COMMINUTION AND FLUIDIZATION OF GRANULAR FAULT MATERIALS: IMPLICATIONS FOR FAULT SLIP BEHAVIOR

Abstract

Whilst faulting in the shallow crust is inevitably associated with comminution of rocks, the mechanical properties of the comminuted granular materials themselves affect the slip behavior of faults. Therefore, the mechanical behavior of any fault progresses along an evolutionary path. We analyzed granular fault rocks from four faults, and deduced an evolutionary trend of fractal size frequency. Comminution of fault rocks starts at a fractal dimension close to 1.5 (2-D measurement), at which a given grain is supported by the maximum number of grains attainable and hence is at its strongest. As comminution proceeds, the fractal dimension increases, and hence comminution itself is a slip weakening mechanism. Under the appropriate conditions, comminuted granular materials may be fluidized during seismic slip events. In this paper, we develop a new method to identify the granular fault rocks that have experienced fluidization, where the detection probability of fragmented counterparts is a key parameter. This method was applied to four fault rock samples and a successful result was obtained. Knowledge from powder technology teaches us that the volume fraction of grains normalized by maximum volume fraction attainable is the most important parameter for dynamic properties of granular materials, and once granular fault materials are fluidized, the fault plane becomes nearly frictionless. A small decrease in the normalized volume fraction of grains from 1 is a necessary condition for the phase transition to fluidization from the deformation mechanism governed by grain friction and crushing by contact stresses. This condition can be realized only when shearing proceeds under unconstrained conditions, and this demands that the gap between fault walls is widened. Normal interface vibration proposed by Brune et al. [Tectonophysics 218 (1993) 59] appears to be the most appropriate cause of this, and we presented two lines of field evidence that support this mechanism to work in nature.