Earthquakes in Switzerland and surrounding regions during 2019 and 2020
Tobias Diehl, Carlo Cauzzi, John Clinton, Toni Kraft, Philipp Kästli, Frédérick Massin, Federica Lanza, Verena Simon, Francesco Grigoli, Manuel Hobiger, Philippe Roth, Florian Haslinger, Donat Fäh, Stefan Wiemer
Abstract
Abstract This report summarizes the seismicity in Switzerland and surrounding regions in the years 2019 and 2020. In 2019 and 2020, the Swiss Seismological Service detected and located 1660 and 1407 earthquakes in the region under consideration, respectively. The strongest event in the analysed period was the M L 4.3 Elm/Steinibach earthquake, which occurred in the Glarus Alps in eastern Switzerland on October 25, 2020. Received felt reports suggest intensities up to degree V for this earthquake. Modelled and instrumentally measured ground motions, however, hint at intensities approaching degree VI–VII at the epicentre. Derived focal mechanisms and relative hypocentre relocations of fore- and aftershocks image a dextral WSW–ENE to W–E striking multi-segment strike-slip fault zone with a total length of about 3.5 km. Well-constrained focal depths of 1–2 km indicate that the fault zone likely locates in the uppermost part of the crystalline basement of the eastern Aar Massif. Another exceptional earthquake sequence occurred between Anzère and Sanetschpass in the Rawil Depression in November 2019. Within 10 days, more than 300 earthquakes occurred in this cluster and 16 of those events reached M L magnitudes between 2.5 and 3.3. Focal mechanisms and relative hypocentre relocations derived for this sequence image the reactivation of a contractional stepover. The imaged stepover confirms the previously proposed segmented nature of the Rawil Fault Zone north of the Rhône valley in SW Switzerland. The M L 4.2 Novel earthquake, which occurred in the Préalpes region south of Lake Geneva on May 28, 2019, provides additional evidence for the recently proposed domain of NE–SW oriented extensional to transtensional deformation along the Alpine Front in the transition zone between Central and Western Alps. Evidence for transtensional deformation along the SW edge of the Mont-Blanc Massif is provided by another remarkable earthquake cluster near the Grandes Jorasses Mountain in the border region between France and Italy. The transtensional deformation of the Hegau-Bodensee Graben in the northern foreland is revealed by a vigorous earthquake sequence on the Bodanrück Peninsula in southern Germany in 2019. Finally, evidence for unusually shallow seismicity in the domain of the Dent-Blanche nappe is provided by the M L 3.5 Arolla earthquake. In conclusion, the seismic activity during the period 2019–2020 is exceptional in terms of absolute numbers of earthquakes as well as number of events with M L ≥ 2.5.