Earthquakes were growing stronger and more frequent at Mount St. Helens Thursday, hitting at the rate of three or four per minute, KIRO 7 Eyewitness News reported.

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Jeff Wynn, chief scientist at the U.S. Geologic Survey's Cascade Volcano Observatory in Vancouver, Wash., said larger quakes, with magnitudes of 3 to 3.3, were happening every three or four minutes.

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At a news conference Thursday afternoon at the Cascades Volcano Observatory in Vancouver, Cynthia Gardner of the U.S. Geological Survey said a third flight over the volcano detected no sign of gasses from magma.

Scientist believe the quakes are a sign the volcano may be building to a small eruption. They put the chances of an eruption at 70 percent within a few days to a month.

Measurements show the 975-foot lava dome in the volcano's crater has moved 21/2 inches to the north since Monday.

The USGS has been monitoring St. Helens closely since last Thursday, 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.

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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 visitors center at Johnston Ridge remained open after geologists raised the alert level at Mount Saint Helens to a "volcano advisory," the second highest warning of a possible eruption, KIRO 7 Eyewitness News reported.

The heightened alert has drawn a throng of 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.

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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.

Scientists did not expect anything like the mountain's devastating eruption in 1980, which killed 57 people.

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.