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A series of stimulation treatments were performed as part of the Engineered Geothermal System (EGS) experiment in the shallow open-hole section of Desert Peak well 27-15 (September 2010–November 2012). These injections at variable wellhead pressures, both below and above the magnitude of the least horizontal principal stress (Shmin), produced injectivity gains consistent with hydraulically induced mechanical shear and tensile failure in the surrounding rock. A conceptual framework for the overall Desert Peak EGS experiment is developed and tested based on a synthesis of available structural and geological data. These data include down-hole fracture attributes, in situ stress conditions, pressure interference tests, geochemical tracer studies, and observed induced seismicity. Induced seismicity plays a key role in identifying the geometry of large-scale geological structures that could potentially serve as preferential flow paths during some of the stimulation phases. The numerical code FLAC3D is implemented to simulate the reservoir response to hydraulic stimulation and to investigate in situ conditions conducive to both tensile and shear failure. Results from the numerical analysis show that conditions for shear failure could have occurred along fractures associated with a large northeast-trending normal fault structure located ∼400 m below the injection interval which coincides with the locations of most of the observed micro-seismicity. This structure may also provide a hydrologic connection between EGS well 27-15 and injection/production wells further to the south–southwest.

Creation of an Enhanced Geothermal System relies on stimulation of fracture permeability through self-propping shear failure that creates a complex fracture network with high surface area for efficient heat transfer. In 2010, shear stimulation was carried out in well 27-15 at Desert Peak geothermal field, Nevada, by injecting cold water at pressure less than the minimum principal stress. An order-of-magnitude improvement in well injectivity was recorded. Here, we describe a numerical model that accounts for injection-induced stress changes and permeability enhancement during this stimulation. We use the coupled thermo-hydrological–mechanical simulator FEHM to (i) construct a wellbore model for non-steady bottom-hole temperature and pressure conditions during the injection, and (ii) apply these pressures and temperatures as a source term in a numerical model of the stimulation. A Mohr–Coulomb failure criterion and empirical fracture permeability is developed to describe permeability evolution of the fractured rock. The numerical model is calibrated using laboratory measurements of material properties on representative core samples and wellhead records of injection pressure and mass flow during the shear stimulation. The model captures both the absence of stimulation at low wellhead pressure (WHP ≤1.7 and ≤2.4 MPa) as well as the timing and magnitude of injectivity rise at medium WHP (3.1 MPa). Results indicate that thermoelastic effects near the wellbore and the associated non-local stresses further from the well combine to propagate a failure front away from the injection well. Elevated WHP promotes failure, increases the injection rate, and cools the wellbore; however, as the overpressure drops off with distance, thermal and non-local stresses play an ongoing role in promoting shear failure at increasing distance from the well.

Sequestration of CO2 into subsurface reservoirs can play an important role in limiting future emission of CO2 into the atmosphere (e.g., Benson and Cole, 2008). For geologic sequestration to become a viable option to reduce greenhouse gas emissions, large‐volume injection of supercritical CO2 into deep sedimentary formations is required. These formations offer large pore volumes and good pore connectivity and are abundant (Bachu, 2003; U.S. Geological Survey Geologic Carbon Dioxide Storage Resources Assessment Team, 2013). However, hazards associated with injection of CO2 into deep formations require evaluation before widespread sequestration can be adopted safely (Zoback and Gorelick, 2012). One of these hazards is the potential to induce seismicity on pre‐existing faults or fractures. If these faults or fractures are large and critically stressed, seismic events can occur with magnitudes large enough to pose a hazard to surface installations and, possibly more critical, the seal integrity of the cap rock. The Decatur, Illinois, carbon capture and storage (CCS) demonstration site is the first, and to date, only CCS project in the United States that injects a large volume of supercritical CO2 into a regionally extensive, undisturbed saline formation. The first phase of the Decatur CCS project was completed in November 2014 after injecting a million metric tons of supercritical CO2 over three years. This phase was led by the Illinois State Geological Survey (ISGS) and included seismic monitoring using deep borehole sensors, with a few sensors installed within the injection horizon. Although the deep borehole network provides a more comprehensive seismic catalog than is presented in this paper, these deep data are not publically available. We contend that for monitoring induced microseismicity as a possible seismic hazard and to elucidate the general patterns of microseismicity, the U.S. Geological Survey (USGS) surface and shallow borehole …

Earthquake recurrence requires interseismic fault restrengthening which results from solid-state deformation in room temperature friction and indentation experiments. In contrast, exhumed fault zones show solution-transport processes such as pressure solution, and contact overgrowths influence fault zone properties. In the absence of fluid flow, overgrowths are driven by gradients in surface curvature where material is dissolved, diffuses, and precipitates at the contact without convergence normal to the contact. To determine the rate of overgrowth for quartz, we conducted single-contact experiments in an externally heated pressure vessel. Convergence was continuously monitored using reflected light interferometry through a long-working-distance microscope. Contact normal force was constant with an initial effective normal stress of 1.7 MPa, temperature was between 350 and 530°C, and water pressure was constant at 150 MPa. Two control experiments were conducted: one dry at 425°C and one bimaterial (sapphire) at 425°C and 150 MPa water pressure. No contact growth or convergence was observed in the controls. For wet single-phase contacts, growth was initially rapid and then decreased with time. No convergence was observed. Fluid inclusions indicate that the contact is not uniformly wetted. The contact is bounded by small regions of high aperture, reflecting local free-face dissolution as the source for the overgrowth. The apparent activation energy is ~125 kJ/mol. Extrapolation predicts rates of contact area increase orders of magnitude faster than in dry, room temperature and hydrothermal friction experiments, suggesting that natural strength recovery near the base of the seismogenic zone could be dominated by contact overgrowth.

