Fractal particle size distribution of pulverized fault rocks as a function of distance from the fault core

The size distributions of particle in pulverized rocks from the San Andreas fault and the Arima-Takatsuki Tectonic Line were measured. The rocks are characterized by the development of opening mode fractures with an apparent lack of shear. Fragments in the rocks in both fault zones show a fractal size distribution down to the micron scale. Fractal dimensions, dependent on mineral type, decrease from 2.92 to 1.97 with increasing distance normal to the fault core. The fractal dimensions of the rocks are higher than those of both natural and experimentally created fault gouges measured in previous studies. Moreover, the dimensions are higher than the theoretically estimated upper fractal limit under confined comminution. Dimensions close to 3.0 have been reported in impact loading experiments. The observed characteristics indicate that pulverization is likely to have occurred by a dynamic stress pulse with instantaneous volumetric expansion, possibly during seismic rupture propagation similar to impact loading.