The major seismic events at Mount St. Helens were:
- A gradual increase in daily earthquake counts from March 15 to
March 20, 1980.
- The first magnitude-4.2 earthquake, located at shallow depth beneath
Mount St. Helens, 1547 PST on March 20.
- An increase in hourly counts primarily of low-magnitude tectonic-like
earthquakes starting March 21.
- Numerous earthquakes saturating the SHW seismograph record at
approximately noon on March 25.
- Earthquake counts peak on March 27, the day of the first observed
eruption activity.
- Gradual decrease in earthquake counts after March 27, accompanied by
and increasing number of larger magnitude earthquakes.
- 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.
- Largest magnitude earthquake, 5.1, occurring on May 18 at 0832 PDT.
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