The San Andreas Fault Observatory at Depth (SAFOD) scientific borehole near Parkfield, California crosses two actively creeping shear zones at a depth of 2.7 km. Core samples retrieved from these active strands consist of a foliated, Mg-clay-rich gouge containing porphyroclasts of serpentinite and sedimentary rock. The adjacent damage zone and country rocks are comprised of variably deformed, fine-grained sandstones, siltstones, and mudstones. We conducted laboratory tests to measure the permeability of representative samples from each structural unit at effective confining pressures, Pe up to the maximum estimated in situ Pe of 120 MPa. Permeability values of intact samples adjacent to the creeping strands ranged from 10−18 to 10−21 m2 at Pe = 10 MPa and decreased with applied confining pressure to 10−20–10−22 m2 at 120 MPa. Values for intact foliated gouge samples (10−21–6 × 10−23 m2 over the same pressure range) were distinctly lower than those for the surrounding rocks due to their fine-grained, clay-rich character. Permeability of both intact and crushed-and-sieved foliated gouge measured during shearing at Pe ≥ 70 MPa ranged from 2 to 4 × 10−22 m2 in the direction perpendicular to shearing and was largely insensitive to shear displacement out to a maximum displacement of 10 mm. The weak, actively-deforming foliated gouge zones have ultra-low permeability, making the active strands of the San Andreas Fault effective barriers to cross-fault fluid flow. The low matrix permeability of the San Andreas Fault creeping zones and adjacent rock combined with observations of abundant fractures in the core over a range of scales suggests that fluid flow outside of the actively-deforming gouge zones is probably fracture dominated.

Frictional sliding on quasi-statically deforming faults and fractures can be modeled efficiently using a linear complementarity formulation. We review the formulation in two dimensions and expand the formulation to three-dimensional problems including problems of orthotropic friction. This formulation accurately reproduces analytical solutions to static Coulomb friction sliding problems. The formulation accounts for opening displacements that can occur near regions of non-planarity even under large confining pressures. Such problems are difficult to solve owing to the coupling of relative displacements and tractions; thus, many geomechanical problems tend to neglect these effects. Simple test cases highlight the importance of including friction and allowing for opening when solving quasi-static fault mechanics models. These results also underscore the importance of considering the effects of non-planarity in modeling processes associated with crustal faulting.

The objective of the Unzen Scientific Drilling Project (USDP) is not only to reveal the structure and eruption history of the Unzen volcano but also to clarify the ascent and degassing mechanisms of the magma conduit. Conduit drilling (USDP-4) was conducted in 2004, which targeted the magma conduit for the 1990-95 eruption. The total drilled length of USDP-4 was 1995.75 m. Geophysical well logging, including resistivity, gamma-ray, spontaneous potential, sonic-wave velocity, density, neutron porosity, and Fullbore Formation MicroImager (FMI), was conducted at each drilling stage. Variations in the physical properties of the rocks were revealed by the well-log data, which correlated with not only large-scale formation boundaries but also small-scale changes in lithology. Such variations were evident in the lava dike, pyroclastic rocks, and breccias over depth intervals ranging from 1 to 40 m. These data support previous models for structure of the lava conduit, in that they indicate the existence of alternating layers of high-resistivity and high P-wave velocity rocks corresponding to the lava dikes, in proximity to narrower zones exhibiting high porosity, low resistivity, and low P-wave velocity. These narrow, low-porosity zones are presumably higher in permeability than the adjacent rocks and may form preferential conduits for degassing during magma ascent.

Detailed study of core samples of silicic tuff recovered from three geothermal wells along the strike-slip Great Sumatran fault zone near Silangkitang, North Sumatra, supports a model for enhanced hydrothermal circulation adjacent to this major plate-boundary fault. Two wells (A and C) were drilled nearly vertically ∼1 km southwest of the eastern (i.e., the principal) fault trace, and the third, directional well (B) was drilled eastward from the site of well A to within ∼100 m of the principal fault trace. The examined core samples come from depths of 1650–2120 m at measured well temperatures of 180–320 °C. The samples collected near the principal fault trace have the highest temperatures, the largest amount of secondary pore space that correlates with high secondary permeability, and the most extensive hydrothermal mineral development. Secondary permeability and the degree of hydrothermal alteration decrease toward the southwestern margin of the fault zone. These features indicate episodic, localized flow of hot, possibly CO 2 -rich fluids within the fault zone. The microstructure populations identified in the core samples correlate to the subsidiary fault patterns typical of strike-slip faults. The geothermal reservoir appears to be centered on the fault zone, with the principal fault strands and adjoining, highly fractured and hydrothermally altered rock serving as the main conduits for vertical fluid flow and advective heat transport from deeper magmatic sources.

We have re-examined asperity deformation predicted by joint closure models based on Greenwood and Williamson [1966] which use a statistical representation of loaded, rough surfaces. Although such models assume small elastic strains within contacting asperities (Hertzian contact) and well predict the observed dependence of closure on normal stress, large elastic normal strains measured in experiments violate the model assumptions. This inconsistency between observations and models can be resolved. The model dependence of closure on macroscopic normal stress results primarily from the statistics of the surface topography, and the functional dependence of closure on normal stress can be independent of assumed contact-scale elastic interactions. Thus, a joint model of the Greenwood and Williamson kind, modified to allow a portion of the elastic deformation to occur outside of the asperity contact region, predicts macroscopic behavior consistent with Hertzian models. Contact stresses derived from previously published models of this kind may be in error.

This year, the world has faced energetic and destructive earthquakes almost every month. In January, an M = 7.0 event rocked Haiti, killing an estimated 230,000 people. In February, an M = 8.8 earthquake and tsunami claimed over 500 lives and caused billions of dollars of damage in Chile. Fatal earthquakes also occurred in Turkey in March and in China and Mexico in April.

