Induced earthquakes are still highly unpredictable, and often caused by variations in pore fluid pressure. Monitoring and understanding the mechanisms of fluid-induced fault slip is essential for seismic risk mitigation and seismicity forecasting. Fluid-induced slip experiments were performed on critically stressed faulted sandstone samples, and the evolution of the actively sent ultrasonic waves throughout the experiment was measured. Two different fault types were used: smooth saw-cut fault samples at a 35º angle, and a rough fault created by in-situ faulting of the samples. Variations in the seismic slip velocity and friction along the fault plane were identified by the coda of the ultrasonic waves. Additionally, ultrasonic amplitudes show precursory signals to laboratory fault reactivation. Our results show that small and local variations in stress before fault failure can be inferred using coda wave interferometry for time-lapse monitoring, as coda waves are more sensitive to small perturbations in a medium than direct waves. Hence, these signals can be used as precursors to laboratory fault slip and to give insight into reactivation mechanisms. Our results show that time-lapse monitoring of coda waves can be used to monitor local stress changes associated with fault reactivation in this laboratory setting of fluid-induced fault reactivation. This is a critical first step towards a method for continuous monitoring of natural fault zones, contributing to seismic risk mitigation of induced and natural earthquakes.

Predicting stress changes in the subsurface leading to failure or seismicity remains challenging. Developing a robust monitoring method can help the prediction and thus mitigation of natural hazards. Ultrasonic transmission experiments were performed on Red Pfaelzer sandstones to investigate the forecasting potential to failure at different confining pressures. The forecasting potential for failure of the energy of the direct and coda wave, the transmissivity, Q-factor, coda wave decorrelation coefficient, and velocity change by coda wave interferometry are investigated and compared. Our results show the failure of the tested samples can be forecasted from 40 to 70% of the failure point. Small differences are visible in the precursors between the tested confining pressures, but as the trends are very similar, a robust prediction of failure can be made by combining the various analyses techniques. In this paper, we propose a traffic light forecasting system using active acoustic monitoring which is applicable for forecasting failure at various depths and or stress conditions, for a better prediction of small stress-induced changes in the subsurface and thus mitigation of failure (and seismicity) in the subsurface.