GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 83 The hypocentral distribution shows that focii are confined between 10 and 20 km depth (red circles in Fig. 2, bottom). Two clusters of events can be observed. The deeper one is at around 16-19 km, and includes the hypocentral depth for the two main-shocks (about 18 km). A well-defined shallower concentration of events is about 14-15 km depth and shows a sub-horizontal trend. Very few events occurred between 5-10 km depth and no events are located above 5 km and below 20 km in depth. The hypocentral distribution in a cross-section orthogonal to the main regional structures outlines a steep (about 75°-80°) southwest-dipping alignment between 10-20 km (Fig. 2 bottom). The computed fault-plane solutions show normal dip-slip mechanisms (Fig. 2 top, right panel). The focal mechanism of the first main-shock has about NW-SE striking planes (n. 1 in Fig. 2 top, right panel), as well as those related to events with M L > 2.8 (n. 2, 3, and 5 to 8). Instead, solutions n. 4 and 9 show about N-S striking planes. Fault-plane solutions relative to the second main-shock and those related to two events who followed it, have almost the same NW-SE striking planes like the previous (n. 10 to 12 in Fig. 2 top, right panel). The nodal planes of almost all computed fault-plane solutions agree with the sequence alignment and the P- and T-axes position of each focal mechanism are consistent with the large-scale extensional regime that characterize the Apennine Chain. Discussion and concluding remarks. The sequences occurred in 2013-14 and 1997-98, although the epicentral areas are very close, shown considerable differences between them. The main differences concern: their temporal duration, very few days the first and over a year the second, the number of event, about 1000 vs more than 4000, the epicentral distribution, about NW-SE vs NNE-SSW, and the depth of the events. With respect to the last, the seismicity of the 2013-14 sequence does not propagate towards the surface being confined between 10 and 20 km in depth (red circles in Fig. 2, bottom). On the contrary, the seismic events of the 1997-98 sequence propagate towards the surface starting from the depth of about 15 km (blue circles in Fig. 2, bottom). This depth correspond to the depth of the M D = 4.1 earthquake that triggered the sequence. ��� ����������� ������������ �� ��� ������ ��������� �� ��� ������� �������� The hypocentral distribution ��� ������� �������� of the events belonging to the 1997-98 sequence shows that seismic activity ���� ����� ����� � ����� ������� �������� ��� ������� ������� took place along a plane NNE-SSW striking and dipping steeply eastward, ������� ��� ����������� ������������ �� ��� ������ ��������� �� ��� ������� �������� ������� ��� �� ��� ������� �������� whereas the hypocentral ������������ �� ��� ������ ��������� distribution of the events belonging to the 2013-14 sequence suggest that seismic activity took place along a near vertical plane NW-SE striking and gently dipping south-westward. Further, strain analysis of faults applied to the focal mechanisms of the 1997-98 sequence is consistent with a local NNW-SSE extension, as also reveled for the 2001 sequence, whereas the focal mechanism of the investigated sequence agree with the NE- SW large-scale extensional regime. The spatial distribution of the seismicity since 1996, particularly, the epicentral areas of the 1997-98, between the seismogenic sources of the 1805 and 1688 events, and the 2001, at the NW tip of the 1805 source, sequences suggest that, before to the beginning of the 2013-14 sequence, the seismic release in the Matese Massif was concentrated in its WNW and ESE edges. Tomographic image of the Matese area and surrounding (Chiarabba and Amato, 1997; Bisio et al. , 2004) evidence the presence of positive velocity anomalies all NW-elongated and the observation of high Vp regions at depths below 6 km beneath the Matese Massif. In the tomographic images proposed by Bisio et al. (2004), the most interesting feature is the high heterogeneity observed between 6 and 15 km depth. In particular, at about 9-10 km depth, where the Apulian platform is expected, the large NW-SE striking high velocity anomaly is found in the central part of the model. This structure extends parallel to the chain beneath the Matese Massif and is composed by two local highs that, laterally, �� �� ���� ����� ��� �� �� go up till about 6-7 km in depth. ����� ����� ���� �� These local high Vp regions are located close to the 1688 and 1805 seismogenic sources, and close to the 1688 seismogenic source, and are characterized by scarce seismicity, it being concentrated at their borders as showed by instrumental seismicity occurred in the last about 20 years. As evidenced by Bisio et al. (2004), the high Vp regions can be interpreted as high strength materials able to store large stress, representing the main asperities. The presence of high Vp bodies at 6 to 9 km depth beneath the Matese Massif was interpreted by Chiarabba