GNGTS 2015 - Atti del 34° Convegno Nazionale

168 GNGTS 2015 S essione 1.3 cumulative strain release occurred. This increase could, at least partially, be due to the increase in the number of seismic stations in southern Sicily, and therefore improvement in the detection capability of the Italian Seismic Network (Amato and Mele, 2008). It is worth highlighting that the seismicity shows a prevalent release characterized by isolated earthquakes rather than swarms. Most of earthquakes have magnitude ranging between 2.0 and 4.0 and located within 30 km of depth. In general, the epicentres show a widespread distribution, even if an approximately N-S oriented clustering, between Linosa and Pantelleria islands, can be observed (Fig. 3a). However, a comparison between historical and instrumental seismicity shows that the seismicity seems to move from the coast to offshore, confirming that the shocks located in Sciacca occurred offshore. Zooming in on the Pantelleria area (Fig. 3a), the epicentral map of earthquakes indicates a very low level of seismicity, particularly if compared to other tectonic and volcanic areas of eastern Sicily (i.e. Mt. Etna). Moreover, the distribution of epicentres is scattered widely around the island, with few foci close to or on the island itself. This pattern concurs with the results of the analysis performed on the continuous seismic data recorded by PTMD station. We have scrutinized the seismograms of this station for the period 2010-2014, in order to highlight signals attributable to local earthquakes. Only six seismic signals of this kind have been recognized. Since these signals were recorded by only one station, it was not possible to obtain any analytical location. Discussion and conclusion. Only few detailed studies deal with seismicity and historical eruptions of the Sicily Channel. For this reason, the aim of this study is to highlight the main features of the instrumental and historical seismicity of the Sicily Channel and provide an overview of the historical eruptions affecting this geodynamic sector. South-western Sicily is characterised by seismogenic zones having different seismotectonic behaviour. Onshore, the strongest earthquakes are located in the Belice Valley, where six events with magnitudes ranging from 5.2 to 6.1 occurred in 1968. This intense seismic sequence seemed almost unexpected from a historical seismicity viewpoint, since no other strong earthquakes are reported in the catalogue for the previous period (Rovida et al. , 2011). Most of the historical earthquakes seem to be located in the southern coast of Sicily. The seismicity is characterised by low magnitude seismic swarms affecting mostly the area of Sciacca. Earthquake magnitudes did not exceed 5.1, and they were probably located in the near offshore. The swarm-like features and vague references to submarine degassing phenomena suggest relationships with the volcanic activity in the Sicily Channel, even if there is evidence only for the 1831 and 1891 earthquakes and probably for the 1816-17 event. Moreover, the historical information, which was gathered along the coasts and islands of the Sicily Channel, may be biased if the actual seismogenic sources are offshore. The instrumental seismicity recorded by the INGV seismic network during the period 1983-2014 in the Sicily Channel and during the experiment carried out in the Pantelleria island show that the earthquakes are shallow, few and isolated events rather than swarms. They are characterised by low magnitudes (M~4.0) and are more densely located along the “separation zone” trending N-S, located between the troughs of Pantelleria, Linosa and Malta. These results are in good agreement with those obtained by Calò and Parisi (2014) which performed a relocation of the earthquakes. A comparison of data from the permanent network with the mobile one allows stating that the Pantelleria island and the Sicily Channel are characterized by a low rate of seismicity linked to tectonic release and that the current seismicity is not due to volcanic processes. Historical data indicate that seismic swarms and increasing fumarolic temperature preceded the eruptions, both in Pantelleria and in the Adventure Bank. We cannot rule out a priori the existence at Pantelleria of seismic signals related to hydrothermal activity. We believe that the installation of a local permanent seismic network