Mt St Helens Level 3 Explanation
Posted by Petra on October 03, 2004 at 01:05:20:

Hi All,

This is why they decided to go to Level 3 and issue further evacuation notices...Petra (see full story via the link below)

Silence followed but was quickly replaced by an hourlong harmonic tremor — steady, rumbling seismic activity — signaling to scientists the upward movement of fresh magma into the mountain. Monitors more than 70 miles away registered the low, steady rumble, prompting the increased warning and evacuation.