GNGTS 2016 - Atti del 35° Convegno Nazionale

150 GNGTS 2016 S essione 1.1 The epicenters of the sequence formed a ~10 km long NNW-SSE trending alignment and the hypocenters are confined in the 10-20 km depth range. In spite of this high seismicity, also testified by moderate energy events (4<M L <5) occurred in the last century (Rovida et al. , 2016), the knowledge of the location, geometry and kinematics of the active fault segments in the Sannio-Matese area is scarce with respect to other seismogenic areas of Italy. The study of the 2016 low magnitude seismic activity may offers the opportunity to deepen the knowledge on the seismotectonic and to add new information to improve the valuation of the seismic hazard of the area. Inspecting the seismic activity before the January 16, 2016 (18:55 UTC) event, we note that the sequence was not triggered by the mainshock, as this was preceded by an increment of the seismicity started about a week before. This increase in seismic activity was particularly significant in the 36 hours leading up to the main event. In the whole, about 40 events, with magnitude 1.5<M L <2.8, have occurred since 10 January, more than 60% of which in the 36 hours before the mainshock. After this and until the end of January, more than 150 events, with magnitude 1.2<M L <2.8, have been recorded. Relatively to this time-period, only 5 events, included the mainshock, have M L >3.0. After the end of January, the seismic activity was characterized by few low magnitude single events. A new swarm-type earthquake activity, constituted by about 55 events with M L <2.1, occurred in the time period 3-12 April. In order to obtain reliable hypocenters location and fault plane solutions, the waveforms of the events belonging to this sequence have been collected to perform a re-picking of the P- and S- phases. The epicentral distribution of about 200 best located events occurred in January Fig. 1 – Map showing the main structural lineaments of the Sannio-Matese area (redraw after CNR-PFG, 1983) on which is reported the epicentral distribution of the seismicity that occurred in the last 20 years (data from Milano, 2014 and references therein) and that occurred to the January 2016. The rectangles with the years represent the main seismogenic sources of the strong earthquakes that struck the area (DISS Working Group, 2015). The epicenters of the earthquakes with 4<M<5 that occurred in the last century are also reported (blue circles with a date); the black circles represent single events. The focal mechanism is related to the January 16, 2016 event.