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Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S243–S264 S249

convex, polygons whose vertices coordinates are given in NNE–SSW striking P axes (Fig. 3b). Fault plane solu-
Fig. 5. The eastern ABT and the southern SBT boundaries tions, which indicate a dominant nearly N–S regional
simply coincide with the corresponding thrust front, compression, are available for some minor earthquakes
whereas the western ABT and the northern SBT boundaries (M B 3.6) recorded beneath the western side of Mt Etna
correspond to the surface projection of the 25 km depth at depths between 10 and 30 km (see inset in Fig. 3)
contour line of the underlying basal thrust. The ABT (Neri et al. 2005). N–S seismogenic compression was
province, as drawn in Fig. 5, has a surface areal extent of recognised in central and western Sicily, as well
2
nearly 14.400 km with an along-strike extent (l 1 )of (Caccamo et al. 1996).
*240 km, measured along the thrust front and an average In the Southern Tyrrhenian province, the 2002 Palermo
width (l 2 )of *60 km. The SBT province has a surface sequence shows reverse faulting on E to NE striking
2
areal extent of nearly 16.200 km with l 1 , measured along planes, consistent with the dominant reverse faulting of the
the thrust front, of *270 km and l 2 *60 km. Given the area, as highlighted by the focal mechanisms of events
wedge shape of both seismogenic volumes, controlled by (Montone et al. 2004; Pondrelli et al. 2006) with similar
the deepening of the underlying basal thrust from near the energy recorded in recent years in the area between Ustica
surface to a depth of at least 25 km, an average vertical and the Aeolian Islands (Fig. 3).

extent (l 3 ) equal to 12.5 km, may be reasonably assumed For each province, we derived the moment tensor F ij ;
for both seismogenic volumes. which is a function of strike, dip and rake of the fault plane
The Southern Tyrrhenian Compressional (STC) prov- solutions, from the calculation of an average focal mech-
ince, as drawn in Fig. 5, has a total areal extent of nearly anism through the application of a Linked Bingham Dis-
2
13.000 km with an E–W length (l 1 )of *260 km, and tribution (LBD) procedure, which is the equivalent of an
an average width (l 2 )of *50 km. We do not have any unweighted moment tensor summation. The overall utilised
geologic constraint on the maximum depth of the prov- dataset of focal mechanisms is reported in Table 2
ince. Exclusively, based onto the earthquake depth (Figs. 3a, b and references therein). They refer to moderate
events (M w C 4.0), background seismicity and minor
distribution data (Castello et al. 2005) an average l 3
value of 20 km has been assumed in the forthcoming sequences. We were forced to consider also the moderate
calculations. events and even the minor ones as the studied areas have
not been struck by strong earthquakes since instrumental
Fault plane solutions times and, on the other hand, local, permanent and tem-
porary stations have registered good-quality data elabo-
We selected all the focal mechanisms available in the lit- rated in terms of focal mechanisms in the literature (Neri
erature for the events with moment magnitudes M w C4.0 et al. 2005).
which occurred in the time interval 1968–2006 within the The computed average focal mechanisms show that all
three studied areas (Fig. 3; Table 2). In the Marche–Adri- the provinces are mostly accommodating reverse dis-
atic area, prevailing strike–slip kinematics characterise the placement in response to a maximum sub-horizontal
1972 Ancona seismic sequence (M w 4.8, Gasparini et al. compression oriented approximately WSW–ENE across
1985), whereas prevailing reverse solutions are typical of the ABT province, N–S across the SBT province and NW–
the 1987 Porto S. Giorgio sequence (M w 4.6, Riguzzi et al. SE across the Southern Tyrrhenian Compressional
1989) and of the remaining selected events. In any case, all province.
the mechanisms show nearly-horizontal NE–SW to E–W
trending P axes. Earthquake dataset
In Sicily, there are significantly different interpretations
of the focal mechanism of the highly destructive 1968 The earthquake dataset compiled for each province, with
Belice seismic sequence (M w 5.5 in Anderson and Jackson the main aim of computing the scalar seismic moment

_
1987; M w 5.6 in Morelli and Pondrelli 1998). Some rate M o ; consists of all the known crustal earthquakes
authors evaluate it as right lateral transpression on a with M w C 4.5 occurred since 217 BC within the surface
NNE-striking plane (McKenzie 1972; Gasparini et al. boundary of the province. The information was extracted
1985), others as pure thrusting on north-dipping planes from the Italian Parametric Catalogue CPTI04 (Working
(Anderson and Jackson 1987) or as oblique thrusting on a Group CPTI 2004a), which contains information on the
north-dipping plane (Morelli and Pondrelli 1998). In all Italian earthquakes from 217 BC to 2002, and subordinately
cases, even if data are not clear enough, the average P from other historical regional catalogues, as well as from
axis was nearly N–S and sub-horizontal. The few other specific papers on major historical earthquakes (see
selected events in central and eastern Sicily show reverse- Table 1 for the references). Information on the instru-
oblique and strike–slip kinematics with NNW–SSE to mental events was obtained from regional and local

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