What caused the partial collapse of the Van Norman Dam during the 1971 earthquake?

The partial collapse of the Van Norman Dam (also known as the lower San Fernando Dam) during the 1971 earthquake was caused by the liquefaction of its saturated soil structure.1

The dam was constructed using hydraulic fill technology, a method that leaves a significant amount of water trapped within the soil. When subjected to seismic shock, this saturated material lost its strength and stability, causing a large quantity of the hydraulic fill to slip and leading to the dam's partial failure. This event highlights the specific vulnerability of hydraulic-fill embankment dams to seismic activity compared to other dam designs.12