Microseismic triggering by small sinusoidal stress perturbations at the laboratory scale

Small transient stress perturbations are prone to trigger (micro)seismicity. In the Earth's crust, these stress perturbations can be caused by various sources such as the passage of seismic waves, tidal forcing, or hydrological seasonal loads. A better understanding of the dynamic of earthquake triggering by transient stress perturbations is essential to improve our understanding of earthquake physics and our consideration of seismic hazard.

Here, we study an experimental sandstone-gouge-filled fault system that undergoes creep with combined far field loading and periodic stress perturbations at crustal pressure conditions. This complex loading is analogous to the loading experienced by faults in the natural setting.

Microseismicity — in the form of acoustic emissions (AE) — strains, and stresses, are continuously recorded to study the response of the system as a function of loading rate, amplitude, and frequency of a periodic stress perturbation. 

The observed temporal distributions of the AEs disagree with the theoretical results of a Coulomb failure model considering both constant loading and oscillation-induced strain rates. This implies that the stress perturbations are of shorter period than the nucleation time of the AEs of the system. A susceptibility of the system’s AE response to confinement pressure amplitude is estimated, which highlights a linear relation between confinement pressure amplitude and the AE response amplitude, observations which are consistent with recent higher frequency experimental results on dynamic triggering.

The magnitude-frequency distribution of AEs is also computed. The Gutenberg-Richter b-value oscillates with the stress perturbations, the amplitude of the b-value oscillations increasing with the amplitude of the stress perturbations, as observed in natural catalogues with large stress oscillations. 

Our experiments may complement our understanding of the influence of low inertia stress phenomena on the distribution of seismicity, such as observations of dynamic triggering and seismicity modulation by solid earth tides or seasonal loading.