compressive and tensile failure of inclined well bores and determination of in situ stress and rock strength in this paper we investigate the occurrence of compressive and tensile failures of arbitrarily inclined well bores under a wide variety of stress conditions the principal assumptions in this analysis are that the rock is isotropic and that it deforms elastically to the point of failure as has been shown by previous investigators for a given stress state and well bore orientation it is straightforward to predict the orientation of the failures around the well bore as well as whether failure is likely to occur depending on such parameters as rock strength and borehole fluid pressure however as the stress state is almost never known in situ we demonstrate how observations of compressive and tensile wall failures in inclined holes can be used to constrain in situ stress orientations and magnitudes if there are independent data on the magnitude of the least principal stress from either leak off or microfrac tests and on the formation pore pressure we further demonstrate how once the stress state is determined it is possible to assess both an upper bound on the effective in situ rock strength and the degree to which increasing the borehole fluid pressure or mud weight can reduce the likelihood of borehole failure through application of this methodology to an inclined well bore in an area of complex faulting in the gulf of mexico we illustrate how it is possible to utilize observations of borehole failures to determine the magnitude and orientation of the stress tensor in areas such as offshore sedimentary basins where drilling inclined well bores is quite common

earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

inversion of borehole breakout orientation data well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross sectional shape of the borehole from circular to roughly elliptical in vertical holes drilled in areas where one principal stress is vertical breakout orientations tend to parallel the direction of the regional minimum principal stress compressive such breakout orientations are often valuable indicators of the direction of the principal stresses we will show in this paper that the variation of the breakout direction as the borehole deviates from vertical gives further information about the relative magnitudes of all the three principal stresses based upon the variation of breakout directions as a function of deviations and azimuths of the borehole along its depth an inverse problem is formulated to estimate both the direction of the minimum horizontal stress and the relative magnitudes of the horizontal stresses to the vertical stress tests with synthetic data sets show that the power of the method is dependent on the stress regime strike slip normal and thrust generally the inverse problem in the case of thrust faulting is less constrained whereas in the case of normal faulting it allows for a determination of all three parameters sh sv sk sv and with a reasonable certainty interpretation of the data from the siljan deep drilling project in sweden shows that the azimuth of the minimum principal horizontal stress is very well constrained to n 18 4 0 3 e while the ratio of the maximum principal horizontal stress to the vertical stress is constrained to 1 1 0 1 the ratio between minimum principal horizontal stress and principal vertical stress is poorly constrained to the interval 0 0 0 95 information of drilling mud leakage and borehole stability analysis provides a value of 0 61 stephansson et al 1990 these results are in fair agreement with the results from earthquake fault plane solutions in south central fennoscandia with dominant strike slip faulting and an inferred minimum horizontal stress direction of n45 e slunga 1981

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

polar margin dunes and winds on mars the approximately concentric arrangement of layered deposits and dune fields at the two martian poles may reflect a nearly steady state dispersal of material from the polar deposits data on effective surface winds from high resolution viking images combined with theory of local winds suggest that the northern dunes are in part confined to a latitude band by winds generated by their own low albedo dispersal of the dark sand from the southern polar region is not subject to this kind of feedback because the irregular topography prevents areal accumulations sufficiently extensive to produce winds

shear wave splitting observations and implications on stress regimes in the los angeles basin california a systematic analysis of three component seismograms recorded at 15 stations from earthquakes occurring at depths of 5 to 18 km beneath the los angeles basin and adjacent areas during the period between 1988 and 1994 shows 20 to 160 ms shear wave splitting shallow events exhibit little splitting while deeper events show progressively greater splitting with depth the preferred polarization direction of the fast shear wave is nearly n s consistent with the direction of the regional maximum horizontal compressive stress but independent of the azimuth between the event and station we interpret that the shear wave splitting is caused by fluid filled crustal microcracks and macrofractures aligned in the n s direction the shear wave splitting observations of 2 8 to 7 8 ms km can be explained in terms of an anisotropic crust containing vertical cracks with the apparent crack density of 0 023 0 08 on a regional basis the crack density may vary from station to station but we find that the apparent crack density in the strike slip region of the newport inglewood fault and the whittier fault is higher than in the reverse thrusting santa monica mountains and palos verdes hills no systematic change of shear wave splitting in the los angeles basin is found in this study

