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Marine incursions of the past 1500 years and evidence of tsunamis at Suijin-numa, a coastal lake facing the Japan Trench

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan, yuki.sawai{at}aist.go.jp

Yushiro Fujii

International Institute of Seismology and Earthquake Engineering, Building Research Institute, 1 Tatehara, Tsukuba, Ibaraki 305-0802, Japan

Osamu Fujiwara

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

Takanobu Kamataki

OYO Corporation, Daitakubo 2-2-19, Minami-ku, Saitama, Saitama, 336-0015, Japan

Junko Komatsubara

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

Yukinobu Okamura

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

Kenji Satake

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

Masanobu Shishikura

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Site C7 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

Sandy deposits of marine origin underlie the floor of Suijin-numa, a coastal lake midway along the subduction zone marked by the Japan Trench. The deposits form three units that are interbedded with lacustrine peat and mud above a foundation of marine, probably littoral sand. Unlike the lacustrine deposits, all three sandy units contain marine and brackish diatoms. The middle unit (B) contains, in addition, graded beds suggestive of multiple waves of long wavelength and period. The uppermost unit (C) probably dates to a time in the area's written history when the lake was separated from the sea by a beach-ridge plain at least 0.5 km wide and several metres high. Units A and B postdate AD 540—870, and unit C postdates AD 1030—1640 as judged from radiocarbon dating of leaves and seeds. Unit B pre-dates AD 915 and unit C postdates that year as judged from a tephra within the peat that separates units B and C. The age constraints permit correlation of unit B with a tsunami in AD 869 that reportedly devastated at least 100 km of coast approximately centred on Sendai. Unit C may represent a later catastrophic tsunami in 1611, or perhaps a storm surge that inundated much of Sendai. The lake lacks obvious signs of tsunamis from the region's largest twentieth-century earthquakes, which were centred to the north in 1933 (M 8.1) and directly offshore in 1936 (M 7.5), and 1978 (Mw 7.6).

Key Words: Tsunami deposits • diatoms • coastal lakes • historical records • Japan Trench • late Holocene.

The Holocene, Vol. 18, No. 4, 517-528 (2008)
DOI: 10.1177/0959683608089206


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