[MIS11-P20] Tsunami deposits detection along the west coast of Kyushu, Japan
Keywords:Kyushu, event deposit, tsunami deposit
We conducted a boring survey at 14 areas on the northwest coastal region of Kyushu and identified event deposits at 6 areas (Nijinomatsubara: ca. 7.1/2.3/2.2/0.6ka, Minatomachi: ca. 6.2/3.9/3.4ka, Fuonji: ca. 8.2/6.4/6.1ka, Hoshikachoshimodamen: 4 layers about 7.0ka under study, Ikitsukichomisaki: 3 layers about 7.0ka under study, Yahataura: ca. 2.8/0.8ka). According to the tsunami deposit identification flow (Goto et al., 2017), event layer of Minatomachi ca. 6.2 ka was of “There are any materials that are characterized by landward sediment transport from the sea (group B),” and others were of “Presence of characteristic sedimentary features such as basement erosion and upward grading is evaluated (group C).” Additionally, at the Minatomachi ca. 3.4 ka event layer have a possibility of a correlation with event layer of ca. 3.5ka reported at Iki and Shimonoseki [URL1].
We conducted a boring survey at 16 areas on the southwest coastal region of Kyushu and identified event deposits at 6 areas (Teuchi: 9 layers between ca. 6.7-0.6ka, Nakayama: many layers between ca. 6.2-0.5ka, Nakakoshiki: ca. 4.3/3.8/3.4/2.5ka, Hashima: ca. 7.4/7.3ka, Ikedaike: many layers between ca. 8.0-0.5ka, Gumizakicho: 12 layers between ca. 9.6-1.3ka). According to the tsunami deposit identification flow, event layers of Hashima at ca. 7.3ka and Gumizakicho at ca. 7.3/6.3ka were of “The deposit may show evidence of distinctive tsunami origin in some cases (group A1),” the several layers between Nakayama at ca. 3.0-0.5ka, the Nakakoshiki ca. 4.3ka, and Hashima and Gumizakicho ca. 7.4ka were of “group B,” and others were of “group C” or “The deposit that was instantaneously formed during a geological event (group X).” Hashima and Gumizakicho at ca. 7.4ka, Hashima, Gumizakicho, and Ikedaike at ca. 7.3ka, Teuchi at ca. 6.2ka and Gumizakicho at ca. 6.3ka, and Teuchi and Nakakoshiki ca. 3.4ka event layers have a possibility of correlation with each other. Additionally, Hashima, Gumizakicho and Ikedaike ca. 7.3 ka event layer include Kikai-Akahoya Tephra-derived pumices and volcanic glasses that are not found in a lower layer, and may be correlated of 7.3ka Kikai caldera eruption-related event (e.g., Maeno et al., 2006; Naruo and Kobayashi, 2002).
Acknowledgments: We are thankful to Central Research Institute of Electric Power Industry for kindly permitting us to use their X-ray CT imaging system in this study.
References: Goto, K. et al., 2017, Tsunami engineering technical report, 33, 45-54.; Maeno, F. et al., 2006, Earth, Planets and Space, 58, 1013-1024.; Naruo, H. and Kobayashi, T., 2002, Quaternary Research 41, 4, 287-299.
[URL1]: Earthquake Research Institute, University of Tokyo "The integrated research project on seismic and tsunami hazards around the Sea of Japan" 2017 First Steering Committee Documents "2-1-2 Tsunami Deposits Survey", Urabe, A., 2017.9.29, http://www.eri.u-tokyo.ac.jp/project/Japan_Sea/2-1-2nihonkai_170929.pdf