Mapping Liquefaction Hazards in Simi Valley, Ventura County, California

Abstract

Liquefaction-related ground settlement and displacement associated with the Northridge earthquake of January 17,1994 caused considerable damage to private and public property in nearby Simi Valley, California. This damage primarily occurred in saturated Holocene stream deposits and overlying artificial fill at the eastern end of the valley. Detailed geologic mapping of late Quaternary deposits and analysis of geotechnical borehole data show that similar saturated, potentially liquefiable sediments are present elsewhere in Simi Valley. Liquefaction hazards in Simi Valley are governed by unique geologic and hydrologic conditions that have helped shape the valley. Locally thick accumulations of young stream deposits in the eastern and western ends of Simi Valley coincide with areas of shallow ground water. Liquefaction-related damage caused by the Northridge earthquake in eastern Simi Valley was confined to a small alluvial basin. This basin is separated from the main Simi Valley basin by a partially buried bedrock ridge that inhibits westward groundwater flow, resulting in locally saturated conditions. Similarly, unconfined ground water ponded behind the Simi fault saturates deposits at the western end of Simi Valley. Analyses of boring logs show that these deposits are poorly consolidated and contain sandy layers. The combination of high ground water within loose sandy sediments constitutes a significant liquefaction hazard beneath the western and eastern portions of Simi Valley.