stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

stress field constraints on intraplate seismicity in eastern north america focal mechanisms of 32 north american midplate earthquakes mo 3 8 6 5 were evaluated to determine if slip is compatible with a broad scale regional stress field derived from plate driving forces and if so under what conditions stress regime pore pressure and frictional coefficient using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes 10o the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes defined by the parameterb s2 s3 s s3 from the fault stress geometry as long as the focal mechanism has a component of oblique slip i e the b axis does not coincide with the intermediate principal stress direction this calculation identifies which of the two nodal planes is a geometrically possible slip plane gephart 1985 slip in a majority of the earthquakes 25 of 32 was found to be geometrically compatible with reactivation of favorably oriented preexisting fault planes in response to the broad scale uniform regional stress field slip in five events was clearly inconsistent with the regional stress field and appears to require a localized stress anomaly to explain the seismicity significantly all five of these events occurred prior to 1970 when many regional networks were installed and their focal mechanisms are inconsistent with more recent solutions of nearby smaller events the frictional likelihood of the geometrically possible slip on the selected fault planes was evaluated in the context of conventional frictional faulting theory the ratio of shear to normal stress on the fault planes at hypocentral depth was calculated relative to an assumed regional stress field regional stress magnitudes were determined from 1 s s3 ratios based on the frictional strength of optimally oriented faults the basis for the linear brittle portion of lithospheric strength profiles 2 the computed relative stress magnitude b values and 3 a vertical principal stress assumed equal to the lithostat two end member possibilities were examined to explain the observed slip in these less than optimally oriented fault planes first the frictional coefficient was held constant on all faults hydrostatic pore pressure was assumed regionally and the fault zone pore pressure was determined since pore pressure is a measurable quantity with real limits in the crust p0 s3 this end member case was used to determine which of the geometrically possible slip planes were frictionally likely slip planes alternately pore pressure was fixed at hydrostatic everywhere and the required relative lowered frictional coefficient of the fault zone was computed slip in 23 of the 25 geometrically compatible earthquakes was determined to also be frictionally likely in response to an approximately horizontal and vertical regional stress field derived from plate driving forces whose magnitudes are constrained by the frictional strength of optimally oriented faults assuming hydrostatic pore pressure regionally the conditions for slip on these 23 relatively well oriented earthquake faults were determined relative to this regional crustal strength model and require only moderate increases in pore pressure between about 0 4 0 8 of lithostatic hydrostatic is about 0 37 of lithostatic or alternately moderate lowering 50 of the frictional coefficient on the faults which slipped superlithostatic pore pressures are not required focal mechanisms for the two other earthquakes with slip vectors geometrically consistent with the regional stress field however did require pore pressures far exceeding the least principal stress or extremely low coefficients of friction these events may reflect either local stress rotations undetected with current sampling or poorly constrained focal mechanisms the analysis also confirmed a roughly north to south contrast in stress regime between the central eastern united states and southeastern canada previously inferred from a contrast in focal mecha nisms between the two areas most central eastern united states earthquakes occur in response to a strike slip stress regime whereas the southeastern canadian events require a thrust faulting stress regime this contrast in stress regime with a constant maximum horizontal stress orientation determined by far field plate driving forces requires a systematic lateral variation in relative stress magnitudes superposition of stresses due to simple flexural models of glacial rebound stresses are of the correct sense to explain the observed lateral variation but maximum computed rebound related stress magnitude changes are quite small about 10 mpa and do not appear large enough to account for the stress regime change if commonly assumed stress magnitudes determined from frictional strength apply to the crust at seismogenic depths

a note on induced stress changes in hydrocarbon and geothermal reservoirs abstract earthquakes have been induced by oil and gas production where pore pressures have decreased in some cases by several tens of mpa it has previously been suggested that such earthquakes are caused by poroelastic stressing of crust surrounding the reservoir induced earthquakes are also common in geothermal fields such as the geysers where strong correlations between both steam production and condensate injection and earthquake activity have been observed over the last several decades stress measurements within hydrocarbon reservoirs show that the least horizontal stress decreases with declining reservoir pressure as predicted by poroelasticity for circular disk shaped reservoirs isothermal reduction in pore pressure induces a relative horizontal tension within the reservoir production induced stressing may promote frictional sliding on pre existing faults within the reservoir itself normal faulting is promoted if the regional stress is extensional and the biot coefficient is sufficiently large 0 85 for reasonable coefficients of friction on the other hand dilatant fracturing and normal faulting are always promoted in extensional environments near the edge of the reservoir or in regions of high pore pressure gradient it is suggested that such fracturing could enhance fracture permeability in tight rocks adjacent to portions of the reservoir that experience large reductions in pore pressure due to production in regional compressional environments production modestly favors reverse faulting above and below the reservoir the ratio of thermal to poroelastic stress can be quite large in geothermal reservoirs such as the geysers reservoir wide energy balance considerations suggest that the average temperature has declined at the geysers by 6 c during the past 20 years reservoir average stress changes are thus on the order of 2 mpa and are certainly much larger near injection wells and steam producing fractures

