During the 16 April Kumamoto earthquake (Mj7.3), ～30-km-long surface ruptures with right-lateral slip appeared along the previously mapped Futagawa and Hinagu faults. In Mashiki Town, surface ruptures also appeared north of the Futagawa fault across the alluvial plain of the Kiyama River where no tectonic geomorphic features were identified. In order to reveal shallow subsurface structure and past movements of the 2016 surface ruptures north of the Futagawa fault, we conducted a trenching survey at Jichu, Mashiki Town. Fluvial deposits derived from the Kiyama River and Aso-4 pyroclastic flow deposits were deformed by moderately to steeply south-dipping reverse faults. While the vertical offset during the 2016 earthquake was ～30 cm up on the south, older strata exposed on the trench walls were offset more than 2 m, suggesting that reverse faulting events occurred repeatedly in the late Quaternary. Based on the deformational features of the exposed strata, we identified three surface-rupturing events, including the 2016 earthquake. Radiocarbon dating of the strata suggests that at least two faulting events occurred after ～9,000 yBP. Since there were no strata deposited after ～8,500 yBP, it is possible that there were more seismic events than we identified during the middle to late Holocene. We suggest that the small vertical displacement associated with individual events, sedimentation/erosion by the Kiyama River, and artificial modification contributed to no tectonic geomorphic features along the surface rupture that appeared across the alluvial plain of the Kiyama River.
