Elevated Pore Pressures Research Articles

Analysis of the peat slide at Pollatomish, County Mayo, Ireland

A major landslide event occurred at Pollatomish, County Mayo, Ireland in September 2003, during a period of intense rainfall. It comprised about 30 significant individual longitudinal planar type slides of peat and weathered rock. Relatively simple limit state stability analyses, using the method of slices and an infinite slope analysis, were used to model the slide and it was found that the features observed on site could easily be reproduced. These included confirmation that thin layers of peat could be stable on steep slopes but the margin of safety reduces rapidly under elevated pore pressure conditions. As was observed in the field, the analyses suggested the most vulnerable zone was the upper layer of weathered rock but that slides could occur in the peat if its thickness was appreciable. Careful site characterization is vital in such studies. Here efforts have been made to understand the effect of fibers on the peat strength and some sensitivity analyes have been performed to assess the critical engineering parameters of the peat.

Fault rock analysis of the northern part of the Chelungpu Fault and its relation to earthquake faulting of the 1999 Chi‐Chi earthquake, Taiwan

Abstract The 1999 Chi‐Chi earthquake in Taiwan (Mw = 7.6) produced a surface rupture along the north–south‐striking Chelungpu thrust fault with pure dip‐slip (east side up) and left lateral strike‐slip displacements. Near‐field strong‐motion data for the northern part of the fault illustrate a distinct lack of the high‐frequency seismic radiation associated with a large slip (10–15 m) and a rapid slip velocity (2–4 m/s), suggesting a smooth seismic slip associated with low dynamic frictional resistance on the fault. A drillhole was constructed at shallow depths in the possible fault zones of the northern part of the Chelungpu Fault, which may have slipped during the 1999 earthquake. One of the zones consists of a 20‐cm‐thick, unconsolidated fault breccia with a chaotic texture lacking both discrete slip surfaces (e.g. Riedel shears) and grain crushing. Other possible fault zones are marked by the narrow (less than a few centimeters) gouge zone in which clayey material intrudes into the damaged zone outside of the gouge zone. These characteristic fault rock textures suggest that the slip mechanisms at shallow levels during the earthquake involved either granular flow of initially unconsolidated material or slip localization under elevated pore pressure along the narrow clayey gouge zone. Because both mechanisms lead to low dynamic frictional resistance on the fault, the rapid seismic slip in the deep portions of the fault (i.e. the source region of strong‐motion radiation) could have been accommodated by frictionless slip on the shallow portions of the fault. The combination of strong‐motion data and fault rock analysis suggests that smooth slip associated with low dynamic friction occurred on both the deep and shallow portions of the fault, resulting in a large slip between the source region and the surface in the northern region.

Flank spreading and collapse of weak-cored volcanoes

Volcanoes subjected to hydrothermal activity develop weak cores as a result of alteration and due to elevated pore pressures. Edifices constructed at the angle of repose of volcanoclastics, or at even more gentle slopes, respond to internal weakening by initially deforming slowly, but may then collapse catastrophically. Such a process has so far been described for only a few volcanoes, such as Casita, Nicaragua; however, the conditions for flank spreading are widespread and many, if not most volcanoes should suffer some alteration-related flank spreading. We provide analogue models that characterise the structure — surface deformation fields and internal structures — of a spreading flank. Deformation creates a characteristic concave-convex-concave flank profile producing structures such as basal thrusts, summit normal faults, grabens and strike-slip relay faults. Three deformation regimes are found: a ‘pit collapse’ regime is associated with very small volumes of ductile material located far from the edifice surface. This would not appear in nature, as time for deformation is greater than the lifetime of a volcano, unless very low rock viscosities are present. The other two regimes are flank spreading regimes, one symmetric and one asymmetric. The latter is the most common, as most volcanic structures are asymmetrical in form and in distribution of physical properties. The deformation is controlled by altered region dimensions, volume and position relative to the edifice, and to a lesser extent by its shape. As the flanks spread, landslides are created, initially on the steepened portion, but also from fault scarps. Major flank collapse may occur leading to explosive hydrothermal decompression and to a debris avalanche rich in hydrothermally altered material. We provide several new examples of volcanoes that have structures and morphologies compatible with flank spreading. We suggest that it is a common feature, important in the tectonics and hazards of many volcanoes.

