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162 G. Lavecchia et al. / Tectonophysics 445 (2007) 145–167

the central portion compared to steeper lateral portions, the only exclusion of the Hyblean, Etna and Peloritani
which is also well evident from the shape of the SBT areas. However, in these works a possible regional-scale
depth-contour lines (Fig. 6). Moreover, the uncertainties seismogenic source had not been identified and its
of instrumental data, which may arise in the earthquake association with historical and instrumental earthquakes
location and detection in this sub-province due to the not had not been discussed.
uniform configuration of the seismic networks, have to be In this paper, based on a recognition of the long-term
considered. history of deformation, the kinematics and the likely
deep-crust deformation style of the Sicilian fold-and-
6. Discussion and conclusions thrust belt system, we point out a likely configuration
and shape at depth of the Sicilian Basal Thrust, the SBT
Mainland and central-southern Sicily has been struck the outermost, and still active, of a system of inward-
by various earthquakes in the past millennium, but dipping thrusts active since Miocene times. We analyse
available data, also those regarding strong earthquakes, the historical and instrumental earthquakes which occur
are few and often not well constrained. The long-term in the area above the SBT 0-to-30 km depth-contour
history of deformation is very complicated and the lines and we identify just in the SBT a likely source for
present geological state of strain highly controversial, as the seismicity in mainland and central-southern Sicily.
demonstrated by a wide number of contrasting papers. We base our seismogenic hypothesis on a few simple
In such a situation, different possible seismotectonic points: 1) the fold-and-thrust system at the hangingwall
models can be build up and have, as a matter of fact, of the SBT (kinematic unit 1 in Fig.4 and crustal slice 1
been proposed in the literature. For a long time, the in Fig. 5) may be considered active as testified by the
hypothesis of foreland differential flexural retreat presence, although sporadic, of field evidence of com-
associated to regional-scale N–S strike slip faulting pressional deformations in Quaternary times; 2) the
had been accepted and considered responsible for major SBT, as reconstructed in this paper, has a thick-skinned
Sicilian earthquakes, such as the Belice 1968 event geometry and dips northward connecting the Gela–
(Gasparini et al., 1985; Meletti et al., 2000). This point Catania front with the base of the crust beneath northern
of view has strongly conditioned the seismotectonic Sicily (Fig. 5); 3) N–S seismogenic compression
interpretation of Sicily. In fact, in the zonations of the characterises both the western and eastern sides of
entire Italian territory elaborated in the frame of mainland Sicily, e.g. the Mazara–Belice area and the
government projects for seismic hazard assessment deep Etna area (Figs. 6 and 8); 4) the hypocentral
purposes, central Sicily appears as a large aseismic distribution of the deep Etna seismicity identifies a
domain, with the only exclusion of a narrow N–S strip north-dipping seismogenic volume which well fits with
across the Belice area (Scandone and Stucchi, 2000; the independently reconstructed section-view geometry
Meletti et al., 2000; Working Group MPS, 2004). A of the SBT shown here (Fig. 8); 5) the inferred depth
seismotectonic approach similar to that proposed in this distribution of the major historical and instrumental
paper had been followed by Cristofolini et al. (1985). events occurred above the SBT also suggest a northward
These authors were the first to interpret the present deepening of the compressional sources; 6) the attitude
seismicity of Sicily in the frame of a progressive long and location of the seismogenic sources reconstructed in
history of deformation characterized by the outward the literature for the Belice 1968 (M aw 6.1), Catania
migration with time of the thrust belt-foredeep system 1818 (M aw 6.0) and Mineo 1624 (M aw 5.6) earthquakes
synchronous to the co-axial extension in the rear and to on the basis of the analysis of the intensity data points
give a 3D deformation field insight to the definition of distribution (Monaco et al., 1996; Azzaro and Barbano,
the seismotectonic domains of Sicily. More recently, a 2000; Barbano and Rigano, 2001) are very compatible
NW–SE to N–S seismogenic compressional state of with the geometry of the underlying portion of the SBT.
stress across western and eastern Sicily, also inclusive of All things considered, we have drawn the boundary
the deep Etna area, has been identified by Neri et al. of a compressional seismotectonic province, which ref-
(2005a) from the detailed analysis of fault plane lects the depth geometry of the underlying seismogenic
solutions. Still, Jenny et al. (2006), by integrating SBT, from the surface to a depth of 25 km. We have also
constraints on tectonic style and deformation rates from suggested to schematically subdivide the province into a
geodetic and geologic data with the traditional con- Shallow and a Deep sub-province, which represent the
straints from seismic catalogues, identified a very large surface projection of the upper crust and of the mid-to-
unique domain undergoing N–S compression across the deep crust segment of the SBT, respectively. The
entire Sicilian region and its northern offshore area, with identification of an individual regional scale thrust