An Induced Mw -0.4 Earthquake Under a Microscope at the BedrettoLab

The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) is a deep underground laboratory in the southern Swiss alps that allows to induce and study rock deformation processes in situ, from exceptionally short distances, at 1 km depth. In the context of the Fault Activation and Earthquake Rupture (FEAR) project, we have conducted a series of fluid injection experiments, with the goal of inducing an M_L~0.0 earthquake. In this 'M-zero' experiment series, we have used a custom-built remote control system to implement fluid injection protocols that were informed by 2D continuum hydro-mechanical earthquake cycle models, with the target of increasing the likelihood of inducing a larger event. In the first of two major experiments, after 16 hours of high pressure (20 MPa) fluid injection, we have induced a M_w~ -0.4 "main shock", which triggered an aftershock sequence with >100 events in the first 3 seconds alone. The fluid injection was stopped 10 minutes after this main shock.

The experiment and earthquake sequence was recorded on an extensive multi-domain and multi-scale 3D monitoring array, which combines various types of seismic sensors with distributed acoustic, strain and temperature sensing, Fibre Bragg Grating strain sensors, as well as pressure and flow monitoring. Although the main shock magnitude is smaller than the target magnitude (which was unclear at the time of its occurrence), the wide range of observations made, including high-resolution micro-seismicity catalogues covering 5 units of magnitude, offer a detailed look at seismological and rock mechanic processes before, during and after the main shock. In this talk we summarize some of the key observations and insights including i) the foreshock sequence, which shows signatures of a triggering cascade but which did not accelerate before the main shock; ii) a distinct Gutenberg-Richter b-value anomaly where the main shock occurred, and which may reflect the structural inventory and stress state of the activated fracture network; iii) the first-order consistency between early aftershock distributions and kinematic rupture models from spectral fitting, and a pronounced aftershock gap across a 2-by-2 metre patch where the main shock occurred, iv) static deformation patterns resolved on the strain sensor network, and their consistency with the focal mechanisms inferred from seismic data. We also discuss the implications for the planned fault activation experiments in the coming three years at the BedrettoLab.