EARTHQUAKE / THE ROAD TO RECOVERY : Seismic Experts Differ on Pacoima Dam : Temblor: Caltech professor says it might not have held if full. Officials marvel at massive ground motions.

Pacoima Dam survived a pounding by the Northridge earthquake, but the huge ground motions recorded there Jan. 17 should cause dam builders and operators in seismic zones worldwide to re-examine the safety of their structures, a dam expert said Thursday.

Appearing in Burbank before the state Seismic Safety Commission, Caltech engineering professor John F. Hall said he believed the 365-foot-tall dam in the San Gabriel Mountains, which will require costly repairs, might not “have remained intact” if the quake had struck on one of the rare occasions when its reservoir was full.

But Vernon H. Persson, state dam safety chief, said he believed the massive impoundment would not “have failed even if full.”

While noting that area dams came through the quake without a single failure, commissioners and earthquake experts marveled at the enormous ground accelerations measured at Pacoima Dam, which reached a peak level of twice the force of gravity.

When “people around the world involved in dam engineering see the records, they’re just going to be amazed,” Hall said in an interview.

Experts say the quake caused shaking far stronger than would be expected in a 6.8 quake. Ground motion recordings also confirmed that there is something about the setting of the dam--perhaps involving the steep canyon walls--that amplifies earthquake shaking. Ground motion recordings were so high there during the 1971 Sylmar quake that some experts attributed them to instrument error.

Hall, who led a quake reconnaissance team for the Earthquake Engineering Research Institute, said ground motion readings at the dam should not be dismissed as a local anomaly because many dams bridge steep canyons.

Persson, chief of dam safety for the state Department of Water Resources, said that dams, in general, did well in the quake.

Of 108 state-supervised dams in the quake area, only five, including Pacoima, sustained enough damage to need detailed inspection and repairs--and Persson said even those pose no immediate threat to public safety.

The state lacks jurisdiction over about a dozen federal dams in the region, and a commission staff member said the Army Corps of Engineers had declined a request to address the panel. Persson said corps officials had inspected the area’s federal dams “and said they were in good shape.”

The Los Angeles County Department of Public Works, which uses Pacoima Dam for flood control, has hired consultants to assess damage to the 65-year-old structure--once the tallest concrete arch dam in the world.

The quake created a two-inch gap between the top of the dam and a massive concrete block connecting it to the canyon wall. It also caused that concrete connection to move about a half-inch downstream relative to the dam’s crest.

Damage from the Sylmar quake was similar, although less severe, and the dam was repaired and strengthened at a cost of about $3 million. Ken Swanson, a supervising engineer with the public works department, said officials hope by the end of the month to have a preliminary estimate of repairs needed after the Northridge quake.

As a precaution, the reservoir pool is being kept at a maximum elevation of 1,884 feet--66 feet below the spillway--where it was when the earthquake struck.

For several years after the Sylmar quake, state officials restricted water levels behind the dam, until convinced it had been adequately repaired and strengthened.

The four other dams include the Los Angeles Reservoir in Sylmar and two old, adjacent dams now used as storm drains, according to Mitchell Sakado, an engineer with the city Department of Water and Power, who said repairs to the structures will cost more than $1 million.

The fifth dam, built in 1888 and empty, is on Porter Ranch Development Co. property at the western end of Chatsworth Street.

* RELATED STORY: A1