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Hundreds of buildings in West Los Angeles and the San Fernando Valley suffered hidden structural damage in the Jan. 17 Northridge earthquake and are likely to collapse if another large earthquake occurs in their vicinity, says one of the nation's leading earthquake experts.

"It is a serious problem that we've known about for at least six months," Thomas Heaton, a seismologist at the U.S. Geological Survey office in Pasadena, Calif., said this week. "But very little has been done about it."Many owners of affected buildings have been notified about the problem, Heaton said, but there is no law requiring them to tell occupants about the damage.

Many thousands of people may be living or working in such buildings but have not been told of the danger, Heaton said. Nor is there any way for them to find out.

Heaton described the problem in two talks this week at the annual meeting of the American Geophysical Union, which is being held in San Francisco. He elaborated on those remarks in the telephone interview Thursday.

Asked why he is now stressing the problem, Heaton said he was feeling some frustration that more had not been done about it.

William Rome, director of building and safety for the city of Santa Monca, said owners of steel- and concrete-frame buildings have hired private consultants to inspect their buildings. Whether or not tenants are told of any damages, he said, depends on terms in the landlord-tenant leases. Rome said that he did not know if people were being advised about cracks in the steel.

The city, meanwhile is waiting for the state's structural engineers to come up with recommendations for how to repair damaged buildings, Rome said. Once those are in place, the city will send notices to building owners who then have two to four years to make the repairs, he said.

After the Jan. 17 Northridge earthquake, Heaton said, building inspectors were shocked to discover fractures in the steel welds of hundreds of buildings throughout the San Fernando Valley and Santa Monica.

The problem is found in buildings whose frames are made of steel or concrete, Heaton said. It is a very common type of construction for office buildings, and the major type for high-rise buildings in Los Angeles. Quite a few apartment buildings also have steel or concrete frames, he said. Wood-frame buildings and those made from continuous walls of concrete or other sheet materials are not at issue, he said.

When inspectors ripped away the outer walls of dozens of steel-and concrete-frame buildings, Heaton said, they found that the welds and joints between the columns and beams had cracked. "The cracks literally ran from the welds into the steel material," he said.

This means that the buildings have been substantially weakened, the scientist said. If they experience another episode of severe ground shaking, as found in an earthquake with a magnitude of seven on the Richter scale, "they could collapse," Heaton said.