstract to determine the role of strain localization during dynamic weakening of calcite gouge at seismic slip rates single slide and slide hold slide experiments were conducted on 2 3 mm thick layers of calcite gouge at normal stresses up to 26 mpa and slip rates up to 1 m s 1 microstructures were analyzed from short displacement 35 cm experiments stopped prior to and during the transition to dynamic weakening in fresh calcite gouge layers dynamic weakening occurs after a prolonged strengthening phase that becomes shorter with increasing normal stress and decreasing layer thickness strain is initially distributed across the full thickness of the gouge layer but within a few millimeters displacement the strain becomes localized to a boundary parallel high strain shear band c 20 m wide during the strengthening phase which lasts between 3 and 30 cm under the investigated conditions the shear band broadens to become c 100 m wide at peak stress the transition to dynamic weakening in calcite gouges is associated with the nucleation of micro slip surfaces dispersed throughout the c 100 m wide shear band each slip surface is surrounded by aggregates of extremely fine grained and tightly packed calcite interpreted to result from grain welding driven by local frictional heating in the shear band by the end of dynamic weakening strain is localized to a single 2 3 m wide principal slip surface flanked by layers of recrystallized gouge calcite gouge layers re sheared following a hold period weaken nearly instantaneously much like solid cylinders of calcite marble deformed under the same experimental conditions this is due to reactivation of the recrystallized and cohesive principal slip surface that formed during the first slide reducing the effective gouge layer thickness to a few microns our results suggest that formation of a high strain shear band is a critical precursor to dynamic weakening in calcite gouges microstructures are most compatible with dynamic weakening resulting from a thermally triggered mechanism such as flash heating that requires both a high degree of strain localization and a minimum slip velocity to activate the delayed onset of dynamic weakening in fresh calcite gouge layers particularly at low normal stresses may inhibit large coseismic slip at shallow crustal levels in calcite bearing fault zones

the crucial role of temperature in high velocity weakening of faults experiments on gouge using host blocks with different thermal conductivities we study the important role of temperature rise in the dynamic weakening of fault gouge at seismic slip rates by using host blocks composed of brass stainless steel titanium alloy and gabbro with thermal conductivities h of 123 15 5 8 and 3 25 w m k respectively our experiments are performed mostly on fault gouge collected from the longmenshan fault sichuan china consisting primarily of illite and quartz high velocity weakening of gouge becomes more pronounced as h decreases because the temperature in the gouge increases microstructure observations reveal welded slip zone material and more compact slip surfaces for the gouge deformed with low h host blocks which is probably caused by a sintering process indicative of higher temperatures these conclusions are supported by temperature calculation performed using the finite element method the observed differences in frictional behaviors deformation microstructures and calculated temperature demonstrate that temperature rise driven by frictional heating is essential in causing dynamic weakening of gouge at seismic velocities we show that our data are in good agreement with the flash heating model though thermochemical pressurization may also be important some of our experiments where nanoparticles are present but show negligible weakening demonstrate that the presence of nanoparticles alone is not sufficient to cause dynamic weakening of faults

dynamic weakening of smectite bearing faults at intermediate velocities implications for subduction zone earthquakes the hydrous clay mineral smectite which is pervasive in sediments on subducting oceanic plates is thought to weaken and stabilize subduction thrust faults however these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 m s to 1 3 m s for water saturated smectite quartz mixtures with various smectite contents so as to shed light on the frictional response of smectite bearing faults at intermediate to high slip rates at slip rates of 30 to 150 m s the friction coefficients decreased gradually from 0 5 0 6 to 0 1 with an increase in smectite content from 20 to 50 wt in contrast at slip rates higher than 1 3 mm s friction exhibited marked slip weakening resulting in low friction coefficients of 0 1 0 05 even for low smectite contents roughly 30 wt drastic slip weakening occurred at smectite contents of 10 30 wt at slip rates of 10 mm s which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types the intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear enhanced compaction and microscopic thermal pressurization of pore fluids this process could weaken the fault even below seismic slip rates leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench

