deformation textures and mechanical behavior of a hydrated amorphous silica formed along an experimentally produced fault in chert 1 we conducted a series of friction experiments on chert at intermediate to high slip velocities v 0 87 104 mm s and at low normal stress n 1 5 mpa to better understand the process of fault weakening by silica gel formation as identified in previous friction experiments on quartz rocks fault weakening in chert samples occurred in association with the formation of a 0 1 mm thick fault gouge layer that contains a thin foliated layer of fine grained material and a cataclastically fragmented zone characterized by clast matrix texture mft ir and xrd analyses reveal that the fault gouge consists of a mixture of hydrated amorphous silica and quartz grains the mechanical behavior of the fault gouge at a small magnitude of strain 0 008 as examined independently of the friction experiments using a rheometer is characterized by a negative dependence of shear stress on strain rate with a notable hysteresis behavior

silica diagenesis ii general mechanisms abstract amorphous silica phases opal a precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions opal a dissolves and yields solutions of still relatively high silica content in pore waters containing abundant cations open framework polymers form which flocculate to yield opal ct opal ct becomes increasingly ordered primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase opal ct dissolves to yield pore waters of low silica concentration which allows slow growth of quartz crystals from monomeric solution the quartz crystals then slowly increase in size and crystallinity carbonates appear to enhance opal ct formation possibly due to the activity of positively charged hydroxyl complexes hence polymerization in relatively pure systems is involved in opal a formation and polymerization in impure systems is involved in opal ct formation and slow growth from monomeric low silica concentration solutions is involved in precipitation of quartz in sedimentary realms

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

flow and deformation of viscous silica oversaturated dispersions in low grade faults abstract cryptocrystalline silica veins occur along shear faults and in dilatational cracks in low grade cataclasites derived from leucocratic granitoids the silica minerals occur as radiate bladed quartz and as spherulitic and sheaf like clusters of neocrystallites that nucleated on crush fragments in a matrix of opaline and chalcedony this texture is indicative of rapid crystallization from a viscous oversaturated fluid the veins are clast loaded and typically demonstrate a banding defined by variation in the amount and size of feldspar crush fragments three types of veins are distinguished that differ in relative timing of viscous fluid flow deformation and crystallization type 1 veins are dilatational cracks in which static settling and crystallization took place an undisturbed planar layering is found which has no fixed orientation with respect to the vein wall this banding is commonly associated with a layering defined by periodic variations in hematite precipitates type 2 veins show a gradual decrease in grain size of the crush fragments from one vein wall to the other this banding shows pinch and swell structures micro boudinage and folding suggestive of flow type 3 veins occur in shear faults and demonstrate a banding which is asymmetric with respect to the vein walls apart from pinch and swell structures and folds the foliation shows sigmoidal deflection similar to that in shear bands in ductile mylonites it is proposed that the cryptocrystalline fragment loaded veins are formed by solidification of a silica sol or hydrogel that was formed by quenching of a hot fragment bearing solution during brittle failure the effect of a viscous fluid in fault rock is discussed in terms of rheology and of seismic periodicity

interannual variability of the south china sea in a data assimilation model 1 sea surface height ssh variations in the south china sea are examined using results from a data assimilation model after the ssh data have had the annual cycle removed principal component analysis illustrates two distinct anomaly patterns the first mode strongest off vietnam would affect the basin wide gyre intensity the second mode consists of a meridional dipole feature with a nodal line around 12 n and is related to the development of the eastward jet and upwelling off vietnam both eof modes have significant interannual variations and are highly correlated to the enso events the leading phase of mode 2 coefficient provides preliminary evidence for the air sea interaction and the pacific east asian teleconnection an innovative mechanism is proposed to describe a weakened upwelling off luzon in winter 1998 an extra strong basin wide warming in summer 1998 persists into fall and winter preventing the formation of winter upwelling off luzon in that year

the relationship between slickenside surfaces in fine grained quartz and the seismic cycle abstract well polished or reflective fault surfaces with or without linear striae indicating the sliding direction are commonly referred to as slickensides this study examines the development of slickensides which are now exposed along the surface trace of a large seismically active normal fault zone in dixie valley nevada u s a geologic and mineralogic constraints indicate the slickenside surfaces formed at depths of less than 2 km and temperatures less than 270 c the slickenside surface material is composed of greater than 98 quartz with less than 2 kaolinite and iron oxide transmission electron microscope tem observations reveal that the slickenside surface material has an extremely fine yet variable grain size 0 01 1 m and an unusual nonequilibrium texture characterized by irregular grain boundaries and low dislocation density angular fragments in the cataclasite to either side of the slickensides provide clear evidence of cataclasis crystallographic preferred orientation in the slickenside surface material indicates that non brittle continuous deformation occurred within 0 1 10 mm of the fault surfaces in many areas non brittle continuous deformation must have alternated with cataclasis because some fragments in the cataclasite have strong preferred orientations we suggest that in the dixie valley slickensides continuous deformation and the development of crystallographic preferred orientations occurred at relatively low strain rates during the interseismic period while cataclasis occurred at higher strain rates associated with seismic events the wide range of strain rates occurring within a fault zone during the earthquake cycle may be an essential element in the formation of fine grained glassy slickensides both at dixie valley nevada and many other areas

