Relocation of the 26 July 2001 Skyros Island (Greece) earthquake 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 2001 M 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].