impact of poroelastic response of sandstones on geothermal power production during geothermal power production using a borehole doublet consisting of a production and injection well the reservoir conditions such as permeability k porosity and skempton coefficient b at the geothermal research site gross schoenebeck germany will change besides a temperature decrease at the injection well and a change of the chemical equilibrium also the pore pressure pp will vary in a range of approximately 44 mpa 10 mpa in our reservoir at 3850 to 4258 m depth this leads to a poroelastic response of the reservoir rocks depending on effective pressure p eff difference between mean stress and pore pressure resulting in a change in permeability k porosity and the poroelastic parameter skempton coefficient b hence we investigated the effective pressure dependency of flechtinger sandstone an outcropping equivalent of the reservoir rock via laboratory experiments the permeability decreased by 21 at an effective pressure range from 3 to 30 mpa the porosity decreased by 11 p eff 6 to 65 mpa and the skempton coefficient decreased by 24 p eff 4 to 25 mpa we will show which mechanisms lead to the change of the mentioned hydraulic and poroelastic parameters and the influence of these changes on the productivity of the reservoir the most significant changes occur at low effective pressures until 15 to 20 mpa for our in situ reservoir conditions p eff 43 mpa a change of 10 mpa effective pressure will result in a change in matrix permeability of less than 4 and in matrix porosity of less than 2 besides natural fracture systems fault zones and induced hydraulic fractures the rock matrix its only one part of geothermal systems all components can be influenced by pressure temperature and chemical reactions therefore the determined small poroelastic response of rock matrix does not significantly influence the sustainability of the geothermal reservoir

numerical simulation of seismic cycles with a composite rate and state dependent friction law properties of rate and state dependent friction laws are examined through numerical simulations among several versions of the rate and state dependent friction laws we use the slowness law the slip law and the composite law the last was recently proposed by kato and tullis 2001 for better explaining experimental observations of rock friction in the laboratory two kinds of simulations are done one is for cyclic stick slip motion of a single degree of freedom spring block system the other is for seismic cycles at a subduction zone where a 2d model of uniform elastic half space is used assuming the same values of friction parameters we compare the simulation results for the three friction laws from the two kinds of simulations we find that the recurrence interval of simulated earthquakes is the longest and the magnitude of preseismic sliding is the smallest for the composite law

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

analysis of a heavy rainfall event during tamex abstract a heavy rainfall event during the taiwan area mesoscale experiment intensive observing period 13 has been studied using upper air surface mesonet and dual doppler radar data the heavy rainfall 231 mm day 1 occurred over northwestern taiwan with the maximum rainfall along the northwestern coast and was caused by a long lived convective rainband in the prefrontal atmosphere it occurred in an upper level divergence region and along the axis of the maximum equivalent potential temperature at the 850 hpa level as a mei yu front advanced southeastward the postfrontal cold air in the lowest levels was retarded by the hilly terrain along the southeastern china coast as a result a low level wind shift line associated with a pressure trough at the 850 hpa level moved over the taiwan strait before the arrival of the surface front the westerly flow behind the trough interacted with a barrier jet along the northwestern coast of taiwan the barrier jet is caused by the interaction between the pref

linking dynamic elastic parameters to static state of stress toward an integrated approach to subsurface stress analysis abstract stress is the most important parameter to understand basin dynamics and the evolution of hydrocarbon systems the state of stress can be quantified by numerical geo mechanical modelling techniques these techniques require static elastic parameters of the rocks as input while tectonic and gravitational forces are given as explicit boundary conditions to compute the local state of stress at different scales we developed a technique to determine the density and elastic constants at seismic frequencies using full zoeppritz inversion on angle dependent seismic reflection data the dynamic elastic parameters as obtained from seismic data differ from their static equivalents which are necessary to determine the static state of stress the dynamic elastic parameters are related to their static equivalents through experimentally obtained relations in these rock physics experiments the static and dynamic elastic parameters are measured simultaneously during different external loading conditions the experiments used here are all carried out in a tri axial pressure machine under equal axial stresses then pre stack seismic data analysis in combination with the relation between the static and dynamic elastic parameters from the rock physics experiments provides the input parameters for geo mechanical modelling

statistical properties of seismicity of fault zones at different evolutionary stages summary we perform a systematic parameter space study of the seismic response of a large fault with different levels of heterogeneity using a 3 d elastic framework within the continuum limit the fault is governed by rate and state friction and simulations are performed for model realizations with frictional and large scale properties characterized by different ranges of size scales we use a number of seismicity and stress functions to characterize different types of seismic responses and test the correlation between hypocenter locations and the employed distributions of model parameters the simulated hypocenters are found to correlate significantly with small l values of the rate and state friction the final sizes of earthquakes are correlated with physical properties at their nucleation sites the obtained stacked scaling relations are overall self similar and have good correspondence with properties of natural earthquakes

