The Fault Activation and Earthquake Rupture (FEAR) project aims to re-activate a natural, densely instrumented fault in the Swiss Alps to observe earthquake processes in close proximity, in order to advance our physical understanding of earthquake nucleation and propagation processes.
Earthquakes are among the most destructive natural hazards that humanity is confronted with. Although scientists have been trying to predict earthquakes for decades, no existing method can predict with certainty and precision where and when a major quake will happen. Lack of understanding of the earthquake-generation process is a major obstacle for large-scale use of deep geothermal energy in hot but low permeable reservoirs - an almost inexhaustible energy source that could potentially be harvested with a very small ecological footprint.
To a large extent, our inability to predict and control earthquakes results from our limited understanding of earthquake physics. Earthquakes are inherently complex, and often occur many kilometres beneath the Earth's surface. It is typically not possible to deploy instruments close enough to the rupture-generating faults to get high-resolution measurements of the rupture process. Instead, earthquakes can usually only be observed via sensors on the Earth’s surface, located at considerable distance from the quakes themselves. This 'blurs' the images we can extract from our measurements and limits the resolution with which we can observe earthquake processes.
Underground earthquake laboratories
The Fault Activation and Earthquake Rupture (FEAR) project is designed to overcome some of these limitations by studying a natural fault system in-situ, and with instruments that are installed in close proximity to a fault zone. Since 2018, researchers and engineers from ETH Zurich, along with numerous partner organizations, have been building the Bedretto Underground Laboratory for Geophysics & Geoenergies (BULGG/BedrettoLab). Located in the heart of the Swiss Alps, under the steep crests of the Pizzo Rotondo, a 5 km long tunnel provides access to rock formations that are buried under more than a kilometre of granite. In this natural environment we can study geological fault zones, geothermal fluid transport, and a wide range of other natural processes. This unique underground laboratory sets the stage for the FEAR project.
Activating a natural fault
About 2.5 kilometres into the Bedretto tunnel, we have identified a natural fault system which - as our laboratory studies show - has hosted earthquakes in the past. Controlled water injections will be used to re-activate this fault and induce small, non-damaging earthquakes. A 200m long, fault-parallel side tunnel will facilitate direct access to the fault. A dense multidisciplinary monitoring system will be installed to collect high-resolution in-close observations of processes directly on the fault and to monitor variations of physical parameters in the surrounding rock volume, before, during and after an earthquake.
This experimental set up will allow us to observe the full range of relevant processes from unusually close distances, and with unusually high resolution. We will be able to observe and quantify exactly what happens as a natural fault approaches failure. The custom monitoring system will be able to detect micro-quakes across 9 orders of magnitude (M-5 to M1), and provide very high frequency recordings of seismic waves, which cannot be recorded with standard instruments, and which in usual earthquake observatories have already decayed long before they reach the sensors. Using multiple fluid injection/extraction boreholes, we will attempt to condition the fault and to control its response to the stimulations.
The novel observations from the FEAR experiments will allow us to solve some long-standing questions in earthquake physics. We will be able to map out the most subtle precursory signals, in terms of foreshocks, pressure-, temperature- or strain-changes, as well as diagnostic chemical signals. Large and destructive earthquakes sometimes seem to occur without any precursory signals whatsoever. It is plausible, however, that this absence of precursors is only apparent, due to the limited sensitivity and resolution of standard monitoring systems. The FEAR experimental set up will allow us to constrain in a much more comprehensive sense, what kind of precursory signals are observed, and under which conditions they do or do not occur. If there are precursory signals, our close distance monitoring system will detect them. The insights we gain will guide our search for reliable, diagnostic signals at the larger scales, for the large and damaging earthquakes that scientists have so far failed to predict.
The observations we make will also shed light on how an earthquake unfolds in detail. Once the rupture has nucleated, how does it accelerate, how does it propagate through a complex fault system, and, most importantly, how and why does it stop? To what extent is the rupture process constrained to a single, thin rupture plane, as is typically assumed in theoretical and numerical models? With our monitoring system, we will be able to track how much radiated seismic energy comes from off the main rupture plane, and to what extent rupture acceleration and deceleration phases correlate with structural complexities of the fault system. Do the ruptures stop at obvious structural barriers, or do other aspects dominate rupture termination? And, how does a fault interact with neighboring and cross cutting faults in the network?
Earthquake predictability and control
The critical test for our understanding of the physical processes will be to what extent we can forecast, and potentially even control, the behaviour of the fault system. We will test various strategies to condition the system, by means of different fluid injection and extraction protocols, with targeted, local stress reduction schemes, e.g. through tunnelling, heating and cooling, as well as using stress redistributions from previous experiments to set the stage for an earthquake rupture. Can we increase the fraction of elastic energy that is released in slow, non-seismic creep, rather than in potentially dangerous earthquakes? Can we shift a fault’s behaviour towards producing a larger number of smaller events, and reduce the number of larger ones? Finally, can we get a fault system to produce reliable precursory signals, such that we can recognize when we are heading towards a larger event, and take preventative action?
Progress on any of these questions would have a major impact on how our societies can live with the earthquake threat, and on how we can manage and control it. Findings from the FEAR experiments will also find direct application in improvements on how to operate deep geothermal reservoirs in a safe and sustainable manner. Whether in geothermal projects, in our cities and villages, or wherever vulnerable infrastructure is at stake, the insights we gain from the FEAR experiments will help us design mitigation strategies for earthquake risks.