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G. Lavecchia et al. / Tectonophysics 445 (2007) 145–167 153

front, may also be found in the literature. The pointed out 1997; Azzaro, 1999; Obrizzo et al., 2001; Patanè and
evidence mainly consists in high-resolution seismic Privitera, 2001; Patanè et al., 2004; Monaco et al., 2005).
profiles which image compressive deformation and Our final selected dataset is projected in the epicentral
folding of Holocene turbidites at the thrust front (Cita map in Fig. 6 and consists of nearly 500 earthquakes with
et al., 1984; Hieke et al., 2005), as well as in a possible 2.5≤Ml≤4.6. The maximum concentration of low to
association with historical earthquakes (Gutscher et al., moderate events (Ml max 4.6) is observed in the Etna
2006). area and in the surrounding regions in eastern Sicily;
The presence of seismic activity related to active some seismicity is recorded in northern-central Sicily
shearing along the SBT and its inner splay might (Ml max 4.1) and in the Marsala–Mazara area in western
evidently be a strong argument in support of a still active Sicily (Ml max 4.0). Only poor background seismicity
SBT. As a matter of fact, seismogenic N–S compression characterizes the area where the Belice 1968 seismic
is documented beneath the Etna volcanic area, at depths sequence occurred. It consisted of eight major events
between 10 and 30 km (Cocina et al., 1997; Neri et al., with moment magnitude (M w ) between 4.7 and 5.5 from
2005a), as well as it is pointed out in western and central the 14th to 25th of January (Anderson and Jackson,
Sicily (Monaco et al., 1996; Caccamo et al., 1996; Jenny 1987), followed by hundreds of aftershocks until the
et al., 2006). With the approach of this paper, in order to beginning of June (De Panfilis and Marcelli, 1968), with
detect any eventual indication of seismogenic shearing a cumulative moment magnitude of nearly 6.1, as esti-
on the SBT and on its hangingwall splay, we will ana- mated by Working Group CPTI (2004). The Agrigento–
lyze the present and the long-term seismicity which Licata–Canicattì area in central-southern Sicily is almost
occurs in the area corresponding to the SBT surface completely aseismic. The number of events and the
projection, between the contour depth lines 0-to-30 km. energy release, evaluated using the magnitude–energy
The major historical and early instrumental earthquakes relationship log E=9.9+1.9Ml+0.024Ml 2 valid for
occurred in the time period 217 B.C to 2006, and also M≤4.5 (Richter, 1958), are shown for classes of depths
the background seismicity recorded from 1981 to 2006, in the histograms of Fig. 7. The two histograms depict a
will be considered sourcing information from seismic rather similar bimodal pattern: 30% of events occur
catalogues and detailed papers. within the upper crust at depths less than ~ 10 km, the
remaining at depths between 10 and 20 km, with a
4. The seismological dataset highest concentration within the lower crust, at nearly
25 km. Almost 65% of the total energy is released within
4.1. Instrumental seismicity the lower crust, approximately at depths between 20 and
30 km, but a small peak may also be observed at nearly
From two instrumental seismic catalogues available 10 km.
on line (CSI 1.1 1981–2002 by Castello et al., 2006; In the area across the Etna volcanic edifice, it has
Bollettino Sismico, 2003–2006), we extracted the events been possible to perform a more detailed analysis of the
that occurred during the time interval 1981 to 2006 depth distribution of the seismicity and to speculate on
within the area above the 0-to-30 km SBT depth-contour the likely relationships with the geometry of the active
lines (grey area in Fig. 6), at depth b 40 km. In order to tectonic structures. This because a large number of
obtain a rather homogenous and good quality dataset papers based on microseismic data gathered in this zone
without losing information, we applied and compared is available and, in general, the instrument quality and
various selection criteria; we changed, for example, the number of measurement points of the local seismic
numbers of minimum phase readings, the azimuthal gap networks have been highly improved during the last few
and the magnitude threshold. All the obtained datasets years. In the map of Fig. 8a and in the section A–A' of
gave comparable information on the surface and depth Fig. 8b, we have projected a selection of good quality
earthquake distribution. Therefore, we decided to use crustal microseimic events (Ml N 2.5, depth b 40 km,
the selection that the majority of the literature adopts gap b 100°, phase readings N 10, RMS b 0.5s) extracted
(RMS b 0.5s, gap b 200°, phase readings N 10 and from Bollettino Sismico (2003–2006) and recorded
magnitude Ml≥2.5). From the dataset selected with within a half-width of 35 km from the trace of section
this criterion, we excluded the events sited at depths A–A' by the Eastern Sicilian Permanent Seismic
shallower than 8 km beneath the Etna volcanic edifice Network and by the Italian National Permanent Seismic
(hatched circle area in Fig. 6), as these events would be Network during the time between 2005 and 2006. In the
more probably linked with the volcanic dynamics rather same map and section, we have also projected good-
than with the regional tectonic process (Castellano et al., quality microseismic events extracted from background