constraining continental margin seismicity by extending on shore seismograph stations to critical off shore sites the westernmost north american plate margin in central california includes major faults with both onshore and offshore segments prior to deployment during 1997 and 1998 of a suite of ocean bottom seismometers within monterey bay the distribution and focal mechanisms of offshore earthquakes were poorly constrained the use of ocean bottom seismic stations improves the accuracy of locations for events in monterey bay especially for those far offshore and permits more robust focal mechanism solutions by reducing uncertainties in strike dip and rake these initial results using u s geological survey crustal velocity models and location parameters reduce the apparent scatter of seismic events and focal mechanisms and the more consistent solutions provide valuable information on fault characteristics for the san gregorio fault sgf and monterey bay fault zone mbfz most seismic events are relocated to the northern section of the sgf defining a zone of compressional deformation and relatively high microseismicity the mbfz is the source for fewer but important and well located events surprisingly no events were observed on the southern sgf offshore of carmel and pebble beach despite the well defined trace of the san gregorio along carmel canyon

residual mean solutions for the antarctic circumpolar current and its associated overturning circulation abstract residual mean theory is applied to the streamwise averaged antarctic circumpolar current to arrive at a concise description of the processes that set up its stratification and meridional overturning circulation on an f plane simple solutions are found in which transfer by geostrophic eddies colludes with applied winds and buoyancy fluxes to determine the depth and stratification of the thermocline and the pattern of associated residual meridional overturning circulation

effects of slip slip rate and shear heating on the friction of granite the stability of fault slip is sensitive to the way in which frictional strength responds to changes in slip rate and in particular to the effective velocity dependence of steady state friction ss ln v this quantity can vary substantially with displacement temperature and slip rate to investigate the physical basis for this behavior and the possible influence of shear heating we slid initially bare granite surfaces in unconfined rotary shear to displacements of hundreds of millimeters at normal stresses n of 10 and 25 mpa and at room temperature we imposed step changes in slip rate within the range 10 2 to 103 5 m s and also monitored frictional heating with thermistors embedded in the granite the transient response of to slip rate steps was fit to a rate and state dependent friction law using two state variables to estimate the values of several parameters in the constitutive law the first 20 mm of slip shows rising friction and falling ss ln v further slip shows roughly constant friction ss ln v and parameter values suggesting that a steady state condition is reached on the fault surface at v 10 m s ss ln v 0 004 0 001 at higher rates the response is sensitive to normal stress at n 25 mpa granite shows a transition to effective velocity strengthening ss ln v 0 008 0 004 at the highest slip rates tested at 10 mpa granite shows a less dramatic change to ss ln v 0 at the highest rates the maximum temperature measured in the granite is 60 c at 25 mpa and 103 5 m s temperatures are in general agreement with a numerical model of heat conduction which assumes spatially homogeneous frictional heating over the sliding surface the simplest interpretation of our measurements of ss ln v is that the granite is inherently velocity weakening ss ln v 0 mimics velocity strengthening these results have implications for the frictional behavior of faults during earthquakes high slip rates may cause a switch to effective velocity strengthening which could limit peak coseismic slip rate and stress drop for fluid saturated faults strengthening by this mechanism may be partly or fully offset by weakening due to thermal pressurization of a poorly drained pore fluid

recent wind driven variability in atlantic water mass distribution and meridional overturning circulation abstractinterannual variability in the volumetric water mass distribution within the north atlantic subtropical gyre is described in relation to variability in the atlantic meridional overturning circulation the relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an argo based climatology and an ocean state estimate ecco version 4 this highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre a strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and ekman pumping over the gyre suggests that wind driven heave drives transport anomalies at the gyre boundaries this wind driven heaving contributes significantly to variations in the heat content of the gyre as do anomalies in the air sea fluxes the analysis presented suggests tha

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

a 3 d study of the effects of thermomechanical loads on fracture slip in enhanced geothermal reservoirs heat extraction from enhanced geothermal systems egs can greatly affect the behavior of joints and other discontinuities in the reservoir fracture permeability can change in response to fluid injection extraction rock cooling variations of stress field and mineral dissolution precipitation the reduction in effective stress caused by pore pressure increase can cause the slippage of discontinuities thus inducing seismicity studies have shown that thermal stresses generated by cold water injection have a similar effect in order to assess the influence of thermal stresses on fracture opening and slippage a 3 d coupled heat extraction thermal stress elastic displacement discontinuity model is used in this study the effects of each mechanism on fracture slip is estimated with particular reference to the coso geothermal field the results indicate that under typical field conditions a substantial increase in fracture slip is observed when thermal stresses are taken into account the temporal evolution of the thermal stresses suggests that the rock mass deformation will not cease upon stoppage of water injection which can be a cause of delayed seismic activity

the role of mesoscale eddies in the rectification of the southern ocean response to climate change abstract simulations from a fine resolution global coupled model the geophysical fluid dynamics laboratory climate model version 2 4 cm2 4 are presented and the results are compared with a coarse version of the same coupled model cm2 1 under idealized climate change scenarios a particular focus is given to the dynamical response of the southern ocean and the role played by the eddies parameterized or permitted in setting the residual circulation and meridional density structure compared to the case in which eddies are parameterized and consistent with recent observational and idealized modeling studies the eddy permitting integrations of cm2 4 show that eddy activity is greatly energized with increasing mechanical and buoyancy forcings buffering the ocean to atmospheric changes and the magnitude of the residual oceanic circulation response is thus greatly reduced although compensation is far from being perfect changes in poleward eddy fluxes partially compensate for the enhanced equatorward

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

effects of variable thermal conductivity on the mineralogy of subducting slabs and implications for mechanisms of deep earthquakes abstract thermal and thermo kinetic models of subduction zones have been based in general on a constant value of thermal conductivity e g 3 138 w m k for solving the conservation of energy equation in order to make such models more realistic we have used variable thermal conductivities k that depend on pressure temperature and mineralogy olivine wadsleyite or ringwoodite as determined recently by xu et al xu y shankland t j linhardt s rubie d c langenhorst f klasinski k 2004 thermal diffusivity and conductivity of olivine wadsleyite and ringwoodite to 20 gpa and 1373 k phys earth planet inter 143 144 321 336 we have used these variable conductivities to model a variety of slabs with thermal parameters vertical subduction rate age of lithosphere at the trench ranging between 3500 and 17 000 km our models show that when radiative transfer is included k could plausibly be increased by as much as 2 w m k relative to a standard constant lithospheric value 3 138 w m k stein c a stein s 1992 a model for the global variation in oceanic depth and heat flow with lithospheric age nature 359 123 129 causing temperatures in the interiors of the slabs to increase by 50 100 c in turn the volume of metastable olivine in the slabs is reduced by as much as 20 30 with the maximum depth of olivine metastability decreasing by 30 50 km in general the maximum depths of olivine metastability are 160 230 km less than the depths of the deepest seismicity even in the extreme case of tonga metastable olivine falls short of the deepest earthquakes by 140 km in our model even taking the various uncertainties into account these results indicate that deep focus earthquakes occur in regions already transformed to wadsleyite or ringwoodite and must therefore be caused by a mechanism other than transformational faulting

