sequence using the double-difference technique

Routine catalog phase data from the permanent stations of the Greek National Seismological Network were used to relocate 445 aftershocks of the 26 July 2001M 6.5 Skyros (North Aegean Sea) earthquake, using the double-difference (DD) algorithm of Waldhauser and Ellsworth [Bull. Seism. Soc. Am. 19 (2000) 1353]. The relocated epicenters define two zones: (a) a main NW–SB cluster of∼27 km length and (b) a secondary less pronounced NE–SW cluster of∼15 km length, which was activated within 24 h after the mainshock occurrence. Redistribution of stresses due to the occurrence of the mainshock certainly played a role in this activation. The Skyros mainshock occurred in a region controlled by the activity of the Aegean Sea strands of the dextral strike-slip North Anatolian Fault (NAF). The main aftershock zone is almost perpendicular to the main tectonic NAF strands, and confirms that the causative fault strikes NW–SE associated with sinistral strike-slip motion. The 2001 Skyros earthquake is the first instrumentally recorded event that indicates re-activation of the old tectonic NW–SE trending structures of continental Greece under the presently acting stress field in a way sketched by Kiratzi [Geophys. J. Int. 151 (2002) 360]