Many recent studies of stress-triggered seismicity rely on a fault failure model with a single free parameter, the apparent coefficient of friction, presumed to be a material constant with possible values 0 ≤ μ′ ≤ 1. These studies may present a misleading view of fault strength and the role of pore fluid pressure in earthquake failure. The parameter μ′ is intended to incorporate the effects of both friction and pore pressure, but is a material constant only if changes in pore fluid pressure induced by changes in stress are proportional to the normal stress change across the potential failure plane. Although specific models of fault zones permit such a relation, neither is it known that fault zones within the Earth behave this way, nor is this behavior expected in all cases. In contrast, for an isotropic homogeneous poroelastic model the pore pressure changes are proportional to changes in mean stress, μ′ is not a material constant, and −∞ ≤ μ′ ≤ +∞. Analysis of the change in Coulomb failure stress for tectonically loaded reverse and strike-slip faults shows considerable differences between these two pore pressure models, suggesting that such models might be distinguished from one another using observations of triggered seismicity (e.g., aftershocks). We conclude that using the constant apparent friction model exclusively in studies of Coulomb failure stress is unwise and could lead to significant errors in estimated stress change and seismic hazard.

Large areas of the United States long considered geologically stable with little or no detected seismicity have recently become seismically active. The increase in earthquake activity began in the mid-continent starting in 2001 ( 1 ) and has continued to rise. In 2014, the rate of occurrence of earthquakes with magnitudes ( M ) of 3 and greater in Oklahoma exceeded that in California (see the figure). This elevated activity includes larger earthquakes, several with M > 5, that have caused significant damage ( 2 , 3 ). To a large extent, the increasing rate of earthquakes in the mid-continent is due to fluid-injection activities used in modern energy production ( 1 , 4 , 5 ). We explore potential avenues for mitigating effects of induced seismicity. Although the United States is our focus here, Canada, China, the UK, and others confront similar problems associated with oil and gas production, whereas quakes induced by geothermal activities affect Switzerland, Germany, and others.

In the Eurasian part of the World Stress Map almost the whole region east of the Tornquist-Teisseyre line is terra incognita. The closure of this information gap is of fundamental importance to the understanding of the geodynamics of the Eurasian continent. A detailed analysis of stress-induced wellbore breakouts has been performed over a 4.1-km-long depth interval in the Vorotilov drillhole (VGS). The borehole is located in the central part of the Russian platform, right in the center of the Vorotilov meteorite impact crater 60 km to the NNE of the city of Nizni Novgorod. An ultrasonic borehole televiewer (BHTV) was used to obtain high-resolution acoustical images from the borehole wall. With an interactive system for analyzing BHTV data the azimuth and shape of borehole breakouts occurring in the depth range of 1.3–4.8 km were analyzed. A statistical analysis of the resulting orientation profile of the breakout azimuths yields an overall direction of the maximum horizontal principal stress S H of N 137°E ± 15°. Variations of breakout orientation with depth ranging from a few degrees up to more than 90° are seen on various depth scales. The observed stress direction of N 137°E agrees very well with the average S H orientation of N 145°E in Central Europe. If this measurement is taken as representative for the Russian platform, the stress field in Russia is only slightly rotated in comparison to Central Europe. This can possibly be interpreted as indicative for the stress field to be governed by broad scale tectonic forces, such as a strong contribution from the forces exerted by the collision zone in the Alpine-Himalayan belt and by the Mid-Atlantic ridge.

The authors are leading a new international initiative to conduct scientific drilling within the San Andreas fault zone at depths of up to 10 km. This project is motivated by the need to understand the physical and chemical processes operating within the fault zone and to answer fundamental questions about earthquake generation along major plate-boundary faults. Through a comprehensive program of coring, fluid sampling, downhole measurements, laboratory experimentation, and long-term monitoring, the authors hope to obtain critical information on the structure, composition, mechanical behavior and physical state of the San Andreas fault system at depths comparable to the nucleation zones of great earthquakes. The drilling, sampling and observational requirements needed to ensure the success of this project are stringent. These include: (1) drilling stable vertical holes to depths of about 9 km in fractured rock at temperatures of up to 300 C; (2) continuous coring and completion of inclined holes branched off these vertical boreholes to intersect the fault at depths of 3, 6, and 9 km; (3) conducting sophisticated borehole geophysical measurements and fluid/rock sampling at high temperatures and pressures; and (4) instrumenting some or all of these inclined core holes for continuous monitoring of earthquake activity, fluid pressure, more » deformation and other parameters for periods of up to several decades. For all of these tasks, because of the overpressured clay-rich formations anticipated within the fault zone at depth, the authors expect to encounter difficult drilling, coring and hole-completion conditions in the region of greatest scientific interest. « less

