HR: 17:15h
AN: NG12C-06    [PDF]
TI: Scaling Particle Size in Fault Gouge: Variable Fractal Dimension or Non-Fractal Distribution?
AU: * Dewers, T
EM: tdewers@ou.edu
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AU: Wilson, B
EM: brentwilson@ou.edu
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AU: Reches, Z
EM: reches@pangea.Stanford.EDU
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AB: The particulate nature of fault-gouges is believed to be the product as well as the control of earthquake rupture and fault slip. It is expected that the particle-size distribution (PSD) will display a fractal dimension that develops by grain comminution and progressive fault slip. We examine this expectation by measuring the PSD with laser particle size analysis (with 0.04 to 2,000.00 microns range), and observations with scanning and transmission electron microscopy. The gouges of two faults were studied: (1) The exhumed fault-zone of the San Andreas at Tejon Pass, California, with $> 80$ samples collected along a 70 m fault-normal traverse and a few sub-meter exposures within the pulverized Cretaceous Tejon Lookout granite (Wilson et al, 2003, Fall meeting, AGU); and (2) A "new-born" fault formed during the M=3.7 1997 earthquake in Hartebeestfontein gold mine, Klerksdorp, South Africa. The quartzitic gouge of this fault was collected at the focal zone, which was mined more than one year after the earthquake. We ran the laser particle size analysis for continuous periods up to three days while conducting multiple PSD measurements of a single sample. The main results are: (1) The PSD of the gouge powders from both faults systematically vary with measurement time due to progressive grain disaggregation; e.g., the mean grain size (by volume) drops from an initial value of 5.9+/-22.8 micron to 0.5+/-0.2 microns after 72 hours. The submicron nature of the gouges is verified by SEM and TEM. (2) PSD data for a wide, relevant range (0.04-2,000 microns) revealed that fractal dimensions of a single sample could range from 1.7 to 3.6 during the initial measurement. (3) The grain disaggregation (with running time in the laser analyzer) led to bi-modal fractal distributions with anomalous values as well as non-fractal distributions. We conclude that the frequently observed fractal nature of a gouge reflects the particulate agglomeration of finely fragmented grains, and does not represent the true gouge dimensionality or the gouge comminution associated with earthquake rupture. Finally, the intense pulverization with generation of large surface area in the studied gouges could contribute significantly to the earthquake energy balance.
DE: 3250 Fractals and multifractals
DE: 3902 Creep and deformation
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting