tem study of in situ organic matter on continental margins occurrence and the monolayer hypothesis abstract tem photomicrographs show that organic matter in the continental margin sediments that we have examined is primarily patchy in distribution and occurs as 1 discrete discontinuous blebs and micro blebs of differentiated and undifferentiated protoplasm 2 bacterial cells and associated muco polysaccharide networks and 3 localized smears generally associated with clay minerals and domain junctions in clay rich flocs not as thin uniform grain coatings or as infillings of nicks or etch pits on sediment grains with high degrees of surface roughness this occurrence contradicts recent assertions that most organic matter in continental margins occurs as monolayer or monolayer equivalent coatings on mineral grains and inorganic bioclastic particles our results are based on tem photomicrographs of samples from the northern california continental slope that fall within the monolayer equivalent envelope defined by mayer 1994 marine snow from the overlying nepheloid layer and sediments from eckernforde bay in the baltic sea these samples were prepared specifically to preserve and image in situ sediment fabric and organic constituents textural evidence suggests that low permeabilities caused by the interaction between the organic inorganic and biologic components of the clay size fraction play a major role in determining organic matter preservation on continental margins

high resolution particle size analysis of naturally occurring very fine grained sediment through laser diffractometry abstract in this paper we present results from a large number of experiments aimed at quantifying method and instrument uncertainty associated with laser diffraction analysis we analyzed the size distribution of fine grained sediment 24 hours prior to analysis and using 60 seconds of ultrasonication during analysis 2 obscuration a measure of the concentration of the suspension during analysis produced the most reproducible results at about 20 3 variations in refractive index settings can significantly alter estimated grain size distributions 4 assumed values for absorption the degree to which sediment grains absorb the light can have a profound effect on grain size results absorption settings near 0 resulted in unexpected bimodal grain size distributions for sediments in the 10 m size fraction and significantly skewed the fine grained tail of coarser samples probably because of sub optimal diffraction by particles with a diameter similar in size to the laser wavelength absorption settings closer to 1 produced very reproducible results and unimodal grain size distributions over a wide range of refractive indexes our study has shown that laser diffraction can measure very fine grained sediments 10 m quickly with high precision 5 at 2 standard deviations and without the need for extensive mineralogical determinations these results make possible a new generation of studies in which high resolution time series data sets of sediment grain size can be used to infer subtle changes in paleohydrology

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

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

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

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

the role of mineral composition on the frictional and stability properties of powdered reservoir rocks national natural science foundation of china 41672268 41772286 u s department of energy doe de fe0023354

crystallographic controls on the frictional behavior of dry and water saturated sheet structure minerals 1 we compare the frictional strengths of 17 sheet structure mineral powders measured under dry and water saturated conditions to identify the factors that cause many of them to be relatively weak the dry coefficient of friction ranges upward from 0 2 for graphite leveling off at 0 8 for margarite clintonite gibbsite kaolinite and lizardite the values of dry correlate directly with calculated 001 interlayer bond strengths of the minerals this correlation occurs because shear becomes localized along boundary and riedel shears and the platy minerals in them rotate into alignment with the shear planes for those gouges with dry 0 8 shear occurs by breaking the interlayer bonds to form new cleavage surfaces where dry 0 8 consistent with byerlee s law the interlayer bonds are sufficiently strong that other frictional processes dominate the transition in dry friction mechanisms corresponds to calculated surface energies of 2 3 j m2 adding water causes to decrease for every mineral tested except graphite if the minerals are separated into groups with similar crystal structures wet increases with increasing interlayer bond strength within each group this relationship also holds for the swelling clay montmorillonite whose water saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure water in the saturated gouges forms thin structured films between the plate surfaces the polar water molecules are bonded to the plate surfaces in proportion to the mineral s surface energy and wet reflects the stresses required to shear through the water films

frictional properties of simulated fault gouges from the seismogenic groningen gas field under in situ p t chemical conditions we investigated the frictional properties of simulated fault gouges derived from the main lithologies present in the seismogenic groningen gas field ne netherlands employing in situ p t conditions and varying pore fluid salinity direct shear experiments were performed on gouges prepared from the carboniferous shale siltstone substrate the upper rotliegend slochteren sandstone reservoir the overlying ten boer claystone and the basal zechstein anhydrite carbonate caprock at 100 c 40 mpa effective normal stress and sliding velocities of 0 1 10 m s as pore fluids we used pure water 0 5 6 2 m nacl solutions and a 6 9 m mixed chloride brine mimicking the formation fluid our results show a marked mechanical stratigraphy with a maximum friction coefficient of 0 66 for the basal zechstein a minimum of 0 37 for the ten boer claystone 0 6 for the reservoir sandstone and 0 5 for the carboniferous mixed gouges showed intermediate values pore fluid salinity had no effect on frictional strength most gouges showed velocity strengthening behavior with little systematic effect of pore fluid salinity or sliding velocity on a b however basal zechstein gouge showed velocity weakening at low salinities and or sliding velocities as did 50 50 mixtures with sandstone gouge tested with the 6 9 m reservoir brine from a rate and state friction viewpoint our results imply that faults incorporating basal zechstein anhydrite carbonate material at the top of the reservoir are the most prone to accelerating slip that is have the highest seismogenic potential the results are equally relevant to other rotliegend fields in the netherlands and n sea region and to similar sequences globally

