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Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S243–S264 S261

being in the order of *0.1 mm/year across mainland Sicily structural-kinematic maps of the region (Lavecchia et al.
and *0.3 mm/year across the southern Tyrrhenian belt. 2007b). A rough evaluation of the horizontal displacement
In summary, the values of seismic strain rate computed accommodated by folded Quaternary terrains exposed in
in this paper substantially fit with those of Westaway the Catania plain (Catalano et al. 2007) allows us to cal-
(1992) and Jenny et al. (2006), but are one order of mag- culate shortening in the order of 0.1–0.3 mm/year. All
nitude different from those of Kiratzi (1994). This may things considered, it is evident that although the long-term
reflect different choices in the seismic catalogues and in geological history not only can help but is also essential for
M s –M o relationships, and/or in the geometry and azimuth an understanding of the tectonic pattern and the style of the
of the estimated provinces. Furthermore, in the last deforming volumes, it cannot help to definitively constrain
15 years, the focal mechanism dataset is greatly improved, active strain rates in these areas.
allowing a more constrained computation of the deforma- Different is the situation of the South Tyrrhenian belt
tion tensor. Our results are also in full agreement with where the onset of the compressional tectonics cannot be
those obtained by Barba et al. (2007), who applied an younger than early Pleistocene times. In fact, this is the age
independent finite element modelling approach to active of the extensional fault structure now inverted in com-
deformation pattern and seismotectonic sources. They pression (Billi et al. 2007 and reference therein). From the
calculated that the Northern Apennine region is experi- data analysed in this paper, we cannot distinguish between
encing NE–SW active compression of about 0.1–0.2 mm/ the various kinematic interpretations proposed for the belt
year, the mainland Sicilian region is undergoing N–S (north-verging or south-verging thrust), but we can observe
shortening at rate of *0.1 mm/year and the southern that the associated seismic deformation budget (*0.2 mm/
Tyrrhenian Sea is undergoing a N–S contraction of about year) is similar to those across the ABT (*0.3 mm/year)
0.3 mm/year. and the SBT (*0.1 mm/year). This seismic budget appears
To compare the present seismic strain rates with the especially modest given that according to geodetic data at
long-term geologic strain rates is difficult, because the least *2 mm/year of the Africa–Nubia convergence would
considered seismotectonic provinces are partially (ABT be accommodated across the Southern Tyrrhenian strip
and SBT provinces) or entirely (STC province) located (Serpelloni et al. 2007). Also we wish to stress that a
offshore. Furthermore, in the literature there is a general physical continuity between the SBT and STC provinces is
lack of agreement about the tectonic style of the Apennine obstructed by the interposition of an area in northern Sicily
compressional deformation (thick-skinned vs. thin-skin- which is undergoing N–S extension, as clearly shown by
ned) and consequently about the long-term shortening the focal mechanism data (Fig. 1).
values. Those calculated for the Umbria–Marche–Adriatic In general, active strain rates computed across the ABT,
thrust belt range from *10 to *85 km for a time interval SBT and STC seismotectonic provinces through the anal-
of *12 Ma (late Miocene–early Pleistocene) (Barchi et al. ysis of geodetic data (Serpelloni et al. 2005, 2007; Jenny
1998; Decandia et al. 1998; Finetti et al. 2001; Lavecchia et al. 2006; Devoti et al. 2008; Ferranti et al. 2008) indicate
et al. 2003; Basili and Barba 2007), with corresponding velocity in the order of some mm/year that are close to the
velocities ranging from 0.8 to 7 mm/year. Considering the geological values, but are on average one order of mag-
only middle Pliocene–Quaternary compressional stage, nitude higher than those obtained from seismic data.
which has determined the nucleation of the NE-verging Considering that the seismic budget represents only a
fold-and-thrust structure of the outer thrust system at the percentage of the overall strain, this discrepancy may
hanging-wall of the ABT, a shortening in the range of 6– imply either that a large part of the total deformation is
10 km in a time interval of nearly 2.5 Ma has been com- expressed aseismically, or that the seismic deformation is
puted by Lavecchia et al. (2004), which is equivalent to underestimated owing to catalogue incompleteness and
shortening values of 2.5–4 mm/year. Whether this value longer-term recurrence interval of strong earthquakes, or
may be extended also to the late Quaternary fold-and-thrust that the seismic deformation might be driven by small-
belt deformation is hard to decide. For some researchers, scale tectonic processes. (Selvaggi 1998; Papanikolaou
the central Apennine compression has completely ceased et al. 2005). Therefore, we cannot exclude a large amount
since Early Pleistocene times, for others it has had a pro- of seismic release in the near future.
gressive decrease since Miocene times, for others it has
started again (since about 250,000 years) after a stop dur-
ing middle Pleistocene times. The picture is still more Conclusions
complicated and controversial in the case of Sicily. An
average southward migration rate of the late Miocene– Historical and instrumental seismicity (last 400 year
Pleistocene compressional activity in the order of records) were used to compute the strain rate and velocity
*4.5 mm/year may be calculated from available tensors in three areas of the Italian territory undergoing

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