SEISMICALLY INDUCED HAZARDS IN BALZERS, LIECHTENSTEIN: WHY ANCIENT BALZERS WAS SAFER

The recent recognition of sizeable earthquakes in Switzerland, near the Liechtenstein border, has highlighted the little studied threat of seismically induced geological hazards in the Liechtenstein valley. While Switzerland experiences seismic activity regularly, Liechtenstein last experienced an earthquake (mag. 3.6) in 2005. Roughly half the country is within the Rhine River valley floodplain; with the other half is made of peaks belonging to the Rhatikon Range. The valley floor contains oversaturated sediments, which can liquefy during an earthquake. Rock fall and avalanche hazards are higher than average due to seismically induced weakening of the rock face or snow pack. Focus for studying increased natural hazard extent was centered on the town of Balzers, located on the Rhine River floodplain in the south of the country. This small valley, inhabited since roughly the Bronze Age, is exposed to some of the steepest relief in the country paired with a high number of geological hazards, making it have a higher natural hazard risk than widely assumed. Not only are the majority of the city’s buildings built on soils indicated as high to moderate liquefaction risk, they are also within zones mapped to be of high rock fall and avalanche hazard. Building codes were recently updated due to recent tectonic activity, and because of the recent construction boom within the city. This has highlighted an unusual situation not commonly seen in the majority of ancient cities – the older buildings of the city survive natural hazard events better than the modern buildings. The older buildings did not have the protection offered by modern mitigation, limited though that might be, so why are they surviving seismic events with little to no damage, while the up-to-code modern buildings display damage after relatively minor events? The answer may lie in the distinct but unusual pattern and interaction of all of the potential hazards that can be seismically triggered in this valley, the ancient building methods employed by inhabitants, and the fact that modern mitigation methods may not be effective in this unique setting.