UPDATED GEOLOGIC MAPS OF THE LOS ANGELES AND LONG BEACH 30’X60’ QUADRANGLES

The Los Angeles and Long Beach 30’ x 60’ quadrangles together cover approximately 10,000 km² including much of the densely populated urban and suburban areas of the southern California megalopolis. The population of Los Angeles County is estimated at over 10 million people, most of whom reside in the area defined by these quadrangles. Residents and visitors are subject to potential hazards from earthquakes, debris flows and other landslides, floods, wildfires, subsidence from ground water and petroleum withdrawal, and swelling soils. Coastal areas are exposed to flooding and erosion by storm and tsunami waves. New versions of the geologic maps of these quadrangles provide a regional geologic framework to better evaluate the potential for hazards from active earth processes. The maps, issued by the California Geological Survey in 2014 and 2016, illustrate the distribution of the rocks and surficial deposits of the area and their structural and stratigraphic relationships. The maps are compiled from dozens of individual sources, generally mapped at 1:24,000 scale or larger. Revised mapping of younger Quaternary units, including the contact between late Quaternary alluvium and bedrock, was a focus of new mapping for seismic hazard zoning, because these contacts can define the boundaries of Liquefaction Hazard Zones. Revised mapping of some active faults has been prompted by evidence for Holocene activity and the need to establish Alquist-Priolo Earthquake Fault Zones around active faults. The 30’ x 60’ quadrangles are presented at 1:100,000 scale, but as digital products they preserve all of the detail of the original source maps. The maps have been released for review and comments on the CGS web page: http://www.conservation.ca.gov/cgs/rghm/rgm/Pages/preliminary_geologic_map