Testing Different Tectonic Models for the Source of the M_w 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 subsurface 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 décollement 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. This may be a common occurrence in this region of the chain, where the inception of extension is as recent as Middle‐Upper Pleistocene.

Fine‐Scale Structure of the 2016–2017 Central Italy Seismic Sequence From Data Recorded at the Italian National Network

“…It is still under discussion to what extent, if at all, the Sibillini thrust is controlling the extensional deformation in the overlying crust and potentially hosting coseismic slip during the M W 6.5 event (Tinti et al, 2016; Chiaraluce, Di Stefano, et al, 2017, Pizzi et al, 2017; Cheloni, Falcucci, et al, 2019) as a secondary fault. This latter case would imply the reactivation in terms of kinematic inversion, of the preexisting ST oblique ramp, formed during the late Pliocene compressive phase (Bonini et al, 2019; Buttinelli et al, 2018; Chiarabba et al, 2018) as originally proposed by Scognamiglio et al (2018). This model is partially corroborated by the earthquakes alignments we highlighted in Figure 6 (see earthquakes clustered within the cyan box).…”

“…These sequences belong to a unique 150 km long normal fault system composed by individual contiguous and/or sub parallel, 10–30 km long, SW‐dipping fault segments. Their static and dynamic interaction (Cocco et al, 2000; De Natale et al, 2011; Hernandez et al, 2004; Lavecchia et al, 2012; Mildon et al, 2017; Nostro et al, 2005; Pace et al, 2014; Papadopoulos et al, 2017; Pino et al, 1999; Verdecchia et al, 2018) as well as their interference with compressional structures formed during the previous tectonic phase is still debated (Chiaraluce et al, 2005, Chiaraluce et al, 2011; Chiaraluce, Di Stefano, et al, 2017 Scognamiglio et al, 2018; Buttinelli et al, 2018; Bonini et al, 2019).…”

“…and Messinian siliciclastic deposits (Laga Fm.). The role of the ST structure in constraining the extensional deformation, mainly occurring along NW‐trending normal faults segments, is still debated (Tinti et al, 2016; Chiaraluce, Di Stefano, et al, 2017, Pizzi et al, 2017; Bonini et al, 2019), as well as the nature of the normal faults possibly reutilizing preexisting steep ramps formed during the late Pliocene compressive phase (Buttinelli et al, 2018; Chiarabba et al, 2018). In addition to this, Scognamiglio et al (2018) suggested an active role of a portion of the compressional structure, proposing a composite rupture model for the strongest Norcia earthquake: a main NW‐trending normal fault segment, coinciding with the extensional VBFS, and a secondary plane coinciding with the deep portion of the NNE‐trending sector of the ST, implying at depth a kinematic inversion of the thrust ramp (see Figures 6a and 6b).…”

Accounting for uncertain fault geometry in earthquake source inversions – II: application to the Mw 6.2 Amatrice earthquake, central Italy

“…Power et al 1987;Candela et al 2012). The variability of proposed fault morphology for many events, such as the 2009 M w 6.3 L'Aquila (Lavecchia et al 2012), the October 2016 Norcia (Bonini et al 2019), the 2011 M w 9.0 Tohoku-Oki (e.g. Lay 2018), the 1999 M w 7.4 Izmit (e.g.…”

Constraints on the Structure of the Shallow Crust in Central Italy from Geophysical Log Data

“…A very large number of papers have been produced after the seismic sequence, containing different seismotectonic interpretations and models [e.g. 25,[30][31][32][33][34][35][36] ]. However, in the field a surface normal faulting pattern striking N135-165, SW-dipping and subordinately NE-dipping, along ~28 km of the active Mt.…”