Scientific Drilling Into the San Andreas Fault Zone

Zoback, Mark D.; Hickman, Stephen H.; Ellsworth, William L.

doi:10.1029/2010eo220001

Cited by 130 publications

(122 citation statements)

References 25 publications

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“…One possibility is a greatly reduced effective normal stress eff due to the presence of near-lithostatic pore fluid pressure within fault zones [Byerlee, 1990;Rice, 1992;Faulkner and Rutter, 2001]. That explanation does not seem likely, at least for crustal faults, as measurements of pore pressure within Earth's crust generally indicate hydrostatic conditions [Townend and Zoback, 2000], even in the SAF itself [Zoback et al, 2010]. Other theories appeal to anomalously low frictional resistance, from either frictionally weak clayrich materials [Morrow et al, 2000;Carpenter et al, 2011] or weak fault zone fabrics [Collettini et al, 2009] at seismogenic depths.…”

Section: Introductionmentioning

confidence: 99%

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

2013

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[1] The majority of crustal faults host earthquakes when the ratio of average background shear stress b to effective normal stress eff is b / eff 0.6. In contrast, mature plate-boundary faults like the San Andreas Fault (SAF) operate at b / eff 0.2. Dynamic weakening, the dramatic reduction in frictional resistance at coseismic slip velocities that is commonly observed in laboratory experiments, provides a leading explanation for low stress levels on mature faults. Strongly velocity-weakening friction laws permit rupture propagation on flat faults above a critical stress level pulse / eff 0.25. Provided that dynamic weakening is not restricted to mature faults, the higher stress levels on most faults are puzzling. In this work, we present a self-consistent explanation for the relatively high stress levels on immature faults that is compatible with low coseismic frictional resistance, from dynamic weakening, for all faults. We appeal to differences in structural complexity with the premise that geometric irregularities introduce resistance to slip in addition to frictional resistance. This general idea is quantified for the special case of self-similar fractal roughness of the fault surface. Natural faults have roughness characterized by amplitude-to-wavelength ratios˛between 10 -3 and 10 -2 . Through a second-order boundary perturbation analysis of quasi-static frictionless sliding across a band-limited self-similar interface in an ideally elastic solid, we demonstrate that roughness induces an additional shear resistance to slip, or roughness drag, given by drag = 8 3˛2 G * / min , for G * = G/(1 -) with shear modulus G and Poisson's ratio , slip , and minimum roughness wavelength min . The influence of roughness drag on fault mechanics is verified through an extensive set of dynamic rupture simulations of earthquakes on strongly rate-weakening fractal faults with elastic-plastic off-fault response. The simulations suggest that fault rupture, in the form of self-healing slip pulses, becomes probable above a background stress level b pulse + drag . For the smoothest faults (˛ 10 -3 ), drag is negligible compared to frictional resistance, so that b pulse 0.25 eff . However, on rougher faults (˛ 10 -2 ), roughness drag can exceed frictional resistance. We expect that drag ultimately departs from the predicted scaling when roughness-induced stress perturbations activate pervasive off-fault inelastic deformation, such that background stress saturates at a limit ( b 0.6 eff ) determined by the finite strength of the off-fault material. We speculate that this strength, and not the much smaller dynamically weakened frictional strength, determines the stress levels at which the majority of faults operate.Citation: Fang, Z., and E. M. Dunham (2013), Additional shear resistance from fault roughness and stress levels on geometrically complex faults,

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Section: Introductionmentioning

confidence: 99%

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

2013

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“…The wider fault gouges of the SAF are likely caused by the cumulative displacement, which is much greater along the SAF than along the Chelungpu fault (Zoback et al, 2010).…”

