Interactions of asperities controlling on fault stability: An experimental approach

The transition between seismic slip and aseismic creep of faults in the Earth crust suggests a strong time-dependent mechanism for the underlying physics and corresponding mechanical response of fault slip. Asperities establish the real contact on the slipping interface of a fault and serve as stress concentrators that control the initiation of earthquakes. Investigating the interactions between individual asperities and how the global stability of a fault is controlled by the collective effects of their local behaviors are essential for understanding the intrinsic relationships between earthquake swarms and faulting. Here we design a novel direct-shear experimental setup, which allows a thick PMMA (poly-methyl-methacrylate) plate to slide slowly on a customized surface, on which asperities are modeled by spherical PMMA beads and embedded in a softer polymer base, for analogizing tectonic faults. We perform various experiments by applying multiple normal loads and loading rates, with a high-resolution camera employed to capture the detailed activities of asperities. We demonstrate the global stability of a fault could be described by the synthesized behaviors of local asperities. We also prove, for the same asperity, it can experience different slip modes at different time periods. We generate a catalog of fault slip events defined by the slipping velocity of each asperity derived from the image correlation technology, and then we determine slip episodes based on time and space successively. Furthermore, we investigate the distributions of various parameters of the determined slip episodes, including the number of slipping asperities, as well as the duration, mean slip displacement, and moment of slip episodes. We explore the spatiotemporal variations of b-value within one analog seismic cycle and under different normal loads and loading rates. We link the findings at local scales with the bulk mechanical response of the whole fault. Our results bring new insights into the physics and mechanics of seismic and aseismic faulting.