Map showing late Quaternary faults and 1978-84 seismicity of the Los Angeles region, California

The Los Angeles region of California faces the greatest seismic risk of any part of the United States. The region is inhabited by more than 11 million people and is one of the Nation's key commercial and industrial centers. It lies astride a web of potentially active faults, including those segments of the San Andreas fault with the highest probability for generating a great earthquake during the next 30 years (Lindh, 1983; Sykes and Nishenko, 1984; Wesson and Wallace, 1985; U.S. Geological Survey, 1988). Moreover, many potentially active faults that can generate moderate-size, but damaging earthquakes lie within the metropolitan areas. Earthquakes along some of these faults—for example, a magnitude 6.5 event on the Newport-Inglewood zone—could produce losses exceeding those from a great earthquake on the more distant San Andreas fault (Evernden and Thomson, 1985).

This map shows, at 1:250,000 scale, known or suspected late Quaternary faults of the Los Angeles region, the ages of their most recent surface movements, and the associated earthquake activity for a recent 7-year period (1978 through 1984). The geologic and seismologic character of these faults and their potential for generating damaging earthquakes recently were evaluated by Ziony and Yerkes (1985). The map is intended primarily to inform scientists, engineers, and planners of the distribution of those faults that may have a potential for generating damaging earthquakes and (or) displacements of the Earth's surface. The map data also should contribute to further investigations of the seismotectonic setting of the Los Angeles region.

The mapped area extends from lat 33°15' N. to 34°45' N. and from long 116°45' W. to 120°00' W. This region encompasses parts of the Transverse Ranges (including the Santa Ynez, Santa Susana, Santa Monica, San Gabriel, and San Bernardino Mountains), the Los Angeles basin, part of the Mojave Desert, and segments of the Peninsular Ranges (including the Puente Hills, Santa Ana Mountains, and the northern San Jacinto Mountains). Offshore, it includes the eastern Santa Barbara Channel, several of the Channel Islands, the Santa Monica and San Pedro basins, Santa Catalina Island, and part of the Gulf of Santa Catalina.

In addition to the Los Angeles metropolitan area, the cities of Santa Barbara, Ventura, Riverside, and San Bernardino are within the map area.