signature of coseismic decarbonation in dolomitic fault rocks of the naukluft thrust namibia unequivocal geological signatures of seismic slip are rare exceptionally so in carbonate hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt this thermal dissociation leads to significant fault weakening by increased fluid pressure and or nanoparticle lubrication preventing further heating of the fault surface pseudotachylyte is therefore unlikely to form in carbonate hosted faults and other evidence for seismic slip must be identified we studied the lower cambrian naukluft thrust which crops out in central namibia it contains a cataclastic dolomite fault rock referred to as gritty dolomite which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid rock interactions the fault was active at ambient temperatures below 2001c gritty dolomite contains rounded low aspect ratio dolomite clasts with a uniform fe rich dolomite coating euhedral to subhedral magnetite quartz and k feldspar in a fine grained massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement the fault rock texture combined with evidence of injectites of gritty dolomite into the wallrock indicates the cataclasite deformed as a fluidized granular flow at seismic slip velocities frictional heating caused dissociation of dolomite to co 2 and ca fe and mg oxides this release of co2 decreased the ph of the pore fluid in the fault causing dissolution and rounding of dolomite clasts within an inertial grain flow and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and co2 escape examples of similar rocks having some if not all of these characteristics have been described from other carbonate hosted faults the geological setting of the naukluft thrust is unique in spatial extent and quality of exposure allowing us to eliminate alternative hypotheses for sources of co2 to drive fluidization

fault slip distribution and fault roughness summary we present analysis of the spatial correlations of seismological slip maps and fault topography roughness illuminating their identical self affine exponent though the complexity of the coseismic spatial slip distribution can be intuitively associated with geometrical or stress heterogeneities along the fault surface this has never been demonstrated based on new measurements of fault surface topography and on statistical analyses of kinematic inversions of slip maps we propose a model which quantitatively characterizes the link between slip distribution and fault surface roughness our approach can be divided into two complementary steps i using a numerical computation we estimate the influence of fault roughness on the frictional strength pre stress we model a fault as a rough interface where elastic asperities are squeezed the hurst exponent h characterizing the self affinity of the frictional strength field approaches h h 1 where h is the roughness exponent of the fault surface in the direction of slip ii using a quasi static model of fault propagation which includes the effect of long range elastic interactions and spatial correlations in the frictional strength the spatial slip correlation is observed to scale as hs h 1 where hs represents the hurst exponent of the slip distribution under the assumption that the origin of the spatial fluctuations in frictional strength along faults is the elastic squeeze of fault asperities we show that self affine geometrical properties of fault surface roughness control slip correlations and that hs h given that h 0 6 for a wide range of faults various accumulated displacement host rock and slip movement we predict that hs 0 6 even if our quasi static fault model is more relevant for creeping faults the spatial slip correlations observed are consistent with those of seismological slip maps a consequence is that the self affinity property of slip roughness may be explained by fault geometry without considering dynamical effects produced during an earthquake

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

colloidal gold and silica in mesothermal vein systems some of the textural features of mesothermal gold quartz veins may be best explained by the initial precipitation of amorphous silica gel colloid which subsequently crystallizes to quartz this can occur in brittle ductile shear zones where a significant fluid pressure drop occurs during stick slip failure such a process rapidly supersaturates the hydrothermal fluid with respect to amorphous silica which precipitates instead of quartz owing to favorable kinetics depressurization also commonly leads to fluid unmixing and destabilization of soluble gold complexes however the presence of colloidal silica can stabilize gold colloid allowing further transport of particulate gold in suspension in the hydrothermal fluid silica gel would be highly unstable under mesothermal conditions and would undergo rapid syneresis and crystallization to form quartz solid impurities would tend to be expelled toward grain boundaries this model can account for the primary anhedral aggregate textures typical of mesothermal quartz veins the concentration of gold along grain boundaries and the formation of discrete gold nuggets and the rare occurrence of low order silica polymorphs and relict spheroidal structures the transport of gold in colloidal form may be one reason for the frequently consistent bulk grade distribution in gold quartz vein systems over many hundreds of metres in some cases kilometres of depth in addition the formation of charged colloidal particles may help to explain the attraction of gold grains to specific mineral surfaces

