Testing different tectonic models for the source of the Mw 6.5, 30 October 2016, Norcia earthquake (central Italy): A youthful normal fault, or negative inversion of an old thrust?

Abstract

We adopted a multidisciplinary approach to investigate the seismotectonic scenario of the 30 October 2016, Mw 6.5, Norcia earthquake, the largest shock of the 2016-2017 central Italy earthquake sequence. First, we used seismological and geodetic data to infer the dip of the main slip patch of the seismogenic fault, that turned out to be rather low-angle (37°). To evaluate whether this is an acceptable dip for the main seismogenic source, we modeled earthquake deformation using single- and multiple-fault models deduced from aftershock pattern analyses. These models show that the coseismic deformation generated by the Norcia earthquake is coherent with slip along a rather shallow-dipping plane. To understand the geological significance of this solution we reconstructed the sub-surface architecture of the epicentral area. As the available data are not robust enough to converge on a single fault model, we built three different models encompassing all major geological evidence and the associated uncertainties, including the tectonic style and the location of major decollement levels. In all models the structures derived from the contractional phase play a significant role: from controlling segmentation to partially reusing inherited faults, to fully reactivating in extension a regional thrust, geometrically compatible with the source of the Norcia earthquake. Based on our conclusions, some additional seismogenic sources falling in the eastern, external portions of the Apennines may coincide with inherited structures. A circumstance that may be a common occurrence in this region of the chain, where the inception of extension is as recent as Middle-Upper Pleistocene.