SEATTLE - Scientists studying Mount St. Helens say small earthquakes have started rumbling more frequently beneath the volcano in recent days, though they're not sure what it means.

Some scientists think the change is significant, while others say it's just more of the same and that the slow, steady growth of the lava dome in the volcano's crater could continue for as long as a year.

Jeff Wynn, chief scientist for volcano hazards at the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash., said the seismic and electronic signals being picked up from the volcano aren't like anything scientists have seen since the mountain reawakened 31/2 weeks ago.

Four days ago, a seismograph reported a chain of earthquakes looking like a series of heartbeats - seismic activity rising and falling. On Monday, a seismograph hooked up to the same part of the mountain showed what looked like a chain of pearls - steady, more frequent earthquakes but without the more dramatic rising and falling of activity.

Steve Malone, director of the Pacific Northwest Seismograph Network, based at the University of Washington in Seattle, said only a highly skilled observer could detect the slight changes that have occurred in recent days. "This is really not anything significant," he said.

Geologists have said there is little chance of anything similar to the eruption that blew 1,300 feet off the top of the peak and killed 57 people on May 18, 1980.

Wynn said the new signals showed a steady flurry of small quakes, similar to ones recorded in 1984. The earthquakes have been less than a magnitude 1, but Wynn said they've been happening more frequently - about every 10 minutes - than they were several days ago.

Thousands of earthquakes - many of them tiny, some of them topping magnitude 3 - have shaken Mount St. Helens since Sept. 23, apparently from magma breaking through rock as it rose toward the surface. Several steam bursts followed, and geologists detected lava at the surface last Tuesday.

The previous round of dome-building lasted six years, beginning in the months after the May 1980 eruption.

Clouds and fog covering the crater over the weekend and on Monday prevented scientists from seeing exactly what was happening there. The weather also prevented helicopters from flying over to give scientists a closer look.

The National Weather Service reported the clouds may clear away on Wednesday. Wynn said a cloud break would give scientists a chance to experiment with a new way of looking at the volcano using small, unmanned aircraft.

The drones, 5 feet long with an 8-foot wingspan, were developed for the military. If the experiment works, the small unmanned craft could help scientists keep track of what's happening at Mount St. Helens during the fall and winter months.

"Normally, you can't see this volcano for six months out of the year because it's socked in the clouds," Wynn said. "This is the Pacific Northwest."

Meanwhile, parts of the mountain were blanketed with snow, which is expected to fall every day, but probably will not have much impact other than setting off steam when it touches the hot surface of the growing dome.

Wynn said the last reading of surface temperature on the new lava dome registered 730 degrees Celsius, or about 1,350 degrees Fahrenheit.

In December or January, when more snow starts to accumulate, there could be more debris flowing into rivers and streams if the snow falls while lava is making its way out of the volcano, said Jon Major, a USGS geologist. Wynn said sediment flowing into rivers probably would not threaten people downstream.

The Johnston Ridge Observatory, the closest observatory to Mount St. Helens at five miles away, probably won't reopen this year, said Forest Service spokesman Tom Knappenberger.

Johnston Ridge normally closes at the end of October when snow makes it hard to reach the observatory, which sits at an elevation of 4,200 feet.