Characteristics of particle size distribution of the Wenchuan earthquake fault zone: insights for cataclastic mechanisms

Particle size distribution(PSD)of fault rocks has been widely used to investigate the fracturing mechanism, frictional property and energetic partitioning of earthquakes. Sieve-weighting and laser diffraction analysis are two methods of quantifying 3-D PSD of fault rocks. However, each of them can only cover grain size in roughly three orders of magnitude, so they cannot reflect the full range characteristics of typical fault rocks, from nanometer scale to millimeter scale. This study performs measurements combining these two methods, on natural fault rocks collected from two surface exposures of the Wenchuan earthquake fault zone, with particle size measured ranging from 0.2μm to 16mm. Our measurements apparently show the following results: 1)There exists a critical particular size dc(0.95~1.90μm), and particles with larger and smaller size than dc follow different power law relations between particle number(Nd)and particle size(d), indicating that the PSDs of the fault rocks studied are non-self-similar. 2)The fractal dimensions(D-value)obtained from the particles with d>dc are well relevant to the fault rock types. This means that the D-values obtained by sieve-weighting method and by laser diffraction analysis are consistent with one another. The D-values of fault rocks analyzed show an increase trend towards the slip surface, from 2.6 for fractured breccia, to 3.0 for cataclastic breccia, and to nearly 3.5 for fresh fault gouges. By contrast, particles with d<dc have low D-values of 1.7~2.1. We infer that variation in fractal dimension for different particle size intervals reflects the change in cataclasis mechanisms. The critical particle size dc, probably corresponds to the grind limit of the gouge material. 3)From the PSD results, the surface fracture energy calculated is 0.63MJ/m2 for the Wenchuan earthquake fault gouge.