Abstract
In the years between 2011 and 2014, at the edge between the Apennines collapsing chain and the subducting Calabrian arc, intense seismic swarms occurred in the Pollino mountain belt. In this key region, <2.5 mm/yr of NE-trending extension is accommodated on an intricate network of normal faults, having almost the same direction as the mountain belt. The long-lasting seismic release consisted of different swarm episodes, where the strongest event coinciding with a ML 5.0 shock occurred in October 2012. This latter comes after a ML four nucleated in May 2012 and followed by aseismic slip episodes. In this study, we present accurate relocations for ∼6,000 earthquakes and shear-wave splitting analysis for ∼22,600 event-station pairs. The seismicity distribution delineates two main clusters around the major shocks: in the north-western area, where the ML 5.0 occurred, the hypocenters are localized in a ball-shaped volume of seismicity without defining any planar distribution, whilst in the eastern area, where the ML 4.3 nucleates, the hypocenters define several faults of a complex system of thrusts and back-thrusts. This different behavior is also imaged by the anisotropic parameters results: a strong variability of fast directions is observed in the western sector, while stable orientations are visible in the eastern cluster. This tectonic system possibly formed as a positive flower structure but as of today, it accommodates stress on normal faults. The deep structure imaged by refined locations is overall consistent with the complex fault system recently mapped at the surface and with patterns of crustal anisotropy depicting fractures alignment at depth. The possible reactivation of inherited structures supports the important role of the Pollino fault as a composite wrench fault system along which, in the lower Pleistocene, the southward retreat of the ionian slab was accommodated; in this contest, the inversion of the faults kinematics indicates a probable southward shift of the slab edge. This interpretation may help to comprehend the physical mechanisms behind the seismic swarms of the region and defining the seismic hazard of the Pollino range: nowadays a region of high seismic hazard although no strong earthquakes are present in the historical record.
Highlights
In a long period between 2011 and 2014, intense seismic swarms developed at the edge between the Calabrian Arc and the Apennines (Govoni et al, 2013; Passarelli et al, 2015).This area is a crucial node at the rim of the ionian subduction (Totaro et al, 2014; Chiarabba et al, 2016; Palano et al, 2017)
Common tectonic models are based on the continuity of deformation between the southern Apennines and Calabrian arc (i.e., Valensise and Pantosti, 2001); lateral heterogeneities across the Pollino mountain range instead suggest that the strikeslip shear zone permits a kinematic decoupling between the extending Apennines and that retreating Calabrian block (Chiarabba et al, 2016), as signaled by the imprint of toroidal flow at the ionian slab edge on GPS data (Palano et al, 2017)
The seismic swarms occurred within the gap area, along the Pollino shear zone
Summary
In a long period between 2011 and 2014, intense seismic swarms developed at the edge between the Calabrian Arc and the Apennines (Govoni et al, 2013; Passarelli et al, 2015). During these four years of intense seismic activity, a group of temporary INGV seismic stations (INGV Seismological Data Center, 2006) and Deutsches Geo Forschungs Zentrum (GFZ) seismic stations and array (Passarelli et al, 2012) were installed, for the purpose of enhancing the earthquakes identification capability of the Italian National Seismic Network there in real-time (Figure 4, see Govoni et al, 2013 for details) and to refine the accuracy of the locations using off-line data. The existence of this network of fractures and/or porous fluid-filled is invoked, i. e, by Della Vedova et al (2001) to interpret the unusual low geothermal gradient of the region, by Barberi et al (2004) and Totaro et al (2014) to explain an high Vp/Vs value observed in the shallowest crust and by Napolitano et al (2020) to justify the scatter and absorption anomalies
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