[M-IS23_2PM1] 津波堆積物

[MIS23-15] The assemblages of foraminifera in paleo-tsunami sediments on Ishigaki island

*Tu Yoko¹、安藤 雅孝²、Chien Chih-Wei³、北村 晃寿⁴、宍倉 正展⁵、中村 衛⁶、新城 安尚⁶ (1.Department of Natural History Sciences, Hokkaido University, Japan.、2.Center for Integrated Research and Education of Natural Hazards, Shizuoka University, Japan、3.Department of Earth Sciences, National Chen Kung University, Taiwan、4.Department of Geosciences, Shizuoka University, Japan、5.Activity Fault and Earthquake Research Center, The National Institute of Advanced Industrial Scie、6.Department of Physics and Earth Sciences, Ryukyu University, Japan)

キーワード：tsunami sediments, foraminifera, Ryukyu dubduction zone, paleo-tsunami, 1771 tsunami

The Ryukyu subduction zone is generally believed to be aseismic because no large thrust earthquake (M>8) has recently occurred; GPS velocity vectors on the islands are parallel but opposite to the relative motion of the oceanic plate. These observations support the idea that the Ryukyu trench is aseismic or unlocked. However, in 1771 a tsunami struck Ishigaki and Miyako islands with the maximum run-up height of 30 m and caused destructive disaster, which implies that a significant earthquake occurred along the Ryukyu subduction zone. According to Nakamura (2009), the source of this event is a tsunami (slow) earthquake near the Ryukyu trench. Moreover, slow-slip events at depths of 30km (Heki and Kataoka, 2009) and very-low frequency earthquakes at shallow depths near the trench axis (Ando et al., 2012) have been identified in the western Ryukyu trench. These findings suggest that the western Ryukyu subduction zone has a potential to generate large thrust earthquakes. To estimate recurrence intervals and sizes of paleo-tsunamis near the Ryukyu trench, we excavated Holocene deposits at 5 sites on Ishigaki Island during the years of 2011 to 2013. We analyzed the assemblages of foraminifera in the sediments that were transported by tsunamis from the deep seafloor. Most of foraminifera detected from the deposits are benthonic and planktonic foraminifera are rare in all samples at the excavation sites. Species of benthonic foraminifer such as Calcarina defrancii (living at 15 to 50 m depths) are dominant in the tsunami deposits compared to the current beach sand. In addition, some mesopelagic species that commonly live at continental shelf depths are also identified from the tsunami sediments. We found that the percentage of mid epipelagic and mesopelagic species in the deposits can provide a significant key to identify paleo-tsunamis. On the western Ishigaki Island, if the population density of these species in a deposit exceeds 10 %, it can be concluded as a tsunami origin, while on the eastern coast if the population density exceeds 20 %, it can be a tsunami deposit because of the bathymetric reasons. Together with the results of stratigaraphic facies and C14 dating data of the above tsunami sediments, we identified three large tsunamis (similar to the 1771 tsunami) in the past 2000-3000 years: in 1771, between 10-11^th C and between 2000 and 2900 cal. B.P. The average recurrence interval of large earthquake was found to be very long, 500 to 1000-2000 years along the western Ryukyu trench.