A dark coat of ash blankets the southeast flank of Mount St. Helens follwoing the first explosion. USGS photo courtesy of Dan Miller.

Earthquakes to Explosions - Unlike a typical aftershock sequence in which the number of earthquakes decreases with time, the number of earthquakes following the 4.1 ⁽¹⁾ earthquake on March 20 increased. Scientists were uncertain as to whether or not the earthquakes would lead to renewed volcanic activity. By March 24, however, they became convinced that the current earthquake sequence could be a precursor to renewed eruptive activity at Mount St. Helens. On March 27 their suspicions were confirmed when Mount St. Helens began a sequence of phreatic explosions that would continue intermittently until May 14.

Monitoring Earthquakes at Mount St. Helens

Earthquakes at Mount St. Helens were monitored by the University of Washington in cooperation with the US Geological Survey. Data were received at the UW Seismology Lab in Seattle and recorded on helicorder drums and in digital format. Seismologists used this data to locate and determine the magnitude of most of the larger earthquakes.

Paper and ink seismograms of earthquakes from Mount St. Helens were generated on helicorder drums. University of Washington photo courtesy of Steve Malone.

Earthquakes were located with the help of computers. University of Washington photo courtesy of Steve Malone.

Data from the many earthquakes were stored on magnetic computer tapes. University of Washington photo courtesy of Steve Malone.

A Dramatic Increase in Seismicity

Following the first large earthquake on March 20 the number of earthquakes recorded beneath the volcano increased steadily. Earthquake counts went from several to 15 per hour over the course of the next few days. Most were less than magnitude 2.0. By March 24 as many as 20 per hour were recorded. That same day another magnitude 4.0 earthquake was recorded at shallow depth beneath the mountain. On March 25 at approximately noon a large number of events saturated the record of the SHW drum recorder. Between March 20 and 25 there were only 10 earthquakes of magnitude 2.6 or larger recorded. During the two days prior to the March 27 explosion there were 174 earthquakes with similar magnitudes.

Number of earthquakes per hour from March 21 to approximately 1PM on March 25, when the record from seismic station SHW became saturated with nearly continuous earthquake activity. (figure modified from USGS Professional Paper 1250, p.100)

Earthquake records from seismic station SHW for March 22 through 25 show this dramatic increase in the number of earthquakes occurring beneath the volcano. Click on the image or header below to see a larger version of the seismograms (215 and 261KB respectively).

Daily Log

Saturday, March 22 - A second earthquake larger than magnitude 4.0 occurred beneath Mount St. Helens. UW seismologists concluded that this was not an earthquake-aftershock sequence, but rather a swarm of individual earthquakes. The danger from earthquake-triggered avalanches remained high.

Sunday, March 23 - UW seismologists visited the three new portable seismic data recorder units to retrieve data today, but were frustrated as the stations were not functioning properly. Other telemetered data showed a sharp increase in activity, including 5 earthquakes larger than magnitude 3.0 and one larger than magnitude 4.0. UW seismologists determined that the current earthquake sequence could be precursory to volcanic activity.

Monday, March 24 - Earthquake activity continued to increase, with 10 earthquakes larger than magnitude 3.0 and 4 larger than magnitude 4.0 (including one of magnitude 4.7). The University of Washington seismology lab was being staffed 24 hours a day to handle the amount of incoming data. One new station was added and one of the portable seismic data recorders replaced. UW seismologists advised the US Geological Survey's Volcano Hazard Project in Denver and Menlo Park and the USFS that a "tremendous increase in seismicity" has been observed and that it appeared to be a volcanic earthquake sequence. Seismologists and USFS officials began to ask people to stay away from Spirit Lake and Mount St. Helens.

Seismologist examines a malfunctioning portable data recorder unit for one of the new seismic stations. University of Washington photo courtesy of Steve Malone.

Tuesday, March 25 - Five earthquakes larger than magnitude 4.0 occurred within a one hour period and as many as 22 occurred within one 8 hour period (the total number of earthquakes larger than magnitude 4.0 was 25). As of 1PM earthquakes occurred so frequently that the record for seismic station SHW became saturated. Individual earthquakes with a magnitude less than 3.5 were lost due to the continuous traces of larger ones. Donal Mullineaux, co-author of the USGS Bulletin "Future Eruptions of Mount St. Helens, Washington," left Denver for Vancouver to advise the USFS and local officials on the developing situation. USFS officials closed the Spirit Lake Information Center, access to the mountain above tree line, and several forest roads leading to Mount St. Helens. Due to increased interest from the media, photographers and scientists, the FAA imposed special flight restrictions near the volcano. Bud Kimball, a commercial aerial photographer, reported to the USGS office in Tacoma that he had photographed a large crack in the snow across the top of the volcano.

The Spirit Lake Information Center on the south shore of Spirit Lake was closed due to fears of earthquake-generated avalanches. Photograph courtesy of the USFS.

