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

Sub-province D is not characterized at the surface by completeness of each class (ΔTC) per unit area (A).
Plio-Quaternary compressional structures, but active Therefore, the 17 and 361 earthquakes, as well as all
compression at depth in the area is testified by available the others occurring before the time window considered
focal mechanisms, which show a predominance of complete, were excluded from the computation.
thrust and strike–slip focal solutions, with average low- ER was evaluated using the Gutenberg and Richter
dip P-axes orientated around N–S(Anderson and (1956) empirical relationship, log E=11.8+1.5Ms for
Jackson, 1987; Frepoli and Amato, 2000; Montone MsN4.5, that relates energy to instrumental magnitude
et al., 2004; Pondrelli et al., 2004; Neri et al., 2005a) (Ms). The conversion of the moment magnitude (M w or
(Fig. 6). It is characterized by a slightly thickened crust M aw ) or the local magnitude (Ml) values into surface-
(about 35–40 km thick, Finetti, 2005) and by low heath wave magnitude (Ms) values, when the latter was not
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flow values (~ 50 mW/m , Della Vedova et al., 2001). In directly calculated and available in literature, was
the last four hundred years, the western sector of sub- carried out using the empirical relations given by the
province D was shaken by 11 earthquakes with Working Group MPS (2004). In the case of seismic
4.5≤M aw b5.5, which occurred in the Egadi–Marsala, sequences lasting several months and consisting of
in the Corleone and in the Mazara–Belice areas and by many energetic events, such as the one occurring in
the 1968 Belice earthquake (M aw =6.1). The central 1968 in Belice, we used in the computation the M aw
sector is characterized by 3 events of moderate value reported by the CPTI04 catalogue for the main
magnitude (4.5≤M aw b5.0) in the Enna–Caltannissetta shock (Working Group CPTI, 2004). This choice did not
area, but also probably by the 361 event (M aw 6.6). The imply a deficit in the computation of energy release. In
eastern sector of the province is characterized by 6 fact, using only the main shock CPTI04 value (Working
moderate to large events (4.5≤ M aw b5.5), which Group CPTI, 2004), we computed a level of energy
occurred in the Regalbuto–Raddusa areas and by the comparable with that which might have been obtained
1716 (M aw 5.17) and 1818 (M aw 6.0) Catania earth- using all the instrumental magnitude listed in Anderson
quakes in the Deep Etna area. and Jackson (1987) and in De Panfilis and Marcelli
In the Nebrodi–Peloritani area, the deep compres- (1968).
sional sub-province is overlapped by an upper crust The computed yearly energy release (ER/ΔTC) value
seismogenic domain (Fig. 10) which is undergoing is ~ 0.4×10 18 erg/year in sub-province S and ~ 4.7×
NNW–SSE to WNW–ESE extension, as testified by 10 18 erg/year in sub-province D. The value normalized to
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geological data and focal mechanisms (Monaco and the areal extent (ER/ΔTC/A) is ~ 0.5×10 erg/y/km for
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Tortorici, 2000; Neri et al., 2005a; Scarfi et al., 2005; sub-province S (A~6900 km )and ~5×10 14 erg/y/km 2
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Pondrelli et al., 2006; Sgroi et al., 2006). The for sub-province D (A~9300 km ). In order to verify if
coexistence of coeval Plio-Quaternary tension and this strong discrepancy might be dependent on the almost
compression within a same area, but at different depths, total lack of seismicity in the Agrigento–Licata zone
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is well evident in the crustal sections C, D and E in (dotted area in Fig. 9, A~3100 km ), we also evaluated
Fig. 5, as well as in section A–A' in Fig. 8. For example, the yearly ER for unit area in the sub-province S not
in section A–A' along an ideal vertical line across the considering this zone. The ER value obtained for the
Etna volcanic edifice, the normal–oblique Caccamo– western plus the eastern zones of sub-province S is equal
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Caltavuro–Troina fault zone is encountered first, at to ~ 1×10 14 erg/y/km and therefore still significantly
depths of nearly 10 km, and subsequently, at depths of lower than that of sub-province D. The very subordinate
20–25 km, the SBT is encountered. As already pointed level of seismic energy released in the Shallow sub-
out by Cristofolini et al. (1985), an overlapping province compared to the Deep one may be speculatively
epicentral pattern from different surfaces of weakness attributed to different causes, ranging from catalogue
at different depths is the predictable result of the incompleteness (missing earthquakes), to longer recur-
asymmetry of the deformation. rence times intervals and or to different rheological
In order to quantify the level of seismic activity within behaviour with prevailing brittle seismic shearing along
the SBTseismotectonic province and to confront the two the deep SBT portion. Still, the almost complete absence
sub-provinces, we selected from the merged dataset in of seismicity within the Agrigento–Licata zone may be
Table 1 the compressional events occurred within the attributed to ductile reverse shearing of the underlying
time windows of catalogue completeness for magnitude portion of the SBT upper crust segment compared with
classes. Starting from the selected events, resumed in the some brittle shearing of the lateral Mazara–Sciacca and
table of Fig. 10, we calculated the energy release (ER) for Niscemi–Catania portions. This different behaviour might
classes of magnitude normalized to the time windows of be controlled by the flatter geometry of the thrust plane in