Induced fault slip events and their deformation fields: insights from FEAR experiments

Understanding the strain field induced by earthquakes and aseismic slip events is crucial for interpreting deformation data, constraining source parameters, and calculating the induced static stress, which may lead to further seismicity. The Fault Activation and Earthquake Rupture (FEAR) experiments at the Bedretto Underground Laboratory for Geosciences and Geoenergies (“BedrettoLab”) provide a unique opportunity to directly observe and study strain fields resulting from fluid-induced slow and fast fault slip. Understanding strain evolution before, during, and after seismic and aseismic slip events is critical for identifying patterns of deformation localization and rupture nucleation, which are key to advancing physical models of earthquake processes and hazards. In two preparatory experiments before the main fault activation experiments, we attempted to trigger an ML~0.0 quake with model-informed fluid injection protocols. During the first experiment, after 4.5 days of injection with 15 MPa injection pressure, and 18 hours at 20 MPa of injection, we triggered an event with Mw~ -0.4, after which we stopped the stimulation. The observed co-seismic static deformation confirmed our monitoring network’s sensitivity to detect strain changes induced by earthquakes of at least Mw ~-0.4, laying the groundwork for subsequent strain modeling analyses.

We modeled the displacement, strain, and stress fields resulting from the mainshock of the first experiment, treating it as a generic uniform dislocation source. To parameterize the source, we utilized a focal mechanism derived from P-wave first motion polarities and a radius estimate obtained through spectral fitting.

Our analysis focused on comparing strain data from a network of Fiber Bragg Grating (FBG) sensors located at 18 to 31 m from the quake, with analytical models simulating the static deformation induced in an elastic full space by the earthquake. Preliminary results show a strong correlation between the observed and modeled strain, validating the reliability of our simple shear dislocation model. By optimizing the model parameters, particularly the fault's rake, we improved the fit between the predicted and observed strain profiles, refining our estimates of the M-zero earthquake’s source characteristics.

Furthermore, our models enabled comparisons between the spatial patterns of seismicity and the stress fields induced by fault slip deformation—both seismic (Mw~–0.4 event, triggered in the M-zero experiment) and aseismic (triggered in the FEAR1 experiment). These comparisons contribute to understanding the mechanisms driving induced seismicity behaviors.

By modeling co-seismic strain and validating these models against novel observational data, this work lays the groundwork for future strain inversions, including those targeting the pre-seismic phase, to better constrain the physical mechanisms underlying fault slip behavior. This work highlights the potential of underground facilities like BedrettoLab to deliver detailed insights into fault slip behavior, setting the stage for further analyses using data from the recently completed FEAR1 experiment.