VANCOUVER, Wash. (AP) - Earthquake activity increased further early Thursday at Mount St. Helens, and one scientist put the chance of a small eruption at 70 percent.

Jeff Wynn, chief scientist at the U.S. Geologic Survey's Cascade Volcano Observatory in Vancouver, Wash., said tiny quakes were occurring at the rate of three or four a minute. Larger quakes, with magnitudes of 3 to 3.3, were occurring every three or four minutes, he said.

New measurements show the 975-foot lava dome in the volcano's crater had moved 2.5 inches to the north since Monday, Wynn said.

"Six centimeters may not sound like a lot, but imagine taking a 1,000-foot-high pile of rocks and moving it 2.5f inches. For a geologist, that's a lot of energy," Wynn said.

Wynn estimated there was a 70 percent chance the activity will result in an eruption.

Scientists did not expect anything like the mountain's devastating eruption in 1980, which killed 57 people and coated much of the Northwest with ash. On Wednesday, they warned that a small or moderate blast from the southwest Washington mountain could spew ash and rock as far as three miles from the crater at the 8,364-foot peak.

Scientists planned to fly over the volcano again Thursday to test for gasses that could indicate the presence of magma moving beneath the volcano.

The volcano began rumbling more intensely Wednesday, with earthquakes ranging from magnitude 2 to 2.8 coming about four times a minute and possibly weakening the lava dome in the crater of the 8,364-foot mountain, the U.S. Geological Survey said.

The heightened alert has drawn sightseers to observation areas, especially along Washington 504 leading from Interstate 5 eastward toward the volcano, Dawn Smith, co-owner of Eco Park Resort told The News Tribune of Tacoma.

"It's just been crazy the past couple of days," Smith said.

A sign in front of the business read, "Here we go again."

Few people live near the mountain, the centerpiece of the Mount St. Helens National Volcanic Monument in the Gifford Pinchot National Forest about 100 miles south of Seattle. The closest structure is the Johnston Ridge Observatory, about five miles from the crater.

"Whoa, look at that one," said Barbara Wilson of Sebastopol, Calif., one of a number of visitors huddled around a seismograph registering the quakes at Johnston Ridge on Wednesday.

She said she was not at all concerned about her safety, but another in the group, Harold Hassold, 70, a retired certified public accountant from Vancouver, Wash., was more cautious.

"I don't think anyone really knows what's going to happen with the activity that's going on," he told the Tacoma newspaper. "If they knew what was going to happen, they wouldn't have all those scientific instruments up here."

The Geological Survey raised the mountain's eruption advisory from Level 2 to Level 3 out of a possible 4 on Wednesday, prompting officials to begin notifying various state and federal agencies of a possible eruption. The USGS also has asked the National Weather Service to be ready to track an ash plume with its radar system.

In addition, scientists called off a plan to have two researchers study water rushing from the crater's north face for signs of magma. A plane was still able to fly over the crater Wednesday to collect gas samples. Negligible amounts of volcanic gas were found.

The USGS has been monitoring St. Helens closely since Sept. 23, when swarms of tiny earthquakes were first recorded. On Sunday, scientists issued a notice of volcanic unrest, closing the crater and upper flanks of the volcano to hikers and climbers.

Scientists said they believe the seismic activity is being caused by pressure from a reservoir of molten rock a little more than a mile below the crater. That magma apparently rose from a depth of about six miles in 1998, but never reached the surface, Wynn said.

The mountain's eruption on May 18, 1980, blasted away its top 1,300 feet, spawned mudflows that choked the Columbia River shipping channel, leveled hundreds of square miles of forests and paralyzed towns and cities more than 250 miles to the east with volcanic ash.

On the Net:

http://vulcan.wr.usgs.gov/Volcanoes/Cascades/CurrentActivity

http://www.pnsn.org/HELENS/welcome.html