HISTORICAL DEVELOPMENT OF THE SEISMIC HAZARD ZONATION UNIT, CALIFORNIA GEOLOGICAL SURVEY

In 1955, the Division of Mines, now the California Geologic Survey (CGS), published a report on the Arvin-Tehachapi Earthquake of 1952. This was the first earthquake investigation conducted by the agency since its creation in 1860. Additional changes in the Division’s mission were introduced by legislative action after the 1971 San Fernando Earthquake with law enactments addressing various seismic safety issues, including the Alquist-Priolo Earthquake Fault Zoning Act of 1972 (AP). During the next two decades, the scope of the agency’s seismic hazard assessment was periodically expanded by legislation in response to a series of damaging earthquakes (1979 Imperial Valley; 1980 Mammoth Lakes; 1983 Coalinga; 1984 Morgan Hill; 1987 Whittier; and 1989 Loma Preita).

While success of AP active fault zonation was widely recognized, it became clear there was a strong need to generate other types of geologic/seismic hazard information relevant to local planning. Although the Division had been conducting cooperative engineering geologic investigations with some local governments during the 1970s, support for these investigations dropped sharply after the 1977 Proposition 13 tax revolt. Not until after the 1987 Whittier Earthquake did a legislative hearing lead to the funding of a study addressing other seismic hazards. The results were released as CDMG Special Publication 108: Seismic Hazard Information Needs of the Insurance Industry, Local Government, and Property Owners in California. The report recommended creation of a program modeled after the AP program to address liquefaction, landslides, and other seismic hazards. SP 108 was used to pave the way for passage of the Seismic Hazards Mapping Act of 1990. Although half of the initial funding source of the program was lost due to repeal of the Residential Earthquake Recovery Act, a $20 million FEMA hazard mitigation grant awarded after the 1994 Northridge Earthquake jumpstarted the program, which has thus far completed 116 seismic hazard zone maps covering 175 cities in the Los Angeles and San Francisco Bay metropolitan areas. Today, program products and services are integrated into the California Building Code, local land-use planning, and building plan review and approval for nearly all proposed construction of commercial buildings, public schools, and hospitals.