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Weakness of the San Andreas Fault revealed by samples from the active fault zone

Nature Geoscience volume 4, pages 251–254 (2011)Cite this article

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Abstract

Understanding the strength and slip behaviour of tectonic faults is a central problem in earthquake physics and seismic-hazard assessment1,2,3,4,5,6,7,8. Many major faults, including the San Andreas Fault, are weak compared with the surrounding rock, but the cause of this weakness is debated1. Previous measurements of the frictional strength of San Andreas Fault rocks are too high to explain the observed weakness1,9,10,11,12,13. However, these measurements relied on samples taken at a distance from the active fault or from weathered surface samples. Recent drilling into the San Andreas Fault9 has provided material from the actively slipping fault at seismogenic depths. Here we present systematic measurements of the frictional properties and composition of the San Andreas Fault at 2.7 km depth, including the wall rock and active fault. We find that the fault is weak relative to the surrounding rock and that the fault rock exhibits stable sliding friction behaviour. The fault zone contains the weak mineral smectite and exhibits no frictional healing—bonds in the material do not heal after rupture. Taken together, the low inherent strength and lack of healing of the fault-zone material could explain why the San Andreas Fault slips by aseismic creep and small earthquakes in central California, rather than by large, destructive earthquakes.

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Figure 1: Geology of the SAF at the SAFOD borehole.
Figure 2: Experimental data and frictional-healing determination.
Figure 3: Frictional strength and healing behaviour.

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Acknowledgements

We thank S. Haines for X-ray diffraction work and discussions and D. Moore for providing the transmission scanning electron microscope image of the fault-zone material. We also wish to thank N. de Paola and one anonymous reviewer for their comments, which helped improve this manuscript. Finally, we thank S. Hickman, W. Elsworth and M. Zoback for their efforts to initiate and carry out the SAFOD project. This research was funded by the NSF under grants EAR054570, EAR0746192 and OCE-0648331.

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  1. Department of Geosciences and Center for Geomechanics, Geofluids, and Geohazards, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

    B. M. Carpenter, C. Marone & D. M. Saffer

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  1. B. M. Carpenter
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  2. C. Marone
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  3. D. M. Saffer
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All authors contributed to data analysis and writing. B.M.C. conducted experiments and data analysis.

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Correspondence to B. M. Carpenter.

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Carpenter, B., Marone, C. & Saffer, D. Weakness of the San Andreas Fault revealed by samples from the active fault zone. Nature Geosci 4, 251–254 (2011). https://doi.org/10.1038/ngeo1089

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