Seismic swarms in the Pollino seismic gap: Positive fault inversion within a popup structure

Seismic swarms frequently occur along continental fault systems and their
relation with large earthquakes is often contradictory. Such a case is
documented in the Pollino mountain range of southern Italy, a decoupling
zone where the belt-normal stretching drastically rotates accommodating the
differential SE-retreat of the Ionian slab. The paucity of historical large
earthquakes has led to hypothesize the presence of a seismic gap. A long-
lasting seismic swarm that climaxed with a ML = 5.2 earthquake in October
2012 was therefore thought as a possible signal of an impending large
earthquake filling the gap. Seismicity data collected during a 4-years long
monitoring are a powerful microscope to look through the seismic swarm.
In this study, we present accurate relocations for 2385 earthquakes and high-
resolution Vp and Vp/Vs models of the fault system. Seismicity occurred on two
separate normal faults that were formerly part of a thrusts and back-thrusts
system, originally formed as a pop-up at restraining bends of the Pollino fault, a
wrench fault system that inverted the original left lateral sense of slip
accommodating a differential motion induced by the southward retreat of
the Ionian slab.