HR: 13:55h
AN: G22D-02    [PDF]
TI: Recurrent Coastal Uplift Events in Eastern Hokkaido, along Kuril Subduction Zone, as Inferred from Litho- and Biostratigraphy
AU: * Sawai, Y
EM: yuki.sawai@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Nasu, H
EM: nhiroo@nichibun.ac.jp
AF: Department of Japanese Studies, The Graduate University for Advanced Studies (Sokendai), 3-2 Oeyama-cho, Goryo, Nishikyo-ku, Kyoto, 610-1192 Japan
AU: Satake, K
EM: kenji.satake@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Shishikura, M
EM: m.shishikura@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Kamataki, T
EM: t.kamataki@aist.go.jp
AF: Active Fault Research Center, GSJ/AIST, Site C7 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Horton, B P
EM: b.p.horton@durham.ac.uk
AF: Department of Geography, University of Durham, South Road, Durham, DH1 3LE United Kingdom
AB: Litho- and biostratigraphy beneath coastal salt marshes in eastern Hokkaido record at least six emergence events during the past 4000 years, probably resulted from deep aseismic slip following infrequent unusual earthquakes along the Kuril subduction zone. The stratigraphy in Akkeshi and Onnetoh marshes was studied using over 50 cores. Four emergence events in Akkeshi were marked by estuary-wide stratigraphic contacts between mud and peat layers, and by diatom assemblages in the deposits. Sharp peat-over-mud contacts clearly separate the diatom dominance: salt-tolerant diatoms in lower mud and freshwater diatoms in upper peat. Radiocarbon ages of plant macrofossils and tephra show the dates of these four events are 2300-1700, 1300-1000, 700-500 and ca. 300 cal yr B.P. In Onnetoh, about 70 km east, six similar emergent events were identified. They provided contrasting diatom and plant macrofossil assemblages. Salt-tolerant diatoms and vascular plants dominated in mud below the contact, whereas a conifer Picea glehnii and freshwater-aerophilous diatoms dominate in peat above the contacts. Radiocarbon ages and tephra show that these six events, characterized by gradual changes in environment from brackish water to freshwater forests, date to 3900-3600, 2700-2400, 2000-1700, 1400-1000, 700-500, and 300 cal yr B.P. Such emergence events may solve a conflict of vertical crustal movement in eastern Hokkaido. This tectonically active area, where the Pacific plate subducts at 8 cm/yr, has been steady submerging at fast rate of 8-9 mm/yr in the twentieth century as recorded by tide gauges. Pleistocene marine terraces, however, imply 0.3-0.5 mm/yr of net uplift during the past 125,000 years. To balance these two opposite movements, emergence events as found this study were needed. The coastal uplift, the most likely cause of the emergence in this area, might be generated by infrequent a-seismic slip in the Kuril subduction zones, although such large-scale emergence events have not been recorded in interplate earthquake cycle of 19-20th century. Unusual tsunami deposits, recently found along the Pacific coast of eastern Hokkaido (Nanayama et al., 2003) support this hypothesis. The tsunami deposits indicate that unusual earthquake occurred every about 500 years over the past 7000 years. The interval of the unusual earthquake coincided with the emergence events of this study.
DE: 7221 Paleoseismology
SC: Geodesy [G]
MN: 2003 Fall Meeting