Unraveling the subsurface fault geometry of small to moderate strike-slip earthquakes: an example from the Valdelsa basin in Southern Tuscany (Italy)

The connection between faults at surface, their subsurface geometry and earthquakes is a long-debated issue. The attempt of making such correlation is even more difficult when earthquakes are not strong enough to reach and break the topographic surface. Even in the latter case, the subsurface geometry of earthquake-causative-faults is not a trivial issue because of the difficulty of imaging the subsurface setting at seismogenic depths.
We take as an example the area of southern Tuscany in Central Italy where several M≈4 strike-slip earthquakes were registered recently, the latest of which occurred in May 2022.
The seismogenic role of transversal SW-NE striking faults in this area is debated as they do not show clear surface evidence even when releasing earthquakes and their recent and/or Quaternary evidence often a matter of discussion. For these reasons they can be extremely dangerous as they receive relatively little attention and are difficult to identify.
We integrate seismic reflection profiles, surface kinematic data and the relocation of seismological data in order to identify and characterize strike-slip active faults geometry at depth in the Valdelsa basin of southern Tuscany. We show that the Montespertoli NE-trending fault, part of a wider (15–20 km) crustal-scale shear zone, is possibly responsible for the 2016 M=3.9 Castelfiorentino earthquake and discuss the role of transversal SW-NE striking faults in controlling the inner Northern Apennines seismicity.