An Intense Earthquake Swarm in the Southernmost Apennines: Fault Architecture from High-Resolution Hypocenters and Focal Mechanisms

Abstract

Between 2010 and 2013, the Pollino Mountains region (south Italy), already proposed as a seismic gap, was affected by a seismic crisis of more than 5000 small-to-moderate earthquakes (maximum magnitude ML 5.0). Preliminary analyses performed in a previous work highlighted that this activity can be ascribed to normal faulting on north-northwest-trending west-dipping dislocation surfaces consistent with the general seismotectonic frame of the southern Apennines. This work contributes additional data and a more sophisticated analyses that highlight new features of the seismic swarm and support a new interpretation for the study area. We obtained high-precision locations and focal mechanisms using the double-difference method and the cut-and-paste waveform inversion method, respectively. The 3D patterns of hypocenters and focal mechanisms consistently image an ∼10-km-long north-northwest-striking and west-dipping fault zone between 5 and 10 km depth, with predominantly extensional kinematics. The high-resolution data show that this zone broadens from north to south as a result of secondary faulting. The depicted geometry, with preliminary geological observation, leads to the hypothesis of multiple seismogenic normal faults rooted into more regional shallow-dipping detachments inherited from the pre-existing Apennine thrust tectonics.