EARTHQUAKE / LIFELINES OF L.A. : THE FAULT LINE : Tug of War Centered on Caltrans Seismic Map

In the immediate aftermath of the Northridge earthquake, the chief bridge engineer of Caltrans would blame an unmapped fault for the violent shaking that brought down key links in the area’s freeway network.

Engineer James E. Roberts would say later that he based that assessment on early information from seismologists who were studying where the earth moved before dawn on Jan. 17.

Although the precise fault line may have been a mystery, the potential for a severe quake and intense ground motion centered in the northern San Fernando Valley was not a surprise.

To assess the seismic hazard to the state’s freeways, Caltrans for years has been using the most comprehensive map ever prepared by the state detailing the maximum earthquake possible along 225 fault lines and zones in California.

Even before it was finished, the wall-sized map and an accompanying report sparked a behind-the-scenes controversy among geologists and seismologists. Some feared that the information would be misinterpreted by engineers and consultants in designing buildings.

Although Caltrans embraced the map’s conservative approach, some experts expressed concern that it does not indicate a key factor--the probability of a temblor occurring during a specified time period.

In stark black lines, the map shows that the Los Angeles region is riddled with faults capable of generating a quake of the same magnitude as the Northridge temblor or greater.

At least four fault zones believed to be capable of producing a 6.75-magnitude shock or greater crisscross the northern and eastern San Fernando Valley, including the Simi-Santa Rosa-Northridge Fault zone close to the epicenter of last month’s devastating quake. The map lists that fault zone as capable of generating a 7.5-magnitude quake. The Verdugo, San Fernando-Sierra Madre and Santa Susana thrust faults also are shown in the area.

Caltrans commissioned the map in 1982 for use in designing freeways and establishing priorities for reinforcing highway bridges and overpasses to withstand quakes. But it took the state Division of Mines and Geology 10 years--until 1992--to publish the work.

In July of 1989, less than three months before the Loma Prieta quake rocked Northern California, James H. Gates, chief of structural mechanics engineering for Caltrans, demanded the release of the map in a strongly worded memo to Joseph I. Ziony, then-assistant director of the state Department of Conservation.

“I find it incredible that anyone would question the desirability of releasing (the map). The map was developed using public funds and it exists,” Gates wrote.

“I am not a legal expert, but it seems to me that if a significant (quake) were to occur, the Division of Mines and Geology could be held responsible for withholding information which could have reduced damage or saved lives.”

Gates said the data on levels of ground shaking contained on the map is basic and “can be applied with engineering judgment to any structure, just as we do.”

Ziony no longer works for the state. After reviewing the memo, state geologist James F. Davis recalled that Gates “wanted the map to come out. His interests were taken into account.”

But when released by the Division of Mines and Geology, the report was labeled for Caltrans’ “internal use” and the map carried a prominent bold-faced disclaimer saying it was produced exclusively for Caltrans’ use in the design of highway bridges and overpasses.

“General use of this (map) for seismic hazard evaluation is not recommended because this map does not take into account the probability that a particular level of shaking will occur,” the disclaimer reads.

Davis said some members of a special advisory committee to the state’s Mining and Geology Board were concerned that broader use of the map might mislead engineering consultants and others in designing critical facilities, such as hospitals.

“That was a very special purpose map,” he said. “One should not rely on a single map for anything.” Instead, he suggested that engineering consultants make a full examination of available geologic information.

Davis said the Division of Mines and Geology is preparing a new map that will factor in the probability of a certain level of ground shaking occurring during a specified period of time.

Gates, now chief of the Office of Earthquake Engineering, said Caltrans is pleased with the map. The department has since hired the seismologist who prepared the report.

“We are interested in continuing development on that kind of map,” he said.

Caltrans officials say the information is critical in designing freeways and in assessing the seismic hazard to highway bridges and overpasses.

Since its development, the map has been used as one element in a complex formula to determine priorities for retrofitting freeways to resist quake damage, Gates said.

Charting the Faults

After a behind-the-scenes struggle with the state Division of Mines and Geology, Caltrans received the most comprehensive map ever prepared by the state listing the maximum earthquake expected along major fault lines in California. The map shows that the Los Angeles region, including the San Fernando Valley, is riddled with fault systems capable of generating a quake of the same magnitude as the Jan. 17 Northridge temblor or greater.

Source: California Division of Mines and Geology