finite dispersal of a separative nepheloid plume by an internal hydraulic jump in a tropical mountainous river estuary this paper investigates the dynamics of an internal hydraulic jump in a river plume and associated suspended sediment dispersal field investigations were undertaken into the river plume generated by the herbert river australia following a moderate flood event induced by cyclone fritz in 2004 the forced plume experiences an abrupt transition from supercritical to subcritical via an internal hydraulic jump as defined by a mode 1 internal froude number computed using the phase speeds from the taylor goldstein equation the hydraulic theory of a two layer stratified flow was used to identify the plume shape and the mechanical energy loss within the jump the hydraulic jump energy loss is primarily transferred to the buoyancy driven potential energy uplifting the river plume intense stratification decreases the bottom stress damping the resuspension therefore a separative nepheloid dispersal system occurs at the jump section both the upper and lower nepheloid flows are confined to the inner shelf but have different dispersal behaviors and mechanisms the upper nepheloid flow which is primarily controlled by advection and settling satisfies an exponential decay law of the total suspended sediment concentrations versus the offshore distance the lower nepheloid flow dominated by deposition is detached seaward near the lift off point of the river plume a turbidity front associated with the jump may accumulate a large quantity of suspended sediments enhancing sediment release from the river plume these findings will promote in depth understanding of both the cross shelf sediment dispersal and muddy deposit on the shelf

on the transport and angular momentum balance of channel models of the antarctic circumpolar current abstract angular momentum balances are discussed both in general as well as in the context of simple channel models of the antarctic circumpolar current acc particular emphasis is placed on the close relationship between the angular momentum balance and the meridional circulation it is found that topographic form drag is established very early in the integration whereas interfacial form drag can take much longer to develop restrictions on the geostrophic portion of the meridional circulation imposed by zonally reconnecting potential vorticity contours in the upper ocean allow derivation of an estimate for the steady state transport the estimate assumes there to be little or no circumpolar flow at great depth an assumption that stems from the belief that the band of zonally reconnecting geostrophic contours in the southern ocean does not extend to the ocean floor the predicted transport is proportional to the strength of the stratification and compares favorably with numerical results in the lite

global ocean meridional overturning abstract a decade mean global ocean circulation is estimated using inverse techniques incorporating air sea fluxes of heat and freshwater recent hydrographic sections and direct current measurements this information is used to determine mass heat freshwater and other chemical transports and to constrain boundary currents and dense overflows the 18 boxes defined by these sections are divided into 45 isopycnal neutral density layers diapycnal transfers within the boxes are allowed representing advective fluxes and mixing processes air sea fluxes at the surface produce transfers between outcropping layers the model obtains a global overturning circulation consistent with the various observations revealing two global scale meridional circulation cells an upper cell with sinking in the arctic and subarctic regions and upwelling in the southern ocean and a lower cell with sinking around the antarctic continent and abyssal upwelling mainly below the crests of the major bathymetric ridges

effects of shear stress applied to surfaces in stationary contact on rock friction in this paper we show how the applied shear stress affects the frictional strength of granite sliding surfaces initially held in stationary contact we develop an experimental scheme to perform a slide and hold friction test whereby the shear stress is servocontrolled during the hold period of the test a series of experiments conducted at a single normal stress of 5 mpa show the following effects first unloading the shear stress during the hold period increases frictional resistance by up to 10 20 a decrease in shear stress linearly increases the frictional resistance of the interface secondly the static friction coefficient increases with the logarithm of contact time the friction coefficient increases by 0 01 per decade of contact time at near zero shear stress levels and by more than 0 02 at shear stress levels that are greater than half the sliding friction

quantifying the interaction of an ocean bottom seismometer with the seabed a theory is presented for the coupling between an ocean bottom seismometer obs the sediments upon which it rests and the surrounding water assuming that rotational and tilt effects are negligible or have been made negligible through instrument design the response of the obs to forced harmonic motion is considered in both horizontal and vertical directions under these conditions it is concluded that the measured ratio of velocities when the obs is on the seabed and when it is freely suspended in water for an identical force completely characterizes the obs seabed interaction this enables the velocity transfer functions to be directly calculated without recourse to a detailed model of sediment structure interaction an obs was designed and constructed with good coupling to seabed motion and reduction of rocking effects as principal design criteria including an onboard shaker to conduct in situ coupling experiments the amplitude and phase of coupling data collected on a clay seabed provide transfer functions due to horizontal and vertical seabed motion and horizontal water motion in addition a simple mass spring dashpot model of obs seabed interaction permits the analysis of amplitude only coupling data good vertical coupling can be achieved over a wide bandwidth by designing the sensor package to have a large hydrodynamic added mass in the vertical good coupling to horizontal seabed motion is more difficult to achieve but is possible within a limited bandwidth even on very soft seabeds finally an example of seismoacoustic noise is presented hydrophone signals are compared with horizontal geophone signals received from sources within the ocean and within the seabed and the differences are explained in terms of the coupling transfer functions

eddy saturation of equilibrated circumpolar currents abstractthis study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in southern ocean wind stress and global diapycnal mixing at eddy permitting and finer resolution the sensitivity of circumpolar transport to forcing magnitude is drastically reduced at sufficiently high resolution there is little or no sensitivity of circumpolar transport to wind stress even in the limit of no wind in contrast the meridional overturning circulation continues to vary with southern ocean wind stress but with reduced sensitivity in the limit of high wind stress both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions the circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution although sensitivity always remains in contrast the overturning circulation is more sensitive to change in diapycnal

the dependence of southern ocean meridional overturning on wind stress an eddy resolving numerical model of a zonal flow meant to resemble the antarctic circumpolar current is described andanalyzedusing the framework of j marshallandt radko in additionto windand buoyancy forcing at the surface the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation moc to exist at depth the strength of the residual moc is diagnosed for different strengths of surface wind stress it is found that the eddy circulation largely compensates for the changes in ekman circulation the extent of the compensation and thus the sensitivity of the moc to the winds depend on the surface boundary condition a fixed heat flux surface boundary severely limits the ability of the moc to change an interactive heat flux leads to greater sensitivity to explain the moc sensitivity to the wind strength under the interactive heat flux transformed eulerian mean theory is applied in which the eddy diffusivity plays a central role in determining the eddy response a scaling theory for the eddy diffusivity based on the mechanical energy balance is developed and tested the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress the moc sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments

size matters another reason why the atlantic is saltier than the pacific abstractthe surface salinity in the north atlantic controls the position of the sinking branch of the meridional overturning circulation moc the north atlantic has higher salinity so deep water formation occurs there rather than in the north pacific here it is shown that in a 3d primitive equation model of two basins of different widths connected by a reentrant channel there is a preference for sinking in the narrow basin even under zonally uniform surface forcing this preference is linked to the details of the velocity and salinity fields in the sinking basin the southward western boundary current associated with the wind driven subpolar gyre has higher velocity in the wide basin than in the narrow basin it overwhelms the northward western boundary current associated with the moc for wide basin sinking so freshwater is brought from the far north of the domain southward and forms a pool on the western boundary in the wide basin the fresh pool suppresses local convection and spreads eastward

what sets the strength of the middepth stratification and overturning circulation in eddying ocean models abstract the processes maintaining stratification in the oceanic middepth between approximately 1000 and 3000 m are explored using an eddy resolving general circulation model composed of a two hemisphere semienclosed basin with a zonal reentrant channel in the southernmost eighth of the domain the middepth region lies below the wind driven main thermocline but above the diffusively driven abyssal ocean here it is argued that middepth stratification is determined primarily in the model s antarctic circumpolar current competition between mean and eddy overturning in the channel leads to steeper isotherms and thus deeper stratification throughout the basin than would exist without the channel isotherms that outcrop only in the channel are nearly horizontal in the semienclosed portion of the domain whereas isotherms that also outcrop in the northern hemisphere deviate from horizontal and are accompanied by geostrophically balanced meridional transport a northern source of deep water water with temp

