GNGTS 2013 - Atti del 32° Convegno Nazionale

(Wessel and Bercovici, 1998). Sites showing large uncertainties have not been used into the velocity gradient tensor computation. As a final step, we computed the average 2D strain-rate tensor as derivative of the velocities at the nodes of each grid cell. The estimated strain-rates are shown in Fig. 3b. The geodetic velocity field, referred to a fixed Eurasian reference frame depicts a general NNW-directed motion with an N-S decreasing gradient across the Scordia-Lentini Graben, passing from values of ca. 5.5 mm/yr, along the southern-central sector of the Hyblean Plateau, to values of ca. 3.0 mm/yr on the northern rim of the plateau itself (Fig. 3a). The strain-rate field (Fig. 3b) clearly evidences as Scordia-Lentini Graben is dominated by a ~140 nanostrain/yr contractional belt with shortening axis oriented along the prevailing N-S direction. The southern-central sector of the plateau is characterized by a positive areal change of ~90 nanostrain/yr; westward this area seems delimited by the Scicli-Ragusa Fault System. Westward the Scicli-Ragusa Fault System, the plateau shows a gentle negative areal change (~30 nanostrain/yr). Discussion and conclusions. This study is mainly devoted to investigate the current tectonic setting of a large sector of southern Sicily, most including the Hyblean Foreland and the front of the Appennine-Maghrebian Chain, as well as the Ionian Sea offshore, by using an extensive GPS-based and a seismological dataset. In detail, GPS velocity field depicts a general NNW-directed motion in good agreement with the horizontal stress direction inferred from the inversion of focal mechanism solutions. Also, these results well map some interesting features related to the different deformation patterns observed on the investigated area. In particular, a fine-scale analysis of seismic and GPS data indicate that Hyblean Foreland wedge is separate in two crustal blocks (western and eastern) by the Scicli-Ragusa Fault System. Earthquakes distribution, as well as the Vp anomalies, depict the geometry of this important structural lineament, while the focal mechanisms clearly indicate its kinematics, which is mainly left- lateral strike-slip. The left-lateral strike-slip behavior is also confirmed by GPS data, which show a tensile and a strike-slip component of 3 mm/yr and 1 mm/yr (positive for left-lateral Fig. 3 – a) GPS velocities and 95% confidence ellipses in a fixed Eurasian reference frame; red diamonds for survey GPS sites, yellow circles for continuous GPS stations and light-blue stars for solutions coming from Ferranti et al. (2008). b) Geodetic strain-rate parameters: the colour in background shows the rate of areal change, while arrows represent the greatest extensional (red) and contractional (blue) horizontal strain-rates. 219 GNGTS 2013 S essione 1.2