Pressure solution is widely regarded as a potentially important deformation mechanism along crustal faults and during aliagenesis, yet the mechanisms and kinetics of this process remain highly controversial. To better understand the fundamental factors controWing the rates of pressure solution at the grain-to-grain scale, we conducted experiments in which convex halite lenses were pressed against fiats of fused silica in brine. Fluid pressures were maintained at 0.1 MPa; temperatures and mean contact normal stresses ranged from 8.3 o to 90.2oC and 0.5 to 13.5 MPa, respectively. The geometry and growth rate of the contact spot between the two lenses and the rate at which the lenses approached one another (convergence) were monitored using reflected light interferometry and transmitted light photomicrography. Convergence occurred when halite and silica lenses were pressed together in brine (halite/silica experiments). No undercutting was observed, and dry control experiments indicated negligible dislocation creep. Convergence rates in experiments at 50.2oC ranged from 0.01 to 0.05/m/d, depending on mean normal stress and contact spot radius. The data are consistent with intergranular pressure solution (IPS) rate-limited by diffusion through an intergranular film with a very high diffusion coefficient ( 10 -5- 10 -7 cm2/s). The data further suggest that the diffusion coefficient and/or thickness of this film increases with decreasing normal stresses, at least for normal stresses less than about 4 MPa. As no island-channel boundary structures were observed, we propose that this film consists of a continuous layer of strongly adsorbed (i.e., structured) water that is maintained between the halite and silica lenses during deformation. Convergence rates in similar experiments conducted at a constant load of 0.11 N but at 8.3 o, 50.2 o, and 90.2oC were approximately constant at a given normal stress and contact spot size. The cause of this temperature insensitivity is unknown but might result from changes in interphase boundary structure or thickness with increasing temperature that are sufficient to offset the expected thermal activation. An experiment was also conducted in which a halite lens was pressed against a fused silica fiat coated with an 0.8-/m-thick film of Na-montmorillonite in brine. This clay film produced an approximately fivefold increase in convergence rates over those observed in a halite/ silica experiment conducted without clay at the same load and temperature. The strong sensitivity of IPS rates both to contact spot radius and to the presence of second phases along grain boundaries suggests that fine-grained, clay-rich fault gouges and multiphase granular aggregates should be particularly susceptible to pressure solution creep in the middle to upper crust.

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. These include the long term structural and compositional evolution of fault zones; fault creep; and the nucleation, propagation, arrest, and recurrence of earthquake ruptures. Besides the widely recognized physical role of fluid pressures in controlling the strength of crustal fault zones, it is also apparent that fluids can exert mechanical influence through a variety of chemical effects. The United States Geological Survey sponsored a Conference on the Mechanical Effects of Fluids in Faulting under the auspices of the National Earthquake Hazards Reduction Program at Fish Camp, California, from June 6 to 10, 1993. The purpose of the conference was to draw together and to evaluate the disparate evidence for the involvement of fluids in faulting; to establish communication on the importance of fluids in the mechanics of faulting between the different disciplines concerned with fault zone processes; and to help define future critical investigations, experiments, and observational procedures for evaluating the role of fluids in faulting. This conference drew together a diverse group of 45 scientists, with expertise in electrical and magnetic methods, geochemistry, hydrology, ore deposits, rock mechanics, seismology, and structural geology. Some of the outstanding questions addressed at this workshop included the following: 1. What are fluid pressures at different levels within seismically active fault zones? Do they remain hydrostatic throughout the full depth extent of the seismogenic regime, or are they generally superhydrostatic at depths in excess of a few kilometers? 2. Are fluid pressures at depth within fault zones constant through an earthquake cycle, or are they time-dependent? What is the spatial variability in fluid pressures? 3. What is the role of crustal fluids in the overall process of stress accumulation, release, and transfer during the earthquake cycle? Through what mechanisms might fluid pressure act to control the processes of rupture nucleation, propagation, and arrest? 4. What is the chemical role of fluids in facilitating fault creep, including their role in aiding solid-state creep and brittle fracture processes and in facilitating solution-transport deformation mechanisms? 5. What are the chemical effects of aqueous fluids on constitutive response, fractional stability, and long-term fault strength? 6. What are the compositions and physical properties of faultfluids at different crustal levels? 7. What are the mechanisms by which porosity and permeability are either created or destroyed in the middle to lower crust? What factors control the rates of these processes? How should these effects be incorporated into models of time-dependent fluid transport in fault zones? 8. What roles do faults play in distributing fluids in the crust and in altering pressure domains? In other words, when and by what mechanisms do faults aid in or inhibit fluid migration? What are the typical fluid/rock ratios, flow rates, and discharges for fault zones acting as fluid conduits? 9. Are fluids present in the subseismogenic crust, and by what transformation and/or transport processes are they incorporated into the shallower seismogenic portions of faults?

As part of the government response to the Deepwater Horizon blowout, a Well Integrity Team evaluated the geologic hazards of shutting in the Macondo Well at the seafloor and determined the conditions under which it could safely be undertaken. Of particular concern was the possibility that, under the anticipated high shut-in pressures, oil could leak out of the well casing below the seafloor. Such a leak could lead to new geologic pathways for hydrocarbon release to the Gulf of Mexico. Evaluating this hazard required analyses of 2D and 3D seismic surveys, seafloor bathymetry, sediment properties, geophysical well logs, and drilling data to assess the geological, hydrological, and geomechanical conditions around the Macondo Well. After the well was successfully capped and shut in on July 15, 2010, a variety of monitoring activities were used to assess subsurface well integrity. These activities included acquisition of wellhead pressure data, marine multichannel seismic profiles, seafloor and water-column sonar surveys, and wellhead visual/acoustic monitoring. These data showed that the Macondo Well was not leaking after shut in, and therefore, it could remain safely shut until reservoir pressures were suppressed (killed) with heavy drilling mud and the well was sealed with cement.

