drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

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

analysis of borehole breakouts boreholes drilled into rock which is subjected to stresses that amount to a significant fraction of the strength of the rock may cause the rock to fail adjacent to the borehole surface often this results in the elongation of the cross section of the borehole in the direction of the minimum principal compressive stress orthogonal to the borehole axis such breakouts are valuable indicators of the direction of the minimum compressive stress orthogonal to the axis of the borehole their shapes may provide information about the magnitudes of both the maximum and minimum stresses relative to the strength of the rock borehole breakouts also may be impediments to drilling and to in situ measurement techniques such as hydraulic fracturing observations and analyses of borehole breakouts raise three important questions first how does the shape of the borehole breakout evolve second why are breakout shapes stable despite the very high compressive stress concentrations that they produce third how is the shape of the breakout related to the magnitudes of the stresses in the rock in this paper extensile splitting of rock in unconfined plane strain compression is assumed to be the process of rock failure adjacent to the circumference of the borehole by which a breakout forms to simulate the evolution of a borehole breakout this process is combined with a numerical boundary element analysis of the stresses around a borehole as its cross section evolves from the originally circular shape to that of a stable breakout the tangential stresses around a stable breakout cross section are found to be everywhere less than the unconfined plane strain tensile or compressive strength of the rock the stresses outside the stable breakout are found to be everywhere less than the limiting values of shear strength given by a mohr coulomb criterion in the regions of great stress concentrations at the ends of a breakout cross section which have a pointed shape the state of stress approaches that of equal biaxial compression in plane strain as it does ahead of a mathematical crack or notch the fact that the stresses around a breakout are less than the relevant strength establishes both the stability of the final breakout cross section and the appropriateness of an elastic analysis of the stresses according to this model the cross sectional shapes of stable breakouts are not related uniquely to the state of stress and the strength of the rock for example stable breakouts created instantly in rock already subjected to stress are much larger than stable breakouts created in the same rock with a preexisting borehole by subsequently increasing the stresses to the same values the results of drilling into an actual rock probably lie between these extremes modest changes in borehole cross section as a result of breakout do not alter significantly the minimum tangential tensile stress around a borehole with internal pressure from that given by the kirsch solution for a circular hole subjected to the same stresses therefore hydraulic fracturing interpretations based on the kirsch solution give the correct values for the far field stresses

impact of in situ stress and fault reactivation on seal integrity in the east irish sea basin uk abstract despite having been affected by several stages of exhumation during the cretaceous and cenozoic the contemporary stress state of the east irish sea basin eisb is poorly characterised as the basin is mature in terms of exploitation of hydrocarbons future exploration beyond the conventional sherwood sandstone group reservoir triassic necessitates a greater understanding of the in situ stress field while proposed natural gas storage and carbon sequestration schemes also require detailed stress field information using petroleum well data the in situ stress field of the eisb has been characterised to assess the mechanical seal integrity a strike slip stress regime most likely prevails in the basin meaning the maximum horizontal stress shmax is the greatest of the principal stresses interpretation of stress orientation data suggests that shmax is oriented 152 12 consistent with mean stress orientations across the wider region associated with plate boundary forces some degree of structural control appears to influence the orientation of shmax with orientations locally aligned sub parallel to major permo triassic basin bounding faults fault reactivation risk is evaluated through modelling the pore pressure increase required to induce failure on pre existing faults vertical faults striking 30 from shmax are optimally oriented to become reactivated under elevated pore pressure conditions for any project relying on an element of fault seal for the containment of buoyant fluids at the average reservoir depth of 800 m pore pressure increase should be less than 3 3 mpa to avoid reactivating pre existing optimally oriented faults higher pressure increases would be required to initiate reactivation of faults with other orientations vertical faults striking perpendicular to shmax are least likely to become reactivated and in the absence of halite seal integrity would instead be limited by caprock strength and capillary entry pressure major faults affecting the basin have been analysed for their slip tendency ratio of shear to normal stress which provides an indication of their susceptibility to become reactivated although the analysis is limited due to lack of an accurate 3d representation of the fault network the results suggest that many of the fault orientations observed in the eisb exhibit high slip tendencies including n s striking faults to the north and west of the east deemster fault where the shmax orientation is nw se faults striking perpendicular to shmax such as the lagman fault are least likely to become reactivated due to higher normal stresses that inhibit frictional sliding while faults striking parallel or very close to shmax also exhibit low slip tendency as they are not subjected to significant shear stresses

orientation of in situ stresses in the oceanic crust two in situ measurements of principal stress directions have been made in dsdp holes 504b south of the costa rica rift on the nazca plate and 597c west of the east pacific rise on the pacific plate in both cases the orientations of in situ principal stresses determined from borehole breakouts are consistent with the stress directions inferred from intraplate earthquakes located near the sites