mass removal and clay mineral dehydration rehydration in carbonate rich surface exposures of the 2008 wenchuan earthquake fault geochemical evidence and implications for fault zone evolution and coseismic slip 1 we report variations in the mineralogical geochemical and isotopic 13c 18o composition of fault rocks sampled in transects across the zhaojiagou and pingxi exposures of the wenchuan earthquake or longmenshan fault zone where the gouge rich fault core and principal slip surface cuts through carbonate rich strata pervasive fluid infiltration was found to modify the mineralogical and geochemical architecture of the fault zones studied enrichment depletion patterns element partitioning and a very large implied volume loss are quite different from those characterizing faults in granites and clastic sedimentary rocks and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution an increasing enrichment in smectite observed toward the principal slip surface a high abundance of elements such as ba mg and f the deposition of minerals such as barite and fluorapatite as well as the distinct depletion in 13c in vein material consistently suggest reactions involving a hydrothermal fluid originating at depth illitization of black gouges caused by coseismic frictional heating was found to be widespread we propose that coseismic frictional heating along with the action of postseismic hydrothermal fluids controlled the transformation and distribution of smectite and illite within the fault core of the longmenshan fault zone the coseismic dewatering reactions are expected to have been more extensive at depth possibly helping generate excess pore pressure assisting dynamic slip weakening during the wenchuan earthquake

structural and fluid evolution of a young accretionary complex the hoh rock assemblage of the western olympic peninsula washington the hoh rock assemblage of the western olympic peninsula washington was deposited in eocene to miocene time and accreted deformed uplifted and eroded by the late miocene northeast dipping fault zones within the hoh are characterized by intensely deformed mud matrix melange and they range from 200 m to 1 000 m thick kinematic indicators within the matrix indicate thrusting to the west southwest both hanging wall and footwall sections are strongly folded and faulted wall rock sections are lithostratigraphically distinct with hanging walls composed of massive coarse sandstone and conglomerate and footwall sections composed of thin to medium bedded sandstone and siltstone folds within the wall rock sections are oblique to and are truncated by the later fault zone melanges indicating that the fault zones postdate folding evidence of fluid involvement in the hoh rocks includes petroliferous outcrops carbonate and clay cement and veins of carbonate and laumontite the progressive evolution of veins and cements paragenetic sequence can be interpreted in terms of crystallization under conditions of increasing consolidation decreasing salinity and changing fluid composition suppressed carbonate luminescence may reflect crystallization under reducing conditions perhaps related to the sulfate reduction zone the presence of hydrocarbons or clay dehydration although deformational features may be associated with the earliest phase of veining most of the localized grain breakage and matrix shear zones formed after the early micritic veins and before the last phase of sparitic carbonate laumontite veins laumontite occurred syn deformationally and in equilibrium with carbonate and also post deformationally the paragenetic sequences observed in all samples from the hoh rock assemblage can be correlated suggesting that the sources and processes controlling the fluid emplacement were similar across a wide region in addition the paragenetic sequence in the hoh rocks is similar to that documented in autochthonous coeval basins of southwest washington this similarity suggests that the precipitates formed during the evolution of the hoh rocks involved fluids that originated within the matrix and did not involve exotic or far traveled fluids

fracture surface energy of the punchbowl fault san andreas system the energetics of earthquake rupture slip weakening and rupture surface formation are poorly understood by measuring the energy associated with fracture damage along the punchbowl fault in the devil s punchbowl los angeles county park part of the san andreas system chester et al show that the fracture surface energy for rupture along a mature fault of this type accounts for only a small fraction of the energy released during large earthquakes as determined by seismologists this suggests that mechanisms such as melt lubrication and thermal pressurization are responsible for slip weakening fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening1 2 3 fracture energy has been estimated from seismological and experimental rock deformation data4 5 6 7 8 yet its magnitude mechanisms of rupture surface formation and processes leading to slip weakening are not well defined8 9 10 here we quantify structural observations of the punchbowl fault a large displacement exhumed fault11 12 in the san andreas fault system and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake if fracture energy is attributed entirely to the production of fracture surfaces then all of the fracture surface area in the punchbowl fault could have been produced by earthquake displacements totalling 1 km but this would only account for a small fraction of the total energy budget and therefore additional processes probably contributed to slip weakening during earthquake rupture

frictional properties of natural fault gouge from a low angle normal fault panamint valley california 1 we investigate the relationship between frictional strength and clay mineralogy of natural fault gouge from a low angle normal fault in panamint valley california gouge samples were collected from the fault zone at five locations along a north south transect of the range bounding fault system spanning a variety of bedrock lithologies samples were powdered and sheared in the double direct shear configuration at room temperature and humidity the coefficient of friction was measured at a range of normal stresses from 5 to 150 mpa for all samples our results reinforce the intuitive understanding that natural fault gouge zones are inherently heterogeneous samples from a single location exhibit dramatic differences in behavior yet all three were collected within a meter of the fault core for most of the samples friction varies from 0 6 to 0 7 consistent with byerlee s law however samples with greater than 50 wt total clay content were much weaker 0 2 0 4 expandable clay content of the samples ranged from 10 to 40 wt frictional weakness did not correlate with expandable clays our results indicate that friction decreases with increasing total clay content rather than with the abundance of expandable clays the combination of field relations analytical results and friction measurements indicates a positive correlation between clay content fabric intensity and localization of strain in the fault core a mechanism based upon foliations enveloping angular elements to reduce friction is suggested for weakening of fault gouge composed of mixed clay and granular material we provide broad constraints of 1 5 km on the depth of gouge generation and the depth at which fault weakness initiates we show that slip on the panamint valley fault and similar low angle normal faults is mechanically feasible in the mid upper crust if the strength of the fault is limited by weak clay rich fault gouge