Earthquake nucleation by transient deformations caused by the M = 7.9 Denali, Alaska, earthquake.

The permanent and dynamic (transient) stress changes inferred to trigger earthquakes are usually orders of magnitude smaller than the stresses relaxed by the earthquakes themselves, implying that triggering occurs on critically stressed faults. Triggered seismicity rate increases may therefore be most likely to occur in areas where loading rates are highest and elevated pore pressures, perhaps facilitated by high-temperature fluids, reduce frictional stresses and promote failure. Here we show that the 2002 magnitude M = 7.9 Denali, Alaska, earthquake triggered widespread seismicity rate increases throughout British Columbia and into the western United States. Dynamic triggering by seismic waves should be enhanced in directions where rupture directivity focuses radiated energy, and we verify this using seismic and new high-sample GPS recordings of the Denali mainshock. These observations are comparable in scale only to the triggering caused by the 1992 M = 7.4 Landers, California, earthquake, and demonstrate that Landers triggering did not reflect some peculiarity of the region or the earthquake. However, the rate increases triggered by the Denali earthquake occurred in areas not obviously tectonically active, implying that even in areas of low ambient stressing rates, faults may still be critically stressed and that dynamic triggering may be ubiquitous and unpredictable.

Evolution of strength recovery and permeability during fluid–rock reaction in experimental fault zones

Physical and chemical fluid–rock interactions are implicated in controlling earthquake nucleation and recurrence. In particular, interseismic compaction, sealing and healing of fractured fault rocks can lead to strength recovery and stabilisation of fault zones. In contrast, these same processes can also assist increases in pore fluid pressures and consequent destabilisation of faults. Here, we present high-temperature, hydrothermal experiments designed to assess the evolution of strength of fault zones in previously intact rock, and also characterise the associated changes to porosity and permeability. Cores of Fontainebleau sandstone were initially loaded to failure in a high-pressure gas–medium apparatus. The failed specimens were then hydrothermally reacted at 927°C for variable duration under isostatic conditions, and subsequently re-fractured to determine the ‘interseismic’ strength recovery. In the most extreme case, hydrothermally induced gouge compaction, cementation and crack healing resulted in 75% strength recovery after reaction for 6 h. Isostatic hydrothermal treatment also resulted in dramatic reduction in porosity and permeability. Strength of the fault zone following hydrothermal reaction appears to be closely correlated to porosity, consistent with previous studies on brittle failure of porous aggregates. The experimental results show how hydrothermal reactions in fault zones may lead to two competing, time-dependent effects; fault strengthening due to increased cohesion in the fault zone and fault weakening arising from elevated pore pressures within a well cemented, low-permeability gouge layer.

Pressure and porosity influences on VP−VS ratio in unconsolidated sands

Elevated pore pressures, commonly encountered in the shallow, unconsolidated section of the sedimentary column, present a significant hazard during the drilling and completion of offshore wells. The porosity of the ocean-floor sediments is high and a cover of low-permeability clay can prevent the underlying sediments from draining, even at very shallow depths below the seafloor. As further deposition loads the sediment, the entrapped fluids impede normal compaction by becoming pressurized. The lowered effective stress that results from the higher pore fluid pressure produces a proportional drop in the strength of non-cohesive sediments, resulting in very weak shallow sands. This weakness can result in washouts or fracturing in the shallow strata, potentially leading to the loss of the well and of neighboring wells.

