Stress rotation and recovery in conjunction with the 1999 Izmit Mw 7.4 earthquake

Local rotations of the stress field might serve as an indicator to characterize the physical status of individual fault segments during the seismic cycle. In this study we focus on the pre-, 2-month aftershock-and post-seismic phase of the 1999 M(w)7.4 Izmit earthquake in northwestern Turkey. Using a compilation of focal mechanism data we investigate spatiotemporal changes of the stress field orientations and find distinct variations along individual fault segments. Whereas the regional stress field prior to the Izmit earthquake and following the 2-month aftershock sequence reflects a stable strike-slip regime, the early aftershock period is dominated by EW-extension below the Akyazi Basin. During the 2-month aftershock period we find significant changes from strike-slip to normal-faulting during the main shock following by a systematic backrotation to the pre-main shock stress regime. This backrotation commences first in the Akyazi Plain hosting a co-seismic slip deficit of <= 3 m and propagates then further to the east towards the Karadere and Duzce faults where the Duzce M-w 7.1 main shock nucleated 87 d later. Our results confirm that spatiotemporal stress field rotations are a useful indicator for variations of the seismotectonic setting during the seismic cycle.