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Fault Activation and Earthquake Rupture

Periodic Reporting for period 1 - FEAR (Fault Activation and Earthquake Rupture)

Reporting period: 2020-09-01 to 2022-02-28

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 close distance. The Bedretto Underground Laboratory for Geosciences and Geoenergy (BedrettoLab / BULGG) is a deep underground experimental facility being constructed in the Bedretto tunnel, at 1’000m depth in the Swiss Alps. The BedrettoLab 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 fault properties and estimate pre- and post-stimulation stress conditions, injectivity, and permeability. In Experiment II, the fault segment intersecting the Bedretto tunnel will be stimulated to test to what extent the tunnel-induced pore pressure depletion acts as a stress barrier. In Experiment III, fault criticality will be locally increased by circulating cold water in the surrounding rock mass for several weeks. The goal is to reach a temperature reduction of 10°C, and substantially reduce the effective normal stress on the fault. In Experiment IV, stress along the target fault will be altered by stimulating one or several neighboring faults.

To facilitate the various components of an integrated monitoring system, a 150 - 200m tunnel will be excavated parallel to the target fault, at a distance of about 50m from the fault. From this access tunnel, numerous monitoring and stimulation boreholes will be drilled. The side tunnel and boreholes will facilitate the deployment of a dense network of multidisciplinary sensors to capture the rupture preparation phase, the earthquake rupture, 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 seismcity, serving as a unique testbed for state-of-the-art earthquake forecasts. 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 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 scales from laboratory samples to earthquakes on natural faults.

The science objectives of the FEAR project address key questions in 6 areas of earthquake and fault science

Earthquake physics: How do earthquakes nucleate, propagate, and arrest? What is the role of pre-stress conditions and geometrical/rheological complexities (i.e. barriers)? Is there a scale dependence in earthquake mechanics and scaling of source parameters? How do fault zone parameters and heterogeneity control the development and arrest of dynamic ruptures?

Role of fluids: What roles do fluids, pore-pressure changes, heterogeneity of frictional properties and dynamic parameters play for the evolution of individual earthquake ruptures, and for seismicity sequences? Can we image fluids using ambient noise-based techniques? How do fluids change the earthquake size distribution?

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 do fault zone parameters control the degree of cross coupling between microseismic swarms, transient creep, and pore pressure transients? Can we modify the mode of slip release?

How do we best forecast earthquakes? What are the most successful earthquake forecast models? Can we design robust multi-component ensemble forecast models? How can we test earthquake forecasts in near-real-time? What is the current, and what is the inherent, limit of the predictability of earthquakes?

Implications for induced seismicity in geoenergy applications: What stress and injection conditions produce larger magnitude events? How is permeability evolution related to seismic or aseismic slip? To what extent, and how, can induced earthquakes, and seismic and aseismic slip be controlled?
The FEAR project spans 6 years, with a timescale dictated by the experiments in the Bedretto tunnel and divided in three successive work phases. This report covers activities in the first 18 months of the project.

In the first year, the FEAR administration and scientific working group structures were defined, the administrative and research teams assembled, and members of the External Advisory Board selected. The FEAR website has been published. An in-depth geological survey was conducted in the Bedretto tunnel and 3 candidate faults selected. The testing of the FEAR monitoring systems in other on-going projects, development of the detailed experimental setup/design for the 4 planned experiments, and the optimization of the locations of boreholes and the side tunnel, are underway. Adjustments to the drilling schedule for boreholes and the side tunnel were necessary due to pandemic-related delays as well the scheduling considerations of contractors with the specialized equipment and expertise required. Drilling of the first prospection borehole is now planned for summer 2022. Findings from the first prospection borehole will be used to finalize the location, geometry, and design of the side tunnel and instrumentation boreholes. Construction concessions and tenders are required for the construction of the side tunnel. These processes will be initiated once the technical design specifications of the side tunnel are finalized. The first FEAR-related experiment - testing the FEAR hydraulic injection protocols - is currently underway in a part of the Bedretto tunnel already instrumented for another project.
FEAR will be the first program to (i) perform 50-100m scale fault stimulation experiments in basement rock at over 1’000m depth, (ii) pre-condition the stress distribution on the target faults to perform real-time tests of different physical source and forecasting hypotheses, (iii) deploy data-driven approaches and real-time modelling to conduct structured prospective earthquake forecasting experiments, and (iv) integrate and and validate results from deep underground experiments, experimental rock-deformation laboratories, numerical physics and dynamic modelling, and observations from natural earthquakes.
proposed set-up for FEAR fault stimulation experiments I-IV