Susceptibility of microseismic triggering to small sinusoidal stress perturbations at the laboratory scale 

Abstract

<div> <p><span>Small transient stress perturbations are prone to trigger (micro)seismicity. In the Earth's crust, these transient stress changes can be caused by various sources : passing of seismic waves, forcing by tides, hydrological load, and other seasonal climatic loads. A better understanding of the dynamic of earthquake triggering by transient stress perturbations is essential in order to improve our understanding of earthquake physics and our consideration of seismic hazard.</span></p> <p><span>Here, we study an experimental fault system, which mimics the natural seismic cycle, that is to say periodic stress perturbations (of variable amplitude and frequency) superimposed to a far field loading, at crustal pressure conditions. The experimental system is instrumented so that microseismicity </span><span>—</span><span> in the form of </span><span>acoustic emissions (AE) </span><span>— </span><span>deformations, and stresses, are continuously recorded during the experiments, in order to study the response of a system producing microseismicity as a function of loading rate, amplitude and frequency of a periodic stress perturbation. </span></p> </div><div> <p><span>In practice, </span><span>confinement pressure oscillation experiments are undertaken on Fontainebleau sandstone gouge-filled artificial faults undergoing axial loading steps with mean slip speeds ranging from 1.9 μm/s to 1.4e-2 μm/s. The amplitude and period of the oscillations range from 0.1 MPa to 2.5 MPa and from 20s to 500s respectively between experiments. In each experiment, at least several hundreds of AEs are detected at each loading rate step. </span></p> </div><div> <p><span>A susceptibility of the system’s AE response to confinement pressure amplitude is derived. Predictions of a Coulomb stress failure model criterion describing the ratio of constant loading and oscillation-induced strain rates do not match the observed AE distributions. Susceptibility exhibits a linear relation between confinement pressure amplitude and the acoustic emission modulation amplitude, observations with which recent higher frequency experimental results on dynamic triggering agree. </span></p> </div><div> <p><span>Our experiments may help further 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 loadings.</span></p> </div>