how wide is the calaveras fault zone evidence for distributed shear along a major fault in central california high angle and reverse faults offset pleistocene and holocene deposits across a zone several kilometres wide east of the calaveras fault near sunol california alignment of recent epicenters with these and other fault traces suggests that a band of contemporary deformation roughly parallels the calaveras fault these exposures provide evidence that active deformation associated with this major transverse fault is distributed over a broad zone this complex pattern of faulting suggests that other faults throughout central california may be more active than is recognized in seismic hazard assessments

the critical slip distance for seismic faulting experimentally based friction laws1 2 have been found to predict virtually the entire range of observed behaviour of natural faults3 4 these laws contain a critical slip distance l which plays a key role in determining the degree of fault instability the size of the zone of earthquake nucleation the frictional breakdown width and the proportion of pre and post seismic slip to co seismic slip in laboratory measurements l is found to be about 10 5m but modelling results show that it must be about 10 2m if natural earthquake behaviour is to be simulated the discovery that fault surfaces are fractal over the scale range 10 5 105 refs 5 6 even for faults with large net slip has confused the problem of scaling this parameter from laboratory experiments to natural faults because fractal surfaces have no characteristic length here i show that geometrically unmated fractal surfaces when in contact under a normal load develop a characteristic length in their contact because long wavelength apertures close under load whereas short wavelength apertures may remain open this critical distance may be identified with l and calculations based on fault topography data show that at seismogenic depths it will be in the range anticipated from the earthquake modelling studies

effects of simulated fault gouge on the sliding behavior of tennessee sandstone nonclay gouges stick slip has been considered to be a mechanism of shallow focus earthquakes along pre existing faults and thus laboratory studies relevant to stick slip may lead to a better understanding of the mechanisms of the earthquakes and eventually to an effective prediction and or control of them this paper deals with the effects of various simulated gouges on the sliding behavior of rocks with special emphasis on how fault gouge alters potentially unstable behavior of a brittle rock about 150 specimens of tennessee sandstone with various simulated gouges along a 35 precut are deformed dry in a triaxial apparatus at room temperature with shortening rate of about 2 10 4cm s and at confining pressures to 300 mpa tennessee sandstone is a brittle and strong rock quartz cemented quartzose sandstone which consistently shows stick slip without simulated gouge at confining pressures above 30 40 mpa sliding behavior with a gouge layer depends markedly on the material used to simulate the gouge experimental results from monomineralic gouges show that mohs hardness of a mineral is a useful parameter to predict the general sliding behavior the most unstable sliding is associated with gouge composed of minerals with intermediate hardness dolomite anhydrite and calcite these gouges undergo the transition from stable sliding to stick slip at pressures of 50 80 mpa gouge composed of hard mineral such as quartz or feldspar behave more stably but stick slip occurs at pressures above 250 mpa only stable sliding is observed for gouge composed of soft minerals like halite although anhydrite gouge shows violent stick slip at 100 mpa pressure whereas only stable sliding occurs for quartz at the same pressure when they are mixed the behavior changes systematically with composition the inclusion of only 10 anhydrite permits stick slip and hence even a minor constituent mineral of fault gouge may alter the sliding mode for gouge composed of two distinct layers the more stabilizing member of the two dominates the behavior and the behavior is considerably different from that of homogeneously mixed gouge with the same bulk composition this suggests the significance of internal structures of natural fault zone in understanding the effects of fault zone materials on the fault motion the gouge composed of calcite or a mineral harder than calcite all show high and approximately the same ultimate frictional strength with an exception of fluorite the frictional strength is intermediate for fluorite galena and barite gouges and it is low for halite gouge

