Multiple Lines of Evidence for a Potentially Seismogenic Fault Along the Central-Apennine (Italy) Active Extensional Belt–An Unexpected Outcome of the MW6.5 Norcia 2016 Earthquake

Federica Ferrarini,J Ramón Arrowsmith,Francesco Brozzetti,Daniele Cirillo,Giusy Lavecchia,Rita De Nardis

doi:10.3389/feart.2021.642243

Abstract

The Apenninic chain, in central Italy, has been recently struck by the Norcia 2016 seismic sequence. Three mainshocks, in 2016, occurred on August 24 (MW6.0), October 26 (MW5.9) and October 30 (MW6.5) along well-known late Quaternary active WSW-dipping normal faults. Coseismic fractures and hypocentral seismicity distribution are mostly associated with failure along the Mt Vettore-Mt Bove (VBF) fault. Nevertheless, following the October 26 shock, the aftershock spatial distribution suggests the activation of a source not previously mapped beyond the northern tip of the VBF system. In this area, a remarkable seismicity rate was observed also during 2017 and 2018, the most energetic event being the April 10, 2018 (MW4.6) normal fault earthquake. In this paper, we advance the hypothesis that the Norcia seismic sequence activated a previously unknown seismogenic source. We constrain its geometry and seismogenic behavior by exploiting: 1) morphometric analysis of high-resolution topographic data; 2) field geologic- and morphotectonic evidence within the context of long-term deformation constraints; 3) 3D seismological validation of fault activity, and 4) Coulomb stress transfer modeling. Our results support the existence of distributed and subtle deformation along normal fault segments related to an immature structure, the Pievebovigliana fault (PBF). The fault strikes in NNW-SSE direction, dips to SW and is in right-lateral en echelon setting with the VBF system. Its activation has been highlighted by most of the seismicity observed in the sector. The geometry and location are compatible with volumes of enhanced stress identified by Coulomb stress-transfer computations. Its reconstructed length (at least 13 km) is compatible with the occurrence of MW≥6.0 earthquakes in a sector heretofore characterized by low seismic activity. The evidence for PBF is a new observation associated with the Norcia 2016 seismic sequence and is consistent with the overall tectonic setting of the area. Its existence implies a northward extent of the intra-Apennine extensional domain and should be considered to address seismic hazard assessments in central Italy.

Highlights

Identification and geometric reconstruction of active faults are one of the major concerns in Italy given the evident earthquake hazard (MPS, Gruppo di Lavoro, 2004; DISS Working Group, 2018) and the occurrence of several earthquakes among the most energetic (MW≥6.5) in the Mediterranean region (Figure 1A)
We investigate the location and geometry of the highest stress changes within the study area and assess if they were compatible with the Pievebovigliana fault (PBF) activation, computing the stress transfer induced by the largest subsequent earthquakes of Norcia 2016 seismic sequence EQ1, EQ2 and EQ3 on the study area
We introduced the evidence of the PBF normal faulting as we deduced from the field survey and morphotectonic analysis

Summary

Introduction

Identification and geometric reconstruction of active faults are one of the major concerns in Italy given the evident earthquake hazard (MPS, Gruppo di Lavoro, 2004; DISS Working Group, 2018) and the occurrence of several earthquakes among the most energetic (MW≥6.5) in the Mediterranean region (Figure 1A). The majority of the hypocentral distribution (Chiarabba et al, 2005) concentrates at upper crustal depths along the intraApennine Quaternary active extensional belt which, in central Italy (Figure 1B), is mostly represented by W-dipping highangle- and E-dipping low-angle normal faults (Boncio and Lavecchia, 2000; Collettini et al, 2006; Mirabella et al, 2011; Lavecchia et al, 2017; Lavecchia et al, 2020). Three normal fault earthquakes on 24 August (MW6.0, 01:36:32 UTC), 26 October (MW5.9, 19:18:07 UTC) and 30 October (MW6.5, 06:40:18 UTC) (hereinafter EQ1, EQ2, EQ3, respectively) nucleated along two late Quaternary active WSWdipping faults belonging to the outer extensional alignment (Figure 1C): the northern strand of the Mt Gorzano (GF) fault and the Mt Vettore-Mt Bove (VBF) (Lavecchia et al, 2016; Pizzi et al, 2017; Brozzetti et al, 2019). Surface coseismic displacements, associated with both the August- and 30 October events, were extensively observed along the VBF (Emergeo Working Group, 2017; Pizzi et al, 2017; Pucci et al, 2017; Civico et al, 2018; Villani et al, 2018; Brozzetti et al, 2019)

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