historical pattern and behavior of earthquake ruptures along the eastern boundary of the sichuan yunnan faulted block southwestern china abstract the eastern boundary of the sichuan yunnan faulted block in southwestern china which contains four major fault zones named xianshuihe anninghe zemuhe and xiaojiang behaves as a huge left lateral strike slip active fault system where the most frequent seismicity in continental china occurs to uncover the history of earthquake ruptures of this fault system we developed an empirical relation between rupture extent and seismic intensity distribution based on data of those earthquakes whose rupture extents and intensity distributions are well known through analyzing various types of data including distributions of damage or intensity of historical earthquakes as well as surface ruptures and aftershock zones of modern earthquakes we determined systematically the locations and spatial extents of ruptures for 36 main earthquakes and built up a spatial temporal pattern of the rupture history of the fault system for the last several hundred years the spatial temporal rupture pattern contains information of multi cycle and broad scale ruptures from which we find that for most fault segments scales of segment ruptures are time or cycle variable and only in two or three cases do scales of segment ruptures in successive cycles seem to be characteristic in infrequent cases several adjacent and connecting fault units may rupture simultaneously resulting in cascading ruptures triggering of ruptures is common along the fault system but differs in various periods even on a single fault zone triggered ruptures may not occur on fault segments adjacent to a preceding rupture of major earthquake but on other segments at some distance away from the preceding rupture implying that the rupture history determines whether a segment can be triggered or not recurrence intervals of major segment ruptures are longer on the anninghe and zemuhe fault zones and on the southernmost segment of the xiaojiang fault zone than that on the xianshuihe fault zone and the northern and middle segments of the xiaojiang fault zone probably due to the partitioning of slip along major and secondary faults and their complicated fault geometry four seismic gaps along the fault system are recognized where major earthquakes have been absent for a long time

a composite rate and state dependent law for rock friction existing constitutive laws of rock friction cannot describe both observed strengthening in the absence of slip and frictional behavior around steady state sliding combining existing rate and state dependent friction laws we propose a new friction law which expresses logarithmically time dependent healing at sliding velocities smaller than a cut off velocity v c and approaches the so called slip law at velocities larger than v c numerical simulations show that the new friction law adequately explains experimental results of velocity stepping tests and slide hold slide tests which have widely been used to examine velocity time and displacement dependent property of rock friction because the new friction law has a simple form and appears appropriate for a wide range of conditions it is useful for application to numerical modeling of seismic cycles

geodetic measurement of crustal motion in southwest china global positioning system measurements performed over a 2 4 yr period confirm the xianshuihe xiaojiang fault system as one of the primary active structures in southwest china stations southwest of these faults show southerly motions of 5 15 mm yr relative to the western sichuan basin and stations in a 200 km geodetic network located northwest of the basin move at only 0 5 mm yr these results are consistent with clockwise rotation of southwestern sichuan and western yunnan about the eastern tibetan syntaxis accommodated by left lateral slip of 12 4 mm yr on the xianshuihe xiaojiang fault system the results imply that if eastward extrusion of crustal material from the plateau occurs at present it is not accommodated by east west shortening along the margin of the plateau and must involve wholesale eastward motion of low lying regions to the east

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

effective stress law for the permeability of clay rich sandstones 1 two models of clay rich sandstones are analyzed to explain the relative sensitivity of permeability to pore pressure and confining pressure in one model the clay lines the entire pore wall in a layer of uniform thickness and in the second model the clay is distributed in the form of particles that are only weakly coupled to the pore walls equations of elasticity and fluid flow are solved for both models giving expressions for the effective stress coefficients in terms of clay content and the elastic moduli of the rock and clay both models predict that the permeability will be much more sensitive to changes in pore pressure than to changes in confining pressure the clay particle model gives somewhat better agreement with data from the literature and with new data on a stainton sandstone having a solid volume fraction of 8 clay

earthquake depths and the relation to strain accumulation and stress near strike slip faults in southern california earthquakes in the major fault zones are predominantly deep earthquakes in the crustal blocks bounding the fault zones are predominantly shallow in the san jacinto fault zone maximum earthquake depths correlate with surface heat flow these relations together with focal mechanisms geodetic strain measurements and fault zone models are consistent with the following ideas 1 interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes 2 the aseismic slip in a narrow fault zone in the brittle plastic transition region concentrates strain at the base of the brittle fault zone 3 deep earthquakes occur in thelower part of the brittle fault zone due to stick slip failure of highly stressed patches 4 background earhtquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding deep and stick slip shallow regions of the fault zone furthermore the difference in seismicity between the san jacinto and southern san andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases 5 shear stress may be lessmore in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks 6 in the stress field produced by plate motion and slip in the deep fault zone the upper brittle fault zone is not oriented favorably for shear failure lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation less

undrained poroelastic response of sandstones to deviatoric stress change deformation of porous crustal rock through diagenesis tectonic loading or other processes can change pore volume and affect fluid pressure the largest stress induced pore pressure changes occur when fluid is trapped in pores in an undrained condition we have measured the undrained poroelastic response of two sandstones to changes in mean and deviatoric stress pore pressure was found to respond to mean stress m in the usual manner p b m b ranging from 0 4 to 0 7 nearly independent of the ambient deviatoric stress state however variations in deviatoric stress d 1 3 2 at constant mean stress were also found to induce a reversible elastic pore pressure response to stress levels up to and exceeding 80 percent failure stress i e p d m const the coefficient became more negative with increasing deviatoric stress level in sandstone and ottawa sand samples that is represents a dilatant response where increased deviatoric stress causes a decrease in pore pressure the poroelastic response to deviatoric stress is explained in terms of anisotropic matrix stiffening due to closure of crack like pore space or flattening of grain contacts at high ambient stress levels and can be important in calculations of earthquake stress transfer

a numerical study of traveling microbursts abstract an analysis of traveling microbursts in unidirectionally sheared environments is undertaken using a three dimensional numerical model with 50 m resolution in a 19 12 4 km domain for each run the cooling source is centered at a height of 2 km and travels in an eastward direction of cm where cm 3 6 9 12 and 15 m s 1 environmental winds above 2 km are equal to cm and decay linearly to 0 m s 1 below 2 km the authors examine the kinetic energy budget of each run focusing on the dynamic features that are not found in a static microburst simulation as the source speed cm increases from 0 to 9 m s 1 the magnitude of the surface horizontal winds increase in the direction of source movement an examination of the dynamic pressure equation shows that rotationally induced pressure work forces are primarily responsible for increasing surface horizontal winds for the moving source microbursts in a similar form to previous studies of vertical perturbations in a sheared environment elevated h

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

hydraulic fracturing in a sedimentary geothermal reservoir results and implications field experiments in a geothermal research well were conducted to enhance the inflow performance of a clastic sedimentary reservoir section due to depths exceeding 4050 m bottom hole temperatures exceeding 140 c and open hole section dual zone technically demanding and somewhat unprecedented conditions had to be managed the fracturing operations were successful fractures were created in two isolated borehole intervals and the inflow behaviour of the reservoir was decisively enhanced the effective pressures applied for fracture initiation and propagation were only slightly above in situ pore pressures nevertheless the stimulation ratio predicted by fracture performance modelling could not be achieved multiple reasons could be identified that account for the mismatch an insufficient fracture tie back as well as chemical and mechanical processes during closure led to reduced fracture conductivities and therefore diminished productivity the insights gained are the basis for further fracture design concepts at the given and geologic comparable sites

modeling porous rock fracturing induced by fluid injection abstract in this paper we describe and implement a theoretical model that captures the physical processes occurring during hydraulic fracturing of saturated and unsaturated porous rock the model is based on a poro elastic plastic rheology with a mohr coulomb yield function and unassociated flow rule and includes frictional hardening cohesion weakening damage and mobilized dilatancy effects in addition fluid injection is described using the richard s approximation we numerically implement the model using a finite difference scheme on a staggered grid and compare the model results with laboratory experiments published in stanchits et al 2011 16 from various experiments performed by stanchits et al we use three different laboratory configurations for comparing our model results and observations a triaxial compression of a drained rock sample b low pressure fluid injection into a drained critically stressed rock and c high pressure fluid injection into a saturated rock sample we find good agreement between model and observations for all cases indicating a proper formulation and implementation of the dominant physical mechanisms acting the model reproduces experimental observations of macroscopic fracture fluid front localization and the stress strain response curves matching observations from laboratory scale experiments establishes a benchmark and a calibrated model for numerically simulating experiments performed at the larger field scale

the evolution and structure of a bow echo microburst event part i the microburst abstract on 14 july 1982 a comprehensive multi doppler radar dataset was collected during the life cycle of an intense microburst producing thunderstorm during the joint airport weather studies jaws project this is believed to be one of the first attempts to study the temporal and spatial evolution of an entire microburst producing thunderstorm in addition the radar echo of the parent storm evolved into a bow shaped echo thus providing the first detailed dataset on this phenomenon this microburst was numerically simulated using srivastava s one dimensional downdraft model in an attempt to quantify microphysical processes and downdraft propagation in the subcloud layer analysis of the stormwide structure using a standard multi doppler kinematic analysis technique and the gal chen thermodynamic retrieval technique is stressed in this paper which is part 1 of a two part study part ii examines the evolution of the stormwide vorticity during the formation of the bow echo the microburst downdraft wa

numerical simulation of tornadogenesis in a high precipitation supercell part i storm evolution and transition into a bow echo abstract a nested grid primitive equation model rams version 3b was used to simulate a high precipitation hp supercell which produced two weak tornadoes six telescoping nested grids allowed atmospheric flows ranging from the synoptic scale down to the tornadic scale to be represented in the simulation all convection in the simulation was initiated with resolved vertical motion and subsequent condensation latent heating from the model microphysics no warm bubbles or cumulus parameterizations were used part i of this study focuses on the simulated storm evolution and its transition into a bow echo the simulation initially produced a classic supercell that developed at the intersection between a stationary front and an outflow boundary as the simulation progressed additional storms developed and interacted with the main storm to produce a single supercell this storm had many characteristics of an hp supercell and eventually evolved into a bow echo with a rotating comma head structure an analysi

