Abstract
In this paper, the paleo-environmental changes were presumed by the diatom assemblages taken from the tsunami borehole deposits of 1960 Chili Earthquake Tsunami at the three sites in the Kesennuma Bay, along the Pacific Coast, north east Japan. The tsunami deposits contained a lot of fresh water diatom valves, and their volumes to the all diatom valves exceed 60 %. Because the average frequency of fresh water diatoms in the present bay surface deposits are about 20 %, the tsunami deposits composed by mainly inland deposits those were washed into the bay by backwashed current of tsunami waves. In addition to it, marine species such as Thalassiosira spp., those live at offshore areas along the Pacific Coast increased cyclically in the cores of tsunami deposits. Those peaks of marine species presumably showed the repetition of tsunami waves during the tsunami hazard. The 1960 Chili Earthquake Tsunami continued about two days, and tsunami waves attacked with 1-2 hours intervals to the Kesennuma area.
The three cores of tsunami deposits could not reach the basement of the tsunami deposits presume by diatom assemblages. We planned the further drilling in this area, however the bay topography of the bay was drastically changed by the East Japan Earthquake Tsunami, caused on March 11, 2011. Our drilling data archived the sedimentary conditions of the bay just before the huge tsunami in 2011.
