Fluid-induced aseismic slip and seismicity on a natural fault: insights from the FEAR1 experiment at BedrettoLab

Aseismic slip is increasingly recognized as a fundamental driver of earthquake nucleation, affecting the spatio-temporal evolution of seismicity, yet its direct observation remains rare due to limited strain measurements close to a natural fault system. Here, we present results from the 'FEAR1' experiment conducted at the Bedretto Underground Laboratory for Geosciences and Geoenergies (Switzerland), where we used fluid injections to activate a natural fault and fracture network in crystalline rock under in-situ stress conditions at ~1 km depth. This experimental setting is particularly well suited to investigate induced seismicity and the role of aseismic processes in fault activation, thanks to dense near- and on-fault strain, pressure, and seismic monitoring.

During several injections performed in FEAR1, we observed the activation of a steeply dipping, highly permeable fracture zone, which intersects a densely instrumented borehole. Hydraulic stimulations triggered seismicity (−4.9 < Mw < −2.3) that organized along a plane whose orientation is consistent with geological observations in boreholes cores, logs and on the laboratory tunnel wall. Simultaneously, high-resolution Fiber Bragg Grating strain measurements revealed progressive, irreversible tensile deformation localized near the fracture intersection with the monitoring borehole, reaching nearly 1000 µε over the course of the experiment.

Static elastic modeling demonstrates that the cumulative strain produced by the recorded earthquakes accounts for less than 1% of the observed deformation, indicating that fault slip was dominantly aseismic. The spatial and temporal evolution of seismicity shows systematic up-dip migration toward the strain concentration zone and the emergence of families of repeating earthquakes. The recurrence rate and cumulative slip of these repeaters correlate with the measured strain rate and strain, suggesting a scenario where seismic asperities are embedded within a creeping fault segment sustained by pore pressure stress perturbations.

Inversions of irreversible strain for simplified slip sources indicate a predominantly strike-slip mechanism consistent with the estimated local stress field, although trade-offs between source location, source dimension and slip direction highlight the limits of 1D strain observations. Our results provide direct experimental evidence for fluid-driven aseismic slip on a natural fault and demonstrate how microseismicity and repeaters can serve as indirect proxies for underlying slow deformation.