ultracataclasite structure and friction processes of the punchbowl fault san andreas system california abstract the punchbowl fault is an exhumed 40 km displacement fault of the san andreas system in the devil s punchbowl the fault contains a continuous ultracataclasite layer along which the punchbowl formation sandstone and an igneous and metamorphic basement complex are juxtaposed the fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer by analogy with nearby active faults we assume that the punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures we have mapped the ultracataclasite layer at 1 1 and 1 10 to determine the mode of failure and to constrain the processes of seismic slip on the basis of color cohesion fracture and vein fabric and porphyroclast lithology two main types of ultracataclasite are distinguished in the layer an olive black ultracataclasite in contact with the basement and a dark yellowish brown ultracataclasite in contact with the sandstone the two are juxtaposed along a continuous contact that is often coincident with a single continuous nearly planar prominent fracture surface pfs that extends the length of the ultracataclasite layer in all exposures no significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other the ultracataclasites are cohesive throughout except for thin accumulations of less cohesive reworked ultracataclasite along the pfs structural relations suggest that 1 the black and brown ultracataclasite were derived from the basement and sandstone respectively 2 the black and brown ultracataclasites were juxtaposed along the pfs 3 the subsequent final several kilometers of slip on the punchbowl fault occurred along the pfs and 4 earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite by comparison with rock friction experiments the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values and thus relatively small nucleation and breakdown dimensions for earthquake ruptures of the various mechanisms proposed to explain the low strength of the san andreas and to produce dynamic weakening of faults those that require or assume extreme localization of slip are most compatible with our observations

superplastic nanofibrous slip zones control seismogenic fault friction understanding the internal mechanisms controlling fault friction is crucial for understanding seismogenic slip on active faults displacement in such fault zones is frequently localized on highly reflective mirrorlike slip surfaces coated with thin films of nanogranular fault rock we show that mirror slip surfaces developed in experimentally simulated calcite faults consist of aligned nanogranular chains or fibers that are ductile at room conditions these microstructures and associated frictional data suggest a fault slip mechanism resembling classical ashby verrall superplasticity capable of producing unstable fault slip diffusive mass transfer in nanocrystalline calcite gouge is shown to be fast enough for this mechanism to control seismogenesis in limestone terrains with nanogranular fault surfaces becoming increasingly recognized in crustal faults the proposed mechanism may be generally relevant to crustal seismogenesis

effects of chemically active solutions on shearing behavior of a sandstone to evaluate the effect of chemical solutions on the frictional properties of a quartz rich sandstone triaxial compression tests have been performed on a sandstone saturated with distilled water solutions of nacl cacl2 and na2so4 at varying ionic strengths and ph values the sandstone although containing 90 quartz also bears about 7 clay results from these tests indicate that the presence of water alone reduces the ultimate strength by 33 the nacl solution of low ionic strength 0 2 m produces similar reductions in strength the samples saturated with nacl solutions of intermediate ionic strength 1 0 m are up to 20 stronger than those samples saturated with distilled water the samples saturated with solutions of the high ionic strength 5 0 m display the greatest weakening 41 46 from the case of the dry specimen this is a reduction of up to 20 compared to the water saturated sample reductions in ultimate strength due to changes in ph are only apparent for samples saturated with solutions of intermediate ionic strength results from the samples saturated with the divalent solutions show strength reductions of up to 52 compared to dry specimens in addition the tests demonstrate that the presence of either cations or anions appears to produce an equivalent weakening in the samples in all cases the samples saturated with the solutions of intermediate ionic strength 1 0 m display the greatest ultimate strength results from fracture tests on intact specimens indicate that the ultimate strength of nominally dry samples is approximately 15 greater than that of samples from all other environments there is no discernable difference between the strengths of specimens saturated with distilled water and those treated with more chemically active solutions scanning electron microscopic observations of the friction surfaces demonstrate that those solutions which produce the weakest mechanical behavior generate either very rough surfaces high ionic strengths or significant amounts of gouge low ionic strengths the former appear to be the result of solution of quartz which is enhanced by the clays in the sandstone these results suggest that the frictional resistance to sliding of the sandstone seems to be primarily controlled by the ionic strength of the pore fluid under most conditions with secondary control by the ph the same solution chemistry may weaken a fault zone at low concentrations but strengthen it at higher concentrations in the former case fault displacement would be facilitated and strain accumulated reduced this is believed to be more common in crustal environments

