Seismic monitoring of the rock instability at the former Mitholz ammunition depot

There is a higher risk of explosion from ammunition ordnance in the former Mitholz ammunition depot than previously assumed. For this reason, it was decided to completely clear the site. The results published in the work by Glueer et al. (2021) show that a possible earthquake-induced rockfall should be taken into account in risk assessments. Large amplifications of seismic waves are observed on the rock mass above the cavern at Mitholz, the origin of which is the strong internal disruption of the rock mass due to the explosions in 1947. The Federal Department of Defence, Civil Protection and Sport (DDPS) and ETH Zurich are therefore conducting a joint seismological research and measurement project. This includes an alarming after an earthquake, the installation of permanent and temporary seismic sensors and the development of a 3D model of the underground.

Alarming after an earthquake is triggered if defined threshold values are exceeded. This is integrated into the standard operation system within SED. Seismic stations are being installed on the rock formation above the caverns in the Mitholz area. These allow the measurement of earthquake amplification and its temporal variability, in particular the investigation of the dependence on water in the fissures and the recording of changes in the behaviour of the local structure during the extraction of rock material at the rock face. During the seismic measurements, the dynamic behaviour of the local structure at the site on the rock face is continuously derived from the ambient vibrations, enabling the detection of possible changes in the behaviour of the rock mass.

To optimise the positioning of the permanent seismic stations, ambient vibration measurements are carried out over the entire rock formation above the caverns using dense arrays of seismic sensors. The aim of the measurements is to map the natural frequencies and polarisation of the wave field as a whole. We use the scientific methods developed at the SED, and thus to obtain information about the internal structure of the rock mass, particularly regarding the potential disruption of the material at the rock front and deep fracture structures that delimit the large volumes. These investigations are combined with engineering-geological studies. They lead to the development of a three-dimensional model, which is used for modelling earthquake effects.