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

thermo poroelastic numerical modelling for enhanced geothermal system performance case study of the gros schonebeck reservoir abstract significant pressure and temperature changes can occur within geothermal reservoirs caused by injection and production of fluid which can affect transport properties of the rocks and therefore alter reservoir performance and sustainability to understand the coupling between transport properties evolution and state variable changes a complete description of the mechanical behavior of the reservoir is required which should consider thermo and poroelastic effects this study aims to integrate transport properties evolution for coupled thermo hydro mechanical thm process modelling of fluid bearing reservoirs this approach is here applied to the geothermal research site of gros schonebeck 40 km north of berlin germany which consists of a doublet system at a target depth of about 4100 m in which both injection and production wells have been hydraulically stimulated a 3d reservoir model including the main geological units major natural fault zones and hydraulic fractures is integrated in the finite element method based simulator opengeosys for modelling coupled thm processes during geothermal activity one challenge of this study is to integrate both hydro geological and physical complexity to better describe the dynamic behavior of the geothermal reservoir from the results of the simulation thermal breakthrough is observed after 18 years of injection and life time of the system has been evaluated as 50 years furthermore a 5 5 increase of porosity around the injection well is observed as well as an increase of the anisotropy ratio for permeability k z k xy of about 2 these transport properties enhancements lead to a decrease of the thermal breakthrough time around 8 and life time of the system 14 compared to classic thermo hydro simulations with constant transport properties the results presented here provide therefore valuable insights for understanding porosity and permeability distributions and evolutions during injection and production of geothermal fluids and related impacts on reservoir performance

a constitutive scaling law and a unified comprehension for frictional slip failure shear fracture of intact rock and earthquake rupture 1 it is widely recognized that some of the physical quantities inherent in a rupture are scale dependent and the scale dependence is one of the most no facts and features of rupture phenomena the paper addresses how such scale dependent shear rupture of a broad range from laboratory scale frictional slip failure and shear fracture of intact rock to field scale rupture as an earthquake source can be unified by a single constitutive law noting that the earthquake rupture is a mixed process between frictional slip failure and the shear fracture of intact rock it is concluded that the constitutive law for the earthquake rupture be formulated as a unifying law that governs both frictional slip failure and shear fracture of intact rock it is demonstrated that the slip dependent constitutive law is such a unifying law and a constitutive scaling law is derived from laboratory data on both frictional slip failure and shear fracture of intact rock this constitutive scaling law enables one to provide a consistent and unified comprehension for scale dependent physical quantities inherent in the rupture over a broad range from small scale frictional slip failure and shear fracture in the laboratory to large scale earthquake rupture in the field

fracture energies at the rupture nucleation points of large interplate earthquakes abstract numerical simulations of recurrence of interplate earthquakes in a subduction zone are conducted using a rate and state dependent friction law to estimate the fracture energies at the rupture nucleation points during an interseismic period shear stress is concentrated near the downdip of the locked portion of the plate boundary and seismic rupture initiates when the energy release rate by rupture extension exceeds the fracture energy the simulation results indicate that the fracture energy is proportional to the square of the amount of slip in the deep aseismic zone during the preceding interseismic period this suggests that the fracture energy at the nucleation point can be estimated from the recurrence interval of earthquakes and the relative plate velocity the fracture energies at the rupture nucleation points of great m w 8 nankai earthquakes in southwestern japan are estimated to be 0 1 1 mj m 2 and that of the 2011 m w 9 tohoku oki earthquake to be of the order of 10 mj m 2 the results suggest that fracture energy tends to increase with earthquake magnitude

the role of offshore convergence on coastal rainfall during tamex iop 3 abstract the role of offshore convergence on the coastal rainfall maximum along the northwestern coast of taiwan is analyzed based on tamex taiwan area mesoscale experiment data and numerical experiments using the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 from a case study during tamex iop 3 moderate rainfalls 40 mm h 1 are observed along the northwestern coast of taiwan associated with the arrival of three rainbands in succession during 1000 1600 lst these rainbands form over the taiwan strait with northeast southwest or north south orientation they intensify off the northwestern coast there is a tendency for the most intense echoes to align in a northeast southwest orientation off the northwestern coast an orographically enhanced convergence zone with a northeast southwest orientation occurs in the area where the deflected southerly flow converges with the prevailing southwesterly flow that is modified by the storm