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

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

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

theory and observations of controls on lightning flash size spectra previous analyses of very high frequency vhf lightning mapping array lma observations relative to the location of deep convective updrafts have noted a systematic pattern in flash characteristics in and near strong updrafts flashes tend to be smaller and more frequent while flashes far from strong vertical drafts exhibit the opposite tendency this study quantitatively tests these past anecdotal observations using lma data for two supercell storms that occurred in oklahoma in 2004 the data support a prediction from electrostatics that frequent breakdown and large flash extents are opposed an energetic scaling that combines flash rate and flash area exhibits a 5 3 power law scaling regime on scales of a few kilometers and a maximum inflashenergyatabout10km thespectralshapeis surprisinglyconsistentacrossarangeofmoderatetolarge flash rates the shape of this lightning flash energy spectrum is similar to that expected of turbulent kinetic energy spectra in thunderstorms in line with the hypothesized role of convective motions as the generator of thunderstorm electrical energy the correspondence between kinematic and electrical energy spectra suggests that advection of charge bearing precipitation by the storm s flow including in turbulent eddies couples the electrical and kinematic properties of a thunderstorm

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

effects of temperature and co2 on the frictional behavior of simulated anhydrite fault rock the frictional behavior of anhydrite bearing faults is of interest to a the safety and effectiveness of co2 storage in anhydrite capped reservoirs b seismicity induced by hydrocarbon production and c natural seismicity nucleated in evaporite formations we performed direct shear experiments on simulated anhydrite fault gouges at a range of temperatures 80 150 c and sliding velocities 0 2 10 ms 1 under a fixed effective normal stress of 25 mpa four types of experiments were conducted 1 dry experiments 2 experiments pressurized with water 3 dry experiments pressurized with co2 and 4 wet experiments pressurized with co2 fluid pressures of 15 mpa were used when applied over the temperature range investigated water saturated samples were found to be up to 15 frictionally weaker than dry equivalents wet samples containing co2 were also up to 15 weaker than co2 free equivalents dry sample strength without co2 was independent of temperature whereas wet samples without co2 strengthened 10 from 80 to 150 c samples containing co2 weakened by 4 dry and 10 wet from 80 to 150 c under the p t conditions investigated only dry anhydrite fault gouge showed velocity weakening behavior above 120 c required for faults to potentially generate earthquakes assuming natural fault gouges are wet in situ seismicity is unlikely to nucleate in anhydrite rich faults though the presence of dolomite or reaction produced calcite may change seismic potential co2 penetration into wet anhydrite rich faults may trigger slip on critically stressed faults due to the observed short term co2 weakening effects excluding possible formation of secondary minerals but is not expected to influence slip stability

three dimensional kinematic and microphysical evolution of florida cumulonimbus part i spatial distribution of updrafts downdrafts and precipitation abstract this paper is the first in a three part study that examines the kinematic and microphysical evolution of florida cumulonimbus and focuses on the convective to stratiform transition of the storm this first paper lays the groundwork for the subsequent papers by defining the problem under study delineating the setting for the storm and describing the spatial distribution of updrafts downdrafts and precipitation high resolution radar data of a typical line of storms associated with the florida sea breeze is the centerpiece of this study the high resolution data reveal details of the internal structure of the squall line that were beyond the resolution of previous squall line studies radar reflectivity filled in between cells at upper levels as the storm evolved reflectivity values were only weakly associated with updraft and downdraft magnitude the updrafts and downdrafts in the storm tended to be irregular in their three dimensional shape and less than 5 km in horizontal extent at any giv

chemistry and isotopic composition of rotliegend and upper carboniferous formation waters from the north german basin abstract element concentrations and isotopic composition d 18 o 15 n of ammonium 87 sr 86 sr have been determined in highly saline formation waters from the rotliegend altmark gasfield and the upper carboniferous husum schneeren gasfield in the north german basin both reservoirs have been covered by huge zechstein evaporite caps since the late permian formation waters from the altmark gasfield are characterized by high total dissolved solids and are enriched in 18 o relative to meteoric waters and either show positive or negative d values which typically follow the proposed d vs 18 o paths for seawater evaporation formation waters stored in upper carboniferous sandstones show much more variable element concentrations and 18 o and d values than the rotliegend formation waters the br vs cl concentrations of the brines suggest seawater evaporation being the principal source of salinity at both sites the ammonium concentrations in the analyzed brines are highly variable and the 15 n isotopic composition of dissolved ammonium is controlled either by the isotopic composition of the gas in the reservoir rotliegend or that of the host rocks upper carboniferous at both sites early formation waters interacted with the sedimentary host rocks after deposition 87 sr 86 sr ratios suggest that re equilibration between fluids and their host rocks occurred in pre triassic times there is no chemical or isotopic evidence that original formation waters which occur in sandstone below the overlying zechstein evaporites were flushed out or mixed by later meteoric water as has been documented in deep seated mesozoic and other paleozoic aquifers of the north german basin