seismicity in the war wink gas field delaware basin west texas and its relationship to petroleum production abstract this study reevaluates seismicity recorded in the war wink gas field ward county texas between 1976 and 1979 earthquakes were relocated using a velocity model from sonic well logs in the vicinity of the field well located earthquakes were compared with a geologic model built from well control and with oil field operations the new locations reveal that the seismicity occur in three depth intervals earthquakes in the lowest zone occur in early paleozoic carbonates 4 3 to 6 2 km below sea level and have no apparent relation to oil field operations in the middle zone of overpressured shales and limestones 2 4 to 4 3 km below sea level earthquakes have predominantly normal composite mechanisms with north south oriented extension and may be related to differential compaction over an underlying anticline and fluid pressure reduction from oil production earthquakes in the upper zone of permian aged carbonates and clastics have predominantly strike slip composite mechanisms and appear to be related to compaction of and to high fluid pressures leaking upwards from the underlying overpressure zone an inversion for stress orientation using the nine composite mechanisms determined for the field suggests that 3 is oriented n10 e and is in good agreement with 3 orientations determined in other studies of oil fields within the permian basin

induced microearthquake patterns and oil producing fracture systems in the austin chalk abstract microearthquakes collected during hydraulic stimulation allowed us to study fracture zones in austin chalk oil reservoirs at two sites in the giddings field texas we deployed three component downhole geophone tools in production wells at depths of 2100 m and greater one near cook s point and two on the matcek lease near caldwell at cook s point we collected 482 microseismic events during a 4000 m 3 25 000 bbl hydraulic stimulation in an offset well we collected 770 events during a similar operation on the matcek lease many seismograms contained reflected phases that constrained location depths to the production zone at the base of the austin chalk by restricting all microearthquake locations to production depths we located 20 of the cook s point events and over 60 of the matcek events at both sites we observed only the fracture wing closest to the observation stations locations formed elongated patterns extending up to 1 km from the stimulation well and trending n60 e parallel to the known regional fracture trend the cook s point seismic zone measured over 100 m in width while long stretches of the matcek seismic zone narrowed to 30 m or less we believe that the width of the seismic zone reflected the density of conductive fractures and thus the volume of the reservoir accessed by the stimulation indeed production rates in the first year following stimulation were much higher at cook s point where we observed the wider of the two seismic zones

structural uplift beneath the chicxulub impact structure models of the central structure of large impact craters are poorly constrained partly because of the lack of well preserved terrestrial examples and partly because of the extreme nature of impact events even large impact craters take only a few minutes to form during which time rocks from the deep crust move upward many kilometers interacting with impact melts and breccias before settling to their final position we construct a new model of central uplift beneath the chicxulub crater based upon a well constrained 3 d velocity model obtained by jointly inverting seismic traveltime and gravity data the input tomographic data set has good resolution and many rays cross the central uplift in many directions we use laboratory measurements to convert between velocity and density our velocity model possesses a high velocity zone near the crater center and velocity gradually decreases outside this zone we use regional refraction data to interpret these velocities in terms of a broad 80 km wide zone of structural uplift in which the central rocks originate from the lower crust and the surrounding rocks from the midcrust and upper crust this is in contrast with previous models in which the zone of central uplift is either 40 50 km or 150 km wide our interpretation is consistent with scaling laws yucatan basement lithology other velocity data observations at similar sized terrestrial craters and dynamic modeling of peak ring formation our model of the uplift at chicxulub can be used to help distinguish between competing models of effective target strength in numerical models of crater formation