comparison between connected and overall porosity of thermally stressed granites abstract the structural evolution of four granites heated from 20 to 700 c was examined after cooling the granites have four typical initial structures which can be sub divided into two groups rocks with high connected porosity comprising cracks with low aspect ratio and alteration pores type a pores and a few cracks type b or cracks type c and rocks with low connected porosity with cracks type d the connected porosity was measured by mercury injection and the total porosity by x ray computerized tomography at low temperature t the connected porosity decreases with increasing temperature particularly for the material with high crack density and connected porosity a and b the total porosity increases for b c and d and decreases for the material with low aspect ratio cracks a above 300 c both porosities increase the data show that the thermal history of a granite can be decomposed into two phases in the first stage the connected porosity decreases with increasing temperature cracks are initiated at low temperature and they form unconnected networks in the second stage the connected and total porosities increase the previously initiated cracks form a connected network by opening and they interact with newly created cracks

displacement velocity effects on rock fracture shear strengths abstract triaxial shear tests are performed to assess the effects of displacement velocity and confining pressure on shear strengths and dilations of tension induced fractures and smooth saw cut surfaces prepared in granite sandstone and marl specimens a polyaxial load frame is used to apply confining pressures between 1 and 18 mpa with displacement velocities ranging from 1 15 10 5 to 1 15 10 2 mm s the results indicate that the shearing resistances of smooth saw cut surfaces tend to be independent of the displacement velocity and confining pressure under each confinement the peak and residual shear strengths and dilation rates of rough fractures increase with displacement velocities the sheared off areas increase when the confining pressure increases and the displacement rate decreases the velocity dependent shear strengths tend to act more under high confining pressures for the rough fractures in strong rock granite than for the smoother fractures in weaker rocks sandstone and marl an empirical criterion that explicitly incorporates the effects of shear velocity is proposed to describe the peak and residual shear strengths the criterion fits well to the test results for the three tested rocks

decline of subpolar north atlantic circulation during the 1990s observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid to high latitude north atlantic ocean topex poseidon altimeter data show that subpolar sea surface height increased during the 1990s and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation combining the data from earlier satellites we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s direct current meter observations in the boundary current of the labrador sea support the weakening circulation trend of the 1990s and together with hydrographic data show that the mid to late 1990s decline extends deep in the water column analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations a weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation

a comparative study of island seafloor and subseafloor ambient noise levels abstract a study of seafloor and island stations shows that for the frequency band 0 1 to 10 hz the seismic noise levels on islands are comparable to the levels on the seafloor the microseism peak at the seafloor appears to be comparable to the highest levels observed on small islands for this band seafloor stations are realistic alternatives when island sites are not available seven year averages of the ambient noise levels recorded by seismic research observatory sro stations on three islands guam gumo taiwan tato and new zealand9s north island snzo are compared with those recorded by the international deployment of accelerometers ida station on easter island and on and beneath the ocean floor by ocean bottom seismometers obss and the marine seismic system mss deployed in a south pacific dsdp drill hole at 23 8 s 165 5 w adair et al 1986 from 0 3 to 2 hz the sro displacement power levels fall in the range historically observed by the scripps9 obss decreasing at 70 db decade from 1 by 10 6 nm 2 hz at 0 3 hz to 1 nm 2 hz at 2 hz and are 10 to 15 db above mss levels above 2 hz it appears that the same ratios hold the sro power levels decrease at 70 db decade to 1 by 10 3 nm 2 hz at a frequency of 10 hz although this correlation is based on very limited high gain short period data at frequencies below 0 3 hz the sro noise levels peak and decrease to approximately 2 by 10 3 nm 2 hz at 40 mhz the noise levels recorded at easter island are somewhat higher decreasing at 70 db decade from 1 by 10 7 nm 2 hz at 0 2 hz to 1 nm 2 hz at 10 hz and to 1 by 10 5 nm 2 hz at 50 mhz at the microseism peak near 0 2 hz the mss levels are from 15 to 20 db higher than observed by the sro stations and equivalent to those recorded at easter island there appears to be little dependence of the variance in noise level estimates on frequency the upper 95 per cent confidence limit generally lies 10 db above the average noise levels for all island stations all island noise level curves are dominated by the broad double frequency microseism peak centered between 0 15 to 0 2 hz the single frequency peak ranges from absent easter island to discernable guam and new zealand to obvious at taiwan the center frequency of this peak ranges from 0 07 hz at guam and new zealand to 0 1 hz at taiwan we speculate that the increased amplitude and frequency of the single frequency microseism peak is due to the interaction between the shallow continental shelf and surface gravity waves and or the presence of taiwan in a region of limited fetch

temperature fields generated by the elastodynamic propagation of shear cracks in the earth w 1 the temperature maximum shifts toward the crack tip for faults having slip zone thickness of the order of centimeters or less the onset of thermally induced phenomena e g frictional melting thermal pressurization etc may occur at any point along the rupture depending on the degree of slip localization and rupture duration in the absence of significant increases in the pore fluid pressure localized fault slip may raise temperature by several hundred degrees sufficient to cause melting the onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area and high viscosity of silicate melts near solidus the inferred transient increases in the dynamic friction viscous braking are consistent with results of high speed rock sliding experiments and might explain field observations of the fault wall rip out structures associated with pseudotachylites possible effects of viscous braking on the earthquake rupture dynamics include 1 delocalization of slip and increases in the effective fracture energy 2 transition from a crack like to a pulse like rupture propagation or 3 ultimate rupture arrest assuming that the pulse like ruptures heal by incipient fusion the seismologic observations can be used to place a lower bound on the dynamic fault friction this bound is found to be of the order of several megapascals essentially independent of the earthquake size further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength index terms 3210 mathematical geophysics modeling 5104 physical properties of rocks fracture and flow 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 8010 structural geology fractures and faults

microseismicity linked to gas migration and leakage on the western svalbard shelf the continental margin off prins karls forland western svalbard is characterized by widespread natural gas seepage into the water column at and upslope of the gas hydrate stability zone we deployed an ocean bottom seismometer integrated into the masox monitoring arctic seafloor ocean exchange automated seabed observatory at the pinch out of this zone at 389 m water depth to investigate passive seismicity over a continuous 297 day period from 13 october 2010 an automated triggering algorithm was applied to detect over 220 000 short duration events sdes defined as having a duration of less than 1 s the analysis reveals two different types of sdes each with a distinctive characteristic seismic signature we infer that the first type consists of vocal signals generated by moving mammals likely finback whales the second type corresponds to signals with a source within a few hundred meters of the seismometer either due east or west that vary on short tens of days and seasonal time scales based on evidence of prevalent seafloor seepage and subseafloor gas accumulations we hypothesize that the second type of sdes is related to subseafloor fluid migration and gas seepage furthermore we postulate that the observed temporal variations in microseismicity are driven by transient fluid release and due to the dynamics of thermally forced seasonal gas hydrate decomposition our analysis presents a novel technique for monitoring the duration intensity and periodicity of fluid migration and seepage at the seabed and can help elucidate the environmental controls on gas hydrate decomposition and release

inter book normal fault related shear heating in brittle bookshelf faults abstract this work models shear heat related temperature rise due to inter book dip slip normal faults by a bookshelf mechanism such faults are characterized by temporally changing the dip of the inter book fault planes considerable temperature can be produced by such faulting although the effects of high temperature have not yet been reported around such fault planes this may mean that fluids flowing through such planes do not allow shear heating to accumulate

thermal effects on shearing resistance of fractures in tak granite abstract triaxial shear tests have been performed on tension induced fractures and smooth saw cut surfaces in tak granite under temperatures up to 773 k the objective is to gain an understanding of the movement of shallow faults that cause seismic activities in the tak batholith in the north of thailand the results indicate that the peak and residual shear strengths and fracture dilations notably decrease as the temperatures increase the thermal effect is enhanced under higher confining pressures the areas of the sheared off asperities increase with temperature and confining pressure a power equation can describe the increase of shear strengths with normal stress where the normal stress exponent is a linear function of the temperature the strain energy principle is applied to incorporate the principal stresses and strains into a strength criterion a linear relation between the distortional strain energy wd and the mean strain energy wm of the fractures is obtained the wd wm slope depends on the fracture roughness and strength of the asperities which can be defined as a function of shear and mean strains and dilation of the fractures this may allow predicting the peak strength of the shallow faults in the tak batholith