P wave velocity and orthogonally polarized S wave velocities were measured on 12 cores recovered from the Kola superdeep well at depths of 0 to 12 km. Measurements were made along the core axis at a frequency of 1 MHz, at confining pressures ranging from 2 to 100 MPa, and under dry and water-saturated conditions. Cores were chosen to sample a variety of lithologies and were used to estimate interval velocities based on a simplified geological column of the well. These interval velocities were then compared with sonic log and vertical seismic profile (VSP) data. High-pressure lab velocities correlated primarily with rock composition and texture. These laboratory velocities are generally in good agreement with both sonic log and VSP data, suggesting that extremely low velocities, as measured in unconfined laboratory samples or at low confining pressure, are the result of drilling and core-recovery-induced damage. The magnitude of this microcrack-induced damage generally increases with depth in a stepwise manner but with a few notable inversions. These inversions are characterized by a relatively small reduction in dry unconfined velocities compared to the in situ velocities. We interpret these inversions to be due to localized in situ stress relief related to faulting, fracturing, and/or hydrothermal alteration. We also observed pronounced S wave splitting in the cores, the analysis of which suggests that the stress relief microcracks tend to be aligned parallel to the foliation in gneisses and amphibolites (dip angle 28°–45°) rather then being subhorizontal These observations have important implications for the nature of gently dipping seismic reflections detected in the immediate vicinity of the Kola well.

[1] We analyzed measurements of the direction of maximum horizontal compressive stress as a function of depth in two scientific research wells near the San Andreas Fault in central and southern California. We found that the stress orientations exhibit scale-invariant fluctuations over intervals from tens of cm to several km. Similarity between the scaling of the stress orientation fluctuations and the scaling of earthquake frequency with fault size suggests that these fluctuations are controlled by stress perturbations caused by slip on faults of various sizes in the critically-stressed crust adjacent to the fault. The apparent difference in stress scaling parameters between the two studies wells seem to correspond to differences in the earthquake magnitude-frequency statistics for the creeping versus locked sections of the fault along which these two wells are located. This suggests that stress heterogeneity adjacent to active faults like the San Andreas may reflect variations in stresses and loading conditions along the fault.

Laboratory measurements of the strength of core samples from a drill hole located northwest of Parkfield, California, near the southern end of a creeping zone of the San Andreas fault, demonstrate that the fault is profoundly weak at this location and depth. This is because of the presence of the smectite clay mineral saponite — one of the weakest phyllosilicates known. The finding suggests that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals, rather than by high fluid pressure or other proposed mechanisms. This study reports on laboratory-strength measurements of fault core materials from a drill hole located northwest of Parkfield, California, near the southern end of a creeping zone of the San Andreas fault. It is found that the fault is profoundly weak at this location and depth, owing to the presence of the smectite clay mineral saponite—one of the weakest phyllosilicates known. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The San Andreas fault accommodates 28–34 mm yr−1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault2,3. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms1. The combination of these measurements of fault core strength with borehole observations1,4,5 yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

Acoustic and electrical image logs in fractured granitic rocks penetrated by U.S. Navy well 58A-10, Coso Wash," in the eastern margin of the Coso geothermal field, California, were compared to evaluate their relative ability to characterize fractures and fault rock textures and to measure stress orientations from borehole failure. Electrical image logs are sensitive to variations in mineralogy or porosity, which affect conductivity. Thus, they capture both open and healed natural fractures as well as rock foliation. In acoustic image logs, fractures and faults are principally revealed by increased roughness of the borehole wall and acoustic impedance contrasts caused by increased microcrack density or hydrothermally altered fault rock. Thus, they reveal rock fabric and healed fractures relatively poorly while favoring open fractures and well-developed fault zones. These tools are thus complementary and fracture characterization benefits from using both. Drilling-induced structures such as breakouts and tensile fractures that form at the borehole wall and petal-centerline fractures that form just ahead of the borehole floor record the orientation of the principal stresses. Although both types of logs produce good images of drilling-induced tensile fractures, acoustic logs are superior to electrical logs in recording the distribution and geometry of borehole breakouts and petal-centerline fractures because they produce a full 360 image of borehole wall reflectivity and radius. Analyses of repeat temperature logs reveal that zones of localized fluid flow coincide with large faults visible in both types of image logs. These faults are characterized by distinctive brittle fracture texture and are well oriented for slip.

To investigate the mechanisms and kinetics of pressure solution, we measured deformation at contacts between polished convex lenses of halite and polished flat lenses of either halite or fused silica in saturated brine at 50.2 ± 0.2°C and fluid pressures of 0.1 MPa. Loads, applied using small springs, ranged from 0.1–4.2 N; mean effective normal stresses within the contact zone ranged from 1–14 MPa. The geometry and diametric growth rate of the contact spot (neck growth) and the rate at which the lenses approached one another (convergence) were monitored during deformation using transmitted and reflected light photomicrography. Time-dependent convergence did not occur at measurable rates when two halite lenses were pressed together in saturated brine but did occur when halite and fused silica lenses were pressed together in brine. Convergence rates in the halite/silica experiments were 0.01–0.05 μm/day. Although the initial deformation during loading involved elastic and plastic processes, control experiments without brine showed no time-dependent convergence, indicating that dislocation creep did not contribute to the observed rates. No undercutting or cataclasis was observed in any experiments. Residual fluid inclusions were formed along grain boundaries between two halite lenses loaded in brine, indicating non-zero wetting angles.

We have conducted constant flow-rate permeability measurements on three core samples taken from the 12-km-deep well on the Kola Peninsula, USSR. All cores are from 11.4– 2.0 km depth. Pore pressures, Pp, used in these permeability measurements ranged from 112–117 MPa. Measurements were performed at effective confining pressures (Peff=pc-Pp) ranging from 10 to 400 MPa. The resulting permeabilities varied from approximately 2.5 × 10−17 m2 (25 μDa) at a Peff of 10 MPa to 1 × 10−22 m2 (0.1 nDa) at a Peff of 300 MPa. The unusual sensitivity of permeability to Peff exhibited by these samples is most likely the result of severe stress-relief crack damage that occurred during coring and sample retrieval. This strong pressure dependence underscores the importance of measuring permeability at in situ pressures to obtain meaningful bounds on in situ matrix permeabilities in this well. In an attempt to infer closure pressure of these stress-relief cracks, and provide estimates of in situ Peff, we have analyzed the pressure dependence of permeability, resistivity and crack porosity. By applying an equivalent-channel-model analysis, these data appear consistent with either an in situ pore pressure exceeding the hydrostat by as much as 100 MPa or a hydrostatic Pp and sub-lithostatic in situ vertical stress. In either case, if this closure pressure accurately reflects the in situ confining pressure, then an upper bound on in situ matrix permeabilities would be 1 × 10−20 to 2 × 10−19 m2 for the samples studied.