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

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

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

electrical structure in thunderstorm convective regions 3 synthesis in this paper results from nearly 50 electric field soundings through convective regions of mesoscale convective systems mcss isolated supercells and isolated new mexican mountain storms are compared and synthesized these three types of thunderstorm convection are found to have a common basic electrical structure within convective updrafts the basic charge structure has four charge regions alternating in polarity and the lowest is positive outside updrafts of convection there are typically at least six charge regions alternating in polarity and the lowest is again positive among the three storm types there are differences in the heights and temperatures at which the basic four charge regions are found in updrafts the height temperature of the center of the main negative charge region averages 6 93 km 16 c in mcs convective region updrafts 9 12 km 22 c in supercell updrafts and 6 05 km 7 c in new mexican mountain storm updrafts in updraft soundings through all three storm types the center height of the main negative charge region increases with increasing average balloon ascent rate and updraft speed at a rate of about 0 3 km per 1 m s 1 with a correlation coefficient of 0 94 a schematic illustrates the basic four and six charge structure for thunderstorm convective regions and it is offered as an improved model for thunderstorm charge structure

frictional strengths of talc serpentine and talc quartz mixtures 1 talc is a constituent of faults in a variety of settings and it may be an effective weakening agent depending on its abundance and distribution within a fault we conducted frictional strength experiments under hydrothermal conditions to determine the effect of talc on the strengths of synthetic gouges of lizardite and antigorite serpentinites and of quartz small amounts of talc weaken serpentinite gouges substantially more than predicted by simple weight averaging in comparison mixtures of quartz and talc show a linear trend of strength reduction at talc concentrations 15 wt and enhanced weakening at higher concentrations all of the strength data are fit by a modified version of the reuss mixing law that allows for the dominance of one mineral over the other the difference in the behavior of serpentinite talc and quartz talc mixtures at low talc concentrations is a reflection of their different textures lizardite antigorite and talc all have platy habits and displacement within gouges composed of these minerals is localized to narrow shears along which the platy grains have rotated into alignment with the shear surfaces the shears in the mixed phyllosilicate gouges maximize the proportion of the weaker mineral within them when mixed with a strong rounded mineral such as quartz some minimum concentration of talc is needed to form connected pathways that enhance strength reductions the typical development of talc by the reaction of si rich fluids with serpentinite or dolomite would tend to localize its occurrence in a natural fault and result in enhanced weakening

influence of mg2 on the kinetics of calcite precipitation and calcite crystal morphology abstract the kinetics of calcite growth in the presence of mg2 has been studied by both a ph free drift method and by visual observation our experiments show that the calcite growth rate is reduced by the presence of mg2 the higher the mg ca ratio in the solution the lower the growth rate in a caco3 supersaturated solution the inhibition of calcite growth by the presence of mg2 is caused by mg2 being incorporated into the original calcite seed surfaces and developing new crystal surfaces the mg2 distribution on the calcite surfaces is not uniform as there is likely to be different mg2 densities on the newly developed surfaces than on the original surfaces of the calcite seeds this causes the calcite crystal morphology to change the significance of creating error in using the initial rate data rather than the equilibrium rate data for the prediction of the calcite precipitation kinetics is discussed