changes in compressional and shear wave velocities and dynamic moduli during operation of a hot dry rock geothermal system compressional and shear wave velocities measured during cross well seismic surveys are found to decrease during operation of a hot dry rock geothermal system the relative p and s wave velocity decrease is very close to values predicted by o connell and budiansky 1974 who modeled velocities in rocks containing fluid saturated and dry cracks because our data fit their model quite well we attribute the velocity decrease to an increased density of fluid filled fractures in the reservoir fracture density parameters of o connell and budiansky range as high as 0 365 near the main heat transfer areas of the geothermal reservoir the dynamic shear and young s modulus are significantly lower near the principal heat transfer surfaces but the bulk modulus is largely unaffected these observations suggest that the velocity differences are caused by fully saturated fractures we demonstrate that the fractures inferred by this survey may have lengths as large as 70 cm these fractures may have been caused by shear failure microcracking associated with thermal drawdown or thermal stress cracking

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

α β phase transition in quartz monitored using acoustic emissions summary it is usually suggested that thermal cracking in a quartz bearing rock results from the anomalously high volumetric expansion coefficients of quartz e g simmons cooper 1978 it has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient friedrich wong 1986 experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures e g friedrich johnson 1978 homand etienne troalan 1984 atkinson mcdonald meredith 1984 meredith atkinson 1985 here we report on acoustic emission experiments that monitor the process of thermal cracking as it occurs during heating supported by measurements of crack surface area pore fluid permeability porosity and surface conductivity carried out on rock samples treated to various peak temperatures the acoustic emission measurements show a strong peak of microcracking at the phase transition temperature for quartz 573 c superimposed upon a background of microcracking due to thermal expansion there is also a clear peak of microcracking at higher temperatures 800 c that can be attributed to oxidation dehydroxylation reactions of hornblende and chlorite measurements of fluid permeability pore surface area porosity and electrical conductivity made on samples that have been heat treated to various maximum temperatures show increases associated with a major episode of cracking in the 500 600 c temperature range indicating that the new cracks form a well interconnected network this has been confirmed by sem and optical microscopy these results have implications for the electrical conductivity of the continental crust providing a mechanism enabling the high pore fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth it should be recognized however that th se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures

investigating the role of the tibetan plateau in the formation of atlantic meridional overturning circulation abstractthe tibetan plateau tp over the eurasian continent has significant effects on both regional and global climate it can even affect the remote atlantic meridional overturning circulation

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

a direct determination of the world ocean barotropic circulation abstractthe time mean argo float displacements and the world ocean atlas 2009 temperature salinity climatology are used to obtain the total top to bottom mass transports outside of an equatorial band the total transports are the sum of the vertical integrals of geostrophic and wind driven ekman currents however these transports are generally divergent and to obtain a mass conserving circulation a poisson equation is solved for the streamfunction with dirichlet boundary conditions at solid boundaries the value of the streamfunction on islands is also part of the unknowns this study presents and discusses an energetic circulation in three basins the north atlantic the north pacific and the southern ocean this global method leads to new estimations of the time mean western eulerian boundary current transports maxima of 97 sverdrups sv 1 sv 106 m3 s 1 at 60 w for the gulf stream 84 sv at 157 e for the kuroshio 80 sv for the agulhas current between 32 and 36 s and finally 175 sv for the

ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in lac du bonnet granite concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined 15 cm cube of lac du bonnet granite from atomic energy of canada limited s underground research laboratory an electrical resistance cartridge heater placed in a central vertical borehole was used to cycle the sample to progressively higher peak temperatures between 75 c and 175 c tomography data were collected at room temperature before and after each thermal cycle acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle microfractures opened above 80 c and eventually coalesced into a macroscopic fracture plane the macroscopic fracture originated at the outer edges of the sample and then extended inward parallel to the fast velocity direction and eventually intersected the borehole both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane we attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle anisotropic medium

the 20 hz signals of finback whales balaenoptera physalus the 20 hz signals of finback whales balaenoptera physalus were analyzed from more than 25 years of recordings at a variety of geographic locations on near surface hydrophones close to whales and on deep hydrophone systems these signals were composed of 1 s pulses of sinusoidal waveform with downward sweeping frequency from approximately 23 to 18 hz at variable source levels up to 186 db re 1 pa at 1 m usually with slightly lower levels for the pulses at the beginning and end of sequences these 20 hz pulses were produced in signal bouts separated by more than 2 h lasting as long as 32 5 h bouts were composed of regularly repeated pulses at intervals of 7 26 s typically either at one nominal pulse rate or at two alternating doublet pulse intervals signal bouts were interrupted by rests of 1 20 min at roughly 15 min intervals and by irregular gaps lasting between 20 and 120 min the distribution of these signals throughout the year and their temporal sequence were analyzed from the cont

fracture toughness and fracture roughness interrelationship in thermally treated westerly granite this paper presents an experimental work aimed at assessing the correlation between fracture toughness k ic and fracture roughness for a series of westerly granite specimens thermally treated up to 850 c mode i fracture toughness as a function of thermal treatment is determined using cracked chevron notched brazilian disc specimens the degree of roughness of the resultant fracture surfaces is analyzed with the aid of a high accuracy high precision stereo topometric measurement system roughness and toughness values display a negative correlation as a function of temperature fracture toughness decreases with increasing temperature due to the gradual opening of grain grain boundaries in response to thermal stresses mode i fractures preferentially follow these weakened grain grain boundaries which in addition to the thermal expansion of individual grains result in rougher failure profiles with increasing temperature at low temperature a distinct anisotropy in roughness was observed in all fracture surfaces with higher roughness value perpendicular to the direction of fracture propagation however higher treatment temperatures resulted in the homogenization of fracture roughness in all directions these results confirm the important link among petrofabric analysis fracture toughness and fracture roughness in response to thermal treatment

acoustic properties of fine grained sediments from emerald basin toward an inversion for physical properties using the biot stoll model acoustic data from two long cores comprising marine clays and silts taken from emerald basin off nova scotia are presented high resolution measurements of compressional wave velocity attenuation and power law exponent are made using ultrasonic frequencies between 100 to 1000 khz the observed values of the frequency dependence of attenuation suggest that a nonconstant q mechanism is needed to explain these data and biot stoll theory is used to model the experimental results an inversion scheme is used to constrain physical parameters in the biot stoll dispersion relation the inversion shows that there is a restricted range of permeability and grain size by assigning reasonable values for grain size in the inversion the biot stoll model predicts unique values for the permeability and frame bulk modulus that agree well with estimates made by other means

brittle rotational faults and the associated shear heating abstract brittle faulting related shear heating is important in petroleum geosciences tectonics and seismic studies temporal variation of shear heat related temperature rise for rotational and roto translational faults are investigated in this work for planar fault planes devoid of gouge and any secondary faulting temperature rise is proportional to the coefficient of friction and rate of angular slip tectonically realistic physical parameters for rotational faults especially prolonged faulting can significantly increase temperature by shear heating at shallow crustal depth capable of thermal maturity of hydrocarbons