(1) We present a protocol for using paleomagnetic analysis to determine the absolute orientation of core recovered from the SAFOD borehole. Our approach is based on determining the direction of the primary remanent magnetization of a spot core recovered from the Great Valley Sequence during SAFOD Phase 2 and comparing its direction to the expected reference field direction for the Late Cretaceous in North America. Both thermal and alternating field demagnetization provide equally resolved magnetization, possibly residing in magnetite, that allow reorientation. Because compositionally similar siltstones and fine-grained sandstones were encountered in the San Andreas Fault Zone during Stage 2 rotary drilling, we expect that paleomagnetic reorientation will yield reliable core orientations for continuous core acquired from directly within and adjacent to the San Andreas Fault during SAFOD Phase 3, which will be key to interpretation of spatial properties of these rocks. Citation: Pares, J. M., A. M. Schleicher, B. A. van der Pluijm, and S. Hickman (2008), Paleomagnetic reorientation of San Andreas Fault Observatory at Depth (SAFOD) core, Geophys. Res. Lett., 35, L02306, doi:10.1029/2007GL030921. During Phase 3 of SAFOD, which is being conducted in the summer of 2007, � 600 m of continuous core will be acquired from directly within and adjacent to the active San Andreas Fault Zone in side tracks drilled off the existing borehole. These cores will be extensively tested in the laboratory to determine their mineralogy, geochem- ical composition, deformation mechanisms, frictional be- havior and physical properties. (3) Several physical properties measured in the core have a directional nature, including bedding, faults, veins, micro- fractures, seismic velocities, and permeability. Reconstruct- ing the in-situ orientation of recovered drill core (termed here reorientation) is hence of great importance when interpreting the structural, deformational and physical prop- erties of fault and country rocks. Paleomagnetism can be used to reconstruct the absolute orientation of the core by providing a reference direction relative to geographic coor- dinates. The paleomagnetic core reorientation method has been successfully used for a number of years (e.g., Fuller, 1969; Kodama, 1984; Shibuya et al., 1991) and offers distinct advantages over other methods. Measurements are made on samples from the cores following recovery from the borehole, and hence do not have any impact on the coring process itself (unlike, for example, scribing techni- ques that can lead to core jamming and poor recovery, especially in highly fractured rock). Because cores are oriented one piece at a time, paleomagnetic core reorienta- tion is generally more reliable than traditional scribed-core techniques, which require extremely precise correlations between core depths and orientation data acquired with a downhole orientation tool. This technique can be applied to recently drilled cores and stored old cores, even in the absence of real-time core orientation or borehole image log data. In this paper we report results of a paleomagnetic and mineralogic (SEM) study aimed at providing a reliable method for SAFOD core reorientation.

[1] We developed a robust Bayesian inversion scheme to plan and analyze laboratory creep compaction experiments. We chose a simple creep law that features the main parameters of interest when trying to identify rate-controlling mechanisms from experimental data. By integrating the chosen creep law or an approximation thereof, one can use all the data, either simultaneously or in overlapping subsets, thus making more complete use of the experiment data and propagating statistical variations in the data through to the final rate constants. Despite the nonlinearity of the problem, with this technique one can retrieve accurate estimates of both the stress exponent and the activation energy, even when the porosity time series data are noisy. Whereas adding observation points and/or experiments reduces the uncertainty on all parameters, enlarging the range of temperature or effective stress significantly reduces the covariance between stress exponent and activation energy. We apply this methodology to hydrothermal creep compaction data on quartz to obtain a quantitative, semiempirical law for fault zone compaction in the interseismic period. Incorporating this law into a simple direct rupture model, we find marginal distributions of the time to failure that are robust with respect to errors in the initial fault zone porosity.

[1] The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 Mw 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500–1900 m, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction (N115E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 m and 1310 m are close to the Chinshui Shale’s upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake. Citation: Wu, H.-Y., K.-F. Ma, M. Zoback, N. Boness, H. Ito, J.-H. Hung, and S. Hickman (2007), Stress orientations of Taiwan Chelungpu-Fault Drilling Project (TCDP) hole-A as observed from geophysical logs, Geophys. Res. Lett., 34, L01303, doi:10.1029/2006GL028050.

The objective of fault-zone drilling projects is to directly study the physical and chemical processes that control deformation and earthquake generation within active fault zones. An enormous amount of field, laboratory, and theoretical work has been directed toward the mechanical and hydrological behavior of faults over the past several decades. Nonetheless, it is currently impossible to differentiate between – or even adequately constrain – the numerous conceptual models of active faults proposed over the years. For this reason, the Earth science community is left in the untenable position of having no generally accepted paradigm for the mechanical behavior of faults at depth. One of the primary causes for this dilemma is the difficulty of either directly observing or inferring physical properties and deformation mechanisms along faults at depth, as well as the need to observe directly key parameters such as the state of stress acting on faults at depth, pore fluid pressure (and its possible variation in space and time), and processes associated with earthquake nucleation and rupture. Today, we know very little about the composition of active faults at depth, their constitutive properties, the state of in situ stress or pore pressure within fault zones, the origin of fault-zone pore fluids, or the nature and significance of time-dependent fault-zone processes.