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

influence of pore fluid salt content on compaction creep of calcite aggregates in the presence of supercritical co2 abstract we have performed uniaxial compaction experiments on simulated porous carbonate rocks crushed and sieved carrara marble using pore fluids of varying salinity saturated with co2 in order to investigate the long term chemically coupled effects of co2 on creep of such rock materials experiments were performed at reservoir conditions equivalent to 3 km depth which is up to 30 mpa effective pressure and a temperature of 80 c on samples with mean grain sizes of 28 37 50 and 106 m the tests were carried out using saline pore fluid solutions containing 0 0 6 1 2 and 3 m nacl or 0 1 and 2 m mgcl2 with and without supercritical co2 at a pressure of 10 mpa addition of either salt nacl or mgcl2 to the pore fluid in samples compacted with co2 caused a reduction in compaction creep rate at low salt concentrations 0 6 1 m as salt concentration was increased to values of 2 3 m creep rates increased to approach or exceed values obtained in salt free experiments by comparison of our data on the compaction creep of carbonate aggregates in the presence of added salts and supercritical co2 with microphysical models and previous work on fluid enhanced deformation mechanisms we suggest that the effect of increasing salt concentration in samples tested with supercritical co2 is the net result of progressive inhibition of pressure solution and enhancement of the role of subcritical crack growth the present results imply that storage of co2 in highly porous carbonate formations increases creep rates by up to ten times depending on the salt concentration however it remains difficult to quantify the rate of compaction creep of specific carbonate reservoirs in the context of co2 storage as compaction behaviour is strongly influenced by porosity and other microstructural and compositional factors as well as by salinity

correlation between hydrocarbon reservoir properties and induced seismicity in the netherlands earthquakes induced by gas production are a social concern in the netherlands over the last two decades a total of about 350 such earthquakes have been recorded with magnitudes ranging up to 3 5 on richter s scale the new dutch mining law prescribes the operators to give a quantitative estimation of the likelihood of future seismic activity hazard and the associated damage risk this estimation has to be given for every onshore field producing or to be produced a traditional probabilistic seismic hazard analysis psha can not give an estimation of the hazard for a field before the occurrence of seismic activity we have therefore investigated the correlation between parameters related to reservoir and production properties and the occurrence of induced seismicity in a hydrocarbon field statistically using bayes theorem and the rule of succession three key parameters have been identified that show a good correlation with the occurrence of earthquakes pressure drop fault density of the reservoir and stiffness ratio between seal and reservoir rock based on the observed correlation a probability for the occurrence of earthquakes in fields that have no historical earthquake record has been calculated this has resulted in the definition of four groups of hydrocarbon fields having all a different probability 2006

the role of fault zone fabric and lithification state on frictional strength constitutive behavior and deformation microstructure 1 we examine the frictional behavior of a range of lithified rocks used as analogs for fault rocks cataclasites and ultracataclasites at seismogenic depths and compare them with gouge powders commonly used in experimental studies of faults at normal stresses of 50 mpa the frictional strength of lithified isotropic hard rocks is generally higher than their powdered equivalents whereas foliated phyllosilicate rich fault rocks are generally weaker than powdered fault gouge depending on foliation intensity most samples exhibit velocity strengthening frictional behavior in which sliding friction increases with slip velocity with velocity weakening limited to phyllosilicate poor samples this suggests that lithification of phyllosilicate rich fault gouge alone is insufficient to allow earthquake nucleation microstructural observations show prominent throughgoing shear planes and grain comminution in the r1 riedel orientation and some evidence of boundary shear in phyllosilicate poor samples while more complicated anastomosing features at lower angles are common for phyllosilicate rich samples comparison between powdered gouges of differing thicknesses shows that higher riedel shear angles correlate with lower apparent coefficients of friction in thick fault zones this suggests that the difference between the measured apparent friction and the true internal friction depends on the orientation of internal deformation structures consistent with theoretical considerations of stress rotation

induced seismicity in the geysers geothermal area california a simultaneous inversion for hypocenters velocities and station delays was performed from the p arrival times of 55 microearthquakes and four explosions distributed throughout a 90 by 100 km region centered at the geysers geothermal area in northern california the resulting one dimensional layered model shows velocity increasing with depth from 4 4 km s at the surface to 5 9 km s at 8 km and deeper we relocated 7215 earthquakes which occurred between may 1975 and february 1982 using the new velocity model these relocations show a significant spatial clustering near production wells in the steam field that was not apparent in earlier studies we infer from the spatial and temporal pattern of seismicity that the seismicity at the geysers is induced by geothermal production activities with the expansion of power production by approximately 70 in 1979 1980 seismic activity has developed near the new production areas seismicity has also expanded northwestward of the present production area although there are no active steam wells in this area statistical cross correlations between the number of earthquakes per month and both steam withdrawal per month and injection volume per month were calculated for the wells studied we find no consistent pattern of correlation between injection and seismicity likewise we find little or no correlation between steam withdrawal and seismicity for steam wells in production longer than 7 years two mechanisms remain plausible for inducing seismicity at the geysers volumetric contraction due to mass withdrawal may perturb the stress field and cause faulting in reservoir rock already near failure due to the regional stress field alternatively aseismic deformation due to regional tectonic stress may be converted to stick slip seismic deformation due to an increase of coefficient of friction along fault surfaces such as could occur from exsolution of dissolved silica into fractures neither mechanism would be likely to show strong correlation between production and seismicity rates however we expect seismicity to continue in the geysers and for seismicity to appear in areas where new production occurs

