3. Results
All the sections of the core were composed of clayey
materials intercalated by several thin turbidite and
volcanic ash layers. Based on sedimentary structure
and authigenic mineral composition observed under the
light microscope, a turbidite layer at 115–108 cm deep
was correlated with that formed by the historical
earthquake in 1662 AD detected by Fukusawa et al.
(1994). The turbidite is overlain by brackish sediments,
because seawater began to come up through the artificial
canal constructed 2 years after the earthquake (Editorial
Committee for History of Mikata Town, 1990). X-ray
images of the sediment core (Fig. 1) revealed laminated structures, especially in the uppermost sections from 12
to 161 cm deep. Based on counting these varves, detailed
chronology with 1-year resolution was established, from
1590 AD to 1729 AD. A tephra layer intercalated at
44 cm deep corresponds to 1774 AD based on the
chronology. Among these laminated sections, distinct
differences in lamina structure and thickness were
observed, with 1–2mm thick laminae with very clear
contrast of dark- and light-colored laminae at 21–25,
31–32, 38–45 cm deep, and 3–6mm thick gradually
changing to the base of superimposed lamina at 32–38,
45–108, and 118–161 cm depth (Fig. 1). In the lower
sections, including turbidite layers, laminated structure
was not recognized on X-ray images. (p.22)