Seismicity distribution in conjunction with spatiotemporal variations of coseismic slip and postseismic creep along the combined 1999 Izmit-Düzce rupture

- We study seismicity related to the 1999 M > 7 earthquakes based on seismicity extending along the combined rupture zone.
- We identify distinct seismically (in-)active fault patches related to tectonic setting and co- and postseismic deformation.
- Four seismically inactive patches extending 30-50 kilometers along the rupture zone and down to 10 kilometers depth are identified with a systematic spatial shift between them introduced by the Düzce mainshock.
- The cumulative distribution of shows that the entire crust is activated between rupture initiation and a two-year postseismic period.
- Our findings suggest that a coseismically introduced lateral and vertical slip deficit is systematically compensated postseismically in both the brittle and ductile portion of the crust.

The North Anatolian Fault Zone (NAFZ) in NW Turkey as one of the most active and best studied strike-slip faults provides a unique opportunity to study earthquake related relaxation processes through analyzing co- and postseismic deformation. We study the spatial and temporal distributions of seismicity related to the two consecutive 1999 M > 7 Izmit and Düzce earthquakes. A high-resolution aftershock catalogue including ~ 10,000 hypocenters extending along the combined rupture zone and extending from prior to the Izmit event to after the Düzce event is studied. Spatial and temporal distributions of events allow to identify distinct seismically active and inactive fault patches. Their location is related to the co- and postseismic deformation within and below the seismogenic layer, respectively. Four seismically inactive patches extending 30-50 km along the rupture zone and down to 10 km depth are identified with a systematic spatial shift between them introduced by the Düzce mainshock. The cumulative distribution of sub-areas hosting coseismic slip, aftershock clusters and postseismic creep shows that the entire upper (seismogenic) and lower (ductile) portions of the crust along the combined Izmit and Düzce rupture zone are activated between rupture initiation and a two-year postseismic period. This observation was only achieved due to the subsequent occurrence of two adjacent M > 7 strike-slip earthquakes in combination with a distinct local seismic and geodetic monitoring. Our findings suggest that a coseismically introduced lateral and vertical slip deficit is systematically compensated postseismically in both the brittle and ductile portions of the crust.