Many spike-like anomalies of grain densities and clay mineral concentrations in silty sediments have been recognized in the uppermost part (60cm in length) of a piston core, named SG2, in Lake Suigetsu of Mikata area, Fukui Prefecture, central Japan. Grain density spikes indicate large precipitations of iron sulfides and carbonates from interstitial waters in the upper part of muddy turbidites, which were suddenly caused by earthquakes. Spikes of Fe- and Mgrich chlorite/illite ratio indicate large accumulations of detrital clay particles which flowed into lake by sudden flood.
After the Urami Canal was constructed between Lake Suigetsu and Lake Kukushi, which connects both lakes directly to the Sea of Japan, in 1664 A. D., sea water began to enter Lake Suigetsu and the lake changed its water conditions from fresh-to brackish-water. Iron mineral concentrations changed from siderite (FeCO3
) to pyrite (FeS2
) which was formed under sulfate reduction condition, in 35cm depth of a core column. Core column from 29cm to 35cm in depth shows a silty layer. Macroscopic observations and historical informations presumed a silty layer to be a muddy turbidite which was probably formed by Kanbun Earthquake (June 6, 1662 A. D.). This interpretation suggests that 29cm depth of core column represents deposition surface in 1662 A. D.
Based on relative spacing between 0cm (=1991 A. D.) and 29cm (=1662 A. D.) depths, formation ages of grain density and chlorite/illite ratio spikes can be dated. Grain density spikes are interpreted to represent historical earthquake events of 1449, 1532, 1586, 1605 (Keicho Earthquake), 1662, 1683, 1707 (Houei Earthquake), 1819, 1854 (Ansei Earthquake), 1891 (Nou-bi Earthquake), 1909 (Gonou Earthquake) and 1963 (Echizenmisakioki Earthquake) of the Wakasa region. Chlorite/Mite ratio spikes are interpreted to represent historical flood events of 1633, 1691, 1701, 1729, 1735, 1786, 1791, 1825, 1842, 1853, 1866, 1895, 1953 (Typhoon No.13), 1959 (Isewan Typhoon), 1965 (Typhoon No.23 and No.24) in the Wakasa region. Five spikes cannot be correlated to historical earthquakes and floods. These events that represent depositions in 1646, 1799, 1848, 1931 and 1936 are probably correlated with historical engineering works of 1642, 1801, 1848, 1932 and 1934-35 around Lake Suigetsu.
All events suggest that non-bioturbated fine-graind sediments of meromictic lake near human communities recorded any earthquakes, floods and human activities over a long periods of which no historical literature exists.
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