GNGTS 2016 - Atti del 35° Convegno Nazionale

18 GNGTS 2016 S essione A matrice very close to the bedrock fault plane, but sometimes at a distance of several meters. In the first days, the observed offsets were from 2 to 25 cm, while heave reached 10 cm. These ruptures were followed northward from the road SP34 to the end of the Mt. Vettoretto western slope, where the ruptures were bending downslope to a ca. WNWdirection (see Fig. 1). This portion of the rupture is almost continuous for about 1.7 km. Southward from SP34, NNW-SSE trending discontinuous ruptures were followed for about 0.8 km. The latter are quite well aligned with the major rupture, but generally not related to a bedrock fault plane and sometimes associated to evident gravity-driven slope movements. Along the Mt. Vettoretto fault, repeated field surveys have clearly pointed out a post- seismic evolution of the slip, whose vertical separation is continuing to increase with time. The phenomenon is particularly evident and monitored where the fractures have affected the road SP34. Here, on August 24, i.e., a few hours after the earthquake, it was observed the widening of a pre-existing asphalt crack with about no vertical displacement and a very thin new oblique crack. On August 31, a downwarping to the west was observed together with the widening of the oblique crack (4 to 5 cm downthrow) and the appearing of new cracks in the ground on both sides of the road. One of the issues debated by the scientific community is whether the “Cordone del Vettore” and Mt. Vettoretto ruptures are directly linked or not to the seismogenic source(s). Based on the observed phenomena and the literature data, we put forward here some considerations about the origin of the surface ruptures, discussing separately the two cases. According to the currently available seismological, geodetic and geologic data, the seismogenic structure of the Mw 6 (01:36 UTC) mainshock ruptured across a relay zone between two major NNW trending normal faults: the Vettore and Laga faults (total rupture length was 20-21 km and rupture width ca. 9 km - Gruppo di Lavoro INGV sul terremoto di Amatrice, 2016). A second mainshock occurred at 02:33 UTC, with Mw 5.3. (Fig. 2 left) The Mt. Vettore rupture is likely connected to the first main shock (Fig. 2 left), i.e., the only one with a magnitude that can be reasonably associated to the observed surface faulting. The rupture of the second shock, despite much closer to the Vettore Fault, and very likely occurred on it, had a magnitude (5.3) and a depth (8 km) hardly able to reach the surface. Fig. 1 – Left: Oblique view of the Castelluccio Basin bounded, to the east, by the Mt. Vettore fault escarpment. Capable faults (after ITHACA database - https://goo.gl/55FwDH ) are shown in red, with the surface fracturing along the “Cordone del Vettore and Mt. Vettoretto faults overprinted in yellow. Right (a): detail of the ground ruptures pattern observed along the “Cordone del Vettore” and Mt. Vettoretto faults.