The distribution of historical earthquakes indicates that a large portion of the Italian territory could be hit by strong earthquakes (e.g., Guidoboni et al. 2007). Since a large part of the building patrimony in Italy was realized without taking into\ud account adequate antiseismic criteria (e.g., Di Pasquale et al. 2005; Crowley et al. 2009), it would be economically very difficult in the near future to achieve a significant mitigation of seismic risk in such a large zone. This objective could more\ud easily be obtained if reliable information was available about the zones most proneto next strong earthquakes, where the limited resources now available could be concentrated. An attempt at obtaining such kind of information could be made by taking into account the possible connections between the spatio-temporal distribution of major shocks and the progressive development of tectonic processes in the central Mediterranean region, which are mainly related to the complex short-term kinematics of the Adriatic plate. It is known that each strong shock triggers a perturbation of the strain field that propagates in the surrounding zones (post-seismic relaxation, e.g., Pollitz et al. 2006; Ryder et al. 2007; Ergintav et al. 2009; Ozawa et al. 2011). When the effects of such perturbation reaches other seismic zones they may modify the probability of fault activation or even cause an earthquake when the fault involved is close to seismic failure. The possibility that this phenomenon induces seismicity has been pointed out in a number of papers (e.g., Anderson 1975; Rydelek and Sacks 1990; Pollitz et al. 1998, 2004, 2012; Mikumo et al. 2002; Freed 2005; Freed et al. 2007; Brodsky 2009; Lay et al. 2009; Durand et al. 2010; Luo and Liu 2010; Viti et al. 2012, 2013), which show that the time and place of occurrence of a number of major shocks are compatible with the expected effects of post-seismic relaxation induced by triggering events. In particular, this phenomenon has been recognized for some Italian zones (Southern Apennines and Calabria) whose seismic activity seems to be significantly influenced by major seismic crises in Hellenic and Dinaric zones respectively (Viti et al. 2003; Mantovani et al. 2008, 2010, 2012). The fact that past seismic activity may affect the spatio-temporal distribution of next shocks in the tectonic context here considered is instead supported by the time pattern of major earthquakes that occurred at the main periAdriatic zones since 1400 A.D. In the next section, we describe the evidence that may support the plausibility of the proposed approach and we discuss on how it may provide insights into the location of next strong earthquakes in the Italian peninsula

The evolution of the Mediterranean area since the Oligocene-Lower Miocene has been driven by the convergence of the surrounding plates. This implies that the observed deformation pattern in that region must be the most convenient shortening pattern, i.e. the one controlled by the minimum action principle. To understand why the fulfilment of such condition has required a complex spatio-temporal distribution of major tectonic events, such as uplift, lateral displacement and bending of orogenic belts, consumption of large lithospheric domains and formation of back arc basins, it may be very useful to take into account a basic tectonic concept, which helps to identify the process that can minimize the resistance of tectonic forces. Such concept starts from the fact that the most convenient consumption process is the one that involves low buoyancy oceanic lithosphere (Tethyan domains). However, such process is highly favoured where the oceanic lithosphere is stressed by vertical forces, a situation that develops when orogenic wedges are forced to over thrust and load the oceanic domain to be consumed. This interpretation can provide plausible and coherent explanations for the complex pattern of the observed deformations. In this view, the generation of back arc basins is taken as a side effect of an extrusion process, as suggested by numerical and mechanical experiments

No abstract

Major seismic activity in the Northern Apennines concentrates in few zones, distributed in a peculiar way. It is argued that such context may be plausibly explained as an effect of belt-parallel shortening, which has caused oroclinal bending of the longitudinal ridges formed during the Late Miocene to Lower Pliocene evolutionary phase. The main effects of this process, developed since the upper Pliocene, have mainly affected the outer sectors of the belt. The major seismic sources have generated in the zones where different oroclinal bendings of adjacent ridges have produced extensional/transtensional deformation. In the inner side of the Northern Apennines, belt parallel shortening has occurred at a lower rate. The main effects have resulted from the shortening of the Albano-Chianti-Rapolano-Cetona ridge. In particular, the proposed tectonic setting may account for the moderate seismic activity that occurs in the Firenze, Elsa, Pesa, Siena and Radicofani basins.

In previous papers, we have argued that a close connection may exist between the discontinuous northward displacement of the Adria plate and the spatio-temporal distribution of major earthquakes in the periAdriatic regions [1]-[3]. In particular, five seismic sequences are tentatively recognized in the post 1400 A.D. seismic history, each characterized by a progressive migration of major shocks along the eastern (Hellenides, Dinarides), western (Apennines) and northern (Eastern Southern Alps) boundaries of Adria. In this work, we describe an attempt at gaining insights into the short-term evolution of the strain field that underlies the migration of seismicity in the Apennine belt. The results of this study suggest that seismicity in the study area is mainly conditioned by the fact that the outer (Adriatic) sector of the Apennine belt, driven by the Adria plate, is moving faster than the inner (Tyrrhenian) belt. This kinematics is consistent with the observed Pleistocene deformation pattern and the velocity field inferred by GPS data. The spatio-temporal distribution of major shocks during the last still ongoing seismic sequence (post 1930) suggests that at present the probability of next major shocks is highest in the Northern Apennines. Within this area, we suggest that seismic hazard is higher in the zones located around the outer sector of the Romagna-Marche-Umbria units (RMU), since that wedge is undergoing an accelerated relative motion with respect to the inner Apennine belt. This hypothesis may also account for the pattern of background seismicity in the Northern Apennines. This last activity might indicate that the Upper Tiber Valley fault system is the most resisted boundary sector of the RMU mobile wedge, implying an higher probability of major earthquakes.