fluidization of granular material in a subduction thrust at seismogenic depths abstract subduction thrusts exhumed from seismogenic depths may record the dynamic slip processes during subduction zone earthquakes the thrust faults in the shimanto accretionary complex of southwest japan imbricate the melange derived from sedimentary rocks and basalt which represent a duplex underplating associated with the downward step of the subduction thrust at seismogenic depths 4 6 km in depth temperature 130 150 c one well exposed duplex fault zone consists mainly of foliated cataclasites and ultracataclasites derived from basalt the fault zone structure suggests slip localization along the 2 20 cm thick ultracataclasite layer at the boundary between different lithologies the common features observed along the ultracataclasite layer are the injection of granular material into the melange without the sorting of fine grained particles along with a small detection probability of the fragmented counterparts in the injected materials which suggests a rapid injection of fluidized granular material during faulting the ultracataclasite layer is marked by fluid related processes as indicated by the alteration and precipitation of amorphous silica gel along the ultracataclasite and the concentration of veins near the ultracataclasite the fluidization during localized slip under fluid infiltrated conditions could be due to thermal pressurization which causes the fluid pressure to approach lithostatic values the injection of fluidized granular material may result from hydraulic fracturing in response to an increase in the fluid pressure associated with thermal pressurization the plate boundary subduction thrust could be lubricated when the granular material fluidizes due to thermal pressurization during earthquakes

semi brittle flow of granitoid fault rocks in experiments field studies and seismic data show that semi brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so called frictional viscous transition to understand the physical and chemical processes accommodating semi brittle flow we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space temperature t 300 400 500 and 600 c confining pressure pc 300 500 1000 and 1500 mpa shear strain rate 10 3 10 4 10 5 and 10 6 s 1 to finite shear strains 0 5 the experiments have been carried out using a granular material with grain size smaller than 200 m with a little h2o added 0 2wt only two experiments performed at the fastest strain rates and lowest temperatures have failed abruptly right after reaching peak strength 1400 mpa all other samples reach high shear stresses 570 1600 mpa then weaken slightly by 10 190 mpa and continue to deform at a more or less steady state stress level clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed in order to express this relationship the strain rate stress sensitivity has been fit with a stress exponent assuming n and yields high stress exponents n 10 140 which decrease with increasing temperature the microstructures show widespread comminution strain partitioning and localization into slip zones containing at first nanocrystalline and partly amorphous material later during continued deformation fully amorphous material in slip zones despite the mechanical steady state conditions the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below 5 within the slip zones the fault rock material progressively transforms from a crystalline solid to an amorphous material we present and interpret the experimental results both in terms of sliding friction and viscous flow and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation

deformation and hydrofracture in a subduction thrust at seismogenic depths the rodeo cove thrust zone marin headlands california we have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths the rodeo cove thrust zone which outcrops north of the golden gate bridge of san francisco imbricates two basalt chertsandstone sequences belonging to the marin headlands terrane franciscan complex the thrust outcrop is a 200 m thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop dominated by basaltic lithologies where zones of concentration of deformation have been mapped disruption is made by variably dense discrete fault systems synthetic to the main thrust r and p fractures these faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defi ned by chlorite and pumpellyite which also constrain the depth of faulting 8 10 km t 200 250 c within the seismogenic zone the central part of the fault also features the densest system of carbonate fi lled veins veins occur in the broken formation matrix and fragments in both cases parallel to the foliation the veins are either folded truncated or pressure solved along the cleavage cementation and hardening of shear surfaces of the fault core may have caused the distribution as opposed to localization of subsequent slip events the fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fl uids occurring through systems of multiple dilatant fractures grossly parallel to the decollement zone the crosscutting relationships between veining and foliation suggest that fl uid injection is cyclic and consequently that large transient variations in permeability and cohesion may occur the repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress or by difference in tensional strength parallel to and perpendicular to the foliation both of which would require extremely high fl uid pressure we interpret the features of the rodeo cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress fi eld in the interseismic phase and an extensional stress fi eld in the immediately postseismic phase

inferring earthquake physics and chemistry using an integrated field and laboratory approach abstract earthquakes are the result of a combination of 1 physico chemical processes operating in fault zones which allow ruptures to nucleate and rock friction to decrease with increasing slip or slip rate and 2 of the geometrical complexity of fault zones in this review paper we summarize recent experimental findings from high velocity conducted at about 1 m s slip rate or typical seismic slip rates rock friction experiments with an emphasis on potential dynamic weakening mechanisms melt lubrication nanopowder lubrication etc and how these mechanisms might be recognized by means of microstructural and mineralogical studies in exhumed fault zones we discuss how earthquake source parameters coseismic fault strength weakening distances energy budgets etc might be derived from the field and laboratory experiments additionally we discuss what needs to be considered in terms of fault zone geometry and morphology focusing on fault surface roughness in order to develop models of realistic fault surfaces and present theoretical considerations for microphysical modeling of laboratory data at seismic slip rates with an emphasis on the case of melt lubrication all experimental data and in the case of melt lubrication microphysical models indicate that faults must be very weak

