GNGTS 2014 - Atti del 33° Convegno Nazionale

NW-SE oriented seismic boundary, roughly located between Cefalù and Mt. Etna volcano. In particular, this boundary splits a sector with high concentration of earthquakes northwards from a zone with relative paucity of seismicity southward (s ee Fig. 2 A). The possible presence of a major structural discontinuity is emphasized also by lateral velocity perturbations found in the same area and visible down to about 20-30 km of depth (Fig. 2 B). Moreover, looking to the vertical cross-sections, the obtained velocity structure clearly points out to a crustal thinning; i.e., high velocity layers are shallower towards northeast. To further investigate this issue, we considered the kinematics characterizing the events located in the studied seismic sector, analyzing the focal mechanisms available in the Sicily and Calabria focal mechanism database by Scarfì et al. (2013), including some data from Neri et al. (2005) and from Musumeci et al. (2014). All types of mechanisms are represented, although it is noteworthy that the land stripe between Cefalù and the Ionian Sea is characterized by a strike-slip regime (Fig. 3), while further northeast normal type focal mechanisms prevail, with nodal planes striking NW-SE. Fig. 3 – Focal solutions for the studied area, as extracted from the INGV database (http://sismoweb.ct.ingv.it/focal/ index.asp). Plunges of P- and T-axes have been used to divide focal mechanism datasets into the main stress regime, according to the Zoback (1992) classification: red, strike-slip fault; blue, normal fault; black, inverse fault. Conclusion. A preliminary analysis of large scale geological features and seismological data provides clues for a regionally-extended structural discontinuity slicing through all over the central-eastern portion of the Sicilian Fold and Thrust Belt, between the Madonie Mts. range and south of Mt. Etna region. Along this area, major NE-SW trending Mio-Pliocene compressive structures (thrust-top and related syncline) are systematically dragged and rotated according to vertical axis until to assume a hook-shape at their terminations. The clockwise sense of rotation of such major structures suggests the occurrence of regional, near 100 km long, dextral wrench fault zone with a NW-SE direction. More detailed field surveys performed in key sectors (e.g. Madonie Mts. and Mt. Judica region) reveal that this crustal discontinuity probably consists of a wider deformation band that in the near-surface scattered into a series of discrete segments which sometimes overlap to form zone of releasing/restraining stopovers. 132 GNGTS 2014 S essione 1.2