Wednesday, March 26 - The University of Washington recorded the 100th earthquake larger than magnitude 3.5, as well as 7 larger than magnitude 4.0. Depths for most quakes were less than 3 miles. Additional recording equipment was installed at the UW Seismology Lab to handle the amount of incoming data. Seismologists were working around the clock to locate earthquakes. Federal, state, and county emergency services officials met in Vancouver to hear USGS scientists explain their interpretation of the recent seismic events and potential hazards.

Press conference in Vancouver, Washington. Photograph courtesy of the USFS.

Thursday, March 27 - USGS officials issued a Hazards Watch to public officials. At 11:20AM an observer in an Army National Guard reconnaissance plane reported seeing a hole in the icecap near the summit and a gray streak extending southeast from the hole. At approximately 12:30 people near the volcano reported hearing a loud "boom," which probably marked the first sighted explosion. Mike Beard, a Portland radio reporter, was flying over the cloud-shrouded mountain and reported seeing "ash and smoke spewing out, a little like smoke out of a chimney." At 2:00PM UW seismologists recorded the second strongest earthquake to date (a magnitude 4.7). Following the earthquake a black plume was observed to rise about 7,000 feet above the volcano.

Earthquakes occurred so frequently that one University of Washington seismologist was quoted as saying the instruments had detected earthquakes so often that it "was difficult to tell where one ends and another begins."

Following the explosion a crater about 200 to 250 feet wide was reported near the summit. A series of cracks criss-crossed the summit from east to west and volcanic ash had darkened snow in a band across the crater and down the volcano's southeast slope.

Concerns over the increased potential for large rock avalanches (as a result of the cracks forming near the summit) and widespread flooding (due to heat from the volcano melting snow and ice) prompted the evacuation of hundreds within a 15 mile radius from the volcano. Among these were USFS employees and their families at the Pine Creek Ranger Station at the head of Swift Reservoir, 300 or so loggers from three Weyerhaeuser camps, and 20 employees from the state fish hatchery about 30 miles downstream on the North Fork Toutle River. Skamania County Sheriffs also evacuated about 45 people, mostly scientists and reporters, from the Spirit Lake area while Cowlitz County Sheriffs evacuated those further downstream on the North Fork Toutle River. Skamania County Sheriffs established a roadblock on State Route 504 approximately 7 miles from the volcano. By evening, however, USGS hydrologists had determined that there was no rise in streams draining the volcano.

Additional scientists arrived in the evening to study the continued seismic activity, measure possible ground deformation on the volcano, determine the chemical composition of volcanic ash and gases, and to monitor heat emissions, water levels and water quality.

Pacific Power and Light began drawing down all three reservoirs on the Lewis River on the south side of the volcano to accommodate flooding and mudflows from a possible eruption. Company representatives stated that the dams should withstand the most severe earthquakes expected in the area. The Oregon Museum of Science and Industry installed a seismograph exhibit with an expert on duty to field questions.

A Portland radio reporter described the first sighted explosion: "ash and smoke spewing out, a little like smoke out of a chimney." USGS photo courtesy of Dan Miller.

The first explosion blasted out a new crater 200 to 250 feet wide. University of Washington photo courtesy of J. Booker.

East-west trending crack systems crossed the summit. Photo courtesy of Pierre Stephenson.

Skamania County Sheriffs' roadblock on State Route 504. USGS photo coutesy of Pete Lipman.

Exposed shoreline along Swift Reservoir as Pacific Power and Light lowered the water level to make room for possible mudflows. USGS photo courtesy of Don Swanson.

Friday, March 28 Another interval of frequent phreatic explosions began at 3AM and lasted nearly 2 hours. Observations were hampered by darkness and bad weather, but airborne observers saw a cloud of ash and steam that rose more than a mile above the volcano. By nightfall at least 12 additional explosions had occurred, with columns of steam and ash reaching to nearly 10,000 feet above the mountain. One that occurred just before sunset threw out rocks as large as 3 feet onto the flanks of the volcano and sent ash as far east as Trout Lake (35 miles). Several ash-darkened avalanches were visible on the east flank. Some reached the 6400-foot level near the Plains of Abraham. Some observers mistook these for lava or mud flows, further adding to concerns of flooding in valleys draining the mountain.

Scientists from the University of Washington Cloud Physics Group flew over the volcano to collect samples of escaping gases. They found that sulfur dioxide was the main species of sulfur present. This was the first indication that a high-temperature magmatic source was releasing gas during explosions from somewhere inside the volcano. Scientists flew to the 4,000 foot level despite bad weather to collect ash samples for analysis to determine whether the volcanic ash was from new magma or older, pulverized rock.

One of at least 13 explosions from March 28. USGS photo courtesy of Dan Miller.

Aerial view of ash-darkened avalanches on the Plains of Abraham. USGS photo courtesy of Richard Waitt.

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

• Ahead to March 29 - April 4

• Back to March 15 - 21

• Return to Introduction

• Return to Menu

• Images

• Helicorders   (seismicity)

• References