Western termination of the Mw 7.4, 1999 İzmit earthquake rupture: Implications for the expected large earthquake in the Sea of Marmara

The Mw 7.4, August 17, 1999 İzmit earthquake ruptured a ~100-km-long onshore section of the North Anatolian Fault (NAF) in the eastern Marmara region, causing the loss of more than 20,000 people and extensive destruction. Th e western termination and total length of the earthquake rupture is still a matter of debate because the surface rupture goes off shore in the Gulf of İzmit aft er displaying a coseismic displacement of ~5 m. Such a considerable slip implies that the fault rupture must definitely continue some distance westward on the sea floor, but where exactly it terminated is difficult to determine. This issue is critical for determining the size of the Marmara seismic gap, south of İstanbul. Therefore, to explore the fault scarps associated with the 1999 rupture on the sea floor, we have studied ultra-high resolution bathymetry (0.5 m resolution) acquired with a remotely operated submersible during the MARMARASCARPS cruise, an innovative approach which proved to be useful in seeking earthquake surface deformation on the sea floor. The analysis of microbathymetry suggests that the 1999 İzmit earthquake rupture extended westward at least to 29.38°E longitude about 10 km west of the Hersek Delta in the Gulf of İzmit. It is clearly expressed as a sharp fault break with a 50 cm apparent throw across the bottom of a submarine canyon. Further west, a pronounced and linear fault rupture zone was observed, along with fresh en-échelon cumulative fault scarps. We infer that the seismic break continues westwards, reaching a total length of ~145 km at around 29.24°E longitude, consistent with the 1999 rupture deduced from SAR interferometry. It appears to stop at the entrance of the Çi{dotless}narci{dotless}k Basin where a normal faulting component prevails. We suggest that fault complexity at the junction between dominant strike-slip faulting along the İzmit fault and significant normal faulting in the Çi{dotless}narci{dotless}k Basin may act as a barrier to rupture propagation of large earthquakes.