Earthquake Damages Related to Site Effects: A Case Study for an Alpine Valley

Abstract eng:
The Valais in Switzerland is the area of largest seismic hazard in Switzerland and has experienced a magnitude 6 or larger event every 100 years (1524, 1584, 1685, 1755, 1855, 1946). The Visp event of 1855 was the largest in Switzerland for the last 300 years. Besides its seismic activity, the Valais is characterized by several factors adding to the total hazard level: rough topography, unstable and steep slopes, and deep sediment-filled valleys. During the large earthquakes of the past the area experienced great damage from earthquake ground motion and different secondary phenomena such as liquefaction in the Rhone plain, landslide reactivation and extended rock fall. From the detailed historical reconstruction of the large events in 1855 and 1946, and macroseismic investigations related to smaller events in the past 150 years, it became obvious that the major effects were concentrated in some villages in the alluvial planes. In order to analyse this pattern we combined measurements of ambient noise, weak motion records and numerical wave propagation modeling to analyze the past earthquakes. The results provide clear evidence that the observed damage is caused by local site effects induced by two- and three-dimensional wave propagation phenomena. The results of this work demonstrate the complexity of wave propagation effects that cause amplification of seismic waves in irregularly shaped structures such as mountain valleys, and the importance to combine different analysis techniques and data. Due to river regulations and engineering progress in the last century, seismically unfavorable sites have become attractive for expanded settlement and industries. Future earthquakes will therefore cause more damage than was observed in the past. For this reason it is important to learn from past events, to recognize and map potential areas at risk, and to estimate ground motion and non-linear behaviour for engineers and planners.

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