Injection operations started on Wednesday afternoon, 22 April. During the following night, the experiment was temporarily interrupted due to an unexpected  power outage. After repair work was completed, injection was resumed on Thursday, 23 April in the late afternoon. The simultaneous injections via two stimulation intervals in two separate boreholes led to the activation of several structures across the heavily instrumented experimental volume.  

The experiment was stopped on Saturday morning at 08:30, after a total of 750 m³ of water had been injected over approximately 50 hours of pump operation. A few thousand very small seismic events were induced, with local magnitudes ranging from −5 to −0.14. While some seismic events occurred on the target fault zone, a large number of events took place on neighbouring geological structures activated by the fluid injection. The decision to stop the experiment was taken when an increasing number of seismic events occurred outside of the core measurement network, limiting their scientific analysis.  

Overall, the experiment confirmed that faults can be activated at the BedrettoLabs' Earthquake Physics Testbed, and that resulting seismicity remains within the expected range. The ground shaking produced outside the tunnel was 5’000 to 6’000 times below the design ground acceleration value from the Swiss Norms, with peak ground acceleration values of 0.000014g at the tunnel entrance, 0.0000167g at the top of the mountain, and 0.0000172g at the Furka Base Tunnel entrance. These values are approximately 700 times below the level associated with perceptible and approximately 7’000 times below levels associated with damaging earthquakes. 

The research team has begun a first analysis of the collected data, which will inform the preparation of the next FEAR 2 experiment planned for June or September. 

To carry out this work, the team has built a new 120 metre long tunnel starting 2.2 km from the entrance of the main Bedretto tunnel, providing direct access to the fault. A dense monitoring network consisting of several types of sensors placed on and around the fault, in over 40 boreholes, will measure rock deformation, fluid pressure, temperature, seismic activity, and biochemical changes. These observations will help scientists study the entire sequence of processes leading up to and following an earthquake. 

The FEAR-2 experiment has been preceded by hydromechanical tests to understand how fluid injection affects the fault’s pressure conditions, deformation, and stability, and how these coupled processes may influence seismic activity. 

The subsequent main experiment, hydraulic stimulation, will consist of water being injected over a period of several days to weeks. This process will operate around the clock and be monitored in real time. The team will start by injecting at low pressure, gradually increasing it until the target magnitude of 1 is reached or until 2000 m³ of water has been injected. Numerous much smaller earthquakes are expected to occur before the target magnitude is achieved. 

The experiment has undergone a detailed safety and risk assessment, and several layers of safety measures are in place. All high-pressure injection activities are controlled remotely from Zurich, which means that no personnel are present in the tunnel during the stimulations. 

According to the safety and risk assessment, the targeted magnitude 1.0 event would likely cause no damage, although very minor damage to the Bedretto and FEAR tunnels, located 30 to 50 meters from the rupture plane, is possible. Severe damage to the tunnels would only be expected in the case of a significantly larger event, with a magnitude of 2.5 or greater. The probability of such an event is estimated at around 1 percent. 

We do not expect earthquakes related to the FEAR-2 experiment to be felt or to cause any damage at the surface, in the Furka Base tunnel (2 to 2.5 kilometres away), or in nearby buildings. The probability of an earthquake causing damage outside the Bedretto tunnel is about 0.01 percent (1 in 10,000) and is therefore extremely low. It is much lower than the probability that a natural earthquake occurring over the same period would be felt or cause damage in the area. 

Overall, the analysis shows that the FEAR-2 experiments can be carried out safely, with a very low and acceptable level of seismic risk for the Bedretto tunnel, the Furka Base Tunnel, and nearby buildings. The experiment will be closely monitored at all times and will follow strict safety procedures. If any safety threshold is exceeded, injection will be stopped immediately.