熊本平野地下における第4系の最近15万年間の沈降速度

概要

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Many active faults are distributed in and around the Kumamoto Plain (Watanabe et al., 1979; Watanabe, 1984, 1987 and others). In this study, more than 50 drilling cores were re-examined, and some valuable key beds for correlation and age determination were recognized. They are the Akahoya Ash, which originated from the Kikai caldera about 6, 300 years BP, and Aso-1 to Aso-4 pyroclastic flow deposits, which were erupted from the Aso caldera in the past 300, 000 years. From their radiometric ages and differences in depths between the drilling cores, the subsidence rate of the Quaternary deposits in the Kumamoto Plain has been estimated. The average subsidence rate is about 0.2-0.5mm/year in the southern part of the Kumamoto Plain, which corresponds to the Kiyama-Kashima graben (Watanabe et al., 1979) and westward. In the western part of the Kumamoto Plain, the subsidence rate has been calculated at 0.2-0.3mm/year. The subsidence of these Quaternary deposits was probably caused by the movement of some active faults below the ground surface.
日本第四紀学会の論文