Where does the active North Aegean Sea shear stop? Geodynamic and seismotectonic implications from recent strike-slip earthquake occurrences and GPS-based geodetic analysis in Euboea, Phthiotis and Boeotia, Central Greece

Abstract

Since 2008, eight strike-slip earthquake sequences/swarms have occurred in the broader regions of Boeotia and Euboea, where shear from the North Aegean Sea intersects the extensional province of Central Greece. Notably, the 2023 Elatia sequence unfolded within an area previously considered dominated by pure extension. In broader Euboea, the seismotectonic context has remained ambiguous, with limited implications for the southwestward continuation of North Aegean shear as far as the North Euboean Gulf. We reassessed these sequences/swarms through hypocenter relocation and recalculated moment tensor solutions, integrated with GPS data analysis. Our findings reveal that shear and extension coexist throughout much of the study area, with GPS velocities indicating southeastward increasing movement segmented by four wide and diffuse dextral shear zones. These ‘soft’ boundaries facilitate progressive slip between five compartments, enhancing extension in the North and South Euboean Gulfs. Seismic ruptures of the eight cases occurred on either dextral NE–SW-trending faults or sinistral NW–SE faults, consistent with a ‘wrench’ tectonic model. In this framework, fault patterns correspond to Riedel shears (R) and their conjugates (R′), while the coexisting normal faulting reflects transtensional deformation. The North and South Euboean Gulfs act as rifts, accommodating differential motion between Euboea Island and the mainland. We also conclude that the southwestward propagation of the North Aegean shear zone is a young geodynamic process potentially reaching, at least, the northeastern coast of the Corinth Gulf.