GNGTS 2013 - Atti del 32° Convegno Nazionale

the northern Dinarides (by dextral transpressional fault systems) and from the Alpine edifice (underthrusting the Eastern Southern Alps). On the western Adriatic side, the decoupling mechanism of Adria is more complex, since the motion of this plate forces the outer sector of the Apennine belt to extrude roughly NE ward (Fig. 1). This process is accommodated by extensional deformation at the inner border (located in the axial zone of the chain) of the mobilized wedges and thrusting at the outer margin, where such wedges overthrust the Adriatic lithosphere. One may expect that in the framework of the proposed context the spatio-temporal distribution of seismicity along the periAdriatic zones may present significant regularities, with particular regard to the tendency of major decoupling earthquakes to migrate from South to North, along the eastern and western periAdriatic boundaries, up to reach the Eastern Southern Alps. This hypothesis is supported by the time patterns of the seismicity that since 1400 A.D. has occurred in the main periAdriatic zones (Fig. 2). In order to provide an information more representative of the kinematic implications of major earthquakes, the diagrams in Fig. 2 report the seismic slip of events, tentatively determined by the empirical relation between magnitude and average fault slip suggested by Wells and Coppersmith (1994). This choice aims at giving insights into the time concentration of seismic slips, by reporting their sum over intervals of ten years. In particular, such parameter may be useful to evaluate how rapidly the structures surrounding the epicentre may have been stressed by the effects of the triggering earthquake, given that the brittle behaviour of rocks primarily depends on the value of the induced strain rate (e.g., Pollitz et al. , 1998). The patterns shown in Fig. 2 point out that in the zones considered seismic activity is mostly discontinuous over time, with periods of high activity separated by almost quiescent phases. Furthermore, one could recognize (by the help of coloured bands) a progressive northward migration of seismic crises along the Dinaric and Apennine sectors, up to reach the northern front of Adria. In our view, the series of periAdriatic decoupling earthquakes involved by each sequence allows the whole Adria plate to make a step towards Europe. The first presumed seismic sequence (grey in Fig. 2) can only be recognized for the last 5 zones, where a significant increase of seismic activity took place, from about the half of XV century in the Southern Dinarides, to the beginning of the XVI century in the northern Adriatic front. This seismic sequence was followed in most periAdriatic zones by a long period of moderate activity. The first complete strong sequence (green in Fig. 2) started with a considerable increase of seismic activity at the Hellenides during the first decades of the XVII century. This first crisis was followed by a significant increase of seismic activity in all other periAdriatic zones, up to reach the northern front of Adria in the first part of the XVIII century. During this period, Calabria was hit by one of the most intense crises (1638, M=6.9, 7.0) of its known seismic history. At the northern Adriatic front, seismic activity lasted up to about the end of the XVIII century, after which seismicity underwent a drastic reduction for a relatively long period, up to 1870. The beginning of a new seismic sequence (yellow in Fig. 2) was marked by a drastic increase of seismic activity in the Hellenides in the last decades of the XVIII century. In this case too, the Hellenic crisis was accompanied by strong earthquakes in Calabria (1783, M=7.0, 7.1, 6.6). Soon after the first crisis, a new seismic period occurred in the Hellenides, from 1815 to 1826, that was again followed by other major shocks in Calabria (1854, M=6.2, 1870, M=6.1). The sequence then continued with several major events in the Southern Dinarides and Southern Apennines. In the Central Apennines, seismic activity was moderate for a relatively long period, until the occurrence of the strongest shock ever recorded in that zone (Fucino 1915, M=7.0), which was followed by a series of strong shocks in the Northern Apennines (Viti et al. , 2012, 2013). 96 GNGTS 2013 S essione 2.1