Permanent seismic displacements in volcanic lake-bed deposits of Aso Caldera, Kumamoto, Japan

Ground deformation in the Aso Caldera triggered by the 2016 M7 Kumamoto earthquake formed a graben-structure approximately 10-km-long. This ‘depression’-zone projects northeastward from where the Futagawa Fault ruptured into the caldera to the northern end of the caldera. Several competing models for the cause of the graben-structure have been proposed, including vertical fault offset (Lin et al. 2016) and horizontal sliding (Tsuji et al., 2016). In this paper, we explore the hypothesis that the extensional boundary of a large seismic displacement. We investigated the area north and downslope of the graben for evidence of compression and modeled the potential for large displacements on gentle slopes within a multidirectional compliant seismic displacement analysis. Compression occurred continuously near the Kurogawa river, north of the graben. Ground motions recorded near the graben indicate directivity effects that create a large velocity pulse capable of driving ground displacements toward the north, consistent with field observations.