Tectonic activity in western Tuscany, Italy: A tectono-geomorphic and seismotectonic analysis

The Inner Northern Apennines in Tuscany (Italy) are part of a mountain range building process which, due to its complexity, still poses open questions. Neither the long-term evolution of the Apennines is completely understood yet, nor the influence the different stages of deformation have on the ongoing development e.g. through remnants such as inherited structures. Are inherited structures reactivated in the current stress field? Do they pose a potential seismic hazard? And how do they influence the landscape evolution? These are all unexplored questions limiting our understanding of this backarc region. Especially, the active tectonics of western Tuscany is very poorly understood, likely induced by its slow deformation rates and the relatively low to low-moderate earthquake hazard postulated for the region implying relative seismic quiescence. However, this assessment might lead to a false sense of security as the historic record shows that the area has indeed been hit by larger magnitude earthquakes in the past. The largest known event is the 1846 Orciano Pisano earthquake which had an estimated magnitude of 6. Despite the significance of the event and the reported extensive destruction, the knowledge of the earthquake is limited to the general source area, the Val di Fine Basin. This lack of knowledge limits our structural and seismological understanding of the region leading to a possible underestimation of the earthquake hazard for the area. This thesis focuses on improving our tectonic understanding of the region through conducting a detailed analysis of the long- and short-term evolution of the Val di Fine area as well as exploring the current activity and kinematics of the anti-Apennine structures. Anti-Apennine structures are likely inherited lithospheric structures, oriented E-W to NESW, and crossing the dominant N-S to NNW-SSE oriented fault trend in the Northern Apennines. To conduct this analysis an interdisciplinary approach was chosen combining tectono-geomorphic with seismological methods. For the seismological analysis, the seismicity of the past nearly 40 years in western Tuscany was examined and several seismic clusters were reprocessed leading to the successful relocation of several earthquake swarms. The relocation results of two clusters clearly show seismic activity along a NE-SW oriented, anti-Apennine fault system, which likely acts as a transfer fault between two major basins influencing their development. In the tectono-geomorphic analysis a detailed study of the Val di Fine area was conducted. The analysis shows that the Val di Fine Basin has a very complex history and experienced several large-scale changes during its evolution leading to tilt and river reorganisation. The structural analysis identified oblique to strike-slip movement along most of the basin bounding faults, this indicates that the basin likely experienced oblique rifting during its evolution. This finding is in line with the increased recognition of non-normal fault kinematics in western Tuscany. The results of the analysis furthermore suggest that the source of the Orciano Pisano earthquake might be an anti-Apennine fault presumed to be located under a low elevation water divide at the centre of the Val di Fine Basin. If this theory is confirmed, it would strongly ask for a re-evaluation of the significance of the traversal structures in the Inner Northern Apennines as they might pose a so far unrecognised seismic hazard for the region.