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

ultrasonic borehole televiewer investigation of oceanic crustal layer 2a costa rica rift in situ lithostratigraphy and the distribution of fracture and void zones in oceanic layer 2a were examined with an ultrasonic borehole televiewer at ipod sites 501 and 504b on the south flank of the costa rica rift these records indicate a decrease in the size of basalt pillows with depth and a corresponding increase in fracture and void density the observed increase in p wave velocity with depth in this hole as layer 2a merges into 2b is best explained by an increase in the degree to which clays and zeolites fill fractures and voids this also explains the order of magnitude drop in permeability observed in hole 504b an increase in alteration filling with age could explain the transition from convective to conductive heat flow and the correlative disappearance of layer 2a on the flanks of oceanic spreading centres

integrating borehole image logs with core a method to enhance subsurface fracture characterization image logs provide critical data for characterizing fractured reservoirs however the nature of such logs coupled with borehole modified stresses creates challenges that may lead to erroneous fracture interpretations we developed a methodology to integrate image logs core and geomechanical models to obtain high confidence fracture data sets derived from image logs the methodology starts with a detailed comparison of overlapping image logs and core first to properly shift them into alignment core to log shift and second to determine which features interpreted as fractures on image logs picks can be validated as fractures in core then we derive guidelines for interpreting image logs without core control to reduce the likelihood of false fracture picks geomechanical modeling is used to assist with quality control and interpretation of image logs where drilling induced tensile fractures occur we use a fractured carbonate reservoir to demonstrate the application of this methodology comparison of image log fracture picks with core led to an average rejection rate of greater than 50 of fractures picked on the image logs these high rejection rates are mostly caused by overinterpretation of discontinuous sinusoids and misidentification of sedimentary features and induced fractures we derived rules for interpreting high quality fractures 1 only sinusoids with continuity greater than 50 should be interpreted and 2 sinusoids with dip angles less than 55 should be excluded from the interpretation application of this methodology has provided much higher quality fracture orientation and frequency data than that resulting from interpretation of borehole images alone the improved fracture data set provides a more reliable basis for testing hypotheses and deriving models of fracture distribution

direct numerical simulation of the plumb mcewan laboratory analog of the qbo abstract the laboratory experiment of plumb and mcewan demonstrates the principal mechanism of periodically reversing winds observed in the stratosphere the quasi biennial oscillation qbo however despite numerous studies some aspects of the qbo and the connection to its laboratory analog remain unclear incorporating the rapidly undulating boundaries of the laboratory experiment into the numerical algorithm via time dependent curvilinear coordinates allows for the reproduction of the experimental setup while minimizing numerical uncertainties results are presented of the first direct numerical simulation of the phenomena that lead to the zonal mean flow reversal in the laboratory analog the aim of this research is to narrow the widening gap between the theoretical understanding of laboratory scale internal gravity wave processes and the complexity of global scale circulations a detailed study is presented on the parametric and numerical sensitivities of the oscillation the results confirm a numb

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

stress orientations from oil well fractures in alberta and texas many oil wells in alberta exhibit spalling of the walls known as breakouts which elongates the holes with the longer axis aligned northwest southeast this alignment is observed over an area in excess of 4 105 km2 in siltstones sandstones carbonate sediments and one shale formation through the stratigraphic column from devonian to cretaceous it is unrelated to dip of the beds we have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls in a stress field having large unequal horizontal principal stresses and with the larger compression oriented northeast southwest it is probable that this northeast southwest principal stress is 1 this paper adds new data from oil wells in alberta and northern british columbia and shows that the breakouts and by inference the stress orientations are consistent through much of the western canadian sedimentary basin it also uses evidence from hydraulic fracturing in west central alberta and from steam injec

