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. Introduction The earthquake series and its parametersIt was already dark when the region of the central Valais was hit by a strong earthquake on January 25, 1946 at 5.32 pm. In sion, the capital city of the Valais canton and in many other villages nearby the power supply broke down immediately. People rushing out of the houses found themselves on streets covered with rubble of destroyed chimneys, thrown off roof tiles and chipped façade pieces. For many hours, confusion reigned. communication lines were overstrained and for some time out of order. It took several days for the extent of the consequences to become clear with respect to the event's geographical compass and severity of damage. Fortunately, the consequences appeared more minor than originally reported by newspapers within the first days after the earthquake. Althoug...

The Chironico landslide (Valle Leventina, southern Swiss Alps): age and evolution

“…Gunzburger et al 2005;Prager et al 2008;Korup et al 2010). Important triggering factors are heavy rainfall or earthquakes, which have as a consequence a direct collapse of the slope, as shown by more recent events in Alpine realm (Fritsche and Fäh 2009).…”

Historical intensity VIII earthquakes along the Rhone valley (Valais, Switzerland): primary and secondary effects

“…Taken from Fritsche and Fäh (2009) earthquake in 1855 and the Sierre event in 1946. We assume that our list for this size of earthquakes is complete for this period.…”

“…The soil conditions on these sites make amplification effects likely. Modified from Fritsche and Fäh (2009) area, consisting of thick sedimentary soils, behaves disadvantageously during earthquakes, when compared with the valley slope. Moreover important infrastructure is located below instable rock and soil slopes that might be triggered by strong earthquakes.…”

“…Approximately 3,500 of 22,000 buildings in Canton Valais sustained damage. In general the results Fritsche and Fäh (2009) show that damage in districts with a high percentage of infrastructure on the lacustrine and fluviatile deposits of the Rhone valley floor show higher loss than other districts (Fritsche and Fäh 2009). Regarding the cities of Sierre and Sion, two case studies led to more detailed results.…”

“…1). These are the events of 1584 (Aigle I = VIII, see Schwarz-Zanetti 2008a;Schwarz-Zanetti and Fäh 2011), 1755 (Brig I = VIII, see Gisler et al 2004;Gisler 2008;, 1855 (Visp I = VIII, see Fritsche et al 2005Fritsche et al , 2006Gisler and1946 (Sierre I = VIII and I = VII, Fritsche and Fäh 2009). The present paper provides a summary of these publications focusing on the main characteristics of the observed primary and secondary effects.…”