importance of thermochemical pressurization in the dynamic weakening of the longmenshan fault during the 2008 wenchuan earthquake inferences from experiments and modeling 1 we determined the internal structure and mineral composition of the yingxiu beichuan fault zone at the zhaojiagou exposure and measured frictional and transport properties of the fault rocks collected to gain a better understanding of dynamic weakening mechanisms during seismic fault motion this fault is a major fault in the longmenshan fault system that caused the 2008 wenchuan earthquake the exposure studied is located midway along the surface rupture close to where the largest coseismic displacement occurred high velocity friction experiments reveal exponential slip weakening from a peak friction toward a steady state value slip weakening is more pronounced for water dampened gouge than dry gouge suggesting thermal pressurization the fault gouge has a very low permeability 10 21 m2 at 165 mpa effective pressure and is surrounded by fault breccia with a permeability of 10 19 to 10 17 m2 grading into less permeable fractured country rocks the fault zone thus exhibits a conduit barrier structure allowing fluid flow only in the breccia zone we numerically modeled coseismic slip weakening including thermal pressurization and mineral dehydration decarbonation basing our calculation on measured frictional and transport properties and on the slip history inferred for the wenchuan earthquake the results indicate that 1 thermochemical pressurization played an important role in causing dynamic slip weakening 2 the slip weakening distance is similar to the seismologically determined values and 3 pore pressures might have exceeded the normal stress thus maintaining temperatures below 600 c interestingly enough heat was generated to fully remove and thermally pressurize the interlayer water from smectite contributing an excess pore pressure of 6 mpa in addition we found that the incorporation of state dependent fluid properties predicts much more efficient fluid pressurization than using constant properties the dramatic weakening predicted probably offers a compelling explanation for the large coseismic displacement and slip acceleration observed near beichuan city

earthquake slip weakening and asperities explained by thermal pressurization for the earth s crust to release accumulated stress in the form of an earthquake a fault must weaken and give way otherwise the movement would quickly grind to a halt due to the surrounding pressure this weakening is not really understood laboratory experiments simply don t match seismologists estimates of the distance a fault must slip before it becomes very weak a new study concludes that the missing ingredient is water geological studies and measurements show the slip zones to be narrow impermeable structures water already in the fault zone can quickly become pressurized by frictional heating as the rocks start to move catalysing large earthquakes from minor movements an earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion1 2 this slip weakening occurs over a critical distance dc understanding the controls on dc in nature is severely limited however because the physical mechanism of weakening is unconstrained conventional friction experiments typically conducted at slow slip rates and small displacements have obtained dc values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes3 4 5 6 here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the median tectonic line fault zone japan we show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid7 8 9 10 is the slip weakening mechanism our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity

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

mineralogy porosity and fluid control on thermal conductivity of sedimentary rocks summary estimates of thermal conductivity of the main non clay and clay sedimentary minerals are presented especially for the case of argillaceous ones these estimates are averaged estimates based on the interpretation of laboratory conductivity porosity and mineralogy measurements performed on small volumes of the order of 100cm3 of water saturated moist and air saturated samples the sampling is representative of the main sedimentary rocks sandstones carbonates evaporites and shales and it is composed of samples considered as structurally isotropic material first we experimentally verify the first order control of mineralogy porosity and fluid content on bulk conductivity and we demonstrate that such influences may be predicted accurately using a geometric mean model as long as isotropic samples are used then we interpret the data using an inverse method in order to estimate the average mineral conductivities of the main non clay and clay minerals which give the best fit to the individual laboratory measurements through the geometric mean model the analysis is based on measurements on 82 non clay samples and 29 clay samples taken on oil well cores or cuttings on outcrops and on artificially recompacted samples estimates of average mineral conductivities returned by the inversion process for the main non clay minerals are similar to generally admitted values 7 7 w m k for quartz 3 3 w m i k for calcite 5 3 w m l k for dolomite and 6 3 w m k for anhydrite values obtained for the clay minerals are systematically lower than those obtained for non clay ones they are of the order of 1 9 w m k for illite and smectite 2 6 w m k for kaolinite and 3 3 w m l k for chlorite these estimates combined with porosity and mineralogy data are used with the geometric mean model in order to verify its first order validity for predicting thermal conductivity of small volumes of porous isotropic sedimentary rocks the bulk conductivity is predicted with an accuracy of the order of f10 per cent for moist or water saturated samples and of the order of f20 per cent for air saturated samples

long term temperature records following the mw 7 9 wenchuan china earthquake are consistent with low friction this is the publisher s final pdf the published article is copyrighted by the geological society of america and can be found at http geology gsapubs org

