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Roland Bürgmann - One of the best experts on this subject based on the ideXlab platform.
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a revised position for the primary strand of the pleistocene holocene San Andreas Fault in southern california
Science Advances, 2021Co-Authors: Kimberly Blisniuk, Katherine M Scharer, Roland Bürgmann, Warren D Sharp, Colin B Amos, M J RymerAbstract:The San Andreas Fault has the highest calculated time-dependent probability for large-magnitude earthquakes in southern California. However, where the Fault is multistranded east of the Los Angeles metropolitan area, it has been uncertain which strand has the fastest slip rate and, therefore, which has the highest probability of a destructive earthquake. Reconstruction of offset Pleistocene-Holocene landforms dated using the uranium-thorium soil carbonate and beryllium-10 surface exposure techniques indicates slip rates of 24.1 ± 3 millimeter per year for the San Andreas Fault, with 21.6 ± 2 and 2.5 ± 1 millimeters per year for the Mission Creek and Banning strands, respectively. These data establish the Mission Creek strand as the primary Fault bounding the Pacific and North American plates at this latitude and imply that 6 to 9 meters of elastic strain has accumulated along the Fault since the most recent surface-rupturing earthquake, highlighting the potential for large earthquakes along this strand.
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Slow slip events in the roots of the San Andreas Fault
Science advances, 2019Co-Authors: Baptiste Rousset, Roland Bürgmann, Michel CampilloAbstract:Episodic tremor and accompanying slow slip are observed at the down-dip edge of subduction seismogenic zones. While tremors are the seismic signature of this phenomenon, they correspond to a small fraction of the moment released; thus, the associated Fault slip can be quantified only by geodetic observations. On continental strike-slip Faults, tremors have been observed in the roots of the Parkfield segment of the San Andreas Fault. However, associated transient aseismic slip has never been detected. By making use of the timing of transient tremor activity and the dense Parkfield-area global positioning system network, we can detect deep slow slip events (SSEs) at 16-km depth on the Parkfield segment with an average moment equivalent to Mw 4.90 ± 0.08. Characterization of transient SSEs below the Parkfield locked asperity, at the transition with the creeping section of the San Andreas Fault, provides new constraints on the seismic cycle in this region.
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slow and go pulsing slip rates on the creeping section of the San Andreas Fault
AGU Fall Meeting Abstracts, 2014Co-Authors: R C Turner, Robert M. Nadeau, Manoochehr Shirzaei, Roland BürgmannAbstract:Rising and falling slip rates on the creeping section of the San Andreas Fault have been inferred from variations of recurrence intervals of characteristically repeating microearthquakes, but this observation has not previously been confirmed using modern geodetic data. Here we report on observations of this “pulsing” slip obtained from advanced multitemporal interferometric synthetic aperture radar (InSAR) data, confirmed using continuous GPS sites of the Plate Boundary Observatory. The surface deformation time series show a strong correlation to the previously documented slip rate variations derived from repeating earthquakes on the Fault interface, at various spatial and temporal scales. Time series and spectral analyses of repeating earthquake and InSAR data reveal a quasiperiodic pulsing with a roughly 2 year period along some sections of the Fault, with the earthquakes on the Fault interface lagging behind the far-field deformation by about 6 months. This suggests a temporal delay between the pulsing crustal strain generated by deep-seated shear and the time-variable slip on the shallow Fault interface, and that at least in some places this process may be cyclical. There exist potential impacts for time-dependent seismic hazard forecasting in California and, as it becomes better validated in the richly instrumented natural laboratory of the central San Andreas Fault, the process used here will be even more helpful in characterizing hazard and Fault zone rheology in areas without California's geodetic infrastructure.
