Present-day seismic activity in the Mugello Basin and adjoining areas (Northern Apennines, Italy)

The Northern Apennines is a NW-SE striking fold-and-thrust belt that developed during the Cenozoic collision between the European plate and the Adria plate, currently characterized by a crustal shortening in the external, Adriatic part, and an extension in the internal, Tyrrhenian side. One of the most relevant seismogenic areas is the Mugello basin region (Northern Tuscany), close to the city of Florence. Historically, two major earthquakes occurred in this area: M=6.0 in 1542 and M=6.4 in 1919. Unresolved issues are (i) how the earthquake distribution, depth, slip style, and seismic structure of the crust, relate to the transition from an extensional stress regime at the internal side of the belt to a compressional one at the external side, and (ii) the exact location and geometry of the major seismogenic fault(s) that rupture with larger earthquakes. To address these topics we integrated all the available seismic catalogs for the area, obtaining more than 12000 earthquakes spanning the 2005-2019 time interval. These data were then used to derive a minimum-misfit, 1-D velocity model and for a travel times inversion 3D tomography. Then, we complemented the permanent seismic recording network in the area with 9 new temporary stations near the Mugello basin, which helped in recording the Mw=4.5 earthquake on Dec. 9, 2019 at the NW margin of the basin. The mainshock and the ~200 aftershocks precisely delineate a 5-km-long, NW-striking and SW-dipping fault which extends 6-9 km depth. The focal mechanism of the mainshock is a normal fault striking N105°E and dipping about 45°. This fault appears to be distinct from those activated during the two last important sequences in the area, which occurred in 2008 and 2009. Given the proximity of the aforementioned fault to that inferred for the 1542 (and, conceivably, 1919) earthquake(s), a detailed study of the 2019 seismic sequence is expected to shed new light into the overall dynamics of the basin.