scales of stress heterogeneity near active faults in the santa barbara channel southern california the santa barbara channel represents the offshore portion of the ventura basin in southern california ongoing transpression related to a regional left step in the san andreas fault has led to the formation of e w trending en echelon fault systems that accommodate localized shortening across the basin recent studies have suggested that faults within the channel could be capable of a multisegment rupture and producing a m w 7 7 8 1 tsunamigenic earthquake however dynamic rupture models producing these results do not account for stress heterogeneity with only sparse information available on the stress field in this region further borehole derived stress constraints are essential for obtaining a more comprehensive understanding of the hazards related to the complex fault systems we used caliper logs from 19 wells obtained from industry to identify stress induced borehole breakouts beneath the holly and gail oil platforms in the channel our newly developed forward modeling technique provides constraints on the orientations and relative magnitudes of the three principal stresses at gail we determine a reverse faulting stress regime s hmax 1 7 s hmin 1 6 sv 1 0 and an s hmax azimuth of n45 e our results are consistent with local structures which reflect deeper regional scale trends and with similar studies onshore nearby at holly an s hmax rotation from n36 w to n57 e occurs across 100 m depth in a single well and differs from nearby results indicating that short length scale 10 km laterally and 1 km in depth stress heterogeneity is associated with complex changes in fault geometry

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

compression directions north of the san fernando valley determined from borehole breakouts borehole breakouts in 4 nearly vertical oil wells and several other deviated holes in the san fernando valley and santa susana mountains suggest a maximum horizontal compressive stress direction s h of n49 w these wells provide information about the stress field from 1974 to 1983 in the uppermost crust 3 km depth near the aftershock zones of the january 1994 northridge earthquake and the february 1971 sylmar earthquake this direction of s h is anomalous with respect to the n to nne directions of s h seen in other regional data but is consistent with the structural complexity of this zone including local changes in strike of major thrust fault zones and the presence of lateral ramps in both the santa susana and san fernando faults

compression directions in southern california from santa barbara to los angeles basin obtained from borehole breakouts borehole elongation in 71 drill holes was used to infer breakout orientation and directions of maximum horizontal principal stress s h for six areas west of the san andreas fault in southern california santa barbara ojai central ventura basin east ventura basin west los angeles basin and east los angeles basin breakouts were determined from analysis of oriented four arm caliper data the breakouts form at the position of the maximum compressive stress on the borehole wall if the borehole is vertical and parallel to one of the principal stress directions the breakouts will form parallel to the minimum horizontal principal stress s h orthogonal to the maximum horizontal principal stress s h observations from deviated boreholes permit some constraints on the relative magnitudes of the principal stresses in most cases the data permit either a thrust faulting s v s h s h or strike slip faulting s h s v s h stress regime with ne to nw directions of s h these results are broadly consistent with results from focal mechanism studies hauksson 1990 li 1996 and with breakout and focal mechanism data present in the world stress map database zoback 1992 however we find systematic variations in s h directions suggestive of strong heterogeneity in the stress field at shallow depths similar to that present in the cajon pass borehole shamir and zoback 1992 anomalous nw directions of s h in the san fernando valley region and near the whittier fault may be related to structural complexities and or lateral ramps in nearby fault systems

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

estimation of maximum horizontal principal stress magnitude from stress induced well bore breakouts in the cajon pass scientific research borehole in this paper we utilize the mechanical properties of core samples detailed observations of stress induced well bore breakouts and estimates of the magnitude of shmin from hydraulic fracturing experiments to construct a vertical profile of the maximum horizontal principal stress shmax to 3 5 km depth in the cajon pass borehole as in essentially all other boreholes the hydraulic fracturing stress measurements in the cajon pass borehole yielded appreciably more and more reliable data on the magnitude of the least principal horizontal stress shmin than the maximum principal horizontal stress shmax to utilize the breakout observations to constrain the magnitude of shmax a brittle failure criterion was used that is based on the effective strain energy concept this failure criterion also allows us to account for the polyaxial stress around the borehole and to incorporate the role of pore fluid pressure on rock strength in various ways a comparison of the shmax profile estimated from the breakouts with estimates of shmax obtained from the hydraulic fracturing tests indicates that they compare fairly well for the case when the effect of pore pressure on hydraulic fracture initiation is negligible the overall stress state to a depth of 3 5 km in the well corresponds to a normal strike slip faulting regime the shmax profile indicates several marked decreases in stress magnitude associated with major fault zones an observation consistent with the shmin profile obtained from hydraulic fracturing tests

