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Weakness of the San Andreas Fault Revealed by Samples from the Active Fault Zone B

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Weakness of the San Andreas Fault Revealed by Samples from the Active Fault Zone B LETTERS PUBLISHED ONLINE: 27 FEBRUARY 2011 | DOI: 10.1038/NGEO1089 Weakness of the San Andreas Fault revealed by samples from the active fault zone B. M. Carpenter*, C. Marone and D. M. Saffer Understanding the strength and slip behaviour of tectonic Phase III of the San Andreas Fault Observatory at Depth faults is a central problem in earthquake physics and seismic- (SAFOD) drilling project near Parkfield, California, provided direct hazard assessment1–8. Many major faults, including the San access to rocks from the active SAF zone. Here, we describe the first Andreas Fault, are weak compared with the surrounding rock, friction measurements on samples obtained during SAFOD Phase but the cause of this weakness is debated1. Previous measure- III drilling through the actively slipping fault. Sample locations ments of the frictional strength of San Andreas Fault rocks are constrained by borehole logging and by direct correlation of are too high to explain the observed weakness1,9–13. However, the cuttings with core obtained across the active fault (Hole G, these measurements relied on samples taken at a distance from Core Runs 4–6; ref. 13). the active fault or from weathered surface samples. Recent The SAFOD borehole (Fig. 1a) penetrated two actively creeping drilling into the San Andreas Fault9 has provided material from strands of the SAF at approximately 2.7 km vertical depth, returning the actively slipping fault at seismogenic depths. Here we cuttings and core of the wall rock and fault zone. Geophysical present systematic measurements of the frictional properties logs indicate a ∼200-m-wide damage zone characterized by low and composition of the San Andreas Fault at 2.7 km depth, seismic velocity and electrical resistivity surrounding two active including the wall rock and active fault. We find that the fault fault strands (Fig. 1a inset). The lower fault strand, at 3,296.3– is weak relative to the surrounding rock and that the fault 3,298.9 m drilling depth, is the wider of the two and accommodates rock exhibits stable sliding friction behaviour. The fault zone nearly all fault motion (Fig. 1b; ref. 13). Core from this fault contains the weak mineral smectite and exhibits no frictional contains highly sheared rock with penetrative fabric characterized healing—bonds in the material do not heal after rupture. Taken by clay coatings and striations (Fig. 1c; ref. 13). Wall rock together, the low inherent strength and lack of healing of southwest of the fault consists of a sheared sandstone and siltstone the fault-zone material could explain why the San Andreas sequence; rock northeast of the fault consists of highly sheared Fault slips by aseismic creep and small earthquakes in central siltstone and mudstone. California, rather than by large, destructive earthquakes. We conducted measurements on cuttings returned from the The apparent weakness of major tectonic faults, as suggested lower fault and surrounding wall rock. The relative depths of by heat-flow measurements (see for example refs 10,11) and stress the cutting samples are known with high precision (to within orientations (see for example ref. 12), has been widely debated1. One ∼0:5 m); however, absolute locations are less well constrained of the fundamental unknowns is the absolute magnitude of shear owing to uncertainties associated with drilling-mud circulation stress acting on faults. Earthquake stress drops are generally between times. We use two approaches to locate the cuttings relative to 1 and 10 MPa; however, the shear stress that is relieved during an the active fault zone, and to accurately register the cutting depth earthquake is thought to be a small percentage of the absolute fault (CD) in a reference frame of measured depth, as defined from strength. If the major faults that bound Earth's tectonic plates are core and geophysical logging data. First, we conducted X-ray strong, shear stresses at seismogenic depths are likely to be 100 MPa diffraction analyses for all cutting samples (Supplementary Fig. S1 or higher, whereas this number could be a factor of 10 or more lower and Table S1). Only the sample from the fault zone (at 3,304.8 m if these faults are frictionally weak. CD) contains di-octahedral smectite (montmorillonite), and it is A number of studies have concluded that the San Andreas comprised of ∼14 wt% smectite. Existing analysis of the intact core Fault (SAF) is mechanically weak10–12. Several mechanisms have recovered from the fault zone also shows that this smectite is found been proposed, including the presence of weak minerals such