seismicity on a fault controlled by rate and state dependent friction with spatial variations of the critical slip distance 1 we perform systematic simulations of slip using a quasi dynamic continuum model of a two dimensional 2 d strike slip fault governed by rate and state dependent friction the depth dependence of the a b and l frictional parameters are treated in an innovative way that is consistent with available laboratory data and multidisciplinary field observations various realizations of heterogeneous l distributions are used to study effects of structural variations of fault zones on spatiotemporal evolution of slip we demonstrate that such realizations can produce within the continuum class of models realistic features of seismicity and slip distributions on a fault we explore effects of three types of variable l distributions 1 a depth dependent l profile accounting for the variable width of fault zones with depth 2 uncorrelated 2 d random distributions of l with different degrees of heterogeneity and 3 a hybrid distribution combining the depth dependent l profile with the 2 d random l distributions the first type of l distribution with relatively small l over the depth range corresponding to the seismogenic zone and larger l elsewhere generates stick slip events in the seismogenic zone and ongoing creep above and below that region the 2 d heterogeneous parameterizations generate frequency size statistics with event sizes spanning 4 orders of magnitude our results indicate that different degrees of heterogeneity of l distributions control 1 the number of simulated events and 2 the overall stress level and fluctuations other observable trends are 3 the dependency of hypocenter location on l and 4 different nucleation phases for small and large events in heterogeneous distributions

a poroelastic description of permeability evolution pore pressure changes in a geothermal reservoir as a result of injection and or production of water result in changes of stress acting on the reservoir rock and consequently changes in the mechanical and transport properties of the rock bulk modulus and permeability were measured at different pressures and temperatures an outcropping equivalent of rotliegend reservoir rock in the north german basin flechtinger sandstone was used to perform hydrostatic tests and steady state fluid flow tests permeability measurements were conducted while cycling confining pressure the dependence of permeability on stress was determined at a constant downstream pressure of 1 mpa also temperature was increased stepwise from 30 to 140 c and crack porosity was calculated at different temperatures although changes in the volumes of cracks are not significant the cracks control fluid flow pathways and consequently the permeability of the rock a new model was derived which relates microstructure of porosity the stress strain curve and permeability porosity change was described by the first derivative of the stress strain curve permeability evolution was ascribed to crack closure and was related to the second derivative of the stress strain curve the porosity and permeability of flechtinger sandstone were reduced by increasing the effective pressure and decreased after each pressure cycle

estimation of radiated energy of finite source earthquake models we estimated seismically radiated energy for kinematic earthquake rupture models instead of integrating far field waveforms we used an expression for radiated energy with slip and stress on the fault plane using an analytic crack model of sato and hirasawa 1973 we show that the finite difference method can estimate the radiated energy fairly well however if the rupture propagation velocity is near the s wave velocity and the grid interval is coarse we may underestimate the radiated energy substantially we analyzed five models of three earthquakes of m w 5 7 for the 1995 kobe earthquake we used three different kinematic models and obtained consistent values of radiated energy of 2 4 3 6 1014 j there are common features among the three models 1 more efficient energy radiation from the deeper regions of the fault and 2 little energy radiation beneath the city of kobe the models of the kobe earthquake and the other two earthquakes show that energy radiation is concentrated near the hypocenter and the initiation point of asperities we also find the area of negative energy radiation energy absorption near the edge of large slipped areas although our estimates tend to be smaller by about a factor of 3 than previous estimates the difference is reasonable because the frequency range is not complete and we cannot deal with detailed rupture behavior such as rapid rupture acceleration and deceleration manuscript received 18 january 2002

field study of a highly active fault zone the xianshuihe fault of southwestern china the xianshuihe fault of western sichuan province china is one of the world s most active faults having produced 4 earthquakes during this century of magnitude 7 along a 350 km length of the fault at least 8 such events have occurred since 1725 in the more limited 150 km long segment including luhuo and daofu major earthquakes in 1904 1923 1973 and 1981 m 7 7 7 6 6 9 were associated with overlapping surficial fault ruptures and with individual left lateral displacements as large as 3 6 m field studies indicate that this high degree of activity is typical of the fault s longer term history the holocene left lateral slip rate on the north western segment of the fault has been 15 5 mm yr decreasing to about 5 mm yr on its southeastern segment based on radiometrically dated offset stream channel and terrace deposits and on offset glacial moraines physiographic features of active faulting are fully as diagrammatic as those of california s san andreas fault mainly because of high altitude preservation and the absence of cultural modification on this eastern margin of the tibetan plateau detailed en echelon tensional and pushup features resulting from surface ruptures in 1973 1955 1923 and 1893 can still be recognized today and new data have been collected bearing on the offsets and fault rupture lengths during these and other events the locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the xianshuihe fault obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events whereas segmentation control for others remains obscure based on the historic record repeat times estimated from slip rates and current seismic gaps two segments are particularly likely sites for m 7 earthquakes in the near future the 65 km long segment between daofu and qianning and the 135 km long segment bracketing kangding continuing creep has been documented along some segments of the fault and this together with the high degree of activity and other unique attributes makes the xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard evaluation studies

