Late quaternary sedimentation and liquefaction hazard in the San Fernando Valley, Los Angeles County, California

A new map of late Quaternary deposits of the San Fernando Valley has been p repared from interpretation of geomorphology as expressed on aerial photogr aphs and pre-development topographic maps, combined with subsurface informa tion from thousands of boreholes. This new, detailed map shows that the San Fernando Valley is a structural trough that has been filled from the sides , with the major source of sediment being large drainages in the San Gabrie l Mountains, Deposition on the major alluvial fan of Tujunga Wash and Pacoi ma Wash, which issues from the San Gabriel Mountains, and on smaller fans, has been influenced by ongoing compressional tectonics in the valley. Late Pleistocene deposits have been cut by active faults and warped over growing folds. Holocene fans are locally ponded behind active uplifts. The resulti ng complex pattern of deposits has a major effect on liquefaction hazards, Young sandy sediments generally are highly susceptible to liquefaction wher e they are saturated, but the distribution of young deposits, their grain s ize characteristics, and the level of ground water all are complexly depend ent on the tectonics of the valley. Completion of a detailed map of late Qu aternary deposits is a necessary first step in preparing a map of liquefact ion susceptibility. It also has provided clues to the details of ongoing de formation within the valley.