关键词: 应力扰动, 断层泥, 应变场, 声发射, 摩擦实验

Abstract:

Industrial activities(e.g., natural gas storage and wastewater injection)and certain natural processes(e.g., solid tides and volcanic activity)can induce stress perturbations on faults, potentially influencing seismic activity in affected regions. Understanding how fault systems respond to these stress perturbations is essential for improving the accuracy of seismic event prediction and mitigation strategies. Previous laboratory studies have primarily focused on the impact of stress perturbations on the mechanical properties of fault stick-slip behavior, such as shear stress drop and the timing of periodic stick-slip cycles. Most of these studies have modeled faults using bare rock surfaces. However, obtaining accurate stress measurements on natural faults is challenging due to the limitations of commonly used seismic monitoring technologies, such as GPS, dense seismic arrays, and distributed optical fibers, which are typically deployed on the Earth's surface or in boreholes. This complicates the extrapolation of laboratory findings to field conditions. Moreover, there is a lack of research on the role of fault gouge, which is prevalent in natural fault zones.

This study aims to investigate how stress perturbations influence the physical fields(strain and acoustic emissions)adjacent to a simulated fault containing fault gouge. A double-direct shear sample configuration was employed to replicate the fault, using two types of fault gouge: a natural fault gouge(composed mainly of dolomite, bassanite, calcite, and small amounts of quartz)and pure halite gouge. During quasi-static loading, the natural fault gouge exhibited stick-slip behavior characterized by lower stress drops and self-sustained oscillations during the “stick” phase, while the halite gouge demonstrated regular stick slip with large stress drops. Strain rosettes and acoustic sensors were used to measure shear strain along the gouge layer and detect acoustic emissions(AE)events. In each experiment, a sinusoidal normal stress oscillation with varying amplitudes was applied after several regular stick-slip cycles.

The results indicate that both gouge types exhibited similar mechanical behavior: shear stress drops, fault weakening, and an increase in stick-slip velocity with higher normal stress oscillation amplitudes. However, distinct differences were observed in the strain field response and the distribution of AE events. As the oscillation amplitude increased, the fault with natural fault gouge showed a growing number of nucleation zones, with no AE events occurring during nucleation. Fault rupture eventually became more homogeneous after the stress drop. In contrast, the halite gouge maintained a constant number of nucleation zones, but both the nucleation time and area increased with higher oscillation amplitudes. AE events persisted throughout the nucleation phase, and their locations corresponded closely with the nucleation zones. The fault rupture process remained homogeneous under all conditions for the halite gouge.

Analysis of the normal strain evolution and Young's moduli for the two gouge materials revealed that the natural fault gouge exhibited significantly higher Young's modulus than that of the halite gouge. This suggests that fault heterogeneity and plastic deformation are key factors in the evolution of the strain field. Materials with higher modulus enhance strain field heterogeneity as normal stress oscillation amplitude increases, since asperities in these materials are more responsive to stress variations. In contrast, the softer halite gouge exhibits less sensitivity to stress changes, leading to fewer variations in strain. This mechanism can be well reflected by the spatial-temporal evolution of normal strain for two types of fault gouge layers. This explains why the number of nucleation zones increases with higher oscillation amplitudes in natural gouge but remains unchanged in halite gouge. These findings offer valuable insights for the development of near-surface monitoring programs aimed at assessing seismic activity.

Key words: Stress oscillation, fault gouge, strain field, acoustic events, friction experiment