Active Subaquatic Fault Segments in Lake Iznik Along the Middle Strand of the North Anatolian Fault, NW Turkey

The seismic activity of the middle strand of the North Anatolian Fault (MNAF), Northwestern Turkey, is debated because of its quiescence during the instrumental period, in contrast to a significant historical activity documented by several chronicles over the last two millennia. Here, we focus on Lake Iznik, bordered by the MNAF, to get a new insight into its long‐term seismicity and its tectonic setting. The study of lacustrine sediment cores reveals 14 earthquake‐induced turbidites since their ages correspond to seismic events during the past two millennia. Bathymetry and high‐resolution seismic reflection data allow to describe two hitherto unknown subaquatic active fault structures (the South Boyalica and Iznik faults) that belong to the MNAF system. Sediment cores sampled on both sides of the Iznik Fault document an event deposit and a sedimentary unit vertically offset of ~40 cm interpreted as the last rupture during the 1065 CE destructive earthquake. Older events are supposed on this fault more than thousand years ago. Further studies will help to estimate the horizontal co‐seismic offset of this oblique‐slip fault and the calendar of older ruptures. The current seismic gap of thousand years on this segment greatly increases the seismic hazard in this region and must be considered in the seismic risk assessment of the NAF system. Plain Language Summary During large earthquakes, sediments are generally transported from lake slopes to the lake basin. The resulting event deposits can provide information on the recurrence of past regional earthquakes, which is crucial for seismic hazard assessment. In this study, we discovered two underwater fault structures in Lake Iznik, using geophysical methods. Studying the sedimentation on both sides of the southernmost fault, we observed an increased sedimentation rate on the hanging wall of the fault immediately after an event deposit, dated to 1077 ± 77 cal. CE. We interpreted these indicators as resulting from a co‐seismic displacement along the fault plane, linked to the 1065 CE historical earthquake, which had significantly impacted the city of Iznik. We also show that most of the other event deposits in the sediment cores are confidently associated to 14 historical earthquakes since 2,000 years. Key Points Previously unknown faults that belong to the North Anatolian Fault system have been discovered in Lake Iznik through geophysical surveys (from multibeam bathymetry and high‐frequency seismic reflection d