Gouge Powder from Earthquakes Rupture-zones and Laboratory Rupture Experiments: Sub- microscopic Observations and Particle size Distribution

We examine grain features and estimate the particle size distribution (PSD) in gouge collected from three sources: rupture zones of two recent earthquakes in South African mines (m3.7 event in 1997 and m2.2 event in 2004); unstable faulting experiments of quartzite cylinders; and the San Andreas fault-zone at Tejon Pass, California. Studies in nanotechnology indicate that PSD measurements of fine powders are susceptible to major errors due to aggregation and agglomeration of the fine particles that cause a systematic bias toward coarser PSD. Thus, our central analytical objective is to determine the true grain size of the gouge that forms during an earthquake and we have employed several techniques. In the TEM (Jeol-2010F) we use bright-field and dark-field modes for magnifications smaller than 200,000, and FFT-filtering mode for high-resolution (HREM) magnifications of 200,000-500,000. We also test several methods to disperse the agglomerates (with ethanol, silanes and toluene) and measure the PDS in the Dynamic Light Scattering method (using the Zeta particle analyzer ZetaPALS-90Plus of Brookhaven Instruments). The grains in the TEM analyses of eight gouge samples display similar shapes: aspect ratios range up to 1:3 with small amounts of very elongated grains, and most grains are angular to very angular while grains of the rock mechanics experiment are the most angular. The samples display wide ranges of grain sizes (5 nm to 5 microns), however, quantitative PSD cannot be determined as most (if not all) grains appear in clusters and aggregates that are hardly separable in the bright-field mode. We thus use the dark-field and FFT-filter methods to map the internal structure of tens of grains ranging in size from a few nanometers to about 2 microns. With a few exceptions, all examined grains are composites of 3 to more than 15 (and probably more) secondary grains that are as small as a few nanometers in size. The PSD of these samples is measured with the Zeta analyzer with resolution range of 1 nanometer to 3 micron. For half of the samples we use 'as is' gouge powder and for the other half we use the gouge fraction that passes 63 micron sieve. Several dispersion methods are used. The observed PSD is multimodal, and with the exception of one sample, the grain sizes are 0.1-1.0 micron. The TEM direct observations of the agglomerate internal structure indicate that the true grain size of gouge powder is in the sub-micron range as confirmed by the Zeta analyzer. We currently attempt to quantify the abundance of particles in the 10-100 nanometer range in gouge powder by testing nanotechnology dispersion techniques. We discuss the significant implications of such fine grains to earthquake energy balance and to slip weakening mechanisms during earthquakes. This study was supported by a SCEC 2007 grant and by NSF Continental Dynamic grant 0409605.