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

identification of wrench faults using subsurface structural data criteria and pitfalls 1 many structures imaged with reflection seismic can resemble a wrench fault system because of the great structural diversity of strike slip faults and the wide range of contractional and extensional features associated with these faults because of these resemblances numerous faults are misidentified as wrench faults by workers using subsurface structural data credible identification requires combinations of profile and map criteria 1 narrow long straight throughgoing solitary master fault or zone of deformation 2 steep to moderate dip of the master fault at depth 3 offset of the top of basement 4 changes in relative upthrown side separation sense and or fault dip direction at depth or along the strike of the master fault 5 narrow fault slices within the master fault zone that steepen and join at depth negative and positive flower structures 6 different separation sense and orientation of relative upthrown side at these fault slices and 7 coeval en echelon flanking structures the identification should be confirmed with other criteria that refute alternative interpretations the main pitfalls to a reliable identification are not considering alternative structural styles and the misinterpretation of features or patterns that superficially resemble wrench systems the profile geometries of basement involved reverse or normal faults particularly those with structural inversion can resemble the single strand of convergent or divergent wrench faults profile geometries of flower structures can be mimicked by narrow contractional horsts or narrow extensional grabens complexly faulted anticlines or synclines splay faults fault zones intruded by salt or shale and overprinted structures pitfalls in interpreting map relationships are possible when structural patterns combine several tectonic influences are controlled by rock anisotropy or include elements hat are not coeval and if the patterns characteristic of other styles are misinterpreted many erroneous identifications of wrench faults can be eliminated by demonstrating that the investigated fault or feature lacks a straight trace lateral length or throughgoing continuity

ten years of seismic monitoring over a gas field abstract ten years of seismic recording allows us to locate more than 800 tremors with magnitudes ranging from 1 0 to 4 2 in a small 10 by 10 by 5 km volume near a major gas field lacq france the seismic activity began 10 yr after gas extraction began nearly all of the epicenters are located within the lateral extent of the gas field the only perturbation of the stress or strain fields which has the same spatial and temporal scale as the seismicity is the 500 bar drop in gas pressure due to 20 yr of gas exploitation demonstrating that these events are induced the network established to monitor this activity consists of eight telemetered stations including a well bottom geophone located at 3200 m depth using three dimensional relocation techniques we computed new hypocenter positions for more than 280 events these better located hypocenters form a diffuse zone that mimics the local dome structure taking the regional tectonic stresses into account we suggest that these ruptures may be the result of unfolding of the dome structure

poroelastic stressing and induced seismicity near the lacq gas field southwestern france o hundreds of shallow small to moderate earthquakes have occurred near the lacq deep gas field in southwestern france since 1969 these earthquakes are clearly separated from tectonic seismicity occurring in the pyrenees 25 km to the southwest the induced seismicity began when the reservoir pressure had declined by 30 mpa repeated levehng over the field shows localized subsidence reaching a maximum of 60 mm in 1989 segall 1989 suggested that poroelastic stressing associated with volumetric contraction of the reservoir rocks is responsible for induced seismicity associated with fluid extraction to test this model we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields we find that the relationship between average reservoir pressure drop and subsidence is remarkably linear lending support to the linear poroelastic model displacements and stresses are computed based on a priori knowledge of the reservoir geometry materim properties and reservoir pressure changes the computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling stress perturbations accompanying gas extraction computed using the same parameters are found to be 0 2 mpa or less changes in coulomb failure stress are computed assuming that shp occurs on optimally oriented planes the predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compresslye stress axis is vertical accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state

induced stresses due to fluid extraction from axisymmetric reservoirs earthquakes can be induced by fluid extraction as well as by fluid injection segall 1989 proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction here i present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries the results ofgeertsma 1973 for a thin disk reservoir with uniform pressure drop are recovered as a special case predicted surface subsidence agrees very well with measured leveling changes over the deep lacq gas field in southwestern france the induced stresses are finite if the reservoir pressure changes are continuous computed stress changes are on the order of several bars suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production

the evolution of seismic barriers and asperities caused by the depressuring of fault planes in oil and gas fields of south texas fluid pressures within some oil and gas fields of south texas have dropped to less than 20 per cent of their original values producing earthquakes with magnitudes up to 3 9 in recent years differential compaction of the depressurized region may be sufficient to result in the number and size of earthquakes generated or the faults may have been creeping prior to depressurization in either model the depressuring of fluids strengthens a fault and at first produces a barrier to slip as strain accumulates due to compaction or the continued aseismic slip of nearby portions of the fault stress builds up along the locked portions eventually forming high stress regions or asperities the asperities ultimately fail and earthquakes occur the process is repeated as long as the faults are active as the fluid pressures continue to decrease the barriers and subsequent asperities may increase in size and strength resulting in increasingly large and frequent earthquakes