frictional properties of sedimentary rocks and natural fault gouge from the longmen shan fault zone sichuan china in this paper we report friction experiments performed on samples collected from the region hit by the 2008 wenchuan earthquake in the longmen shan fault zone lfz of sichuan southwestern china the materials tested consisted of simulated gouges prepared from intact clay rich mudstone and sandstone a calcite limestone plus a natural fault gouge from a trenched surface rupture cutting the mudstone and sandstone the clay rich samples including the natural gouge were dominated by illite and quartz in our experiments we sheared 1 mm thick gouge layers between saw cut driver blocks using a triaxial testing machine at conditions corresponding to 2 km depth in the lfz temperature was varied from 25 c to 150 c and to investigate the velocity dependence of friction we stepped the shear displacement rate between 1 22 and 0 122 m s our results show that the natural gouge was more illite rich and much weaker than the protolith mudstone and sandstone and showed a steady state friction coefficient of 0 4 compared with 0 6 for the latter the limestone gouge displayed values of 0 6 0 7 all samples except the limestone showed stable velocity strengthening slip the limestone showed velocity strengthening at 25 c 50 c but quasi static oscillations at 100 c 150 c along with velocity weakening behavior at 150 c we apply our results to discuss the role of the sedimentary rocks studied during events such as the wenchuan earthquake and argue that the clay rich sediments of the region may have a damping effect upon ruptures propagating from depth whereas the limestone may accelerate propagation producing significant stress drops

hydraulic properties of a low permeable rupture zone on the yingxiu beichuan fault activated during the wenchuan earthquake china implications for fluid conduction fault sealing and dynamic weakening mechanisms abstract fluid transport properties of fault rocks are crucial parameters that affect earthquake nucleation and rupture propagation in this study we examined the internal structure mineral composition and fluid transport properties of fault rocks collected from two shallow boreholes penetrating a granitic rupture zone on the yingxiu beichuan fault ybf that was activated during the 2008 wenchuan earthquake fluid transport properties were measured using water as pore fluid at effective pressures p e ranging from 10 mpa to 165 mpa permeabilities of fault rocks exhibit a wide variation from 2 1 10 22 m 2 to 4 6 10 17 m 2 strongly depending on rock types and overburden pressure specifically at p e of 165 mpa the damage zone samples have permeabilities from 5 0 10 21 m 2 to 1 2 10 17 m 2 and the fault gouges are between 2 1 10 22 m 2 and 3 1 10 19 m 2 thus the ybf consists of a low permeability fault core acting as fluid barrier and surrounding high permeability damage zones acting as fluid conduits combining the structural and compositional results and transport data together we propose that the interplay between cataclasis and fluid rock interactions controls the hydraulic properties and their response to the fault zone evolution it is noteworthy that we measured extremely low permeabilities but high porosities and high specific storages for the gouges the cemented cataclasites which are inferred to be equivalent to the rocks in which the wenchuan earthquake nucleated also have low permeabilities suggesting the fault zone is a potential area for fluid storage and capable of generating high pore pressure at depths according to our laboratory data we found fluid pressurization could occur at depths below 2 7 km we suggest thermal pressurization has played an important role in causing the dynamic weakening of the wenchuan earthquake

mineralogical characterization of protolith and fault rocks from the safod main hole 1 washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the san andreas fault observatory at depth safod both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology p t conditions deformation mechanisms and fluid composition and abundance regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults in addition both smectites occurring as mixed layer phases and serpentine minerals are found in association with active strands of the san andreas fault that were intersected during drilling although their rheological influence is not yet fully known faults containing these mineralogical phases are prime candidates for continuous coring during phase 3 of safod drilling in the summer of 2007

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

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

isotope geochemistry of mercury and its relation to earthquake in the wenchuan earthquake fault scientific drilling project hole 1 wfsd 1 abstract based on the wenchuan earthquake fault scientific drilling project wfsd we first investigated the distribution of the total mercury thg from 600 m to 1035 m of wfsd hole 1 core wfsd 1 the concentrations of thg in the fault gouge are significantly higher than in the sandstone shale siltstone and cataclasite in wfsd 1 the thg concentrations in the main seismic fault zone range from 9 9 to 73 5 ng g while those under the main fault range from 20 5 to 36 ng g to determine the source of hg typical rocks from different depths were chosen for hg isotope analysis the results show that 202hg in the main seismic fault zone range from 0 11 to 2 68 and below the seismic fault zone 202hg are from 0 64 to 1 33 the seismic fault rocks are more enriched in thg and have a larger variation of 202hg compared to the other rocks mercury has different isotopic compositions and fractionation mechanisms in the fault zone which are mainly affected by the earthquake and the fluid the mercury isotope compositions reflect the fluid activities at the main fault zone we demonstrate that mercury stable isotope ratios could serve as an effective tool for tracing mercury sources and monitoring and predicting earthquakes

