Radiocarbon dating of coastal boulders from Kouzushima and Miyake islands off Tokyo Metropolitan Area, Japan: Implications for coastal hazard risk

Over the last two decades, two giant tsunamis generated by earthquakes of approximately Mw 9 (the 2004 Sumatra–Andaman and 2011 Tohoku earthquakes), and a large storm surge associated with the 2013 Super Typhoon Haiyan, have caused catastrophic damage to infrastructure, property, and industry in many areas of the western Pacific. If we are to improve coastal hazard assessment for the Pacific coast of the southwestern Japanese mainland, a reconstruction of the history of tsunamis and storm surges during the late Holocene is required. This study surveyed coastal boulders to determine whether such events have previously affected the islands of Niijima, Shikinejima, Kouzushima, and Miyakejima that lie offshore from the Tokyo Metropolitan Area. Coastal boulders on Kouzushima and Miyakejima were found with marine organisms attached. Radiocarbon dating of these organisms indicates that the boulders were emplaced during the periods AD 1694–Modern and Modern, respectively. The boulder on Kouzushima (13.3 ton at 1.4 m ground elevation) was transported by historical tsunamis or severe storm surges, whereas the boulder on Miyakejima (33.4 ton at 7.1 m ground elevation) was probably transported by a storm surge associated with Typhoon 7920 in 1979. An emerged marine sessile assemblage on Miyakejima (2.31–3.06 m above mean sea-level (amsl)) was dated to ca. 3900–3500 years BP. This high relative sea-level can be explained by a mid-Holocene highstand and uplift associated with volcanic activity.