Earthquake activity has increased at Mount St Helens, but scientists said on Saturday there was no reason to raise the volcano’s alert level.
Scientists said earthquake activity had been low until Friday, indicating molten rock was moving upward with little resistance. By Saturday, however, quakes of magnitude 2,4 were occurring every one to two minutes, they said.
“It’s at levels equal to or higher than the October 5 steam-and-ash eruption,” said Jeff Wynn, the United States Geological Survey’s (USGS) chief scientist for volcano hazards at Vancouver.
A bubble on the south side of the dome has also risen to at least 100m since scientists first spotted it on September 30 and is now almost as tall as the dome’s 300m summit, said USGS geologist John Pallister.
“The blister is a rather remarkable event,” he said, saying that it infers magma is less than 1,6km below the surface.
Scientists will monitor the bulge to see how its movement relates to the seismic activity, Pallister said.
Scientists have said earthquake activity is expected to ebb and flow, and the most likely scenario now is weeks or months of occasional steam blasts and possibly some eruptions of fresh volcanic rock. In order to raise the alert level, scientists said an eruption would have to be imminent or occurring.
Thousands of small earthquakes shook the peak as the 1Â 509m volcano in south-west Washington spewed clouds of steam and small amounts of volcanic ash each day from October 1 to 5, forcing the evacuation of thousands of people.
An hour-long blast on October 5 sent a thick cloud hundreds of metres into the air.
Seismic activity diminished afterward. On Wednesday, scientists lowered the alert level from “volcano alert” to “volcano advisory”, saying earthquake activity that could endanger lives was down to the lowest level since October 2.
There was no indication on Saturday that a dangerous eruption was imminent, said Larry Mastin, a USGS expert on the physics of volcano eruptions.
Cloudy weather on Saturday hampered visibility for scientists hoping to monitor developments.
Closures around the volcano — including the Johnston Ridge observatory north of the crater — remained in effect.
Johnston Ridge is usually closed for the winter by the end of October.
Geologists have said there is little chance of anything similar to the blast on May 18 1980, which blew 400m off the top of the peak, killed 57 people and covered much of the inland Pacific Northwest with gritty volcanic ash. — Sapa-AP
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