a note on the stress dilatancy relation for simulated fault gouge theoretical constraints on the stress dilation relation for a deforming coulomb material requirev ifc 0 andv sin 1 m m always wherev is the dilation angle is the friction angle c is cohesion m is the maximum shear stress and m is the mean effective stress recent laboratory measurements of friction and dilatancy of simulated fault gouge show that small amplitude shear load cycling causes compaction and consolidation comparison of the data with theory indicates that such load cycling produces 1 increased coefficient of friction or friction angle 2 increased cohesion and 3 increased dilatancy rate or dilation angle under certain conditions of load cycling without significant plastic shear strain accumulation p 0 005 we find thatv exceeds both and in contrast to theory sin 1 m m this result is interpreted in terms of enhanced cohesion and overconsolidation which lead to residual stresses within the gouge an analogy is drawn between these special loading conditions and those extant on natural faults in particular our results imply that jostling and minor stress variations associated with microearthquakes may produce strengthening of fault gouge and changes in the fault zone s stress dilatancy relation hence compaction associated with microseismicity may lead to subsequent dilation of fault gouge even for faults with large displacement rates and large net offsets e g san andreas in regions where such dilation persists over sufficient displacements on the order of the critical slip distance for seismic faulting it may tend to inhibit unstable slip

three dimensional terrain corrections in resistivity surveys a three dimensional finite element computer algorithm which can accommodate arbitrarily complex topography and subsurface structure has been developed to model the resistivity response of the earth the algorithm has undergone extensive evaluation and is believed to provide accurate results for realistic earth models testing included comparison to scale model measurements analytically calculated solutions and results calculated numerically by other independent means computer modeling experiments have demonstrated that it is possible to remove the effect of topography on resistivity data even under conditions where such effects are large with respect to the subsurface responses this can be done without resorting to lengthy and costly trial and error computer modeling after correction the data can be interpreted as if the anomalies are due only to subsurface structure the results of case studies on field data measured in high relief topography indicate the following 1 pre or postsurvey 3 d compu

on the mechanics of earthquake afterslip we propose a model for earthquake afterslip based on rate and state variable friction laws in the model afterslip is attributed to the interaction of a velocity weakening region at depth within which earthquakes nucleate with an upper region of velocity strengthening frictional behavior the existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments in our model afterslip is the result of relaxation of a stress perturbation within the velocity strengthening region which arises when an earthquake propagates into that region from below we derive the stress perturbation and its decay from the friction constitutive law using a simple 1 degree of freedom approximation for the elastic interaction between the fault and its surroundings this approximation is based on thickness averaged displacements and slip velocities within the velocity strengthening region which is assumed to slip as a rigid block coseismic and postseismic slip are coupled through the thickness averaged stiffness k of the velocity strengthening region we assume k to be inversely proportional to the thickness of this region which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip we model the afterslip time histories of the 1966 parkfield and 1987 superstition hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults in accord with field observations our model predicts 1 that afterslip on some faults scales with the thickness of the unconsolidated sedimentary cover and 2 that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth velocity strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones about 500 m within their shallow regions 3 5 km combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip

three dimensional terrain corrections for mise a la masse and magnetometric resistivity surveys three dimensional modeling of topographic effects in mise a la masse and magnetometric resistivity surveys is accomplished using the surface integral equation method the technique provides a means for 1 analyzing these effects on earth models of homogeneous conductivity and 2 removing terrain effects from field data a new method combining current source images with surface charge is developed to treat the electric field boundary conditions at the air earth interface the method uses an image of each subsurface current source positioned above the surface so as to induce a surface charge distribution which approximately cancels the charge distribution induced by the subsurface current source the resulting residual surface charge distribution varies spatially more gradually than either of the original charge distributions and hence may be represented accurately on a coarsely segmented model surface with simple basis functions the topographic surface is modeled by a finite number of facets each wi

the effect of water on strain localization in calcite fault gouge sheared at seismic slip rates abstract strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating the earthquake energy budget and the evolution of fault rock microstructure to assess the nature of strain localization during shearing of calcite fault gouges high velocity v max 1 m s rotary shear experiments at normal stresses of 3 20 mpa were conducted under room dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers when sheared at 1 m s the room dry gouges showed a prolonged strengthening phase prior to dynamic weakening whereas the wet gouges weakened nearly instantaneously microstructural analysis revealed that a thin

