Slip rates and late Quaternary activity of the Unzen active faults in western Kyushu

概要

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The Unzen Graben, which is bounded by normal faults to the north and south, is located at the western end of the Beppu-Shimabara Graben. The faults in the Unzen Graben have developed in association with the growth of the Unzen volcanoes and deformed volcanic materials such as lavas and pyroclastic deposits. The detailed location and vertical offsets of the active faults have been reported by previous studies, but the timing of faulting was poorly constrained. In this study, we calculate vertical slip rates of the faults based on recently obtained radiometric ages of volcanic rocks and sediments. The active faults in the Unzen Graben can be divided into five groups, based on the slip rates and the timing of faulting. The faults located on the northwestern and southeastern margins of the Unzen Graben have vertical slip rates as high as 0.12 to 1.0 mm/yr. However, the faults located on the northeastern margin of the graben are less active and have slip rates of 0.28 to 0.38 mm/yr. The faults inside the graben have slip rates of 0.085 to 0.46 mm/yr, except for the Akamatsudani fault which has a slip rate of 0.91 to 1.7 mm/yr. These faults show cumulative vertical offsets and their slip rates tend to be higher closer to the Fugen Volcano. On the other hand, the faults outside the Unzen Graben have considerably lower slip rates of 0.026 to 0.068 mm/yr.