Frictional strength and sliding behaviors of an analogue rock-fault structure: A laboratory study

Abstract

Preexisting faults exert a dominant control on frictional strength and stability of crustal faults. However, the complex interplay between fault and host rocks and their influences on frictional behaviors of fault zones remain poorly known. Here, we mimic natural fault-rock binary structures with rock-gouge assemblages made of fractured granite with or without gouge sandwiched inside. Then, we quantitatively assessed the frictional strength and sliding stability of the granite fractures by performing velocity stepping (VS) and slide-hold-slide (SHS) experiments. The VS results reveal a positive relationship between friction coefficient and velocity jump, indicating a velocity strengthening behavior. Besides, friction coefficient of fractured granite is found to be susceptible to gouge thickness and fracture roughness, while it is less affected by normal stress. According to the SHS experiment, frictional healing (Δμ) is scaled logarithmically with hole time, this linear link is mainly caused by the logarithmic grow in contact area during the hold period. By comparing the pre- and post-shear surfaces, it is found that the fractures with rougher surface and thinner gouge layer suffer more severe damage in the post-shear surface, reflecting the dominant impacts of gouge and fracture roughness on the frictional sliding behaviors. Accordingly, three modes, namely fracture-surface-dominant, mix, and gouge-dominant modes, are proposed to characterize frictional sliding of fractured granite. Moreover, a threshold, termed as the critical gouge thickness, is identified based on frictional strength evolutions, exceeding of the critical gouge thickness will motivate the transitions of the sliding modes, and thus affect the frictional strength and stability. This study designed and conducted rock friction experiments, as a laboratory earthquake, on rock-gouge assemblages, the frictional strength and sliding stability of which were thoroughly explored, and therefore it shed light on the complex interplay of surrounding rocks with gouge in dominating the frictional properties of natural fault zones.