Geodynamic and seismotectonic implications from recent strike-slip earthquake swarms and GPS-based geodetic analysis in Euboea, Phthiotis and Boeotia, central Greece

Eight strike-slip earthquake swarms since 2008 in the broader region of Euboea, Phthiotis and Boeotia (central Greece) strongly suggest that the North Aegean Sea shear continues further southwestwards in mainland central Greece. Although most of the swarms are produced by NE-SW-striking dextral faults, as expected, three swarms are produced by the conjugate NE-SW-striking sinistral faults. In fact, one of these occurred in the Greek mainland where extension was considered the dominant stress regime as suggested by previously studied large normal fault zones. GPS strain rates show that dilatation and shear variably coexist, suggesting a transtensional regime. We interpret this seismotectonic setting with the ‘wrench’ tectonic model and the intense accumulated simple-shear deformation deriving from the North Aegean Sea; thus, the dextral faults represent the Riedl shears (R) and the sinistral faults the conjugate Riedl shears (R’). Based on the same model, the co-existence of WNW-ESE-oriented normal faults in the same area can also be explained.

The GPS velocities in the study area revealed increasing values along a NW-SE trending profile, parallel to the Euboea Island axis, in two ways: i) from NW to SE, the profile-normal (NE-SW) component demonstrates stepwise increasing values in station groups on both Euboea and mainland Greece (Phthiotis-Boeotia-Attica, eastern Sterea Hellas), and ii) within these groups, the stations on mainland Greece move faster toward SW than the respective ones on Euboea. These two observations led us to the partitioning of the study area into five compartments, sliding to each other along ‘soft’, NE-SW-trending dextral shear boundaries with increasing rate towards the SE, and a further division of the three middle compartments by the two rifts, i.e. the North and South Euboean Gulfs. This interpretation also agrees with the aforementioned ‘wrench’ model and the NE-SW-oriented shear.