EARTHQUAKES IN SOUTHERN CALIFORNIA, 1979-1989

Sand Boil Near El Centro, 1979

Earthquake of October 15, 1979, Imperial Valley, California. A sand boil near El Centro provides graphic evidence of liquefaction. Such boils were found especially along the southern extent of the Imperial Fault. Liquefaction occurs when earthquake ground shaking distorts the granular structure of the ground causing some loosely packed groups of grains to collapse together. 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 and liquefaction has taken place. Photograph Credit: U.S. Geological Survey (G. Reagor).