Faults segmentation in active external extensional front: insights from seismicity in the Sant'Anna Pelago area, North Apennines

The Neogene-to-Recent tectonic evolution of the Northern Apennines has been characterized by contraction in the foreland, accompanied by extension in the internal domain. While the architectural and kinematic aspects of these two distinct sectors are better-known, uncertainties persist regarding the transition between them. Understanding the tectonics is made more complex since the width of the internal extensional domain increases south-eastward as the Northern Apennines steps eastward across Italy. The Sant'Anna Pelago area, located on the Tuscan-Emilian ridge between Alpe di Succiso (NW) and Monte Cimone (SE), exhibits pronounced instrumental seismicity with over a thousand recorded events over the last ~15 years, forming the focus of this study. Sant'Anna Pelago represents a critical zone as the extensional front is situated near the outermost out-of-sequence contractional front affecting the Tuscan Nappe. The region locates close to the orographic divide, and also where the width of the internal extension starts widening significantly eastward. Seismicity in Sant’Anna is expressed through three major seismic sequences over a 10-year period from 2012 to 2022, with events concentrated in 2013, 2018, and 2022. P and S arrival times from 56 evenly distributed stations within 130 km radius from the publicly available INGV database were utilized to perform a preliminary relocation of seismic clusters using NonLinLoc and a local velocity model. Subsequent precise relocations were conducted using differential arrival times through HypoDD. The relocation revealed three primary deep clusters and several minor aligned ones. The 2013 sequence is 8-km-long, 10-17 km deep, strikes parallel (NW) with the Apenninic trend, and dips 50 degrees towards the SW. This structure aligns prominently with the southern tip of the Zola master fault, surfacing near Pieve Pelago. Earthquakes are particularly dense at the southern tip of this structure. The subsequent clusters are in the footwall of the 2013 sequence, and at similar depths. These show several discrete, sub-vertical structures oriented E-W, each approximately 3 km long. These clusters geometrically resemble synthetic en-echelon faults situated in the footwall of the Zola fault. The western tips of these structures align along a potential envelope segment linking them to the 2013 cluster, a transverse SW-NE structure orthogonal to the Apenninic structure and the Zola Fault. We interpret the 2013 sequence as normal slip on a reactivated NW striking Apenninic contractional structure, and the subsequent en-echelon sequences on E-W faults as mostly normal slip in a dextral stepping zone of local re-orientation of stress. The interpretations will however, be tested with earthquake focal mechanisms and field structural geology.