Grabens that formed in a caldera of the Aso during the 2016 Kumamoto Earthquake in Japan

Abstract

Many grabens − approximately 50 m wide, several hundred meters long, and 1.5 m deep − formed in the caldera of the Aso Volcano during the 2016 Kumamoto Earthquake. Following the subsidence of the ground surface, the houses in the grabens sank without significant damage; there was no damage to the glass windows. After the earthquake, the authors conducted thorough on-site investigation to determine the mechanism of the grabens and to propose an appropriate method to restore the affected ground to a usable state. First, the distribution of the ground surface displacement was measured by laser scanning from satellites. This measurement revealed horizontal displacements of approximately 2 m to 3 m at, and around, the grabens. A surface wave survey clarified that the shallow soil layers under the grabens were looser than the soil layers surrounding the grabens. Therefore, it was estimated that the grabens were formed by the horizontal extension force from local large displacements. Deep borings also showed that a thick clayey layer was deposited from GL-17 m to GL-50 m in the zones where large horizontal displacements occurred. The bottom of this clayey layer was inclined because volcanic ash had deposited in a large lake several thousand years ago.