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

low to high velocity frictional properties of the clay rich gouges from the slipping zone of the 1963 vaiont slide northern italy the final slip of about 450 m at about 30 m s of the 1963 vaiont landslide italy was preceded by 3 year long creeping phase which was localized in centimeter thick clay rich layers 60 70 smectites 20 30 calcite and quartz here we investigate the frictional properties of the clay rich layers under similar deformation conditions as during the landslide 1 5 mpa normal stress 2 10 7 to 1 31 m s slip rate and displacements up to 34 m experiments were performed at room humidity and wet conditions with biaxial torsion and rotary shear apparatus the clay rich gouge was velocity independent to velocity weakening in both room humidity and wet conditions in room humidity experiments the coefficient of friction decreased from 0 47 at v 0 70 m s full lubrication results from the formation of a continuous water film in the gouge the vaiont landslide occurred under wet to saturated conditions the unstable behavior of the landslide is explained by the velocity weakening behavior of the vaiont clay rich gouges the formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero even without invoking the activation of thermal pressurization this explains the extraordinary high velocity achieved by the slide during the final collapse

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

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

thermal decomposition of serpentine during coseismic faulting nanostructures and mineral reactions abstract this paper reports a detailed characterization of an antigorite bearing serpentinite deformed at seismic slip rate 1 1 m s in a high velocity friction apparatus micro nanostructural investigation of the slip zone 200 m thick revealed a zonal arrangement with a close juxtaposition of horizons with significantly different strength respectively consisting of amorphous to poorly crystalline phases with bulk anhydrous composition close to starting antigorite and of highly crystalline assemblages of forsterite and disordered enstatite 200 nm in size and in polygonal like nanotextures the slip zone also hosts micro nanometre sized cr magnetite grains aligned at low angle with respect to the slipping surface and inherited from the host serpentinite overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820 1200 c in agreement with flash temperature theory responsible for serpentine complete dehydration and amorphization followed by crystallization of forsterite and enstatite under post deformation static conditions the results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite hosted faults through the seismic cycle

sims pb pb and u pb age determination of eucrite zircons at 5 μm scale and the first 50 ma of the thermal history of vesta eucrites a subgroup of basaltic achondrites represent the remnants of the earliest magmatic stage of the solar system formation possibly on the asteroid 4 vesta the precise determination of their crystallization age is therefore important for constraining the evolutionary history of the asteroid including melting and differentiation processes of the parent body zircons would be the best mineral for such purpose however extra terrestrial zircons are rare and those found in eucrites are typically small 5 m using a cameca ims 1280 high resolution ion microprobe precise uranium lead u pb and pb pb ages of the non cumulate basaltic eucrite zircons in bereba cachari caldera camel donga and juvinas were determined successfully at scale of 5 m we obtained a weighted average 207pb 206pb age of 4541 11 ma and u pb concordia age of 4525 24 ma for the five eucrite samples studied a synthesis of available age data of howardites eucrites and diogenites indicate a protracted magmatic history on vesta with a most pronounced core mantle differentiation event marked at 4564 2 ma followed by a peak of basaltic magmatism at 4552 7 ma which gradually diminished over the next 50 ma thermal modeling of vesta suggests that partial melt fraction of greater than 10 with temperature well exceeding 1500 k could be maintained over this period throughout its interior this is consistent with the chronological records of eucrites suggesting persistent magmatism on vesta during the first 50 ma in the early solar system

