Aftershock relocation and frequency–size distribution, stress inversion and seismotectonic setting of the 7 August 2013 M=5.4 earthquake in Kallidromon Mountain, central Greece

On August 7, 2013 a moderate earthquake (NOA ML=5.1, NOA Mw=5.4) occurred in central Kallidromon Mountain, in the Pthiotis region of central Greece. 2270 aftershocks were relocated using a modified 1-D velocity model for this area. The b-value of the aftershock sequence was b=0.85 for a completeness magnitude of Mc=1.7. The rate of aftershock decay was determined at p=0.63. The spatial distribution of the aftershock sequence points towards the reactivation of a N70°±10°E striking normal fault at crustal depths between 8 and 13km. A NNW–SSE cross-section imaged the activation of a steep, south dipping normal fault. A stress inversion analysis of 12 focal mechanisms showed that the minimum horizontal stress is extensional at N173°E. No primary surface ruptures were observed in the field; however, the earthquake caused severe damage in the villages of the Kallidromon area. The imaged fault strike and the orientation of the long-axis of the aftershock sequence distribution are both at a high-angle to the strike of known active faults in this area of central Greece. We interpret the Kallidromon seismic sequence as release of extensional seismic strain on secondary, steep faults inside the Fokida–Viotia crustal block. •Reactivation of a N70°±10°E striking normal fault at 8–13 km depths.•A NNW–SSE cross-section revealed the activation of a south dipping normal fault.•The b-value is b=0.85 for Mc=1.7. The rate of decay was determined at p=0.63.•From a stress inversion technique we obtain that the tension (S3) is at N173°E.•The Kallidromon sequence was due to strain release inside the Fokida–Viotia block.