Late Quaternary average vertical slip rate of the Fukaya fault, central Japan, revealed by drilling survey of fluvial terraces

概要

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On the Arakawa Alluvial fan, northwestern part of the Kanto Plain, late Quaternary fluvial terraces are widely distributed along the Arakawa river and a part of them are deformed by the Fukaya fault. The ages of the terraces have rarely been reported in this area, due to the scarcity of outcrops and lack of visible tephras. In this paper, we carried out a drilling survey on terraces to obtain samples of the overlying loamy soil and upper part of terrace deposits. From these samples, we extracted some well-known widespread volcanic ash, from which we were able to estimate the approximate age of the terraces and the vertical slip rate of the active faults. Late Quaternary fluvial terraces in the Arakawa fan are divided into 5 levels: from the Osato terrace to the Hanazono terrace in descending order. Widespread tephras such as the Hr-Hp (50ka) is contained in the lowest part of the loamy soil in the Kushibiki terrace and the As-BP (20-25ka) is present in the lower part of the loamy soil in the Miizugahara 1 terrace. From the ages and the vertical displacements of the fluvial terraces, the late Quaternary average vertical slip rates of the Fakaya fault are estimated to be 0.25-0.30 m/kyr (Kushibiki terrace, the minimum value), and 0.36-0.45 m/kyr (Miizugahara 1 terrace), respectively.