Damage field and site effects: multidisciplinary studies of the 1964 earthquake series in Central Switzerland |
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| Authors: | Stefan Fritsche Donat Fäh Brian Steiner Domenico Giardini |
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| Institution: | 1.Swiss Seismological Service, Institute of Geophysics,Zurich,Switzerland |
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| Abstract: | Central Switzerland shows comparatively high seismic activity by Swiss standards. Many historical earthquakes are known and
several of them caused damage. The last major event dates back to 1964 and has the characteristics of an earthquake swarm.
Among dozens of felt shocks were two main shocks (Mw = 5 and 5.7) that moderately damaged a limited area with hundreds of
buildings suffering loss. Our aim here was to reconstruct the damage field and to analyze whether it was influenced by site
effects. Given the existence of a contemporary damage assessment and other historical sources, we could describe the damage
field in detail. For about 95% of the affected buildings, we could reconstruct the location and extent of loss, using assessments
from the European Macroseismic Scale (EMS 98). Spatial analysis of the resulting data showed that most losses were concentrated
in the villages of Sarnen and Kerns. Damage to residential houses and barns was by far most frequent (90%), but expensive
losses to the relatively few sacral buildings were responsible for almost 50% of the repair costs. We compared the damage
data with deposit thickness and soil composition and carried out field experiments using H/V spectral ratios to measure the
fundamental frequency of ground resonance at 75 sites to estimate the frequency band in which amplification occurs. Our results
show that locations on both thick fluviatile sediments and large alluvial cones showed higher intensities than did other ground
types. Moreover, at some sites, intensity was probably increased by a layer of weathered rock below thin deposits. |
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| Keywords: | Earthquakes Historical earthquakes Seismology History Damage fields Damage assessment EMS Site effects Switzerland |
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