GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 65 interpreted as able to stop the propagation of coseismic ruptures and referred to as geometric and structural barriers (Aki, 1979). On the other hand, both theory and field observation show that the same geometric and structural barriers can act as points of rupture nucleation, also producing multisegment earthquakes, probably because they are zones of stress concentration (e.g. Aki, 1979; Boatwright and Cocco, 1996; Perrin et al. , 2016 with references). This dual behavior of the barrier, however, is not yet fully understood. The great quantity of geologic, geodetic and seismological data that came from the 2016 Central Italy seismic sequence provides a unique opportunity to better understand the relation between earthquakes, fault segmentation and structural barriers.We compare the slip distribution of the three main events obtained by the finite-extent fault inversion of the strong-motion data with the pattern of coseismic ruptures observed in the field and geological long-term data. In general, our results seem to confirm the dual role exerted by barriers that act not only as obstacle to the rupture propagation, but can also concentrate stress and localize the rupture causing twin earthquakes. Geological and seismotectonic framework. The threemain shocks of the sequence occurred in the axial zone of the Central–Northern Apennines affected by (N)NW–(S)SE active normal fault systems. Here the structural architecture is dominated by a curve-shaped fold-and-thrust system, delimited to the east by the Miocene-Pliocene Sibillini Mts thrust (MST). South of Mt. Vettore, the MST is a crustal-scale regional thrust ramp oblique to the normal fault systems interpreted as a pre-existing structural barriers that segmented active and seismogenic normal faults at depth, controlling their growth, development and associated seismicity (e.g., Pizzi and Galadini, 2009; Di Domenica et al. , 2012). We distinguish the Mt. Vettore–Mt. Bove fault system (VBF) from the south-easternmost segment, i.e., the Cordone del Vettore fault, which is characterized by a very clear and continuous fault scarp (CVF). Within the VBF we further distinguish the northernmost Cupi segment (CF), activated during the October 26th event, based on coseismic ruptures mapped in the field, from the central–southern sector (Mt. Bove segment: BF). In the MST footwall, the Laga Mts fault system (LF) extends from Accumoli to Campotosto with a NNE–SSW trend and with lengths comparable to the Mt. Vettore–Mt. Bove system. Data analysis. A finite fault inversion, using the LinSlipInv method (Gallovič et al. , 2015; https://github.com/fgallovic/LinSlipInv ), has been performed for the three largest events of the 2016 Central Italy seismic sequence, utilizing the strong-motion data recorded by the Rete Accelerometrica Nazionale (RAN) and the Rete Sismometrica Nazionale (RSN), the former operated by the Italian Department of Civil Protection and the latter from the Engineering Strong-Motion database by the Istituto Nazionale di Geofisica e Vulcanologia (Luzi et al. , 2016a, 2016b). The obtained slip models are superimposed on the aftershocks distribution (from Chiaraluce et al. , 2017) and the main tectonic features of the area along a cross-section parallel to the VBF-CVF-LF strike, in order to explain the relationship between the pre-existing geological structures and the rupture processes of the three events (Fig. 1; Pizzi et al. , in press). Although this analysis suffers from limitations and assumptions, the obtained results seem to be sufficiently robust as they have been obtained comparing independent data and methods. Slip inversion of the Amatrice Mw 6.0 event shows that almost the whole slip occurred in the footwall block of the MST with a maximum value of 40-50 cm at a depth of 1 to 5 km (with respect to sea level) and a rupture length of 25-30 km (Fig. 1a). The northward propagation of the slip along the CVF was (almost completely) halted at the high-angle deeper part of the MST (Figs. 1a and b), which hence acted as a mechanical structural barrier, although the very shallow low-angle part of the MST was clearly displaced by the CVF. These geometric relationships are strongly supported by the surface rupture pattern following the August 24th earthquake, which showed the reactivation of the CVF, through the MST trace, for a surface length of about 5-6 km, starting ca. 2 km north of the Arquata del Tronto village up to the northernmost sector of the Mt. Vettore, with a maximum throw of ca. 20 cm (Fig. 1a).