laboratory investigations of impact generated plasma the production of magnetic fields by hypervelocity meteoroid impact has been proposed to explain the presence of paleomagnetic fields in some lunar samples as well as on the lunar surface impact generated magnetic fields also may be significant for the paleomagnetic record on a variety of cratered surfaces in the solar system such as the moon mercury phobos and asteroids previous experiments with the two stage hydrogen light gas gun at the nasa ames vertical gun range demonstrated that hypervelocity impacts can produce impact generated magnetic fields by the expansion of an impact derived ionized vapor cloud impact generated plasma here we further characterize impact generated magnetic fields and plasma with hypervelocity impact experiments at the nasa ames vertical gun spherical projectiles 0 32 cm fe cu 0 64 cm al nylon with velocities from 5 2 to 6 0 km s were impacted at impact angles of 15 30 60 and 90 measured from horizontal into powdered dolomite silica sand and aluminum plate three sets of experiments using search coils as magnetic detectors characterized impact generated magnetic fields as functions of ambient field strength and orientation projectile target composition and impact angle experiments using langmuir probes indicated a charged particle density between 109 and 1011 ions el cm 3 and an electron temperature 4500 k of the impact generated plasma the inferred source of impact induced magnetic fields these new results demonstrate that impact generated magnetic fields at the laboratory scale exhibit spatial and temporal complexity dependent on impact angle velocity and projectile target composition thereby suggesting that crater related paleomagnetism associated with this mechanism should exhibit similar complexity with spatial wavelengths on the order of a fraction of the crater radius

electric fields cloud microphysics and reflectivity in anvils of florida thunderstorms into the interior electric fields very frequently increased abruptly from 1 to 10 kv m 1 even though the particle concentrations and radar reflectivity increased smoothly the abrupt increase in field usually occurred when the aircraft entered regions with a reflectivity of 10 15 dbz we suggest that the abrupt increase in electric field was because the charge advection from the convective core did not occur across the entire breadth of the anvil and because the advection of charge was not constant in time also some long lived anvils showed enhancement of electric field and reflectivity far downwind of the convective core screening layers were not detected near the edges of the anvils comparisons of electric field magnitude with particle concentration or reflectivity for a combined data set that included all anvil measurements showed a threshold behavior when the average reflectivity such as in a 3 km cube was less than approximately 5 dbz the electric field magnitude was 3 kv m 1 based on these

residual strength of kaolin and bentonite the influence of their constituent pore fluid the residual strength of clays is affected by their mineral composition and by the nature of their constituent pore fluid in principle both factors may influence either strength parameters or stresses but it is not yet completely clear how this influence works previous publications have elucidated some aspects of clay residual strength behaviour this paper investigates further the roles of minerals and pore fluid composition on the residual strength of clayey soils it describes and analyses tests on kaolin bentonite and their mixtures exposed to distilled water and sodium choride solutions with given concentrations the results show that the shear strength of kaolin is not affected by the solutions used whereas the residual strength of bentonite varies greatly because of the inward salt diffusion towards the clay further for the clay mixtures with any of the pore fluids considered the weaker component has the greater influence on the behaviour of the mixture la resistance residuelle des argile

