Texture and Energetics of Gouge Powder from Earthquake Rupture Zones
Abstract
Standard methods for the analysis of particle-size-distribution (PSD) in fine-grained gouge (microscopic, sieving and laser particle analyzer) have several pitfalls. For example, our analysis of 145 gouge samples from the San Andreas fault zone shows that mean grain size dropped by 26 percent during 0.5 hr of continuous measurement with a laser particle size analyzer. This time drift reflects disaggregation of extremely fine particles into primary grains, and thus questions remain open with regard to size, PSD, and surface area of primary particles in fault gouge. To determine these properties accurately, we developed a new procedure for extended PSD measurements (up to 190 hr in the laser particle size analyzer), validated by SEM observations of gouge grains. Pristine gouge is difficult to find as chemical alteration and lithification may alter the gouge texture in exhumed, inactive fault zones. The present analysis is conducted on gouge from two fault systems that partly remove these limitations: (1) the active segment of the San Andreas fault-zone in the Tejon Pass region, southern California, with about 160 km slip and uplifted from a 2-4 km depth; and (2) the rupture zone of a "new borne" fault formed during the 1997 M=3.7 earthquake in Hartebeestfontein gold mine, South Africa, with about 0.4 m slip. The present study includes PSD measurements of approximately 250 gouge samples from both faults; 155 samples were measured for 0.5 hr or more, with eight samples being measured for 45-190 hr. Both faults display strikingly similar gouge characteristics: grain size distribution is non-fractal, fine grains approach the nanometer scale, and gouge surface areas approach 80 m2/g. These observations challenge common precepts that gouge texture is fractal and that gouge surface energy (evaluated here as 2¬10 MJ/m2) is a negligible contributor to the earthquake energy budget. We propose that the observed fine-grain gouge is not related to quasi-static cumulative slip, but rather formed by dynamic rock pulverization during the propagation of a single earthquake (Reches and Dewers, this meeting, session S03).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.T21D..07W
- Keywords:
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- 8123 Dynamics;
- seismotectonics;
- 7209 Earthquake dynamics and mechanics