Mt St Helens - More than 400 earthquakes, yet no imminent eruption

VANCOUVER, Wash. (AP) — More than 400 earthquakes have been detected beneath Washington’s Mount St. Helens in recent months, though there are no signs of an imminent eruption, according to the U.S. Geological Survey.

Most of the quakes over a three-month span beginning in mid-July were less than magnitude 1.0 and too small to be felt at the surface, the agency reported last week.

The sudden reawakening of Mount St. Helens in late September 2004 was surprising because the preceding four years had seen the fewest earthquakes since the 1980-86 eruption ended.

In the early hours of 23 September 2004, a swarm of small-magnitude (< 1), shallow earthquakes (< 1 km or about 0.5 mi below the surface) began beneath the 1980-1986 lava dome. Over the next seven days, earthquakefrequency and size increased and a growing welt formed on the SE margin of the 1980-86 lava dome and nearby portions of Crater Glacier. By September 26, 2004, the rise in activity led scientists to warn of an increased chance of explosions from the lava dome.

On October 1, 2004, the first of several explosions shot a plume of volcanic ash and gases into the atmosphere from a vent on the southwest margin of the growing welt. Four additional steam and ash explosions occurred through October 5th, and three produced noticeable fallout of fine ash downwind.