Seismo-tectonic activity along transversal structures in the Inner Northern Apennines

The Inner Northern Apennines (Italy) are a region with a dominant N-S to NNW-SSE fault system. However, E-W to NE-SW oriented structures crossing and dissecting the dominant fault trend have long been recognised. These transversal structures are often believed to be inherited, but our knowledge about their nature, activity and role in current tectonic motions is still limited and debated. Especially their activity and seismotectonic relevance is unclear.

A new seismo-tectonic analysis identified and relocated two distinct earthquake clusters in the Viareggio Basin, western Tuscany, which clearly showed activity along NE-SW oriented fault systems in the vicinity of one of the major transverse structures in the Inner Northern Apennines, the Livorno – Empoli lineament. The relocated clusters show mostly oblique slip, with a depth-dependent switch of motion direction in one cluster, as well as a localised change of the stress field. The position of the clusters and their onshore-to-offshore nature suggest the identified fault system to be related to the reactivation of pre-existing structures. These results show that the transversal structures of the Inner Northern Apennines, even when they do not show a morphologic surface expression, are seismogenic.

Moreover, a new tectono-geomorphic analysis was carried out in the presumed source area of the 1846 ~M6 Orciano Pisano earthquake (Val di Fine Basin), the biggest and most destructive recorded earthquake in western Tuscany. Our investigation, combining an in-field structural analysis with a detailed qualitative and quantitative geomorphic approach (e.g. stream network analysis, knickpoint calculation, slope map) based on a 10x10 m DTM, could not confirm the general assumption which attributes this event to one of the N-S striking faults bounding the basin. Instead, large and small scale geomorphic features as well as structural observations (e.g. an E-W trending water divide, diverted rivers, a newly identified fault) suggest that the event might have originated along a, so far unrecognised, transversal structure with an oblique right-lateral motion direction located at the centre of the basin.

The combined results of the seismo-tectonic & tectono-geomorphic analysis show that a more detailed investigation of the, often elusive and therefore easily overlooked, transversal structures in the Inner Northern Apennines is necessary, as they seem to be holding a seismogenic potential and might pose a so far uncared seismic hazard for the region.