Mapping liquefaction hazards in Simi Valley, Ventura County, California

Liquefaction-related ground settlement and displacement associated with the Northridge earthquake of January 17, 1994 caused considerable damage to pr ivate and public property in nearby Simi Valley, California. This damage pr imarily occurred in saturated Holocene stream deposits and overlying artifi cial fill at the eastern end of the valley, Detailed geologic mapping of la te 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, Loca lly thick accumulations of young stream deposits in the eastern and western ends of Simi Valley coincide with areas of shallow ground water. Liquefact ion-related damage caused by the Northridge earthquake in eastern Simi Vall ey 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 w estward groundwater flow, resulting in locally saturated conditions. Simila rly, unconfined ground water ponded behind the Simi fault saturates deposit s at the western end of Simi Valley. Analyses of boring logs show that thes e deposits are poorly consolidated and contain sandy layers. The combinatio n of high ground water within loose sandy sediments constitutes a significa nt liquefaction hazard beneath the western and eastern portions of Simi Val ley.