• 3D tomography
• Installation seismic network
• Mugello basin
• Seismic sequence analysis

Within the geodynamic context of the Northern Apennines (Italy), a relevant seismogenic area is the Mugello basin (North-Eastern Tuscany). The area has a well-documented record of seismicity; the two major historical earthquakes occurred in 1542 (Mw=6.0) and in 1919 (Mw=6.4). The proximity of the Mugello Basin to densely-urbanized areas and the potential impact of strong earthquakes on the cultural heritage in the nearby (~30km) city of Florence makes a better knowledge of the seismicity in that area an important target. Within this context, one unresolved issue concerns the exact location and geometry of the major seismogenic fault(s) that rupture with larger earthquakes. The present-day seismicity in the region is characterized by short-duration sequences, exhibiting a clear mainshock-aftershock behaviour. Their accurate study is particularly relevant since it may provide constraints on fault location, geometry, and kinematics, which are necessary ingredients for seismic hazard assessment. To address these topics, I integrated all the available seismic catalogs for the area, obtaining more than 12000 earthquakes spanning the 2005-2019 time interval in a single catalog. Then, I participated to the installation of 9 temporary stations that complemented the permanent seismic recording network in the area in the period spanning 2019-2021. From integration of both archive (for the period 2005-2019) and newly-acquired (2019) sets of phase arrival times I obtained an improved seismic catalog, which has been inverted for hypocentral location using a specific velocity model. The subsequent relocation with a double-difference algorithm (HypoDD) of this data set allowed for accurate analyses of the most relevant seismic sequences which affected the study area in March 2008, September 2009, 2015-2017 and December 2019. These sequences are associated with the activation of adjacent segments of a larger NW-striking fault system. These results show that the present-day seismicity is driven by the fault system bordering the NE margin of the basin (Ronta fault system). The temporary network provided high-quality recordings of the 2019 sequence which, coupled to advanced detection methods (e.g. Matched Filter technique), permitted improving the catalog completeness by a factor greater than 3 in the number of locations, and a decrease of 1.2 in magnitude of completeness. Finally, the arrival times of local earthquakes from the merged catalog have been inverted to derive a 3D seismic velocity model for the Northern Apennines crust. The model broadly has seismic velocities within the range expected for continental crust, except for shallow high P-wave velocity lobes in the upper crust near to and striking parallel to the
Apennines, which likely correspond with crystalline basement at the core of major anticlines associated with the Apennines thrust system. The images also show a distinctive, deeper increase of seismic velocity consistent with the base of the crust, which deepens from ~20-25 km to ~30 km beneath the Apennines, thus confirming local crustal thickening beneath the belt.