pre devonian landscape of the baltic oil shale basin nw of the russian platform the erosional relief of ordovician and silurian deposits in estonia was developed during the continental period in late silurian and early middle devonian times the uplift of the area and marine regression were induced by compressional tectonics in the continental interior related to the closure of the iapetus and tornqvist oceans in the northern part of the baltic sedimentary basin estonia on the gentle southerly dipping slope between the fennoscandian shield finland and baltic syneclise latvia a pre devonian slightly rugged erosional relief with few cuesta was developed the predevonian erosional landforms hills depressions and escarpments reaching 150m in height were probably buried under the devonian deposits and then partly re exposed by pre quaternary erosion these landforms are described in detail using data from several thousands of cores drilled in the course of oil shale and phosphorite exploration and mining erosional relief on ordovician and silurian deposits in the northwest of the east european craton was developed during the continental period of exposure in the late silurian and early middle devonian uplift of the territory and marine regression were induced by compressional tectonics in the continental interior due to the closure of the external iapetus ocean to the northwest phillips et al 1976 and the tornqvist ocean to the southwest ziegler 1990 schwan 1997 at the beginning of pre devonian erosion the land surface was of generally low relief due to ordovician and silurian near shore marine sedimentation structurally induced local landforms were almost non existent before the late caledonian phase of compression before the late silurian early devonian phase of deformation the whole east baltic area was a shallow intracontinental seabed or near shore continental flat with few structural complications in the early palaeozoic sedimentary cover the compressional tectonics along the craton margins caused uplift of the region and deformation of the crystalline basement and sedimentary cover in the western corner of the russian platform east of the present baltic sea at least two areas of different types of structure were formed in the central part of the baltic sedimentary basin i e in latvia tectonic deformation was most extensive and fault throws reached 700m suveizdis 1979 after early devonian erosion the faultand dome related residual escarpments and hills had altitudes of up to 150m polivko ulst 1969 in the northern part of the sedimentary basin estonia within the gentle southerly dipping slope between the fennoscandian shield finland and baltic syneclise latvia a flatter hilly relief developed the influence of faults with throws of up to 50 m on the formation of the pre devonian topography was negligible these oldest erosional landforms are now partly buried under devonian deposits and partly re exposed during late phanerozoic erosion following the previous preliminary interpretations published in estonian and russian puura 1974 puura et al 1987 tuuling 1988 this paper discusses sedimentary bedrock structure topography relationships and reconstruction of the pre devonian landscape geological and geomorphological setting the study area is situated in the northwestern part of the russian platform of the east european craton south of the fennoscandian shield fig 1 oil shale deposits of the baltic oil shale basin occur in northeastern estonia and adjacent areas of northwestern russia extending from 25 59 to 29 59 fig 2 it is the best known portion of the basin because of extensive exploration for oil shale and phosphorite over 15000 boreholes in 5000 km2 the oil shale basin is located in the sub surface of a slightly hilly plain south of the coastal cliffs of the gulf of finland the highest point of the plain is 166 m above sea level on the pandivere heights ph fig 2 the devonian is distributed around the periphery of the bedrock hills south of the pandivere heights and between the ahtme elevation ae and izhora heights ih from smith b j whalley w b warke p a eds 1999 uplift erosion and stability perspectives on longterm landscape development geological society london special publications 162 75 83 1 86239 030 4 99 15 00 9 the geological society of london 1999 by guest on january 6 2014 http sp lyellcollection org downloaded from

intraplate seismicity of the southern part of the pacific plate we present a compilation of all the known seismicity of the part of the pacific plate located south of the equator this includes historical earthquakes dating back to 1937 in addition to the gilbert islands swarm of 1981 1983 we identify 60 events at 32 epicenters in the magnitude range 4 3 6 9 our results indicate that the level of seismicity of the southern pacific plate is comparable to that of other plates including continent bearing ones most of the seismic activity is concentrated in the youngest parts of the plate but a few large events are present in cretaceous lithosphere with the exception of events from the gilbert islands swarm earthquakes located in lithosphere 35 ma and older are usually compatible with the release of horizontal compressional stress due to the gravitational process known as ridge push in younger areas the situation is much less clear cut the final geometry of the focal mechanisms is controlled by local tectonics in particular due to the miocene reorientation of the spreading at the east pacific rise the bulk of the pacific plate is particularly vulnerable to ridge push and this explains the numerous strike slip events and the absence of magnitude 6 events in most of the plate correlations of epicenters with zones of weakness or bathymetric features are frequent but not universal finally a number of areas with high seismic release are identified as potential targets for future exploration

