Harmonic tremor describes continuous rhythmic earthquakes that can be detected by seismographs. Harmonic tremors often precede or accompany volcanic eruptions.
During the first week of April a different type of seismic signature was showing up on seismograms from Mount St. Helens. On April 1 the first weak harmonic tremor was detected. Stronger bursts of harmonic tremor were recorded on the 2nd. The seismogram above is an example of harmonic tremor recorded at seismic station RAN on April 2. (Image courtesy of the Volcano Hazards Program website)