Abstract
We analyze the seismicity of southern Calabria, the most active area of Italy from the seismic point of view, and compare it with geodetic data available from literature. Our analysis focuses both on the strongest earthquakes of the last centuries reported in the Italian seismic catalog and on seismicity recorded in the last decades by instrumental networks. The data highlight that strong shallow seismicity of southern Calabria, imputed to normal faulting by previous investigators, corresponds to low values of local, geodetic horizontal strain rate. This situation is quite different from that observed along the Apennines from Umbria to northern Calabria, where normal-faulting strong earthquake activity corresponds to relatively large values of extensional strain rate. On the other hand, the strong earthquake activity of southern Calabria corresponds to marked variation of vertical displacement rates detected from west to east in the same area. We frame these evidences into the regional geodynamic model assuming the coexistence of Africa-Europe NNW-trending plate convergence and SE-ward residual rollback of the Ionian lithospheric slab subducting underneath the Tyrrhenian-Calabria unit. Taking also benefit from the recently found relationship between the two strongest earthquakes of the twentieth century in Italy (the southern Calabria earthquakes of 1905 and 1908 of magnitude 7.5 and 7.1, respectively), we propose that instabilities of the upper bending part of the subduction slab may perturb shallow normal faults in the overriding plate and concur to shallow seismicity of southern Calabria jointly with the dynamics of differential vertical motion marked by geodetic data. The opposite action of lithosphere convergence and rollback may justify low values of horizontal strain rate in the low coupling scenario of the Ionian and Tyrrhenian-Calabria units.
Highlights
In the last decade, taking benefit from large improvement of geophysical datasets and methods, researchers have much deepened the knowledge of the structure and dynamics of the Calabrian Arc subduction zone (Figures 1 and 2), the most active area of Italy from the seismic point of view. Neri et al (2009, 2012) have found that southern Calabria is the only place in the Calabrian Arc subduction zone where the subduction slab is still continuous over depth, while detachment of the deepest portion of subducting lithosphere has already occurred beneath the Arc edges
While marked extension is observed along the Apennine chain between central Italy and northern Calabria with areal change values as large as 55–35 nanostrain/yr, and this closely corresponds to normal-faulting strong seismicity, the southern Calabria segment of the chain displays relatively low extension between 0 and 15 nanostrain/yr normal-faulting earthquake activity is here even stronger than along the rest of the Apennine chain
Based on recent results concerning relationships between intermediate and shallow earthquakes occurring in southern Calabria, we suggest that the observed patterns of ground deformation and seismicity in this specific segment of the chain may be explained with low coupling between the Ionian subducting slab and the Tyrrhenian overriding plate
Summary
In the last decade, taking benefit from large improvement of geophysical datasets and methods, researchers have much deepened the knowledge of the structure and dynamics of the Calabrian Arc subduction zone (Figures 1 and 2), the most active area of Italy from the seismic point of view. Neri et al (2009, 2012) have found that southern Calabria is the only place in the Calabrian Arc subduction zone where the subduction slab is still continuous over depth, while detachment of the deepest portion of subducting lithosphere has already occurred beneath the Arc edges (northern Calabria and northeastern Sicily, respectively; Figure 1). 4a–c in Figure 1 and Table 1) is even more debated: i) epicenter location onshore in the Capo Vaticano promontory area (4a in Figure 1) or offshore in the Tyrrhenian sea (4b and 4c, same figure); ii) depth estimates in the upper crust of the overriding plate (in the case of locations 4a and 4b) or at the top of the Ionian subducting slab (4c); iii) magnitude values between 6.7 and 7.5 (DISS Working Group, 2018, and references therein). As reported in a previous section, the zones of uplift in Calabria are predicted by numerical modeling of lithosphere motions and strains based on the assumption of coexistence of continental plate convergence and rollback of an in-depth continuous subducting slab beneath the Calabrian Arc (Negredo et al, 1999)
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