内陸堆積層の分布高度から求めた中部九州地溝内沈降域の変位  [in Japanese] Displacement of the depressed areas in the graben, estimated by the altitude of the inland sediments in central Kyushu, Japan  [in Japanese]

中部九州の玖珠盆地の堆積層と久住および大分川上流域の地下を構成する堆積層との対比から, 玖珠盆地に比べて両地域がそれぞれ約300万年前以降0.2 mm/yrおよび約120万年前以降1.1 mm/yrの割合で沈降していることが明らかになった。これらの変位は負の重力異常から考えられている久住および庄内沈降帯の沈降によるものと考えられる。同時期の内湾性堆積層である大分市鶴崎地域の鶴崎層と熊本平野地下の水前寺層間の変位は1.2 mm/yrと見積られる。さらに, 熊本地域の中期更新世の古地理を復元して, 当時形成された堆積層の推定高度と現在の分布高度の差から水前寺地区で0.4 mm/yr, 力合地区で0.7 mm/yrの変位が考えられる。

There are many lacustrine deposits and some sediments formed in bays in the Late Cenozoic era in the graben of central Kyushu. The formations distributed in the underground of the southern foot of Mt. Kuju and in the upper stream of the Oita River are correlated with the formations formed in the Pliocene and Pleistocene periods in the Kusu Basin based on their stratigraphic positions. The difference between the altitude of the tops of the Otagawa, Kimida, Iwamuro, Hosenji and Nogami Formations in the Kusu Basin and those of their equivalents in the southern foot of Mt. Kuju are assumed to show the depression of the southern part of Mt. Kuju against the Kusu Basin at the rate of O.2 mm/year during 3 million years. In the same way, the difference between the altitude of the Kimida, Iwamuro and Hosenji Formations in the Kusu Basin and their equivalents in the upper stream of the Oita River are assumed to show the depression of the upper stream of the Oita River at the rate of 1.1 mm/year during 1.2 million years. The Suizenji Formation in the middle Pleistocene in the underground of the Kumamoto Plain is correlated with the Tsurusaki Formation distributed in Oita city based on stratigraphic and paleoenvironmental characteristics. Based on the vertical drop, the rate of relative depression of the Suizenji Formation under the Rikogo area in Kumamoto city is 1.2 mm/year against the Tsurusaki Formation in the Tsurusaki plateau in Oita city and 1.O mm/year against the Tsumori Formation on the other side of the Futagawa Fault during about O.35 million years. The Suizenji Formation is depressing from its original level of deposition at the rate of 0.7 mm/year in the Rikigo area and of 0.4 mm/year in the Suizenji area.