Structure of the San Andreas fault zone at SAFOD from a seismic refraction survey

Hole, J. A.; Ryberg, Trond; Fuis, Gary S.; Bleibinhaus, Florian; Sharma, Antara

doi:10.1029/2005gl025194

Cited by 51 publications

(67 citation statements)

References 21 publications

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“…The Hole (1992) backprojection approach produces a minimum-structure model, whereas the Zelt and Barton (1998) matrix inversion can adjust spatial smoothing to enable additional nonrequired structure to be added to the model. Where nonuniqueness exists, these algorithms can fit the data with different structures (e.g., Hole et al, 2006). Excellent ray coverage, like that along line 4, reduces nonuniqueness, whereas poorer coverage, like that along line 6, allows a wider range of models to match the data.…”

Section: Refraction Analysismentioning

confidence: 99%

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

Fuis¹,

Bauer²,

Goldman³

et al. 2017

Bulletin of the Seismological Society of America

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The San Andreas fault (SAF) is one of the most studied strike-slip faults in the world; yet its subsurface geometry is still uncertain in most locations. The Salton Seismic Imaging Project (SSIP) was undertaken to image the structure surrounding the SAF and also its subsurface geometry. We present SSIP studies at two locations in the Coachella Valley of the northern Salton trough. On our line 4, a fault-crossing profile just north of the Salton Sea, sedimentary basin depth reaches 4 km southwest of the SAF. On our line 6, a fault-crossing profile at the north end of the Coachella Valley, sedimentary basin depth is ∼2-3 km and centered on the central, most active trace of the SAF. Subsurface geometry of the SAF and nearby faults along these two lines is determined using a new method of seismic-reflection imaging, combined with potential-field studies and earthquakes. Below a 6-9 km depth range, the SAF dips ∼50°-60°NE, and above this depth range it dips more steeply. Nearby faults are also imaged in the upper 10 km, many of which dip steeply and project to mapped surface fault traces. These secondary faults may join the SAF at depths below about 10 km to form a flower-like structure. In Appendix D, we show that rupture on a northeast-dipping SAF, using a single plane that approximates the two dips seen in our study, produces shaking that differs from shaking calculated for the Great California ShakeOut, for which the southern SAF was modeled as vertical in most places: shorter-period (T < 1 s) shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall, compared to shaking calculated for a vertical fault.

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Section: Refraction Analysismentioning

confidence: 99%

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

Fuis¹,

Bauer²,

Goldman³

et al. 2017

Bulletin of the Seismological Society of America

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“…To the southeast of the fault, seismic refl ection and refraction studies reveal a step-like feature in the P-wave velocities across the site, with a shallow, high-velocity region likely underlain by Salinian rocks to the southwest, and a low-velocity region to the east of the SAFOD boreholes ( Fig. 1B; Thurber et al, 2004;Zhang and Thurber, 2005;Hole et al, 2006). Hole et al (2001Hole et al ( , 2006 use seismic data to show a moderately northeast-dipping transition from high-to low-velocity rocks at the SAFOD site (Fig.…”

Section: Geological and Geophysical Settingmentioning

confidence: 99%

“…1B; Thurber et al, 2004;Zhang and Thurber, 2005;Hole et al, 2006). Hole et al (2001Hole et al ( , 2006 use seismic data to show a moderately northeast-dipping transition from high-to low-velocity rocks at the SAFOD site (Fig. 1B).…”

Section: Geological and Geophysical Settingmentioning

confidence: 99%

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models

et al. 2007

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We examine drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes to determine the lithology and deformational textures inthe fault zones and host rocks. Cutting samples represent the lithologies from 1.7-km map distance and 3.2-km vertical depth adjacent to the San Andreas Fault. We analyzed two hundred and sixty-six grain-mount thin-sections at an average of 30-m-cuttings sample spacing from the vertical 2.2-km-deep Pilot Hole and the 3.99-km-long Main Hole. We identify Quaternary and Tertiary(?) sedimentary rocks in the upper 700 m of the holes; granitic rocks from 760-1920 m measured depth; arkosic and lithic arenites, interbedded with siltstone sequences, from 1920 to ~3150 m measured depth; and interbedded siltstones, mudstones, and shales from 3150 m to 3987 m measured depth. We also infer the presence of at least fi ve fault zones, which include regions of damage zone and fault core on the basis of percent of cata-clasite abundances, presence of deformed grains, and presence of alteration phases at 1050, 1600-2000, 2200-2500, 2700-3000, 3050-3350, and 3500 m measured depth in the Main Hole. These zones are correlated with borehole geophysical signatures that are consistent with the presence of faults. If the deeper zones of cataclasite and alteration intensity connect to the surface trace of the San Andreas Fault, then this fault zone dips 80-85° southwest, and consists of multiple slip surfaces in a damage zone ~250-300 m thick. This interpretation is supported by borehole geophysical studies, which show this area is a region of low seismic velocities, reduced resistivity, and variable porosity.