Using Automated, High-precision Repicking to Improve Delineation of Microseismic Structures at the Soultz Geothermal Reservoir

— An automatic, adaptive, correlation-based algorithm for adjusting phase picks in large digital seismic data sets provides significant improvement in resolution of microseismic structures using only a small fraction of the time and manpower which would be required to re-analyze waveforms manually or semi-automatically. We apply this technique to induced seismicity at the Soultz-sous-Forets geothermal site, France. The method is first applied to a small, previously manually repicked subset of the catalogue so that we may compare our results to those obtained from painstaking, visual, cross-correlation-based techniques. Relative centroid-adjusted hypocenters show a decrease in median mislocation from 31 to 7 m for preliminary and automatically adjusted picks, respectively, compared to the manual results. Narrow, intersecting joint features not observed in the preliminary hypocenter cloud, but revealed through manual repicking, are also recovered using the automatic method. We then address a larger catalogue of ∼7000 microearthquakes. After relocating the events using automatic repicks, the percentage of events clustering within 5 m of their nearest neighbor increases form 5 to 26% of the catalogue. Hypocenter relocations delineate narrow, linear features previously obscured within the seismic cloud, interpreted as faults or fractures which may correspond to fluid propagation paths, or to changes in stress as a result of elevated pore pressures. RMS travel-time residuals for the larger data set are reduced by only 0.2%; however, phase-pick biases in the preliminary catalogue have influenced both the velocity model and station correction calculations, which will affect location residuals. These pick biases are apparent on the adjusted, stacked waveforms and correcting them will be important prior to future velocity model refinements.

Fault strength and transpressional tectonics along the Castle Mountain strike-slip fault, southern Alaska

Analyses of structures along the Castle Mountain fault reveal the mechanical relations and relative timing of orogen-parallel strike-slip faulting to distributed wrenching and shortening and to plate motion in the southern Alaskan transpressive margin. The Castle Mountain fault is a >200-km- long, orogen-parallel, right-lateral seismogenic fault in the southern Alaskan plate margin. Exposures of the fault in the study area have been exhumed from 3−4 km depth and ∼80 °C. Moment tensor summations and stress inversions of slip data from fault networks within 400 m of the Castle Mountain fault and analyses of structures in forearc deposits 2−4 km from the fault yield maximum incremental shortening and maximum compressive stress (σ 1 ) axes that are subhorizontal and trend ∼325°. This trend is close to the 340° North American–Pacific plate convergence direction. The inferred σ 1 makes a 70°−80° angle with the ∼070°-striking Castle Mountain fault, indicating that the fault slips at a lower shear stress than predicted by laboratory rock friction experiments if hydrostatic pore pressure exists in the fault (i.e., coefficient of friction ∼0.85, Byerlee9s Law). The forearc structures record coaxial strain and apparently remain in their formative orientations, showing that previously documented distributed shearing in the form of vertical-axis block rotations in the forearc ceased prior to formation of the structures in late Oligocene time. The mechanical weakness of the Castle Mountain fault probably results in part from its clay- rich gouge, which averages about 43 wt% clay phases. However, the gouge composition does not account for slip in response to σ 1 at 70°−80° to the fault. Another mechanism, such as elevated pore pressure, further weakens the fault and enables it to slip right laterally in response to the small component of oblique motion across the plate boundary. Upon its inception in early Tertiary time, the fault could not have formed a >200-km-long planar zone of low-friction clay-rich gouge or elevated pore pressure; thus, the mechanisms that weaken it must have developed over time. The distributed shearing in the forearc may have ended in response to the weakening and localization of dextral slip onto the Castle Mountain fault.

Anomalously high porosities in the proto‐decollement zone of the Barbados Accretionary Complex: Do they indicate overpressures?

Elevated pore pressures have been hypothesized to play a key role in the development of the decollement zones of accretionary complexes. Drilling within the proto‐decollement zone of the Barbados accretionary complex revealed anomalously high porosities up to 6 km seaward of the deformation front. Defining whether the high porosities are a reflection of overpressures is critical for understanding the seaward propagation of the decollement zone. In this investigation, we used numerical modeling to assess whether the high porosities reflect overpressures due to sedimentation. From this work, we concluded that reported permeabilities are not low enough to produce overpressures, even if additional fluid were contributed from areas of greater sedimentation. Furthermore, overpressures within the proto‐decollement zone are demonstrated to be inconsistent with normally compacted underlying sediments. Locally anomalous sediment strength provides a more likely explanation for the observed high porosities.

Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea

Research Article| July 01, 2000 Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea David Wiprut; David Wiprut 1Department of Geophysics, Stanford University, Stanford, California 94305-2215, USA Search for other works by this author on: GSW Google Scholar Mark D. Zoback Mark D. Zoback 1Department of Geophysics, Stanford University, Stanford, California 94305-2215, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (7): 595–598. https://doi.org/10.1130/0091-7613(2000)28<595:FRAFFA>2.0.CO;2 Article history received: 10 Dec 1999 rev-recd: 06 Apr 2000 accepted: 13 Apr 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation David Wiprut, Mark D. Zoback; Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea. Geology 2000;; 28 (7): 595–598. doi: https://doi.org/10.1130/0091-7613(2000)28<595:FRAFFA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Detailed seismic imaging and in situ stress and pore-pressure measurements are used to analyze reverse-fault reactivation of a long-dormant normal fault in the northern North Sea. Fault reactivation is caused by three factors: (1) a recent increase in the compressional stress in the area associated with postglacial rebound, (2) locally elevated pore pressure due to the presence of natural gas in a hydrocarbon reservoir on the footwall side of the fault, and (3) a fault orientation that is nearly optimally oriented for frictional slip in the present-day stress field. We demonstrate that the combination of these three factors induces fault slippage and gas leakage along sections of the previously sealing reservoir-bounding fault. We argue that similar pore-pressure triggering of fault slip in the crust may occur because of the accumulation of gas columns of, e.g., CO2 and He in the vicinity of tectonic faults. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Effects of pore pressure on compressional wave attenuation in a young oceanic basalt

Laboratory data are reported for ultrasonic compressional wave attenuation (αp) as a function of pore pressure in a Juan de Fuca Ridge dredge basalt. Four experiments have been made to determine the relationships between attenuation and quality factor (Qp) and confining and pore pressures in the shallow ocean crust. Attenuation was measured at 1) a constant differential pressure of 40 MPa; 2) confining pressures to 120 MPa and atmospheric pore pressures; 3) a constant confining pressure of 50 MPa while varying pore pressures; 4) a constant confining pressure of 100 MPa while varying pore pressures. For atmospheric pore pressures, αp ranges from 1.64 dB/cm to 7.08 dB/cm (Qp = 24 to 10). In addition, attenuation increases systematically with increasing pore pressure and decreasing differential pressure (confining pressure ‐ pore pressure). Results from DSDP and ODP downhole packer experiments suggest that the hydrostatic pore pressure regime may best approximate in situ conditions for young oceanic crust. Hydrostatic pore pressures (@ 5000 meters depth) reduce Qp as much as 35% from normal atmospheric pressure conditions; therefore, pore pressures generated in the upper oceanic crust may be responsible in part for the observed low seismic Qp within layer 2A. Qp measurements at elevated pore pressures agree well with seismic Qp data.

Geodetic observations of interseismic strain segmentation at the Sumatra Subduction Zone

Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5°S and half as much to the north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw&gt;8) thrust earthquakes. The rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle, suggesting that seismic segmentation is controlled by properties of the plate interface that persist through more than one earthquake cycle. Structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north.

Continental subduction and three‐dimensional crustal structure: The northern South Island, New Zealand

The three‐dimensional Vp and Vp/Vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment, and 3146 earthquakes have been relocated. Between 40 km and 100 km depth, the subducted plate is imaged as a relatively low‐velocity feature in the uppermost mantle, reflecting the continental nature of the subducted crust in this region. An increase in amplitude of this low‐velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction. Velocity variations within the subducted and overlying plates show some spatial correlation. This suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath Marlborough is currently not accommodating much active subduction. In the overlying plate, the Awatere fault is a major structural feature, associated with a low‐velocity zone extending to 23 km depth. There is a marked change in structure near this fault, with seismic velocities being lower to the southeast. A relatively high level of seismicity occurs in this region of lower seismic velocities, suggesting a relationship between the two. A possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust. The low‐velocity region in the overlying plate coincides with the region of most intense active deformation, suggesting it is relatively weak.

