On Friday evening, January 25 in 1946 it was already dark and there was snow on the ground in Sierre when at around 6.32 p.m. the ground shook for several seconds. People ran out into the streets, chimneys and tiles fell from the rooftops and the streets were strewn with rubble. There was a power cut, and for 10 minutes the city was plunged into complete darkness. Very soon, the telephone lines became so overburdened that for a number of hours great uncertainty and chaos ensued.
Only the next day did the full impact of the earthquake, which had a magnitude of 5.8, become clear: 4 fatalities and 3,500 damaged buildings. The total damage caused totalled around CHF 26 million in today's money.
Unlike in 1946, the floor of the Rhone Valley is now densely populated, as well as being home to some large-scale industrial plants, so the impact of such an earthquake would be much greater. On top of this, the valley has an unfavourable substratum: so-called 'site effects' can increase the amplitude of earthquake shockwaves along the soft sediment of the valley floor by a factor of up to 10. This means the vibrations would be much stronger here than in rocky areas, resulting in greater damage to buildings and possibly causing houses to collapse. Since even many new buildings could probably not withstand a strong earthquake under these conditions, the death toll would probably be far higher than it was in 1946.
A magnitude-5.8 earthquake like the one in 1946 would be felt throughout Switzerland. Moderate to severe damage would be expected over a wide area around the epicentre. Assuming that around 7,000 buildings were moderately to severely damaged, the cost of building damage would amount to approximately CHF 2.5 billion. In addition, some 300 people could be injured and up to 40 killed, with around 9,000 people seeking shelter.
On average, an earthquake with a magnitude of 5.8 can be expected within a radius of 50 km of Sierre every 235 years.
The whole scenario at a glance (PDF).