Fault orientation in earthquake seismic precursors: insights from the laboratory

Faults in the brittle crust lie at any orientation to the far-field stress. However, laboratory experiments designed to investigate earthquake physics commonly simulate favorably oriented faults, potentially overlooking the complexity of natural fault behavior. Here, we assess the role of stress field orientation in fault reactivation and earthquake precursors by conducting triaxial saw-cut experiments with laboratory faults oriented at different angles to the maximum principal stress, ranging from 30° to 70°. The samples were instrumented with strain gauges and piezo-electric sensors. Laboratory well-oriented faults describe a rather simple system in which the elastic energy is stored via the deformation of the surrounding host rock during the inter-seismic period and released via on-fault slip during the co-seismic phase with associated precursor acoustic activity. Consistent with previous laboratory data, an abrupt increase in the on-fault acoustic emission rate occurs shortly before the laboratory earthquake. A more complex picture emerges when deforming laboratory misoriented faults. Particularly, acoustic emissions and strain gauge data indicate that when the fault is misoriented, off-fault permanent deformation occurs well before fault reactivation. The stress state in the host rock surrounding the fault is indeed far beyond the one required for the onset of inelastic deformation. In this case, acoustic activity distributed in the rock volume during the pre-seismic phase is associated with permanent deformation in the critically stressed host rock and is not a direct precursor to the following laboratory earthquake. Unlike well-oriented faults, laboratory mis-oriented faults lack detectable seismic precursors. The laboratory-observed increase in acoustic activity prior to, but not precursor of, mis-oriented fault reactivation impacts our understanding of earthquake precursors in natural faults.