Sand Boil Near El Centro, 1979 Earthquake of October 15, 1979, Imperial Valley, California. A sand boil near El Centro. Such boils were found along the southern extent of the Imperial Fault. Each collapse transfers pressure from the grain groups to the water. When the pore-water pressures reach a critical level, the granular material suddenly behaves as a liquid rather than a solid. Liquefaction has taken place.