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

the origin of tectonic stress in the lithosphere abstract the sources of lithospheric stress and their distinctive features are briefly reviewed it is suggested that there are two main categories of lithospheric stress renewable stress which persists despite continuing stress relaxation and non renewable stress which can be dissipated by relief of the initial strain the two most important types of renewable stress arise from plate boundary forces and from isostatically compensated loads non renewable stress systems include bending stresses membrane stresses and thermal stresses an important phenomenon generating large stresses at shallow depth is stress amplification caused by lower lithospheric creep this applies to renewable stresses but not to the non renewable type it is suggested that only renewable stresses contribute significantly to tectonic activity however bending and thermal stresses are locally important in subducting lithosphere

in situ stress orientation near the san andreas fault preliminary results to 2 1 km depth from the cajon pass scientific drillhole stress induced wellbore breakouts were analyzed in the cajon pass scientific drillhole 4 3km ne of the san andreas fault to determine the orientation of the principal horizontal stresses using borehole televiewer digitized data in the depth interval 1750 2115 m the mean orientation of the maximum horizontal stress is 073 the scatter in the orientation of breakouts is as much as 37 and variations occur with wavelengths of few hundred meters in this interval reflecting either true changes in stress orientation with depth or the variability of rock anisotropy all the measured orientations however result in left lateral shear stress resolved onto planes parallel to the san andreas fault contrary to its post miocene right lateral slip of 300 km the stress orientation is consistent however with reported left lateral displacement across the cleghorn fault 3 km northeast of and possibly subparallel to the san andreas near cajon pass since the pleistocene the cajon pass stress orientation may reflect the current state of stress across the san andreas fault in which case it should change prior to right lateral slip on this fault or a shallow local perturbation to the san andreas stress field in which case it should rotate couterclockwise with depth determination of the stress orientations at greater depths in the cajon pass drillhole will better sample the near field of the san andreas fault and help resolve this question

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

multi scalar structure at dsdp odp site 504 costa rica rift iii faulting and fluid circulation constraints from integration of fms images geophysical logs and core data

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

state of lithospheric stress and borehole stability at deep sea drilling project site 504b eastern equatorial pacific hole 504b in the eastern equatorial pacific is the deepest hole to penetrate oceanic basement extending more than 1500 m beneath the seafloor two borehole televiewer bhtv logs were obtained in this hole one through the upper 1000 m of basement during deep sea drilling project leg 83 and the other extending an additional 260 m deeper into the crust during ocean drilling program leg 111 the two bhtv records have been combined and processed in terms of both acoustic amplitude and travel time in order to evaluate the extent and distribution of rock failure along the borehole wall a histogram of borehole enlargements versus azimuth depicts a dominant breakout azimuth of n122 5 e which corresponds to the direction of minimum principal stress sh and correlates well with stress orientations inferred from solutions to regional earthquake focal mechanisms furthermore the bimodality of this histogram with a secondary mode orthogonal to sh indicates that a significant number of enlargements are coalesced tensile fractures occurring along the orientation of sh the maximum principal stress the appearance of this orthogonal bimodal distribution suggests that the regional horizontal stress field is highly anisotropic a condition supported by seismic data which show all of the earthquakes in the area to be strike slip the frequency of borehole enlargements increases with increasing depth and depicts a systematic structural deterioration of the well bore the tensile fractures along the sh azimuth contribute to this degradation and appear to be induced by thermal stresses due to the injection of cold water into hot rock the frequency of these extensional features does not increase with depth rather their appearances can be directly correlated with shipboard efforts at deliberately cooling the well and or with the sudden resumption of drilling after the hole had been allowed to reequilibrate thermally for several days these latter borehole enlargements are more pronounced then those commonly associated with hydraulic fracturing the mechanisms for fracture initiation and growth based upon temperature contrasts between the well bore fluid and the adjacent rock may enhance rock failure

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

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

localized rotation of principal stress around faults and fractures determined from borehole breakouts in hole b of the taiwan chelungpu fault drilling project tcdp abstract to reveal details of stress perturbations associated with faults and fractures we investigated the faults and large fractures accompanied by stress induced borehole breakouts or drilling induced tensile fractures in hole b of the taiwan chelungpu fault drilling project tcdp then we determined the relationship between the faults and fractures and stress orientation changes we identified faults and fractures from electrical images of the borehole wall obtained by downhole logging but also from photographs and descriptions of retrieved core samples and measured the variations in the principal horizontal stress orientation ascertained from borehole breakouts observed on the electrical images in the vicinity of the faults and fractures identification of geological structures faults fractures and lithologic boundaries by electrical images only is difficult and may sometimes yield incorrect results in a novel approach therefore we used both the electrical images and core photographs to identify geological structures we found four patterns of stress orientation change or no change in the vicinity of faults and fractures in tcdp hole b i abrupt discontinuous rotation in the vicinity of faults or fractures ii gradual rotation iii suppression of breakouts at faults fractures or lithologic boundaries and iv no change in the stress orientation we recognized stress fluctuations that is heterogeneous mesoscale 10 cm stress distributions with respect to both stress orientation and magnitude in addition we found that stress state changes occurred frequently in the vicinity of faults fractures and lithologic boundaries

