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Particle size and energetics of gouge from earthquake rupture zones

Abstract

Grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales1,2,3,4, and most slip along mature faults is observed to have been localized within gouge zones5,6. This fine-grain gouge is thought to control earthquake instability3,6,7,8, and thus understanding its properties is central to an understanding of the earthquake process7,9. Here we show that gouge from the San Andreas fault, California, with 160 km slip, and the rupture zone of a recent earthquake in a South African mine with only 0.4 m slip, display similar characteristics, in that ultrafine grains approach the nanometre scale, gouge surface areas approach 80 m2 g-1, and grain size distribution is non-fractal. These observations challenge the common perception that gouge texture is fractal10,11 and that gouge surface energy is a negligible contributor to the earthquake energy budget3,9,12. We propose that the observed fine-grain gouge is not related to quasi-static cumulative slip, but is instead formed by dynamic rock pulverization during the propagation of a single earthquake.

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Figure 1: Field setting of the investigated faults.
Figure 2: Particle size distribution (PSD) of two representative gouge samples measured for extended periods in a laser PSD analyser (see Methods).
Figure 3: Time drift for four gouge samples measured for 72–190 h.
Figure 4: Scanning electron microscope images of untreated gouge from San Andreas gouge, with an order-of-magnitude resolution increase from left to right.

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Acknowledgements

We thank the US National Science Foundation and the Southern California Earthquake Center for supporting this research.Authors' contributions All authors contributed equally to this work.

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Correspondence to Ze'ev Reches.

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Wilson, B., Dewers, T., Reches, Z. et al. Particle size and energetics of gouge from earthquake rupture zones. Nature 434, 749–752 (2005). https://doi.org/10.1038/nature03433

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