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Geologic versus geodetic deformation adjacent to the San Andreas Fault, central California
Geological Society of America Bulletin, 2011Co-Authors: Sarah J. Titus, Mark Dyson, Charles Demets, Basil Tikoff, Frédérique Rolandone, Roland BürgmannAbstract:We combine geologic and global positioning system (GPS) data to characterize the style and magnitude of off-Fault deformation across the San Andreas Fault system in central California. Geologic structures record ∼12 km of both Fault-parallel and Fault-perpendicular displacements across creeping and locked portions of the San Andreas Fault. Analysis of 150 GPS site velocities suggests that the borderlands record 4–6 mm/yr of Fault-parallel and 3–5 mm/yr of Fault-perpendicular motion alongside the creeping segment, where elastic strain is minimized. The distribution of both long-term geologic and short-term geodetic deformation is affected by basement type, where more deformation is concentrated northeast of the San Andreas Fault on Franciscan basement. We suggest that at least half the Fault-parallel GPS deformation measured by GPS bordering the creeping segment must be accommodated by geologic structures; this permanent deformation needs to be incorporated into dynamic models of the Fault system. Elastic modeling of the San Andreas Fault in central California, which incorporates its well-known transition from locked to creeping behavior near Parkfield, predicts first-order variations in the GPS velocity field along the Fault and corresponding variations in dilatational strain rates. The strain rate pattern is dominated by a large contractional region northeast of the transition from locked to creeping behavior and a large extensional region southwest of the transition. The former coincides with the Coalinga and Kettleman Hills anticlines, the growth and development of which seem to have occurred under at least two kinematic conditions. We suggest that the onset of Fault creep in central California promoted the growth of these folds. By implication, Fault creep has been active over geologic time scales.
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tremor tide correlations and near lithostatic pore pressure on the deep San Andreas Fault
Nature, 2009Co-Authors: Amanda M Thomas, Robert M. Nadeau, Roland BürgmannAbstract:Amanda Thomas and colleagues identify a correlation between non-volcanic tremor activity near Parkfield, California, and extremely small, tidally induced shear stress parallel to the San Andreas Fault. Non-volcanic tremor is a weak seismic signal observed periodically on some major Faults. Thomas et al. suggest that the Parkfield tremors may represent shear failure on a critically stressed Fault in the presence of near-lithostatic pore pressure. Similarities with tremor in subduction-zone environments, such as Cascadia and Japan, indicate that these findings may also be relevant to other tectonic settings. Non-volcanic tremor was discovered nearly a decade ago; however, a thorough explanation of the geologic process responsible for tremor generation has yet to be determined. A robust correlation is now identified between extremely small, tidally induced shear stress parallel to the San Andreas Fault and non-volcanic tremor activity near Parkfield, California. Such tremor may represent shear failure on a critically stressed Fault in the presence of near-lithostatic pore pressure. Since its initial discovery nearly a decade ago1, non-volcanic tremor has provided information about a region of the Earth that was previously thought incapable of generating seismic radiation. A thorough explanation of the geologic process responsible for tremor generation has, however, yet to be determined. Owing to their location at the plate interface, temporal correlation with geodetically measured slow-slip events and dominant shear wave energy, tremor observations in southwest Japan have been interpreted as a superposition of many low-frequency earthquakes that represent slip on a Fault surface2,3. Fluids may also be fundamental to the failure process in subduction zone environments, as teleseismic and tidal modulation of tremor in Cascadia and Japan and high Poisson ratios in both source regions are indicative of pressurized pore fluids3,4,5,6,7. Here we identify a robust correlation between extremely small, tidally induced shear stress parallel to the San Andreas Fault and non-volcanic tremor activity near Parkfield, California. We suggest that this tremor represents shear failure on a critically stressed Fault in the presence of near-lithostatic pore pressure. There are a number of similarities between tremor in subduction zone environments, such as Cascadia and Japan, and tremor on the deep San Andreas transform3,4,5,6,7,8,9,10,11,12, suggesting that the results presented here may also be applicable in other tectonic settings.
M J Rymer - One of the best experts on this subject based on the ideXlab platform.
