Late Quaternary Sedimentation and Liquefaction Hazard in the San Fernando Valley, Los Angeles County, California

Abstract

A new map of late Quaternary deposits of the San Fernando Valley has been prepared from interpretation of geomorphology as expressed on aerial photographs and pre-development topographic maps, combined with subsurface information 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 Gabriel Mountains. Deposition on the major alluvial fan of Tujunga Wash and Pacoima Wash, which issues from the San Gabriel Mountains, and on smaller fans, has been influenced by ongoing com-pressional 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 resulting complex pattern of deposits has a major effect on liquefaction hazards. Young sandy sediments generally are highly susceptible to liquefaction where they are saturated, but the distribution of young deposits, their grain size characteristics, and the level of ground water all are complexly dependent on the tectonics of the valley. Completion of a detailed map of late Quaternary deposits is a necessary first step in preparing a map of liquefaction susceptibility. It also has provided clues to the details of ongoing deformation within the valley.