Late Quaternary movement of active faults in the Unzen Graben, western Kyushu, based on trenching study at Chijiwa Town and observation of a fault outcrop at the north slope of the Mt. Fugendake

Abstract

The Unzen Graben is located on the western end of the Beppu-Shimabara Graben and is bounded by normal faults on the north and south sides. The faults in the Unzen Graben have developed in association with the growth of the Unzen volcanoes and offset volcanic materials such as lavas and pyroclastic deposits. The detailed location and amount of offsets of these active faults have been reported by previous studies, but the timing of faulting was poorly constrained.
The Ogura fault is located on the northern margin of the Unzen Graben and offsets the alluvial fan surfaces formed by the Chijiwa River. We excavated a trench across the Ogura fault at Chijiwa Town in order to reveal the timing of recent faulting events. The faults exposed on the trench walls strike N70° W-EW and dip 70° -90° S, and offset the terrace gravels of the Chijiwa River. These faults may have moved after the deposition of the AT volcanic ash (26-29ka), which was found in fine sands filling coseismic open cracks.
We also found an outcrop across the Kusenbu fault inside the Unzen Graben. A layer containing K-Ah volcanic ash (7.3ka) is offset by the Kusenbu fault, suggesting that the fault has moved during the Holocene.