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a revised position for the primary strand of the pleistocene holocene San Andreas Fault in southern california
Science Advances, 2021Co-Authors: Kimberly Blisniuk, Katherine M Scharer, Roland Bürgmann, Warren D Sharp, Colin B Amos, M J RymerAbstract:The San Andreas Fault has the highest calculated time-dependent probability for large-magnitude earthquakes in southern California. However, where the Fault is multistranded east of the Los Angeles metropolitan area, it has been uncertain which strand has the fastest slip rate and, therefore, which has the highest probability of a destructive earthquake. Reconstruction of offset Pleistocene-Holocene landforms dated using the uranium-thorium soil carbonate and beryllium-10 surface exposure techniques indicates slip rates of 24.1 ± 3 millimeter per year for the San Andreas Fault, with 21.6 ± 2 and 2.5 ± 1 millimeters per year for the Mission Creek and Banning strands, respectively. These data establish the Mission Creek strand as the primary Fault bounding the Pacific and North American plates at this latitude and imply that 6 to 9 meters of elastic strain has accumulated along the Fault since the most recent surface-rupturing earthquake, highlighting the potential for large earthquakes along this strand.
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correlation of clayey gouge in a surface exposure of serpentinite in the San Andreas Fault with gouge from the San Andreas Fault observatory at depth safod
Journal of Structural Geology, 2012Co-Authors: Diane E Moore, M J RymerAbstract:Abstract Magnesium-rich clayey gouge similar to that comprising the two actively creeping strands of the San Andreas Fault in drill core from the San Andreas Fault Observatory at Depth (SAFOD) has been identified in a nearby outcrop of serpentinite within the Fault zone at Nelson Creek. Each occurrence of the gouge consists of porphyroclasts of serpentinite and sedimentary rocks dispersed in a fine-grained, foliated matrix of Mg-rich smectitic clays. The clay minerals in all three gouges are interpreted to be the product of fluid-assisted, shear-enhanced reactions between quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the Fault from a source in the Coast Range Ophiolite. We infer that the gouge at Nelson Creek connects to one or both of the gouge zones in the SAFOD core, and that similar gouge may occur at depths in between. The special significance of the outcrop is that it preserves the early stages of mineral reactions that are greatly advanced at depth, and it confirms the involvement of serpentinite and the Mg-rich phyllosilicate minerals that replace it in promoting creep along the central San Andreas Fault.
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talc bearing serpentinite and the creeping section of the San Andreas Fault
Nature, 2007Co-Authors: Diane E Moore, M J RymerAbstract:High rates of creep along parts of the San Andreas Fault have been attributed to low Fault strength associated with serpentinized rocks. This is problematic because the frictional strength of serpentine minerals is considered too high to account for any weakness. Diane Moore and Michael Rymer now report that talc — the soft magnesium silicate mineral familiar in its pure form as talcum powder — may be behind the high creep rate. They discovered talc in serpentinite samples collected during drilling of the SAFOD (San Andreas Fault Observatory at Depth) main hole in 2005. The frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep. This paper reports the discovery of talc in cuttings of serpentinite collected during drilling of the San Andreas Fault Observatory at Depth main hole in 2005. The frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas Fault. The section of the San Andreas Fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr-1 (ref. 1), and it is also the segment that yields the best evidence for being a weak Fault embedded in a strong crust2,3,4,5. Serpentinized ultramafic rocks have been associated with creeping Faults in central and northern California6,7,8, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas Fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on Fault strength, and these minerals also have the potential for unstable slip under some conditions9,10. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas Fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the Fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep11. Talc may therefore provide the connection between serpentinite and creep in the San Andreas Fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the Fault zone.
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talc bearing serpentinite and the creeping section of the San Andreas Fault
Nature, 2007Co-Authors: Diane E Moore, M J RymerAbstract:The section of the San Andreas Fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak Fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping Faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas Fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on Fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas Fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the Fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas Fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the Fault zone.
