GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 81 a maximum striking NE-SW and a second-order maximum striking WNW-ESE; 2) rakes have two maxima with values of about –50 and –120; 3) P axes are sub-vertical to oblique and show two preferred strike: NNE-SSW and ESE-WNW; 4) T axes are sub-horizontal with a maximum strike striking NNW-SSE. These results suggest that the strain field is heterogeneous even if a clear preferred rupture direction striking NE-SW is recognized. Rakes associated to this rupture as well as the preferred strike and dip of rupture plains are consistent with a NNE- SSW striking fault dipping towards SE and with normal to oblique dextral movements (Milano et al. , 1999; Milano et al. , 2002). Between January and September 2001 the area to NE of Isernia was interested by a seismic sequence (Fig. 1, green circles). The seismic activity started with a sensible increase of the background seismicity but the magnitude of the events was comparable with the magnitude of the background seismicity of the area. The sequence consisted of about 1000 events with M D between 0.8 and 3.6 and only 5 events had M D > 3.0. Both the epicentral and hypocentral distributions of the events belonging to the sequence suggest that seismic activity took place along a near vertical plane NNE-SSW striking and dipping steeply eastward (Fig. 1, green circles) located ����� �� ��� ��� ���� �� ��� ������ ������� �������� �������������� ���������� close to the WNW edge of the Matese Massif. �������� �������������� ���������� Standard statististical techniques of strain analysis of faults applied to the nodal planes of 30 well constrained focal mechanisms of events with M D > 2.5, shown that: 1) poles and strike evidence a maximum striking NE-SW with a maximum in rakes between –50 and –100; 2) P axes are sub-vertical to obliques and strike N-S; 3) T axes are sub-horizontal with a NW-SE preferred strike. The 2001 events record ruptures along a NE-SW preferred plane dipping towards SE. The rake values are consistent with prevailing normal movements. These ruptures are consistent with a NW-SE striking extension (Milano et al. , 2005). The 2013-2014 sequence. ��� �������� ������� �� �������� ��� ���� ������ ���� ���� The sequence started on December 29, 2013 (17:08 UTC) with a M L = 4.9 earthquake and was characterized by an intense seismic activity till the first week of January, 2014. The main shock was preceded a few minutes earlier by an event with M L = 2.7 (17:03 UTC). After a period of relative quiescence characterized by very few events, a second earthquake with M L = 4.2 occurred on January 20, 2014 (7:12 UTC), and, in turn, was followed by low seismic activity till January 23. Seismicity totally ceased afterwards. SGG station, the oldest seismic station operating in the area (since 1977; OV-INGV network; Fig. 1) and about 5 km away from the epicentral area of the sequence, recorded few events since January 2013 (about 20; M L ≤ 2.0). No events were detected in the days before the main-shock. With regard to the sequence, this station recorded about 1000 events between December 29, 2013 and the end of January 2014 with magnitude greater than 0.8. More than 50% of the events occurred during the first 3 days of the sequence. The magnitude of the events of the sequence was very low. Till the first week of January, only 8 events had 3.0 < M L < 4.0 and about 100 events had 2.0 < M L < 3.0, whereas, following the M L = 4.2 earthquake of January 20, only 2 events with 3.0 < M L < 4.0 and about 15 events with 2.0 < M L <3.0 occurred. With the exception of the two main shocks, no events with 4.0 < M L < 4.9 were recorded. For location purposes, data recorded by the Italian Telemetered Seismic Network (ITSN) operated by Istituto Nazionale di Geofisica e Vulcanologia (INGV) have been used. Consisting of more than 300 stations, the present-day ITSN configuration ensures a fairly good azimuthal coverage of the Italian territory. In particular, the present-day spatial distribution of the seismic stations in the Sannio-Matese area (triangles in Fig. 1) allows also to locate low-magnitude events (M L ≤ 2). Data of the permanent OV-INGV seismic stations as well as data of two seismic stations installed in the area the days after the December 29, 2014 event (De Gori et al. , 2014) have been used. Digital waveforms of recorded data related to the sequence have been collected to perform a re-picking of the events, focusing the attention on S-phase, for the reliability of the focal depth. To compare the results of this study with the results of previous studies performed on the area, the procedure reported in Milano et al. (2008) has been adopted. Events recorded by a minimum of five stations and with at least five P- and four S-phase readings have been located