In-situ testing and observations of the impact of earthquakes on a shallow segment of the San Andreas fault

Numerous studies reported that traveling seismic waves produced by major earthquakes may significantly affect the hydromechanical properties of faults. This paper presents some initial results from a dedicated field test site, which monitors activation of shallow faults in the San Andreas Fault (SAF) system. The area features regular local earthquakes with magnitudes Mw 3.5-4.5 as well as strong earthquakes Mw~7 originated within 500 km from the site. To monitor their effects on faults behavior, we drilled three dedicated boreholes hosting pressure and strain monitoring sensors as well as distributed acoustic sensing and optical seismometers for characterization of the incident signals. The core component of the monitoring network, DORSA (Downhole Robotic Stress Analyzer), enabled high-precision tracking of 3D displacements of the fault. Based on the pump tests, history-matched poro-elasto-plastic simulations estimated the poroelastic parameters of the target fault. The fault stiffness and relatively small friction angle explain why relatively small-amplitude earthquakes may produce clear irreversible slip on the studied fault.