Gravity versus tectonics. The case of 2016 Amatrice and Norcia (Central Italy) earthquakes surface coseismic fractures

The 2016 central Apennines earthquake sequence was caused by slip on an extensional faultsystem and resulted in sizable coseismic surface deformation. The most evident effects occurred along thewestern slope of Mount Vettore, a geologically and morphologically complex mountain ridge. Steeptopography and rheological contrasts are known to have strongly controlled the coseismic deformationpattern during a number of different earthquakes that occurred in mountainous areas worldwide.Nevertheless, so far the role of seismically induced slope failures has not been taken into account in theinterpretation of the surface fractures caused by the 2016 earthquake sequence. We modeled the static anddynamic slope stability along the westernflank of Mount Vettore and in the underlying Piano Grande plain.Combining the slope stability analysis with geomorphic and geological analyses, we show that thecoseismic fractures are distributed along the most unstable areas of the westernflank of Mount Vettore andcan be partly explained by shaking‐induced mechanisms such as gravity‐driven displacement, compaction,and secondary ground failure. Conversely, in the Piano Grande plain the fracture pattern is not affectedby topography or rheology contrasts, suggesting that it is positively caused by tectonic faulting. Differentprocesses, such as gravitational and erosional‐depositional phenomena, may contribute to the exposure offault scarps during both the coseismic and interseismic periods. Attributing the surface deformation entirelyto tectonic faulting, especially in complex mountainous terrains such as the Apennines may lead to anincorrect assessment of fault displacement and fault slip rate and hence of seismic hazard