GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.1 149 GEOMETRY AND STRUCTURE OF A FAULT-BOUNDED EXTENSIONAL BASIN BY INTEGRATING GEOPHYSICAL SURVEYS AND SEISMIC ANISOTROPY ACROSS THE 30 OCTOBER 2016 MW 6.5 EARTHQUAKE FAULT (CENTRAL ITALY): THE PIAN GRANDE DI CASTELLUCCIO BASIN F. Villani, V. Sapia, P. Baccheschi, R. Civico, G. Di Giulio, M. Vassallo, M. Marchetti, D. Pantosti Istituto Nazionale di Geofisica e Vulcanologia, Italy Introduction. In the past twenty years, the central Apennines (Italy) were hit by three important seismic sequences affecting a region >100 km-long parallel to the chain axis. Those earthquakes caused heavy damage, economic losses and >600 deaths (Colfiorito sequence, 1997, M w 6.0; L’Aquila sequence, 2009, M w 6.1; Amatrice-Visso-Norcia sequence, 2016, M w 6.1, 5.9 and 6.5). The mainshocks of those sequences were generated by the in-cascade activation of segmented normal fault-systems bounding Quaternary extensional basins (Chiaraluce et al. , 2017). The Pian Grande di Castelluccio (PGC) basin is the main Quaternary depocenter of the Mt. Vettore - Mt. Bove normal fault-system (VBFS), responsible for the 30 October 2016 M w 6.5 Norcia earthquake (Fig. 1). The widespread coseismic surface faulting following this earthquake (Civico et al. , 2018; Villani et al. , 2018a,b) affected also part of this basin, thus highlighting the existence of active fault splays capable of rupturing the surface and linked to the seismogenic fault at depth. Unveiling the subsurface structure of the PGC basin is crucial to infer the long-term behavior of the VBFS. We performed a geophysical transect in the PGC b asin across the coseismic ruptures, by integrating electrical resistivity tomography (ERT), time-domain electromagnetic soundings (TDEM), and horizontal-to-vertical spectral ratios of ambient vibrations (HVSR). We also analyzed shear-wave splitting of S-phases (fast direction φ and delay time δt) from local earthquakes recorded during our surveys to better constrain the fracture field and the properties of the inferred fault zones. Fig. 1 - Geological map of the Pian Grande di Castelluccio (geology simplified after Pierantoni et al. , 2013) with the location of the geophysical surveys and main faults.