M 7.6 earthquake. Saturated sand below buildings turned into a viscous fluid; entire apartment blocks tilted over without breaking apart, becoming the textbook image of soil liquefaction.
Niigata sits on Holocene loose sand with a high water table. When the seismic shear waves arrived, pore-water pressure built up faster than it could dissipate. The cyclic stress ratio (CSR) exceeded the cyclic resistance ratio (CRR) for layers 5-15 m deep, the soil temporarily lost shear strength, and structures founded on shallow foundations rotated as the ground gave way beneath them.
Liquefaction-susceptible soil was not screened. Modern NCEER simplified procedure with PGA = 0.16g and N₆₀ ≈ 10 in the affected layers gives CRR ~ 0.12 and CSR ~ 0.18, so FoS_liq ≈ 0.67 — far below the 1.0 threshold.
IS 1893 / NCEER liquefaction screening — CSR vs CRR per layer with FoS_liq output.
Open Geotechnical Analysis