[1] Washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the San Andreas Fault Observatory at Depth (SAFOD). Both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology, P-T conditions, deformation mechanisms, and fluid composition and abundance. Regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults. In addition, both smectites occurring as mixed-layer phases and serpentine minerals are found in association with active strands of the San Andreas Fault that were intersected during drilling, although their rheological influence is not yet fully known. Faults containing these mineralogical phases are prime candidates for continuous coring during Phase 3 of SAFOD drilling in the summer of 2007.

7209 Seismology: Earthquake dynamics andmechanics;7230Seismology:Seismicityandseismotectonics;8010Structural Geology: Fractures and faults. Citation: Hickman, S.,M. Zoback, and W. Ellsworth (2004), Introduction to specialsection: Preparing for the San Andreas Fault Observatory atDepth, Geophys. Res. Lett., 31, L12S01, doi:10.1029/2004GL020688.

[1] Time-dependent closure of fractures in quartz was measured in situ at 22–530°C temperature and 0.1–150 MPa water pressure. Unlike previous crack healing and rock permeability studies, in this study, fracture aperture is monitored directly and continuously using a windowed pressure vessel, a long-working-distance microscope, and reflected-light interferometry. Thus the fracture volume and geometry can be measured as a function of time, temperature, and water pressure. Relatively uniform closure occurs rapidly at temperatures and pressures where quartz becomes significantly soluble in water. During closure the aperture is reduced by as much as 80% in a few hours. We infer that this closure results from the dissolution of small particles or asperities that prop the fracture open. The driving force for closure via dissolution of the prop is the sum of three chemical potential terms: (1) the dissolution potential, proportional to the logarithm of the degree of undersaturation of the solution; (2) the coarsening potential, proportional to the radius of curvature of the prop; and (3) the pressure solution potential, proportional to the effective normal stress at the contact between propping particles and the fracture wall. Our observations suggest that closure is controlled by a pressure solution-like process. The aperture of dilatant fractures and microcracks in the Earth that are similar to those in our experiments, such as ones generated from thermal stressing or brittle failure during earthquake rupture and slip, will decrease rapidly with time, especially if the macroscopic stress is nonhydrostatic.

[1] Stress measurements made in the SAFOD pilot hole provide an opportunity to study the relation between crustal stress outside the fault zone and the stress state within it using an integrated mechanical model of a transform fault loaded in transpression. The results of this modeling indicate that only a fault model in which the effective friction is very low (

[1] Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, S Hmax , but show a generalized increase in the angle between S Hmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ± 10° at 1000-1150 m to a maximum of 69 ± 14° at 2050-2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in our estimates of horizontal stress magnitudes. If stress observations near the bottom of the pilot hole are representative of stresses acting at greater depth, then they are consistent with regional stress field indicators and an anomalously weak San Andreas Fault in an otherwise strong crust.

[1] Evidence for a weak San Andreas Fault includes (1) borehole heat flow measurements that show no evidence for a frictionally generated heat flow anomaly and (2) the inferred orientation of σ1 nearly perpendicular to the fault trace. Interpretations of the stress orientation data remain controversial, at least in close proximity to the fault, leading some researchers to hypothesize that the San Andreas Fault is, in fact, strong and that its thermal signature may be removed or redistributed by topographically driven groundwater flow in areas of rugged topography, such as typify the San Andreas Fault system. To evaluate this scenario, we use a steady state, two-dimensional model of coupled heat and fluid flow within cross sections oriented perpendicular to the fault and to the primary regional topography. Our results show that existing heat flow data near Parkfield, California, do not readily discriminate between the expected thermal signature of a strong fault and that of a weak fault. In contrast, for a wide range of groundwater flow scenarios in the Mojave Desert, models that include frictional heat generation along a strong fault are inconsistent with existing heat flow data, suggesting that the San Andreas Fault at this location is indeed weak. In both areas, comparison of modeling results and heat flow data suggest that advective redistribution of heat is minimal. The robust results for the Mojave region demonstrate that topographically driven groundwater flow, at least in two dimensions, is inadequate to obscure the frictionally generated heat flow anomaly from a strong fault. However, our results do not preclude the possibility of transient advective heat transport associated with earthquakes.

Abstract To determine factors controlling permeability variations within and adjacent to a fault-hosted geothermal reservoir at Dixie Valley, Nevada, we conducted borehole televiewer observations of wellbore failure (breakouts and cooling cracks) together with hydraulic fracturing stress measurements in six wells drilled into the Stillwater fault zone at depths of 2 to 3 km. Measurements in highly permeable wells penetrating the main geothermal reservoir indicate that the local orientation of the least horizontal principal stress, Shmin, is nearly optimal for normal faulting on the Stillwater fault. Hydraulic fracturing tests from these wells further show that the magnitude of Shmin is low enough to lead to frictional failure on the Stillwater and nearby subparallel faults, suggesting that fault slip is responsible for the high reservoir productivity. Similar measurements were conducted in two wells penetrating a relatively impermeable segment of the Stillwater fault zone, located 8 and 20 km southwest of the geothermal reservoir (wells 66–21 and 45–14, respectively). The orientation of Shmin in well 66–21 is near optimal for normal faulting on the nearby Stillwater fault, but the magnitude of Shmin is too high to result in incipient frictional failure. In contrast, although the magnitude of Shmin, in well 45–14 is low enough to lead to normal faulting on optimally oriented faults, the orientation of the Stillwater fault near this well is rotated by 40 from the optimal orientation for normal faulting. This misorientation, coupled with an apparent increase in the magnitude of the greatest horizontal principal stress in going from the producing to nonproducing wells, acts to inhibit frictional failure on the Stillwater fault zone in proximity to well 45–14. Taken together, data from the nonproducing and producing wells thus suggest that a necessary condition for high reservoir permeability is that the Stillwater fault zone be critically stressed for frictional failure in the current stress field. P. 79

