SEDIMENTOLOGICAL IMPACTS OF LARGE SCALE INUNDATION EVENTS: RECORDS OF TSUNAMI AND TYPHOON FLOODING FROM LAKE RYUUOO, JAPAN

Along the Nankai Trough, the subduction of the Filipino plate beneath the Eurasian plate generates large earthquakes (M_W>8), which are accompanied by tsunamis that flood coastal areas in southwestern Japan every ~100 to 150 years. Historical eyewitness accounts from this area provide estimates for maximum run-up elevations for events more recent than 1605 AD. A tsunami generated by the 1707 Hoei earthquake is largely thought to be the most extreme flooding within this time period, although observations are limited.

To provide further constraints on the 1707 AD event, we have collected three sediment cores from Lake Ryuuoo (33.372°N, 132.360°E), a freshwater pond located within the Bungo Channel on the Pacific coast of southwestern Japan. The lake is susceptible to flooding from Nankai Trough earthquake-generated tsunamis but is somewhat sheltered from a direct typhoon landfall; thus, major event deposits in Lake Ryuuoo are likely tsunami deposits. Based on a multi-proxy approach for identifying event deposits that includes loss on ignition, X-Ray fluorescence, X-radiograph, grain size, and magnetic susceptibility, preliminary results from one of the Lake Ryuuoo cores yield a record that spans from ~1070 AD – present and contains 18 event deposits. The deposits consistently correspond to spikes in grain size, percent inorganic sediment, strontium abundance, and magnetic susceptibility in the core.

Here, we present a record of coastal flooding for Lake Ryuuoo from all three cores, using the same multi-proxy approach. We compare the sedimentary signatures of individual event layers across the three cores in order to identify spatial grain size trends. These results will be used to inversely model the tsunami heights required to produce observed grain size trends, and model results for historic tsunamis will be compared with eyewitness accounts to evaluate the model’s accuracy.