warm and saline events embedded in the meridional circulation of the northern north atlantic 1 ocean state estimates from 1958 to 2005 from the simple ocean assimilation system soda system are analyzed to understand circulation between subtropical and subpolar atlantic and their connection with atmospheric forcing this analysis shows three periods 1960s around 1980 and 2000s with enhanced warm saline waters reaching high latitudes alternating with freshwater events originating at high latitudes it complements surface drifter and altimetry data showing the subtropical subpolar exchange leading to a significant temperature and salinity increase in the northeast atlantic after 2001 the warm water limb of the atlantic meridional overturning cell represented by soda expanded in density salinity space during these warm events tracer simulations using soda velocities also show decadal variation of the gulf stream waters reaching the subpolar gyre and nordic seas the negative phase of the north atlantic oscillation index usually invoked in such variability fails to predict the warming and salinization in the early 2000s with salinities not seen since the 1960s wind stress curl variability provided a linkage to this subtropical subpolar gyre exchange as illustrated using an idealized two layer circulation model the ocean response to the modulation of the climatological wind stress curl pattern was found to be such that the northward penetration of subtropical tracers is enhanced when amplitude of the wind stress curl is weaker than normal in this case both the subtropical and subpolar gyres weaken and the subpolar density surfaces relax hence the polar front moves westward opening an enhanced northward access of the subtropical waters in the eastern boundary current

quantitative three dimensional description of a rough surface and parameter evolution with shearing note roches reference lmr article 2002 005view record in web of science record created on 2006 11 09 modified on 2016 08 08

frictional behavior of simulated biotite fault gouge under hydrothermal conditions abstract we investigated frictional properties of biotite under hydrothermal conditions by shearing simulated biotite gouge sandwiched between saw cut driving blocks using a testing system with argon as confining medium experiments were conducted under an effective normal stress of 200 mpa with pore pressure of 30 mpa temperatures were varied from room temperature to 600 c with shear displacement rate stepped at standard velocities 1 22 0 122 m s and slow velocities 0 244 0 0488 m s to acquire rate dependence of friction our results show that the friction coefficient of biotite ranges from 0 25 to 0 44 significantly lower than that of muscovite documented in previous studies the steady state rate dependence shows velocity strengthening at temperatures of 25 c to 200 c and transitions to velocity weakening at temperatures above 300 c the velocity weakening is quite minor with a b ranging from 4 10 4 to 9 10 4 and stick slip motions at temperatures from 400 c to 600 c are found to be related to submicron dc values corresponding to critical stiffness higher than the actual stiffness surrounding the fault thus leading to unstable slips microstructures of deformed samples show three types corresponding respectively to three different temperature ranges while brittle structures are dominantly associated with deformation at 100 c significant plastic deformation occurred at higher temperatures deformation at temperatures of 200 c to 400 c are characterized with shear fracture zones accompanied with plastic deformation mostly associated with basal glide intensively deformed zones are the characteristic structures for samples deformed at 500 c to 600 c accompanied with shear fracture zones and moderately deformed zones it is evident that biotite alone is not weak enough to explain a weak fault like the san andreas fault thus overpressured pore fluid or another weaker mineral is needed to produce a weak fault with apparent friction coefficient of 0 1 0 2 the around neutral velocity weakening found for biotite may be appropriate in modeling fault creep transients in the mid crust

an exploration of the role of transient eddies in determining the transport of a zonally reentrant current the meridional ekman transport in a zonally reentrant channel may be balanced by diabatic circulations standing eddies associated with topography or by lagrangian mean eddy mass fluxes a simple model is used to explore the interaction between these mechanisms a key assumption of this study is that diabatic forcing in the poleward edge of the channel acts to create lighter fluid as is the case with net freshwater fluxes into the southern ocean for weak wind forcing or strong diabatic constraint a simple scaling argument accurately predicts the level of baroclinic shear however given our understanding of the relative magnitudes of ekman flux and deep upwelling this is not the appropriate parameter range for the antarctic circumpolar current with stronger wind stresses eddies are prominent with baroclinic instability initially developing in the vicinity of large topography arguments have been advanced by a number of authors that baroclinic instability should limit the velocity shear leading to a stiff upper limit on the transport of the current however in the simulations presented here baroclinic instability is largely confined to the region of topographic highs and the approach to a current that is independent of the wind stress occurs gradually several recent parameterizations of transient eddy fluxes do not reproduce key features of the observed behavior

the effect of northern hemisphere winds on the meridional overturning circulation and stratification abstractthe current paradigm for the meridional overturning cell and the associated middepth stratification is that the wind stress in the subpolar region of the southern ocean drives a northward e

reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria it has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides however it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation which depends on thermohaline feedbacks and is more consistent with a buoyancy driven view of the circulation both theories have been illustrated by box models in the past stommel in tellus 13 224 230 1961 gnanadesikan in science 283 2077 2079 1999 here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation the box model has two equilibrium solutions one with sinking at high northern latitudes as in the present day atlantic and one without the circulation is mechanically driven but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect while the source of energy comes from mechanical forcing the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks in some areas of parameter space an oscillation between the model s two circulation regimes occurs reminiscent of a bipolar seesaw

oceanic isopycnal mixing by coordinate rotation abstract current numerical models of oceanic circulation differentiate between the eddy diffusion and viscosity transport along the geopotential horizontal and vertical directions only in order to model the effect of anisotropic turbulence as diffusive transport along and across density surfaces the isopycnal mixing tensor has been transformed from a diagonal second rank tensor in the isopycnal coordinate system to a tensor containing off diagonal elements in the geopotential coordinate system

the role of mesoscale eddies in the remote oceanic response to altered southern hemisphere winds abstract it has been suggested that a strengthening of the southern hemisphere winds would induce a more vigorous overturning through an increased northward ekman flux bringing more light waters into the oceanic basins and enhancing the upwelling of north atlantic deep water in the southern ocean thereby increasing ocean ventilation simulations from a coarse and a fine resolution version of a coupled model subject to idealized wind stress changes in the southern ocean are presented in the fine resolution eddy permitting model changes in poleward eddy fluxes largely compensate for the enhanced equatorward ekman transport in the southern ocean as a consequence northward transport of light waters pycnocline depth northern hemisphere overturning and southern ocean upwelling anomalies are much reduced compared with simulations in the coarse resolution model with parameterized eddies these results suggest a relatively weak sensitivity of present day global ocean overturning circulation to the proj

laboratory study of fault healing and lithification in simulated fault gouge under hydrothermal conditions abstract seismic data geologic observations and laboratory friction studies suggest that faults lithify and strengthen heal during the interseismic period of the earthquake cycle we report on experiments to investigate the influence of healing duration and temperature on the strength and healing rate of simulated faults layers of m sized quartz powder were used to simulate granular fault gouge gouge layers were sheared within sioux quartzite in a triaxial pressure vessel at elevated pressures p c 250 mpa elevated temperatures 230 636 c and in the presence of water p h 2 o 75 mpa we performed hold slide experiments in which samples were subjected to a period of healing under hydrostatic load followed by shear deformation healing times ranged from a few hundred s to 10 5 s to isolate the effects of lithification and temperature on friction we ran two types of hold slide experiments 1 samples were subjected to healing and shear deformation at elevated temperature and 2 following healing at elevated temperature temperature was reduced prior to shear we also ran slide hold slide experiments in which samples were healed under shear load at elevated temperature 636 c experiments in which deformation was carried out at a lower temperature than healing show that both the static and sliding coefficient of friction increase with heal time samples healed and deformed at higher temperature showed lower peak strengths and within the scatter of our data did not exhibit time dependent strengthening these data are interpreted to result from enhanced lithification rates at elevated temperature samples healed under shear load exhibited a log linear decrease in friction with hold time modeling using rate and state dependent friction laws indicates velocity strengthening behavior with friction a b values of 0 05 to 0 08 our data indicate that at high temperature the effective state evolution term b is negative moreover the data indicate the rate of frictional strengthening varies significantly for hydrostatic and non hydrostatic load conditions our measurements of healing are consistent with seismic estimates of fault healing rates if stress drop is a fraction of the total shear stress and effective fault normal stress is 60 to 100 mpa

