Four surface ruptures associated with the 2016 Kumamoto earthquake that extend across the alluvial plain of Kiyama River in Mashiki Town, Japan, are investigated via a seismic reflection survey to elucidate their fault subsurface geometries and their interrelationship with each other. Three of these surface ruptures are subparallel, ENE-WSW-striking faults: a right-lateral fault along the southern margin of the plain (fault Fb), a normal fault on the flank of the southern mountainous area (fault Fa), and a right-lateral fault along the northern part of the plain (fault Fc). The faults Fa and Fb correspond to the previously mapped Futagawa fault that are identified by fault landforms. However, the fault Fc is not topographically defined. The fourth surface rupture (fault Fx) is highly oblique to the ENE-WSW-striking faults and cuts across the alluvial plain left laterally, extending from fault Fb to fault Fc. The processed seismic reflection profiles image a basin structure beneath the alluvial plain that is bounded by the faults Fb and Fc. The near-vertical right-lateral fault Fb and a north-dipping normal fault Fa converge at 350 m depth. The fault Fc may be the Kiyama fault due to its location and south-dipping structure. A comparison of the coseismic displacement during the 2016 earthquake and the subsurface geometry of the fault traces highlights that the oblique slip on the north-dipping source fault of the Kumamoto earthquake may be partitioned into the three subparallel faults at the surface. Therefore, it is important to consider the presence of subparallel slip-partitioned faults in a given area and comprehensively evaluate the amount of slip along each fault when conducting seismic hazard assessments via active fault investigations.