a new method to estimate location and slip of simulated rock failure events abstract at the laboratory scale identifying and locating acoustic emissions aes is a common method for short term prediction of failure in geomaterials above average ae typically precedes the failure process and is easily measured at larger scales increase in micro seismic activity sometimes precedes large earthquakes e g tohoku l aquilla oceanic transforms and can be used to assess seismic risk the goal of this work is to develop a methodology and numerical algorithms for extracting a measurable quantity analogous to ae arising from the solution of equations governing rock deformation since there is no physical property to quantify ae derivable from the governing equations an appropriate rock mechanical analog needs to be found in this work we identify a general behavior of the ae generation process preceding rock failure this behavior includes arbitrary localization of low magnitude events during pre failure stage followed by increase in number and amplitude and finally localization around the incipient failure plane during macroscopic failure we propose deviatoric strain rate as the numerical analog that mimics this behavior and develop two different algorithms designed to detect rapid increases in deviatoric strain using moving averages the numerical model solves a fully poro elasto plastic continuum model and is coupled to a two phase flow model we test our model by comparing simulation results with experimental data of drained compression and of fluid injection experiments we find for both cases that occurrence and amplitude of our ae analog mimic the observed general behavior of the ae generation process our technique can be extended to modeling at the field scale possibly providing a mechanistic basis for seismic hazard assessment from seismicity that occasionally precedes large earthquakes

fluid circulation in the depths of accretionary prisms an example of the shimanto belt kyushu japan accretionary prisms constitute ideal targets to study fluid circulation and fluid rock interactions at depths beyond the reach of active margin deep drilling the highest grade rocks from the shimanto belt on kyushu were buried under 3 5 kbars at 300 c toriumi and teruya 1988 they contain abundant quartz veins formed throughout burial and exhumation and variably affected by brittle and ductile deformation cathodoluminescence cl reveals the existence of two distinct types of quartz characterized by a blue and brown color respectively cl blue quartz fills macro veins width 10 m while cl brown quartz is present in micro veins width 1 10 m and ductilely recrystallized domains on the basis of microstructures the fluids associated with the cl blue and cl brown quartz are interpreted as external and local respectively quartz growth rims of alternating cl colors as well as mutually cross cutting veins show that the two fluids cyclically wetted the host rock from fluid inclusions analysis the fluid associated with cl blue quartz has a salinity similar to seawater while the fluid associated with cl brown quartz is less saline in addition cl blue quartz is richer in aluminum than the cl brown one in contrast to the salinity aluminum signature the 18o isotopic signature of both quartz types is similar and buffered by host rock the difference between the preservation of the salinity signature of the fluid and the loss of its 18o signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration the local fluid associated with cl brown quartz reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals the external fluid associated with cl blue quartz is interpreted as seawater or pore water from shallow depth 1 2 km below seafloor sediments we propose that downward percolation of shallow water to depths 10km is a transient process associated with mega earthquakes

geodetic imaging of potential seismogenic asperities on the xianshuihe anninghe zemuhe fault system southwest china with a new 3 d viscoelastic interseismic coupling model we use gps and interferometric synthetic aperture radar insar measurements to image the spatial variation of interseismic coupling on the xianshuihe anninghe zemuhe xaz fault system a new 3 d viscoelastic interseismic deformation model is developed to infer the rotation and strain rates of blocks postseismic viscoelastic relaxation and interseismic slip deficit on the fault surface discretized with triangular dislocation patches the inversions of synthetic data show that the optimal weight ratio and smoothing factor are both 1 the successive joint inversions of geodetic data with different viscosities reveal six potential fully coupled asperities on the xaz fault system among them the potential asperity between shimian and mianning which does not exist in the case of 1019 pa s is confirmed by the published microearthquake depth profile besides there is another potential partially coupled asperity between daofu and kangding with a length scale up to 140 km all these asperity sizes are larger than the minimum resolvable wavelength the minimum and maximum slip deficit rates near the moxi town are 7 0 and 12 7 mm yr respectively different viscosities have little influence on the roughness of the slip deficit rate distribution and the fitting residuals which probably suggests that our observations cannot provide a good constraint on the viscosity of the middle lower crust the calculation of seismic moment accumulation on each segment indicates that the songlinkou selaha s4 shimian mianning s7 and mianning xichang s8 segments are very close to the rupture of characteristic earthquakes however the confidence level is confined by sparse near fault observations

