Study of fault gouge influences on mechanical and frictional behavior of granular shear zones using the distinct element method

Abstract

Studies of fault gouge and its role in shear zone deformation are the key to understanding the mechanics of earthquakes and fault zone evolution. With the advantage of exploring the micromechanical process of gouge deformation in "real time", the combination of the Distinct Element Method (DEM) and linear elastic contact bonds provides an opportunity to deform complex, heterogeneous granular assemblages that approximate natural shear zones in a more realistic way, and to study gouge deformation processes that are responsible for unstable sliding of fault zones. Granular assemblages of multiple shaped grains were sheared over a range of normal stresses, sigman, in order to examine the influences of sigman gouge grain shape, grain comminution, and associated dynamic changes in grain characteristics on the frictional behavior of granular shear zones. The results show an inverse power law relationship between sigman and maximum sliding friction, where both its coefficient and exponent are dependent on gouge angularity. Enhanced grain rolling alone does not explain the low frictional strengths of simulated granular assemblages. Shear zone strength is dependent on the competition between strength reduction by fracturing and strength variation by changes in grain characteristics that are related to the partitioning of different deformation mechanisms. DEM experiments were also conducted to simulate the growth of fault gouge zones, for the purposes of studying the processes of gouge zone evolution, and its dependence on sigman and uniaxial compressive strength, sigmaucs. The simulated fault gouge zones exhibit two distinct stages of evolution, i.e., fast growth and slow growth, distinguished by a switch in deformation mechanism from dominantly wear of the fault blocks to dominantly shearing of existing fault gouge. During the fast growth stage, the rates of gouge thickening and bond breakage decrease exponentially and are proportional to sigman and inversely proportional to sigmaucs the rates become relatively constant and the dependency reverses during the slow growth stage. Gouge properties show complex correlations and dependences on shear displacement, sigman and sigma ucs, demonstrating the important effects of depth, mechanical properties of fault rocks, and gouge properties on the evolution and stability of natural faults.