as only in the fault rock18,19. Second, because we know the measured talc2 or various clays3, the formation of fabric within the fault depths at which drilling began and ended (3,294.9 m and 3,311.8 m, zone4,5 or pore pressure far in excess of hydrostatic6–8. Each of respectively), we can register the cutting depths to these positions these mechanisms is supported by different aspects of a currently and thus to the reported fault-zone depth as defined by analysis of incomplete understanding of fault properties at seismogenic depths. the drill core and downhole logs. Six of our samples (Supplementary One central problem in resolving the debate about the strength Fig. S1) come from the cored interval, drilling depth 3,294.9– of the SAF is that there are no measurements of frictional 3,311.8 m. Two further samples were obtained beyond the cored strength for carefully located samples from the actively slipping interval, at 3,317.1 and 3,320.8 m, to ensure that they were from fault at seismogenic depths. Existing data are limited to shear the wall rock to the northeast of the fault. On the basis of zones in the country rock away from the active fault and/or these registered depths (herein termed `cutting depths', CD), we to samples from weathered surface outcrop exposures (Fig. 1a), constrain the depth of origin for our cuttings to 3,294.9–3,320.8 m and indicate friction coefficients of ∼0:30–0.55 (refs 15–17), CD, with a sample from the active SAF zone at 3,304.8 m CD. which are considerably higher than those required to satisfy Samples were washed on site during the drilling operations in a weak-fault models10–12. 106 µm sieve and then air-dried. Before our experiments, we ran Department of Geosciences and Center for Geomechanics, Geofluids, and Geohazards, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. *e-mail: [email protected]. NATURE GEOSCIENCE j ADVANCE ONLINE PUBLICATION j www.nature.com/naturegeoscience 1 © 2011 Macmillan Publishers Limited. All rights reserved. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1089 a NE SW Buzzard Canyon Gold Hill SAF SAFOD Fault Fault 0 Quaternary alluvium ? Paso 500 ? Santa Margarita Formation Robles ntiated) Formation Tertiary (Undiffere? 1,000 Salinian granite and granodiorite Etchegoin Formation 1,500 100 m) 10 Ω True vertical depth (m) ( Resistivity 1 ? 150 2,000 100 Great 50 Petroleum Valley Institute units) 0 Gamma (American ) 4 Formation ¬1 3 (km s 2 2,500 s 1 ? 6 )V ¬1 5 Arkosic sandstone and conglomerate (km s 4 p Franciscan 3,000 V 3 3,100 3,150 3,200 3,250 3,300 3,350 3,400 3,450 3,500 Formation Measured depth (m) b c 500 μm Figure 1 j Geology of the SAF at the SAFOD borehole. a, Geological cross-section9,14 at the SAFOD borehole (blue line)9,14, showing cored fault strands (red lines), locations of cuttings used in this study (yellow circle) and the approximate boundary of the geophysical logs (black box). Inset: Geophysical logs from SAFOD Phase II borehole. Red lines indicate active fault strands and the bracket (at the top) indicates the approximate locations of the drill cuttings we studied. b, SAFOD Phase III, Hole G, Run 4, Section 3 core from the main creeping strand of the SAF (ref. 13). c, Transmission scanning electron microscope image taken from a gap in the core showing anastomosing shear zones in clay-rich foliated gouge13. 2 NATURE GEOSCIENCE j ADVANCE ONLINE PUBLICATION j www.nature.com/naturegeoscience © 2011 Macmillan Publishers Limited. All rights reserved. NATURE GEOSCIENCE DOI: 10.1038/NGEO1089 LETTERS 20 ab Velocity steps (μm s¬1 ) Slide¬hold¬slide (s) 3,290 3,290 v = 10 μm s¬1 10 μm s¬1 100 10 10 μ 15 ss 1 1 3,300 3,300 3 10 Cutting depth (m) 30 Fault zone 100 300 0.012 CD: 3,294.9 m 3,310 3,310 10 CD: 3,304.8 m 0.008 Cutting depth (m) Shear stress (MPa) μ SAFOD Phase II Δ Δμ 0.004 3,320 3,320 μ inor fault 300 s M Major fault 5 hold Cuttings Displacement (mm) Hole 0 G = 50 MPa p2329 σN' CD: 3,297.7 m 1 10 100 1,000 3,330 3,330 = 50 MPa 0.20 0.25 0.30 0.35 0.40 0 0.002 0.004 0.006 σN' t (s) h μ Healing rate (Δ μ /decade) 0 16 18 20 22 24 Shear displacement (mm) Figure 3 j Frictional strength and healing behaviour. a,b, Coefficient of friction (a) and healing rate (b) as a function of location across the fault Figure 2 j Experimental data and frictional-healing determination. Raw zone. The frictional strength and rate of frictional healing are significantly data for a representative shearing experiment run at an effective normal lower in the fault zone compared with the adjacent wall rock. Inset: 0 D −1 stress σN of 50 MPa. After a ‘run-in’ at loading rate v 10 µm s , velocity Schematic diagram of SAFOD boreholes in relation to the active SAF steps are used to evaluate the frictional velocity dependence, followed by a strands and the cuttings studied here.
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