neotectonics of the san cayetano fault transverse ranges california study of late quaternary alluvial deposits cut by the san cayetano reverse fault indicates that the dip slip rate of faulting increases from 1 05 0 2 mm yr at sisar creek near the western end of the fault eastward to 1 35 0 4 mm yr at bear canyon and 2 35 0 55 to 4 15 0 85 mm yr at mud creek at timber canyon near the central part of the fault a minimum slip rate of 3 6 0 4 mm yr is suggested by fanhead segmentation however a higher slip rate of 8 75 1 95 mm yr is suggested by stratigraphic evidence stratigraphic separation of 7 5 km in less than 1 m y across the fault between fillmore and piru indicates a slip rate of at least 7 5 mm yr for the eastern section of the fault recent activity is shown by faulted holocene or latest pleistocene fan alluvium and colluvium at bear canyon and the silverthread oil field respectively and inferred at timber canyon by fanhead deposition active folding of the footwall is demonstrated by tilted and faulted holocene and pleistocene alluvial fans present tilt rates approach 1 10 ka yr and associated bedding plane faulting rates are about 0 5 to 1 5 mm yr the hanging wall however does not appear to be involved in active deformation inasmuch as pleistocene deposits as old as several hundred thousand years are not tilted or affected by bedding plane faulting the only exception is in the silverthread field area where hanging wall deformation is directly relatable to fault geometry not folding of the hanging wall rocks the lack of documented moderate to large magnitude historical earthquakes on this fault is in strong contrast with its demonstrated late quaternary activity based on its length slip rate overall geometry and segmentation into two parts a recurrence interval of between 200 and 600 yr for the western segment and about 230 yr for the eastern segment is suggested

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

shear wave splitting observations in the northern los angeles basin southern california abstract shear wave splitting with 20 to 120 msec time separation is observed on 3 component seismograms from earthquakes occurring at depths of 5 to 18 km in the crystalline basement beneath the los angeles basin shallow events that occur in the basement at the base of the 5 km thick sedimentary section exhibit little splitting while deeper events show progressively greater splitting with depth the polarization direction of the fast shear wave is n s independent of the azimuth of ray path between the event and station we interpret that the shear wave splitting occurs mainly in the crystalline basement and is the result of vertical crustal microcracks aligned in n s direction under the assumption that the alignment of microcracks is due to the subsurface stress regime the results from the shear wave splitting data yield a maximum principal stress direction of n s 15 at depth beneath the northern los angeles basin consistent with results from geological mapping and fault plane solutions ray trace modeling indicates that the observations of shear wave splitting can be explained in terms of an anisotropic basement containing vertical microcracks aligned in the n s direction and with a crack density of 0 04 overlain by a weakly anisotropic sedimentary section with a crack density less than 0 02 during the last 4 yr three major earthquakes occurred in the los angeles basin area they are the m 4 6 and m 4 4 montebello double earthquakes of 1989 the m 5 5 upland earthquake of 1990 and the m 5 8 sierra madre earthquake of 1991 with epicenters of 10 55 and 40 km away from the recorder site scs located in the northern los angeles basin respectively we did not observe significant variations of shear wave splitting at scs before and after the upland and sierra madre earthquakes however we found fairly rapid variations of shear wave splitting following the montebello events the time separation between the split shear waves sharply decreased from 70 msec to less than 20 msec after the mainshock and then returned to the average level gradually this rapid decrease in shear wave splitting may be a temporal variation or a result of location variations of the montebello events that were located approximately where the line singularity is expected

are scale invariant stress orientations related to seismicity rates near the san andreas fault 1 based on an analysis of the direction of maximum horizontal compressive stress as a function of depth as observed at different scientific wells along the san andreas fault it has recently been suggested that the scale invariant fluctuations in the stress orientation over intervals from tens of centimeters to several kilometers are directly related to the local earthquake magnitude frequency statistics here we mathematically analyze the possibility of such a relationship and show that the magnitude frequency statistics alone is insufficient to explain the scaling of the stress orientation fluctuations while stress perturbations caused by slip on adjacent faults of various sizes can still be responsible for these fluctuations the average amplitude of the induced changes in the orientation would have to increase nonlinearly with the fault size as the example of two research wells near the san andreas fault also shows the specific nonlinear form would have to depend sensitively on the specific geographic location we conclude that the observed scale invariant fluctuations in the stress orientation are more likely a consequence of a combination of local seismicity rates and the specific local fault structure

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

effect of borehole deviation on breakout orientations well bore breakouts are zones of spalling and fracture that form on opposite sides of a well bore and tend to change the cross sectional shape of the borehole from circular to roughly elliptical in vertical holes drilled in areas where one principal stress sv is vertical breakouts tend to form at opposite ends of the borehole diameter parallel to the least compressive horizontal principal stress direction sh this paper uses an analytical elastic solution for stress at the wall of a borehole to analyze the rotation of breakout orientations away from the direction of sh as the borehole deviates from the direction of the vertical principal stress the calculated orientations of breakouts in deviated boreholes depend on the type of faulting regime in which the well was drilled i e normal strike slip or thrust on the deviation angle o of the borehole axis from vertical on the angle between the horizontal projection of the borehole axis and the direction of sh and on the relative magnitudes of the three principal stresses sh sh the greatest compressive horizontal stress and sv in the strike slip faulting regime regardless of the values of sv sh and sh a borehole must deviate at least 35 from vertical before the horizontal projection of the breakout orientation differs by more than 10 from sh in the normal and thrust faulting regimes however the borehole deviation angle ocrit required to rotate projected breakout orientations by 10 from sh approaches zero as the value of sh approaches that of sh in the thrust faulting regime only about 3 of all combinations of sv sh and sh will produce ocrit values less than 10 while about 12 of all such combinations will yield ocrit 10 in the normal faulting regime about 12 and 33 of such combinations will yield ocrit 20 in the thrust and normal faulting regimes respectively careful study of changes in breakout orientation as a function of borehole deviation may improve the resolution of inferred stress directions when studying breakouts in deviated boreholes in these two faulting regimes