on the mechanical role of smectite in subduction zones smectites a group of hydrated phyllosilicate minerals are often a main component of sediments entering a subduction zone they are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component smectite rich strata are structurally weak retain water and are highly porous a review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite rich horizons one consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted at deeper structural levels smectite is transformed to illite a mechanically stronger mineral estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces i speculate that the smectite illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity

surficial slip and rupture geometry on the beichuan fault near hongkou during the mw 7 9 wenchuan earthquake chinasurficial slip and rupture geometry on beichuan fault near hongkou during the wenchuan earthquake

frictional behavior of materials in the 3d safod volume 1 we report on frictional properties of rocks within the 3 d crustal volume surrounding the san andreas fault observatory at depth safod samples include lithologies adjacent to the san andreas fault saf in the subsurface safod borehole rocks and synthetic fault gouge composed of talc serpentinite and quartz granodiorite arkosic sandstone and siltstone samples from the safod borehole are frictionally strong 0 56 0 66 sand and clay rich lithologies from outcrop exhibit friction in the range 0 56 0 68 natural serpentinite thought to abut the saf at depth exhibits low friction 0 18 0 26 our results indicate that 1 serpentinite exhibits low strength but is not weak enough to completely satisfy weak fault models 2 all other samples are consistent with a strong fault and crust and 3 if the saf is weak 0 2 due to the presence of serpentinite or talc these minerals would likely need to constitute over 50 by weight of the shear zone

frictional constraints on crustal faulting we consider how variations in fault frictional properties affect the phenomenology of earthquake faulting in particular we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes we model these variations using a rate and state dependent friction law where we differentiate velocity weakening behavior into two fields the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening similarly we differentiate velocity strengthening behavior into two fields the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening the strong seismic field comprises the seismic slip concentrations or asperities the two intermediate fields weak seismic and compliant have frictional velocity dependences that are close to velocity neutral these fields modulate both the tectonic loading and the dynamic rupture process during the interseismic period the weak seismic and compliant regions slip aseismically while the strong seismic regions remain locked evolving into stress concentrations that fail only in main shocks the weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically this mixed frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance the model also provides a mechanism for rupture arrest dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas producing broad areas of accelerated afterslip aftershocks occur on both the weak seismic and compliant areas around a fault but most of the stress is diffused through aseismic slip rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed we test this frictional model by comparing the seismicity and the coseismic slip for the 1966 parkfield 1979 coyote lake and 1984 morgan hill earthquakes the interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip these regions are interpreted as either weak seismic or compliant depending on whether or not they manifest interevent seismicity

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

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

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

frictional properties of the south china sea oceanic basalt and implications for strength of the manila subduction seismogenic zone abstract we conduct friction experiments on a basalt sample no u1431e41r6w from international oceanic discovery program iodp expedition 349 in the temperature range of 100 600 c and effective normal stress of 50 mpa with 100 mpa pore fluid pressure loading rates are stepped up and down in the range of 0 04 1 0 m s to obtain the velocity dependence of friction based on the rate and state friction law we numerically fit the experimental curves to acquire constitutive parameters that cannot be obtained by direct measurement experimental results show that basalt friction coefficient varies from 0 70 to 0 74 similar to the range of gabbro documented in a previous study he et al 2007 stable velocity strengthening behavior is observed at temperatures of 100 c and 200 c and unstable velocity weakening at temperatures above 300 c with occurrences of periodic stick slips both a and b values inferred by numerical fittings show positive temperature dependence in the temperature range of 100 600 c the stick slip motions above 300 c seem to be due to a dramatic decrease in the characteristic slip distance dc from 11 to 15 m to 0 4 2 5 m based on our results and plastic flow laws of mafic rocks from a previous study fault strength profile and slip stability of manila subduction zone are discussed with calculations in the context of a unified friction flow model the inferred lower boundary of seismogenic zone of manila subduction fault is around 40 km which is in a reasonable range compared with seismicity data

fluid rock interactions in seismic faults implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 wenchuan earthquake china we describe the structural features and mineralogical and geochemical compositions of the fault rocks recovered from boreholes at the golden river site on the yingxiu beichuan fault which activated and slipped along a 240 km long main surface rupture zone during the 2008 wenchuan earthquake the fault which accommodated co seismic slip cuts granitic rocks from the pengguan complex in which this earthquake most likely nucleated fault rocks including cohesive cataclasite unconsolidated breccia and three fault gouges with distinct colors were identified from the drilling cores on going uplift and erosion in the area means that the fault rocks formed at different depth were exhumed to the shallow surface during the uplift history of the longmenshan fault zone a clear change from fracturing and comminution in the cataclasites and breccia to more pervasive shear formation of fine grained materials in the gouges has been observed the gouges are distinct and have accommodated significant displacement in multiple increments of shear furthermore fault rocks recovered from the boreholes display numerous features indicative of fluid infiltration and fluid rock interaction toward the fault core clay minerals have replaced feldspars the element enrichment depletion patterns of the fault rocks show general fluid infiltration trends such as 1 mobile elements are generally depleted in the fault rocks 2 the microstructural mineralogical and geochemical results of the fault rocks consistently indicate that pervasive fluid infiltration and fluid rock interactions altered feldspars and mafic minerals to clay minerals the fluid was mg2 and fe2 rich facilitating formation of chlorite isocon analyses further reveal that a large rock volume has been lost which is attributed to the removal of mobile elements associated with fluid infiltration and perhaps enhanced by pressure solution these results reflect the accumulated effects of cataclasis and fluid infiltration fluid rock interaction during the long evolution history of the longmenshan fault zone fluid rock interactions in seismic faults implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 wenchuan earthquake china