synthetic saline aqueous and hydrocarbon fluid inclusions trapped in calcite at temperatures and pressures relevant to hydrocarbon basins a reconnaissance study abstract we conducted reconnaissance experiments to synthesize aqueous and hydrocarbon inclusions trapped in calcite at conditions relevant to petroleum basins and characterize the microthermometric properties of such inclusions fluid inclusions fis were synthesized in a system of saline aqueous solution 5 or 20 wt nacl coexisting with either heavy crude oil or gasoline under gas undersaturated conditions from 90 to 210 c and 200 550 bar the synthetic inclusions are not representative of gas bearing systems and methane ch 4 was not detected in any aqueous inclusions the fis are mainly distributed along planar healed cracks indicating that the inclusions formed by fracture healing in the calcite crystal microthermometric measurements were conducted on coeval aqueous and hydrocarbon inclusions and raman spectroscopic analyses were done on aqueous inclusions to determine the properties of fis trapped at these conditions homogenization temperatures of synthetic fis are mostly lower than the experimental trapping temperature although the fis show high variability in measured homogenization temperature results allow comparison of t h values for each sample with the expected t h isochores and pressure corrections calculated for the system h 2 o nacl the latter parameters are broadly consistent with the known pvtx properties of h 2 o nacl fluids suggesting little effect of hydrocarbons on the homogenization behavior although the low precision of the t h data limits this assessment nevertheless this result is not unexpected considering that light hydrocarbons gas is not present in the experiments as corroborated by raman spectroscopy a consequence of using dead oil in the experiments simulation of gas bearing petroleum basins will require additional protocols for producing gas either by in situ cracking of the starting hydrocarbon material or by other means the reconnaissance experiments documented here provide a basis for such future experiments

effects of temperature and sliding rate on frictional strength of granite layers of artificial granite gouge have been deformed on saw cut granite surfaces inclined 30 to the sample axes samples were deformed at a constant confining pressure of 250 mpa and temperatures of 22 to 845 c the velocity dependence of the steady state coefficient of friction ss was determined by comparing sliding strengths at different sliding rates the results of these measurements are consistent with those reported bysolberg andbyerlee 1984 at room temperature andstesky 1975 between 300 and 400 c stesky found that the slip rate dependence of ss increased above 400 c in the present study however the velocity dependence of ss was nearly independent of temperature

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

slow slip slow earthquakes period two cycles full and partial ruptures and deterministic chaos in a single asperity fault abstract the assimilation of geological geophysical and laboratory data in physics based models of fault dynamics promises increasingly realistic simulations of the seismic cycle to assist this effort i explore the dynamics of a single velocity weakening asperity to delineate the relationship between a fault physical properties and the style of faulting during the seismic cycle based on a micro physical model of rate and state friction from a dimensional analysis four non dimensional parameters control the characteristics of slip events and the behavior during the interseismic period including the dieterich ruina rice number ru which is proportional to the ratio of asperity to critical nucleation size w h and rb b a b where a and b control the direct and local evolutionary effects respectively for sufficiently large rb the ru number controls the spectrum of fault slip from periodic slow slip events to aperiodic fast ruptures in finite faults but not in semi infinite faults the transition between these end members includes a bifurcation that involves period two period four period six cycles and deterministic chaos with increasing ru macroscopic fault slip is stabilized as rb vanishes even as ru 1 for finite faults but not for semi infinite faults the transition between slow and fast ruptures for increasing rb includes another type of bifurcation with period two and period four cycles of slow and fast ruptures for 0

fracture toughness and subcritical crack growth during high temperature tensile deformation of westerly granite and black gabbro abstract the double torsion testing method has been used to determine catastrophic and subcritical crack propagation parameters for pre cracked specimens of westerly granite and black gabbro under a number of environmental conditions the critical stress intensity factor for catastrophic crack propagation fracture toughness of granite and gabbro has been measured at temperatures from 20 to 400 c in a vacuum at 20 c the fracture toughness of westerly granite was 1 79 0 02 mpa m 1 2 and for two blocks of black gabbro it was 3 03 0 08 mpa m 1 2 and 2 71 0 15 mpa m 1 2 respectively these values are very close to those reported by other investigators for tests conducted in air of ambient humidity at room temperature for both rocks fracture toughness at first increased slightly and then decreased steadily on raising the temperature above ambient conditions this behaviour is explained in terms of the density and distribution of thermally induced microcracks as determined by quantitative optical microscopy subcritical crack growth behaviour has been studied at temperatures up to 300 c and under water vapour at pressures of 0 6 to 15 kpa both the load relaxation and incremental constant displacement rate forms of the double torsion testing method were utilised to generate stress intensity factor crack velocity diagrams crack growth was measured over the velocity range 5 10 3 to 10 7 m s 1 increasing both temperature and water vapour pressure resulted in substantially higher crack growth rates the overall effect of raising the temperature over the range studied here 20 300 c was to increase the crack growth rate in granite and gabbro by 5 and 7 orders of magnitude respectively at constant stress intensity factor and vapour pressure of water for both rocks the slopes of stress intensity factor crack velocity curves were sensitive to changes in both temperature and water vapour pressure at low values of the latter parameter slopes fell substantially on raising the water vapour pressure but were relatively insensitive to changes in temperature at these higher pressures no subcritical crack growth limit was encountered estimates of the uncertainty in our experimental data are given from the results of multiple load relaxation experiments on westerly granite specimens we estimate the uncertainty in position of stress intensity factor crack velocity curves along the stress intensity axis to be c 10 of the fracture toughness and the uncertainty in slope of such curves to be c 12 problems associated with the extrapolation of our experimental data to regions of higher effective confining pressure in the earth s crust are discussed

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

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

seismic receiver coupling to the seafloor abstract the presence of geophysical receivers on the seafloor changes the local wave field due to the receiver seafloor interaction the resulting pp and ps wave distortion of the wave field is often referred to as receiver coupling to the seafloor and can make data processing challenging and sometimes impossible this paper provides an overview of the mathematical approaches to describe receiver coupling how to estimate receiver coupling and what the difficulties and possible solutions are the first section shows how the mathematical approach developed from a simple model considering only the vertical receiver component to include all three receiver components and their interactions with the seafloor in the second section i show how receiver coupling can be measured and how it can be improved using mathematical and data driven approaches

the effect of high temperature and pressure on rock friction coefficient a review the coefficient of friction is a parameter of the mechanical state which has an important role in the shear failure of rock in the fields of earth sciences in order to analyze the influence of temperature and pore pressure on the coefficients of friction between solid and gouge data from studies carried out in both china and international contexts are applied analyzed and discussed the survey results show that the coefficient of friction mainly depends on the pore pressure pressure and temperature under the supercritical water state the framework of silicate minerals might be substantially disintegrated which suggests a possible change in the frictional resistance from stick slip to stable sliding the temperature range of 300 450 c could constitute as the critical threshold for shear failure to collected rocks i e gabbro gouge pyroxene gouge gabbro solid biotite gouge structured gouge granitic mylonite mylonite plagioclase gouge granite gouge pre fractured granite westerly granite