Listric normal faulting on the Cascadia continental margin

Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E‐W extension in this region. Fault activity began in the late Miocene and, in some cases, has continued into the Holocene. Most listric faults sole out into a subhorizontal décollement coincident with the upper contact of an Eocene to middle Miocene mélange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF. The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF. Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile. For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low‐angle (0.1°–2.5°) detachment coincident with the upper MBF contact. Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments. No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane. Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction‐related compression.

On the effect of pore pressure on the isotropic behavior of saturated porous media

This paper analyzes the small strain elasticity of fluid saturated porous media and the influence of the fluid static pressure, or pore pressure, on their isotropic constitutive properties. The porous medium is modelled as a lattice assembly of monodisperse spheres that interact through Hertz contacts. The size of the spheres and hence the matrix volume at zero solids stress are shown to be influenced by the pore pressure. Consequently, the structural pressure generated by elastic Hertzian contact deformations is shown to depend on both the volume occupied by the matrix and the pore pressure. This result is then generalized for random assemblies of spheres by using an isotropic constitutive law that is realistic for dry sands. The present findings suggest that the contributions of pore pressure and structural pressure to the total stress do not have a constant weighting as is often assumed, but one that is sensitive to the deformation state of the matrix. The theory also captures the characteristic loss of matrix stiffness during liquefaction at sufficiently elevated pore pressures.

Submarine and coastal sediment failure triggered by the 1995, M s = 6.1 R Aegion earthquake, Gulf of Corinth, Greece

On June 15th, 1995 a locally destructive earthquake of magnitude M s 6.1 on the Richter scale occurred offshore, 7.5 km NNE of the town of Aegion, in the western Gulf of Corinth (Greece). An offshore survey using 3.5 kHz subbottom profiling system and a remote operated vehicle (ROV) has shown that the earthquake caused small sized subaerial to submarine sediment failure in at least four sites, in three fan delta deposits: the Rododafni; the Eliki and the Tolofonas. The fan deltas were located within a radius of about 9 km from the epicentre. The areal size of the four sediment failure sites ranged from 2 × 10 4 m 2 to 6 × 10 5 m 2, whilst the volume of the failed masses ranged from 4 × 10 3 to 3 × 10 6 m 3. The sediment deformation types identified at the failure sites consist of ground cracking, rotational slides, elongated slides, sediment gravity flows and extrusion of mixtures of water and sand (sand boils). The sediment failure in the four sites affected the upper 5–6 m of well layered Holocene (?) topset and foreset fan delta deposits. The failure occurred on slopes ranging from 0.2 ° to 23 ° and the slip planes were all bedding planes which have a gradient from 0.2 ° to 21 °. The dominant instability mechanism that caused the sediment failure in the Rododafni and Eliki fan deltas is considered to be liquefaction of a shallow sub-surface horizon. The liquefaction was caused by elevated pore pressure enhanced perhaps by the presence of gas, resulting from the cyclic loading induced by the earthquake. The liquefied layer is assumed to have temporarily provided a failure surface for sliding to take place on and caused movement of sand that was ejected onto the surface. In the Tolofonas fan delta the causative mechanism in the case of the multi-block rotational slide is considered to be deformation of the underlying sediments caused by remoulding and/or liquefaction while in the case of the elongated slide it is considered to be the result of a combination of shear stress increase and/or strength degradation of the unconsolidated sediment. Both of these conditions could have been generated during the cyclic loading resulting from the Aegion earthquake. The study of historical documents reveals that sediment failure like those described above have also occurred at least four times during the past 2500 years in the same locations. Therefore, it is suggested that they could be repeated in the future by any earthquake event with a magnitude greater than 6 R, depending upon the proximity of the site to the earthquake epicentre.

