Paleoseismic trenching on slip-partitioned surface ruptures associated with the 2016 Kumamoto earthquake

Surface ruptures appear over a wide area in addition to the primary fault during a Large earthquake like the 2016 Kumamoto earthquake. Although the displacement of such distributed surface ruptures is small, information on their paleo activities provides clues for evaluating displacement hazard risk and whether they can be used as a paleoseismic history of the primary fault. We conducted lidar differencing analysis and trench excavation on the Idenokuchi fault, which was activated simultaneously with the primary Futagawa fault, during the 2016 Kumamoto earthquake, and where a slip partition occurred. First, we clarified the 3D displacement field by lidar differencing and discussed quantitatively how the slip partition occurred on both faults. We found that deep oblique slip is completely split into horizontal and vertical components at the ground surface and inferred that the Idenokuchi fault is structurally connected to the Futagawa fault. Then, we excavated a trench on the conjugate surface rupture of the Idenokuchi fault and identified eight faulting events since 15 ka. Finally, we revealed a reliable activity history since 7.3 ka. Our results indicate that the Futagawa fault has ruptured relatively periodically and many surface ruptures have appeared in the last few events, like the 2016 event.