high velocity frictional properties of a clay bearing fault gouge and implications for earthquake mechanics 1 frictional properties of natural kaolinite bearing gouge samples from the median tectonic line sw japan have been studied using a high velocity rotary shear apparatus and deformed samples have been observed with optical and electron scanning and transmission microscopy for a slip velocity of 1 m s 1 and normal stresses from 0 3 to 1 3 mpa a dramatic slip weakening behavior was observed x ray diffraction analysis of deformed samples and additional high velocity friction experiments on pure kaolinite indicate kaolinite dehydration during slip the critical slip weakening distance dc is of the order of 1 to 10 m these values are extrapolated to higher normal stresses assuming that dc is rather a thermal parameter than a parameter related to a true characteristic length the calculation shows that dimensionally dc 1 n2 where n is the normal stress applied on the fault the inferred dc values range from a few centimeters at 10 mpa normal stress to a few hundreds of microns at 100 mpa normal stress microscopic observations show partial amorphization and dramatic grain size reduction down to the nanometer scale localized in a narrow zone of about 1 to 10 m thickness fracture energy gc is calculated from the mechanical curves and compared to surface energy due to grain size reduction and energies of mineralogic transformations we show that most of the fracture energy is either converted into heat or radiated energy the geophysical consequences of thermal dehydration of bonded water during seismic slip are then commented in the light of mineralogical and poromechanical data of several fault zones which tend to show that this phenomenon has to be taken into account in most of subsurface faults and in hydrous rocks of subducted oceanic crust

textural record of the seismic cycle strain rate variation in an ancient subduction thrust active faults slip at different rates over the course of the seismic cycle earthquake slip c 1 m s 1 interseismic creep c 10 100 mm year 1 and intermediate rate transients e g afterslip and slow slip events studies of exhumed faults are sometimes able to identify seismic slip surfaces by the presence of frictional melts and slow creep by textures diagnostic of rate limited plastic processes the pasagshak point thrust preserves three distinct fault rock textures which are mutually cross cutting and can be correlated to different strain rates ultrafine grained black fault rocks including pseudotachylyte were formed during seismic slip on layers up to 30 cm thick well organized s c cataclasites 7 31 m thick were formed by slow creep with pressure solution as a dominant rate limiting mechanism these must have formed at strain rates consistent with long term plate boundary motion but solution creep healing acted to reduce porosity of the cataclasites and eventually restricted fluid connectivity such that creep by this mechanism could not continue disorganized non foliated rounded clast cataclasites were formed at shear rates faster than solution creep and are interpreted as representing shear at intermediate strain rates these could have formed during afterslip or delocalization of slip associated with an earthquake rupture

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

origin of pseudotachylites in slow creep experiments we conducted a series of experiments on granitoid cataclasites under mid crustal conditions pc 500 mpa t 300 c and slow displacement rates 10 8 ms 1 d 10 7 ms 1 i e within an order of magnitude of plate tectonic velocities the samples reach high peak strength 1250 1375 mpa then weaken slightly by 40 mpa and continue to deform without any abrupt stress drops up to a of 4 microstructural observations show that at peak strength nanocrystalline mean grain size 34 nm smallest crystalline particles 8 nm to partly amorphous material develops weakening occurs once this material is abundant enough to form through going anastomozing slip zones where the strain partitions during higher strain deformation deformation localizes further and 1 to 2 multiply connected layers of fragment loaded amorphous material develop this material shares many similarities with naturally occurring pseudotachylites however was formed at velocities up to 8 orders of magnitude slower than those calculated for earthquakes 1 ms 1 we conclude that the use of pseudotachylites as indicators for paleoseismic events should be reconsidered

silica gel formation during fault slip evidence from the rock record dynamic reduction of fault strength is a key process during earthquake rupture many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments including silica gel lubrication however few have been observed in nature here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface the corona heights fault slickenside in san francisco california is covered by a shiny layer of translucent silica microstructures in this layer show fl ow banding armored clasts and extreme comminution compared to adjacent cataclasites the layer is composed of 100 nm to 1 m grains of quartz hydrous crystalline silica and amorphous silica with 10 100 nm inclusions of fe oxides and ellipsoidal silica colloids kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip the layer therefore represents a relict silica gel that formed during fault motion and which could have resulted in frictional instability these observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths