Mount St. Helens Precursory Activity

Mount St. Helens from the north. Photograph courtesy of the USFS.

Early Warnings -Earthquake activity at Mount St. Helens was not monitored by seismologists until seismometers were installed near the volcano in 1972. From January 1975 through early 1980 only 44 earthquakes were located within 35km (22mi) of the volcano. The most recent period of unrest began on March 15, 1980. From the 15th through the 21st over 100 earthquakes were recorded. It was a magnitude 4.1⁽¹⁾ earthquake recorded on March 20 that provided scientists with the first early warning sign that Mount St. Helens might be preparing for an eruption for the first time since 1857.

Although a number of very small earthquakes had been recorded as early as March 15 they were not recognized as immediate precursors to possible volcanic activity. Following this first large earthquake on March 20 the number of earthquakes recorded increased dramatically.

Daily earthquake counts for mid-March 1980. Note the sharp increase in number of earthquakes on March 20 and 21. Figure modified from USGS Professional Paper 1250, p.100.

Seismogram of moderate earthquake recorded on the afternoon of March 20 at station SHW. Click for a larger image (109kb)

Daily Log

Thursday, March 20 - At approximately 3:45 PM a magnitude 4.1 earthquake occurred just northwest of the summit of Mount St. Helens at shallow depth. It was unlike any that had been previously detected in the area. Ground shaking from the earthquake triggered snow avalanches on some flanks of the volcano. Seismologists were uncertain as to whether or not these first earthquakes were related to volcanic activity. They decided to deploy additional seismometers in order to better monitor future activity.

Friday, March 21 - Earthquake activity increased. University of Washington (UW) seismologists warned the US Forest Service that continued ground shaking from sizeable earthquakes could cause new avalanches. Although seismologists were suspicious the earthquakes might be related to volcanic activity, they did not say so publicly. Three new portable seismic recorder stations and one radio-telemetered seismic station were installed. These stations were needed to determine more precisely the location, and more importantly, the cause of the earthquakes. Seismologists also consulted with the US Geological Survey (USGS) in regards to the current earthquake activity. Over the next week, continued concern over earthquake-triggered avalanches would lead the USFS to close the mountain above treeline.

Snow avalanche triggered by earthquakes at Mount St. Helens, 1980. USGS photo courtesy of Richard Waitt.

Univerisity of Washington seismologist installs a new seismic station at Mount St. Helens. University of Washington photo courtesy of Steve Malone.

(1) Preliminary Richter magnitude, subsequent results using a revised coda magnitude relationship yielded a magnitude of 4.2