New seismic maps available for San Francisco area

The U.S. Geological Survey (USGS) and California Geological Survey (CGS) introduced three new seismic-hazard maps of the San Francisco Bay area and announced the online release of seismic-hazard studies resulting from the 1989 Loma Prieta earthquake. That earthquake occurred at 5:04 p.m. on Oct. 17, 1989, as much of the Bay area prepared to watch the San Francisco Giants and Oakland A’s play a World Series game. The magnitude 6.9 earthquake ruptured the Santa Cruz Mountains, killing 62 and causing more than $5.9 billion of damage.

The USGS Professional Paper comprises 162 papers published in four volumes covering the earthquake itself; the effects of the earthquake; the response of buildings, lifelines, and highways; and the response to the earthquake and its economic and societal impact. Although the paper versions of these reports have been available in libraries and for sale at the USGS, the new website makes all 3,000 pages of them available for free on demand.

One lesson learned from the Loma Prieta earthquake was that the resulting landslides and liquefaction tended to occur where they had during the 1906 earthquake along the San Andreas fault. Maps of the 1989 occurrences are now online. This observation led to passage of the Seismic Hazards Mapping Act, which mandates that the CGS publish maps that identify regions prone to landslides and liquefaction during earthquakes.

Three new regulatory Seismic Hazard Zone maps covering parts of the South Bay area-including the communities of Morgan Hill, Menlo Park, Palo Alto, and Redwood City-designate areas where new development and construction must implement special precautions to protect life and property in the event of a large earthquake. Once official, these maps also trigger disclosure requirements that affect property owners and real estate agents.

Ivan Wong, president of the Northern California Chapter of the Earthquake Engineering Research Institute notes that "the 1989 earthquake is important because it is the one event that still remains in the public memory. However, what we saw in that event is just the tip of the iceberg because damage and casualties will be much more severe in the next big Bay area earthquake. We can and must, however, learn from the lessons of the 1989 earthquake. These new publications from the USGS and CGS help us remember and build on those lessons."

To learn more about these seismic hazard maps produced by the CGS, and to download these maps and associated reports, visit https://gmw.consrv.ca.gov/shmp . The new online versions of the USGS Professional Papers on the Loma Prieta Earthquake are available free at https://earthquake.usgs.gov/1989 . This site provides additional information about the Loma Prieta earthquake, including animations showing how the earthquake waves spread away from the earthquake rupture.