Seismicity of the upper lithosphere and its relationships with the crust in the Italian region

In a recent paper, we compared the earthquake hypocenters, plotted according to updated catalogs, with the structure of the Earth�s crust interpreted after the results of seismic exploration (mainly the Deep Seismic Soundings - DSS). The comparison was made along several cross-sections in the Alpine range, the Italian peninsula and the surrounding seas. The main conclusions of that analysis were that 1) the majority of the events is positioned in the upper, rigid crust and 2) the earthquakes tend to concentrate above the discontinuities unveiled by the seismic exploration in the deep crust and at the Moho boundary. In this paper a similar analysis is conducted, even in volumes where DSS information is not available, with the goal of shedding some light on the continuation of these structures with depth. It is apparent that the upper mantle seismicity is very unevenly distributed; therefore, we only focus on the areas where a sub-crustal seismicity is recorded, adding to the seismic models of the crust some information, if available, on the physical characters of the upper lithosphere. Four areas are examined: the well-known Calabrian (Aeolian) Arc where the Ionian plate is subducted beneath the Tyrrhenian, thin crust of oceanic type, the active subduction of the slab being witnessed by deep and very deep earthquakes; the north-central Apennines where the continental crust of the Adria microplate seems also subducted beneath the transitional, peri-Tyrrhenian type of crust but where the observed hypocenters are limited to the depth of about 100 km; the northern Apennines, where the same type of subduction seems to occur beneath the north-eastern slope of the mountain range, though evidenced by an even smaller number of events; finally, the western Alps: also here a small group of foci are recorded in the upper mantle beneath the southern end of the �Ivrea body�. The different behavior of deep seismicity in the four areas confirms that the Italian peninsula is formed by sectors deriving from different geodynamical processes.