Detecting signals of active tectonics with geomorphological methods in the slowly deforming Val di Fine Basin, Northern Apennines – limits to the resolution?

Tectonic and surface processes leave fingerprints on the modern topography. Deciphering tectonic signals becomes especially challenging when mechanisms at different wavelength overlap (e.g. faulting, deep-seated uplift). Nowadays, classic approaches can be combined with new tools and datasets to explore the morpho-structural evolution along a wide range of tectonic settings. The Val di Fine basin in the western side of the Tuscan Northern Apennines (Italy) is a slowly deforming area, generally assumed to be only of moderate to low seismic hazard. However, the 1846 ~M6 Orciano Pisano earthquake, responsible for significant destruction at the time, is strongly challenging this assumption. The event is presumed to have nucleated in the Val di Fine, however its source as well as precise location remain unclear and hence its seismological relevance. To clarify this question, we performed a new geomorphological analysis based on a 10x10 m DSM incorporating qualitative and quantitive methods (e.g. slope map, stream network analysis, knickpoint calculation). We complimented this work by field work, focusing on ground truthing of the geomorphic results and fault mapping, as well as a seismogenic approach relocating the freely available INGV earthquake catalogue. The results of the remote sensing analysis clearly show signs of several hundred meters uplift at the eastern side of the basin since the Pliocene as well as rather unspecific geomorphic features, raising new questions about the topographic development of the basin. The field data and seismogenic record clearly show signs of recent tectonic activity and uplift (newly mapped faults, seismites and small earthquake swarms) as well as clear indications that the basin likely features a more complex fault system then the N-S trending normal faults predominantly recorded in the region. However, these structures and possibly events seem to have left only very limited distinct detectable marks in the geomorphology. This decorrelation between the geomorphic results and field observations prompts the question why the tectono-geomorphic approach seems to be reaching its limit in this region. Factors like intense human activity and a dense vegetation in the area surely increase the noise level however these are common factors to be accounted for using remote sensing data. The cumulative results for this region rather point towards a complex morpho-structural evolution, characterized by a large-wavelength uplift component with local faulting activity. This may induce a complex response in the topographic expression leading to an overblending of the transient uplift signals through mid- and long-term developments making them more challenging to be detected with the current geomorphological methods.