GNGTS 2015 - Atti del 34° Convegno Nazionale

166 GNGTS 2015 S essione 1.3 in the Sicily Channel, as for example in 1831 when the Ferdinandea Island was formed in the sea between Pantelleria and Sciacca (Gemmellaro, 1831; Mercalli, 1883). Evidence of more ancient earthquakes, which damaged the town of Selinunte, located on the coast about 25 km west of Sciacca, are provided by archaeological investigations, which suggest the occurrence of two shocks dated around 400-200 B.C. and 400-1200 A.D. (Guidoboni et al. , 2002; Bottari et al. , 2009), but the seismogenic sources are unknown. The knowledge of the seismicity of Pantelleria, and also in nearby areas of the Sicily Channel, e.g. the Maltese Islands (Galea, 2007), has suffered both from poor accuracy of earthquake locations due to inadequate network coverage (especially before the 1980s) and the difficulty in detecting low magnitude shocks. Instrumental data recorded in the past are insufficient to provide a realistic framework of the seismicity of Pantelleria area and Sicily Channel. In these areas, the set-up of permanent stations dates back to 1980s, when a 1-component short period (1s) sensor was installed at Pantelleria. In August 2010, this analog station was replaced by a digital one (PTMD), equipped with a three-component broadband seismometer, installed in the central part of the island and managed by INGV-OE. However, analytical location of low magnitude earthquakes, and/or typical low energy seismic signals often recorded at active volcanic areas, has been overlooked to date. This is mainly because of the lack of a permanent local seismic network at Pantelleria. According to literature data, seismic activity in the Sicily Channel is characterized by shallow earthquakes (typically less than 25 km), and with magnitudes generally below 5.0 (typically between 2.0 and 4.0). According to Agius and Galea (2011) most of seismicity, especially south of the Maltese islands, is either unreported or badly constrained. These authors refer to many earthquakes detected only by a digital broadband station running since 1995 on Malta Island, belonging to the MedNet network. Besides, few earthquake fault plane solutions are available in order to characterize kinematic features of the Sicily Channel (e.g. Chiarabba et al. , 2005; Pondrelli et al. , 2002). The experiment with a mobile seismic network. Between 2006 and 2007, a seismic experiment was carried out at Pantelleria, using five digital seismic stations equipped with 3-component broadband (20 s) sensors, belonging to the mobile network of INGV-OE. Unfortunately, the day after the installation the station PAN5 (Fig. 1) broke down and no substitution was available. The remaining four stations mainly monitored the central-southern sector of the island. Overall, the array recorded data from June 28, 2006 to February 23, 2007. The mobile network recorded various types of seismic signals (i.e. teleseismic events, regional earthquakes, shocks in the Sicily Channel, and local events). Most of earthquakes were teleseismic, whereas the shocks that can be referred (see - http://iside.rm.ingv.it) to the Sicily Channel seismicity amount to sixteen. During the network operating period, only one earthquake (30/12/2006 – 00:02 UTC), with characteristics suggesting a source located very close to or on the island of Pantelleria, was detected. We could not perform any analytical location of this event, since it was recorded by PAN2 station alone, the other ones being out of order due to power supply failure. The analysis of the frequency content of the three components of the PAN2 records (Fig. 2) show a typical waveform of a low magnitude, local shallow volcano- tectonic earthquake. To obtain some constraints on the epicentral area of the shock, we performed a single-station location, using a ray tracing method. Assuming a Vp equal to 5.2 km/s proposed by Chiarabba and Frepoli (1997), we located the earthquake at a distance of about 4 km offshore from the southwestern coast of Pantelleria. Moreover, the short S-P delay time (about 0.5 s) suggests a focal depth of few kilometres (< 5 km), We evaluated the magnitude Ml of the shock to be no greater than 1.0. Seismic data analysis. To acquire information on the seismic characteristics of the Sicily Channel, and the island of Pantelleria in particular, we carried out a study of the instrumental seismicity by using earthquake catalogues (Castello et al. , 2006), reports, and instrumental