GNGTS 2014 - Atti del 33° Convegno Nazionale

80 GNGTS 2014 S essione 1.1 large scale NE-SW extensional regime affecting the Apennine Chain. Detailed study on the instrumental seismicity occurred since 1996 in the Sannio-Matese area (e.g. Milano et al. , 2008; Fracassi and Milano, 2014) have shown that the seismic activity close to the Matese Massif is characterized by the occurrence of few single events, generally with magnitude less than 3.0, typically 2.5, with hypocenters within the uppermost 15 km of the crust (black dots in Fig. 1). These events occur sparse along the Massif without a preferred alignment even if the general distribution of the seismicity follow the NW-SE Apennine Chain axis. To this background seismicity are superimposed low magnitude seismic sequences and swarms. Key examples are the 1997-98 (M D < 4.2) and 2001 (M D < 3.6) sequences occurred at the eastern border of the Matese Massif and to NE of Isernia, respectively. Examples of low-magnitude swarms (M D < 3.2), lasting few days and consisting of some tens of events, occurred in 1999 and 2005 (Fig. 1). ��� ������� ������� �������� ������� �� ����� ��� ���� �� ����� ����� The 1997-98 seismic sequence started on March 19, 1997 at 23:10 (UTC) with an M D = 4.1 earthquake. The sequence, marked by alternating periods of intense activity and relative quiescence, consisted of more than 4000 events with M D between 0.8 and 3.7, 25 of these had M D between 3.0 and 3.7. The epicentral distribution of the sequence and the hypocentral distribution (Fig. 1, blue circles) show that earthquakes occurred along a nearly vertical plane striking NNE-SSW and dipping towards the east, located �� ��� ��� ���� �� ��� in the ESE edge of the Matese Massif. �������� �������������� ���������� �� ������ �������� �� ������ ������� �� �� ����� Standard statististical techniques of strain analysis of faults applied to 50 focal mechanisms of events with M D > 2.5 shown that: 1) poles and strike of nodal planes evidence Fig. 1 – Top: Main structural lineaments of the Matese Massif (modified from: Neotectonic map of Italy, CNR-PFG, 1983) on which is reported the background seismicity occurred since 1996 (black circles), the 1997-98 (bleu circles) and the 2001 (green circles) seismic sequences. Red ellipse represent the area in which the 2013-2014 seismic sequence occurred. The rectangular box with the year represent the seismogenic sources of the strong historical earthquakes (DISS, 2010). Triangles identify the permanent seismic stations close to the Matese Massif. Bottom: Hypocentral distribution of the events along the SW-NE (A-A’) and NW-SE (B-B’) directions.