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G. Lavecchia et al. / Tectonophysics 445 (2007) 145–167 159

source identified in the literature is controversial. seismicity, which occurs close and along the thrust plane
Azzaro and Barbano (2000) attributed it to activity of within two distinct seismogenic layers, located within
the northernmost segment of the NNW–SSE striking the upper crust (b 10 km) and the upper part of the lower
Malta Escarpment normal fault, but Barbano and crust (15–25 km) (Pace et al., 2006). Then, also in
Rigano (2001), successively, by applying the Gasperini analogy with the Marche–Adriatic zoning, we identify
et al. (1999) inversion procedure to the intensity data above the SBT a compressional seismotectonic prov-
points, calculated a SW–NE striking source. The ince, meaning with this term a large area active in
geometry and position of this source well fit the Quaternary times consistent from the structural, kine-
configuration of the SBT depth-contour lines recon- matic, geophysical and seismological point of view
structed in this paper beneath the Etna volcanic area. which corresponds to a homogeneous regional-scale
Therefore, we suggest associating this event to thrust seismogenic source. The definition of such a large
shearing along the SBT middle crust segment; subse- province allows us to give an uniform seismogenic
quently, within our merged dataset (Table 1), it has been interpretation to the areas above the SBT, which were
categorized as a deep compressional event. struck by earthquakes during historical times (light and
The 1624 Mineo earthquake (M aw 5.6) is a relatively dark grey areas above the SBT depth-contour lines in
strong event located at the boundary between the Gela– Fig. 9) and to those which remained aseismic (white
Catania thrust front and the underlying undeformed areas above the contour depth lines in Fig. 9). We
Gela foredeep terrains (Azzaro and Barbano, 2000; suggest dividing the area above the SBT into two sub-
Working Group CPTI, 2004). It has been related with provinces: a southern one that represents the surface
the external front of the chain by Azzaro et al. (2000b) projection of the SBT upper crust segment down to a
and we share this point of view. In fact, taking into depth of ~ 10 km (Shallow, S, sub-province in Fig. 10)
consideration the location error, it could well be related and a northern one which represents the SBT deeper
to upper crust thrust shearing along the SBT. In Table 1, segment between a depth of ~ 10 to ~ 25 km (Deep, D,
we have scheduled it as shallow compressional event. sub-province in Fig. 10). We schematically chose the
The oldest known Sicilian event, that heavily struck depth threshold of 10 km because available deep crust
central Sicily (Working Group CPTI, 2004), is the 361 seismic refraction and reflection profiles (Chironi et al.,
earthquake (M aw 6.6). Although its epicentral location is 2000; Finetti, 2005; Finetti et al., 2005) show that the
not well-constrained, Guidoboni et al. (1994) pointed base of the sedimentary crust at the hangingwall of the
out archaeological evidence of damage possibly at the outer basal thrust, the SBT, may be in average
great historic roman villa of Casale (III–IV A.D.) in encountered at this depth. The entire SBT province,
Piazza Armerina town, and Jenny et al. (2006) schematized in Fig. 10, has a total areal extent of
2
associated it to N–S compression in mainland Sicily. 16,200 km with an along strike length measured along
If we consider the location of CPTI04 (Working Group the SBT 0-depth-contour line of ~ 270 km and an
CPTI, 2004) macroseimic epicenter, the event results to average perpendicular-to-strike extent of ~ 60 km.
be located above the 15-to-20 km SBT depth-contour Sub-province S is characterized by folds, thrusts and
lines. Therefore, in Table 1 we have highly speculatively strike slip faults, nucleated since late Pliocene times, at
classified it as deep compressional event. However, we upper crust levels within the hangingwall of the SBT
cannot exclude its association with a shallower portion and its splays. It has a normal-to-thick crust (average
of the SBT. 30–35 km, Finetti, 2005) with associated low heat flow
2
values (~ 50 M w /m , Della Vedova et al., 2001). Low-
5.2. The seismotectonic zonation to-moderate seismic activity (M aw max 5.6) charac-
terizes the eastern and western sectors of the sub-
In our opinion, the overall geological and seismo- province. During the last four hundred years, in the
logical information previously discussed converge western sector (Sciacca area) there is knowledge of 5
towards the hypothesis of seismogenic compression in earthquakes with 4.5≤M aw b5.5, whereas in the eastern
mainland and central-southern Sicily linked with sector (Niscemi–Caltagirone, Mineo and Piana di
ongoing shearing along the northward-dipping SBT. Catania areas) there is knowledge of 10 earthquakes
This setting is analogous to that already detected in the with 4.5≤M aw b5.5 and of one slightly stronger event
Marche–Adriatic region in central Italy (Lavecchia with M aw 5.6 (1624 Mineo). The central sector
et al., 2003), where the outermost basal thrust of the (Agrigento–Canicattì–Licata dotted area in Fig. 9)is
Apennine fold-and-thrust system is still active and almost devoid of both historical and instrumental
associated to the reverse, reverse-oblique and strike–slip earthquakes.