DIFFERENTIATING TYPHOON AND TSUNAMI DEPOSITS IN LAKE RYUJIN, KYUSHU, JAPAN

Developing a protocol for differentiating the source of sandy deposits in coastal lakes is vital for estimating intensities and recurrence intervals for typhoons and tsunamis in order to improve coastal hazard assessment. Lake Ryujin is a back-barrier coastal lake in Kyushu, Japan that has been inundated by both typhoons and tsunamis within the historical period. This lake has preserved several potential typhoon and tsunami deposits, including a deposit associated with the 1707 Hoei earthquake, which is widely considered to be the Nankai Trough event of record. Recent research shows that the Nankai trough may produce an event with a Magnitude 8-9, similar to that of the Tohoku event of 2011, in the next few decades. Historical records can be helpful in identifying the coastal hazard risk, however, they can often be sparse or unreliable. The sedimentary record of coastal lakes can help fill in the gaps and extend the record. Identifying past tsunamis from sedimentary deposits in southern Japan is complicated due to the frequent typhoons that also impact the area making protocols for differentiating the two events essential. In this study sandy deposits from cores are analyzed for grainsize, deposit thickness, and elemental composition in order to identify a possible source. Typhoons are associated with large rainfall events that produce deposits that will have a fluvial elemental signature due to higher river discharge. Tsunami deposits in Lake Ryujin are from wave inundation through an inlet and will have a marine elemental signature with thinning away from the inlet. Distinguishing between typhoon and tsunami deposits in Lake Ryujin will enable an assessment of the two hazards at the site.