Over the past 500 years, an earthquake causing major damage to buildings in the Valais has occurred in the middle of each century. Information becomes patchier the further back in time you go. However, it is safe to assume that earthquakes with the potential to cause major damage to buildings also occurred in the Valais prior to 1500. The most recent damaging earthquake in the Valais took place near Brig on 23 March 1960. It had an intensity of VII. Things have been relatively quiet since then, with no earthquake exceeding an intensity of V.
For the earthquake in Visp on 25 July 1855, with an estimated magnitude of 6.2, it has been possible to accurately reconstruct the extent of the damage. This applies to both the geographical extent of the damage area and the scale of the damage within the directly affected localities. The earthquake reached its maximum intensity of VIII in the Vispertal valley, where it destroyed large parts of the settlements of Visp, Stalden, Törbel, Grächen and St. Niklaus. It killed one person and left dozens injured. Hundreds of boulders were dislodged while entire hillsides gave way and slid into the valley, burying roads and causing existing springs to dry up and new ones to emerge. The earthquake produced intensity VI effects (minor damage to buildings) in Geneva, Basel, Zurich and Schaffhausen, and could be felt as far away as Paris, Lyon, Genoa, Ingolstadt and Mainz. Hundreds of aftershocks were felt for years afterwards, including eight intensity VI to VIII quakes which caused further damage to buildings up to the end of 1855.
The earthquake in Sierre on 25 January 1946 was Switzerland's most powerful quake of the 20th century. From a few seismograms recorded abroad, it was possible to calculate the moment magnitude as 5.8, which corresponds to a Richter magnitude of approximately 6.1. The earthquake claimed three lives and caused severe damage to around 3,500 buildings in Sierre, Sion and surrounding areas. Over 500 aftershocks were felt up to the end of 1946 alone, including four with intensities of VI and VII. A particularly noteworthy aftershock occurred on 30 May 1946. This triggered a massive rock avalanche on the Rawilhorn, whose traces can still be seen in the landscape today. Earthquakes had been repeatedly documented in the wider epicentral area of the 1946 quake prior to that event and it is also possible that the cluster of epicentres north of Sion and Sierre, which can still be seen today on maps of instrumentally recorded events, is linked to the 1946 earthquake.
The Sierre earthquake today
The damage caused by the 1946 earthquake would be valued at around CHF 26 million in today's money. As the area is now more built-up, the consequences of an equally powerful quake today would be much greater. Unlike in 1946, the floor of the Rhone Valley is now densely populated and home to large industrial facilities. The subsoil is also unfavourable since the soft sediments of the valley floor amplify seismic waves by up to 10 times compared with a hard subsurface. In other words, shaking here would be much more intense than in places with a rocky substrate. This would lead to greater structural damage and potentially even the collapse of buildings. As even many new buildings would probably not withstand a powerful earthquake under these conditions, the number of victims would likely be much higher than in 1946 and damages up to several billion CHF are possible (Massnahmenkonzept "Erdbeben", Planat, 2000).