GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.2 199 and the hypocentral parameters obtained by LOTOS were then inverted by applying the TomoDDPS algorithm (Zhang et al. , 2009), which has the additional advantage of using a combination of both absolute arrival time readings and differential ones (see Fig. 1). Focal mechanisms were computed for the major and best-recorded earthquakes by using P wave polarity data and their spatial distribution on the focal sphere (Reasenberg and Oppenheimer, 1985). In addition, we compiled a list dating back from 1977 of all moment tensor solutions falling in the studied area, published by the Italian CMT, RCMT and TDMT catalogues (Pondrelli et al. , 2006, 2011; see also http://www.bo.ingv.it/RCMT/ and http://cnt.rm.ingv.it/tdmt) . The results depict new details of a network of faults which enables the concurrent existence of adjacent compressional and extensional domains. In particular, in a large sector between Capo d’Orlando and the Messina Strait, normal type focal mechanisms prevail, while the southern area and the Aeolian Islands area are characterized by a strike-slip regime (Fig. 1). Following, a standard numerical technique (Gephart and Forsyth, 1984) has been applied to invert the 183 selected focal mechanisms and determine the principal stress axes (σ 1 , σ 2 , σ 3 ) and the dimensionless parameter R=(σ 2 −σ 1 ) / (σ 3 −σ 1 ), which represents the shape of the stress ellipse (Fig. 2). The findings are also compared by the inversions on a horizontal grid, obtained by using the software package ZMAP (Wiemer, 2001). All events inside a radius of 14 km around a grid cell 10 × 10 km spacing have been selected and their focal mechanisms inverted using Michael’s method (Michael, 1987). In the northern part an eastward transition from a thrust to a strike-slip faulting stress regime is observed. Here the maximum horizontal stress is aligned in a generally NNW- SSE direction, which becomes roughly N-S south of Lipari. Farther south (north- eastern Sicily and southern Calabria), the Fig. 1 – Sketch summarizing the main features deriving from this study. The VP distribution for two layers (top), the final event locations (red circles in maps and sections) and the most representative FPSs, are shown. Colors of the focal solutions are based on the Zoback (1992) classification, i.e., blue=normal faulting; red=strike-slip; black=thrust faulting. Some focal mechanisms along with the related foci position are also reported in the vertical sections. Contraction south of Etna comes from Musumeci et al. (2014) and De Guidi et al. (2015).