talc friction in the temperature range 25 400 c relevance for fault zone weakening abstract talc is one of the weakest minerals that is associated with fault zones triaxial friction experiments conducted on water saturated talc gouge at room temperature yield values of the coefficient of friction shear stress effective normal stress n in the range 0 16 0 23 and increases with increasing n talc gouge heated to temperatures of 100 400 c is consistently weaker than at room temperature and talc has a temperature pressure range of stability that extends from surficial to eclogite facies conditions making it of potential significance in a variety of faulting environments talc has been identified in exhumed subduction zone thrusts in fault gouge collected from oceanic transform and detachment faults associated with rift systems and recently in serpentinite from the central creeping section of the san andreas fault typically talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica saturated hydrothermal fluids this mode of formation of talc is a prime example of a fault zone weakening process because of its velocity strengthening behavior talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern california that are associated with ophiolitic rocks

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

effects of simulated clay gouges on the sliding behavior of tennessee sandston abstract the effects of simulated fault gouge on the sliding behavior of tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge about 30 specimens with gouge composed of pure clays of homogeneously mixed clay and anhydrite or of layered clay and anhydrite along a 35 precut are deformed dry in a triaxial apparatus at a confining pressure of 100 mpa with a shortening rate of about 5 10 4 sec and at room temperature pure clay gouges exhibit only stable sliding and the ultimate frictional strength is very low for bentonite mont morillonite intermediate for chlorite and illite and considerably higher for kaolinite anhydrite gouge shows violent stick slip at 100 mpa confining pressure when this mineral is mixed homogeneously with clays the frictional coefficient of the mixed gouge determined at its ultimate frictional strength decreases monotonically with an increase in the clay content the sliding mode changes from stick slip to stable sliding when the frictional coefficient of the mixed clay anhydrite gouge is lowered down below 90 95 of the coefficient of anhydrite gouge the stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays that is the effect is conspicuous only for a mineral with low frictional strength only 15 20 of bentonite suppresses the violent stick slip of anhydrite gouge in contrast violent stick slip occurs even if the gouge contains as much as 75 of kaolinite the behavior of illite and chlorite is intermediate between that of kaolinite and bentonite bentonite anhydrite two layer gouge exhibits stable sliding even when the bentonite content is only 5 thus the presence of a thin clay rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge this result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion based on the present experimental results incorporated with some other experimental data it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature the effect should be much less pronounced at elevated temperatures due perhaps to the dewatering of the clays in most geological environments where shallow earthquakes occur the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault

clay mineral anomalies in the fault zone of the chelungpu fault taiwan and their implications 1 the taiwan chelungpu fault drilling project tcdp hole a recovered continuous core samples across the rupture zone of the 1999 chi chi earthquake mw7 6 studying in situ chemical properties sequentially from fresh fault zone materials of the chelungpu fault provides insight into possible faulting mechanisms distinct anomalies of mineral assemblages at the 1111 m fault zone of tcdp hole a are found to be 1 a decrease in clay content within the primary slip zone psz and 2 a significant decline of illite disappearance of chlorite and kaolinite and spike in smectite within the psz meanwhile features relating to melting or amorphous material in the psz have been observed by sem and tem the results suggest that the psz might have experienced generation of glassy materials such as pseudotachylyte by the expense of clay minerals due to strong shear heating then prompt alteration of pseudotachylyte into smectite characteristics of clay minerals and images obtained from electronic microscopes in the psz thus imply that pseudotachylyte possibly developed during the 1999 chi chi earthquake but quickly altered into smectite this particular phenomenon may explain why pseudotachylyte is rarely found in exhumed hydrated fault zones

coseismic reverse and oblique slip surface faulting generated by the 2008 mw 7 9 wenchuan earthquake china the mw 7 9 wenchuan china earthquake ruptured two large thrust faults along the long menshan thrust belt at the eastern margin of the tibetan plateau this earthquake generated a 240 km long surface rupture zone along the beichuan fault and an additional 72 km long surface rupture zone along the pengguan fault maximum vertical and horizontal offsets of 6 5 m and 4 9 m respectively were measured along the beichuan fault a maximum vertical offset of 3 5 m was measured along the pengguan fault coseismic surface ruptures integrated with aftershocks and industry seismic profiles show that two imbricate structures have ruptured simultaneously resulting in the largest continental thrust event ever documented large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region as everywhere along the edge of tibetan plateau