computations of secondary potential for 3d dc resistivity modelling using an incomplete choleski conjugate gradient method an accurate and efficient 3d finite difference fd forward algorithm for dc resistivity modelling is developed in general the most time consuming part of fd calculation is to solve large sets of linear equations where a is a large sparse band symmetric matrix the direct method using complete choleski decomposition is quite slow and requires much more computer storage we have introduced a row indexed sparse storage mode to store the coefficient matrix a and an incomplete choleski conjugate gradient iccg method to solve the large linear systems by taking advantage of the matrix symmetry and sparsity the iccg method converges much more quickly and requires much less computer storage it takes approximately 15 s on a 533 mhz pentium computer for a grid with 46 020 nodes which is approximately 700 times faster than the direct method and 2 5 times faster than the symmetric successive over relaxation ssor conjugate gradient method compared with 3d finite element resistivity modelling with the improved iccg solver our algorithm is more efficient in terms of number of iterations and computer time in addition we solve for the secondary potential in 3d dc resistivity modelling by a simple manipulation of the fd equations two numerical examples of a two layered model and a vertical contact show that the method can achieve much higher accuracy than solving for the total potential directly with the same grid nodes in addition a 3d cubic body is simulated for which the dipole dipole apparent resistivities agree well with the results obtained with the finite element and integral equation methods in conclusion the combination of several techniques provides a rapid and accurate 3d fd forward modelling method which is fundamental to 3d resistivity inversion

constitutive behavior and stability of frictional sliding of granite an understanding of the frictional sliding on faults that can lead to earthquakes requires a knowledge of both constitutive behavior of the sliding surfaces and its mechanical interaction with the loading system we have determined the constitutive parameters for frictional sliding of initially bare surfaces of westerly granite using a recently developed high pressure rotary shear apparatus that allows long distances of sliding and therefore a greater assurance of attaining steady state behavior from experiments conducted at room temperature and normal stresses of 27 84 mpa several important results have been found 1 a gouge layer 100 to 200 m thick was developed from the initially bare rock surfaces after 18 to 70 mm of sliding 2 the steady state frictional resistance attained after about 10 mm of sliding is proportional to the negative of the logarithm of the sliding velocity 3 abrup changes in the velocity of sliding result in initial changes in the frictional resistance which have the same sign as the velocity change and are followed by a gradual decay to a new steady state value over a characteristic distance of sliding this velocity weakening behavior is essentially identical with that found by several previous workers on the same material at lower normal stress 4 our results are well described by a two state variable constitutive law the values of the constitutive parameters are quite similar to those found previously at low normal stress but the characteristic distance is about an order of magnitude smaller than that found at 10 mpa normal stress with thicker layers of coarser gouge 5 we have approximated our results with a one state variable constitutive law and compared the results with the predictions of existing nonlinear stability analysis in addition we have extended the stability analysis to systems possessing two state variables with such formulations good agreement is found between the experimentally observed and theoretically predicted transitions between stable and unstable sliding these results allow a better understanding of the instabilities that lead to earthquakes

