GNGTS 2013 - Atti del 32° Convegno Nazionale

between Hellenic and Calabrian earthquakes has undergone a dramatic worsening, we advance the hypothesis that such anomalies are related with a very rare tectonic event that considerably changed the strain and stress fields in the central Mediterranean area. This event is the large westward displacement of the Anatolian-Aegean system that took place in response to the series of very strong earthquakes that since 1939 activated the entire North Anatolian fault system and its prolongation in the North Aegean zone (e.g., Barka, 1996). That sudden jump presumably induced a strong E-W compressional regime in the Aegean region. From the minimum-work point of view, it is reasonable to suppose that the fast shortening required by this sudden compression could have been achieved by a tectonic processes that can minimize resistance against driving forces. This requirement might be fulfilled by an acceleration of the lateral escape of the Aegean structures that face the low buoyancy Ionian subducting lithosphere and a slow down of the motion of the Hellenic sector (Cephalonia-Albania) that collides with the continental Adriatic domain. This hypothesis is corroborated by the fact that since about 1948 most seismic activity in such area has mainly affected the Aegean structures lying south of the Cephalonia fault system and of the North Aegean trough, while a low to moderate activity has affected the Greek zones lying north of Peloponnesus (Mantovani et al. , 2013b). Conclusions. It is advanced the hypothesis that the next strong earthquake (M≥5.5) in Italy will most probably hit the northern part of the territory, in particular the Northern Apennines. The possibility that such event occur in Calabria and Southern Apennines is recognized as minimal, while an intermediate probability is suggested for the Central Apennines and Eastern Alps. These indications are suggested by the study of the spatio- temporal distribution of major earthquakes in the central Mediterranean area since 1400 A.D., in the framework of the ongoing tectonic setting. The underlying concept is that the activation of a periAdriatic fault system depends on the previous pattern of seismicity in the other decoupling zones of that plate. The interaction between seismic sources is mainly due to post-seismic relaxation, i.e. the fact that each strong earthquake triggers a perturbation of the strain and stress fields that gradually propagates through the surrounding regions. The quantification of the effects of that phenomenon has allowed the recognition of some possible connections between periAdriatic seismic structures. The possible effects of the concepts cited above are tentatively recognized in the time patterns of seismicity at the main periAdriatic zones. Such patterns point out a discontinuos distribution of major earthquakes in the single zones, with seismic crises separated by almost quiescent periods, and the tendency of seismic phases to migrate northward through the eastern (Hellenic-Dinaric) and western (Apennine) boundaries of the Adria plate. In the period considered (1400-2013), it is possible to recognize at least two sequences (roughly developing from 1600 to 1800 and from 1700 to 1900), which involved a significant activation of all periAdriatic boundary sectors, from south to north, which are supposed to correspond to northward steps of the whole Adria plate. The last sequence, roughly started at 1850, has so far involved main seismic activity along the southern and central sectors of the eastern and western Adria boundaries, while scarce activity has so far affected the northern sectors (Northern Apennines, Northern Dinarides and Southern Eastern Alps). In our view, this evidence suggests that at present such zones are the most prone to the next strong earthquake in the study area. This indication is confirmed by the two systematic interaction of seismic sources recognized between major earthquakes in the southern Italian zones (Southern Apenines and Calabria) and the respective eastern Adriatic sectors (Southern Dinarides and Hellenides), which presently imply a low probability of strong earthquakes in Calabria and Southern Apennines. Since the Central Apennines and Eastern Southern Alps have already been partly activated during the ongoing sequence, we suggest an intermediate value of probability for those zones. A detailed description of the evidence and arguments mentioned in this note is given by Mantovani et al. (2013b). 101 GNGTS 2013 S essione 2.1