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

frictional properties and microstructure of calcite rich fault gouges sheared at sub seismic sliding velocities we report an experimental and microstructural study of the frictional properties of simulated fault gouges prepared from natural limestone 96 caco3 and pure calcite our experiments consisted of direct shear tests performed under dry and wet conditions at an effective normal stress of 50 mpa at 18 150 c and sliding velocities of 0 1 10 m s wet experiments used a pore water pressure of 10 mpa wet gouges typically showed a lower steady state frictional strength 0 6 than dry gouges 0 7 0 8 particularly in the case of the pure calcite samples all runs showed a transition from stable velocity strengthening to potentially unstable velocity weakening slip above 80 100 c all recovered samples showed patchy mirror like surfaces marking boundary shear planes optical study of sections cut normal to the shear plane and parallel to the shear direction showed both boundary and inclined shear bands characterized by extreme grain comminution and a crystallographic preferred orientation cross sections of boundary shears cut normal to the shear direction using focused ion beam sem from pure calcite gouges sheared at 18 and 150 c revealed dense arrays of rounded 0 3 m sized particles in the shear band core transmission electron microscopy showed that these particles consist of 5 20 nm sized calcite nanocrystals all samples showed evidence for cataclasis and crystal plasticity comparing our results with previous models for gouge friction we suggest that frictional behaviour was controlled by competition between crystal plastic and granular flow processes active in the shear bands with water facilitating pressure solution subcritical cracking and intergranular lubrication our data have important implications for the depth of the seismogenic zone in tectonically active limestone terrains contrary to recent claims our data also demonstrate that nanocrystalline mirror like slip surfaces in calcite rich faults are not necessarily indicative of seismic slip rates

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

effect of hydration state on the frictional properties of montmorillonite based fault gouge 1 we report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge we tested four mixtures of ca montmorillonite and quartz 100 70 50 and 30 montmorillonite at four hydration states dry 16 0 wt water we controlled the hydration state using either oven drying for 13 wt h2o under conditions of controlled relative humidity for each clay quartz mixture and hydration state we measured frictional properties over a range of normal stresses 5 100 mpa and sliding velocities 1 300 m s we observe a systematic decrease in the coefficient of friction with increasing water content normal stress and clay content values of for 50 50 mixtures range from 0 57 to 0 64 dry and decrease to 0 21 0 55 for the most hydrated cases wet for layers of 100 montmorillonite ranges from 0 41 0 62 dry to 0 03 0 29 wet as water content is increased from 0 to 20 0 wt the friction rate parameter a b becomes increasingly positive variation in a b values decreases dramatically as normal stress increases if our experimental results can be applied to natural fault gouge the combination of stress state hydration state and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite

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

the occurrence of graphite bearing fault rocks in the atotsugawa fault system japan origins and implications for fault creep abstract graphite in fault zones has received little attention even though it is a well known solid lubricant that could affect frictional properties of faults dramatically this paper reports the presence of abundant graphite in fault zones of the atotsugawa fault system central japan mesoscopic and microscopic observations of fault rocks revealed two processes of carbon enrichment in fault zones one is a pressure solution process or diffusive mass transfer in general which removes water soluble minerals such as quartz and carbonates from rocks resulting in the enrichment of insoluble minerals including carbon the other process is precipitation of graphite from a high temperature carbon rich fluid forming graphite filling fractures within cataclasitic fault zones the two processes have led to the concentration up to 12 wt of graphite in the atotsugawa fault zones compared to 0 to 3 wt of carbonaceous materials in the host rocks this concentration is high enough for graphite to affect frictional properties at wide range of slip rates the presence of graphite may provide an explanation for the low resistivity the patterns of microearthquakes and fault creep along the western part of the atotsugawa fault system graphite should receive more attention as a weakening and stabilizing agent of faults

velocity weakening behavior of plagioclase and pyroxene gouges and stabilizing effect of small amounts of quartz under hydrothermal conditions 1 we investigated properties of frictional sliding of plagioclase pyroxene and their mixture gouges with a small amount of hornblende biotite and quartz as accessory minerals under hydrothermal conditions with an effective normal stress of 200 mpa pore pressure of 30 mpa and temperatures from 100 c to 600 c axial loading rate was stepped between 0 001 and 0 0001 mm s to acquire the rate dependence both plagioclase and pyroxene gouges showed velocity weakening behavior in the whole temperature range except the velocity strengthening behavior of pyroxene at 200 c for temperatures above 400 c both plagioclase and pyroxene gouges showed oscillatory slips as a result of small dc values of 3 4 m which make the critical stiffness rise remarkably and approach the system stiffness above 300 c the direct effect of plagioclase shows an increasing trend with temperature indicating control of the deformation process by thermally activated mechanisms as the difference of a and b values here are only 20 at most this trend also applies to the evolution effect our analytical derivation based on the theory of pressure solution shows a log linear contact area growth with time that corresponds to an evolution effect and estimations based on this encompass the plagioclase data though the identification of actual mechanisms is not easy finally it is found that a little quartz 3 5 added to the plagioclase 60 62 pyroxene 35 mixture has a strong stabilizing effect leading to a transition from velocity weakening to velocity strengthening these results may help constrain the depth range of seismic slips on deep faults in the lower crust of gabbroic composition

eddy contributions to the meridional transport of salt in the north atlantic 1 the meridional transport of salt in the atlantic ocean is an important process for climate controlling the stability of the meridional overturning circulation the contribution of transient eddies to this transport is quantified in an eddy resolving north atlantic model at 1 12 resolution natl12 and compared with lower resolution north atlantic and global 1 4 models in natl12 between 10 n and 40 n there is a volume loss by evaporation of 0 6 sverdrups sv the divergence of the eddy flux of salt normalized by a reference salinity of 34 8 is 0 2 sv over the region a significant fraction of the total air seawater exchange but it is compensated by an opposite convergent transport of salt by the mean flow so that the total transport of salt is small the compensation between eddy and mean salt transport is almost complete in a multicentury long global model experiment but less effective in natl12 because the short integration time does not allow the salt content to equilibrate and the model drift is large eddies arising from baroclinic instability contribute to the meridional salt transports at the northern and southern boundary of the subtropical gyre where they appear consistent with a lateral diffusion acting on the mean salinity gradient however the eddy transport of salt is the sum of two terms an advective contribution arising from the correlations of velocity and isopycnal thicknesses and a diffusion along isopycnals both components have the same amplitude at the southern boundary of the subtropical gyre while diffusion is dominant at the northern boundary