fracture energies at the rupture nucleation points of large strike slip earthquakes on the xianshuihe fault southwestern china abstract earthquake cycles along a pure strike slip fault were numerically simulated using a rate and state dependent friction law to obtain the fracture energies at the rupture nucleation points in the model deep aseismic slip is imposed on the fault which generates recurrent earthquakes in the shallower velocity weakening friction region the fracture energy at the rupture nucleation point for each simulated earthquake was calculated using the relation between shear stress and slip which indicates slip weakening behavior the simulation results show that the relation between the fracture energy at the nucleation point and other source parameters is consistent with a theoretical approach based on fracture mechanics in that an earthquake occurs when the energy release rate at the tip of the aseismic slip zone first exceeds the fracture energy because the energy release rate is proportional to the square of the amount of deep aseismic slip during the interseismic period which can be estimated from the recurrence interval of earthquakes and the deep aseismic slip rate the fracture energies for strike slip earthquakes can be calculated using this result we estimated the fracture energies at the nucleation points of large earthquakes on selected segments of the xianshuihe fault southwestern china we find that the estimated fracture energies at the rupture nucleation points are generally smaller than the values of average fracture energy for developed ruptures as estimated in previous studies suggesting that the fracture energy tends to increase with the rupture propagation distance

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

variation of permeability with porosity in sandstone diagenesis interpreted with a fractal pore space model permeability is one of the key rock properties for the management of hydrocarbon and geothermal reservoirs as well as for aquifers the fundamental equation for estimating permeability is the kozeny carman equation it is based on a capillary bundle model and relates permeability to porosity tortuosity and an effective hydraulic pore radius which is defined by this equation whereas in clean sands the effective pore radius can be replaced by the specific surface or by the grain radius in a simple way the resulting equations for permeability cannot be applied to consolidated rocks based on a fractal model for porous media equations were therefore developed which adjust the measure of the specific surface and of the grain radius to the resolution length appropriate for the hydraulic process these equations are calibrated by a large data set for permeability formation factor and porosity determined on sedimentary rocks this fractal model yields tortuosity and effective pore radius as functions of porosity as well as a general permeability porosity relationship the coefficients of which are characteristic for different rock types it can be applied to interpret the diagenetic evolution of the pore space of sedimentary rocks due to mechanical and chemical compaction with respect to porosity and permeability

forecast model for great earthquakes at the nankai trough subduction zone an earthquake instability model is formulated for recurring great earthquakes at the nankai trough subduction zone in southwest japan the model is quasistatic two dimensional and has a displacement and velocity dependent constitutive law applied at the fault plane a constant rate of fault slip at depth represents forcing due to relative motion of the philippine sea and eurasian plates the model simulates fault slip and stress for all parts of repeated earthquake cycles including post inter pre and coseismic stages calculated ground uplift is in agreement with most of the main features of elevation changes observed before and after the m 8 1 1946 nankaido earthquake in model simulations accelerating fault slip has two time scales the first time scale is several years long and is interpreted as an intermediate term precursor the second time scale is a few days long and is interpreted as a short term precursor accelerating fault slip on both time scales causes anomalous elevation changes of the ground surface over the fault plane of 100 mm or less within 50 km of the fault trace

earthquake fracture energy inferred from kinematic rupture models on extended faults 1 we estimate fracture energy on extended faults for several recent earthquakes by retrieving dynamic traction evolution at each point on the fault plane from slip history imaged by inverting ground motion waveforms we define the breakdown work wb as the excess of work over some minimum traction level achieved during slip wb is equivalent to seismological fracture energy g in previous investigations our numerical approach uses slip velocity as a boundary condition on the fault we employ a three dimensional finite difference algorithm to compute the dynamic traction evolution in the time domain during the earthquake rupture we estimate wb by calculating the scalar product between dynamic traction and slip velocity vectors this approach does not require specifying a constitutive law and assuming dynamic traction to be collinear with slip velocity if these vectors are not collinear the inferred breakdown work depends on the initial traction level we show that breakdown work depends on the square of slip the spatial distribution of breakdown work in a single earthquake is strongly correlated with the slip distribution breakdown work density and its integral over the fault breakdown energy scale with seismic moment according to a power law with exponent 0 59 and 1 18 respectively our estimates of breakdown work range between 4 105 and 2 107 j m2 for earthquakes having moment magnitudes between 5 6 and 7 2 we also compare our inferred values with geologic surface energies this comparison might suggest that breakdown work for large earthquakes goes primarily into heat production

