The Natural History of Foster City, California

by James K. Sayre

a draft manuscript

12 May 2005

Chapter 5 - Earthquakes

Earthquakes and damage resulting from them are always a concern of people that live in an active earthquake fault zone such as coastal California. The most recent severe earthquake, as of this writing, occurred in the fall of 1989 on October 17th. Foster City suffered some damage, the city's water mains that cross over local bridges suffered some breaks. This was not the massive damage that some folks had envisioned as the city was designed and constructed back in the 1960's: that Foster City would slide into the San Francisco Bay and never been seen again... Some private dwellings suffered some minor structural damage, but generally the structures built in Foster City maintained their integrity. The levees and the lagoons all stayed in place.

Foster City was created by strengthening the original levees that were built around .i.Brewer's Island; about 1900. The existing hay pastures were then filled in by pumping in millions of tons of bay bottom fill dredged up from nearby parts of the San Francisco Bay. This material was then compacted down. At the same time, the backwards-s-shaped lagoon system was being carved out in the middle of the city.

Bay fill is considered to be very unstable from the viewpoint of earthquake activity - unconsolidated soil, fill and sand is subject to a form of liquefaction - which allows the transmittal of damaging earthquake motions with little or any damping out. However, the buildings constructed in Foster City have the benefit of well-designed foundations, including some with sunken pilings that have provided stability during earth-shaking events. However, it must be noted that the 1989 .i.Loma Prieta earthquake; epicenter was located about forty miles away from Foster City. The earthquake intensity in the Foster City area was estimated at 7 in terms of a Modified Mercalli scale [Bolt]. Foster City has yet to experience a strong local earthquake centered on the .i.San Andreas fault;, which lies about five miles to the west or on the Hayward fault which lies about fifteen miles to the east. A strong, locally-centered earthquake would provide a severe test of the design and construction of structures in Foster City.

The October 17, 1989 Loma Prieta earthquake did damage City facilities to the tune of $329,000 [Santiago]. This damage included water main leaks and breaks as they passed over Foster City bridges, the Foster City Blvd. overpass over Route 92, lift stations that move sewage to San Mateo, fire hydrants, water storage tanks, the gazebo at the Recreation Center and public facilities at the Civic Center. The Federal Government reimbursed the City of Foster City $329,000 through the Federal Emergency Management Administration (FEMA) for its expenses for repairs of damages incurred by the Loma Prieta earthquake.

By comparison, the City of Burlingame facilities suffered damages to the amount of about $150,000, mostly street, curb and gutter repairs east of the Bayshore Freeway [Becker]. Three hotels there suffered significant damage: the Hyatt Hotel suffered shear wall damages to the tune of $10,000,000 to $12,000,000 [Means]. These damages were repaired and the hotel was reopened. The Amfac Hotel suffered damages to its front wall when the water tower on the top of the building fell down in the earthquake. Due to pre-existing code violations, the hotel was demolished rather than pay the costs of repairing the structure and bringing it up to code. The site was sold for the price of the land and it is now paved as a parking lot. The Marriott Hotel suffered some utility line separation during the earthquake. These damages were rapidly repaired and the hotel is still in business. The Loma Prieta earthquake did emphasize seismic structural problems in the Burlingame Fire Station and the Burlingame Public Library. The Fire Station was replaced and the Library will be upgraded to meet modern seismic standards.

The earthquake damages experienced in Foster City were significant, but were minor compared to the damages experienced in San Francisco, Oakland and the Santa Cruz area.

End.

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