On January 17, 1994, a magnitude 6.7 earthquake centered in Northridge struck the southern California area. Known as the Northridge Earthquake, it caused 57 fatalities, and up to $20 billion in damage costs and $40+ billion in economic loss, making it the costliest earthquake disaster in U.S. history.
Intense ground shaking generated widespread ground failures in the region. Thousands of landslides occurred throughout the mountains surrounding the San Fernando Valley and as far north as Castaic Lake Widespread liquefaction (saturated soil that behaves as a liquid when shaken) was triggered as far away as the Port of Los Angeles, some 30 miles from the epicenter. These types of ground failures caused severe damage to building foundations and to vital lifeline infrastructure (transportation corridors, water, gas, and electric transmission facilities). As a consequence of the severe losses caused by this event, the Federal Emergency Management Agency awarded the California Geological Survey nearly $20 million to accelerate the zoning of earthquake hazards under the Seismic Hazards Mapping Act of 1990.
In the 20+ years since the Northridge Earthquake, the California Geological Survey (CGS) has completed 90 maps covering 5,580 square miles that identify landslide and liquefaction hazard zones in developed and developing areas of Los Angeles, Orange, and Ventura counties, and 29 similar maps (1,798 square miles) in the San Francisco Bay area. These Seismic Hazard Zones, or Zones of Required Investigation, delineate where there is a high likelihood of these hazards occurring in future earthquake events. Statewide, CGS has completed mapping liquefaction and landslide hazard zones in eight counties covering 165 incorporated cities and 32 unincorporated communities. Local city and county permitting agencies are required to request “special geological and engineering studies” be performed within these zones prior to land development and construction.
Once special studies are performed by qualified engineers and geologists, measures to physically mitigate these ground-failure hazards are included in the development plans before permits are issued. Recognizing and fixing these potential problems before development occurs is the most cost-effective way to deal with these earthquake hazards. Reducing the impact of these ground-failure hazards, not only save lives and property during future earthquakes, but leads to more sustainable economic communities.
The Seismic Hazard Zonation Program at CGS continues to produce similar maps throughout the Bay Area, and in recent years also has started map production in the fast developing portions of Riverside and San Bernardino counties.
During the Northridge earthquake of 1994, ground motion instruments recorded horizontal accelerations of 1.25g and vertical accelerations of 1.2g (1g = force of gravity). The California Strong Motion Instrumentation Program (SMIP), part of CGS’s Earthquake Engineering Program, was established in 1971 after the devastating San Fernando earthquake and now manages more than 1,000 strong motion instruments statewide. At the time of the 1994 Northridge Earthquake there were approximately 400 stations. The CGS SMIP stations contribute to ShakeMap, a real-time seismic network product that can help guide emergency response efforts within minutes of a strong earthquake.
For more information about the CGS earthquake hazard maps, products and programs, please visit the following pages at the California Geological Survey:
CGS Regulatory Earthquake Fault, Liquefaction, and Landslide Zones Maps
3-D Overview of CSG Seismic Hazard Zones of the Los Angeles Area
http://www.conservation.ca.gov/cgs/geologic_hazards/earthquakes/Pages/3d_snaps.aspxTsunami Inundation Maps
http://www.conservation.ca.gov/cgs/geologic_hazards/Tsunami/Inundation_Maps/Pages/Index.aspxAP Program and Earthquake Fault Zone Maps: http://maps.conservation.ca.gov/cgs/informationwarehouse/Guidelines for Evaluating and Mitigating Seismic Hazards in California
http://www.conservation.ca.gov/cgs/shzp/Documents/sp117.pdfFault-Rupture Hazard Zones in California
ftp://ftp.consrv.ca.gov/pub/dmg/pubs/sp/Sp42.pdfRecommended Criteria for Delineating Seismic Hazard Zones in California
http://www.conservation.ca.gov/cgs/shzp/webdocs/Documents/SP118_Revised.pdfCGS Note 54 – Regulatory Earthquake Hazard Zones, Southern California Region
http://www.conservation.ca.gov/cgs/information/publications/cgs_notes/Documents/CGS_Note_54.pdfEarthquake Shaking Potential for California
http://www.conservation.ca.gov/cgs/information/publications/ms/Documents/MS48_revised.pdfCGS Fault Activity Map of California
http://www.conservation.ca.gov/cgs/smip/Pages/shakemap.aspxCalifornia Strong Motion Data