Tsunami History Associated with Oceanic Intraplate Earthquakes in Beppu Bay, Eastern Kyushu, Japan

Results of earlier acoustic and coring surveys in Beppu Bay suggest that active faults in the center of the bay have been active five times in the past 7000 years. Historical written records document that the tsunami generated by the latest earthquake in AD1596 (M=6.9) affected the entire coastal area of the bay. The aim of this study was to establish a detailed chronology of tsunamigenic intraplate earthquakes that occurred in the bay before AD1596 using data from onshore sediment cores. Ten sediment cores, up to 8.8 m long, were obtained from a 100 m wide coastal freshwater marsh located on the south coast of the bay. At the coring site 170 m from the shoreline, there was a thick organic-rich mud layer at 2.1-4.6 m depth between the recent agricultural soil and 7300-year-old K-Ah tephra, which also contained a 5700-year-old scoria (Kj-DS). Three thin sand layers observed in the organic-rich mud exhibited sharp upper and lower contacts with the surrounding mud, indicating that they were most probably deposited by sudden events. They were characterized by higher magnetic susceptibility associated with higher counts of titanium and iron, as well as higher counts of calcium and strontium, suggesting that the sand grains were transported from different environments. Brackish-marine benthic diatom assemblages in the sand layers support the hypothesis that the sand units have, at least partly, a marine source. The sedimentary record since the K-Ah tephra includes at least three probable prehistorical tsunami deposits from 3300-3450 cal. yr BP, 4400-4530 cal. yr BP, and 5030-5290 cal. yr BP. As the material for radiocarbon dating was collected from just beneath and above the event deposits, results provide higher resolution ages than those from offshore seabed studies.