Localized ground deformation within the Aso caldera during the 2016 Kumamoto Earthquake: Combined effects of irregular basin geometry, soft lacustrine clay, and earthquake sequencing

Graben-type subsidence cracks observed in the Aso Caldera during the 2016 Kumamoto Earthquake were investigated through two-dimensional soil–water coupled elastoplastic simulations using the SYS Cam-clay model. Severe localized ground deformation resulted from the combined effects of (1) irregular basin geometry formed by paleolakes, (2) soft and highly sensitive lacustrine clay layers, and (3) sequential strong earthquakes within a short 28 h interval. The foreshock significantly reduced clay stiffness and mean effective stress, thereby lengthening the natural period of the ground. In contrast, the subsequent mainshock was characterized by long-period component-triggered resonance and amplified surface shaking. Complex wave propagation due to irregular basin geometry, including focusing effects and excitation of long-period surface waves, further intensifies the damage. Additional simulations revealed that if the interval between the foreshock and mainshock were extended to one year or more, stiffness recovery would have suppressed severe surface damage. These findings highlight the critical role of irregular basin geometry and earthquake sequencing in predicting seismic damage, underscoring the need for multidimensional analyses beyond conventional one-dimensional approaches for basin-affected urban areas worldwide.