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high resolution seismic velocities and shallow structure of the San Andreas Fault zone at middle mountain parkfield california
Bulletin of the Seismological Society of America, 2002Co-Authors: Rufus D. Catchings, M J Rymer, J A Hole, M R Goldman, R Huggins, C LippusAbstract:A 5-km-long, high-resolution seismic imaging survey across the San Andreas Fault (SAF) zone and the proposed San Andreas Fault Observatory at Depth (SAFOD) drill site near Parkfield, California, shows that velocities vary both laterally and vertically. Velocities range from 4.0 km/sec) probably correspond to granitic rock of the Salinian block, which is exposed a few kilometers southwest of the SAF. The depth to the top of probable granitic rock varies laterally along the seismic profile but is about 600 m below the surface at the proposed SAFOD site. We observe a prominent, lateral low-velocity zone (LVZ) beneath and southwest of the surface trace of the SAF. The LVZ is about 1.5 km wide at 300-m depth but tapers to about 600 m wide at 750-m depth. At the maximum depth of the velocity model (750 m), the LVZ is centered approximately 400 m southwest of the surface trace of the SAF. Similar velocities and velocity gradients are observed at comparable depths on both sides of the LVZ, suggesting that the LVZ is anomalous relative to rocks on either side of it. Velocities within the LVZ are lower than those of San Andreas Fault gouge, and the LVZ is also anomalous with respect to gravity, magnetic, and resistivity measurements. Because of its proximity to the surface trace of the SAF, it is tempting to suggest that the LVZ represents a zone of fractured crystalline rocks at depth. However, the LVZ instead probably represents a tectonic sliver of sedimentary rock that now rests adjacent to or encompasses the SAF. Such a sliver of sedimentary rock implies Fault strands on both sides and possibly within the sliver, suggesting a zone of Fault strands at least 1.5 km wide at a depth of 300 m, tapering to about 600 m wide at 750-m depth. Fluids within the sedimentary sliver are probably responsible for observed low-resistivity values.
Diane E Moore - One of the best experts on this subject based on the ideXlab platform.
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correlation of clayey gouge in a surface exposure of serpentinite in the San Andreas Fault with gouge from the San Andreas Fault observatory at depth safod
Journal of Structural Geology, 2012Co-Authors: Diane E Moore, M J RymerAbstract:Abstract Magnesium-rich clayey gouge similar to that comprising the two actively creeping strands of the San Andreas Fault in drill core from the San Andreas Fault Observatory at Depth (SAFOD) has been identified in a nearby outcrop of serpentinite within the Fault zone at Nelson Creek. Each occurrence of the gouge consists of porphyroclasts of serpentinite and sedimentary rocks dispersed in a fine-grained, foliated matrix of Mg-rich smectitic clays. The clay minerals in all three gouges are interpreted to be the product of fluid-assisted, shear-enhanced reactions between quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the Fault from a source in the Coast Range Ophiolite. We infer that the gouge at Nelson Creek connects to one or both of the gouge zones in the SAFOD core, and that similar gouge may occur at depths in between. The special significance of the outcrop is that it preserves the early stages of mineral reactions that are greatly advanced at depth, and it confirms the involvement of serpentinite and the Mg-rich phyllosilicate minerals that replace it in promoting creep along the central San Andreas Fault.
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low strength of deep San Andreas Fault gouge from safod core
Nature, 2011Co-Authors: David A Lockner, Diane E Moore, C A Morrow, Stephen H HickmanAbstract: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.
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talc bearing serpentinite and the creeping section of the San Andreas Fault
Nature, 2007Co-Authors: Diane E Moore, M J RymerAbstract:High rates of creep along parts of the San Andreas Fault have been attributed to low Fault strength associated with serpentinized rocks. This is problematic because the frictional strength of serpentine minerals is considered too high to account for any weakness. Diane Moore and Michael Rymer now report that talc — the soft magnesium silicate mineral familiar in its pure form as talcum powder — may be behind the high creep rate. They discovered talc in serpentinite samples collected during drilling of the SAFOD (San Andreas Fault Observatory at Depth) main hole in 2005. The frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep. This paper reports the discovery of talc in cuttings of serpentinite collected during drilling of the San Andreas Fault Observatory at Depth main hole in 2005. The frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas Fault. The section of the San Andreas Fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr-1 (ref. 1), and it is also the segment that yields the best evidence for being a weak Fault embedded in a strong crust2,3,4,5. Serpentinized ultramafic rocks have been associated with creeping Faults in central and northern California6,7,8, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas Fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on Fault strength, and these minerals also have the potential for unstable slip under some conditions9,10. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas Fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the Fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep11. Talc may therefore provide the connection between serpentinite and creep in the San Andreas Fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the Fault zone.
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talc bearing serpentinite and the creeping section of the San Andreas Fault
Nature, 2007Co-Authors: Diane E Moore, M J RymerAbstract:The section of the San Andreas Fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak Fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping Faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas Fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on Fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas Fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the Fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the Fault, and its inherently stable sliding behaviour is consistent with Fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas Fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the Fault zone.
