How far did the surface rupture of the 1999 Izmit earthquake reach in Sea of Marmara?

An open problem concerning the Mw 7.4, 1999 ?zmit earthquake along the North Anatolian Fault (NAF) system is the apparent conflict between estimates of strike-slip deformation based on field and remote sensing data. This is due to the fact that the main strand of the NAF west of the epicenter lies below the Sea of Marmara. Seismological evidence and models based on synthetic aperture radar interferometry suggest that coseismic and early postseismic displacement accumulated after the earthquake could have reached the western end of the ?zmit Gulf and possibly the southern edge of the Ç?narc?k Basin, tapering off along the northern coast of the Armutlu Peninsula, more than 60 km from the epicenter. This scenario is not confirmed by onshore field observations that point toward a termination of the surface rupture around 30 km to the east. These discrepancies convey high uncertainties in the estimate of the tectonic load produced by the ?zmit earthquake on the adjacent fault segment toward Istanbul. We analyzed data from different sources, including high-resolution marine geophysical surveys and two Nautile dives along the fault-controlled canyon that connects ?zmit Ç?narc?k basins. Our observations suggest that the surface rupture of the 1999 ?zmit earthquake propagated through the shallow Gulf but did not reach the deep Marmara basins. In fact, along the slope between Ç?narc?k and the western end of the ?zmit Gulf, we do not observe fault-related ruptures affecting the seafloor but rather a series of active gas seeps and "black patches" that mark the presence of known active faults. Our findings have implications for seismic risk assessment in the highly populated region of Istanbul, both for the estimate of tectonic load transferred to the next fault segments and the location of the next earthquake.