Seismic liquefaction assessment in transitional fluvial–lacustrine depositional settings: A case study from the western shoreline of Lake Iznik, Türkiye

Seismic liquefaction assessment in transitional depositional environments requires careful consideration of complex lithology and sedimentological variability. This study presents a detailed liquefaction potential evaluation of Holocene fluvial–lacustrine deposits along the western shoreline of Lake Iznik, Türkiye. The area features intricate interbedding of fluvial sands, gravels with fluvial/lacustrine silts and clays, shaped by dynamic sedimentary processes including fluctuating water levels and variable sediment supply. Nine boreholes were investigated through Standard Penetration Tests (SPT) and laboratory analyses to characterize the heterogeneous subsurface conditions. Liquefaction susceptibility was assessed using Youd and Perkins's geological and geomorphological criteria, Iwasaki's gradation curve criteria, multiple SPT-based methods, complemented by Bray and Sancio's criteria for fine-grained soils and Ishihara's models for surface manifestations. Findings reveal that loosely packed sandy soils are highly vulnerable to liquefaction, while lacustrine fine-grained layers exhibit moderate susceptibility influenced by plasticity and moisture content. Importantly, non-liquefiable deposits are present near the surface, acting as protective caps that influence the development and severity of surface manifestations. Liquefaction behavior in transitional fluvial–lacustrine settings is shown to be highly site-specific, controlled by depositional heterogeneity rather than random variability. These findings provide novel insight and practical guidance for reliable hazard assessment in complex sedimentary environments.