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

modulation of fault strength during the seismic cycle by grain size evolution around contact junctions abstract earthquake prediction is fundamentally hampered by insufficient knowledge of the constitutive behavior of faults laboratory and theoretical work by dieterich 1978 and ruina 1983 formed the basis of rate and state friction a constitutive framework that captures the healing and weakening of frictional contacts the rate and state law successfully explains the frictional behavior of a wide range of rocks and many phenomena associated with fault slip the physics underlying the rate and state friction law is not fully understood but it is generally accepted that several micro mechanisms operate depending on the texture maturity fluid content and other physical and kinematic conditions of the fault here i formulate a constitutive framework that unifies a number of laboratory experiments on silicate rocks and the gouges derived from these rocks that describe the effect of grain size temperature real area of contact gouge thickness and fault roughness fundamental to the model is that fault strength is controlled by the area of the interfacial contact junctions that support the shear and normal loads the curvature of asperities weakly controls the area of their contact junction and the resulting fault strength as a result the dynamics of grain size evolution around micro asperities enables the seismic cycle the model quantitatively explains the correlation between the characteristic weakening distance and the gouge thickness marone and kilgore 1993 the correlation of the static friction coefficient with the rate dependence parameters ikari et al 2011 the dependence of frictional resistance to sub solidus temperatures chester 1994 and predicts a range of grain sizes that is compatible with observations at exposed fault zones however the dynamics of grain size evolution is still challenging to verify experimentally the model features a regularization at vanishing slip speeds that implies distinct dynamics at low strength for friction coefficients lower than 0 1 seismic cycle simulations predict the emergence of frictional instabilities with complex source time functions this is a significant departure from the predictions of rate and state friction that is most relevant to the dynamics of decollements serpentinized shear zones and other low strength faults

coupled thermal hydrological mechanical analyses of the yucca mountain drift scale test comparison of field measurements to predictions of four different numerical models abstract the yucca mountain drift scale test dst is a multiyear large scale underground heating test designed to study coupled thermal hydrological mechanical chemical behavior in unsaturated fractured and welded tuff as part of the international cooperative code comparison project development of coupled models and their validation against experiments four research teams used four different numerical models to simulate and predict coupled thermal hydrological mechanical thm processes at the dst the simulated processes included heat transfer liquid and vapor water movements rock mass stress and displacement and stress induced changes in fracture permeability model predictions were evaluated by comparison to measurements of temperature water saturation displacement and air permeability the generally good agreement between simulated and measured thm data shows that adopted continuum model approaches are adequate for simulating relevant coupled thm processes at the dst moreover thermal mechanically induced rock mass deformations were reasonably well predicted using elastic models although some individual displacements appeared to be better captured using an elasto plastic model it is concluded that fracture closure opening caused by change in normal stress across fractures is the dominant mechanism for thermal stress induced changes in intrinsic fracture permeability at the dst whereas fracture shear dilation appears to be less significant this indicates that such changes in intrinsic permeability at the dst which are within one order of magnitude are likely to be mostly reversible

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

a theory of the interhemispheric meridional overturning circulation and associated stratification abstracta quantitative theoretical model of the meridional overturning circulation and associated deep stratification in an interhemispheric single basin ocean with a circumpolar channel is presented the theory includes the effects of wind eddies and diapycnal mixing and predicts the deep stratification and overturning streamfunction in terms of the surface forcing and other parameters of the problem it relies on a matching among three regions the circumpolar channel at high southern latitudes a region of isopycnal outcrop at high northern latitudes and the ocean basin between the theory describes both the middepth and abyssal cells of a circulation representing north atlantic deep water and antarctic bottom water it suggests that although the strength of the middepth overturning cell is primarily set by the wind stress in the circumpolar channel middepth stratification results from a balance between the wind driven upwelling in the channel and deep water formation at high northern latitudes d

ultra high temperature overprinting of high pressure pelitic granulites in the huai an complex north china craton evidence from thermodynamic modeling and isotope geochronology abstract paleoproterozoic granulite facies rocks are widely distributed in the north china craton ncc the huai an terrane located within the northern segment of the trans north china orogen tnco a major paleoproterozoic collisional belt in the central ncc expose mafic and pelitic granulites as well as ttg tonalite trondhjemite granodiorite gneisses here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages the prograde metamorphism m1 is recorded by relict biotite and the compositional profile of xca grt isopleths the pmax m2 is distinguished by the xca grt isopleths which corresponds to the kyanite stable area with an inclusion mineral assemblage of grt c ky qz rt kfs liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 c however kyanite is absent in thin sections suggesting its consumption during later stages the tmax metamorphism m3 is characterized by the assemblage grt m qz pl rt kfs sil liq in the garnet mantle and also reflected in the compositional profile two feldspar geothermometry yields a p t range of 940 c 950 c and 9 5 10 5 kbar indicating ultra high temperature uht metamorphic overprinting the subsequent retrograde metamorphic stage m4 is characterized by grt r bt sil kfs pl qz rt ilm with symplectites of bt sil qz in the garnet rim suggesting garnet breakdown with p t conditions estimated as 770 c 840 c and 6 5 8 kbar the pelitic granulites show a clockwise path with p t estimates higher than those in estimated in previous studies using conventional techniques la icp ms u pb analysis of metamorphic zircon grains yield two groups of ages at 1972 9 8 1 ma and 1873 3 9 9 ma we suggest that the protoliths of the manjinggou hp uht granulites were deep subducted where they experienced hp metamorphism associated with the collision of the ordos and yinshan blocks at ca 1 97 ga subsequently the uht metamorphic overprint occurred during the assembly of the unified western and eastern blocks of the ncc along the tnco at ca 1 87 ga

cascadia onshore offshore site response submarine sediment mobilization and earthquake recurrence local geologic structure and topography may modify arriving seismic waves this inherent variation in shaking or site response may affect the distribution of slope failures and redistribution of submarine sediments i used seafloor seismic data from the 2011 to 2015 cascadia initiative and permanent onshore seismic networks to derive estimates of site response denoted sn in low and high frequency 0 02 1 and 1 10 hz passbands for three shaking metrics peak velocity and acceleration and energy density sn varies similarly throughout cascadia and changes primarily in the direction of convergence roughly east west in the two passbands sn patterns offshore are nearly opposite and range over an order of magnitude or more across cascadia sn patterns broadly may be attributed to sediment resonance and attenuation this and an abrupt step in the east west trend of sn suggest that changes in topography and structure at the edge of the continental margin significantly impact shaking these patterns also correlate with gravity lows diagnostic of marginal basins and methane plumes channeled within shelf bounding faults offshore sn exceeds that onshore in both passbands and the steepest slopes and shelf coincide with the relatively greatest and smallest sn estimates at low and high frequencies respectively these results should be considered in submarine shaking triggered slope stability failure studies significant north south sn variations are not apparent but sparse sampling does not permit rejection of the hypothesis that the southerly decrease in intervals between shaking triggered turbidites and great earthquakes inferred by goldfinger et al 2012 2013 2016 and priest et al 2017 is due to inherently stronger shaking southward

melt welt mechanism of extreme weakening of gabbro at seismic slip rates a laboratory study of the frictional properties of the igneous rock gabbro at seismically relevant slip rates suggests that the initial weakening of a fault surface during earthquake rupture may be associated with hotspots and macroscopic streaks of melt which partially unload the rest of the slip interface

effects of healing on the seismogenic potential of carbonate fault rocks experiments on samples from the longmenshan fault sichuan china fault slip and healing history may crucially affect the fault seismogenic potential in the earthquake nucleation regime here we report direct shear friction tests on simulated gouges derived from a carbonate fault breccia and from a clay carbonate fault core gouge retrieved from a surface exposure of the longmenshan fault zone lfz which hosted the 2008 wenchuan earthquake the experiments were conducted under dry and hydrothermal conditions at temperatures up to 140 c at an effective normal stress of 50 mpa and involved sequential velocity stepping vs slide hold slide shs and velocity stepping stages dry tests performed on breccia derived samples showed no dependence of quasi steady state friction ss on shs or vs history and a log linear relation between transient peak healing pk and hold time or classical dieterich type healing behavior by contrast all experiments conducted under hydrothermal conditions were characterized by non dieterich healing behavior this included 1 an increase in ss upon resliding after a hold period and 2 an increase in friction rate parameter a b after shs testing comparison with previous results suggests that the healing behavior seen in our wet tests may be attributed to solution transfer processes occurring during hold periods our findings imply that the shallow portions of faults with carbonate clay rich cores e g the lfz can heal much faster than previously recognized while the upper limit of the seismogenic zone may migrate to deeper levels during interseismic periods these effects have important implications for understanding the seismic cycle in tectonically active carbonate terrains

