High-Rise Quake Risk Greater, Conferees Told

Earthquake scientists painted a somber picture of the Los Angeles Basin’s seismic future at an all-day conference at Caltech on Tuesday, saying that a magnitude 7 quake in the Downtown area would do much greater damage to high-rise buildings than had been thought before the Jan. 17 Northridge temblor.

But at the same time, the speakers acknowledged that they cannot predict earthquakes, and the magnitude 7 earthquake scenario they drew for central Los Angeles probably occurs only once every 1,500 to 4,000 years, Caltech geologist Kerry E. Sieh said.

Nevertheless, Sieh said, taking all fault stress into account, there should be such a quake centered in Ventura, Los Angeles or Orange county every 250 years. Ominously, he said, there is no evidence that such a quake has occurred in the last 2,000 to 3,000 years.

Other large quakes, such as the 1857 Ft. Tejon temblor, the 1952 Tehachapi quake and the 1992 Landers quake, were centered outside the three-county area.

A prospective quake in the heavily populated urban counties particularly concerns the scientists because they observed that the Northridge temblor caused vastly more damage and casualties than Landers.

Seismologist Lucile M. Jones of the U.S. Geological Survey agreed with Sieh that there is a “quake deficit” in the Los Angeles Basin, but she noted that there have been six fairly sizable quakes in the region in the last six years.

She said the deficit could be overcome if there were another Northridge-size earthquake every year for the next 20. The alternative eventually may be a much larger quake than Northridge, she told the 190 people attending the conference.

A series of Caltech and Geological Survey speakers contended that a magnitude 7 quake that hit Downtown could prove, under current building codes, devastating.

Three speakers questioned whether the base-isolation systems being installed in a number of buildings, and planned for Los Angeles City Hall, would be adequate to protect against such a jolt.

David J. Wald of the Geological Survey said that even the four-foot-wide moat that would surround City Hall under the base-isolation plan probably would not prove wide enough to allow for the displacements occurring in that strong of a quake.

Andrew D. Gibson of Caltech said that examinations of seven buildings that have base-isolation systems indicated that several did not have adequate displacement leeway on all sides during the Northridge quake.

George W. Housner, Caltech professor of engineering emeritus, noted that since Northridge, 60 steel-framed buildings have been found to have cracks in the steel, causing “a lot of concern” in two other quake-prone cities, San Francisco and Tokyo, as well as in Los Angeles.

Meanwhile, the Los Angeles City Council on Tuesday approved a motion calling on building and safety officials to draft an ordinance requiring inspections of steel-frame office buildings in certain parts of the city.

Responding to widespread reports that steel building frames cracked during the Northridge quake, Councilman Hal Bernson this month introduced a proposal that 400 steel-frame office buildings across the San Fernando Valley and on the Westside undergo the costly inspections within three months of being notified by city officials.

A Bernson spokeswoman said the council’s action asks that an ordinance be drafted that specifies the exact areas in which inspections would be required, and details how those inspections must be carried out. The spokeswoman said drafting the ordinance could take several months, after which the council would vote on the ordinance.

Times urban affairs writer Sonia Nazario contributed to this story.