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[SSS11-03] Numerical elucidation of the Aso Caldera graben mechanism focusing on stratum irregularities, soft clay and two consecutive earthquakes
Keywords:Kumamoto earthquake, Aso Caldera, Stratum irregularity, Soft clayey soil, Consecutive earthquakes, Seismic response analysis
In the 2016 Kumamoto earthquake, graben damage extending about 10 km occurred in the northwest part of Aso Caldera. Two strong motions measuring maximum seismic intensity of 7 (foreshock and main shock) occurred in rapid succession. It has been reported that the graben damage occurred not during the foreshock, but immediately after the main shock although the shocks had the similar degree of intensity. It was thought that the Futagawa fault zone that caused the Kumamoto earthquake did not reach to the Aso Caldera, and no causal relationship has been identified between the graben damage and the Hinagu fault zone. Therefore, various causes have been considered, such as collapse of underground cavities, surface seismic faults, liquefaction-induced horizontal movement of large-scale geological blocks in deeper ground, etc. However, the definite cause of graben has not been identified yet because no trace of fault movement or liquefaction has been found. On the other hand, Yasuda et al.1) and Ishikawa et al. 2) revealed that most of the grabens appeared at the ground above the old lake which existed about 9,000 years ago. In addition, they have confirmed that the lake sedimentary layer has the property that the shear rigidity decreased sharply with strong repeated shear stress from the laboratory testing, and conducted a reproduction analysis of the graben damage. However, they have still not mentioned why localized and strong repeated shear was acted on the lake sedimentary layer.
The aim of this study is to elucidate the mechanism of graben that occurred in the Aso Caldera in terms of "stratum irregularity" formed by the old lake basin in the Aso Caldera, the "presence of soft clayey soil" of the old lake deposit and the "two consecutive earthquakes" that characterize the Kumamoto earthquakes. Therefore, 2-dimensional seismic response analysis was conducted to elucidate these effects on the surface ground damage. The analysis code employed was the soil-water coupled finite deformation analysis3), which incorporates an elasto-plastic constitutive model that allows description of mechanical behavior of soils ranging from sand through intermediate soils to clay within the same theoretical framework4).
The following conclusions were obtained.
(1) By considering the effect of stratum irregularity, (a) the focal phenomena of the body waves, (b) the excitation of the surface waves and (c) the amplification interference of the body waves and surface waves ("edge effect") were numerically reproduced. Therefore, the wave propagation became complicated and the oscillation became larger at a specific point near the ground surface.
(2) The clayey lake deposit was in a soft condition. Therefore, the effective stress was easily decreased by seismic motions, which causes further increase in natural period of the ground.
(3) As mentioned in (2), natural period of the ground was increased by foreshock. The continuous occurrence of main shocks containing long-period components significantly amplified long-period oscillations. This long-period and large shaking caused graben damage at the ground surface.
It was analytically demonstrated that the graben damage of the Aso Caldera was caused by the combined effect of all of the above three characteristics (1), (2) and (3). In other words, if even one is missing, graben damage may not have occurred.
1) Yasuda, S. et al., Deformation survey of the Kario district in Aso city where caused graben damage during the Kumamoto earthquake, Proc. of 13th JAEE, 2-13, 2017.
2) Ishikawa, K. et al., Residual deformation analysis due to softening of the lacustrine sediment in the Aso caldera in the 2016 Kumamoto earthquake, Proc. of 54th JGS, 1925-1926, 2019.
3) Asaoka, A. et al., An elasto-plastic description of two distinct volume change mechanisms of soils, S&F, 42(5), 47-57, 2002.
4) Noda, T. et al., Soil-water coupled finite deformation analysis based on a rate-type equation of motion incorporating the SYS Cam-clay model, S&F, 48(6), 771-790, 2008.
The aim of this study is to elucidate the mechanism of graben that occurred in the Aso Caldera in terms of "stratum irregularity" formed by the old lake basin in the Aso Caldera, the "presence of soft clayey soil" of the old lake deposit and the "two consecutive earthquakes" that characterize the Kumamoto earthquakes. Therefore, 2-dimensional seismic response analysis was conducted to elucidate these effects on the surface ground damage. The analysis code employed was the soil-water coupled finite deformation analysis3), which incorporates an elasto-plastic constitutive model that allows description of mechanical behavior of soils ranging from sand through intermediate soils to clay within the same theoretical framework4).
The following conclusions were obtained.
(1) By considering the effect of stratum irregularity, (a) the focal phenomena of the body waves, (b) the excitation of the surface waves and (c) the amplification interference of the body waves and surface waves ("edge effect") were numerically reproduced. Therefore, the wave propagation became complicated and the oscillation became larger at a specific point near the ground surface.
(2) The clayey lake deposit was in a soft condition. Therefore, the effective stress was easily decreased by seismic motions, which causes further increase in natural period of the ground.
(3) As mentioned in (2), natural period of the ground was increased by foreshock. The continuous occurrence of main shocks containing long-period components significantly amplified long-period oscillations. This long-period and large shaking caused graben damage at the ground surface.
It was analytically demonstrated that the graben damage of the Aso Caldera was caused by the combined effect of all of the above three characteristics (1), (2) and (3). In other words, if even one is missing, graben damage may not have occurred.
1) Yasuda, S. et al., Deformation survey of the Kario district in Aso city where caused graben damage during the Kumamoto earthquake, Proc. of 13th JAEE, 2-13, 2017.
2) Ishikawa, K. et al., Residual deformation analysis due to softening of the lacustrine sediment in the Aso caldera in the 2016 Kumamoto earthquake, Proc. of 54th JGS, 1925-1926, 2019.
3) Asaoka, A. et al., An elasto-plastic description of two distinct volume change mechanisms of soils, S&F, 42(5), 47-57, 2002.
4) Noda, T. et al., Soil-water coupled finite deformation analysis based on a rate-type equation of motion incorporating the SYS Cam-clay model, S&F, 48(6), 771-790, 2008.