Abstract Borehole televiewer, temperature, and flowmeter data recorded in six wells penetrating a geothermal reservoir associated with the Stillwater fault zone in Dixie Valley, Nevada, were used to investigate the relationship between reservoir permeability and the contemporary in situ stress field. Data from wells drilled into productive and nonproductive segments of the Stillwater fault zone indicate that permeability in all wells is dominated by a relatively small number of fractures striking parallel to the local trend of the fault. However, Coulomb failure analysis using our fracture orientations in conjunction with stress orientations and magnitudes determined by Ref. 1 suggests that fault zone permeability is high only when individual fractures as well as the overall Stillwater fault zone are optimally oriented and critically stressed for frictional failure. Fracture geometry may also play a significant role in determining reservoir productivity. The well-developed populations of low-angle fractures present in wells drilled into the producing segment of the fault are not present within the relatively impermeable segment of the Stillwater fault zone. P. 315

Lithification of a sediment to form a rock may involve cementation, diagenetic reactions, or compaction under load. In these experiments, convex halite lenses were pressed against fiat halite plates at 50° C in a specially designed microscope stage. A saturated brine surrounded the samples, which were observed during the experiment in transmitted and reflected light. No time-dependent convergence was observed between the two crystals, even at mean normal stresses of up to 14 M Pa at the contact. In all experiments, however, the contact (or neck) between the two crystals grew with time as material dissolved from the surrounding lens surfaces, diffused through the pore fluid, and precipitated at the neck. Neck growth rates did not appear to correlate with the applied load, but did systematically increase as the misorientation between the two crystals decreased. Our analysis of the shapes of fluid inclusions formed along the grain boundary within the neck suggests that the grain boundary energy is about 1.8 times greater than the fluid-solid interfacial energy. Neck growth appears to be driven by the reduction of interfacial energy rather than by mechanical loads. Assuming that the interfacial energy is isotropic, and incorporating some geometric simplifications, two models of neck growth were formulated. The rate-controlling steps in the models were either precipitation or diffusion in the pore fluid. The data fit either model equally well. Both models predict that neck growth rate will be rapid at first but will decrease with time, as was observed. Neck growth will lead to an increase in real area of contact between grains in a granular aggregate even without the introduction of supersaturated solutions and may be important in the induration of sediments and the strengthening of fault gouge.

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. These include the long-term structural and compositional evolution of fault zones; fault creep; and the nucleation, propagation, arrest, and recurrence of earthquake ruptures. Besides the widely recognized physical role of fluid pressures in controlling the strength of crustal fault zones, it is also apparent that fluids can exert mechanical influence through a variety of chemical effects. To address these issues, a “Red-Book” Conference on the Mechanical Effects of Fluids in Faulting was sponsored by the U.S. Geological Survey under the auspices of the National Earthquake Hazards Reduction Program at Fish Camp, Calif., from June 6–10, 1993. The coconvenors were Steve Hickman, Rick Sibson, and Ron Bruhn.

Permeability measurements were conducted on intact core samples from the Kola drill hole in Russia and the KTB drill hole in Germany. Samples included granodiorite gneisses, basalts and amphibolites from depths up to 11 km. The tests were intended to determine the pressure sensitivity of permeability and to compare the effects of stress relief and thermal microcracking on the matrix permeability of different rock types and similar samples from different depths. The pore pressure Pp was fixed at the estimated in situ pressure assuming a normal hydrostatic gradient; the confining pressure -Pc was varied to produce effective pressures (-Pe - -Pc- -Pp) of 5 to 300 MPa. The permeability of the basaltic samples was the lowest and most sensitive to pressure, ranging from 10 -2o to 10-23m 2 as effective pressure increased from 5 to only 60 MPa. In contrast, the granodiorite gneiss samples were more permeable and less sensitive to pressure, with permeability values ranging from 10 -l? to 10 -22 rn 2 as effective pressures increased to 300 MPa. Amphibolites displayed intermectiate behavior. There was an abundance of microfractures in the quartz-rich rocks, but a relative paucity of cracks in the mafic rocks, suggesting that the observed differences in permeability are based on rock type and depth, and that stress relief/thermal-cracking damage is correlated with quartz content. By applying the equivalent channel model of Walsh gcl Brace (1984) to the permeability data of the quartz-rich samples, we can estimate the closure pressure of the stress-relief cracks and thereby place bounds on the in situ effective pressure. This method may be useful for drill holes where the fluid pressure is not well constrained, such as at the Kola well. However, the use of crack closure to estimate in situ pressure was not appropriate for the basalt and amphibolite samples, because they are relatively crack-free in situ and remain so even after core retrieval. As a result, their permeability is near or below the measurable lower limit of our apparatus at the estimated in situ pressures of the rocks.

Microcracks in quartz {approximately} 100{mu}m in length and < {approximately}10 {mu}m in width heal in 4 h at 600 C and water pressure of 200 MPa (fluid pressure (P{sub f}) = confining pressure (P{sub c})). Healing is thermally activated; the activation energy is estimated to be between 80 and 35 kJ/mol, depending on the model assumed. Rates also show dependence on fluid pressure, chemistry, and crack dimensions. Faster healing rates are observed in smaller cracks. Thus, when new cracks are not being produced in rocks at elevated temperatures and pressures, fractures will have a vast range of lifetimes: macrofractures transport most of the fluid volume and seal relatively slowly, whereas microcracks allow pervasive penetration of fluid into the rock mass but heal quickly.