The number of small earthquakes at Mount St. Helens increased on Thursday while the volcano's lava crust shifted further, government scientists said, keeping the volcano's alert status at the second-highest level.

There is a 70 percent chance of an eruption or explosion of the volcano's lava crust over the next few days, said Willie Scott, a scientist with the U.S. Geological Survey.

Any eruption would be small compared with the one that dramatically changed the shape of the mountain in May 1980, killed 57 people, destroyed more than 200 homes and flattened acres of evergreen forest in eastern Washington.

Small earthquakes were occurring at the rate of three to four per minute, with larger ones of magnitude 3 to 3.3 detected every three to four minutes, the USGS said in a statement. The lava dome that formed after the 1980 eruption has moved 2.5 inches since Monday.

The eruption on May 18, 1980, blew off the top of the volcano, reducing its summit from 9,677 feet to 8,364 feet.

Any potential eruption would probably be similar to a minor 1986 eruption that disrupted the lava dome in the crater, Scott said.

Ash from the 1980 eruption billowed across North America and was carried as far east as Oklahoma. "Anytime there's ash in the air where there is aircraft, it is a concern," Scott said. Volcanic ash can make aircraft engines stall.

Air traffic officials said they were prepared to divert air traffic in case of an ash-spewing eruption.

Government scientists are also conducting daily helicopter flights over the volcano's horseshoe-shaped crater to try to detect any signs of gases associated with the movement of magma, or molten rock. So far, they have reported nothing out of the ordinary.

Seismologists said there was no connection between activity at Mount St. Helens and a strong earthquake near Parkfield, Calif., or a smaller series of quakes in Alaska earlier this week.

Television trucks line up in sight of Mount St. Helens, which scientists say will likely erupt in the next few days.