From archaeological and historical information of past earthquakes to a seismological description for seismic hazard and risk assessment
Donat Fäh (1) and the team of the Swiss Seismological Service
(1) ETH Zurich
Two case studies of strong earthquakes in Switzerland are presented. One of the most damaging events in intra-plate Europe was the 1356 Basel earthquake (Mw 6.6). Given its significance, an interdisciplinary project re-assessed this event. The efforts aimed to incorporate techniques from history, archaeology, seismology, paleo-seismology and engineering. New and reinterpreted historical data from Basel and its surroundings plus archaeological findings on buildings that survived the event and still exist, enabled a detailed macroseismic assessment. For the surrounding areas, archaeology offers sparse information on some castles and churches, sometimes supported by historical records. Paleo- seismological studies provided additional data. A contemporary source allowed a reconstruction of the stronger foreshocks, important to explain the few fatalities historically reported.
The second example concerns the Roman city Augusta Raurica located east of Basel. One important topic of the city’s history concerns the hypothesis of an earthquake in the middle of the third century A.D. This idea had been formulated according to archaeological findings. These findings were reviewed and dated in order to test the hypothesis of a single event. However, it could not be proven that the features of possible destruction date to the same event. Paleo-seismological findings provide no hints to a large earthquake in the third century. Detailed seismological investigations were performed together with geological and geophysical mapping of the unconsolidated sediments. From all information, a three- dimensional model of the surface geology was compiled and used to simulate earthquake ground motion. If at all, an event with maximum magnitude Mw of about 6.0 or even lower has to be assumed.
Such studies allow a calibration of earthquake risk scenarios. As an application, loss assessments of the school buildings in Basel were performed for different earthquake scenarios of significance. The hazard part of the computations (i.e. ground-motion estimation) benefits from the work performed for the 2016 Swiss seismic hazard assessment. A regional microzonation and recordings of small earthquakes on a dense strong-motion network are used to compute site-amplification factors. The school buildings, which are mainly unreinforced masonry or reinforced concrete shear-wall buildings, have been classified according to a specifically developed taxonomy. Fragility curves have been developed using non-linear static procedures and subsequently, vulnerability curves in terms of human and financial losses are proposed. The scenarios simulate what could happen today when assuming earthquakes of the past. This is a useful tool to increase awareness and acceptance for the mitigation strategies in Basel.