GNGTS 2017 - 36° Convegno Nazionale

124 GNGTS 2017 S essione 1.2 rate and the normal stress rate multiplied by an assumed friction coefficient. The obtained CFF represents a measure of the loading/unloading rate on the fault plane. Hence this analysis can eventually indicate the rate at which each fault is loading or unloading elastic energy. As a possible inference, regions where the current strains explain well the known seismicity can be identified, and areas that are historically quiescent but where stress is consistently building up can be singled out. In such areas the lack of seismicity may result from a limited earthquake coupling – i.e. ongoing strains are consumed aseismically - or from the incompleteness of the earthquake record. Fig. 2 shows the statistical distributions of the CFF in the case of fixed friction of 0.5. We have also tested the option of an optimal friction, i.e. computed on the basis of the angles provided by the DISS for each ISS. the majority of the faults we have examined are positively coupled to the regional field at a relatively modest rate in the 0 : 4 kPa/yr range, with a few notable exceptions in central Italy The notable exceptions are ITIS052 (San Giuliano di Puglia), ITIS053 (Ripabottoni), for which we find a loading rate of 5 kPa/yr, and ITIS094 (Tocco da Casauria) with more than 6 kPa/yr for the variable friction case. Assuming a constant friction only ITIS094 clearly emerges with 5 kPa/yr. Assuming a friction coefficient optimized on the dip and rake of a fault generally increases the CFF by about 1 kPa/yr for some faults that already had a high CFF. The Sources ITIS052 and ITIS053 are both associated with the 2002 San Giuliano di Puglia earthquakes of Mw 5.7 and 5.8 at a depth of 16-20 km, according to DISS. They show a pure strike slip mechanism. Di Luccio et al. (2005) estimate that the minimum stress (i.e. the extensional stress) is elongated in the N-S direction, which very well agrees with the geodetic estimates of the strain rate eigenvectors in Fig. 1. The Source ITIS094 (Tocco da Casauria) is associated with the 5-30 December 1456 earthquakes, the largest destructive event, occurred in the Italian peninsula and among of the strongest of the entire seismic history of Italy. Fracassi and Valensise (2007) propose that the 1456 sequence is composed of various sub-events but that only three main events are responsible for the major damage, between 5 and 30 December 1456. According to these authors the seismogenic style falls between the mainly extensional Apennines axis and the mainly right- lateral strike-slip kinematics occurring in the Apulian foreland. Alignment of the strike to the orientation of the geodetic stress rate field. In areas of high structural complexity the strike angles of the fault plane solutions may differ from the corresponding angles estimated from field survey. The strike of the seismic fault or source may also be offset relative to the direction of the largest eigenvector of the stress or strain rate tensor, for normal and reverse faults. In such cases the seismic slip occurs on inherited faults, or faults which have been reactivated with a different stress regime. An example is given by the structures in Friuli, as discussed by Viganò et al. (2008,) and Restivo et al. (2016). Fig. 2 - Statistical distribution of the ISS’s according to the rate of CFF, for a fixed friction.