GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.2 219 of homogeneous stress regime, to obtain a statistical characterization of the geodetic and geophysical stress data. We summarize our findings in Tab. 1. We consider six tectonic regions: three (northern Sicily, Friuli and Emilia) are subject to compressive stress mostly in a N-S direction; central and southern Apennines are subject to extensional stress orthogonally to the chain; finally Gargano is subject to a dextral shear stress. The polygons in Fig. 1 define these regions. We consider separately borehole breakouts and focal mechanisms, because they are rather different stress indicators. In Tab. 1 we give the number of boreholes or focal mechanisms falling within each of the six tectonic regions, the weighted mean difference between the geophysical and geodetic S h and the weighted rms. Weights were computed on the basis of the uncertainties. Likewise for the focal mechanism data. Here however the nominal uncertainty is fixed to 25 deg. (Montone and Mariucci, 2016). A pictorial example is given in Fig. 3, where we plot the GPS velocities in the central Apennines, the geodetic and geophysical S h and a rose plot with the angular distribution of the difference between the geodetic and geophysical S h . In the central Apennines the number of focal mechanisms is rather large and so are the GPS velocities. The discrepancy between the Fig. 3 – Results of statistical analysis of differences between minimum stress orientation from GPS data and focal mechanisms for the central Apennines. a) Location of focal mechanisms and GPS velocities. b) Geophysical and geodetic Sh estimated at the location of focal mechanisms data. c) Rose histogram showing the angular distribution of the difference between the geodetic and geophysical S h .