HR: 0800h
AN: T11A-0352    [Abstracts]
TI: Map of Tsunami Deposits along Nankai and Sagami Troughs, Japan
AU: * Komatsubara, J
EM: j.komatsubara@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: Fujiwara, O
EM: o.fujiwara@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AU: Kamataki, T
EM: t.kamataki@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, Higashi, Tsukuba, Ibaraki, 305-8567 Japan
AB: We compiled published data and our own preliminary survey results as a map showing the temporal and spatial distributions of tsunami deposits on the Pacific coast of Central to Southwest Japan along Sagami and Nankai troughs. Historical and prehistorical tsunami deposits have been reported from at least 14 sites of coastal lowland including marshes, ponds, lagoons and excavated archaeological sites. Most of them have been found from cored samples with some exception of the emerged Holocene marine successions in a coseismically uplifted area. Tsunami deposits have been recognized as coarse-grained beds mainly composed of sands and gravels in muddy host sediments. Major criterion for identifying the trace of tsunamis is correlation with historical records and coseisimic crustal movements based on their 14C ages. Reported tsunami deposits may include misidentified storm deposits, because the identification of the two is still difficult. Our map shows following insights into the Holocene tsunami history in SW Japan, though it involves above mentioned problems. 1) About 3000-year-long geological history have been revealed from several sites. They are composed of various types of tsunami deposit, from thin (< 10 cm) sand sheets to a conglomeratic mound exceeding 4 m thickness. The longest record, tracing back to nearly 10,000 years ago, was obtained from the Boso Peninsula, east end of the study area. 2) Many of the tsunami deposit were reported from eastern part of the target area. Relatively small amount of data was obtained from western area including Shikoku and Kyushu islands. 3) Recurrence interval of tsunami deposits was generally calculated as 100-200 years_@at the shortest, in good agreement with historical records, although it is often longer than 500 years in prehistorical age. This apparent inconsistency is mainly attributed to the insufficient geological data. More paleo-tsunami data from various depositional sites and synthesis of various types of earthquake-induced event deposits, such as trace of liquefactions and offshore seismo-turbidities, will improve our understanding of earthquake history of the region.
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005