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strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak San Andreas Fault
Geology, 1996Co-Authors: Diane E Moore, David A Lockner, R Summers, M Shengli, J D ByerleeAbstract:Chrysotile-bearing serpentinite is a constituent of the San Andreas Fault zone in central and northern California. At room temperature, chrysotile gouge has a very low coefficient of friction (μ ≈ 0.2), raising the possibility that under hydrothermal conditions μ might be reduced sufficiently (to ≤0.1) to explain the apparent weakness of the Fault. To test this hypothesis, we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 °C and axial-shortening velocities as low as 0.001 μm/s. As temperature increases to ≈ 100 °C, the strength of the chrysotile gouge decreases slightly at low velocities, but at temperatures ≥200 °C, it is substantially stronger and essentially independent of velocity at the lowest velocities tested. We estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 MPa. Thus, on the sole basis of its strength, chrysotile cannot be the cause of a weak San Andreas Fault. However, chrysotile may also contribute to low Fault strength by forming mineral seals that promote the development of high fluid pressures.
Stephen H Hickman - One of the best experts on this subject based on the ideXlab platform.
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low strength of deep San Andreas Fault gouge from safod core
Nature, 2011Co-Authors: David A Lockner, Diane E Moore, C A Morrow, Stephen H HickmanAbstract: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.
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scientific drilling into the San Andreas Fault zone an overview of safod s first five years
Scientific Drilling, 2011Co-Authors: Mark D Zoback, Stephen H Hickman, William L EllsworthAbstract:Abstract. The San Andreas Fault Observatory at Depth (SAFOD) was drilled to study the physical and chemical processes controlling Faulting and earthquake generation along an active, plate-bounding Fault at depth. SAFOD is located near Parkfield, California and penetrates a section of the Fault that is moving due to a combination of repeating microearthquakes and Fault creep. Geophysical logs define the San Andreas Fault Zone to be relatively broad (~200 m), containing several discrete zones only 2–3 m wide that exhibit very low P- and S-wave velocities and low resistivity. Two of these zones have progressively deformed the cemented casing at measured depths of 3192 m and 3302 m. Cores from both deforming zones contain a pervasively sheared, cohesionless, foliated Fault gouge that coincides with casing deformation and explains the observed extremely low seismic velocities and resistivity. These cores are being now extensively tested in laboratories around the world, and their composition, deformation mechanisms, physical properties, and rheological behavior are studied. Downhole measurements show that within 200 m (maximum) of the active Fault trace, the direction of maximum horizontal stress remains at a high angle to the San Andreas Fault, consistent with other measurements. The results from the SAFOD Main Hole, together with the stress state determined in the Pilot Hole, are consistent with a strong crust/weak Fault model of the San Andreas. Seismic instrumentation has been deployed to study physics of Faulting – earthquake nucleation, propagation, and arrest – in order to test how laboratory-derived concepts scale up to earthquakes occurring in nature. doi: 10.2204/iodp.sd.11.02.2011
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scale invariant stress orientations and seismicity rates near the San Andreas Fault
Geophysical Research Letters, 2010Co-Authors: Amy Daylewis, Mark D Zoback, Stephen H HickmanAbstract:[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.
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introduction to special section preparing for the San Andreas Fault observatory at depth
Geophysical Research Letters, 2004Co-Authors: Stephen H Hickman, Mark D Zoback, William L EllsworthAbstract:: 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.
Indrevær Kjetil - One of the best experts on this subject based on the ideXlab platform.
