GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione A matrice 13 Mapping of surface faulting and geochemical monitoring of groundwater following the August 24, 2016 Amatrice earthquake: preliminary results and work in progress G. Binda, F. Brunamonte, M.F. Ferrario, C. Frigerio, F. Livio, R. Gambillara, A.M. Michetti, A. Pozzi, S. Terrana Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, Como, Italy Introduction. On August 24, 2016, a Mw 6.0 earthquake hit central Italy between Norcia and Amatrice, some tens of kilometers NW of L’Aquila, struck by a slightly larger earthquake in 2009 (Mw 6.3). The epicenter of the 2016 normal faulting event was located near the village of Accumoli at the border of Umbria, Marche and Latium regions. One hour later, a strong Mw 5.3 aftershock followed, with epicenter close to Norcia. This region hosts several Quaternary normal faults included in the ITHACA database, that is, the inventory of capable faults in Italy, managed by ISPRA (https://goo.gl/55FwDH ). According to preliminary seismological, geodetic and geologic data, the seismogenic structure ruptured across a relay zone between two major NNW trending normal faults: the Vettore and Laga faults. Total rupture length is ca. 25-30 km and rupture width ca. 10-12 km. The Insubria University group of Environmental Geology and Chemistry conducted analysis of coseismic environmental effects since August 25, 2016. Fieldwork is presently in progress to assess the post-seismic evolution of faulting (i.e., afterslip) and trends in spring hydrogeochemistry. Some preliminary results are described in the following, focusing on specific issues which might be relevant for seismic hazards assessment and the vulnerability of the physical environment in the Central Apennines geological and hydrogeological setting. Surface faulting: a new case study for a long-debated issue. We started surveying the day after the mainshock, and our observations were collected quite continuously for almost 15 days, and work is still in progress, including data on after-slip on mapped ground ruptures. The fieldwork is being conducted in collaboration with ISPRA, INGV, University of Leeds, Birkbeck University of London, CNR – Firenze, University of Camerino (Central Italy Earthquake WG, 2016; EMERGEO WG, 2016). Here we present some preliminary results regarding the Vettore fault (i.e., “Cordone del Vettore”). The western slope of Mt. Vettore (Fig. 1) is the morphologic expression of a NNW- trending, SW dipping major normal fault segment (total length 30 km), cutting through pelagic carbonates of the Umbria-Sabina meso-cenozoic succession with a total stratigraphic offset of ca. 1000 m. A set of ground ruptures was observed on the E slope of Mt. Vettoretto. These ruptures, showing normal displacement (slip vector ca. N240°) with minor left-lateral component, strike ca. N150°, and commonly show en-echelon right-stepping. Along the “Cordone del Vettore” we record an average strike of N134° and the slip vector is ca. N230°. The fractures generally affect colluvium and soil close to the bedrock fault plane or, less frequently, at a distance of a few meters. In the first days, the observed throw ranged from 2 to 25 cm, while heave reached max. 10 cm. These ruptures run continuously, crosscutting the road SP34, up to the end of Mt. Vettoretto slope, where the ruptures bend slightly downslope, for a total distance of at least 1.7 km. In the uphill sectors, ground ruptures were found along the fault scarp, for a length of about 2.4 km. The end-to-end length of this surface rupture zone is thus, at least, 4.1 km. Repeated field surveys have clearly pointed out a post-seismic evolution of the rupture, whose offset is continuing to increase with time. The phenomenon can be clearly observed where the fault crosses the SP34 road near Forca di Presta. The assessed primary surface faulting along the Vettore Fault is significant to earthquake geology and paleoseismology, and therefore to seismic hazard assessment, based on geological investigation. This earthquake clearly confirms that the threshold for surface faulting during extensional events in the Apennines is between Mw 5.5 and 6.0. The comparison between field mapping, InSAR remote sensing analyses, and instrumental