hydraulic fracturing stress measurements at yucca mountain nevada and relationship to the regional stress field hydraulic fracturing stress measurements and acoustic borehole televiewer logs were run in holes usw g 1 and usw g 2 at yucca mountain as part of the nevada nuclear waste storage investigations for the u s department of energy eight tests in the saturated zone at depths from 646 to 1288 m yielded values of the least horizontal stress s h that are considerably lower than the vertical principal stress s v in tests for which the greatest horizontal principal stress s h could be determined it was found to be less than s v indicating a normal faulting stress regime the borehole televiewer logs showed the presence of long in excess of 10 m vertical drilling induced fractures in the first 300 m below the water table these are believed to form by the propagation of small preexisting cracks under the excess downhole fluid pressures up to 5 2 mpa applied during drilling the presence of these drilling induced hydrofractures provides further confirmation of the low value of the least horizontal stresses a least horizontal principal stress direction of n60 w n65 w is indicated by the orientation of the drilling induced hydrofractures n25 e n30 e and the orientation of stress induced well bore breakouts in the lower part of usw g 2 n65 w this direction is in good agreement with indicators of stress direction from elsewhere at the nevada test site the observed stress magnitudes and directions were examined for the possibility of slip on preexisting faults using these data the coulomb criterion for frictional sliding suggests that for coefficients of friction close to 0 6 movement on favorably oriented faults could be expected for coefficients of friction of 1 0 preexisting faults of all orientations should be stable laboratory studies on the yucca mountain tuffs reported elsewhere yield coefficients of friction ranging from 0 6 to 0 9

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

in situ stress conditions at iodp site c0002 reflecting the tectonic evolution of the sedimentary system near the seaward edge of the kumano basin offshore from sw japan this paper presents a complete set of in situ stress calculations for depths of 200 1400 mbsf meters below seafloor at iodp site c0002 near the seaward margin of the kumano forearc basin offshore from southwest japan the vertical stress component was obtained by integrating bulk density calculations from moisture and density logging data and the two horizontal components were stochastically optimized by minimizing misfits between a probabilistic model and measured breakout widths for every 30 m vertical segment of the wellbore our stochastic optimization process reveals that the in situ stress regime is decoupled across an unconformity between an accretionary complex and the overlying kumano forearc basin the stress condition above the unconformity is close to the critical condition for normal faulting while below the unconformity the geologic system is stable in a normal to strike slip fault stress regime the critical state of stress demonstrates that the tectonic evolution of the sedimentary system has been achieved mainly by the regionally continuous action of a major out of sequence thrust fault during sedimentation in the forearc basin the stable stress condition in the accretionary prism is interpreted to have resulted from mechanical decoupling by the accommodation of large displacement along the megasplay fault

stress state and its anomaly observations in the vicinity of a fault in nantroseize expedition 322 to better understand the stress state and geological properties within the shallow shikoku basin southwest of japan two sites c0011a and c0011b were drilled in open ocean sediments using logging while drilling lwd and coring respectively resistivity image logging was performed at c0011a from sea floor to 950 m below sea floor mbsf at c0011b the serial coring was obtained in order to determine physical properties from 340 to 880 mbsf for the lwd images a notable breakout anomaly was observed at a depth of 615 m using resistivity images and a stress polygon the potential horizontal principal stress azimuth and its magnitude within the 500 750 mbsf section of the c0011a borehole were constrained borehole breakout azimuths were observed for the variation by the existence of a fault zone at a depth of 615 mbsf out of this fracture zone the breakout azimuth was located at approximately 109 12 subparallel to the nankai trough convergence vector 300 315 our calculations describe a stress drop was determined based on the fracture geometry a close 90 73 12 rotation implied a 100 stress drop defined as a maximum shear stress drop equal to 1 mpa the magnitude of the horizontal principal stresses near the fracture stress anomaly ranged between 49 and 52 mpa and the bearing to the vertical stress sv 52 mpa was found to be within the normal faulting stress regime low rock strength and a low stress level are necessary to satisfy the observations

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

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