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trending low-dip minimum principal stress tensor (CSI 1981–2002 by Castello et al., 2006; Bollettino
(Patanè et al., 2004; Neri et al., 2005a). Sismico, 2003–2006), although detailed papers on the
In order to define the seismogenic deformation field regional instrumental seismicity were also considered
within the study area at a regional scale, the available (Frepoli and Amato, 2000; Obrizzo et al., 2001; Patanè
focal mechanisms for the events with moment magni- and Privitera, 2001; Scarfì et al., 2003; Patanè et al.,
tude ≥ 4.0 occurred since 1968 above the SBT 0-to- 2004; Pondrelli et al., 2004; Vannucci and Gasperini,
30 km depth-contour lines were considered and 2004; Monaco et al., 2005; Musumeci et al., 2005; Neri
projected in the map in Fig. 6 (Anderson and Jackson, et al., 2005a; Neri et al., 2005b; Scarfi et al., 2005; Sgroi
1987; Castellano et al., 1997; Frepoli and Amato, 2000; et al., 2006). The historical data were mainly extracted
Pondrelli et al., 2004; Neri et al., 2005a). Different focal from the CPTI04 essentially macroseismic catalogue,
solutions have been proposed in the literature for the which contains information about Italian earthquakes
Belice 1968 seismic sequence including almost pure from 217 B.C to 2002 (Working Group CPTI, 2004), as
compressional mechanisms on a E–W striking plane well as from papers that reviewed Sicilian historical
(Anderson and Jackson, 1987) or right lateral transpres- earthquakes (Rigano et al., 1999; Azzaro and Barbano,
sion on a NNW–SSE striking plane (McKenzie, 1972; 2000; Azzaro et al., 2000a; Azzaro et al., 2000b). Some
Gasparini et al., 1985). In both cases, the average P-axis of the historical events (those starred in Table 1) were
is low-dip and nearly N–S trending. An almost pure analyzed ex novo starting from the historical information
reverse mechanism with a nearly N–S P-axis is also and arriving at a re-evaluation of the felt intensities and
shown by the Mazara 1981 earthquake with M w 4.9 to a consequent re-definition of the macroseismic field.
(Pondrelli et al., 2004) located about 40 km eastward The time interval of completeness for magnitude classes
from the Belice seismic area. The other mechanisms, of the earthquakes in the dataset was computed applying
with the only exclusion of the Maletto 1987 event, are the method found in Mulargia's et al. (1987) and it was
characterized by strike–slip and reverse-oblique kine- verified that the here compiled list may be considered
matics compatible with low-dip NNW–SSE to NNE– complete since 1600±200 for events with M aw ≥5.5,
SSW trending P-axes (Fig. 6 and references therein). since 1680±100 for 5.0≤M aw b5.5 and since 1820±60
This deformation pattern fits rather well that from the for 4.5≤M aw b5.0. These completeness intervals are
borehole breakouts (Montone et al., 2004) located in the similar to the ones used to assess seismic hazard by the
Gela–Catania area above the 0-to-10 km depth-contour Working Group MPS (2004).
lines of the SBT, which show an average WNW–ESE The likely depth range of the historical earthquakes
trending minimum horizontal shear stress (Fig. 6). within the studied area has been expressed by quali-
tative classes, inferred by considering the maximum
4.2. Merged instrumental and historical data distance between the epicentre and the lowermost
intensities data points and the epicentral intensity of the
In order to take into consideration all the moderate- associated macroseismic field, which are reported in
to-large earthquakes that might be related to seismo- the DBMI04 (Stucchi et al., 2007), in the Catalogue
genic shearing of the SBT, we compiled a merged list of Strong Italian Earthquakes (Boschi et al., 2000), in
(Table 1)ofthe majorhistorical(217B.C.to1980) the Macroseismic Catalogue of Mt. Etna Earthquakes
and instrumental (1981 to 2006) earthquakes located (Azzaro et al., 2000a), in the aforementioned papers
within the surface area above the SBT depth-contour and in the reviewed events. Obviously, in dependence
lines from 0 to 30 km. Because of our intention to of the characteristic of the macroseismic field (i.e.
describe the regional scale features of the seismogenic number and spatial distribution of intensity points)
process, we have only considered the events with some depth estimations are more reliable than others;
moment magnitude ≥ 4.5. In the case of the historical therefore, we schematically identified only three groups
events, being the moment magnitude value M w based of depth ranges: 1) — a shallow, upper crust, source
on the instrumental record generally not available, the group, labelled as CS in Table 1 which stands for
empirical moment magnitude M aw , derived from the “compressional shallow”;2) — a deep, mid-to-lower
macroseismic field by the Working Group MPS (2004) crust, source group, labelled CD, which stands for
and given in the CPTI04 parametric catalogue of “compressional deep”;3) — another shallow source
Italian earthquakes (Working Group CPTI, 2004), was group consisting of the Etna volcanotectonic events,
used. labelled S-Et, which stands for “shallow Etna events”.
The instrumental data were primarily taken from the The latter is characterized by swarm sequences of mod-
aforementioned seismic catalogues available on-line erate magnitude concentrated within the sedimentary