Abstract
This report summarizes the seismicity in Switzerland and surrounding regions in the years 2015 and 2016. In 2015, the Swiss Seismological Service detected and located 735 earthquakes in the region under consideration. With a total of 20 earthquakes of magnitude ML ≥ 2.5, the seismic activity of potentially felt events in 2015 was close to the average of 23 earthquakes over the previous 40 years. Seismic activity was above average in 2016 with 872 located earthquakes of which 31 events had ML ≥ 2.5. The strongest event in the analyzed period was the ML 4.1 Salgesch earthquake, which occurred northeast of Sierre (VS) in October 2016. The event was felt in large parts of Switzerland and had a maximum intensity of V. Derived focal mechanisms and relative hypocenter relocations of aftershocks image a SSE dipping reverse fault, which likely also hosted an ML 3.9 earthquake in 2003. Another remarkable earthquake sequence in the Valais occurred close to Sion with four felt events (ML 2.7–3.2) in 2015/16. We associate this sequence with a system of WNW-ESE striking fault segments north of the Rhone valley. Similarities with a sequence in 2011, which was located about 10 km to the NE, suggest the existence of an en-echelon system of basement faults accommodating dextral slip along the Rhone-Simplon line in this area. Another exceptional earthquake sequence occurred close to Singen (Germany) in November 2016. Relocated hypocenters and focal mechanisms image a SW dipping transtensional fault segment, which is likely associated with a branch of the Hegau-Bodensee Graben. On the western boundary of this graben, micro-earthquakes close to Schlattingen (TG) in 2015/16 are possibly related to a NE dipping branch of the Neuhausen Fault. Other cases of earthquakes felt by the public during 2015/16 include earthquakes in the region of Biel, Vallorcine, Solothurn, and Savognin.
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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