Linking subsurface structure and active faulting in the intermontane Mugello Basin and its implication for the post-orogenic tectonic evolution of the Northern Apennines (Italy)

The Mugello Basin is a WNW-ESE-striking, seismically active intermontane basin currently experiencing post-orogenic extension that affects the hinterland of the Northern Apennines belt (Italy). It lies near the main watershed of the Northern Apennines, a feature traditionally seen as separating the internal, extension-dominated (to the southwest) part of the belt from the external, contractional (to the northeast) part. Therefore, complex and controversial relationships exist between the recorded seismic activity and the surface manifestations of potentially seismogenic faults at depth in the Mugello region. The ITaly HAzard from CApable faults (ITHACA) catalogue reports active and capable faults along both margins of the basin. Moreover, coseismic surface ruptures were observed during the 1919 earthquake. However, surface expressions of active faults in the Mugello region are less pronounced compared to those associated with similar graben-bounding faults in other intermontane basins of the Northern Apennines. Moreover, the subsurface structure of the basin remains poorly constrained and is widely debated, with a significant lack of reliable geophysical data. This has led to contrasting views on the size, geometry, and orientation of the potential seismogenic sources, according to different researchers. Thus, the Mugello Basin offers an excellent opportunity to apply geophysical surveys to address the gaps in knowledge regarding its subsurface structure.

In this study, we used a combined methodological approach to propose a subsurface model for the basin's geometry and mechanical properties. We performed seismic microtremor measurements to be interpreted in the frame of the H/V and H&V methods along four profiles orthogonal to the basin’s axis (i.e., NNE-SSW) and one parallel to it (i.e., WNW-ESE). By integrating surface geological data and geomorphic analysis of the fluvial network (chi-map) based on a 10-m DEM, we were able to refine the geophysical model and more accurately evaluate the seismic behavior of the basin. We combined microtremor measurements with publicly available well log data and field geology observations, which helped us interpret the normalized H/V values and estimate the thickness of the basin’s sedimentary fill. Preliminary results suggest that the basin exhibits an asymmetric across-strike geometry, with active extensional faults likely concentrated along its northern margin. This is consistent with the epicentral distribution of seismic sequence that affected the area between 2009 and 2019, as highlighted by recent studies. Accordingly, geomorphic analysis shows the highest chi values in the northeast sector. This marks a disequilibrium of the river network which might be associated with active faulting, whereas in the southeast chi-values indicate steady state among rivers. The interpretation of the normalized H/V values in terms of bedrock geometry provides new insights into the basin's subsurface structure and offers constraints that challenge previously proposed models. These results have significant implications for evaluating seismic site effects at the scale of the Mugello Basin. Furthermore, they contribute to understanding the tectonic evolution of the basin within the larger geodynamic context of the Northern Apennines, particularly with respect to the transition from syn-orogenic compressive to post-orogenic extensional tectonics.