earthquakes are always a prelude to an eruption........they are usually lots of small ones though
Mount St. Helens remained dormant from its last period of activity in the 1840s and 1850s until March 1980.[11] Several small earthquakes, beginning on March 15, indicated that magma might have begun moving below the volcano.[12] On March 20, at 3:45 pm Pacific Standard Time, a shallow, magnitude-4.2 earthquake centered below the volcano's north flank,[12] signaled the volcano's return from 123 years of hibernation.[13] A gradually building earthquake swarm saturated area seismographs and started to climax at about noon on March 25, reaching peak levels in the next two days, including an earthquake registering 5.1 on the Richter scale.[14] A total of 174 shocks of magnitude 2.6 or greater was recorded during those two days.[15]

Shocks of magnitude 3.2 or greater occurred at a slightly increasing rate during April and May, with five earthquakes of magnitude 4 or above per day in early April, and eight per day the week before May 18.[13] Initially, no direct sign of eruption was seen, but small earthquake-induced avalanches of snow and ice were reported from aerial observations.