We investigate aftershock focal mechanisms of the Mw=7.4 Izmit earthquake of Aug 17th, 1999, on the western North Anatolian Fault Zone (NAFZ). Spatial clustering and the orientation of 446 fault plane solutions are analyzed. The Izmit mainshock occurred as a right-lateral slip on an EW-trending near-vertical fault plane. Aftershock clusters define 4 individual fault segments. Focal mechanisms surrounding the epicenters of the Izmit and subsequent Düzce mainshock (Mw=7.1, Nov 12th, 1999) indicate predominantly strike-slip but also normal faulting. Aftershocks in the area between the Izmit and Düzce segments are mainly related to EW-oriented normal faulting delineating a small pull-apart structure. Beneath the easternmost Sea of Marmara, alignments of aftershocks suggest branching of the NAFZ into three or more active segments that differ significantly in terms of their focal mechanism characteristic. The distribution of aftershock focal mechanisms corresponds to fault segmentation of the NAFZ in the Izmit-Düzce region produced by coseismic slip. Areas with large amounts of coseismic slip show aftershocks that are predominantly strike-slip, but low-slip barriers show mostly normal faulting aftershocks.

Stress tensor inversions of the aftershock focal mechanisms show rotations of the local stresses following the Izmit mainshock. In the Izmit-Sapanca area, the maximum horizontal compressive stress axis is horizontally rotated counterclockwise by 8° with respect to the coseismic and long-term regional stress field. Towards the eastern end of the rupture (Karadere-Düzce area), stresses are rotated clockwise. We conclude that the Izmit earthquake caused significant stress partitioning along the rupture. The direction of stress rotation is related to the orientation of the individual fault segments along the NAFZ.

Reference: Bohnhoff et al., 2006, Geophys. J. Int. , doi:10.1111/j.1365-246X.2006.03027.x