GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 64 Figure 2 shows geophysical prospecting carried out across the Piedicolle FAC at the paleo- seismological trench site studied by Michetti et al . (1995). This provided useful information also for calibrating the different geophysical survey methodologies. We acquired ERT with dipole- dipole and Wenner-Schlumberger arrays, positioning 64 electrodes with 1 m spacing. The profile (Fig 2a), N-S trending and 64 m long, reaches a depth of investigation of about 8 m for the di- pole - dipole and 15 m for the Wenner - Schlumberger. The 2D section shows an abrupt electrical variation (red arrow in Fig. 2a) from a higher resistivity layer (Res ≥ 400 Ω m; Cretaceous marly limestone, Scaglia Fm.) in the N portion to a lower resistivity layer (Res ≤ 20 Ω m; lower colluvial unit, early Holocene) further S. Above the latter, there is a slightly higher resistivity layer (Res approx. 80 Ω m; upper colluvial unit, late Holocene) with a thickness varying from about 1-2 m., which increases southward. At depth, the profile allows to detect an area with greater resistivity (Res ≥ 250 Ω m), which develops below the body with lower resistivity; this allows to constrain its thickness (constant and equal to about 2 m). These bodies with higher and lower resistivity are interrupted towards the N, along the profile, with an abrupt transition of resistivity characterized by a clear sub-vertical geometry. At the same site, 4 parallel radar profiles were acquired. In all profiles, sudden changes of response in the electromagnetic signal are visible (CR in Fig. 2b). These changes have very steep limits which could be compatible with a fault scenario. Furthermore, those located further N in the investigated area are aligned and consistent with the result obtained from the ERT data (red line and symbol “F”). Also in the S part there is an alignment of contacts between means with dif- ferent electromagnetic characteristics which could be compatible with the (brutal) termination of the conductive zone identified in the dipole-dipole geoelectric prospecting. Figure 3 shows the trench 2 at the Apoleggia site. Preliminary interpretation of the trench exposure suggests that the Apoleggia fault ruptured at surface in recent times (Holocene?), with at least a couple of coseismic displacement events. Discussion and comparison with related investigations in the Rieti Basin The ongoing research project at Rivodutri and Cantalice allows to map at 1:5000 scale the geometry of the Quaternary fault segments bounding the Rieti Basin to the N (Rivodutri area) and to E (Cantalice area). Preliminary results allow to confirm the capability of the Apoleggia and Piedicolle segments. The recent reappraisal of the 1948-1949 Rivodutri earthquake (Io = VIII MCS; Mw = 5.3; no record of significant environmental effects; Bernardini et al ., 2013) provides a calibration for the minimum magnitude associated to earthquake surface faulting along these segments. Data interpretation is in progress for the Cantalice area. AMS dating results will allow to chronologically constrain the last reactivation of the San Liberato Fault and to verify or falsify the capability of this segment. A similar picture is described by Comerci et al . (this volume) for the Rieti – Cittaducale segment along the S border of the Rieti Basin. References Archer C., Noble P., Rosen M., Sagnotti L., Florindo F., Mensing S., Piovesan G., and Michetti A.M., 2019, Lakes as paleoseismic records in a seismically-active, low-relief area (Rieti Basin, central Italy) , Quater- nary Science Reviews, 211, 186-207. Bernardini, F. M. A., Castelli, V., Camassi, R., Caracciolo, C. H., Ercolani, E. (2013). A “forgotten” earthquake rediscovered: The 1948-1949 Monti Reatini (Central Apennines) seismic sequence . BGTA, 54, 3, 229-244. Commissione tecnica per la microzonazione sismica (2015). Linee guida per la gestione del territorio in aree interessate da Faglie Attive e Capaci (FAC), versione 1.0 , Conferenza delle Regioni e delle Province Autonome – Dipartimento della Protezione Civile, Roma, 55 p.