Earthquake recurrence and glacial loading in western Washington

The tectonic setting of the Puget lowland was simpler during late Tertiary time (10 6 –10 7 yr) when the transfer of stress from subduction was not modulated by periodic glacial loading. Each advance of the Cordilleran Ice Sheet during the Quaternary Period created an additional 10–13 MPa of vertical load that may have stabilized the thrust sheet beneath the Puget lowland, causing background tectonic stresses to accumulate beyond the strength of active faults in their unglaciated condition. Conversely, deglaciation may have been accompanied by a brief episode of intense seismicity caused by decreasing overburden stress, elevated pore pressures, and elastic flexure in the shallow crust, and by viscous drag at the base of the crust during relaxation of isostatic anomaly. If accumulated tectonic stress was effectively “erased” during deglaciation, then some, or nearly all, of the present interglacial may have been seismically quiet relative to the modern regime. Such a scenario would complicate the use of paleoseismic approaches to establish the recurrence frequency of damaging earthquakes in this heavily populated region.

Hydrogeologic properties of a thrust fault within the Oregon Accretionary Prism

Two sets of hydrogeologic tests conducted at Ocean Drilling Program (ODP) Hole 892 on the Oregon Accretionary Prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone. The first set of tests consisted of shipboard packer tests conducted during ODP Leg 146 (fall 1992), while the second set of tests were constant‐drawdown and constant‐discharge tests conducted in fall 1993 using the submersible Alvin. Pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure (relative to hydrostatic) dropped below 0.315 to 0.325 MPa (λ* ∼ 0.53 to 0.54, where λ* = (pore pressure ‐ hydrostatic)/(lithostatic‐hydrostatic)). Analysis of the packer test data suggested an apparent background pressure of 0.25 MPa (λ* ∼ 0.42 to 0.50). Because the borehole had been open for 12 hours prior to the packer tests, formation pore pressures may have exceeded this value prior to drilling of the borehole. These overpressures dissipated by the time the second set of tests were conducted. One possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone. The second set of tests were conducted at pressures (≤0.019 MPa or λ* ∼ 0.03) below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests (when fractures were open). Constraints on fluid flow rate along the fault are provided by observed displacement in a bottom‐simulating reflector (BSR) at its intersection with the fault zone. The closed‐fracture transmissivities are insufficient to produce flow rates capable of displacing the BSR; therefore open‐fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed BSR displacement. In addition, calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site; fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor.

The Little Skull Mountain, Nevada, earthquake of 29 June 1992: Aftershock focal mechanisms and tectonic stress field implications

The ML = 5.6 29 June 1992 Little Skull Mountain earthquake occurred in the southern Nevada Test Site, Nye County, Nevada, within 20 km of a potential national nuclear waste repository at Yucca Mountain, Nevada. While microseismicity in the immediate epicentral vicinity occurred for years prior to the mainshock, foreshock activity ( ML 2) were also sufficiently well recorded to provide local network P -wave focal-mechanism solutions. The mainshock and the majority of the aftershocks through September 1992 display predominantly normal-slip focal mechanisms, but unambiguous strike-slip and oblique-slip focal-mechanism solutions are also present in the aftershock series. The mainshock and immediate foreshock depths were 10 ± 1 km below sea level, and depths of focus of aftershocks range from about 6 to 12 km. A large subset of the aftershock hypocenters define a southeast-dipping structure, probably the mainshock fault plane. No variation of mechanism style with depth is apparent. Two mainshock and 84 aftershock focal-mechanism solutions were input into Gephart's (1990) stressfield inversion program, and principal compressional stress directions, σ j , j = 1, 2, 3, were determined, as well as the stress difference ratio R = (σ1 − σ2)/(σ1 − σ3). The results indicate a uniform stress field in which σ3 (minimum stress) is oriented 116°/5° (azi, plunge) and σ1 (maximum stress) is oriented 219°/69° with R = 0.35. The inversion's selection of preferred nodal planes invokes Bott's (1959) criterion, which states that slip occurs in the direction of resolved shear traction on preexisting fault surfaces. For the predominantly normal-slip Little Skull Mountain earthquake mechanisms having ∼northeast-striking planes, the southeast, moderately to steeply dipping (dip > 55°) nodal planes are selected. However, for the dip-slip earthquake mechanisms exhibiting more northerly trending strikes, shallowly to moderately west-dipping (25° < dip < 65°) nodal planes are selected. When a Coulomb-Mohr analysis was applied to the focal mechanisms using the above principal stress directions and an assumed ratio σ1/σ3, the ratio of resolved shear stress to normal stress, τ/σ n , on the southeast-dipping nodal plane of the mainshock exhibited a near-maximum level, suggesting a high fault strength. In contrast, a wide range of fault strengths, or apparent coefficients of friction, was suggested by the τ/σ n ratios for Little Skull Mountain aftershock preferred nodal planes. Seismicity on misoriented faults, i.e., those on which shear stress was relatively low, may have been the result of a temporarily elevated fluid pore pressure following the mainshock.

