Tectonophysics, vol.846, 2023 (SCI-Expanded)
© 2022 Elsevier B.V.Although the North Anatolian Fault is one of the most investigated continental transform faults across the globe, the maximum earthquake magnitude (Mmax) expected and the resulting seismic risk to the nearby big settlements is still a matter of debate. Some part of the problem issues from the relatively short paleoseismological record of this fault while the rest is closely related to the uncertainties of probable multiple segment ruptures. This study addresses this issue through the investigation of the Sünnet-W landslide in terms of age and dynamic triggering conditions in NW Anatolia where similar large bedrock failures abound. This landslide is a rotational failure with a volume of 5.75 million m3 and developed in the Jurassic-Cretaceous carbonate successions 16 km off the NAF. Radiocarbon dating of the earliest sediments of the associated dam lake upstream yields a calibrated age of 8000 ± 35 yr BP for the landslide formation. Pseudo-static back analysis of the failure based on the pre-slide morphology, strength, and discontinuity density of the bedrock revealed horizontal accelerations of 0.484 g and 0.976 g for the initiation of failure. The steep topography and especially the height of the failed slope imply that a topographic amplification of 1.5 times would be reasonable based on the previous numerical models. Moreover, the paleoclimatological conditions of the time are estimated not to be sufficient for the complete saturation of the deep sliding surface. Even after the consideration of these site-specific encouraging conditions, a threshold magnitude of about 8.0 for the triggering earthquake of the Sünnet-W landslide is suggested. This estimate is at least six times larger than the anticipation of the previous paleoseismological studies (M7.4) from the western part of the NAF. We suggest that the triggering earthquake of the Sünnet landslides 8 ky ago may have been a huge cascade rupture that involves many, if not all, of the segments between Erdek and Niksar along the western and central NAF. This type of multi-segment rupture was previously conceptualized throughout the NAF but remained unexemplified up to date. The present study demonstrates that they should be seriously traced in far-reaching paleoseismological records due to their huge impacts on the seismic hazard of the region.