Historical earthquake observations over the past four centuries and instrumental seismological data gathered over the past 30 years clearly indicate that there is naturally occurring earthquake activity in the Lower Rhone Valley between Martigny and Lake Geneva. Earthquakes like the one near Aigle in 1584, at the Lac d'Emosson reservoir or in the Mont Blanc Massif in 1905, which reached a macroseismic intensity of VII (very strong) and thus can be classified as damaging seismic events, bear witness to this. The occurrence of clearly weaker, but near-surface series of quakes in the municipality of Val d'Illiez in 1953-1954 and 1994-1996 was associated with the filling of the Lac de Salanfe reservoir. They show that the Earth's crust in this area is subjected to stress and that even minor disturbances to the stress field or the hydrological conditions may suffice to trigger seismic events.
In principle, deep geothermal energy projects should not hydraulically influence large, active tectonic faults (see Hirschberg et al., 2014, for example). Although very small, most likely very shallow earthquakes occurred at Lavey-Village in 2005-2006, the seismological data available do not indicate the presence of any active fault zone in the immediate vicinity of Lavey-les-Bains. However, the period over which data are available, i.e. a few decades, is very short compared to the recurrence cycles of earthquakes along a given fault (which in Switzerland often last thousands, or even tens of thousands, of years). Local seismological monitoring enables the swift detection of even the smallest changes in natural seismicity and makes it possible to assess whether there may be a connection with geotechnical activities in the context of the geothermal project. The SED informs AGEPP and the public in real time about such changes.