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Abstract
This report summarizes the seismicity in Switzerland and surrounding regions in the years 2017 and 2018. In 2017 and 2018, the Swiss Seismological Service detected and located 1227 and 955 earthquakes in the region under consideration, respectively. The strongest event in the analysed period was the ML 4.6 Urnerboden earthquake, which occurred in the border region of cantons Uri, Glarus and Schwyz on March 6, 2017. The event was the strongest earthquake within Switzerland since the ML 5.0 Vaz earthquake of 1991. Associated ground motions indicating intensity IV were reported in a radius up to about 50 km and locally approached intensity VI in the region close to the epicentre. Derived focal mechanisms and relative hypocentre relocations of the immediate aftershocks image a NNW–SSE striking sinistral strike-slip fault. Together with other past events in this region, the Urnerboden earthquake suggests the existence of a system of sub-parallel strike-slip faults, likely within in the uppermost crystalline basement of the eastern Aar Massif. A vigorous earthquake sequence occurred close to Château-d'Oex in the Préalpes-Romandes region in western Switzerland. With a magnitude of ML 4.3, the strongest earthquake of the sequence occurred on July 1, 2017. Focal mechanism and relative relocations of fore- and aftershocks image a NNE dipping normal fault in about 4 km depth. Two similarly oriented shallow normal-fault events occurred between subalpine Molasse and Préalpes units close to Châtel-St-Denis and St. Silvester in 2017/18. Together, these events indicate a domain of NE–SW oriented extensional to transtensional deformation along the Alpine Front between Lake Geneva in the west and the Fribourg Fault in the east. The structural complexity of the Fribourg Fault is revealed by an ML 2.9 earthquake near Tafers in 2018. The event images a NW–SE striking fault segment within the crystalline basement, which might be related to the Fribourg Fault Zone. Finally, the ML 2.8 Grenchen earthquake of 2017 provides a rare example of shallow thrust faulting along the Jura fold-and-thrust belt, indicating contraction in the northwestern Alpine foreland of Switzerland.
Highlights
Past earthquake activity in Switzerland and surrounding regions has been documented in a series of annual reports since 1879
With a total of 23 and 25 earthquakes of magnitude ML ≥ 2.5, the seismic activity of potentially felt events in either year was close to the yearly average of 23 earthquakes in the same magnitude range over the previous 42 years
The focal mechanism derived for this event indicates dominantly strike-slip faulting with a small normal-fault component (Fig. 4a)
Summary
Past earthquake activity in Switzerland and surrounding regions has been documented in a series of annual reports since 1879. Within the Swiss Strong Motion Network (SSMNet) renewal project, 100 free-field, real-time, very broadband accelerometer stations are being installed over a 10-year timeframe between 2010 and 2021 (e.g., Clinton et al 2011). To improve seismic monitoring in regions of past or future geothermal projects, five new broadband weak-motion stations were installed in 2017/18 (Fig. 1; FULLY, ILLEZ, LAVEY, SAVIG, SGT18). To improve the reliability of locations for events at the periphery of or outside of Switzerland, the SED continues to be engaged in an on-going cross-frontier cooperative effort to exchange seismic data in real-time with foreign seismic networks as documented in detail by Diehl et al (2014).
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