SEISMIC HAZARDS MAPPING ACT

The 1991 Seismic Hazards Mapping Act mandates cities and counties to regulate development projects located in seismic hazard zones. They must withhold development permits until geologic and soil conditions are investigated and mitigation measures incorporated in the development plans. A licensed engineering geologist or civil engineer having competence in this field is required to review the geotechnical reports; submit approved copies to the State of California Geologist.

As of June 21, 2002, there are 66 official maps of seismic hazard zones statewide, affecting more than six counties and 115 cities. For more information regarding these sites visit the California Geological Survey website at gmw.consrv.ca.gov/shmp.

LIQUEFACTION: THE PROCESS

Liquefaction is one geotechnical phenomena regulated by the Seismic Hazard Mapping Act and requires investigation and reporting prior to development of a site. Liquefaction is a process by which water-saturated sediment losses strength and fails during strong ground shaking from earthquakes. Research into the process and consequences of liquefaction was stimulated by catastrophic ground failures triggered by great earthquakes in the early 1960's and 1970's. These events included the 1964 Alaska and Nigata, Japan earthquakes, the 1967 Caracas, Venezuela earthquake, and the 1971 San Fernando earthquake. Subsequent studies linked liquefaction to specific hydrogeologic and geologic conditions. Liquefaction is now relatively well understood. Geologic environments conducive to liquefaction can be delineated. Ground failures resulting from liquefaction can be mitigated.

HISTORIC SEISMIC OCCURRENCES IN SOUTHERN CALIFORNIA

THE GREAT 1857 FORT TEJON EARTHQUAKE
Ground fissuring and sand boils near the Los Angeles, San Gabriel, Santa Clara, and Santa Ana Rivers resulted from the Fort Tejon earthquake of 1857.

1933 LONG BEACH EARTHQUAKE
Liquefaction induced ground failures damaging parts of Compton, Huntington Beach, and Long Beach as a result of the 1933 Long Beach earthquake.

1971 SAN FERNANDO EARTHQUAKE
Liquefaction induced ground failures severely damaging the San Fernando Juvenile Hall, the Jensen Water Filtration Plant, and resulted in near-catastrophic failure of the Lower Van Norman Reservoir Dam as a result of the 1971 San Fernando earthquake.

1973 POINT MUGU EARTHQUAKE
Liquefaction-related ground failures near Port Hueneme and Calleguas Creek as a result of the 1973 Point Mugu earthquake.

1979 AND 1981 IMPERIAL VALLEY EARTHQUAKES
Widespread ground failures that damaged roads, utilities, canals, and agricultural facilities were a result of the 1979 and 1981 Imperial Valley earthquake.