A major earthquake jolted eight northwestern states this morning and unleashed flying debris that killed two Idaho children, the first deaths from a continental U.S. earthquake in 12 years.

The quake dislodged the stone front wall of a building in Challis, Idaho, crushing Tara Leaton, 7, and Travis Franck, 6, as they walked to school. Storefronts collapsed in the tiny town of Mackay, 50 miles southeast of Challis.

Although its center was near Borah Peak in south central Idaho, the quake toppled chimneys and cracked walls in eastern Montana and rocked buildings as far away as Washington, Oregon, Utah, Nevada, Wyoming, North Dakota and parts of Canada.

"It reminded me of one of those sonic booms," said Pat Azevedo, a proprietor of Hawkins Automotive Texaco Station in Mackay. "It roared and started to roar some more, until I realized what it really was." Azevedo said many of the dishes in her cupboard fell, and a crack opened up in the side of her service station so "you can see the sun coming right through."

The U.S. Geological Survey in Golden, Colo., said the temblor struck at 8:06 a.m. MDT, lasted 30 to 60 seconds and registered 6.9 on the Richter scale. It was the worst earthquake in the area since the 1959 Hebgen Lake earthquake in southwestern Montana, and the strongest in the continental United States in 24 years. Edna King of the U.S. Geological Survey in Menlo Park, Calif., said the Hebgen quake registered 7.1 on the Richter scale and killed 21 persons.

The last fatal earthquake in the continental United States occurred in the San Fernando Valley in 1971, registering 6.5 on the Richter scale and causing 65 deaths. Two persons died in Hawaii in 1975 in a seismic sea wave caused by a quake that measured 7.2 on the Richter scale.

Idaho officials had no immediate estimate of damages from the quake, but they said at least three persons had been injured in addition to the two who died.

Mackay city employe Tom Shinderling, who was working near the site where the two children were killed, said that Tara's mother "was watching her walk by on the street when the building collapsed. I ran over and started helping them remove the rocks."

Beverly Bowers, an employe of a nearby gift shop, said a small group of volunteers moving rubble to free the children grew to about 20 in the 45 minutes it took to uncover them. "It took four or five of them to move some of the blocks, they were so big," she said.

Another resident said he was told that the children had been crossing the street before the quake hit. "The boy saw a car coming and screamed at her to get back. If the kids wouldn't have run back to the sidewalk they wouldn't have gotten hit by the building," he said.

Idaho Gov. John V. Evans declared Custer County, where the quake was centered, a disaster area and rescue crews moved through the rubble seeking other victims. Part of a Challis high school collapsed, and dust clouds hung in the air in Challis and Mackay after the quake struck. A section of highway dropped six feet.

Bob Thornborrow, 28, a farmer from Buhl, Idaho, his wife, Carlene, and year-old son, Sonny, were camping at the foot of Borah Peak only three miles from the estimated epicenter of the earthquake. When the shock wave hit, Thornborrow said, "We thought our camper was rolling down the hill at about 30 miles an hour."

They did not realize that it was an earthquake until Carlene Thornborrow ran forward to the driver's seat and stepped on the brakes, finding that they had no effect on the camper's apparent movement.

When they tried to drive back to Mackay, a 15-foot-deep crevice on the exit road cut them off. "Our camper is still up there," Thornborrow said.

Tonight city workers were trying to clean up the debris on Mackay's Main Street. Bricks littered the sidewalk from the collapsed side of Perk's Bar and Mackay Drugs. A fine coating of dust lay over much of the city's downtown area, and a county health worker was distributing notices telling residents to boil their water for the immediate future.

Deputy Sheriff John Burroughs said the only injury to a Mackay resident occurred when a woman parked her car next to Mackay Drugs and walked toward the Idaho First Bank just as the earthquake struck. Bricks from the drugstore wall rained down on her and her car, knocking her to the ground. Burroughs said, however, he understood that she suffered only severe bruises.

Mackay is a small farming community in a valley of Idaho's rolling central mountains. Many of the town's residents work at the military nuclear research plant 20 miles east of Arco.

Police said nearly every building in town has suffered cracked windows or other damage, and at least 10 downtown buildings had brick walls collapse into the streets.

Burroughs said most town residents felt they were lucky to have sustained as little damage and injury as they did.

Local officials said it was fortunate that the quake had occurred in a sparsely populated area. The last major U.S. earthquake, centered near Coalinga, Calif., on May 2, registered 6.5 on the Richter scale, causing $30 million in damage and 45 injuries but no deaths in the small agricultural community.

In Boise, plaster fell, elevators swayed and streets seemed to be bouncing up and down as drivers felt the quake. State employes at the capitol building briefly fled their offices for the safety of the basement, and classrooms at Boise State University were evacuated.

Two of the 15 nuclear reactors at the Idaho National Engineering Laboratory, the military research center about 50 miles southeast of the quake center, were running when the shock hit but seismographic monitors automatically shut them down. Department of Energy spokesman Anne Hofford said there were no radiation leaks.

Harry James, a Challis cattle rancher, said he was loading cattle south of town when the quake hit. "The mountains looked like they were moving," he said, "and suddenly the cattle gate was swinging from side to side. It was kind of a nauseating feeling."

James said he got home to find that his wife had escaped unhurt when a boulder had smashed through the wall of his house. King said U.S. Geological scientists think that the quake may have been caused by slippage on the Lost River fault near Borah Peak, at 12,662 feet the highest point in Idaho. Seismographs recorded more than 15 aftershocks within six hours of the first major earth tremor, the largest of them registering at 5.5.

A helicopter flight today over the quake's epicenter near the peak revealed a fissure about 25 miles long in the sagebrush-covered slopes.

A nearby earthen dam above Mackay was undamaged, but a natural artesian well opened by the quake was spilling water into the reservoir above the dam. Buses waited at Mackay High School, ready to take residents to higher ground if the dam should break.

Although no major quakes had been recorded on the Lost River fault, scientists considered the crack in the earth's crust to be "suspicious" and had evidence that it had moved in the last 10,000 years, King said.

Special correspondents Katharine Macdonald and Hal Berton contributed to this report.