Ban on High-Rise Construction Near Major Faults Urged

SACRAMENTO —  Two Caltech earthquake experts testifying Thursday before the state Seismic Safety Commission called for, respectively, a moratorium on all new high-rise building construction close to major earthquake faults, and a policy of avoiding the use of seismic base isolation in downtown Los Angeles.

The testimony by engineering seismology professor Thomas H. Heaton and civil engineering professor John Hall brought some skeptical reaction--but no direct attack--by industry representatives.

A staff member of the commission had said before the hearing that an attempt would be made to convince Heaton and Hall to express their views more through bureaucratic channels and not so much in public. The hearing was called to examine “near source effects” (very heavy shaking) within six miles of faults during big earthquakes.

Heaton has received substantial attention for statements that California is unwise to have any buildings more than 10 stories high in seismically risky areas. Likewise, Hall has been a sharp critic of weaknesses he perceives in steel frame construction at the Los Angeles Memorial Coliseum and elsewhere.

Thursday’s testimony provided a chance for the two professors to take their views to the state level and before an official agency. The two were granted a cordial hearing and no one suggested, publicly at least, that they quiet down.

At the hearing, it was Heaton--as expected--who suggested the high-rise building moratorium. Hall said base isolation--the cushioning of buildings from heavy shaking by a system of springs and dampers--should be avoided close to quake faults, because, he warned, large ground displacements could overwhelm the protective systems.

The Hall statement amounts to a criticism of plans for the earthquake retrofitting of Los Angeles City Hall. Due to get under way next year, it would incorporate base isolation as a key component.

The Elysian Park fault system, believed capable of generating a magnitude 7.0 quake, passes only about a mile north of City Hall.

Both scientists said not enough is known about shaking motions in earthquakes larger than the magnitude 6.7 Northridge temblor of 1994 to suggest any specific building code revisions at this time.

Hall, in fact, suggested that it is possible nothing would work in the largest earthquakes to prevent severe, and perhaps catastrophic, damage.

“Maybe, it will be necessary simply to acknowledge that risks will remain, and we cannot go beyond a certain level of protection,” he stated.

Even short buildings constructed with the strongest protections will be violently shaken in big quakes, he said, and there will be a great deal of nonstructural damage--to contents.

Testifying later, Loring Wyllie, a Bay Area structural engineer and former president of the Structural Engineers Assn. of California, objected mildly that in some of the biggest earthquakes in the state’s past, “quite a few tall buildings fared well, and some very well.”

He said, for example, that some 17-story high-rises had survived the San Francisco earthquake of 1906. (The San Andreas fault is about seven miles from downtown San Francisco, so downtown buildings would have been on the edge of the strongest near-source effects.)

Wyllie, however, said he feels that some new code revisions and building practices in California have removed certain “redundant protections” that might be advisable to restore.

Norman Abrahamson, a seismologist with the Pacific Gas and Electric utility, said probabilistic hazard studies he has undertaken indicate that the kind of large earthquakes (magnitude 7 and 8), that Heaton and Hall are concerned about will occur very infrequently in urban downtowns, where most high-rise buildings are located.

Abrahamson also noted that for the Heaton and Hall concerns to be realized, a rupture would have to be headed straight for the most urbanized areas, and the energy of the quake would have to be concentrated there. This could happen, he said, but perhaps only at very long intervals. He gave no specific number of years.

Testifying for the California Division of Mines and Geology, Michael Reichle said that in any event, building code upgrades are now largely out of the hands of the state, since there is a new international code system, and that California no longer will generate its own codes.

This drew a rebuttal from one of the commission members, Los Angeles City Councilman Hal Bernson, who said that since California has special quake hazards, it should supplement international codes with its own provisions.