Augustine Volcano began a pulsating eruption yesterday, pumping smoke and ash more than nine miles high and sending a cloud 150 miles long and 25 miles to 35 miles wide over the coastal communities of Homer and Kenai toward Anchorage, scientists said.

The 4,025-foot peak forms an uninhabited island in Cook Inlet about 175 miles southwest of Anchorage. "It's been erupting since midnight, but . . . the eruption waxes and wanes," said state seismologist John Davies.

By early yesterday afternoon, the first faint traces of ash were showing up in Anchorage. Authorities issued a health alert urging people to stay home and avoid exercise.

The Chugach Electric Association, which provides power to most of southcentral Alaska, asked nonessential businesses to close and requested customers to cut power consumption.

A score of airliners flying between the Orient and Europe were diverted from Anchorage to Fairbanks. Pilots were warned to stay out of the ash clouds and to cover their planes while on the ground.

The Air Force hustled several F15 Eagle fighter jets from Elmendorf Air Force Base outside Anchorage to remote stations. The Air Force, Army and Alaska Air National Guard moved aircraft into hangars to protect them from the ash.

Max Olford, assistant director of operations at Anchorage International Airport, said there were no plans to close the airport. "It's going to be up to the airlines to operate or not."

A government report after Augustine's last eruption in 1976 warned that avalanches triggered by a major eruption could generate a devastating tidal wave. Following a major eruption near the turn of the century, residents of Port Graham reported that a 30-foot wall of water crashed into their harbor.