Mount St. Helens, Washington
Summary of Important Seismic Events,
March 20 - May 18, 1980
-- Excerpt from:
Endo, E.T., Malone, S.D., Noson, L.L., and Weaver, C.S., 1981,
Locations, Magnitudes, and Statistics of the March 20 - May 18 Earthquake Sequence:
IN: Lipman, P.W., and Mullineaux, D.R., (eds.),
The 1980 Eruptions of Mount St. Helens, Washington:
U.S. Geological Survey Professional Paper 1250, 844p
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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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