friction properties and deformation mechanisms of halite mica gouges from low to high sliding velocities the evolution of friction as a function of slip rate is important in understanding earthquake nucleation and propagation many laboratory experiments investigating friction of fault rocks are either conducted in the low velocity regime 10 8 10 4 ms 1 or in the high velocity regime 0 01 1 m s 1 here we report on the evolution of friction and corresponding operating deformation mechanisms in analog gouges deformed from low to high slip rates bridging the gap between these low and high velocity regimes we used halite and halite muscovite gouges to simulate processes governing friction active in upper crustal quartzitic fault rocks at conditions accessible in the laboratory the gouges were deformed over a 7 orders of magnitude range of slip rate 10 7 1 m s 1 using a low to high velocity rotary shear apparatus using a normal stress of 5 mpa and room dry humidity microstructural analysis was conducted to study the deformation mechanisms four frictional regimes as a function of slip rate could be recognized from the mechanical data showing a transitional regime and stable sliding 10 7 10 6 m s 1 unstable sliding and weakening 10 6 10 3 m s 1 hardening 10 2 10 1 m s 1 and strong weakening 10 1 1 m s 1 each of the four regimes can be associated with a distinct microstructure reflecting a transition from mainly brittle deformation accompanied by pressure solution healing to temperature activated deformation mechanisms additionally the frictional response of a sliding gouge to a sudden acceleration of slip rate to seismic velocities was investigated these showed an initial strengthening the amount of which depended on the friction level at which the step was made followed by strong slip weakening 2016 elsevier b v

thermochemical pressurization of faults during coseismic slip 1 during earthquakes frictional heating on the fault plane induces a temperature rise and thus a pore pressure rise which is known as thermal pressurization tp coseismic mineral dehydrations may occur because of this temperature increase and are included within the tp framework dehydrations are modeled as a source term for pore pressure because of the total volume change and as a sink term for temperature because they are endothermic the reaction occurs within the slipping zone when a threshold temperature ts is reached dehydration reaction kinetic is modeled using a first order reaction rate using energy and fluid mass conservation we derive analytically the equations of evolution of pore pressure temperature and reaction extent in the undrained adiabatic case using a constant reaction rate we investigate the values of the kinetic rate constant required to produce a significant effect which are much higher than laboratory data reported in the literature on clay serpentine and phyllosilicate dehydration we show however that such high values can be reached if the temperature dependency of the rate constant is taken into account next we include fluid and heat transport and use an arrhenius law to calculate the rate constant as a function of temperature the subsequent set of differential equations is then solved numerically the main effect of dehydration reactions is an increase of pore pressure and a stabilization of the temperature during slip we explore a wide range of parameters in order to determine in which cases dehydration can be considered as a nonnegligible process for high permeability rocks 10 18 m2 and when the amount of water that can be released is of the order of 10 dehydration is an important mechanism as it delays the onset of melting which would normally occur even within the tp framework if the onset temperature is low compared to the initial temperature t0 ts t0 150 c overpressure can occur if the reactions are highly endothermic and if their kinetic is fast enough frictional melting would not occur unless the dehydration reactions are completed within the slipping zone

mineralogical association and ni co deportment in the wingellina oxide type laterite deposit western australia abstract the wingellina deposit currently owned by metals x limited is an oxide type ni co laterite with 168 mt of ore grading 0 98 ni and 0 08 co the laterite profile is derived from the weathering of the olivine rich mafic ultramafic layered intrusion of the giles complex mesoproterozoic and consists of a well developed limonitic ore and a thin saprolite horizon xrpd sem eds and empa analyses revealed that the ore zone is dominated by fe and mn hydr oxides which are the main carriers of both ni and co the mineralogy of the mn hydroxides is complex and high concentrations of co and ni occur mainly in a lithiophorite asbolane intermediate phase which has been subdivided into types i ii iii and iv with significant differences in their co ni ratios and al content the enrichment of metals in the lithiophorite asbolane phases is mainly controlled by the co and ni replacement of al in gibbsite like layers and rarely by ni incorporation in the vacancies of mn hydr oxides other less important metal bearing mn hydr oxides at wingellina include romanechite ernienickelite jianshuiite manganite and birnessite the texture and chemistry of the mn hydr oxides suggest that the lithiophorite asbolane phases were formed at a later stage with respect to the other manganiferous phases after repeated leaching and re precipitation of the mn hydr oxides this process could be one of the main factors controlling the enrichment of co and ni in the limonite zones during lateritization