The 1946 magnitude 6.1 earthquake in the Valais: site-effects as contributor to the damage

Abstract

The January 25, 1946 earthquake in the central Valais region in southwest Switzerland was the strongest for the last 150 years. It reached an epicentral intensity Io of VIII in the area of Sierre. The Swiss Earthquake Catalogue (ECOS 2002) assigns a moment magnitude of Mw = 6.1 to the event. Assessment of recordings from European stations resulted in a moment magnitude of 5.8 (Bernardi et al. 2005). The earthquake caused moderate to high damage within a circle of about a 25 kilometer radius. Slight damage occurred up to a distance of 200 kilometers from the epicenter. The goal of this study was to reconstruct the damage field and consider its possible site-effects. We used an approach combining historical research with seismo-/geological investigation including a large number of experiments measuring the fundamental frequency of resonance and the shear-wave velocities of the sedimentary layers, using the characteristics of ambient vibration. This kind of research is relevant, since a huge alpine valley characterizes the Valais region, showing ground conditions that make site-effects likely for earthquakes. While we searched for damage in an unlimited area, our investigation of site-effects was limited to the Rhone valley and to Sion and Sierre in the central Valais region in particular. Since a contemporary damage assessment has survived in fragments only, the results of our historical investigation are incomplete. Nevertheless, it was possible to describe the losses within the epicentral region adequately enough to discuss possible site-effects. Our results show that Valaisan districts with a high percentage of settlement on the lacustrine and fluviatile deposits of the Rhone valley show higher losses than other districts. The villages of Sierre and Sion we investigated in more detail. Sion is located on compacted sediments with relatively high shear-wave velocities. The city suffered only moderate damage, rather regularly distributed. In Sierre, on the other hand, we identified two zones of significantly increased damage, in which site-effects appear a very probable contributor. Local two- or three-dimensional resonance phenomena play an important role in explaining this feature.