All around California, residents and public safety officials share concerns about the substantial risk associated with the potential collapse of older concrete buildings in future earthquakes.
Older concrete buildings, as a class, pose a higher seismic risk than modern buildings. The line between older and modern buildings can be drawn at approximately 1980. State building code revisions introduced in 1976 that provided for improved safety of this type of building took several years to implement.
For the past six years our team of researchers has investigated the seismic risks posed by concrete buildings constructed before 1980 as part of a study supported by the National Science Foundation.
History shows that fewer than 5 percent of older concrete buildings collapse when shaken by strong earthquakes, but that is still a significant number where there is a large number of these buildings, as there is in major metropolitan areas such as Los Angeles, the San Francisco Bay Area and Sacramento.
The Concrete Coalition, a grass-roots group committed to addressing this important seismic hazard, has estimated there are approximately 22,000 such buildings around the state.
To study the magnitude of earthquake risk in an urban environment, we compiled as part of our research a list of nearly 1,500 older concrete buildings in Los Angeles.
We chose Los Angeles because of the city's location in a high seismic zone and its past leadership in addressing seismic risk. We estimated the numbers of casualties and the dollar losses associated with this building stock.
In a magnitude 7.15 earthquake on the Puente Hills fault (which is beneath downtown Los Angeles), losses in older concrete buildings are estimated to be about $20 billion, with casualties ranging from 300 to 2,000, depending on the time of day the quake hits.
A key goal of the research was to explore ways to mitigate the overall losses. We have identified several steps that government agencies, working closely with property owners and residents, can take to reduce the likelihood of tragedy in case of major earthquakes:
We recognize that retrofit costs may be substantial for those buildings ultimately found to be at-risk. The cost of doing nothing, however, will likely be even higher when the next strong earthquake strikes.
We urge cities to take appropriate action. We stand ready to assist in this process.
Jack Moehle and Mary Comerio are professors of civil engineering and architecture, respectively, at UC Berkeley. Jonathan P. Stewart is chair of the Civil and Environmental Engineering Department at UCLA. Thalia Anagnos is a professor of engineering at San Jose State University.