dynamic modelling of induced seismicity by using seismic efficiency constraints and a new scaling law for slip weakening distance in the numerical simulation of induced seismicity much attention is generally paid to the calibration of the frictional resistance of the causative fault to obtain a seismic moment consistent with that of the actual event whereas sufficient investigation is not made in the estimation of the slip weakening distance dc as well as in the calibration of seismically radiated energy the present study addresses this problem by numerically and analytically investigating the relation between dc and seismic source parameters first this study performs the dynamic simulation of an induced seismic event caused by a decrease in the effective normal stress the analysis demonstrated that seismic efficiency can be used to improve the accuracy of estimating the critical slip weakening distance and the coefficient of kinetic friction d whilst considering not only seismic moment but also radiated energy in the calibration this gave insight into the development of the new calibration method for induced seismicity that considers energy related seismic source parameters furthermore a new scaling law of the slip weakening distance was derived from the theoretical expression of seismic efficiency considering seismic moment mo and scaled energy hat e the proposed scaling law can yield the relation between dc and mo which is shown to be similar to that obtained from a previous study but additionally considers the relation between seismically radiated energy and dc the dependency of dc on seismically radiated energy implied from the proposed scaling law has been verified from the dynamic analyses where 0 06 was used to place a constraint on dc for seismic events with different magnitudes the developed numerical simulation methodology of induced seismicity as well as the scaling law considering the energy indices significantly contributes to improving the accuracy of back analysis thus leading to a more accurate estimation of the mechanical properties of faults and or shear zones in seismically active regions of deep underground mines or reservoirs composed of discontinuous hard rock masses

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

effects of 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

direct shear tests on sand experimental data from unreinforced direct shear tests on sand are presented to show the influence of the boundary conditions on the test measurements the data and the theoretical considerations o

a damage mechanics model for fault zone friction a damage mechanics model for shear failure under compressive loading is used to calculate the shear strength of a fault based on field studies of the structure of natural fault zones the distribution of starter flaws initial damage is assumed to be fractal with dimension d f 2 6 for this fractal dimension the largest flaws dominate the fracture process at low confining stress and the shear strength scales as the inverse square root of the largest flaw size as in tensile loading at higher levels of confining stress appropriate to the base of the seismogenic zone the strength becomes independent of the distribution of flaw sizes and depends only on the density of starter flaws when the initial damage is sufficiently high the damage initiation surface coincides with the failure surface and the fault zone appears to obey the same friction law which controls slip on the individual microfractures the initial damage corresponding to the fractal distribution of flaws measured in a natural fault zone is large enough for this to occur

coulomb constitutive laws for friction contrasts in frictional behavior for distributed and localized shear we describe slip rate dependent friction laws based on the coulomb failure criteria frictional rate dependence is attributed to a rate dependence of cohesionc and friction angle we show that differences in the stress states developed during sliding result in different coulomb friction laws for distributed shear within a thick gouge layer versus localized shear within a narrow shear band or between bare rock surfaces for shear within gouge shear strength is given by c cos n sin whereas for shear between bare rock surfaces the shear strength is c cos n tan where and n are shear and normal stress respectively in the context of rate dependent coulomb friction laws these differences mean that for a given material and rate dependence of the coulomb parameters pervasive shear may exhibit velocity strengthening frictional behavior while localized shear exhibits velocity weakening behavior we derive from experimental data the slip rate dependence and evolution ofc and for distributed and localized shear the data show a positive rate dependence for distributed shear and a negative rate dependence for localized shear indicating that the rate dependence ofc and are not the same for distributed and localized shear even after accounting for differences in stress state our analysis is consistent with the well known association of instability with shear localization in simulated fault gouge and the observation that bare rock surfaces exhibit predominantly velocity weakening frictional behavior whereas simulated fault gouge exhibits velocity strengthening followed by a transition to velocity weakening with increasing displacement natural faults also exhibit displacement dependent frictional behavior and thus the results may prove useful in understanding the seismic evolution of faulting

practical techniques for 3d resistivity surveys and data inversion1 techniques to reduce the time needed to carry out 3d resistivity surveys with a moderate number 25 to 100 of electrodes and the computing time required to interpret the data have been developed the electrodes in a 3d survey are normally arranged in a square grid and the pole pole array is used to make the potential measurements the number of measurements required can be reduced to about one third of the maximum possible number without seriously degrading the resolution of the resulting inversion model by making measurements along the horizontal vertical and 45 diagonal rows of electrodes passing through the current electrode the smoothness constrained least squares inversion method is used for the data interpretation the computing time required by this technique can be greatly reduced by using a homogeneous half space as the starting model so that the jacobian matrix of partial derivatives can be calculated analytically a quasi newton updating method is then used to estimate the partial derivatives for subsequent iterations this inversion technique has been tested on synthetic and field data where a satisfactory model is obtained using a modest amount of computer time on an 80486dx2 66 microcomputer it takes about 20 minutes to invert the data from a 7 by 7 electrode survey grid using the techniques described below 3d resistivity surveys and data inversion can be carried out using commercially available field equipment and an inexpensive microcomputer

