New Earthquake Data in the Calabrian Subduction Zone, Italy, Suggest Revision of the Presumed Dynamics in the Upper Part of the Subducting Slab

The 14 July 2018 magnitude 4.5 southern Calabria earthquake was the only earthquake of magnitude over 4 occurring since 1985 in the same area and depth range (30–70 km) of the 1905 magnitude 7.5 earthquake previously imputed to rupture of the upper part of the Ionian subducting slab. We here apply the Bayloc nonlinear location method for accurate hypocenter location of the 2018 earthquake, and then we estimate its focal mechanism by the cut‐and‐paste waveform inversion method. Moreover, we locate all the seismicity occurring at depth <300  km in the past two decades in the study region. The results, together with the 1905 earthquake information available from previous works, are compared to the seismotomographic structure of the subduction zone taken from the literature. We find that the 2018 earthquake originated at about 60 km depth in close correspondence with the seismogenic inner core of the descending slab, whereas the 1905 earthquake took place at slightly shallower depth near the top of the slab in its elbow zone. The 2018 earthquake focal mechanism indicates down‐dip extension of the descending slab, a process compatible also with the 1905 earthquake rupture mechanism. Down‐dip extension of this upper part of the Ionian subduction slab contrasts with down‐dip compression hypothesized by the previous investigators for the entire slab. On the basis of clear necking of the descending slab and seismicity increase at depths of the order of 150 km, we propose that the slab is approaching detachment at this depth. In this scenario, the part of the slab above necking may be subjected to down‐dip extension under its own weight, which corresponds to an intermediate phase between stages c and d of the basic, Isacks and Molnar’s evolutionary model of subduction.