The 2021–2022 Genoa seismic sequences reveal distributed strike-slip deformation in the Alps-Apennines transition zone, NW Italy

Abstract

The complex tectonic evolution of the Alps-Apennines transition zone in NW Italy is still a matter of debate. In this work, we analyze the 2021–2022 seismic sequences around Genoa to understand how convergence between Africa and Europe is presently accommodated across the Alps-Apennines transition zone. The map-view distribution of HypoDD-relocated seismic events reveals a NE-SW alignment for the Savignone seismic sequence, and NNW-SSE alignments for the Borzonasca and Bargagli sequences. The Borzonasca seismic sequence plots in correspondence of the Villalvernia-Varzi-Ottone Fault, which is often considered as the boundary between the Alps and the Apennines, whereas no seismicity is documented along the Sestri-Voltaggio Fault. The main-shock focal solutions are invariably strike-slip, with near-vertical NNW-SSE and NE-SW to ENE-WSW nodal planes. The evident earthquake alignments in the study area mark active, km-scale fault planes in the upper crust, pointing to a scenario of distributed strike-slip deformation in the transition zone between the Alps and the Apennines. The NE-SW faults are inherited structures that underwent major Neogene rotations and are no longer suitably oriented to accommodate the northward motion of Adria relative to Europe. The Bargagli seismic sequence may reflect the formation of new NNW-SSE strike-slip faults in the upper crust that are more suitably oriented to accommodate the present-day stress field, consistent with the seismotectonic framework outlined by recent works in the nearby regions of the Adria-Europe plate-boundary zone. Our results highlight the important role of strike-slip faulting in the Adria-Europe plate boundary zone not only in the past, but also during its present-day evolution.