static stress drop in the 1994 northridge california aftershock sequence we use time domain pulse widths to estimate static stress drops for 279 ml 2 5 to 4 0 aftershocks of the 17 january 1994 mw 6 7 northridge california earthquake the stress drops obtained range from 0 02 to 40 bars with a log average of 0 75 bar error bars computed for our estimates are typically a factor of 5 indicating that the three order of magnitude scatter in stress drops is not solely a result of measurement errors and that there is a significant amount of heterogeneity in the static stress drops of the aftershocks stress drops might be expected to increase with depth since a fault can maintain a higher shear load at higher confining pressures we observe an increase in log average stress drop at about 15 km depth which is statistically significant at the 80 confidence level the increase is due primarily to a lack of lower stress drop events below this depth and may be controlled by material properties since the northridge aftershocks are observed to intersect an anomalously high velocity body at around this depth hauksson and haase 1997 an apparent increase in stress drop with magnitude is also observed over the entire magnitude range of the study although whether this trend is real or an artifact of attenuation of high frequencies in the upper crust is unresolved

using borehole breakouts to constrain the complete stress tensor results from the sijan deep drilling project and offshore santa maria basin california we use borehole breakouts in nonvertical drill holes to constrain the directions of the three principal stresses and their relative magnitudes in this paper we start by modifying previously published breakout selection criteria to work with highly deviated borehole data we present a forward modeling technique using genetic algorithms and a nongradient n dimensional optimizer to find the best fitting stress state for a set of breakout data the stress state is parameterized by three euler angles and the stress state ratio o a technique is developed to determine the 95 confidence weighted misfit between a model and the data we then map out the 95 misfit confidence limits on the best fitting stress state this technique is applied to data published by qian and pedersen 1991 removing their constraint of a vertical principal stress direction reduces the misfit between the stress state and the breakout data we find that the best fitting stress state they report with a vertical principal stress direction lies outside our 95 confidence limits we also invert breakouts in the offshore santa maria basin california these data show a thrust faulting stress state with the maximum principal stress s 1 at n148 5 e plunging 31 5 the 95 confidence range for the azimuth of s 1 ranges from n143 0 e to n198 1 e the stress ratio o was found to be 0 82l 1 0 584

utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses application to continental deep sea drilling project and ocean drilling program boreholes

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years

thin skinned tectonics of the upper ojai valley and sulphur mountain area ventura basin california by integrating surface mapping with subsurface well data and drawing cross sections and subsurface maps the geometry of shallow structures and their geologic history of the upper ojai valley of california can be reconstructed the geometry of shallow structures the geologic history and the location of earthquake foci then offer constraints on the deep structure of this complex area the upper ojai valley is a tectonic depression between opposing reverse faults its northern border is formed by the active north dipping san cayetano fault which has 6 0 km of stratigraphic separation in the silverthread area of the ojai oil field and 2 6 km of stratigraphic separation west of sisar creek the fault dies out farther west in ojai valley where the south vergent shortening is transferred to a blind thrust the southern border of the upper ojai valley is formed by the quaternary lion fault set sisar big canyon and lion faults which dips south and merges into the sisar decollement within the south dipping ductile lower miocene rincon formation folds with north dipping axial planes including the lion mountain anticline and reeves syncline are middle pleistocen or older and are related to movement on an unnamed fault in the footwall block of the san cayetano fault and on the san cayetano fault respectively by the middle pleistocene the sulphur mountain anticlinorium and the big canyon syncline began forming as a fault propagation fold the fault propagation fold is rooted in the sisar decollement a passive backthrust rising from a blind thrust at depth the formation of the sulphur mountain anticlinorium was followed closely by the ramping of the south dipping lion fault set to the surface over the nonmarine upper pleistocene saugus formation to the east the san cayetano fault overrides and folds the lion fault set near the surface area balancing of the deformation shows shortening of 15 5 km and suggests a 17 km depth to the brittle d ctile transition