nucleation and arrest of dynamic slip on a pressurized fault 1 elevated pore pressure can lead to reactivation of slip on pre existing fractures and faults when the static coulomb failure is reached locally as the pressurized region spreads diffusively slip can accumulate quasi statically paced by the pore fluid diffusion or dynamically in this work we consider a prestressed fault with a locally peaked diffusively spreading pore pressure field to study 1 conditions leading to the escalation of slip and nucleation of dynamic rupture and 2 rupture run out distance before it is arrested nucleation appears in this model when the fault friction decreases from its peak value with slip while arrest of dynamic propagation is imminent on aseismic faults i e such that prestress b is less than the residual fault strength rat ambient conditions when fluid overpressure is a small to moderate fraction of the ambient value of normal effective stress and prestress is large enough for fault slip to be activated by overpressure dynamic rupture always nucleates and the nucleation length increases with decreasing prestress practically independently of the overpressure value transition from the ultimately unstable b r to the ultimately stable b r fault loading is marked by a strong increase of the nucleation length 1 b r 2 as b approaches r from above for aseismic faults b r no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress the largest run out distances of dynamic slip on aseismic faults correspond to overpressure prestress just sufficient for slip activation in such cases the dynamically accumulated slip can lead to enhanced dynamic fault weakening resulting in a sustained dynamic rupture and generating a large earthquake this is consistent with field observations when the largest injection induced seismicity occurred after fluid injection ended

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

effects of normal stress perturbations on the frictional properties of simulated faults 1 we report on laboratory experiments to investigate the frictional response of creeping faults to sudden changes in normal stress experiments were conducted on layers of quartz powder bare surfaces of westerly granite and layers of a 50 50 mixture of quartz powder and smectite clay powder the tests were carried out at room temperature and controlled humidity using a servo controlled double direct shear configuration normal stress perturbations corresponding to loading and unloading of tectonic fault zones were applied during steady sliding at constant loading rate from 3 to 1000 m s shear strain rates of 1 5 10 3 to 0 5 s 1 sudden changes in normal stress resulted in a linear elastic response of shear stress followed by a transient evolution of friction over a characteristic displacement the transient inelastic response is quantified as ln 0 where is the transient change in shear stress following a step change from initial normal stress 0 to final normal stress we find that is independent of sliding velocity and varies with ambient relative humidity and shear loading history for unloading we document a transition from stable to unstable behavior as a function of net slip in the range 3 to 30 mm shear strains of 1 5 to 15 increased humidity led to higher values of for pure quartz gouge but smaller for the quartz clay gouge the effects of shear displacement and humidity are discussed in the context of particle characteristics and gouge fabric development the extended rate and state dependent friction laws using one state variable and the ruina evolution law with normal stress variation describe our observations

evaluation of landfill disposal boundary by means of electrical resistivity imaging this paper deals with an employment of electrical resistivity imaging eri for survey of leachate content on the waste disposal site in northern israel the research consisted of conducting ten eri lines and drilling investigation wells data simulation used a 2d earthimager inversion program analysis of 2d eri interpretation results shows that determination of the boundary between the landfill body bottom intensively saturated with leachates and underlying layers of highly water saturated fat nonconsolidated clays presents a challenge however statistical analysis of eri data indicates that standard deviation and confidence interval of a set of resistivity data measured in the landfill body are significantly larger than those in underlying clays moreover maximum changes of these parameters are found on the boundary between landfill body and underlying soil thus reflecting natural differences in scattering of resistivity data measured in these two objects

