Initial stress distribution dictates nucleation location and complexity of the seismic cycle of long laboratory faults

Many aspects of earthquake physics are still not completely understood given its intrinsically complex nature. Among the others, the nucleation process; when and where an earthquake will occur, as well as its magnitude. Seismology is a commonly used method for studying earthquakes, but it faces challenges in accessing precise information about the physical processes taking place on the fault plane.

Here, we show how laboratory seismology can directly shed light on fault plane dynamics. Our approach involves reproducing in the laboratory on a large biaxial apparatus with a fault length of 2.5 m generated by two analog (PMMA) samples brought into contact. The experimental setup allows to impose both a heterogeneous loading distribution through the use of independent pistons loading the fault in the normal direction and specific boundary conditions (i.e. by modifying stopper and puncher dimensions). The stress state is measured through strain gauges at high frequency (40 KHz) along 15 locations along the fault. The experiments provide insights into two crucial aspects of laboratory earthquakes: (i) the nucleation location of ruptures and (ii) the complexity of the seismic cycle.

Our findings reveal that the initial on-fault stress distribution plays a significant role in both aspects. We observe that ruptures consistently nucleate in locations where the stress ratio τ/σ_n is the highest. Notably, such values change among experiments, challenging the widespread notion that a friction coefficient solely governs the onset of instability. Furthermore, we demonstrate how the heterogeneity of the initial prestress distribution along the fault controls the complexity of the seismic cycle. In certain cases, the seismic cycle manifests as system-size events with complete ruptures occurring regularly in time, devoid of precursors. Conversely, other initial stress distributions generate more complex cycles, characterized by multiple precursors before a main rupture, predominantly occurring in zones of elevated τ/σ_n (referred to as 'friction asperity'). The complexity of the seismic cycle can be described in terms of the number of precursory events, inter-event time, and the size of finite ruptures.

This study, carried out in a long laboratory fault, highlights the complexities that emerge when heterogeneous, hence more realistic, stress conditions are applied, providing valuable insights into the physics of natural earthquakes.