analysis of induced seismicity for stress field determination and pore pressure mapping the focal mechanisms of some one hundred microseismic events induced by various water injections have been determined within the same depth interval numerous stress measurements have been conducted with the htpf method when inverted simultaneously the htpf data and the focal plane solutions help determine the complete stress field in a fairly large volume of rock about 15 106 m3 these results demonstrate that hydraulically conductive fault zones are associated with local stress heterogeneities some of these stress heterogeneities correspond to local stress concentrations with principal stress magnitudes much larger than those of the regional stress field they preclude the determination of the regional stress field from the sole inversion of focal mechanisms in addition to determining the regional stress field the integrated inversion of focal mechanisms and htpf data help identify the fault plane for each for each of the focal mechanisms these slip motions have been demonstrated to be consistent with terzaghi s effective stress principle and a coulomb friction law with a friction coefficient ranging from 0 65 to 0 9 this has been used for mapping the pore pressure in the rock mass this mapping shows that induced seismicity does not outline zones of high flow rate but only zones of high pore pressure for one fault zone where no significant flow has been observed the local pore pressure has been found to be larger than the regional minimum principal stress but no hydraulic fracturing has been detected there

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

wave transformation and shoreline water level on funafuti atoll tuvalu the influence of sea swell ss waves infragravity ig waves and wave setup on maximum runup rmax is investigated across different tidal stages on fatato island funafuti atoll tuvalu field results illustrate that ss waves are tidally modulated at the shoreline with comparatively greater wave attenuation and setup occurring at low tide versus high tide a shoreward increase in ig wave height is observed across the 100 m wide reef flat at all tidal elevations with no tidal modulation of ig wave height at the reef flat or island shoreline a 1 d shock capturing green naghdi solver is used to replicate the field deployment and analyze rmax model outputs for ss wave height ig wave height and setup at the shoreline match field results with model skill 0 96 model outputs for rmax are used to identify the temporal window when geomorphic activity can occur on the beach face during periods of moderate swell energy waves can impact the beach face at spring low tide due to a combination of wave setup and strong ig wave activity under mean wave conditions the combined influence of setup ig waves and ss waves results in interaction with island sediment at midtide at high tide ss and ig waves directly impact the beach face overall wave activity is present on the beach face for 71 of the study period a significantly longer duration than is calculated using mean water level and topographic data

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

reservoir characterization using oil production induced microseismicity clinton county kentucky abstract microseismic monitoring tests were conducted from 1993 to 1995 in the seventy six oil field clinton county kentucky oil is produced from low porosity fractured carbonate rocks at 3 no injection operations were conducted gross changes in production rate correlate with event rate event rate lags changes in production rate by 2 to 3 weeks hypocenters and first motion data have revealed previously undetected low angle thrust faults above and below the currently drained depth intervals production history well logs and drill tests indicate that the seismically active faults or fractures are previously drained intervals that have subsequently recovered to hydrostatic pressure via brine invasion storage capacity computed for one of these drained fractures implies that total oil production represents about 20 of total pore volume correlation of older production intervals and well log porosity anomalies with the seismically active faults indicate that the oil reservoir in the study area is primarily a set of compartmentalized low angle thrust faults although low angle fracture sets have not previously been considered in the exploration and development of the area the mapped thrust faults are consistent with other investigators interpretations of oil associated with secondary fracture sets occurring along deeper seated wrench fault structures stress determined from composite focal mechanisms indicates a near surface

seismic imaging using microearthquakes induced by hydraulic fracturing abstractseismic imaging using microearthquakes induced by hydraulic fracturing produces a three dimensional 3 d s wave velocity model of the fractured zone improves the calculated locations of the microearthquakes and may lead to better estimates of fractureplane orientations fracture density and water flow paths such information is important for predicting the amount of heat energy that may be extracted from geothermal reservoir a fractured zone was created at the los alamos hot dry rock reservoir in north central new mexico within otherwise impermeable basement rock by injecting 21000 m3 of water into a borehole under high pressure at a depth of 3 5 km induced microearthquakes were observed using four borehole seismometers the p wave and s wave arrival times have been inverted to find the 3 d velocity structures and the microearthquake locations and origin times the inversion was implemented using the separation of parameters technique and constraints were incorporated to require smooth vel

