At 3 a.m. on Sunday, Aug. 24, a magnitude-6.0 earthquake centered in the American Canyon, about 9 kilometers south of the city of Napa, awoke much of northern California. The next day, television newscasts focused on damage sustained by a small number of unreinforced masonry buildings in Napa, fire damage to mobile homes in a trailer community, some older wood-frame homes that shifted off their foundations and extensive loss of wine inventory due to failed wine-aging storage tanks and casks.

But contrary to the impression conveyed by the television news and other media, there's good reason to be pleased with the way buildings performed.

HAMBURGER

Considered strong, the earthquake produced Modified Mercalli Intensity VIII shaking in surrounding cities and counties, including Carquinez, Napa and Vallejo. Seismologists recorded peak ground accelerations in excess of 0.6 g in downtown Napa and nearly 1 g at the Carquinez Bridge's south end. The recorded acceleration response spectra in the heavily shaken regions are comparable to design earthquake shaking levels that have been in the California Building Code for 20 years.

Wine-Making Industry

The epicenter was located just to the south of California's adjoining Napa and Sonoma valleys, where winemaking is the main industry. The building stock includes some 19th-century unreinforced masonry and wood-frame structures, many concrete, masonry and timber buildings constructed in the 1930s to 1950s, and a variety of more recent buildings constructed under modern building codes. Many newer building designs mimic historical architecture.

The media failed to emphasize that the region was largely unscathed. Interstate Highway 80 and several state highways transect the region with major bridges over the Sacramento and Napa rivers—none was damaged. Chevron, Conoco-Phillips, Shell, Tesoro and Valero all operate large petroleum refineries, located within 20 miles of the epicenter, and all continued business as usual. Though building officials initially red-tagged a number of buildings in Napa and Vallejo, following more detailed review by engineers retained by the building's owners, most tagged buildings were found to be safe and were restored to service within days. Most other buildings in the region were open for business the day of the earthquake or the next day. The Pacific Gas & Electric Co. suffered only minor damage to its facilities and restored power within hours.

That some wineries lost a large amount of inventory is not a surprise. For years, engineers have noted the vulnerability of thin-walled stainless-steel tanks and stacked wine barrels to earthquake damage. Similar damage was observed in the 1980 Livermore, 1983 Morgan Hill and 2005 Paso Robles, Calif., earthquakes. Some wineries have adopted earthquake-resistant practices in their facilities and managed to escape large losses, while many others chose to ignore the warnings.

Some older buildings, including some that had been retrofitted, did not perform well. But many retrofitted and most modern buildings sustained little or no damage. Given the building code's design philosophy for earthquake resistance that anticipates significant structural and non-structural damage in events of this size and intensity, the performance was excellent. The performance of the buildings and infrastructure is a tribute to the effectiveness of modern building codes and construction practices and our ability to improve the performance of even older construction with substantial vulnerability.

This is not to suggest that the San Francisco Bay area is invulnerable to future earthquakes. The region is traversed by a number of faults that are capable of producing far stronger events, and many older buildings and businesses have not been retrofitted and remain vulnerable to damage. Businesses and communities that have not yet adopted earthquake-resistant design and construction practices and that have not yet retrofitted their vulnerable facilities should note the lessons of this earthquake. Events of this size can strike anywhere and at any time in the U.S. and would cause true disasters in many communities that are not as well prepared. 

Ronald O. Hamburger, S.E., S.E.C.B., is a structural engineer and senior principal with Simpson Gumpertz & Heger's San Francisco office. Hamburger can be reached at 415-495-3700.