toward a better understanding of palaeoclimatic regimes that recharged the fossil aquifers in north africa inferences from stable isotope and remote sensing data abstract the isotopic composition of modern precipitation collected monthly from the international atomic energy agency iaea stations over north africa is enriched average d morocco 25 8 algeria 21 3 tunisia 20 6 libya 17 7 egypt 11 7 compared to fossil groundwater across north africa which shows progressive west to east depletion this indicates that precipitation from paleowind regimes was different from those prevailing in the present day late holocene however a few monthly records march 1982 december 1987 december 1991 march 2002 and april 2002 collected from iaea stations sidi barrani cairo rafah and el arish showed isotopic depletions d 56 to 43 7 approaching those of the fossil groundwater in egypt for each of the five events we investigated the direction of cloud propagation in relation to the isotopic composition of precipitation as it travels over the african continent and surroundings cloud propagation directions and air parcel flow paths were extracted from temporal every 30 min meteosat image products 1 infra red eumetsat 2 cloud motion winds cmw and 3 back trajectory models btm the inferred wind regimes for each of these events were then compared to those for the remaining egyptian records 160 that produced precipitation with enriched to intermediate isotopic compositions d 39 7 to 34 6 the extracted cloud propagation directions and the observed spatial variation in the isotopic compositions of the precipitating rain support a model that attributes previous wet climatic periods over north africa to the intensification of palaeowesterlies during glacial periods

erratum to surficial slip and rupture geometry on the beichuan fault near hongkou during the mw 7 9 wenchuan earthquake china worldwide occurrence and documentation of reverse type ruptures are sparse near hongkou the wenchuan rupture passes through the broad baisha river valley and provides excellent opportunities to trace the surface faulting in fine details for 13 km distance one of the longest continuous sections along the entire rupture in this paper we present the results of our mapping of the surface rupture in this reach based on the discontinuities in slip and geometry the rupture was divided into four sections for convenience from west to east the shenxi gou the miaoba the gaoyuan and the bajiao miao sections respectively the vertical offset is large in the shenxi gou and the bajiao miao sections locally reaching 5 6 m in maxima and generally low in the miaoba section 1 2 m or less in most places the slip gradient for vertical offset is generally 10 3 locally up to 10 1 similar to that in well documented strike slip ruptures near gaoyuan village the surface rupture con sists of two subparallel branches with the northern one exhibiting right lateral slip with minor southeast side up thrusting while the southern one is almost pure south east side up thrusting this pattern mimics the incomplete slip partitioning of oblique thrusting on parallel faults but at a local scale in addition the sense of vertical throw on these two strands is opposite to the general northwest side up thrusting of the wenchuan rupture we propose that it is likely due to the inheritance at shallow depth from the southeast dipping geological faults and that old fault zone structures can have a strong effect on the dynamic rupture by guiding the rupture propagation onto paths of preexisting though locally unfavorable dipping fault planes we also discuss the cross cutting slickenside striations observed near bajiao miao which indicated temporal rake rotation during dynamic rupture and their geological and seismological implications

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

earthquake nucleation on faults with rate and state dependent strength abstract dieterich j h 1992 earthquake nucleation on faults with rate and state dependent strength in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 115 134 faults with rate and state dependent constitutive properties reproduce a range of observed fault slip phenomena including spontaneous nucleation of slip instabilities at stresses above some critical stress level and recovery of strength following slip instability calculations with a plane strain fault model with spatially varying properties demonstrate that accelerating slip precedes instability and becomes localized to a fault patch the dimensions of the fault patch follow scaling relations for the minimum critical length for unstable fault slip the critical length is a function of normal stress loading conditions and constitutive parameters which include dc the characteristic slip distance if slip starts on a patch that exceeds the critical size the length of the rapidly accelerating zone tends to shrink to the characteristic size as the time of instability approaches solutions have been obtained for a uniform fixed patch model that are in good agreement with results from the plane strain model over a wide range of conditions above the steady state stress the logarithm of the time to instability linearly decreases as the initial stress increases because nucleation patch length and premonitory displacement are proportional to dc the moment of premonitory slip scales by d3c the scaling of dc is currently an open question unless dc for earthquake faults is significantly greater than that observed on laboratory faults premonitory strain arising from the nucleation process for earthquakes may by too small to detect using current observation methods excluding the possibility that dc in the nucleation zone controls the magnitude of the subsequent earthquake then the source dimensions of the smallest earthquakes in a region provide an upper limit for the size of the nucleation patch