influence of the atlantic subpolar gyre on the thermohaline circulation during the past decade record high salinities have been observed in the atlantic inflow to the nordic seas and the arctic ocean which feeds the north atlantic thermohaline circulation thc this may counteract the observed long term increase in freshwater supply to the area and tend to stabilize the north atlantic thc here we show that the salinity of the atlantic inflow is tightly linked to the dynamics of the north atlantic subpolar gyre circulation therefore when assessing the future of the north atlantic thc it is essential that the dynamics of the subpolar gyre and its influence on the salinity are taken into account

coupling of ocean bottom seismometers to sediment results of tests with the u s geological survey ocean bottom seismometer abstract the response of an ocean bottom seismometer obs to a transient pull that excites the natural obs sediment coupling resonance can be modeled as a mass spring dashpot system in which the resonant frequency and damping are functions of instrument mass and bearing radius and of the physical properties of the sediment primarily the shear modulus for the very soft sediments sometimes found on the sea floor this resonance may be within the main frequency band of interest 2 to 15 hz for many common instrument configurations to test the model and to find an anchor that would shift the coupling resonance to a higher frequency and decrease its amplitude we conducted a series of tests which measured the response of the vertical and horizontal components of the u s geological survey obs to transient pulls as a function of anchor configuration and sediment properties the tested anchors included a concrete flowerpot a tripod a plate and a perforated plate sites were on soft organic rich ooze and on firm sand several small shots were also fired at the ooze site in order to compare the response of the plate and flowerpot anchors to seismic signals for a given anchor at a given site the observed response was very repeatable we found that the model predicts the vertical coupling response quite well and that good vertical coupling can be achieved with the plate or perforated plate anchors the response to the horizontal pulls however was similar and resonant for all anchors

magma driven hydraulic fracturing and infiltration of fluids into the damaged host rock an example from dronning maud land antarctica abstract excellent outcrops in dronning maud land antarctica provide unique insight into the mode and extent of fluid infiltration into metamorphic and plutonic rocks in the middle crust the fluids are liberated from pegmatitic veins and give rise to alteration halos in the alteration halos the conspicuous change in colour is correlated with 1 hydration mineral reactions and 2 high density of microcracks in quartz and feldspar exceeding that observed in the unaltered host rock by an order of magnitude the field relations indicate that the veins originated as melt driven hydraulic fractures sealed by pegmatite and aplite crystallising from volatile rich melts with the alteration halo being the wake of the process zone formed at the tip of the propagating fractures it is proposed that 1 the size of the alteration zone and the width of the vein are correlated resulting in higher values of both these quantities for cracks propagating at higher velocities and consequently higher crack propagation toughnesses 2 the damage zone is characterised by a transient state of high permeability which was short lived due to rapid healing and sealing of microcracks 3 the infiltration and retrogression of the high grade rocks can be considered as a quasi instantaneous process on geologic time scales with a duration of hours to weeks

the effect of loading rate on static friction and the rate of fault healing during the earthquake cycle the seismic cycle requires that faults strengthen heal between earthquakes and the rate of this healing process plays a key role in determining earthquake stress drop1 2 3 4 rupture characteristics5 6 and seismic scaling relations2 3 4 7 frictional healing as evidenced by increasing static friction during quasi stationary contact between two surfaces1 8 9 10 11 12 is considered the mechanism most likely to be responsible for fault strengthening2 3 13 14 previous studies however have shown a large discrepancy between laboratory and seismic field estimates of the healing rate2 3 4 14 15 in the laboratory rock friction changes by only a few per cent per order of magnitude change in slip rate whereas seismic stress drop increases by a factor of 2 to 5 per order of magnitude increase in earthquake recurrence interval but in such comparisons it is assumed that healing and static friction are independent of loading rate here i summarize laboratory measurements showing that static friction and healing vary with loading rate and time as expected from friction theory16 17 18 applying these results to seismic faulting and accounting for differences in laboratory seismic and tectonic slip rates i demonstrate that post seismic healing is expected to be retardedfor a period of several hundred days following an earthquake in agreement with recent findings from repeating earthquakes13 14 19 20

strain rate effect on frictional strength and the slip nucleation process abstract kato n yamamoto k yamamoto h and hirasawa t 1992 strain rate effect on frictional strength and the slip nucleation process in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 269 282 the strain rate dependence of frictional strength is investigated in relation to sliding behaviour by biaxial compression tests of large scale granite samples frictional sliding is generated by increasing shear strain at a constant rate under a constant normal stress on the fault the ram displacement is held constant for a prescribed time interval between two successive sliding events so as to have the same time effect of stationary contact between sliding surfaces we obtained the following results as the strain rate increases 1 sliding becomes more unstable 2 the nucleation zone size of stick slip decreases and 3 frictional strength logarithmically increases result 3 is consistent with the previous study by dieterich 1979a and 1 and 3 are similar to the strain rate effects on failure characteristics of intact rocks reported in the literature result 3 can be explained by a rate and state dependent friction law our results suggest that the strain rate dependence of the fault strength plays an important role in controlling the nucleation process of faulting

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

shear heat model for gouge free dip slip listric normal faults abstract shear heating due to brittle faulting is important in petroleum geosciences tectonics and seismic studies temporal variation of shear heat in a listric normal fault with a circular arc shaped fault plane is simulated in this work which was not done so far the work is expected to have a far reaching implication in tectonics and petroleum geosciences since listric faults can be closely associated with hydrocarbon reserves e g valdiya and sanwal 2017 for such a fault plane devoid of gouge and any secondary faulting shear heat is proportional to the mass of the hanging wall block the coefficient of friction acting between the hanging wall and the footwall block and radius of the circular arc shear heating intensifies temporally as the hanging wall block slides down and reaches progressively gentler fault dip

surface constraints on the depth of the atlantic meridional overturning circulation southern ocean versus north atlantic abstractpaleoclimate proxy evidence suggests that the atlantic meridional overturning circulation amoc was about 1000 m shallower at the last glacial maximum lgm compared to the present yet it

temperature dependence of frictional healing of westerly granite experimental observations and numerical simulations 1 temperature is believed to have an important control on frictional properties of rocks yet the amount of experimental observations of time dependent rock friction at high temperatures is rather limited in this study we investigated frictional healing of westerly granite in a series of slide hold slide experiments using a direct shear apparatus at ambient temperatures between 20 c and 550 c we observed that at room temperature coefficient of friction increases in proportion to the logarithm of hold time at a rate consistent with findings of previous studies for a given hold time the coefficient of friction linearly increases with temperature but temperature has little effect on the rate of change in static friction with hold time we used a numerical model to investigate whether time dependent increases in real contact area between rough surfaces could account for the observed frictional healing the model incorporates fractal geometry and temperature dependent viscoelasoplastic rheology we explored several candidate rheologies that have been proposed for steady state creep of rocks at high stresses and temperatures none of the tested laws could provide an agreement between the observed and modeled healing behavior given material properties reported in the bulk creep experiments an acceptable fit to the experimental data could be achieved with modified parameters in particular for the power law rheology to provide a reasonable fit to the data the stress exponent needs to be greater than 40 alternative mechanisms include time dependent gouge compaction and increases in bond strength between contacting asperities

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