An 8000 yr lacustrine paleoseismic record (Lake Iznik, NW Türkiye): Implications for the seismic cycle of the Middle Strand of the North Anatolian Fault

Lacustrine sediment sequences can provide long and complete records of earthquake recurrence patterns due to their continuous nature of subaqueous sedimentation. This gives this “natural seismograph” a high sensitivity for registration of seismic shaking and accurate age-depth control, these characteristics are important parameters for high-quality seismic hazard assessments. As the middle strand of the North Anatolian Fault (MNAF) is a slow slipping fault (5 mm/yr), a long-term earthquake chronicle is required to study its behavior. Here, we report on findings from three long sediment cores (from 8.5 to 15 m), retrieved in Lake Iznik (northwest Türkiye), crossing the Iznik underwater fault, that provide insight into the local and regional earthquake cycle and thus improve the paleoseismic history for the NAF system over the last 8000 years. We used radiocarbon-based age-depth models combined with sedimentological observations, X-ray fluorescence elemental data, and seismic reflection data to reconstruct the spatiotemporal rupture variability. Recurring millimetric, coarse-grained laminae identified as event deposits were deposited synchronously at the three coring sites. Over the last 2000 years, the ages of the event deposits recorded in Lake Iznik correspond to the historical evidence of seismic activity reported along the North Anatolian Fault (NAF) system, allowing us to recognize regional earthquake-triggering mechanisms and assess seismic intervals. The event deposits are inferred to be earthquake-triggered, and the sensitivity to earthquake shaking in Lake Iznik is proportional to the sedimentation rate. Our record demonstrates that Lake Iznik has been repeatedly exposed to significant seismic shaking over the past 8000 years. Our data suggest that large earthquakes with moment magnitudes Mw >7 occur on the Iznik fault segment part of the MNAF with a mean recurrence interval of ~1000 years. At the regional scale period of seismic activity alternates with several hundred years of quiescence. This study allows us to investigate the timing of large paleoearthquakes over the last 8000 years, providing important input for seismic hazard assessment and increasing societal awareness and preparedness in the case of future catastrophic events along the MNAF.