Seismic responses to fluid pressure perturbations in a slipping fault

Abstract

Seismicity induced by fluid injection in a natural fault is investigated in situ in the near field of the source. We present synchronous seismic and hydromechanical measurements directly recorded in the decametric injection zone. The three main types of seismic events were recorded during injection and shut-in: high-amplitude and short duration seismic events (SE) (i.e., microearthquakes), low to constant amplitude and 5 to 17s long tremor-like signals (TLS), and long period events (LP) with a narrow-frequency band content. Seismicity first initiates with a sequence of SE and TLS, when pressure is high (similar to 3.5MPa), slip is activated on the fault, which experiences a twentyfold increase of permeability. Then LP events appear to be associated to fluid leakage in the fault caused by dilation during slip. During shut-in, residual pressures as low as 0.6 MPa still trigger SE events. We show that the initial TLS sequence triggers when a progressive transition occurs from rupture controlled by effective stress variations close to the injection source to a large friction weakening-dominated slip on the fault. We conclude that the combination of these different seismic signal types may be a proxy to monitor fault instability associated to fluid pressure perturbations.