appalachian stress study 3 regional scale stress variations and their relation to structure and contemporary tectonics using commercial hydrofracture data to supplement published stress data we map regional variations in least horizontal stress magnitude sh and orientation within the paleozoic sediments of the appalachian plateau systematic trends revealed by the data are examined for correlations with structure the study area includes a prominent salt detachment and tectonic history the dominant stress component present at all depths including basement throughout the region is ene oriented compression and is most likely of tectonic origin no deviations from this rule are found that are statistically significant near surface 600 m stresses are consistently greater than the overburden with strike slip regime conditions prevailing below however where the salina salt beds are present thrust regime conditions extend deeper reaching the salt in most places where data are available limited to where the salt is shallower than 1 5 km at the salt a discontinuity in the trend of sh with depth marks the abrupt transition to the underlying strike slip regime characterized by least stress ratios i e rh sh sv in the range 0 5 0 7 and extending to basement the comparatively high stress found above the salt may reflect a difference between the strain histories of the overlying and underlying sections facilitated by mechanical decoupling by the salt and perhaps includes a component of stress remnant from nw oriented alleghanian shortening of the upper section the lower subsalt stress regime extends beyond the eastern salt margin and displays remarkable uniformity over distances of hundreds of kilometers this is particularly evident across ohio where an rh value of 0 6 0 05 appears valid down to basement north of ohio values increase markedly whereas to the south of the salt pinch out in west virginia and eastern kentucky values as low as 0 35 are found in devonian shales at 1 1 5 km depth although values deeper in the section may be higher the northward increasing trend can be reasonably well modeled as the effect of persisting lithospheric flexure due to the pleistocene ice sheet however flexure was much greater in the past and as devonian shales in south ohio west virginia are currently only marginally stable a modest increase over the present flexure would be expected to cause shear failure this suggests that the very low stress in the shales of south ohio west virginia is of different origin and more recent than the pleistocene

locating microearthquakes induced by hydraulic fracturing in crystalline rock microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3 5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced the induced microseismicity was monitored by a network of five vorehole seismic stations a total of about 800 induced events were reliably located from arrival times event locations show a tabular distribution that strikes 350 sup 0 and dips 65 sup 0 east subparallel to the injection well the injection was intended to produce a fracture system that would hydraulically connect two subparallel wells a lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore the 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties the injected fluid appears to have stimulated a zone of rock rather than simply a single fracture the distribution of microseismic events presumably envelopes the zone of fluid paths created by the fracture experiment

induced seismic deformation in the cogdell oil field of west texas a sequence of induced earthquakes occurred at the cogdell oil field in west texas from about 1974 to 1982 the earthquakes occurred within an array of injecting wells which surrounded the perimeter of the field and which decreased in radius during the life of the injection project we examine this sequence of earthquakes in terms of two failure models in the first model seismic failure occurs when and where fluid pressures exceed some critical value in the second model earthquakes occur at the edges of low fluid pressure areas where stresses have been transferred as a result of aseismic deformation in adjacent high fluid pressure regions we calculate the history of fluid pressures in the cogdell field using the theis equation for fluid flow in an areally infinite aquifer the calculations show that the earthquakes occur in a region of relatively low pressure surrounded by regions of higher fluid pressure the cogdell earthquakes may occur on pre existing faults however fluid pressures are sufficiently high in surrounding portions of the field to also produce failure a long time delay between the start of injection and the onset of seismicity may correspond to the time required for fluid pressures on a pre existing fault to reach critical values or may be the result of an evolving injection pattern whereby the inward migration of injecting wells produces aseismic failure which loads stress onto stronger regions within the center of the field injection of fluids for secondary recovery occurs at over three thousand sites in the state of texas many of these sites are geographically near cogdell and many have higher injection pressures and volumes of injected fluid than cogdell however they do not exhibit any seismicity a failure model which incorporates stress loading suggests that the perimeter injection pattern is responsible for the onset of seismicity a consequence of this interpretation is that many other injection fields may be undergoing large amounts of aseismic deformation