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Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas Fault, southern California
'Geological Society of America', 2019Co-Authors: Bergh, Steffen G, Sylvester, Arthur G., Damte Alula, Indrevær KjetilAbstract:Miocene–Pliocene sedimentary rocks in the Mecca Hills, southern California, were uplifted and deformed by transpression along a restraining bend in the San Andreas Fault trace between the Orocopia and San Bernardino Mountains in Pleistocene time. This paper presents field evidence for three stages of structural evolution of a complex, asymmetric wedge-like flower structure, expressed as: (1) subhorizontal en échelon folds and Faults oblique to the San Andreas Fault; (2) steeply plunging folds subparallel to the San Andreas Fault; and (3) folds and thrust Faults fully parallel to the San Andreas Fault. We argue that the resulting flower-structure deformation formed successively from early distributed transpression through full (?) strain partitioning, rather than from active, synchronous, strike-slip–forming movements, as expected. The model is supported by crosscut relations of major folds and Faults and strain estimates from minor conjugate shear fracture sets. The polyphase evolution initiated on a steep right-lateral strand of the San Andreas Fault, producing thick Fault gouge. Then, the adjacent Neogene strata were folded en échelon outward in a uniformly distributed simple shear strain field. The subsidiary Skeleton Canyon Fault formed along a restraining bend that localized right-lateral shearing along this Fault, and reshaped the en échelon folds into steeply plunging folds almost parallel to the San Andreas Fault in a nascent partly partitioned strain field. The final kinematic stage generated SW-verging folds and thrust Faults trending parallel to the San Andreas Fault and decapitated the en échelon folds and Faults. The switch from early, distributed strike-slip to late-stage regional slip-partitioned shortening (fold-thrust) deformation may have been locally induced by the bending geometry of the Fault. The polyphase structures were active in successive order to balance the driving forces in one or more critical-angled transpressional and fold-and-thrust uplift wedges. Fault-related shortening, uplift, and erosion are still controlled in the Mecca Hills by combining and adjusting the wedges with low convergence angle, transpression, and lateral crustal motion in a San Andreas Fault plate scenario. Our model, therefore, addresses a more nuanced view of a polyphase flower-structure system and highlights the need to more carefully sort out spatially and temporally different kinematic data as a basis for analog and numerical modeling of transpressional uplift areas
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Polyphase kinematic history of transpression along the Mecca Hills segment of the San Andreas Fault, southern California
'Geological Society of America', 2019Co-Authors: Bergh, Steffen G, Sylvester, Arthur G., Damte Alula, Indrevær KjetilAbstract:Source at https://doi.org/10.1130/GES02027.1. Miocene–Pliocene sedimentary rocks in the Mecca Hills, southern California, were uplifted and deformed by transpression along a restraining bend in the San Andreas Fault trace between the Orocopia and San Bernardino Mountains in Pleistocene time. This paper presents field evidence for three stages of structural evolution of a complex, asymmetric wedge-like flower structure, expressed as: (1) subhorizontal en échelon folds and Faults oblique to the San Andreas Fault; (2) steeply plunging folds subparallel to the San Andreas Fault; and (3) folds and thrust Faults fully parallel to the San Andreas Fault. We argue that the resulting flower-structure deformation formed successively from early distributed transpression through full (?) strain partitioning, rather than from active, synchronous, strike-slip–forming movements, as expected. The model is supported by crosscut relations of major folds and Faults and strain estimates from minor conjugate shear fracture sets. The polyphase evolution initiated on a steep right-lateral strand of the San Andreas Fault, producing thick Fault gouge. Then, the adjacent Neogene strata were folded en échelon outward in a uniformly distributed simple shear strain field. The subsidiary Skeleton Canyon Fault formed along a restraining bend that localized right-lateral shearing along this Fault, and reshaped the en échelon folds into steeply plunging folds almost parallel to the San Andreas Fault in a nascent partly partitioned strain field. The final kinematic stage generated SW-verging folds and thrust Faults trending parallel to the San Andreas Fault and decapitated the en échelon folds and Faults. The switch from early, distributed strike-slip to late-stage regional slip-partitioned shortening (fold-thrust) deformation may have been locally induced by the bending geometry of the Fault. The polyphase structures were active in successive order to balance the driving forces in one or more critical-angled transpressional and fold-and-thrust uplift wedges. Fault-related shortening, uplift, and erosion are still controlled in the Mecca Hills by combining and adjusting the wedges with low convergence angle, transpression, and lateral crustal motion in a San Andreas Fault plate scenario. Our model, therefore, addresses a more nuanced view of a polyphase flower-structure system and highlights the need to more carefully sort out spatially and temporally different kinematic data as a basis for analog and numerical modeling of transpressional uplift areas