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“…At SAFOD, in situ fault-related rocks from *3 km depth were sampled along the central creeping segment of the San Andreas Fault (SAF). The surface creep rate of the SAF near SAFOD is 20 mm/yr (TITUS et al 2006), and is interpreted to occur on multiple parallel strands in the subsurface (HOLE et al 2006;MCPHEE et al 2004;THURBER et al 2004;THURBER et al 2006;ZOBACK et al 2005). Within the SAF here, a series of repeating microearthquakes occur near 2,500-2,800 m vertical depth, or about *50-300 m from the actively creeping fault strands intersected by the SAFOD borehole ( Fig.…”

Section: Introductionmentioning

confidence: 99%

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

Bradbury

Davis

Shervais

et al. 2014

Pure Appl. Geophys.

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Abstract-We examine the fine-scale variations in mineralogical composition, geochemical alteration, and texture of the faultrelated rocks from the Phase 3 whole-rock core sampled between 3,187.4 and 3,301.4 m measured depth within the San Andreas Fault Observatory at Depth (SAFOD) borehole near Parkfield, California. This work provides insight into the physical and chemical properties, structural architecture, and fluid-rock interactions associated with the actively deforming traces of the San Andreas Fault zone at depth. Exhumed outcrops within the SAF system comprised of serpentinitebearing protolith are examined for comparison at San Simeon, Goat Rock State Park, and Nelson Creek, California. In the Phase 3 SAFOD drillcore samples, the fault-related rocks consist of multiple juxtaposed lenses of sheared, foliated siltstone and shale with blockin-matrix fabric, black cataclasite to ultracataclasite, and sheared serpentinite-bearing, finely foliated fault gouge. Meters-wide zones of sheared rock and fault gouge correlate to the sites of active borehole casing deformation and are characterized by scaly clay fabric with multiple discrete slip surfaces or anastomosing shear zones that surround conglobulated or rounded clasts of compacted clay and/or serpentinite. The fine gouge matrix is composed of Mgrich clays and serpentine minerals (saponite ± palygorskite, and lizardite ± chrysotile). Whole-rock geochemistry data show increases in Fe-, Mg-, Ni-, and Cr-oxides and hydroxides, Fe-sulfides, and C-rich material, with a total organic content of [1 % locally in the fault-related rocks. The faults sampled in the field are composed of meters-thick zones of cohesive to non-cohesive, serpentinite-bearing foliated clay gouge and black fine-grained fault rock derived from sheared Franciscan Formation or serpentinized Coast Range Ophiolite. X-ray diffraction of outcrop samples shows that the foliated clay gouge is composed primarily of saponite and serpentinite, with localized increases in Ni-and Cr-oxides and C-rich material over several meters. Mesoscopic and microscopic textures and deformation mechanisms interpreted from the outcrop sites are remarkably similar to those observed in the SAFOD core. Microscale to meso-scale fabrics observed in the SAFOD core exhibit textural characteristics that are common in deformed serpentinites and are often attributed to aseismic deformation with episodic seismic slip. The mineralogy and whole-rock geochemistry results indicate that the fault zone experienced transient fluid-rock interactions with fluids of varying chemical composition, including evidence for highly reducing, hydrocarbon-bearing fluids.

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Structure of the San Andreas fault zone at SAFOD from a seismic refraction survey

Cited by 51 publications

References 21 publications

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

Mineralogic and textural analyses of drill cuttings from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Initial interpretations of fault zone composition and constraints on geologic models

Composition, Alteration, and Texture of Fault-Related Rocks from Safod Core and Surface Outcrop Analogs: Evidence for Deformation Processes and Fluid-Rock Interactions

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