Study finds major earthquake risk to bridges and overpasses in Seattle

SEATTLE (AP) - Bridges and overpasses on Interstate 5 would likely collapse in a major earthquake in the Puget Sound region, a consultant has reported.

As many as 13 bridges would probably be badly damaged in a quake of 6.5 magnitude along the Seattle Fault, which extends beneath Puget Sound, the city's downtown area and the suburbs east of Lake Washington, according to findings released by Don Ballantyne, vice president of the risk consulting firm EQE.

A similar earthquake on a fault believed to run beneath Tacoma would probably knock down freeway and railroad bridges over the Puyallup River, said Steve Bailey, county director of emergency management.

Ballantyne and Bailey were among experts presenting findings Wednesday at an earthquake conference sponsored by the U.S. Geological Survey.

The conference also was told that scientists have found evidence of surface ruptures along the Seattle Fault at a construction site in Bellevue, at two sites on Bainbridge Island and beneath I-5 just south of I-90. Surface ruptures indicate a greater likelihood of shallow quakes that could cause greater damage in the immediate area, research geologist Brian Sherrod said.

The frequency of shallow magnitude 6.5 to 7.0 quakes along the Seattle Fault is uncertain, but geologists believe the last occurred about 1,000 years ago, thrusting some shorelines upward by more than 20 feet. The recent breaks may be from that temblor.

So far there is no evidence that the apparent Tacoma fault has triggered significant earthquakes in recent geologic times, said Tom Brocher of the Geological Survey.

Ballantyne is working with the ports of Tacoma and Seattle and King and Pierce counties to study and prepare for earthquake effects on transportation and trade.

Most of the state's freeway bridges were built in the 1950s and '60s before building codes required more earthquake-resistant engineering. The most vulnerable spans are supported by simple round columns, including about one-third of the I-5 overpasses and bridges between Tacoma and I-90, he said.