Particle size and energetics of gouge from earthquake rupture zones

Grain size reduction and gouge formation are found to be ubiquitous in brittle faults at all scales(1-4), and most slip along mature faults is observed to have been localized within gouge zones(5,6). This fine-grain gouge is thought to control earthquake instability(3,6-8), and thus understanding its properties is central to an understanding of the earthquake process(7,9). Here we show that gouge from the San Andreas fault, California, with similar to 160 km slip, and the rupture zone of a recent earthquake in a South African mine with only similar to 0.4 m slip, display similar characteristics, in that ultrafine grains approach the nanometre scale, gouge surface areas approach 80 m(2) g(-1), and grain size distribution is nonfractal. These observations challenge the common perception that gouge texture is fractal(10,11) and that gouge surface energy is a negligible contributor to the earthquake energy budget(3,9,12). We propose that the observed fine-grain gouge is not related to quasistatic cumulative slip, but is instead formed by dynamic rock pulverization during the propagation of a single earthquake.