2020 Volume 2020 Issue 52 Pages 1-8
On the 16 April Kumamoto earthquake (Mj7.3), ～31km-long right-lateral surface ruptures appeared along the previously mapped Futagawa and Hinagu faults. The surface ruptures appeared in Dozon, Mashiki Town, recording 2.2m of right-lateral displacement which is the maximum strike-slip displacement of these surface ruptures. Small surface deformations such as flexure of cultivated land and deformation of the waterway and left-lateral conjugated fault also appeared in this area. In order to reveal distribution and amount of small surface deformations, we created a digital surface model (DSM) based on photographs taken by unmanned aerial vehicle (UAV) and RTK-GPS survey and conducted a field survey. As a result, small and conjugated surface ruptures were observed about 100m northwest of the main trace of the strike-slip fault, and amount of these deformations are each about 5―30cm of north-down displacement. The amount of vertical offset of just above the main trace is 25―30cm of south-down offset but the total vertical offset in Dozon is a north-down vertical offset rather than a south-down when summing the vertical offset of the secondary trace and the main trace. We also conducted a trenching survey across the conjugated fault to reveal surface faulting history. While the vertical offset caused by the 2016 earthquake was 20cm down on the south, older strata exposed on the trench walls were offset more than 40cm. Based on the deformational features of exposed strata, we identified at least four faulting events including the 2016 earthquake. The timing of the event before the 2016 earthquake is 500―10,600yrsBP. It indicates that the conjugated fault is also cumulative. It is likely that the conjugated fault and small surface ruptures have repeatedly ruptured simultaneously with the main trace, because the conjugated fault follows the small surface ruptures and is consistent with the timing of events in the main trace.
