Officials have known as far back as the 1971 Sylmar quake that such buildings can collapse, but efforts to address those weaknesses have stalled because of the high cost of retrofitting the buildings.
Photos: Earthquake and nuclear crisis in Japan
The devastating earthquakes in Japan and New Zealand have focused more attention on the vulnerability of such buildings.
For California, the quake in Christchurch was particularly relevant because it served as a fresh reminder of the major weaknesses in so-called non-ductile concrete buildings. Numerous concrete structures failed in New Zealand during the Feb. 22, magnitude-6.3 earthquake, including the six-story Canterbury Television building, where 100 people were believed to have been buried, including many TV station employees and 60 students.
The collapse of the CTV building, which housed a local TV station, clinic and English-language school, was particularly shocking. It was built in 1986, 15 years after the Sylmar earthquake, and after codes had been changed in New Zealand and California to improve buildings' ability to withstand shaking. Yet the CTV building collapsed in a manner consistent with a brittle concrete building, said Thomas Heaton, professor of engineering seismology at Caltech.
Experts both in Christchurch and Los Angeles said such buildings are worrisome because it's so difficult for people inside to survive if large slabs of concrete fall on top of them.
"They're killers. In my opinion, they could take many thousands of lives in a Southern California earthquake, especially one inside the Los Angeles Basin," Heaton said. "When they fail, the failures are just unsurvivable. You just end up with a pile of floor slabs, one on top of another."
Identifying potentially unsafe buildings is crucial, Heaton said, because being in a building designed to withstand major shaking is a "big part of your survivability in an earthquake," Heaton said. "If you choose poorly, you're definitely in harm's way."
In the last decade, the Los Angeles City Council has considered a plan to identify brittle concrete buildings. But the plan's author, Councilman Greig Smith, was forced to shelve the plan as support evaporated and the recession left little money to address the risk.
The California Seismic Safety Commission has also recommended that the state identify brittle buildings, both public and private, and figure out a way to reduce the risk. But lawmakers have taken little action.
"Given the economic situation now, no one has got any money to do it," said Richard McCarthy, the commission's executive director.
Many of the larger buildings in question were constructed in the 1950s, '60s, and '70s, during the post-World War II building boom in which bigger was better, and companies throughout a growing California needed larger, multistory buildings that could house many more employees and residents, and larger parking structures that could hold more cars.
But in creating taller, slender buildings, engineers unknowingly took greater risks, and some buildings carried a fatal flaw. Made out of a concrete frame or walls but lacking sufficient reinforcing steel, these buildings were found to be brittle during an earthquake, as if the frame was built out of saltine crackers. When shaken during an earthquake, the floors and columns can snap, collapsing the entire building.
And they did.
During the 1971 Sylmar earthquake, the first floor of the just-built six-story Olive View Hospital was pulverized, killing three people. The nearby Veterans Administration hospital all but disintegrated into a pile of rubble. Forty-seven people died there.
California has made efforts to improve the safety of some structures since the Northridge earthquake in 1994, focusing on freeways and hospitals. But some hospitals have struggled to comply with state law that they be quake-resistant. In 2013, 258 buildings at 84 hospitals will remain at significant risk of collapsing in an earthquake, according to data supplied by hospitals to the state. California has 426 hospitals.
Identifying problem buildings requires a review of design records and on-site inspections.
"You cannot go out and look at the building real quickly and in all cases be able to say, 'This is dangerous,' or 'This is not,' " said Craig Comartin, a structural engineer who is former president of the Oakland-based Earthquake Engineering Research Institute.
Fixing those buildings is even more costly. One solution is to wrap the concrete columns with carbon-fiber mesh. The idea is to hold the concrete column together when the building is shaken, like wrapping a bag around a pile of toy marbles.
"The only problem is you have to tear everything out to get to the column," Comartin said. Some business groups who have opposed retrofitting said that in some cases it can be almost as expensive as erecting a new building.
Once vulnerable buildings are identified, officials face a difficult question: Should people who work or live in them be told about the risks?
Heaton said he would like to see it possible in California for workers and residents to know if their buildings are vulnerable in quakes. Placards are required for unreinforced brick buildings in California, but most other buildings aren't labeled.
Because of the lack of information, "it just sounds to the public as if there's nothing they can do about it; that it's all just fate, that it's too hard to build a building that's earthquake-resistant," Heaton said. "In fact, choosing the wrong building can really put you in jeopardy. And if you're in the right building — you can have all kinds of earthquakes — and it's not going to collapse."
The debate over warning signs is also occurring in New Zealand. Graeme Beattie, a former president of the New Zealand Society for Earthquake Engineering, said a temporary placard system is being used in Christchurch to alert residents of potentially dangerous buildings. The system uses green, yellow and red cards to indicate risk levels.
But Beattie said the cards are a short-term solution only because they are based on an "educated guess" by engineers who must evaluate buildings at a rapid pace. Part of the discussion now is about whether a broader and more permanent system should be in place in New Zealand to notify residents of vulnerable buildings.
"There are quite a lot of engineers, including me, who would like to see that done so that the public could see what risk they were taking in going to buildings," Beattie said. "But you have to balance technical issues with the political issues that could be involved."
Photos: Earthquake and nuclear crisis in Japan