The Amatrice 2016 seismic sequence: a preliminary look at the mainshock

Abstract

Three damaging earthquakes occurred in Central Italy between August and October 2016 leaving almost 30,000 homeless. The first event is a Mw 6.0 occurred on August 24th at 01:36 UTC close to Accumoli village; two months later, a Mw 5.9 on October 26th at 19:18 UTC happened 3 km West of Visso and finally a Mw 6.5 on October 30th at 06:40 UTC, 6 km North of Norcia, which is the largest earthquake recorded in Italy since the Mw 6.9 1980 Irpinia event. This paper focuses on the seismicity distribution observed from the beginning of the sequence until the 15th of September 2016, almost six weeks before the occurrence of the largest event. We relocated the aftershocks of the Mw 6.0 Amatrice 2016 main event by inverting, with a non-linear probabilistic location method, P-and S-arrival time readings produced and released in near real-time by the analyst seismologists of INGV on 24H duty in the seismic monitoring room. Earthquake distribution shows the activation of a normal fault system with a main SW-dipping fault extending from Amatrice to NW of Accumoli village for a total length of 40 km. Toward north, in the hanging-wall volume of the main fault, the structure becomes more complex activating an antithetic fault below the Norcia basin. It is worth nothing that below 8-9 km of depth, the whole fault system has an almost continuous sub-horizontal layer interested by an intense seismic activity, about 2 km thick.