Implications on recent tectonics in the Alps from centroid moment tensor inversion of weak to moderate seismicity

Despite recent tectonic activity, the Alpine mountain range in central Europe is mostly characterized by weak to moderate seismicity. Low earthquake magnitudes and the heterogeneous crust comprising of different tectonic units challenge centroid moment tensor inversions in this region. Thanks to the dense AlpArray seismic network, comprising more than 600 stations across the Alps, as well as the adoption of a flexible, bootstrap-based inversion tool, we were able to reduce the magnitude threshold for moment tensor inversion to Mw 3.0. The inversion set-up was implemented after systematic tests of different frequency bands, distance ranges, input data types and azimuthal gaps. We quantified the uncertainties of centroid locations and moment tensors, and assessed the reliability of potential non double couple components. Here, we present ~80 deviatoric moment tensor solutions and compare our results to strain rates, historic and recent seismic activity as well as to other published focal mechanisms. We identify three main seismically active subregions, namely the Western Alps, the Lake Garda region and the SE Alps, and two clusters further away from the study region, in the Dinarides and the Apennines. Seismicity is particularly low in the NE Alps and in parts of the central Alps. Additionally, we apply a focal mechanism clustering algorithm to the joint catalog, including our moment tensor solutions and those from existing catalogs. While typical E-W to ENE-WSW striking thrust faulting is observed in the Friuli area in the SE Alps, strike-slip faulting with a similarly oriented pressure axis is observed along the central Alps and in the Dinarides. NW-SE striking normal faulting is observed in the NW Alps with a similar strike direction as the dominant normal faulting events in the Apennines. In the W Alps as well as in the SE Alps, rotations of mechanisms are observed. Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow, with 80 % of the studied events being shallower than 10 km.