Based on field surveys and satellite-geodetic observations, small surface ruptures associated with the 2016 Kumamoto earthquake sequence were widely identified on unknown distributed faults up to ～15 km further from the main rupture zone. In the Aso caldera, we observed small surface ruptures, which were firstly identified as the short discontinuous traces of fringes of interferometric synthetic aperture radar (InSAR) images, around Miyaji, Aso City, Kumamoto Prefecture. These surface ruptures show vertical and dextral displacements ＜10 cm. To reveal the characteristics of the subsurface continuity of surface ruptures and cumulative slip, we excavated a pit across the surface breaks. As a result, the pit walls exposed flower structures that are composed of near-vertical faults and indicates a dextral slip. At a depth of 1.5 m, a measured vertical separation of the strata is 15-20 cm, which is three times larger than that on the ground surface. We then depicted the relation between the depth and the vertical displacement, suggesting that fault displacement decreases toward ground surface. Consequently, we interpret that the dextral fault movement resulted in surface ruptures around Miyaji. In addition, the tephra and radiocarbon dating revealed that the 2016 Kumamoto earthquake sequence is the only surface-rupturing event at the pit site since 1700-1900 cal. BP.