Section: Structural Setting Of Safod and Tcdp Samplesmentioning

confidence: 99%

“…The main drill hole crosscuts multiple faults including two actively creeping strands, the Southwest Deforming Zone (SDZ) and the Central Deforming Zone (CDZ) (Fig. 1b;Zoback et al, 2010), which were revealed by pronounced and ongoing casing deformation (Bradbury et al, 2007;Zoback et al, 2010). Detailed descriptions of SAFOD Phase 3 core material from Holes E and G (Fig.…”

Section: The Safod Projectmentioning

confidence: 99%

“…As a consequence of this gap was that several international fault zone drilling projects have started to address fundamental questions about physical and chemical processes controlling faulting and earthquake generation (e.g. Hickman et al, 2004;Song et al, 2007;Zoback et al, 2007Zoback et al, , 2010Boullier, 2011). This paper is structured around a combined analysis of core samples from two drilling projects (San Andreas Fault Observatory at Depth/SAFOD; Taiwan Chelungpu-fault Drilling project/TCDP).…”

Section: Introductionmentioning

confidence: 99%

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Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

Janssen

Wirth

Wenk

et al. 2014

Journal of Structural Geology

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a b s t r a c tThe microstructures, mineralogy and chemistry of representative samples collected from the cores of the San Andreas Fault drill hole (SAFOD) and the Taiwan Chelungpu-Fault Drilling project (TCDP) have been studied using optical microscopy, TEM, SEM, XRD and XRF analyses. SAFOD samples provide a transect across undeformed host rock, the fault damage zone and currently active deforming zones of the San Andreas Fault. TCDP samples are retrieved from the principal slip zone (PSZ) and from the surrounding damage zone of the Chelungpu Fault. Substantial differences exist in the clay mineralogy of SAFOD and TCDP fault gouge samples. Amorphous material has been observed in SAFOD as well as TCDP samples. In line with previous publications, we propose that melt, observed in TCDP black gouge samples, was produced by seismic slip (melt origin) whereas amorphous material in SAFOD samples was formed by comminution of grains (crush origin) rather than by melting. Dauphiné twins in quartz grains of SAFOD and TCDP samples may indicate high seismic stress. The differences in the crystallographic preferred orientation of calcite between SAFOD and TCDP samples are significant. Microstructures resulting from dissolutioneprecipitation processes were observed in both faults but are more frequently found in SAFOD samples than in TCDP fault rocks. As already described for many other fault zones clay-gouge fabrics are quite weak in SAFOD and TCDP samples. Clay-clast aggregates (CCAs), proposed to indicate frictional heating and thermal pressurization, occur in material taken from the PSZ of the Chelungpu Fault, as well as within and outside of the SAFOD deforming zones, indicating that these microstructures were formed over a wide range of slip rates.

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“…During Phase 3 the SAFOD engineering and science teams successfully exhumed 39.9 meters of 10-cm-diameter continuous core, including cores from the two actively deforming traces of San Andreas Fault Zone (the SDZ and CDZ; Zoback et al, 2010). Figure 5 shows the sidetracks drilled laterally off the SAFOD main borehole in map and cross-sectional views.…”

Section: Phase 3 -Coring the San Andreas Fault Zonementioning

confidence: 99%

Scientific Drilling Into the San Andreas Fault ZoneAn Overview of SAFODs First Five Years

Zoback

Hickman

Ellsworth

2011

Scientific Drilling

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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.

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Scientific Drilling Into the San Andreas Fault Zone

Cited by 130 publications

References 25 publications

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

Scientific Drilling Into the San Andreas Fault ZoneAn Overview of SAFODs First Five Years

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Scientific Drilling Into the San Andreas Fault Zone

Cited by 130 publications

References 25 publications

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

Additional shear resistance from fault roughness and stress levels on geometrically complex faults

Faulting processes in active faults – Evidences from TCDP and SAFOD drill core samples

Scientific Drilling Into the San Andreas Fault ZoneAn Overview of SAFODs First Five Years

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Scientific Drilling Into the San Andreas Fault ZoneAn Overview of SAFODs First Five Years