GNGTS 2016 - Atti del 35° Convegno Nazionale

296 GNGTS 2016 S essione 2.1 References Corral A.; 2005; Time-decreasing hazard and increasing time until the next earthquake. Physical Review E, 71 , 1, DOI 10.1103/PhysRevE.71.017101. Fracassi U., Valensise G.; 2007: Unveiling the sources of the catastrophic 1456 multiple earthquake: Hints to an unexplored tectonic mechanism in southern Italy . BSSA, 97, 725-748, DOI 10.1785/0120050250. Rovida A., Locati M., Camassi R., Lolli B., Gasperini P. (eds); 2016: CPTI15, the 2015 version of the Parametric Catalogue of Italian Earthquakes . Istituto Nazionale di Geofisica e Vulcanologia. DOI 10.6092 /INGV.IT - CPTI15. Storchak D.A., Di Giacomo D., Bondár I., Engdahl E. R., Harris J., Lee W.H.K., Villaseñor A., Bormann P.; 2013: Public Release of the ISC-GEM Global Instrumental Earthquake Catalogue (1900-2009) . Seism. Res. Lett., 84 , 5, 810-815, DOI 10.1785/0220130034. Tinti S., Mulargia F.; 1985: Confidence intervals of b-values for grouped magnitudes. BSSA, 77 , 2125-2134. Return times of large Italian earthquakes P. Harbaglia 1 , M. Vona 1 , S. Parisi 2 1 Scuola di Ingegneria, Università degli Studi Basilicata, Potenza, Italy 2 PROGEON soc. Coop s.r.l. Potenza, Italy The analysis of the new version of the Parametric Catalogue of Italian Earthquakes, known as CPTI15 (Rovida et al. , 2016) and its associated macroseismic data base (Locati et al. , 2016), led us to realize that in fact large earthquakes, did not repeat on the same fault at least during the last 700 years. The new catalogue seems to be statistically complete since 1300 A.D. for magnitudes of 6 or larger. It is quite clear that some epicenters are rather proximal one to the other, but the observation of the macroseismic field shows that the involved faults are different. One notable example is given by the 1461 and 2012 l’Aquila events. We used all the events reported in the catalog with the notable exception of the 1456 event where we preferred the solution proposed by Fracassi and Valensise (2007). These authors separate the event in three major episodes with E-W trend. In all the other cases we accepted the epicentral locations of the CPTI15 catalogue. To verify whether there were repeated events, first of all we need to find the correct fault direction: we use an algorithm derived by that of Boxer (Gasperini et al. , 2009). We also need to give it the right dimensions. We used in this case the relation of Wells and Coppersmith (1994), in particular those that defines the fault area. Since, according to location and orientation we can usually infer what is its mechanism, we normally used the specific relations for that event. We then adapted length, width and dip according to macroseismic field. A final correction was applied to normal earthquakes, since usually the shallower portion of the faults yields the larger damage and this means that the whole fault must be shifted by about 1/3 of its width in the opposite direction. It is noteworthy that using the Fracassi and Valensise (2007) approach, also the 1688 event is probably an E-W trending event, closely associated to one of the 1456 sources. This lead to the fact that in Southern Appennines, the strike-slip events are at least equally as important as the normal ones. From this analysis it results that only large aftershocks can show same overlapping to the main event such as in 1627. Conversely before 1300 A.D. there might be cases of repetition but unfortunately data are usually extremely scant. In this case it is probably more interesting to follow a geological approach. A good example is Galli et al. (2016) that shows how the 1915 Avezzano Earthquake was preceded in the thirteen century by a similar event. The general conclusion is that apparently no larger events repeated on the same fault in the last seven centuries, not even the August 24 Amatrice one. In fact the 1639 event fault solution