frictional properties of a low angle normal fault under in situ conditions thermally activated velocity weakening the zuccale fault is a regional low angle normal fault exposed on the isle of elba in central italy that accommodated a total shear displacement of 6 8 km the fault zone structure and fault rocks formed at 8 km crustal depth the present day fault structure is the final product of several deformation processes superposed during the fault history in this study we report results from a series of rotary shear experiments performed on 1 mm thick powdered gouges made from several fault rock types obtained from the zuccale fault the tests were done under conditions ranging from room temperature to in situ conditions i e at temperatures up to 300 c applied normal stresses up to 150 mpa and fluid saturated the ratio of fluid pressure to normal stress was held constant at either 0 4 or 0 8 to simulate an overpressurized fault the samples were sheared at a constant sliding velocity of 10 m s for at least 5 mm after which a velocity stepping sequence from 1 to 300 m s was started to determine the velocity dependence of friction this can be represented by the rate and state parameter a b which was determined by an inversion of the data to the rate and state equations friction of the various fault rocks varies between 0 3 and 0 8 similar to values obtained in previous studies and decreases with increasing phyllosilicate content friction decreases mildly with temperature whereas normal stress and fluid pressure do not affect friction values systematically all samples exhibited velocity strengthening conditionally stable behavior under room temperature conditions and a b increased with increasing sliding velocity in contrast velocity weakening accompanied by stick slips was observed for the strongest samples at 300 c and sliding velocities below 10 m s an increase in fluid pressure under these conditions led to a further reduction in a b for all samples so that they exhibited unstable stick slip behavior at low sliding velocity the results suggest that phyllosilicate bearing fault rocks can deform by stable aseismic creep at low resolved shear stress and low shear rate conditions an increase in fluid pressure or loading of stronger portions could lead to a runaway instability the runaway instability might be limited in size because of 1 the fault heterogeneity 2 the observed strengthening at higher sliding velocities and 3 a co seismic drop in pore fluid pressure

slip weakening as a mechanism for slow earthquakes slow earthquakes form part of a spectrum of fault behaviour between steady creep and fast rupture during a normal earthquake laboratory simulations of slow slip in rock samples taken from the nankai subduction zone japan reveal similar characteristics to fast earthquakes implying that some slow slip events could be prematurely arrested earthquakes

fault structure frictional properties and mixed mode fault slip behavior abstract recent high resolution gps and seismological data reveal that tectonic faults exhibit complex multi mode slip behavior including earthquakes creep events slow and silent earthquakes low frequency events and earthquake afterslip the physical processes responsible for this range of behavior and the mechanisms that dictate fault slip rate or rupture propagation velocity are poorly understood one avenue for improving knowledge of these mechanisms involves coupling direct observations of ancient faults exhumed at the earth s surface with laboratory experiments on the frictional properties of the fault rocks here we show that fault zone structure has an important influence on mixed mode fault slip behavior our field studies depict a complex fault zone structure where foliated horizons surround meter to decameter sized lenses of competent material the foliated rocks are composed of weak mineral phases possess low frictional strength and exhibit inherently stable velocity strengthening frictional behavior in contrast the competent lenses are made of strong minerals possess high frictional strength and exhibit potentially unstable velocity weakening frictional behavior tectonic loading of this heterogeneous fault zone may initially result in fault creep along the weak and frictionally stable foliated horizons with continued deformation fault creep will concentrate stress within and around the strong and potentially unstable competent lenses which may lead to earthquake nucleation our studies provide field and mechanical constraints for complex mixed mode fault slip behavior ranging from repeating earthquakes to transient slip episodic slow slip and creep events

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

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

frictional strength of cuttings and core from safod drillhole phases 1 and 2 1 we investigated the frictional properties of drill cuttings and core obtained from 1 85 3 1 km true vertical depth in the safod scientific borehole in central california triaxial frictional sliding experiments were conducted on samples from primary lithologic units and significant shear zones including the inferred active trace of the san andreas fault the samples were deformed at room temperature under constant effective normal stresses of 10 40 and 80 mpa with axial shortening rates of 0 01 1 0 m s 1 the weakest samples were from shale claystone and siltstone units with friction coefficient 0 4 0 55 stronger samples were from quartzo feldspathic rocks with 0 6 materials tested from two shear zones at 2560 and 3067 m measured depth had 0 4 0 55 and velocity strengthening behavior consistent with fault creep at depths 4 km the coefficient of friction for bulk samples from the inferred trace of the san andreas fault was 0 6

effects of dissolution precipitation processes on the strength and mechanical behavior of quartz gouge at high temperature hydrothermal conditions laboratory experiments on simulated quartz gouges at a temperature of 927 c a confining pressure of 300 mpa and a pore water pressure of 200 mpa demonstrate the interactions between cataclastic and dissolution precipitation processes and their effects on the strength and mechanical behavior significant porosity reduction and microstructures including grain interlocking grain interpenetration and widespread growth of euhedral shaped grains indicate the operation of dissolution precipitation processes under conditions which favor the activity of dissolution precipitation processes i e for small grain size or at slow displacement rates deformation is distributed across the whole gouge layer and strength increases continuously with increasing displacement so that coefficients of friction reach to 0 7 in contrast for specimens with large grain size sheared at fast displacement rates deformation is localized along a gouge forcing block interface after a small amount of displacement and results in slip softening and subsequent quasi stable sliding with coefficients of friction as low as 0 45 analysis of our results indicates that where the kinetics of dissolution precipitation is sufficiently fast to partly accommodate shear strain the mechanical behavior and strength of quartz gouge can be characterized by slip hardening and high strength with velocity strengthening in contrast where cataclastic processes predominantly accommodate shear strain the mechanical behavior and strength can be characterized by slip softening and subsequent localized sliding with low strength and velocity strengthening

