Mount St. Helens seismic activity.

The USGS recorded seismic activity in the area of Mount St. Helens shortly before it erupted in 1980. Mount St. Helens erupted on May 18th 1980 and for two (2) months prior to the eruption the area recorded earthquakes at a gradual increase in daily counts from March 15th to march 20th. A magnitude 4.2 earthquake was recorded on March 20th in the afternoon; it was located at a shallow depth beneath Mount St. Helens. On March 21st there was an increase in hourly counts of low magnitude tectonic like earthquakes. Seismic activity increased over the next six (6) days then on March 27th earthquake counts peaked. This is also the first day that people observed eruption activity. After the March 27th volcanic activity there was a decreased count in seismic activity, but the number of larger magnitude earthquakes increased. The largest magnitude earthquake was 5.1 occurring at 8:32am on May 18th this was about 20 seconds before Mount St. Helens’ erupted

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