effects of single vertical fluid filled fractures on full waveform dipole sonic logs we conducted a parametric study of the effects of single finite width adjacent vertical fluid filled fractures on full wave borehole dipole logs using the transformed boundary integral equation method we found that a fracture has significant effects on dipole response if it is within half the wavelength or two to three times the borehole diameter from the borehole center dual flexural waves and a leaky fracture mode resulted from the waveguide composed of the borehole and fracture the first flexural wave was controlled by both the borehole and fracture whereas the second flexural wave was controlled primarily by the borehole but influenced by the fracture the separation of these two flexural waves increased and the strength of the first one decreased when the distance between the fracture and the borehole increased the leaky fracture mode was represented by a sharp minimum in the amplitude spectrum and energy leakage to the fracture reached a maximum when the fracture intersected the borehole

laboratory measurements of a complete set of poroelastic moduli for berea sandstone and indiana limestone measurements have been completed for eight different poroelastic moduli of water saturated berea sandstone and indiana limestone as a function of confining pressure and pore pressure the poroelastic moduli for indiana limestone are generally consistent to 10 which was verified by a formal inversion procedure for independent moduli from the eight measurements for indiana limestone best fit values were drained bulk modulus 21 2 gpa the undrained bulk modulus 31 7 gpa drained poisson s ratio 0 26 undrained poisson s ratio 0 33 and pore pressure buildup coefficient 0 47 at 20 35 mpa effective stress the poroelastic moduli for berea sandstone are generally consistent to 20 the greater inconsistency is most likely caused by the nonlinear variation of the moduli at different strains for berea sandstone best fit values were drained bulk modulus 6 6 gpa undrained bulk modulus 15 8 gpa drained poisson s ratio 0 17 undrained poisson s ratio 0 34 and pore pressure buildup coefficient 0 75 at 10 mpa effective stress

the stress regime in a rotliegend reservoir of the northeast german basin in situ stresses have significant impact either positive or negative on the short and long term behaviour of fractured reservoirs the knowledge of the stress conditions are therefore important for planning and utilization of man made geothermal reservoirs the geothermal field gros schonebeck 40 km north of berlin germany belongs to the key sites in the northeastern german basin we present a stress state determination for this lower permian rotliegend reservoir by an integrated approach of 3d structural modelling 3d fault mapping stress ratio definition based on frictional constraints and slip tendency analysis the results indicate stress ratios of the minimum horizontal stress s hmin being equal or increasing 0 55 times the amount of the vertical stress s v s hmin 0 55s v and of the maximum horizontal stress s hmax 0 78 1 00s v in stress regimes from normal to strike slip faulting thus acting stresses in the 4 100 m deep reservoir are s v 100 mpa s hmin 55 mpa and s hmax 78 100 mpa values from hydraulic fracturing support these results various fault sets of the reservoir are characterized in terms of their potential to conduct geothermal fluids based on their slip and dilatation tendency this combined approach can be adopted to any other geothermal site investigation

rock specific hydraulic fracturing and matrix acidizing to enhance a geothermal system concepts and field results abstract enhanced geothermal systems egs are engineered reservoirs developed to extract economic amounts of heat from low permeability and or porosity geothermal resources to enhance the productivity of reservoirs a site specific concept is necessary to actively make reservoir conditions profitable using specially adjusted stimulation treatments such as multi fracture concepts and site specific well path design the results of previously performed stimulation treatments in the geothermal research well gtgrsk4 05 at gros schonebeck germany are presented the reservoir is located at a 4100 4300 m depth within the lower permian of the ne german basin with a bottom hole temperature of 150 c the reservoir rock is classified by two lithological units from bottom to top volcanic rocks andesitic rocks and siliciclastics ranging from conglomerates to fine grained sandstones fluvial sediments the stimulation treatments included multiple hydraulic stimulations and an acid treatment in order to initiate a cross flow from the sandstone layer the hydraulic stimulations were performed in different depth sections two in the sandstone section and one in the underlying volcanic section in low permeability volcanic rocks a cyclic hydraulic fracturing treatment was performed over 6 days in conjunction with adding quartz in low concentrations to maintain a sustainable fracture performance flow rates of up to 150 l s were realized and a total of 13 170 m3 of water was injected a hydraulic connection to the sandstone layer was successfully achieved in this way however monitoring of the water level in the offsetting well egrsk3 90 which is 475 m apart at the final depth showed a very rapid water level increase due to the stimulation treatment this can be explained by a connected fault zone within the volcanic rocks two gel proppant treatments were performed in the slightly higher permeability sandstones to obtain long term access to the reservoir rocks during each treatment a total of 100 ton of high strength proppants was injected with 500 m3 of cross linked gel the subsequent production test in conjunction with flowmeter logging showed an improvement of productivity by a factor of more than 4 due to assumed residual drilling mud constituents calcite dolomite and aragonite in the near wellbore vicinity an acid matrix stimulation was performed thereafter using a coil tubing unit the following nitrogen lift test demonstrated another increase of productivity by 30 50 to an overall increase by a factor of 5 5 6 2