the effects of sliding velocity on the frictional and physical properties of heated fault gouge the frictional properties of a crushed granite gouge and of gouges rich in montmorillonite illite and serpentine minerals have been investigated at temperatures as high as 600 c confining pressures as high as 2 5 kbar a pore pressure of 30 bar and sliding velocities of 4 8 and 4 8 10 2 m sec the gouges showed nearly identical strength behaviors at the two sliding velocities all four gouges however showed a greater tendency to stick slip movement and somewhat higher stress drops in the experiments at 4 8 10 2 m sec varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges the principal mineralogical difference was that at 400 c and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4 8 10 2 m sec but not in those at 4 8 m sec the textures developed in the gouge layers were in part functions of the gouge type and the temperature but changes in the sliding velocity affected among other features the degree of mineral deformation and the orientation of some fractures

earthquake aftereffects and triggered seismic phenomena the physical basis for delayed aftereffects of large crustal earthquakes is analyzed these effects apparently range in scale from typical aftershocks near the source region of a prior earthquake to the triggering of groundwater and deformation related anomalies possibly including other large earthquakes at locations as much as a few hundred kilometers from the source of a great earthquake observational reports of such aftereffects are reviewed and possible mechanisms for their occurrence are discussed

living microbial ecosystems within the active zone of catagenesis implications for feeding the deep biosphere earth s largest reactive carbon pool marine sedimentary organic matter becomes increasingly recalcitrant during burial making it almost inaccessible as a substrate for microorganisms and thereby limiting metabolic activity in the deep biosphere because elevated temperature acting over geological time leads to the massive thermal breakdown of the organic matter into volatiles including petroleum the question arises whether microorganisms can directly utilize these maturation products as a substrate while migrated thermogenic fluids are known to sustain microbial consortia in shallow sediments an in situ coupling of abiotic generation and microbial utilization has not been demonstrated here we show using a combination of basin modelling kinetic modelling geomicrobiology and biogeochemistry that microorganisms inhabit the active generation zone in the nankai trough offshore japan three sites from odp leg 190 have been evaluated namely 1173 1174 and 1177 drilled in nearly undeformed quaternary and tertiary sedimentary sequences seaward of the nankai trough itself paleotemperatures were reconstructed based on subsidence profiles compaction modelling present day heat flow downhole temperature measurements and organic maturity parameters today s heat flow distribution can be considered mainly conductive and is extremely high in places reaching 180 mw m2 the kinetic parameters describing total hydrocarbon generation determined by laboratory pyrolysis experiments were utilized by the model in order to predict the timing of generation in time and space the model predicts that the onset of present day generation lies between 300 and 500mbelow sea floor 5100 5300mbelow mean sea level depending on well location in the case of site 1174 5 10 conversion has taken place by a present day temperature of ca 85 c predictions were largely validated by on site hydrocarbon gas measurements viable organisms in the same depth range have been proven using 14c radiolabelled substrates for methanogenesis bacterial cell counts and intact phospholipids altogether these results point to an overlap of abiotic thermal degradation reactions going on in the same part of the sedimentary column as where a deep biosphere exists the organic matter preserved in nankai trough sediments is of the type that generates putative feedstocks for microbial activity namely oxygenated compounds and hydrocarbons furthermore the rates of thermal degradation calculated from the kinetic model closely resemble ratesof respiration and electron donor consumption independently measured in other deep biosphere environments we deduce that abiotically driven degradation reactions have provided substrates for microbial activity in deep sediments at this convergent continental margin

a digital linear filter for resistivity sounding with a generalized electrode array this paper presents a digital linear filter which maps composite resistivity transforms to apparent resistivities for any four electrode array over a horizontally layered earth a filter is provided for each of three sampling rates the choice of filter will depend on resistivity contrasts and computational facilities two methods of filter design are compared the wiener hopf least squares method is preferable for low sampling rate filters the fourier transform method is more successful in producing a filter with a high sampling rate which can handle resistivity contrasts of 100 000 1

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake