UPPOSE that thousands of lives depend upon a complex system of electronic equipment, and one day an act of nature — an earthquake, perhaps — requires that the system be moved immediately without compromising safety. What to do?

That was the situation facing workers for the Federal Aviation Administration at Seattle-Tacoma International Airport after an earthquake on Feb. 28 knocked out the control tower and the air traffic control, or A.T.C., system.

"We did a lot of really serious scrambling," said Vic Owen, manager of the airport's Radar Automation Field Office. When the control tower was knocked out, controllers were out of business, Mr. Owen said, but "we have a contingency plan for when we go to A.T.C. zero."

While evacuating the top floor of the control tower, which suffered structural damage, F.A.A. workers set up three portable transceivers inside a hangar on the other side of the airport and tuned them to the radio frequencies for ground control and the tower.

Two hours after the earthquake, the airport reopened and controllers were handling a limited number of takeoffs and landings from their location in the hangar. An airport approach control room, which handled flights several miles from the airport, continued operating in the lower part of the damaged tower.

With no radar, and no overhead view of the airport (being at ground level), the controllers had to rely on the pilots to tell them when they had cleared a runway or a taxiway.

"The controllers basically had those three radios on a table and that was it," Mr. Owen said. "It was enough to talk to the aircraft and get them spaced out a lot further so safety was never compromised."

A mobile backup tower that would put the controllers about 15 feet above ground level — far less than the permanent tower's 100 feet — was brought in from 20 miles away. Primarily used for directing aircraft at forest fires and air shows, the mobile tower contained only rudimentary equipment. So radar display equipment was removed from the damaged tower and installed in the temporary tower.

Even with the mobile tower, controllers faced a hurdle: how to transmit radar signals to the mobile tower from the radar control unit 1,000 feet away. Ground radar, known as Airport Surface Detection Equipment, was crucial, since the controllers had little visibility.

Fortunately, a ring of fiber optic cable had been installed around the airport last year as part of an F.A.A. project to replace copper wire that had been used since the late 1940's. The loop had nodes that the radar system could be connected to. A spool of cable was flown in from San Francisco along with special fiber optic modems from Portland, Ore., and Salt Lake City that convert electronic radar signals to optical signals and back again.

"The most technically difficult part," Mr. Owen said, "was getting the fiber strung and connected from the radar sites to the tower."

The cable was run and modem interfaces built in time for the mobile tower to go into operation on the evening of Feb. 28, less than 12 hours after the earthquake.

"Having the fiber optic loop," Mr. Owen said, "allowed us to restore our radar services in the mobile tower much more quickly than we could have otherwise."

The next step was to move the mobile tower to a higher location. Officials with the Port of Seattle, which operates the airport, hired a construction company to build an embankment on the west side of the airport about 20 feet above the runways. To provide additional height, two ocean shipping containers were brought in and welded together, and a frame was built on top to support the mobile tower. Additional cable was run from the closest radar site in preparation.

Early Saturday morning, the controllers returned to using portable radios while the mobile tower was disconnected and moved to the embankment site and then lifted onto the containers. Other workers set up antennas, connected more cable and fine-tuned the equipment.

By noon Saturday, the new location was up and running, putting controllers about 40 feet above the runway and allowing the airport to run at 90 percent of capacity. The mobile tower is expected to be in operation for at least two months while the permanent tower is repaired.

While fiber optic technology proved invaluable, other technology fell short.

The cellular telephones that controllers had come to rely on were useless after the earthquake, and land lines were jammed. The walkie- talkies they had kept after converting to cell phones were few and old.

"We discovered that reverting back to basic two-way radios was what we really needed," Mr. Owen said. "If there is a next time, we'll have better two-way radio communications."