Journal of Japan Association for Earthquake Engineering
Online ISSN : 1884-6246
ISSN-L : 1884-6246
Technical Papers
Mechanism of the Grabens that formed in the Aso Caldera during the 2016 Kumamoto Earthquake
Susumu YASUDANaoto OHBOMasanobu SHIMADATatsuro CHIBAHideo NAGASESatoshi MURAKAMIShigeki SENNANaoko KITADAKeisuke ISHIKAWA
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2021 Volume 21 Issue 1 Pages 1_135-1_158

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Abstract

Many grabens, which are special and strange, belt-like subsidences, formed in the caldera of the Aso Volcano during the 2016 Kumamoto Earthquake. These grabens caused severe damage to houses, roads, water and sewage pipes, rice fields, and river dikes. The authors conducted various site investigations, soil tests and analyses to determine the mechanism of the grabens. The measurement of the displacement by SAR from satellites showed horizontal displacements of about 2 to 3 m. In this area, an old lake was formed approximately 9,000 years ago. Borings and microtremor measurements demonstrated a special cohesive soil layer with diatom and pumice sediment in the lake, and the bottom of the lake is U-shaped. Soil tests on the lake sediment soil clarified that the shear modulus decreases drastically under cyclic loadings, similar to the liquefaction of loose sand. The authors conducted a seismic response analysis and a residual deformation analysis and concluded that the lake sediment soil deformed to wrap around the U-shaped bottom of the lake due to the sharp decrease in shear stiffness during the earthquake, resulting in horizontal tensile stress on the surface soil and the grabens near the edge of the lake.

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