GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.1 57 A RESISTIVITY CRUSTAL PROFILE ACROSS THE SOURCE OF THE OCTOBER 30, 2016 Mw 6.5 NORCIA EARTHQUAKE (CENTRAL ITALY) S. Tripaldi 1 , M. De Girolamo 2 , L. Improta 3 , G. Romano 1 , V. Sapia 3 , A. Siniscalchi 1 , I. Ventola 1 , F. Villani 3 1 Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari Aldo Moro, Bari, Italy 2 CNR - Istituto di Metodologie per l’Analisi Ambientale, Potenza, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy The Apennine seismic belt of Italy is an extensional province affected by multi-fault normal- faulting earthquakes or sequences (Chiarabba et al. , 2005). The geometrical complexity of the active normal fault systems dissecting the Apennines belt, characterized by a poly-phased tectonic evolution, may control multi-fault ruptures during a single seismic event or sequences spanning a period of days. The 2016-2017 Amatrice-Visso-Norcia (AVN) normal-faulting sequence culminated with the Mw6.5 Norcia mainshock, represents a well-documented case of in-cascade reactivation of multiple fault segments. The geometry of the normal faults ruptured during the Norcia mainshock, the role of inherited thrust faults (i.e., Mt. Sibillini thrust) and of crustal fluids on the rupture process are still debated and contrasting finite-fault source models are present in literature (Walters et al. , 2018; Scognamiglio et al., 2018; Cheloni et al. , 2019). Here we present first results from a magnetotelluric (MT) investigation performed along an approximately 20 km profile that extends from the Norcia basin to the Mt. Sibillini thrust across the Castelluccio basin. Thanks to the extreme sensitivity of the MT method to the physical and chemical rock properties and their interactions with fluids, we aim at reconstructing a reliable resistivity section across the zone of maximum co-seismic deformation of the Norcia earthquake to improve understanding of the rock properties in the crustal volume containing the slipped faults. To this aim, geophysical outcomes from a 3-D geoelectrical survey (Sapia et al. , 2021), time domain electromagnetic soundings (Villani et al. , 2019) and aftershocks locations (Improta et al. , 2019) are integrated with the MT survey. The data consists of 10 broadband MT soundings and 5 audiomagnetotelluric (AMT) soundings. The interdistance between each measurement sites varies from ~4000 to ~500 m, with a denser site spacing located in the Castelluccio basin and theMt. Vettore fault systemarea. The complex MT impedance tensor Z estimates ranges from 10 -4 s to 100 s. The dimensionality and directionality analysis of the geoelectrical structures has been investigated following the approach of the phase tensor analysis introduced by Caldwell et al. , 2005. In the longer period range, the strike direction of regional structures is almost consistent with the N25°W direction. The 2-D resistivity model, obtained using the nonlinear conjugate 2-D inversion algorithm of Rodi and Mackie (2001) must be considered preliminary. A series of tests are ongoing to assess reliability, geometries and depths of the imaged resistivity features that will be compared with the published interpretations of commercial reflection profiles (Porreca et al. , 2018; Buttinelli et al. , 2021). However, the recovered model unravels a remarkable drop of resistivity values just below the Castelluccio basin down to a depth of 6 km and delineates a low-resistivity crustal wedge bordered by the main of clusters of early aftershocks of the Mw6.0 Amatrice and Mw 6.5 Norcia mainshocks. References Buttinelli M., Petracchini L., Maesano F. E., D’Ambrogi C., Scrocca D., Marino M., Capotorti F., Bigi S., Cavinato G.P., Mariucci M.T., Montone P. and Di Bucci D.; 2021: The impact of structural complexity, fault segmentation, and reactivation on seismotectonics: Constraints from the upper crust of the 2016–2017 Central Italy seismic sequence area . Tectonophysics, 810, 228861. Caldwell T. G., Bibby H. M. and Brown, C.; 2004: The magnetotelluric phase tensor . Geophysical Journal International, 158(2), 457-469.