2010-2014 Seismic activity images the activated fault system in the Pollino area, at the Appennines-Calabrian arc boundary region

Abstract

The main goal of this study is to increase the understanding of the physical mechanisms behind the ongoing seismic activity in the Pollino area and its influence on the seismic hazard of the Apennines-Calabrian arc boundary region. The study area, near the Pollino massif, is located at the northernmost edge of the Calabrian Arc, which is the last oceanic subduction segment along the Africa-Eurasian plate. The subduction results from the sinking of the Ionian oceanic plate beneath the Calabrian Arc-Southern Tyrrhenian Sea and is part of the fragmented tectonic boundary between two macro-plates: Africa and Eurasia. The subduction geometry is well-documented by several seismological studies (i.e. Chiarabba et al., 2005), and the lithospheric structure of the area is quietly well known (i.e. Totaro et al., 2014 and Piana Agostinetti and Amato, 2009) Despite the slow N-S convergence between these major plates, the Southern Tyrrhenian Sea is a large basin characterized by E-W extensional tectonic. Since Late Miocene, the Calabrian Arc slab experienced rapid rollback, moving E to SE at a rate of 5-6 cm/yr, which is by far higher than the ~1-2 cm/yr rate of convergence between Africa and Europa (Faccenna et al., 2004). However, during late Pleistocene, rollback and subduction have slowed and is likely proceeding at less than 1 cm/yr (D’Agostino and Selvaggi., 2004). Geodetic measurements show that the Pollino Range is subject to NE-SW anti-apenninic extension. In the region the strain rate field shows a continuous belt of extensional deformation that follows the ridge of the Southern Apennines and extends in the Pollino region. The extension rate appears to decrease from the Southern Apennines to the Calabria- Lucania border region (D’Agostino et al., 2013). This finding indeed reveals that the Pollino region is deforming and accumulating tectonic strain which results in a complex system of normal active faults striking sub-parallel to the Apennines. Two principal normal faults are present in the Italian Database of the Individual Seismogenic Sources DISS version 3.1.1 (DISS Working Group, 2010) in the Pollino area: the Pollino (P) fault and the “Rimendiello-Mormanno” (RM) fault system. The RM fault is an active seismogenic structure it strikes about NNW-SSE and dips toward NE; it has hosted in its northernmost part a M 5.0 earthquake on 9th September 1998. The P fault has similar strike but dips toward SW: it shows no recent seismicity and is hence one of the most prominent seismic gaps in the Italian historical seismic catalogue (Rovida et al., 2011). Paleoseismic studies have shown that the P fault was active in the last ten thousand years and is capable to produce events with magnitude above 6.0. The DISS database reports as debated source also the Piana Perretti fault (Brozzetti et al., 2009). A detailed structural map of the area interested by the seismic sequence shows three fault systems (Brozzetti et al., 2013) consisting of several aligned fault segments that have been active during the Late Pleistocene and are reasonably presently active. The first fault system strikes NW-SE and dips toward SW (including the Piana Perretti fault at the NE edge of the Mercure Basin), the second one has similar strike and NE dip, while the third one strikes about E-W. Earthquakes reported in the historical catalogues for this area are not very strong. Few earthquakes with magnitude probably less than 6 affected the area, including the Mw=5.6 “Mercure” event in 1998 (Brozzetti et al., 2009). The Parametric Catalogue of Italian earthquakes (CPTI11, Rovida et al., 2011), shows very well the lack of strong earthquakes in the region: there is a clear evidence of large earthquakes in the Campania-Basilicata area (M~7.0) and several strong earthquakes in the Sila region and in the whole Calabrian territory. According to the seismic classification of the national territory, the area affected by the 2010-2014 seismic activity have a relatively higher probability to be shaken by a strong acceleration (Gruppo di Lavoro MPS, 2004). Most of the seismic events occurred in areas where the peak ground acceleration having 10% chance of being exceeded in next 50 years is between the values of 0.225 g and 0.275 g.