Earthquakes are among the most significant hazards to human society and continue to remain the most elusive. Advancing our ability to understand the occurrence and severity of future earthquakes is of paramount importance to making society more resilient to earthquake risks. Progress in further understanding of earthquake physics is hindered by the lack of appropriate experimental facilities for observing the earthquake process at a close distance. The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) is a deep underground experimental facility being constructed in the Bedretto tunnel, at 1’000m depth in the Swiss Alps and offers a unique opportunity to perform fault stimulation and earthquake nucleation experiments on a scale and depth not available until now. The core idea of the Fault Activation and Earthquake Rupture (FEAR) project is to gain understanding of how earthquakes start and stop by using hydraulic stimulation to modify stress and initiate small non-damaging earthquakes (magnitude 1 range) on a natural target fault in the vicinity of the BedrettoLab.
A suite of 4 stimulation experiments are planned along subsequent segments of the target fault. In Experiment I, an unperturbed fault section will be stimulated and will serve as a baseline experiment to characterize the fault response to stimulation and estimate pre- and post-stimulation stress conditions, injectivity, and permeability. In Experiment II, a larger fault segment will be stimulated, with up to three simultaneous injection boreholes, with the goal of triggering a non-damaging, larger rupture with a target moment magnitude of ~1.0. Experiment III will take place next to the segment ruptured in Experiment II and will test to what extent the rupture from Experiment II conditioned the adjacent Experiment III fault section for further rupture. In Experiment IV, we attempt to actively condition another segment of the same fault, e.g. by circulating cold and temperate water in the surrounding rock mass for several weeks, to either clamp or unclamp certain portions of the fault.
To facilitate the installation of the multidisciplinary FEAR Integrated Monitoring System, a 120m tunnel (the FEAR tunnel) is excavated parallel to the target fault. Numerous monitoring and stimulation boreholes will be drilled from this FEAR tunnel. The FEAR tunnel and the associated boreholes will facilitate the deployment of a dense network of multidisciplinary sensors to capture the rupture preparation phase, the earthquake rupture itself, and the post-rupture response of the target fault at unprecedentedly close distances.
Real-time data from this instrumentation network will flow as inputs into a real-time adaptive traffic light system for risk-mitigation for induced seismicity, serving as a unique testbed for state-of-the-art earthquake forecast models. In parallel to the in-situ activities in the Bedretto tunnel, rock samples from the target faults will be tested in rock deformation laboratories using state-of-the-art friction and fracture testing machines. Numerical models capturing the strongly coupled non-linear thermo-hydro-mechanical processes involved in fault rupture will be developed to address the question of bridging the gap between investigations at the laboratory scale and those on natural faults.
The science objectives of the FEAR project address key questions in 6 areas of earthquakes and faulting science, among which include:
- Earthquake physics: How do earthquakes nucleate, propagate, and arrest? What is the role of pre-stress conditions and geometrical/rheological complexities (i.e. barriers)?
- Role of fluids: What roles do fluids, pore-pressure changes, heterogeneity of frictional properties and dynamic parameters play in the initiation and evolution of individual earthquake ruptures, and in seismicity patterns?
- Earthquake precursors: Can we observe earthquake precursors, especially ones that have been observed at the laboratory scale but not yet in the field? Are there any specific transient process diagnostics of an impending rupture?
- What happens on and around the fault zone? What is the interrelation between seismic and aseismic deformation within the fault zone and in the surrounding volume? How does deformation localize inside the fault zone? Can we modify the mode of slip behavior?
- How do we best forecast earthquakes? What are the most successful earthquake forecast models? Can we use the unusually dense sensor network to inform a physics-based seismicity forecast model that significantly outperforms the purely statistical models? What is the current, and what is the inherent, limit on the predictability of earthquakes?
- Implications for induced seismicity in geo-energy applications: What stress and injection conditions produce larger magnitude events? Can we recognize activatable fault segments a priori, i.e. before injections? To what extent, and how, can induced earthquakes, and seismic and aseismic slip be controlled?