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

amounts and styles of coseismic deformation along the northern segment of surface rupture of the 2008 wenchuan mw 7 9 earthquake china abstract the may 12 2008 mw 7 9 wenchuan earthquake occurred along the longmen shan fault zone a thrust fault zone on the eastern margin of tibet plateau and ruptured the ground surface for more than 200 km a field investigation suggests that the earthquake also ruptured the northern segment from beichuan to qingchuan along which active evidence had been less identified before 75 95 km long this rupture commonly presents a relatively simple 1 to 6 m high scarp with folding and tension cracks along the crest and with minor deformation on both the hanging wall and the footwall the maximum measured vertical and dextral displacements along this segment are 9 0 m and 3 0 m respectively the amounts of the vertical coseismic displacements decrease from southwest to northeast but the amounts of the dextral displacements increase toward the northeastern end of the rupture the surface rupture apparently terminates as mainly strike slip faulting at the northeastern end the observable rupture zone is about 75 km long between beichuan and shuiguan showing mainly thrust slip with a component of dextral strike slip dipping nw at an angle of 54 84 the surface deformation along this segment is classified into five characteristic styles that range from pure thrusting a mixture of thrust and dextral slip to pure dextral slip the amount of shortening is estimated to be minor 0 2 1 0 m

nucleation and early seismic propagation of small and large events in a crustal earthquake model 1 earthquake nucleation and early seismic propagation are studied in a two dimensional strike slip fault model with depth variable properties the fault is governed by the dieterich ruina rate and state friction law we use an efficient and rigorous numerical procedure for elastodynamic analysis of earthquake sequences on slowly loaded faults developed by lapusta et al 2000 we find that for decreasing values of the characteristic slip distance of the friction law small events appear at the transition from the locked to creeping behavior toward the bottom of the seismogenic zone small and large events have very similar nucleation phases in our simulations here by nucleation phase we mean gradually accelerating aseismic slip in a small slowly expanding zone before the breakout of the dynamic seismically detectable event moment acceleration to which velocity seismograms are proportional in early stages of seismic propagation exhibits irregular fluctuations in the form of speedups and slowdowns in the moment release rate consistently with observations as reported by ellsworth and beroza 1995 our simulations show that such irregular moment acceleration can at least in part be due to the heterogeneous stress distribution imprinted on the fault by the arrest of previous small events and by stress concentrations at the borders of creeping regions and to partial arrest of the rupture in velocity strengthening fault regions which inhibit seismic slip index terms 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 3220 mathematical geophysics nonlinear dynamics 7230 seismology seismicity and seismotectonics 3230 mathematical geophysics numerical solutions 8164 tectonophysics stresses crust and lithosphere keywords earthquake nucleation phase event clustering irregular moment release stress concentrations rate and state friction earthquake sequences

granular nanoparticles lubricate faults during seismic slip nanoparticles are known to form in narrow slip zones in natural and experimental fault zones but their possible role in dynamic weakening of faults during seismic slip remains almost unexplored we conducted friction experiments on periclase mgo nanoparticles 50 nm in average size at rates as high as 1 3 m s 1 a typical speed of seismic slip the nanoparticles were used as the initial gouge to avoid complexities arising from comminution and fluid release the dynamic friction decreased with increasing slip rate and the steady state frictional coefficient reduced to as low as 0 1 at a slip rate of 1 3 m s 1 flash heating is not effective for nanoparticles and we propose that development of slickensides and dominant operation of nanoparticle rolling cause such a weakening nanoparticle lubrication appears to be as effective as melt lubrication and thermal pressurization and the formation of nanoparticles in slip zones may be an important fault lubrication process

structural control of rupturing of the mw7 9 2008 wenchuan earthquake china abstract analyses of global seismic waveform data for the 2008 devastating wenchuan earthquake have indicated where the rupture started and how it expanded here we reveal that the rupture process of this earthquake was strongly controlled by structural heterogeneities of the upper crust we made a tomographic inversion of 200 000 local travel time data according to which the rupture was confined in a presumably brittle layer of high p and s wave velocity anomalies within the upper 15 km depths the rupture propagated northeastward through the southwest sw segment of the fault and then jumped to the northeast ne segment with little slip at the intervening presumably ductile zone characterized by low p and s velocities with high poisson s ratio the rupture initiated near the southwestern end of the sw segment within a body of high poisson s ratio which extends down into the lower crust suggesting upward migration of deep seated aqueous fluid to trigger the earthquake by reducing the mechanical strength of the brittle layer

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

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

structural and geochronological constraints on the mesozoic cenozoic tectonic evolution of the longmen shan thrust belt eastern tibetan plateau 1 the longmen shan thrust belt in the eastern margin of the tibetan plateau underwent deformation of d1 associated with the late triassic collision between the north and south china blocks followed by deformation of d2 and d3 related to the eastward growth of the tibetan plateau the d1 is marked by moderately tight folds f1 spaced cleavage s1 mineral lineation l1 multistage reactivated faults and top to the south thrusting this deformation initiated before 237 ma and ended at 208 ma the 193 159 ma d2 deformation is top to the southeast directed and normal ductile in nature and was associated with the formation of half graben basins the mesozoic deformation of the longmen shan belt was related to the subduction of the paleo tethys oceanic lithosphere followed by the south china block underneath the songpan ganze terrane the d1 was produced in the pro side of the overriding songpan ganze plate most probably driven by slab corner flow above the retreating south china block the d2 and extensional songpan ganze turbidite basin were produced by roll back and migration of the overriding plate above the retreating subduction zone this model for the deformation from d1 to d2 is similar to a mediterranean style model the d3 is characterized by se ward thrusting with stepwise migration of rapid denudation and dextral strike slip faulting during the late cretaceous to present therefore the longmen shan thrust belt involved mesozoic compressional and extensional events that were overprinted by the late cretaceous to cenozoic compressional event