Fault slip mechanics and seismic source characteristics considering fracture zone anisotropy

Abstract

The fracture zone, which constitutes majority of a fault, exhibits intricate deformation and significant non-homogeneity (internal structure, composition, mechanical properties, and permeability), which plays a crucial role in the initiation and evolution of seismic and slip instability. An acoustic emission (AE) localisation experiment was conducted on coal subjected to uniaxial graded compression to analyze the distribution of fracture sources. Simulations were employed to portray the anisotropy in the fault fracture zone. The results reveal that the percentage of AE signals (exceeding 30%) and the source location can serve as criteria to identify the time of pre-slip and the fracture position. Tensile sources are predominant and densely distributed along the fault plane, whereas shear sources constitute the second-largest proportion and are concentrated in the middle of the fault plane. In summary, pre-existing crack (pre-crack) dominates the fracture deformation mode, with the size distribution influencing the fault porosity and crack opening angles. The non-homogeneous proportion correlates closely with the abnormal distribution of local stress caused by material heterogeneity, which induces shear displacement and results in relative dislocation. During the meta-instability stage (from the stress peak to the sudden stress release), the pre-crack tip becomes locked, and middle fractures occur in a discontinuous and gradual manner. That is, the local stress is gradually concentrated and transferred from one micro-crack to another, resulting in point-to-point stress transfer, rather than the crack being instantly penetrated. Tensile force chains dominate the failure, progressing from strong to weak or non-force chain areas. The compressive force chain drives sudden fault slip. The paper provides a high-resolution database for fractures in coal samples made up of grains of different shapes and sizes. These findings provide insights into explanations for fault rupture–development–activation and the disaster-causing evolution in fault fracture zones.