Couplled Geomechanical-thermal Simulation For Deforming Heavy-oil Reservoirs

Abstract The paper describes a novel numerical model for the solution of poro-elasto-plasticity and multiphase, thermal flow in unconsolidated heavy-oil and oil-sand reservoirs. The elasto-plastic deformation is calculated using a finite-element incremental plasticity model with Mohr-Coulomb and Drucker-Prager as the yield criteria. This model is coupled with CMG's thermal simulator STARS which is capable of handling many advanced thermal recovery processes. The geomechanical calculation is in fact an effective stress analysis which solves a set of force balance (equilibrium) equations based on total stresses. These are augmented by the elastic material behaviors, plastic yield criteria, plastic flow rule, work-hardening law, and the strain-displacement relationships. Besides volumetric deformation, the solution gives the displacements in each direction, stresses, and strain distribution in the reservoir. The behaviors of reservoir multiphase flow and heat transfer processes are most significantly affected by the volume change and the associated permeability increase. The volume change is calculated by the plasticity model, whereas the permeability increase is related to the volume change via tabular data. The prediction of shear failure region, the rotation of principal stress axis, and the reduction of effective stresses as a result of pore pressure increase, are all important phenomena affecting the placement of fluid and hear in the formation. Introduction Considerable experiences on the in-situ recovery of heavy oil and bitumen from unconsolidated media have been accumulated both in field operations and in laboratory research over the last two decades. The significance of geomechanical responses on the recovery of these reservoirs is now generally recognized. In order to understand the recovery processes, we must consider the complex interaction of multiphase fluid flow, heat transfer, changes in stress state, and formation deformation as a result of injection and production operations. Previous work on the coupling of stress modeling with reservoir simulation(1,2) uses linear elasticity. However, extensive experimental data on oil sands indicate extremely complex constitutive behaviors at elevated pore pressure and temperature(3–6). Oil sands are dense frictional materials which tend to dilate under shear. This dilatancy is the result of an increase in deviatory stresses which leads to the slippage and loss of grain-to-grain contacts. Since the original sand fabric is disturbed, strains thus accumulated are irrecoverable. More recent contributions to this class of coupled simulations(7–11) treat the nonlinearity and plasticity behaviors of oil sands with varying degrees or sophistication. The stresses and strains are related by the material constructive model which must be satisfied for the geomechanical calculation. In this work, an elastoplastic formulation is chosen such that plastic deformation as a result of shear dilatancy can he modeled. During cyclic steam stimulation, the sand matrix goes through several loading and unloading cycles. This leads to complex deformation history which must be properly accounted for. As a reservoir grid block is loaded in the elastic regime, elastic strains accumulate which are recoverable upon unloading. The stress space is delimited by the yield surface. As the stress state reaches the yield surface, additional loading will cause the material to yield plastically.