Periodic Reporting for period 2 - FEAR (Fault Activation and Earthquake Rupture)
Reporting period: 2022-03-01 to 2023-08-31
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. 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.
A 120m 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. This 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 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 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 FEAR fault activation experiments will shed light and advance understanding in 6 areas of earthquake and fault science: 1) earthquake physics and how earthquake nucleate, propagate, and arrest, 2) the role of fluids in the earthquake process, 3) earthquake precursors and whether they can be observed in both the laboratory and the field, 4) what is the interrelation between seismic and aseismic deformation in the fault zone and surrounding volume, 5) how do we best forecast earthquakes, and 6) implications for induced seismicity in geo energy applications.
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.
A 120m 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. This 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 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 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 FEAR fault activation experiments will shed light and advance understanding in 6 areas of earthquake and fault science: 1) earthquake physics and how earthquake nucleate, propagate, and arrest, 2) the role of fluids in the earthquake process, 3) earthquake precursors and whether they can be observed in both the laboratory and the field, 4) what is the interrelation between seismic and aseismic deformation in the fault zone and surrounding volume, 5) how do we best forecast earthquakes, and 6) implications for induced seismicity in geo energy applications.
Important steps towards the FEAR fault activation experiments were made in the first 36 months of the project. The selection of the multi-strand MC fault, a distinct ~4m wide fault zone showing clear evidence of past fault slip episodes, as the FEAR target fault zone was finalized. A detailed characterization campaign was conducted in summer 2022, involving the drilling of 3 prospection boreholes (total length 376.4m). The data from this campaign confirmed that the MC fault zone extends at least 50m beyond the Bedretto tunnel, possibly significantly further, has sufficient planar geometry, and is structurally and compositionally suitable for fault activation experiments. Extensive surface and subsurface characterization (geologic, geophysical, stress, hydro-bio-geochemical, and laboratory) of the MC fault was conducted. A homogenized 3D fault zone model was developed. FEAR hydraulic injection protocols were tested in an already-instrumented reservoir at the BedrettoLab. On-fault tetrahedral arrays and acoustic emission sensors, the initial components of the multidisciplinary FEAR Integrated Monitoring System, have been installed on the MC fault. The real-time background seismic monitoring system is up and running. Following extensive consideration, the overall geometry of the FEAR experiments was finalized. The 120m FEAR side tunnel will be excavated on the left side of the Bedretto tunnel, with FEAR Experiments 2,3, and 4 also on the left, parallel to the FEAR side tunnel. The FEAR Experiment 1, currently scheduled for Spring 2024, will be conducted on the right side of the Bedretto tunnel to minimize the possibility of triggering the FEAR magnitude 1 main shock before the monitoring system is fully in place. FEAR Experiment 1 will be conducted from the Bedretto tunnel, while the construction of the FEAR tunnel is on-going. Following a public tender, the construction company for the FEAR tunnel has been selected and construction has started as of September 2023. Detailed experimental design of the FEAR experiments, with a strong focus on FEAR Experiment 1, and design and implementation of the FEAR Integrated Monitoring System and the experiment remote control system is underway.
Parallel to the activities in the Bedretto tunnel, laboratory experiments on gouge material from the MC fault have confirmed that the MC fault can be dynamically reactivated if stimulated. Calibration experiments were conducted in a salt mine to enable retrieval of absolute ground motions from dynamic sensors in the 10-100 kHz range. Various numerical codes and models for studying nucleation and rupture, evolution of seismicity, effects of poroelasticity, pre-existing fault structures, injection protocols, and fault conditioning in pre-, co- and post-seismic phases are in development.
Parallel to the activities in the Bedretto tunnel, laboratory experiments on gouge material from the MC fault have confirmed that the MC fault can be dynamically reactivated if stimulated. Calibration experiments were conducted in a salt mine to enable retrieval of absolute ground motions from dynamic sensors in the 10-100 kHz range. Various numerical codes and models for studying nucleation and rupture, evolution of seismicity, effects of poroelasticity, pre-existing fault structures, injection protocols, and fault conditioning in pre-, co- and post-seismic phases are in development.
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 fault to perform real-time tests of different physical sources and forecasting hypotheses, (iii) deploy data-driven approaches and real-time modeling to conduct structured prospective earthquake forecasting experiments, and (iv) integrate and validate results from deep underground experiments, experimental rock-deformation laboratories, numerical physics and dynamic modeling, and observations from natural earthquakes.
The planned FEAR Experiments which will take place in years 4 and 5 of the project, as well as the FEAR Integrated Monitoring System being built for these experiments, are innovative, groundbreaking, and ambitious, and will shed light on very fundamental questions on how earthquakes occur. The preparatory work done in the first three years of the project is absolutely essential in enabling these experiments and are the most significant achievements of the project. This is in line with the Description of Work and activity plan of the project.
The planned FEAR Experiments which will take place in years 4 and 5 of the project, as well as the FEAR Integrated Monitoring System being built for these experiments, are innovative, groundbreaking, and ambitious, and will shed light on very fundamental questions on how earthquakes occur. The preparatory work done in the first three years of the project is absolutely essential in enabling these experiments and are the most significant achievements of the project. This is in line with the Description of Work and activity plan of the project.