A 7,500-year-long record of extreme wave events from tidal Lake Hamana, south-central Japan

Tidal Lake Hamana is located near the Nankai-Suruga Trough, which is repeatedly affected by tsunamogenic megathrust earthquakes. The lake represents a good sedimentary archive for past earthquakes and extreme wave events (EWEs), triggered by tsunamis and tropical storms (typhoons). By applying a broad range of surveying methods, sedimentological analyses and dating techniques, a 7.5-kyr-long record of ≥ 22 megathrust and EWE deposits was extracted. Event deposits are embedded in fine-grained (silty) lacustrine sediments and have a thickness between 1 and ~50 cm. They are often sandy and typically display an erosive base, a fining-upward sequence, semi-parallel to chaotic or deformational layering, stronger X-ray attenuation, an increased magnetic susceptibility and high values for ratios of Ca/Fe and Sr/Fe. They correspond to strong reflectors on seismic images and are interpreted as products of EWEs (sandy, fining-upward) and earthquake shaking (silty, deformed). Most event layers become thinner or even disappear and/or become finer-grained towards ocean-distal lake sites. Radionuclide dating and independent tephrostratigraphy show that ages of event deposits go back to 7.5 ka BP, with main recurrence modes of 150-200 years. Especially going back beyond 5 ka BP, the Lake Hamana record adds complementary information to existing megathrust and EWE records along the Eastern Nankai Trough.