Mount St. Helens, Washington
Summary of Important Seismic Events,
March 20 - May 18, 1980

The major seismic events at Mount St. Helens were:

1. A gradual increase in daily earthquake counts from March 15 to March 20, 1980.

2. The first magnitude-4.2 earthquake, located at shallow depth beneath Mount St. Helens, 1547 PST on March 20.

3. An increase in hourly counts primarily of low-magnitude tectonic-like earthquakes starting March 21.

4. Numerous earthquakes saturating the SHW seismograph record at approximately noon on March 25.

5. Earthquake counts peak on March 27, the day of the first observed eruption activity.

6. Gradual decrease in earthquake counts after March 27, accompanied by and increasing number of larger magnitude earthquakes.

7. Swarm following first eruption, dominated by shallow large-magnitude events with emergent P-phases and lacking clear S-phases. High-frequency components of seismic signals consistently attenuated at more distant seismic stations.

8. Largest magnitude earthquake, 5.1, occurring on May 18 at 0832 PDT.