mechanics of seismic instabilities induced by the recovery of hydrocarbons we review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models poroelastic stress transfer and isostasy theory these three mechanisms trigger or induce seismic instabilities at both local scale d 5 km and at regional scale d 20 km the modeled changes in stress are small 1 mpa whatever the tectonic setting each mechanism corresponds to different production processes 1 local hydraulic fracturing due to fluid injection induces seismic slip on cracks ml 3 within the injected reservoir through decreasing the effective stress 2 pure fluid withdrawal causes pore pressure to decrease within the reservoir it triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion poroelastic mechanisms transfer this stress change from the reservoir to the surrounding levels whereml 5 seismic instabilities occur either above or below the reservoir 3 massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the upper crust it can induce larger earthquakes ml 6 at greater distance from the hydrocarbon fields than the two other mechanisms

the chemical and mineralogical controls upon the residual strength of pure and natural clays recent work has shown that the residual strength of clays is to a large extent determined by the type of clay minerals and the chemistry of the constituent pore water changes in either of these factors cause corresponding alterations in residual shear strength the findings have important implications for the seasonal reactivation of landslides and may also provide an explanation for the long term weathering and shear strength reduction of clay slopes results are presented here to demonstrate the separate effects of clay mineralogy and pore water chemistry upon the residual strength of pure and natural clays explanations are sought to identify the controls of these factors upon residual strength based on brief reviews of the shear strength and physico chemical behaviour of clays the findings strongly suggest that residual strength should be considered a dynamic property with increases or decreases in strength occurring in response to environmental change des etudes recentes ont montre que la resis

felt seismicity associated with shale gas hydraulic fracturing the first documented example in europe we describe the origin of felt seismicity during the hydraulic fracturing of the carboniferous bowland shale at the preese hall 1 exploration well near blackpool in the uk during 2011 the seismicity resulted from the interaction of hydraulic fracturing and a fault the location of which was unknown at the time but has subsequently been located and does not intersect the well borehole waveform cross correlation is used to detect 50 events in the sequence a representative hypocenter and strike slip focal mechanism is calculated using the best recorded seismic event the hypocenter is calculated to lie 300 400 m east and 330 360 m below the injection point and shown to lie on a fault imaged using 3 d seismic at a depth of about 2930 m the 3 d survey shows that not only the event hypocenter but also the focal mechanism correlates strongly with a subsequently identifiable transpressional fault formed during the late carboniferous variscan basin inversion

frictional behaviour of simulated quartz fault gouges under hydrothermal conditions results from ultra high strain rotary shear experiments abstract numerous experimental studies have been conducted to explore the mechanical behaviour and strength properties of rock materials most of these studies however do not take the effects of fluids into account though it has long been recognized that fluids might play an important role in the deformation of crustal rocks under brittle ductile conditions moreover all these studies are limited in the total shear strain that could be reached in this study we report on ultra high strain experiments performed on simulated quartz fault gouges in a rotary shear apparatus under hydrothermal conditions effective normal stresses of 20 100 mpa a fluid pressure of 200 mpa temperatures of 400 600 c sliding velocities of 0 01 1 m s and total shear strains up to 50 the experiments show strain hardening up to a shear strain of 0 6 1 8 followed by strain weakening of up to 30 towards a steady state value at a strain of 8 12 this strain weakening effect is much higher than previously reported for quartz gouge the steady state shear strength increases with decreasing grain size increasing sliding velocity and decreasing temperature the microstructure of the deformed quartz gouge is characterized by the presence of a through going boundary parallel y shear some samples also show riedel shears oriented oblique to the shear zone boundary deformation in these tests was largely by cataclastic processes with most displacement being accommodated along the boundary parallel y shear causing the marked weakening observed intergranular pressure solution did not accommodate significant shear strain but appears to play a role in smoothing and weakening the localized slip surface and in controlling gouge compaction

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